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ELECTRICAL SAFETY FOR AN OFFSHORE POWER SYSTEM

By Lisa Lavie, Dave Burns, P.E., and John Ventura, P.E.

Abstract

For an efficient electrical system, maintenance and surveys must be performed on a

routine basis. One method of preventive maintenance is infrared thermography, a technology

that visually describes heat differentials across a surface. When a junction box contains loose

wire connections or a transformer carries unbalanced loads, electricity will not flow efficiently.

This allows much power to be given off as heat, and that heat can easily be detected using an

infrared camera. In order to see if the connections are giving