HSE Health & Safety

Executive

Offshore gas turbines (and major driven equipment) integrity and

inspection guidance notes

Prepared by ESR Technology Ltd for the Health and Safety Executive 2006

RESEARCH REPORT 430

HSE Health & Safety

Executive

Offshore gas turbines (and major driven equipment) integrity and

inspection guidance notes

Martin Wall, Richard Lee & Simon Frost ESR Technology Ltd

551.11 Harwell International Business Centre Harwell

Oxfordshire OX11 0QJ

Gas turbines are widely used offshore for a variety of purposes including power generation, compression, pumping and water injection. Relatively little information is included in safety cases, for example only the manufacture, model, power rating (MW), fuel types, and installation drawings showing the location of the turbines. Some descriptive text may be included on the power generation package, back-up generators and arrangements for power transmission to satellite platforms. Information on integrity management and maintenance is limited or at a high level.

This Inspection Guidance Note provides a more detailed assessment of gas turbines (GTs) and major driven equipment installed on UK offshore installations, focussing on integrity and maintenance issues. This complements the advice in HSE Guidance Note PM84, recently re-issued, covering control of risks for gas turbines used in power generation and HSE Research Report RR076 which provides general guidance on rotating equipment including turbines. The applications, systems and components of offshore gas turbines are reviewed. Guidance is given on the integrity issues and maintenance typical for different systems. Summaries are given of database information on the turbines installed on UK installations together with recent incident and accident data. Recent experience and anecdotal information from operators is also reviewed. The inspection guidance note is principally designed to provide information for HSE inspectors in safety assessments, incident investigations and prior to site visits. The note may also be of interest to manufacturers, suppliers and operators of gas turbines (GTs) used offshore.

This report and the work it describes was co-funded by the Health and Safety Executive (HSE) and the EUs Fifth Framework Programme of Research. Its contents, including any opinions and/or conclusions expressed, are those of the authors alone and do not necessarily reflect HSE policy.

HSE BOOKS

Crown copyright 2006

First published 2006

All rights reserved. No part of this publication may be

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Applications for reproduction should be made in writing to:

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or by e-mail to hmsolicensing@cabinet-office.x.gsi.gov.uk

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Acknowledgements

The authors would like to thank the HSE inspectors, turbine suppliers, operators and others who have contributed to this report and allowed pictures and other information to be reproduced. In particular we would like to thank the following HSE staff for their contribution: Prem Dua the project technical officer, Jim MacFarlane for advice on rotating equipment issues, Tom Gudgin for his valuable comments on electrical issues and control systems, Stan Cutts for advice in the context of the KP3 initiative, Danny Shuter for handling project issues and HSE inspectors who attended project seminars at Aberdeen, Bootle, Norwich and London for their comments. Rainer Kurz from Solar is thanked specifically for allowing us to use some of the images and introductory information from his IGTI 2004 paper. This project was initiated by the HSE Research Strategy Unit. The authors of HSE Research Report RR076 on rotating equipment are thanked for providing a starting point for present project.

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Foreword

This report covers the inspection and integrity of gas turbines (GTs) and major driven equipment (compressors, pumps, alternators). The focus is on offshore applications including floating installations and FPSOs. The work is directly relevant to HSEs Key Programme 3 (KP3) initiative.

The report is intended principally as an information source for HSE inspectors in safety assessments, incident investigations and prior to site visits. The note may also be of interest to manufacturers, suppliers and operators of gas turbines (GTs) used offshore.

The areas covered include: what can go wrong, typical inspection and maintenance, what is done differently offshore, relevant, codes and standards, hazards and safety concerns, good and best practice, summary of incident and accident data (RIDDOR, DO), a review of the main systems and components and how they work. A summary is given of advice in other HSE documents including PM84 and RR076.

Specific areas covered include: the basics of gas turbines, applications offshore, packaging concepts, electrical and control systems, major driven equipment, GTs on UK installations, safety codes and regulations (including environmental), hazards and failure modes, maintenance and inspection, operational issues and recent trends.

Section 1 provides an introduction and advice on use of the information in the report Section 2 gives an introduction to gas turbines, the types of gas turbines that are used offshore, packaging concepts and their applications. Section 3 summarises the main applications offshore Section 4 describes offshore turbine packages in more detail Section 5 summarises the integrity, safety and maintenance issues for major driven equipment building on the information in RR076 Section 6 addresses the associated electrical systems. Section 7 focuses on control systems a main safety consideration and recent developments including synchronisation and corrected parameter control Section 8 summarises the turbines installed in the UK sector. Section 9 covers safety cases, codes and regulations. Section 10 looks at degradation and failure modes including an analysis of incident, accident dangerous occurrence and reliability data. Summary tables are given by system and component. Section 11 looks at maintenance and inspection practice in-service and at overhaul. Section 12 looks at operational issues including hazards, start-up and shutdown, surge prevention, risk assessment and hazard management. Section 13 reviews recent trends in gas turbines including dry low emissions (DLE), micro-turbines, waste heat recovery systems and combine cycle gas turbines. Section 14 gives operational support guidance based on the principles developed in RR076. Section 15 gives examples of good and best practice with applicable guidance and regulations and references listed in Sections 16 and 17 respectively.

Supplementary information is included in a number of Appendices. Appendix 1 gives a current list of UK installations and Appendix 2 describes what would be included in a typical procurement package technical specification for gas turbines for a UK offshore installation. Appendix 3 reproduces HSE guidance note PM84 on gas turbines, Appendix 4 summarises the main turbine suppliers for UK installations derived from an analysis of DTI emissions data and other sources. The specifications for gas turbines used in the UK sector are summarised in Appendix 5. The key systems and components are described in more detail in Appendix 6.

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CONTENTS LIST

Foreword iv

1 Introduction to Inspection Guidance Notes 1

1.1 BACKGROUND 1

1.2 MAP OF GUIDANCE PROCESS 1

1.3 APPLICATION OF GUIDANCE NOTES 2

2 Basics of Gas turbines 3

2.1 INTRODUCTION 3

2.2 SYSTEMS AND COMPONENTS 4

2.3 HOW A GAS TURBINE WORKS 5

2.4 WORKING CYCLE 6

2.5 PRESSURE, VOLUME AND TEMPERATURE 7

2.6 CHANGES IN VELOCITY AND PRESSURE 7

2.7 GAS TURBINES OFFSHORE 8

2.8 TYPES OF GAS TURBINE 9

2.9 PACKAGING CONCEPTS 9

2.10 TURBINE PACKAGES 10

2.11 DESIGN FACTORS 12

2.12 TURBINE CONFIGURATION 12

2.13 DRIVEN EQUIPMENT 13

2.14 OFFSHORE ENCLOSURES 14

2.15 GAS TURBINE GT CYCLES 14

2.16 FUELS 15

3 Applications Offshore 17

3.1 POWER GENERATION 17

3.2 GAS GATHERING 18

3.3 GAS LIFT 19

3.4 WATERFLOOD 19

3.5 EXPORT COMPRESSION 20

4 Offshore Packages 21

4.1 MODULAR TURBINE PACKAGES 21

4.2 DESIGN OPTIONS 22

4.3 FPSO TURBINE PACKAGES 22

5 Major Driven Equipment 25

5.1 ALTERNATORS 26

5.2 COMPRESSORS 27

Applications 29

Package Elements 29

Package Configuration 30

Hazards 30

PM84 Guidance 31

Components 32

5.3 PUMPS 34

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6 Electrical Systems 35

6.1 ELECTRICAL SYSTEMS 35

6.2 ELECTRICAL SYSTEMS GUIDANCE 36

6.3 ELECTROMAGNETIC RADIATION 37

6.4 MAINTENANCE OF ELECTRICAL SYSTEMS 37

7 Control Systems 41

7.1 PM84 GUIDANCE ON CONTROL SYSTEMS 43

7.2 RECENT DEVELOPMENTS IN CONTROL SYSTEMS 43

Corrected parameter control 43

Control Synchronisation 44

Triple Modular Redundant TMR Control Systems 45

Redundant Network Control 46

Standard Control System 46

Software Architecture for a Standard control system 47

8 Gas Turbines on UK Installations 49

8.1 PACKAGERS 50

8.2 SUPPLIERS 50

9 Safety Cases, Codes and Regulations 53

9.1 RELEVANT UK INSTALLATIONS 53

9.2 INFORMATION FROM SAFETY CASES 53

9.3 HSE GUIDANCE NOTE PM84 53

9.4 DESIGN CODES 54

9.5 EMISSION REGULATIONS 55

9.6 ELECTRICAL REGULATIONS 55

9.7 LEGAL REQUIREMENTS 56

10 Hazards and Failure Modes 59

10.1 WHAT CAN GO WRONG 59

10.2 FAILURE MECHANISMS AND ANALYSIS 59

Creep 59

Thermo-mechanical fatigue 60

High-cycle fatigue 60

Metallurgical embrittlement 60

Environmental attack 60

Foreign body damage 60

Manufacture or repair 60

Failure analysis 60

Materials 61

Air Compressors 62

Combustors 62

Turbines 63

10.3 PM84 ADVICE ON MECHANICAL FAILURES 63

10.4 ANECDOTAL INFORMATION 64

10.5 ACCIDENT, INCIDENT AND DANGEROUS OCCURRENCE DATA 65

Data extracted 65

Analysis of Data 65

10.6 IMIA INDUSTRIAL GAS TURBINE MEMBERS FAILURE STATISTICS 69

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10.7 RELIABILITY DATA FOR GAS TURBINES 70

10.8 SUMMARY TABLES BY SYSTEM AND COMPONENT 70

10.9 OTHER HAZARDS 80

11 Maintenance and Inspection 81

11.1 OVERVIEW 81

11.2 INSPECTION & REPAIR 82

Refurbishment of Gas Turbine Components 82

Evaluation of damage 83

84 84 84 85 85 85 85 85 86

Disassembly

Dimensional checking

Non-destructive testing (NDT)

Metallurgical Examination

Defining of workscope

Processes

Nozzle and Vanes

Buckets and Blades

Quality records

11.3 MAINTENANCE GUIDANCE 86 88 90 90 90

Fuels

Water (or steam) Injection

Cyclic Effects

Rotor

11.4 DISASSEMBLY INSPECTIONS 94 94 94

Combustion Inspection

Hot-Gas-Path Inspection

11.5 MAJOR INSPECTION 96 11.6 TURBINE BORE INSPECTIONS 97 11.7 CLEANING 97 11.8 SUMMARY BY SYSTEM AND COMPONENT 99

12 Operational Issues 105

105 105

106 107

108 108

109 109

12.1 12.2 12.3 12.4 12.5 12.6 12.7 12.8 12.9 RISK ASSESSMENT FOR ROUTINE ACTIVITIES 111 12.10 ACCESS 112 12.11 HAZARD MANAGEMENT IN HOT-SPOTS 112 12.12 PRECAUTIONS AGAINST EXPLOSION 113 12.13 VENTILATION 114 12.14 FUEL SUPPLY SYSTEMS 116

HAZARDSSTART-UP AND SHUT-DOWNSURGE PREVENTIONRECYCLE FACILITYCONTROL SYSTEMSVIBRATION MONITORINGFIRE DETECTION REQUIREMENTSPRECAUTIONS AGAINST FIRE

12.15 GAS FUEL 117

12.16 ADDITIONAL EXPLOSION PRECAUTIONS FOR LIQUID FUELS AND OILS 117

12.17 EMERGENCY PROCEDURES 118

12.18 AIR AND GAS SEALS 118

12.19 CHANGEOVER IN DUEL FUEL SYSTEMS 118

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13 Recent Trends 119

13.1 MICROTURBINE DEVELOPMENT 119 13.2 DRY LOW EMISSIONS (DLE) 119 13.3 STEAM INJECTION FOR EMISSION REDUCTION AND POWER OUTPUT 120 13.4 WASTE HEAT RECOVERY UNITS 120 13.5 COMBINED CYCLE GAS TURBINES 120

14 Operational Support Guidance 123

15 Examples of good and Best practice 127

16 List of Applicable Guidance and Regulations 131

17 References 133

APPENDICES

Appendix 1 List of UK installations A1

Appendix 2 Typical procurement package technical specification A2

Appendix 3 HSE guidance note PM84 on gas turbines A3

Appendix 4 Gas turbine suppliers and summary for UK installations A4

Appendix 5 Specification of turbines used in UK sector A5

Appendix 6 Key systems and components A6

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1 INTRODUCTION TO INSPECTION GUIDANCE NOTES

This Inspection Guidance Note provides a detailed assessment of gas turbines (GTs) and major driven equipment installed on UK offshore installations, covering inspection, integrity and maintenance issues. This complements the advice in HSE Guidance Note PM841, recently re-issued, covering control of risks for gas turbines used in power generation. The report is also complementary to HSE Research Report RR0762,which provides more general advice on machinery and rotating equipment including GTs. The applications, systems and components of offshore gas turbines are reviewed. Guidance is given on the integrity issues and maintenance typical for different systems. Summaries are given of database information on the turbines installed on UK installations together with recent incident and accident data. Recent experience and anecdotal information from operators is also included. The guidance note is aimed at manufacturers, suppliers and operators of gas turbines (GTs) used offshore as well as to provide guidance to HSE inspectors in safety assessments, incident investigations and prior to site visits.

1.1 BACKGROUND

Gas turbines are widely used offshore for a variety of purposes including power generation, compression, pumping and water injection, often in remote locations. GTS are commonly duel fuelled, to run on fuel taken from the production process in normal operation or alternatively on diesel. Electrical power can also be generated to run other systems on the offshore installation. GTs offshore are typically from 1 to 50MW and may be modified aero-engines or industrial. Aeroderivative designs are increasingly used, particularly for the gas-generator. Lightweight industrial designs for offshore use are also available.

Relatively little information is included in safety cases, for example only the manufacture, model, ISO power rating (MW), fuel types, and installation drawings showing the location of the turbines. Some descriptive text may be included on the power generation package, back-up generators and arrangements for power transmission to satellite platforms. Information on integrity management and maintenance is limited or at a high level. This document is intended to provide more detailed information.

1.2 MAP OF GUIDANCE PROCESS

The guidance note is broken down into a number of discrete sections. Section 1 provides an introduction and advice on use of the information in the report. Section 2 gives an introduction to gas turbines, the types of gas turbines that are used offshore, packaging concepts and their applications. The main applications offshore and offshore turbine packages are covered specifically in Sections 3 and 4. The integrity, safety and maintenance issues for major driven equipment is summarised in section 5, building on the information in RR076.

Sections 6 and 7 address the associated electrical and control systems, a main safety consideration. Recent developments including synchronisation and corrected parameter control are included. Section 8 summarises the turbines installed in the UK sector, Section 9 covers safety cases, codes and regulations and Section 10 looks at degradation and failure modes including an analysis of incident, accident dangerous occurrence and reliability data. Summary tables are given by system and component. Section 11 looks at maintenance and inspection practice in-service and at overhaul. Operational issues including hazards, start-up and shutdown, surge prevention, risk assessment and hazard management are covered in Section 12. Recent trends in gas turbines including dry low emissions (DLE), micro-turbines, waste heat recovery

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systems and combine cycle gas turbines are reviewed in Section 13. Section 14 gives operational support guidance based on the principles developed in RR076 with examples of good and best practice in Section 15. Applicable guidance and regulations and references listed in Sections 16 and 17 respectively.

Supplementary information is included in a number of Appendices. Appendix 1 gives a current list of UK installations and Appendix 2 describes what would be included in a typical procurement package technical specification for gas turbines for a UK offshore installation. Appendix 3 reproduces HSE guidance note PM84 on gas turbines, Appendix 4 summarises the main turbine suppliers for UK installations derived from an analysis of DTI emissions data and other sources. The specifications for gas turbines used in the UK sector are summarised in Appendix 5. Appendix 6 describes the key systems and components.

1.3 APPLICATION OF GUIDANCE NOTES

The guidance notes are intended to provide advice to HSE inspectors prior to site visits, in accident investigations and in evaluation of safety cases. The report may also be of interest to other parties including dutyholders, users, manufacturers, suppliers and operators.

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2 BASICS OF GAS TURBINES

2.1 INTRODUCTION

A gas turbine (GT) converts fuel into mechanical output power to drive equipment including pumps, compressors, generators, blowers and fans. Gas turbines are widely used in the oil and gas industry in production, midstream and downstream applications with around 300-400 installed on both fixed and mobile UK offshore installations. A typical gas turbine contains three main systems: the compressor, the combustor otherwise referred to as gas-generator or core engine and the power turbine. These main systems are illustrated schematically in Figure 1. A cross section through an Alstom GTX100 industrial turbine is shown in Figure 2 and for an Avon aeroderivative gas turbine in Figure 3. The gas generator itself for this latter turbine design is shown in Figure 4.

Figure 1 The main systems in a gas turbine used for power generation: compressor, gas generator or combustor and power turbine. Courtesy Solar 5

Figure 2 Alstom GTX100 turbine with cross section through GTX100 gas turbine showing compressor, combustion system and power turbine and bearing

arrangements. Courtesy Alstom A gas turbine is a complex component operating at high speeds and high temperatures. This puts demanding conditions on the materials and components, which need to perform in these environments and maintain tight dimensional tolerances. To function a turbine needs a number of ancillary and support systems. Provision has to be made for air-intake, fuel input, starting and ignition, dispersion of exhaust gases, as well as cooling, lubrication of bearings and sealing.

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This total system forms the turbine package. Packaging concepts are described in more detail in Section 2.10.

Figure 3 Rolls Royce Avon gas generator with RT48 Power Turbine

2.2 SYSTEMS AND COMPONENTS

The gas turbine itself contains three main components:

x Compressor (AC) Compresses the air before combustion and expansion through the turbine

x Gas generator (GG) including combustor and gas turbine (GT). Ignition of air and fuel mixture to give a smooth stream of uniformly heated gas into the power turbine

x Power turbine (PT) The power turbine has the task of providing the power to drive the compressor and accessories and, in the case of driven equipment of providing shaft power for power generation, or driving the compressor or pump. It does this by extracting energy from the hot gases released from the combustion system and expanding them to a lower pressure and temperature.

Other key systems within the package include the fuel system either natural gas or liquid (pumped), the bearing lube oil system including tank and filters, pumps (main, pre/post, backup), the starter (usually either pneumatic, hydraulic or a variable speed ac motor), cooling systems, controls (on-skid, off-skid), driven equipment and the seal gas system (compressors).

There is other ancillary equipment external to the turbine package. This includes: the enclosure and fire protection, the acoustic housing, the inlet system including air-filter (self-cleaning, barrier, inertial) and silencer, the exhaust system including silencer and the exhaust stack, a lube

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oil cooler (water, air), the motor control centre, switchgear, neutral ground resistor and inlet fogger/cooler.

A detailed description of each of the main systems and individual components is given in Reference 3 and Appendix 6.

.

Figure 4 Avon gas generator. Courtesy Rolls Royce

2.3 HOW A GAS TURBINE WORKS

The gas turbine is a heat engine using air as a working fluid to provide thrust (Figure 5). To achieve this the air passing through the engine has to be accelerated. This means that the velocity or kinetic energy of the air is increased. To obtain this increase the pressure energy is first of all increased followed by the addition of heat energy before final conversion back to kinetic energy in the form of a high velocity jet efflux. A good description of the principles, design and detail of gas turbine engines can be found in References 4 and 5.

The working cycle of the gas turbine is similar to that of the four-stroke piston engine. In the gas-turbine engine, combustion occurs at a constant pressure, whereas in the piston engine it occurs at a constant volume. In each case there is air-intake, compression, combustion and exhaust. These processes are intermittent in the case of a piston engine, whereas in a gas turbine they occur continuously giving a much greater power output for the size of engine.

The pressure of the air does not rise during combustion due to the continuous action of the turbine engine and the fact the combustion chamber is not an enclosed space. The volume does increase. This process is known as heating at constant pressure. The lack of pressure fluctuations allows the use of low octane fuels and light fabricated combustion chambers, in contrast to the piston engine.

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Air Intake Compression Combustion Exhaust

Figure 5 Cross section through a gas-turbine showing the continuous process of air-intake, compression, combustion and exhaust in an aeroderivative design. Courtesy

Rolls Royce.

2.4 WORKING CYCLE

The working cycle upon which the gas turbine functions is represented by the cycle shown on the pressure volume diagram in Figure 6 below. Point A represents air at atmospheric pressure that is compressed in the air compressor stage along the line AB. From B to C heat is added to the air in the gas generator by introducing and burning fuel at constant pressure, thereby considerably increasing the volume of air. Pressure losses in the combustion chambers are indicated by the drop between B and C. From C to D the gases resulting from combustion expand through the power turbine and exhaust back to the flare. During this part of the cycle, some of the energy in the expanding gases is turned into mechanical power by the turbine; which can be used for power generation or to drive mechanical equipment such as compressors or pumps.

B C

A D

Pressure

Combustion heat energy added

Expansion through turbine

and nozzle

Compression pressure energy added Ambient Air

Volume

Figure 6 The working cycle for a gas-turbine engine

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2.5 PRESSURE, VOLUME AND TEMPERATURE

The higher the temperature of combustion the greater is the expansion of the gases, because the gas turbine is essentially a heat engine. The gas entry temperature following combustion must not exceed design limits or safe operating limits for materials in the turbine assembly.

The use of air-cooled blades and thermal barrier coatings in the turbine assembly permits a higher gas temperature and consequently a higher thermal efficiency. During the working cycle of the turbine engine, the airflow or working fluid receives and gives up heat, so producing changes in its pressure, volume and temperature. These changes as they occur are closely related through the relationships that apply in Boyles and Charles Laws.

Consequently, the product of the pressure and the volume of the air at the various stages in the working cycle is proportional to the absolute temperature of the air at those stages. This relationship applies for whatever means are used to change the state of the air. For example, whether energy is added by combustion or by compression, or is extracted by the turbine, the heat change is directly proportional to the work added or taken away. It is the change in the momentum of the air that provides the thrust on the turbine. Local decelerations of airflow are also required, as for instance, in the combustion chambers to provide a low velocity zone for the flame to burn.

There are three stages in the turbine working cycle during which these changes occur. During compression, work is done to increase the pressure and decrease the volume of the air. This gives a corresponding rise in the temperature. During combustion, fuel is added to the air and burnt to increase the temperature, there is a corresponding increase in volume whilst the pressure remains almost constant. During expansion, work is taken from the gas stream by the turbine assembly, there is a decrease in temperature and pressure with a corresponding increase in volume.

2.6 CHANGES IN VELOCITY AND PRESSURE

The path of the air through a gas turbine varies according to the design. Changes in the velocity and pressure of air are consequent from aerodynamic and energy requirements. For example, during compression a rise in the pressure of the air is required and not an increase in its velocity. After the air has been heated and its internal energy increased by combustion, an increase in the velocity of the gases is necessary to force the turbine to rotate.

Changes in the temperature and pressure of the air can be traced through an turbine by using an airflow diagram. With the airflow being continuous, volume changes are shown up as changes in velocity. The efficiency with which these changes are made will determine to what extent the desired relations between the pressure, volume and temperature are attained. In an efficient compressor, higher pressure will be generated for a given work input and for a given temperature rise of the air. Conversely, the more efficient the use of the expanding gas by the turbine, the greater the output of work for a given drop of pressure in the gas.

When air is compressed or expanded at 100 per cent efficiency, the process is called adiabatic. An adiabatic change means there are no energy losses in the process, for example by friction, conduction or turbulence. It is obviously impossible to achieve this efficiency in practice. 90 per cent is a good adiabatic efficiency for the compressor and turbine.

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Changes in velocity and pressure within the turbine stages are effected by means of the size and shape of the ducts through which the air passes on its way through the turbine. Where a conversion from velocity (kinetic) energy to pressure is required, the passages are divergent in shape. Conversely, where it is required to convert the energy stored in the combustion gases to velocity energy, a convergent passage or nozzle is used.

The design of the passages and nozzles is of great importance. Their good design will affect the efficiency with which the energy changes are effected. Any interference with the smooth airflow creates a loss in efficiency and could result in component failure due to vibration caused by eddies or turbulence of the airflow.

Figure 7 A gas-turbine driving a generator: 1 Fresh air, 2 compressor, 3 combustion chamber, 4 Burners, 5 frame cylinder, 6 turbine, 7 gas turbine exhaust gas, 8

Generator. Courtesy SWRI 3

2.7 GAS TURBINES OFFSHORE

Gas turbine packages offshore often differ to those used in other applications because of the different drivers 3. Optimum size and high power to weight ratio are key factors offshore, as well as availability, reliability and ruggedness. Efficiency has traditionally not been so critical because of the availability of fuel. The increasing requirement for low emissions has made combustion efficiency an important factor. A decision is needed on whether to go for large turbines with appropriate back-up or a smaller number of lower power turbines for specific applications. Most suppliers have different gas turbine products for the oil and gas market. A recent trend has been towards low-emission turbines driven by recent environmental legislation (SI 2005 No 925 The Greenhouse Gas Emission Trading Scheme Regulations, see Section 9.5). Some of these issues are also relevant onshore.

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Table 1 Main drivers for turbines used in the oil and gas sector. Compared to drivers

for normal industrial applications

Oil & Gas Requirements Industrial Power Generation Requirements

Availability / Reliability Cost of Electricity

Ruggedness Efficiency

High Power/Weight ratio Cost of Operations and Maintenance

Efficiency not Critical

2.8 TYPES OF GAS TURBINE

There are two main types of gas turbine: industrial and aero-derivative. Aeroderivative GTs are a development from aircraft engines and differ in a number of respects to industrial turbines: they are usually lighter than industrial engines, often have power turbines (PTs) manufactured by a different manufacturer and have all anti-friction bearings in the gas producer. There is an increasing trend to use aeroderivative gas turbines offshore in the UK, at least in terms of the gas generator (see Section 8).

This distinction is no longer so clear. It is common practice now to include an aeroderivative gas generator (GG) with a conventional power turbine (PT) such as in the GE PGT series. Industrial GTs for offshore use such as those produced by Solar have moved on in simplicity and design and increasingly mirror aeroderivative designs in size and weight. It is common practice for turbine suppliers to match their power turbine with a standard aero-derivative gas generator, for example the LM2500 from GE utilises a Rolls Royce RB211. Industrial heavy duty gas turbines are referred to as Type H by the American Petroleum Institute API. Modular or aero-derivative gas turbines, are designated Type G.

Coincidentally aero-derivatives usually offer higher efficiency and faster start-up, particularly for larger engines. Major maintenance of aero-derivatives and smaller industrial gas turbines is usually off-site (sometimes with engine exchange). For larger industrial gas turbines major maintenance is usually on-site. In the past industrial gas turbines were preferred to aeroderivative gas turbines in process applications and in mechanical drive applications where a wide range (70% to 100%) speed control was required.

Aeroderivative GTs offer advantages in offshore or oil field applications where allowable mass and available space are limited. The reliability and availability of the specific gas turbine are key criteria in selection. Aero-derivative gas turbines traditionally have required premium gas and liquid fuels. If the gas turbine fuel available is a crude oil, residual fuel oil, very lean gas, refinery mix gas or a gas that is subject to changes then an industrial gas turbines may have advantages. Fuel control is an important factor in low emission or DLE turbines.

2.9 PACKAGING CONCEPTS

Gas turbines for offshore installations are normally provided as part of a turbine package developing a rated power at a rated speed and mounted on a single skid (Figure 8) and are not

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normally custom-built to meet the user's particular power requirements. API RP 11 PGT gives general requirements and limitations in applying these standard turbine designs.

Packaging offers several advantages. It offers a fully integrated system that can be plugged in to the installation. It facilitates a modular approach where the same modular systems can be used in different applications; but configured to fit the fuel and exhaust requirements of the specific installation. It combines systems that have been developed and shown to work together. It is simpler to get safety case approval from regulatory bodies where similar packages have already been used on other installations.

Figure 8 Typical gas turbine package offshore installation. Courtesy Solar

2.10 TURBINE PACKAGES

The systems that would usually be included as part of a gas turbine package are illustrated below in Figure 9. These include:

x Air compressor (AC),

x Gas generator (GG) including combustor and gas turbine (GT),

x Power turbine (PT),

x Fuel system either natural gas or liquid (pumped),

x Bearing lube oil system including tank and filters, pumps (main, pre/post, backup),

x Starter (usually either pneumatic, hydraulic or variable speed ac motor),

x Controls (on-skid, off-skid),

x Driven equipment

x Seal gas system (compressors).

There are requirements for other ancillary equipment external to the turbine package. This includes: the enclosure and fire protection, the inlet system including air-filter (self-cleaning, barrier or inertial) and silencer, the exhaust system including the exhaust stack and silencer, a

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lubricating oil cooler (water, air), the motor control center, switchgear, neutral ground resistor and inlet fogger/cooler. The layout of these systems is illustrated in Figure 10 below.

x

Figure 9 Cross section showing the typical systems included as part of a

turbine package. Courtesy Solar/SwRI

Figure 10 Schematic showing the systems typically included outside the turbine package. Courtesy SWRI3

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2.11 DESIGN FACTORS

Factors that needed to be considered in designing turbines offshore include: low weight and dimensions, minimising vibration, resistance to saltwater, resistance to pitch and roll particularly in floating installations. The use of 3-point mounting is common to isolate the GT from deck movements.

Issues in procurement are considered in Appendix 2. The main basis for procurement is normally API 616. A range of other factors need to be considered dependent on the installation. These may include:

x Operating requirements x Spares inventory x Type selection aeroderivative or industrial, one or two shaft x Site environment and fuel considerations x Power requirements x Installation cranes, safe access, lay down areas, mounting, enclosures, auxiliary

equipment

x Noise levels- limits, support information, general requirements x Oil tank vents x Materials specification,temperature, corrosion and environment resistance, coatings,

certification

x Starting drives - gas expansion starters, hydraulic motors, diesel engines x Foundations, baseplates and mountings x Controls and instrumentation x Inlet system intake location, new configurations, material, leak prevention, joints and

movement allowances

x Air intake grids x Air compressor cleaning x Exhaust system Exhaust emission, height, proximity to process equipment, rain

ingress, maintenance access, recirculation

x Combustion air filtration requirements, anti-icing, shutters x Fire protection ventilation dampers, extinguishing systems, enclosure surveillance x Acoustic enclosures accessibility, ventilation, area classification x Fuels and fuel systems fuel selection, gas fuels and systems, liquid fuels and systems,

dual fuel systems, power augmentation

x Inspection and tests general, combustion tests, complete unit or string tests

Best practice in procurement is often included in the operators design and engineering practices. These advise on the above issues and other factors such as: definitions of vital, non-essential and non-essential services, how these impact on selection, gas turbine enclosure ventilation, mounting and foundation requirements, exhaust stack rain-catcher requirements and key issues for gas turbine washing systems. Diagrams of typical installation arrangements may be included.

2.12 TURBINE CONFIGURATION

Gas turbines offshore are normally installed in an n+1 configuration with the additional unit providing spare capacity in case of shutdown. The number of turbines is typically 3 or 5 offshore, depending on sparing requirements and the power needed. Using a number of smaller turbines gives more flexibility if there is frequent need for turning on and off capacity. Whether

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to go for large or smaller engines depends on the flexibility required. With smaller engines there are more start-ups and shutdowns. There is a trade-off between size and maintainability where requirements exist to reduce topside weight. Standards for gas turbines give limited flexibility, for example A36 steel is defined for the baseplate. Increasingly turbine suppliers such as Solar, Rolls Royce and MAN use a modular approach, with application selectors to assist in the selection of modules, filters, and other ancillary components. In offshore applications there is a trend to use increasingly lighter materials for the casings

2.13 DRIVEN EQUIPMENT

Gas turbines are used in a number of functions offshore including oil field power generation; gas gathering; enhanced oil recovery including gas lift, gas injection and waterflood; export compression; gas plants and gas transport in pipelines. This is an efficient use of gas or liquid fuel which is naturally produced on most oil installations. Typically the GT would drive a compressor or pump, normally with gas fuel. Turbines normally duel-fuel with natural gas as primary. The secondary diesel is used in emergency situations e.g well shut down and in bringing systems up. Then gas is used for fuel.

(a) (b)

(c) Figure 11 Examples of equipment driven by a gas turbine and other methods: (a) centrifugal compressor driven by 2-shaft gas turbine, (b) centrifugal compressor driven by variable speed electric motor (c) reciprocating compressor driven by gas

motor. Courtesy Solar

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In this guidance note the compressors, pumps and other equipment that supplies these functions are referred to as driven equipment where they are mechanically driven by the turbine itself either directly or indirectly. In more recent installations there is a trend to use gas turbines primarily for power generation with other equipment driven electrically particularly for satellite or remote installations. Equipment may also be driven by gas motors.

There can be considerable variation in the size and ratings for a gas turbine

x Available Output Power Range: 20 kW- 250MW (25 hp 350,000 hp) x Smaller units (60MW or below) are typically used offshore. x Typical Gas Turbine Simple Cycle Efficiency: 25- 35% x Output Speed Range: 3000 - 25000 rpm x Fuels: Natural Gas, Liquid Fuels or duel fuel

2.14 OFFSHORE ENCLOSURES

To mitigate the risk in case of turbine failure and to reduce noise it is common but not universal to house offshore GTs in enclosures. Offshore gas turbines may be subject to salt spray. To avoid corrosion damage stainless steel is normally used for the enclosures, bolts and hardware.

In smaller installations and FPSOs there can be advantages not to enclose the gas turbine. This eliminates safety risks associated with access to enclosed spaces, reduces the risk of gas or hydrocarbon build up and simplifies ventilation requirements. Gas turbines generally operate smoothly provided a uniform supply of air, fuel and environmental conditions are maintained, this may be more difficult to achieve if the GT is not enclosed.

Gas turbines emit a noise level which is higher than that normally permitted and acoustic enclosures are invariably required. Particular precautions are required for the enclosure, in which high temperatures may prevail and flammable vapour may be present.

The acoustic enclosure may include the gas turbine, its auxiliaries and driven equipment, or it may have separate compartments for each of these individual units. The nature of the installation, the type of driven equipment and the composition of any flammable vapour which could be released within the enclosure will generally dictate whether the enclosure shall be continuous or shall have separate compartments.

Noise control requirements and ergonomics require the use of off-base mounted turbine enclosures to provide more space for maintenance and better control of noise emission instead of the type of enclosures formerly used which were close-fitted and mounted on the turbine baseplate). The enclosure is often fitted with strategically located lifting beams on which a chain block can be fitted for minor maintenance activities.

2.15 GAS TURBINE GT CYCLES

The generation of electricity by a GT is implemented by several different systems. The simple cycle only generates electricity. In combined heat and power (CHP) plants and with waste heat revovery systems (WHRU) the residual heat in the engine exhaust is used for a variety of purposes ranging from industrial process heating to domestic hot water. Combined cycle gas turbine (CCGT) plant uses the residual heat to raise steam, which drives a steam turbine

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producing further electricity. CHP, WHR and CCGT are increasingly used in offshore applications. More information on these is included in Section 13.

2.16 FUELS

A variety of fuels can be used by a gas turbine. While natural gas is the preferred fuel for most UK plants, liquefied petroleum gas (LPG), refinery gas, gas oil, diesel and naphtha may be used as main, alternate, standby or startup fuels. Hydrogen and biogas derivatives are also increasingly being used and fuel can include waste streams produced on-site. Aero-derivative and low emission turbines have more precise fuel requirements. Fuels are covered in Paragraph 5 of HSE Guidance Notes PM 84 and also in Paragraphs 48 to 53.

The choice is dependent on commercial and environmental considerations. Each type of fuel has its own particular hazards arising from its physical and chemical properties. Offshore the fuel would come from the production process with diesel backup used for startup and production shutdown.

The characteristics of the intended fuel(s) would be stated in the data/requisition sheets. Manufacturers are required to confirm the suitability of the intended fuel(s) and to support this with evidence of prior experience with fuels of similar quality and composition, see ASTM D 2880. The Manufacturer would also advise on any treatment needed for the intended fuel(s) to render it suitable for the proposed application. It also needs to be verified that the smoke emission of the intended fuel is within local regulations.

In marginal cases, it would be investigated whether identical fuels have been used by other operators and any specific design requirements determined, especially in relation to trace elements. Gas turbine hot parts are particularly sensitive to alkaline metals such as sodium and potassium. Other elements may have additional restrictions due to environmental emission limits and the general corrosion requirements of downstream systems. Fuels containing heavy metals may require additional fuel treatment systems. Manufacturers have comprehensive guides to suitable fuels including advice on the permissible level of contaminants and concentration of corrosive agents which can be tolerated in a particular fuel. This advice would be followed in reaching agreement with the gas turbine manufacturer on acceptable levels and concentrations for the intended fuel(s). Fuel composition is usually normalised using the Wobbe Index and evaluated for all operating conditions, including start up

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3 APPLICATIONS OFFSHORE

Gas turbines are used in the gas and petroleum industries to provide pumping and gas compression facilities, often in remote locations such as a pipeline. In this case the GT may run on fuel taken from the pipeline. Electrical power can also be generated if required, for instance on an oil production platform. GTdriven plant can be utilised for local or national power-generation requirements. Turbines up to about 50 megawatts (MW) may be either industrial or modified aero-engines, while larger industrial units up to about 330 MW are purpose-built.

Applications offshore include power generation, gas injection, gas lift, waterflood and export compression 3. A distinction can be made between upstream, midstream and downstream applications. In this context production facilities are upstream with pipelines and transportaion being midstream. The specific applications where gas turbines are used offshore are summarised below.

Upstream applications of gas turbines in the oil and gas industry include the following:

x Self-Generation- Power generation to meet needs of oil field or platform x Enhanced Oil Recovery (EOR)- Advanced technologies to improve oil recovery x Gas Lift - Injecting gas into the production well to help lift the oil x Waterflood - Injection of water into the reservoir to increase reservoir pressure and

improve production

x Gas Re-injection- Re-injection of natural gas into the reservoir to increase the reservoir pressure

x Export Compression- Initial boosting of natural gas pressure from field into pipeline (a.k.a. header compression)

x Gas Gathering- Collecting natural gas from multiple wells x Gas Plant and Gas Boost- Processing of gas to pipeline quality; i.e., removal of

sulphur, water and CO. components

x Gas Storage/Withdrawal- Injecting of gas into underground structure for later use: summer storage, winter withdrawal

In midstream applications gas turbines may be used for:

x Pipeline Compression - Compression stations on pipeline to "pump" natural gas; typically 800-1200 psi compression

x Oil Pipeline Pumping - Pumping of crude or refined oil.

Gas turbines are also used in downstream applications including refineries. These are not covered in the context of this inspection guidance note.

3.1 POWER GENERATION

The primary application of gas turbines offshore is in power generation. The turbine will provide direct drive to an alternator to generate power for the installation. It is normal to have at least two GTs on main platforms with an emergency generator as back-up. Satellite and remote or unmanned platforms are commonly provided with power from the main installation via umbilicals rather than having their own gas turbines.

There will be different power generation requirements for floaters/semi-submersibles, fixed leg platforms and onshore. This will depend on electrical requirements and fuel gas availability

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The typical gas turbine size in this application is 1 MW - 30MW. The number and configuration of turbines depends on the flexibility and redundancy needed and to allow for future platform upgrades.

Figure 12 Array of three gas turbines being used for power generation offshore on an FPSO. Courtesy Solar

3.2 GAS GATHERING

Gas gathering is used to collect natural gas from several wells. Modern offshore installations may produce from 50 or more wells. In gas gathering a turbine of typically 3MW - 20MW would typically be used

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Figure 13 3 Body Compressor Skid for Gas Gathering Application. Courtesy Solar

3.3 GAS LIFT

Gas-lift helps Increase crude oil production by injecting natural gas into the oil well. Reduction in oil density and aeration helps oil flow. Gas is separated and re-injected. A typical gas turbine size of 3MW-20MW would be used in this application.

3.4 WATERFLOOD

Waterflood is another method of enhanced Oil Recovery. A gas turbine drives a centrifugal water pump (usually with gearbox). The pressure is usually up to 600bar. Pump cavitation must be avoided. The typical gas turbine size in this application is: 1 MW-15MW

Figure 14 Schematic illustrating water flooding for enhanced oil recovery. Courtesy

Solar.

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3.5 EXPORT COMPRESSION

Export compression is used to boost the gas pressure to flow gas to plant or pipeline. The typical gas turbine size in this application would be ~ 3MW-30MW with the larger turbines being used in pipeline export.

Figure 15 Gas turbine being used in export compression. Courtesy Solar

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4 OFFSHORE PACKAGES

4.1 MODULAR TURBINE PACKAGES

In oil and gas and other sectors, turbine suppliers are increasingly offering modular turbine packages 6 for both aero-derivative and industrial gas turbines. These offer advantages in terms of short installation time, smaller package size, ease of maintenance, achieving regulatory approval and reduced cost. Such packages can be tailored with a wide range of options to fit the requirements of an individual oil and gas installation. Such packages typically include three modules; a turbine module, a compressor module and an air-intake module.

Figure 16 Example of modular approach for aero-derivative gas generator maintenance. Courtesy Rolls Royce

Within these units smaller modules may be included to facilitate replacement, substitution or maintenance (Figure 16). The following systems can vary:

x Starter x Lube oil x Fuel

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x Air-Intake

x Exhaust

For example, to fit single level or multi-level installations and routes to flare, the option of axial or radial exhaust configurations offers flexibility.

The modules can be pre-installed and packages have a common frame size. The drivers for a modular approach are short installation time and lower total cost. An additional benefit is the short turbine change-out time. The sequence for a modular system would be as follows: shut down, disconnect combustion system, disconnect air-intake module, turbine ready for transport. The time required for installation could typically be as follows6 :

x Installation of Foundation and Generator module 2h x Turbine module and air-intake module 4h x Total installation day x 14 days to start-up

Modular systems also allow a short turbine change-out time. The typical sequence of operations may be as follows :

x Step 1 Shutdown and disconnect combustion system

x Step 2 Disconnect air intake module

x Step 3 Remove turbine ready for transport

4.2 DESIGN OPTIONS

An important option is provision for axial or radial exhaust. This gives flexibility in layout. Radial exhausts are excellent for multilayered systems with the silencer above. Axial exhausts allow direct link to a waste heat recovery units (WHRU) and heavier equipment to be installed on the top deck. Approximately 50% of offshore installations have a WHRU, usually a glycol cleaner. The axial v radial exhaust option in Solar Titan 130 and Taurus 170 gives layout flexibility To aid installation the two exhaust options may be configured to have the same width and external dimensions. For example in the Titan 130 turbine both exhaust modules are 3.12m wide and 14.22m long. In the axial system there is an additional 4.22m to the silencer, with a 13.1m vertical rise to the silencer for the radial exhaust. More information on design options and procurement is given in Appendix 2.

4.3 FPSO TURBINE PACKAGES

Deepwater installations are an area of growth in the oil and gas sector with over 150 new Floating production Systems (FPSs) due to be installed Worldwide in the next 5 years. Floating Production Storage and Offloading (FPSO) vessels are the most significant, followed by Tension Leg Platforms (TLPs) and other options such as semi-submersibles. Worldwide approximately 10-15% of gas turbine packages are on floating installations. For example, Solar currently have nearly 300 turbine packages on FPSs of a total of 2375 turbine packages offshore3. These are mainly Taurus 70 or Titan 130 turbines. For power generation an FPSO will typically have one or more gas turbines, usually including waste heat recovery (WHRU) sytems.

The key design drivers are low topside weight, limited space and resistance to changing weather conditions. These impose specific requirements on the turbine package. A typical turbine

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package can weigh 110 tons. A one ton reduction in topside weight on a floating installation can produce savings of $10,000 in cost.

Location of turbines on FPSOs will depend on the installation. On the Trenergy FPSO3, Solar turbines are installed in the middle of the vessel. Turbines used on FPSOs can be industrial or aeroderivative. It is understood that Solar turbines installed on FPSOs up to Jun 2004 were all industrial 3.

Special mounting procedures are needed on FPSOs to allow for pitch and roll. Baffles are used to stop oil movement, scavenge pumps are used on the drains of engine bearings to ensure oil is always flowing, a 3-point mounting is used verified by finite-element analysis. A single mounting in front with two back mountings gives the maximum flexibility on loading. Multiple base plates are generally used as this is less costly and allows scavenging for spare parts. On offshore platforms and floating production and FPSOs the design of the machinery modules can be significantly simplified if the gas turbine driving train baseplate design is rigid and supported on a three-point mount. Alignment of the driving train is then unaffected by platform movements. Installation of the driving train on a steel structure allows tuning to avoid vibration transmission.

Normally offshore the gas turbine train would allow for continuous operation under a tilt angle of maximum 3 degrees. A structural analysis would be performed to achieve the required stiffness of the baseplate, together with stress analysis of connecting pipe work and cables to ensure that no distortion will occur.

For FPSOs the maximum tilt angle can be substantially larger than 3 degrees. The actual static and dynamic displacement requirements for these applications would be specified separately. As a guide, turbine-driven generator sets in essential services must be capable of normal operation up to and including the maximum angles specified, while generator sets in non-essential services and mechanical-drive packages and compressor sets in process services would be capable of surviving, but not necessarily capable of operating, at these maximum angles.

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5 MAJOR DRIVEN EQUIPMENT

There are two options for equipment driven by gas turbines, either to provide power directly from the turbine, known as single shaft, or to drive indirectly with the driven equipment on a separate shaft, known as two-shaft. Mechanical Drive comprises a packaged gas turbine and rotating equipment driven by it. The base frame will be a common single unit. For larger or on shore units this is often of two or more segments bolted together. The gas turbine may be of two distinct types as below:

Single shaft gas turbine

In a single-shaft gas turbine the Power Turbine (PT) and Gas Generator Turbine (GGT) are combined mechanically on to a single shaft. A single shaft turbine has all internal parts rotating at the same speed. This gives simplicity, but requires the driven equipment to be started and operated at the same time as the turbine core. The main use is for electric power generation. This configuration is used in fixed speed applications (in a range: 90%-100% full speed). For example to produce generator drive via gearbox (1500 rpm - 50 hz, 1800 rpm - 60 hz).

Figure 17 Single shaft turbine with shaft coupling. Courtesy Solar/SwRI

Two-Shaft Gas Turbine (no Shaft Coupling)

A two-shaft gas turbine has no mechanical connection between the power turbine and the hot gas generator, thus permitting the power turbine to rotate on its shaft independently of the hot gas generator. In a two shaft gas turbine the Power Turbine (PT) is independently supported on its own shaft and bearings. This allows variable speed applications (typically in range 25%-100% full speed). This configuration is used for compressor, pump and blower applications. Two Shaft turbines permit the core engine to be started without spinning the driven equipment, This configuration is applicable to mechanical drive packages.

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Figure 18 Two-Shaft Gas Turbine (no Shaft Coupling). Courtesy Solar.

Single and two-shaft gas turbines can be used across the full power range: from 0-100% full load , however efficiency will be low and emissions high at loads below 60%.

5.1 ALTERNATORS

The term power generation package refers to a packaged gas turbine and alternator on a common base. Power generation is the most common application of gas turbines offshore. The turbine package is intended for fixed speed operation for electricity generation. The gas turbine will have matched power turbine. A load gearbox is used to match turbine and alternator shaft speeds. Detailed information on the safety and risk issues associated with alternators can be found in HSE report RR0762 covering inspection guidance on rotating equipment. The alternator is directly driven and mounted on the cold inlet end of the shaft before the compressor.

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Figure 19 Typhoon gas turbine power generation package. Courtesy EGT

Figure 20 Cross section through Typhoon gas turbine power generation package

5.2 COMPRESSORS

The second most common application of gas turbines offshore is in gas compression. A

compressor package is an enclosed gas turbine with one or two gas compressors co-axially on

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the end of the output shaft. All turbine elements are mounted to a common baseframe. An offshore compressor package will typically be provided as a single lift module to give simplified installation and transportation to the platform. This module includes all systems, exhaust and waste heat recovery unit (WHRU). In addition to the GT and the driven compressor the package would include:

x A sub-base providing added stiffness for gas turbine and compressor skids x 3-point mounts to give isolation from twisting and vibration x An inclinometer to give alarm and shutdown at high list, trim, pitch, roll angles x Baffles to provide a continued supply of lube oil at inclined operation x A scavenging pump to give a forced supply of lube oil at inclined operation

Figure 21 Single lift gas turbine compression modules. Courtesy Solar, Rolls Royce

Figure 22 Typical offshore gas-turbine compressor package

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Applications

Gas Compressors are used to increase the pressure of a process gas, in order to drive it into a pipeline system to an onshore process plant, to use on the producing well as gas lift, to re-inject gas for reservoir pressure maintenance or for use as a fuel gas. Centrifugal compressors are preferred for high mass flow systems because of their simplicity and reliability compared with screw or reciprocating compressors. In order to achieve the required pressure ratio, several compression stages may be required, in one or more casings. Each compression stage is carried out by a rotor in a matching diffuser. Mechanically linked compressors, working together with drive and support equipment, may be regarded as a single system for design and safety purposes. More detailed information on compressors can be found in HSE Report RR076.

Package Elements

An offshore gas turbine compressor package used to compress hydrocarbon gas typically comprises a twin shaft aero-derivative gas turbine driving a barrel casing centrifugal compressor. The package would also include the control system & ancillary equipment. The package is mounted on a 3-point mounting skid baseplate. It is normal to enclose the gas turbine is enclosed in an acoustic enclosure with its own fire & gas system. Ancillary equipment and systems will include:

x Inlet Air System & Filter

x Fuel System x Exhaust Duct x Lubricating Oil System x Compressor Dry Gas Seals & Support System x Drive Gearbox ( if required ) x Auxiliary Gearbox x Shaft Couplings x Cooling System x Piping Systems x Condition Monitoring

Figure 23 Offshore gas turbine driven compression package. Courtesy Solar.

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Package Configuration

Figure 24 below shows the typical configuration for an offshore gas turbine compressor package.

Figure 24 Process Schematic Diagram - Gas Turbine Driven Gas Compression System. Courtesy RR0762

Hazards

The major hazards have been evaluated in RR076 2 and relate to the inventory of flammable gas that can be released if there is an equipment failure. Hazard assessment must relate to the complete package and not just the compressor body.

The injury risk from a mechanical failure is relatively low, as the robust casing will retain parts. Hot / moving parts may still cause injury local to the machine. Most compressors have gas seals on moving drive shafts or piston rods. These are safety critical items when handling hazardous materials. The gas turbine is dependent on various ancillary systems for safe operation, operating procedures and control system must ensure that these are operational prior to turbine start, and at all times during operation. Hot surfaces will be fitted with heat shields or thermal insulation. These must be in place for operator safety.

Multi-stage centrifugal gas compressors contain high speed moving parts within a robust casing. Mechanical failure can result in severe internal damage but this is not likely to pose a direct hazard to people who are not close to the equipment. The greatest potential threat is the uncontrolled release of a flammable hydrocarbon gas, particularly if the gas is then able to form an explosive mixture within a relatively enclosed space.

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The risk is reduced by ensuring that compressors are competently operated and maintained, and that protective systems are regularly tested and in good order. The overall system design should provide suitable remote isolations, knockout pots and adequate vent routes. Control system issues are covered in detail in Section 7.

A limited number of safety issues can arise from inclusion of a gearbox within a machine package. The most serious are: the potential for accidental or failure engagement of auxiliary drives, used to rotate the compressor at low speed, leading to massive overspeed and usual disintegration of the drive; bursting of the gear wheels (design or manufacturing flaws); fires due to leakage of lubricating oil.

Misalignment of the main drive coupling, even within its tolerance limits, puts increased loads on adjacent shaft bearings. It also reduces the service life of the coupling, as flexible elements are subjected to greater strains. Coupling lubrication (where required) and inspections must be proactively maintained as the coupling has significant mass and has the potential to become a dangerous missile if it fails. Loss of drive is not normally a safety-related incident; special design requirements apply if drive continuity is critical. More information of flexible couplings can be found in Reference 2.

PM84 Guidance

Paragraph 58 of HSE Guidance Note PM84 notes that those concerned with the supply and operation of gas compressor stations used in UK should be aware that the foreword to BS EN 12583: 2000 Gas supply systems - compressor stations - functional requirements contains the following proviso: `In the UK the national safety body, the Health and Safety Executive (HSE) (see CR 13737), has

required additional precautions at gas turbine driven plant, eg compressors, combined heat and

power (CHP) and combined cycle gas turbine (CCGT), in order to comply with the general

provisions of the Health and Safety at Work etc Act (HSWA). These additional precautions are

contained in HSE Guidance (Control of safety risks at gas turbines used for power generation)'.

Surge in driven compressors

Surge, which is the flow reversal within the compressor, accompanied by high fluctuating load on the compressor bearings, has to be avoided to protect the compressor. Surge avoidance in

.centrifugal compressors driven by a two stage GT has been reviewed and modelled by Kurz7. The possible operating points of a centrifugal gas compressor are limited by maximum and minimum operating speed, maximum available power, choke flow, and stability (surge) limit. The usual method for surge avoidance (anti-surge-control) consists of a recycle loop that can be activated by a fast acting valve (anti-surge valve) when the control system detects that the compressor approaches its surge limit. Typical control systems use suction and discharge pressure

If the surge margin reaches a preset value (often 10%), the anti-surge valve starts to open, thereby reducing the pressure ratio of the compressor and increasing the flow through the compressor. The situation is complicated by the fact that the surge valve also has to be capable of precisely controlling flow. Additionally, some manufacturers place limits on how far into choke (or overload) they allow their compressors to operate. A safety critical situation can arise upon emergency shutdown (ESD) if manufacturers surge prevention measures are not properly adhered to.

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Here, the fuel supply to the gas turbine driver is cut off instantly, thus letting the power turbine and the driven compressor coast down on their own inertia . Because the head-making capability of the compressor is reduced by the square of its running speed, while the pressure ratio across the machine is imposed by the upstream and downstream piping sys-tem, the compressor would surge if the surge valve cannot provide fast relief of the pressure. The deceleration of the compressor as a result of inertia and dissipation are decisive factors. The speed at which the pressure can be relieved of the pressure not only depends on the reaction time of the valve, but also on the time constants imposed by the piping system. The transient behavior of the piping system depends largely on the volumes of gas enclosed by the various components of the piping system, which may include, besides the piping itself, various scrubbers, knockout drums, and coolers. Models allow simulation of such upset situations and avoid their occurrence in service model to simulate shutdown events and define simpler rules that help with proper sizing of upstream and downstream piping systems, as well as the necessary control elements.

Normal practice is to a 5% margin control to the surge limit and protect against the possibility of surge by use of a recycle valve and operating within turbine suppliers safe operating limits. These precautions mitigate against the possibility of surge on emergency shutdown (ESD).

Components

Acoustic enclosure

The acoustic enclosure for an aero-derivative gas turbine is normally close fitting, and fitted out with ventilation and Fire & Gas Detection Systems. The internal space is tightly packed, making access to internal components quite difficult. A problem on one component has the potential to affect adjacent components or systems, either by release of material, vibration or over-heating. It may be necessary to remove a component to work on that component or to gain access to adjacent components.

Figure 25 Typhoon mechanical drive package. Courtesy EGT Acoustic enclosure

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Baseframe

The baseframe needs to be sufficiently rigid to maintain machine alignment, despite movement of the supporting structure or vessel. The 3-point mounting system normally used eliminates the transmission of twisting forces to and from the baseframe.

In order to save space, and the weight of additional bases, as many as possible of the ancillary systems e.g. lubrication oil system, seal gas support system, are built into the main baseframe. The control panel may be built on to the end of the baseframe (which is convenient for pre-wiring) or mounted separately (which permits control panels for separate machines to be grouped together).

Gas Turbine

The configuration for a compression package is identical to turbines in other driven applications. The turbine will have a fuel manifold wrapped around the middle of the machine, with multiple combustor fuel feeds. Flexible connections will link to the inlet and exhaust ducts. The gas turbine is typically centre-line mounted from the baseframe. This ensures internal alignment while permitting thermal expansion of the machine. The main drive shaft will be at the hot or exhaust end for a mechanical drive package and fitted with a flexible coupling. A similar configuration is used for any auxiliary drive shafts.

Figure 26 Cross section through Typhoon gas turbine mechanical drive package.

Courtesy EGT

Any mechanical failure of the turbine, or an explosion within the acoustic enclosure, could disrupt fuel pipework, with the potential for a significant release. Missiles, in the form of ejected compressor blades or other high-speed components, may be thrown in a mainly radial direction, with the potential to damage people or critical systems at some distance from the turbine.

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Gas Compressor

The gas compressor and drive gearbox (if fitted) are normally outside the acoustic enclosure, they may still be very closely packed with service pipework & cable trunking. Good design should permit ready access to compressor bearings, instruments and drive couplings. The air inlet housing is located separate from the turbine next to the external cladding of the process area. A multi-stage barrel type centrifugal gas compressor is centre-line mounted on an extension of the common base-frame, ensuring shaft alignment. Where two compressors are required to achieve the required pressure ratio, the second compressor is likely to be driven from the first compressor shaft, by a mechanical gearbox. All shafts require alignment within the tolerances of the shaft couplings.

Process Pipework

Process pipework is connected to the barrel casings, usually by flanged connections. Fully welded assembly is also possible. Thermal expansion of process pipework must be allowed for by good pipe support and flexibility design; bellows are not preferred. Dependent on operating temperatures the compressor casing and pipework may be lagged. The centre-line support system must not be lagged, as it has to remain at ambient temperatures, so far as is possible.

Gearbox and Auxiliary gearbox

The drive gearbox included within the machine package allows the manufacturer to optimise operating speeds of the gas turbine driver and centrifugal compressor separately. The technical disadvantages of additional skid length, equipment complexity, and weight are offset by the benefits for the design of compressor and turbine. Gas turbine drive packages will include an auxiliary gearbox, normally integral to the cold end of the machine. This provides the necessary linkage for turbine starting, and mechanical drives where required for oil or fuel pumps.

Main Drive Coupling

The use of flexible couplings within a machine package is essential to provide the necessary degrees of freedom to enable the machine elements to be aligned, and compensate for any flexibility inherent in the installation skid.

5.3 PUMPS

Pump packages have a similar configuration to that shown for compressors. Normally these will require a smaller turbine. Detailed guidance on the safety risks associated with turbine driven pumps can be found in HSE inspection guidance document RR076 2. Pumps offer a suitable application for use of micro-turbines.

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6 ELECTRICAL SYSTEMS

6.1 ELECTRICAL SYSTEMS

Gas turbines contain a number of electrical systems associated with control, start-up, anciliary systems and system monitoring. These include ignition, governing, controls and instrumentation systems, fuel pumps, inlet guide vane (IGV) controls for variable stators, lubrication pumps and monitoring systems for speed, torque, thrust and pressure. There will be associated electrical systems for driven equipment. Some of this equipment will be mounted as part of the GT skid with separate systems for example in the control room. The major use of gas turbines offshore is for power generation using an alternator driven by the turbine. The alternator has its own electrical and electromagnetic concerns2.

The associated risks do not differ significantly to electrical systems on other large mechanical equipment installed offshore. Specific risk factors for gas turbines are:

x the potential for gas leakage from the gas turbine and exhaust systems; x the potential for leak of fuel, seal oil or hydraulic oil; x the high temperatures, particularly in the combustor, transition and turbine.

In the presence of flammable substances, electrical equipment can be the source of ignition due to sparking or high temperature surfaces integral to the electrical equipment. It is important that electrical equipment is correctly selected, used and maintained in hazardous areas where there is the potential for flammable substances to be present. Control of gas turbine operation and emissions requires use of sensors and monitoring devices often exposed to high temperatures and environmental attack; this places special requirements on the materials used in such sensors and monitoring devices and the associated electrical systems.

PM84 provides specific guidance on gas turbines including electrical issues. Specific electrical issues covered include:

x Compliance of technical plant to UK and international standards. x Electrical protection systems to avoid overload x Enclosures and hazardous area classification x Site safety rules and operational procedures x Requirements for risk assessment x Identification and labelling systems and the positioning of labels and notices on

switchgear, transformers, control gear and plant

x Legal requirements particularly commissioning and work on live electrical systems

x Electromagnetic radiation and protection measures for live conductors magnetic field risk and corona discharge

x Practices not covered by existing safety rules and operating procedures such as live brush changing in relation to the exciter system of the alternator (not used offshore).

Note that live brush changing is not carried out offshore due to the hazards this would incur. There are few situations that can be envisaged where it is not possible to shut down the exciter system and do in a safe manner. Regulation 16 of the Electricity at Work regulations would apply

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Electromagnetic radiation from close proximity to live conductors is covered by National Radiological Protection Board guidance 32 and discussed in more detail in PM84.

6.2 ELECTRICAL SYSTEMS GUIDANCE

Electrical issues are covered in Paragraphs 69 to 78 of PM84. When the initial tenders for new power generation are drawn up, care should be given to the consideration of the technical specifications for the electrical plant, equipment, installations and systems to be provided. It is essential to establish that what is to be provided and installed will comply with the relevant health and safety legislation in the United Kingdom and relevant national or international standards. The electrical protection system should minimise the risk of potentially damaging overload situations (that may result in catastrophic drive-train failure). Note that no UK offshore installations synchronise with the grid system.

Hazardous area classification should be carried out for all plant items and pipework containing flammable substances such as fuel or oils, whether in an enclosure or otherwise. It should be carried out in accordance with relevant regulations (The Dangerous Substances and Explosive Atmospheres Regulations 2002), the associated Approved Codes of Practice L10121, L13422 and L13823 and current recognised standards such as BS EN 60079-10 1996 Normally, enclosures would be expected to be classified zone 2. In some cases it may be possible to justify the reduction of zone sizes by making a conservative allowance for the effects of the ventilation in accordance with relevant standards andguidance. This must take into account the extent of flammable areas from CFD predictions as described at paragraphs 35-41 above. Zoned areas may be safe when the plant has shut down, if the fuel and other flammables are adequately isolated, as described in paragraph 50 of PM84, and sufficiently de-pressurised. Additional guidance relating to area classification for natural gas is given in IGE/SR/25.20 All electrical equipment should be checked to confirm it is suitable for the area classification.

HSE Offshore Division Operations Note ON58 issued in January 2003 provides a short guide for the offshore industry on the Dangerous Substances and Explosive Atmospheres Regulations 2002 DSEAR. HSE Offshore Division Operations Notes 59 and 63 issued in January and December 2003 respectively provide relevant guidance on the Equipment and Protective Systems intended for use in Potentially Explosive Atmospheres Regulations 1996 EPS.

Before plant is taken into use, site safety rules and operational procedures should be carefully matched to the original specifications for the electrical installation, to avoid misunderstandings by the operators. Specific agreements between users/purchasers and manufacturers as required by some relevant standards, need to be checked to ensure full compliance.

Risk assessment should be carried out on all the electrical systems for the plant. The assessment should also include all the risks arising during system verification and commissioning tests. The user will be responsible for ensuring that the suppliers of electrical systems provide sufficient information to describe the safe use of their equipment.

Identification and labelling systems and the positioning of labels and notices on switchgear, transformers, control gear and plant have been used in the UK which differ from those normally used. It is essential that employees are fully conversant with alternative identification and labelling systems and that labels, notices and instructions are clearly displayed. Where this is a potential problem, systems will have to be replaced with more familiar ones or further training will be needed.

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People carrying out commissioning or live work must be familiar with the plant and systems to be commissioned. They must be trained in using a permit-towork system as described in the regulations referred to in paragraph 87 of PM84. They must also consider the effects the work could have on other people and plant. Adequate documentation and drawings must be available at handover and final documentation must be completed as soon as practicable following completion of commissioning. For the purpose of commissioning activities, inhibits and overrides may need to be temporarily installed in order to prove the system controls. If this is the case, a log should be maintained to ensure that they are removed and the systems reinstated, prior to the equipment being made fully operational.

Existing safety rules and operating procedures may not address the requirements of the plant. It may be necessary to confirm before taking operational responsibility that the rules, procedures and all equipment, including where necessary personal protective equipment, are in place. Staff will also have to be familiar and practised in these matters.

If electrical apparatus is located outside, then some environmental protection will be needed for the appropriate Ingress Protection (IP) code.

6.3 ELECTROMAGNETIC RADIATION

Electromagnetic radiation hazards are covered by Paragraphs 76 to 78 of PM84. Employers should use the guidance published by the National Radiological Protection Board32 when assessing whether there is a risk to health.

Current flows greater than a few hundred amps are capable of producing a significant magnetic field risk at a distance of less than one metre. Bare HV conductors may lead to people being exposed to electric fields which exceed the NRPB investigation levels of 12 kV/m. On GT plant the HV conductors are normally phase segregated and insulated, which will prevent corona discharge. The only exception is the conductors from the transformer bushing to the banking compound where a visible corona may be present.

If the measured field strengths exceed the investigation level, more detailed investigation should be carried out to determine the induced currents arising from potential exposures. These should be compared with the published basic restrictions and, if necessary, preventative measures taken. Such measures could include limiting the proximity at which people may approach live conductors. Restricting the duration of exposure is not an acceptable control strategy. In this case suitable barriers and signs shall be in place to warn of the potential for danger.

6.4 MAINTENANCE OF ELECTRICAL SYSTEMS

Maintenance of electrical systems in hazardous areas is a specialised area and covered by a number of standards and regulations. These include:

BS EN 60079-17: 2003 Electrical apparatus for explosive gas atmospheres. Part 17: inspection and maintenance of electrical installations in hazardous areas (other than mines).

IEC 60079 17 Recommendations for inspections and maintenance of electrical installations in hazardous areas (other than mines).

IEC 60079-19 Repair and overhaul for apparatus used in explosive atmospheres (other than mines or explosives)

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BS 5345 Code of practice for the selection, installation and maintenance of electrical apparatus for use in potentially explosive atmospheres (other than mining applications or explosives processing and manufacture).

Other IEC guidance in regard to flameproof enclosures, increased safety, intrinsic safety, protection is also relevant. Electrical apparatus and hazardous areas has been reviewed by Garside13. Other relevant regulations are listed in Section 16.

Specific advice and information on relevant electrical codes and regulations is given in the HSE guidance on Explosive Atmospheres Classification of hazardous areas (Zoning) and selection of Equipment www.hse.gov.uk/comah/sragtech/techmeasareaclas.htm.

Inspection schedules for different equipment type and locations are given in the Tables in BS EN 60079-17: 2003.

Gas turbines present some specific concerns in regard to electrical equipment. There is the risk of ignition in the event of a leak of gas, fuel or lubricating oil. Gas turbine components and casings get extremely hot during operation, particularly in the hot-gas-path and combustion system. Any on-skid electrical equipment must be suitably protected and enclosed. Particular consideration is needed for sensors, wiring and other electrical equipment associated with control and monitoring systems. HSE guidance note PM84 highlights specific concerns in regard to electrical and control systems in gas turbines. See Section 12 and Appendix 3.

A typical summary of points to look for at routine or periodic inspection of electrical systems13

may include:

x Apparatus Tag number x Cable identifications correct to loop diagram/hook-up drawing x Apparatus has no unauthorised modifications x Any rectification work noted at previous inspections has been carried out suitably x Earth connections secure x No undue corrosion (especially on flanges for Ex d) x Cable entries tight x No degradation of required IP rating x No broken covers or fan cowls x No build up of dirt on cooling fins (especially on motors) x Electrical connections tight (especially Ex e and Ex n) x Correct lamp ratings x No changes to area classification. (If so, the type of protection or its apparatus group or

T-class may not be suitable.)

x No damage to associated cables

x No damage to apparatus

x Covers/lids correctly secured

x Apparatus mounting firm and acceptable

x Filters clean and free from dirt and debris

x Breathing and draining devices clean and free from dirt and debris

x Cable supports OK

x No external obstructions to flamepaths (Ex d)

x No excessive grease on flamepaths (Ex d)

x No hard setting compound on flamepaths (Ex d)

x No unauthorised gaskets (especially Ex d)

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x Gaskets of correct type x No excessive grease on bearings x No signs of excessive temperatures (e.g. brittle or burnt insulation) x No cracks to ceramic feedthroughs or insulators (especially Ex d, Ex e, and Ex de) x No obvious changes to surrounding processes which could affect area classification x No signs of leakage of filling medium (especially Ex o and Ex q) x Filling medium at required level (especially Ex o and Ex q) x No signs of leakage from stopper boxes or stopper glands (especially Ex d) x Electrical aspects remain secure: especially earth loop tests, and especially for Ex ia/ib.

Compare value with that previously noted

x No dirt or obstructions to fan covers x No signs of excessive vibration x Surveillance circuits functioning correctly (especially Ex p) x Correct associated apparatus installed (Ex ia/ib) x Conduit seals satisfactory at passage between non-hazardous area and hazardous area x Cable identifications correct, and no changes to wiring

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7 CONTROL SYSTEMS

Gas turbines are sophisticated pieces of equipment and synchronisation of the key systems (compressor, gas turbine, power turbine) is crucial to ensure smooth operation and avoid surge and other operational issues. This is undertaken using the control system. The control system will also control a range of other functions including start-up, monitoring, fuel flow and ignition, lubrication, emergency shutdown ESD. The current trend is to use distributed control systems, both on and off the skid, with separate modules, actuators and sensors to control individual functions. Turbines are generally replaced after 6-7 years. The control system would be updated more regularly, typically every 3 years.

The operation of all parts of the system may be affected by temperature, environment, air input and other factors. The different systems must respond in a synchronised way to operational changes, changes in loading, start-up and shutdown. This synchronisation is crucial in emergency shutdowns.

(a) (b) (c) Figure 27 Modern gas turbine control system components: 9a) GE control system

(Type IV). Images courtesy GE Power Systems, Woodward.

Machines within a package need to be integrated to function as a complete system. Control systems are designed to provide this essential control and protection for the machine elements. This requires key logical interlocks between the main control system, the turbine control and the compressor control. These will provide for start / run permits and sequence control, e.g. Control Room authorisation for turbine start.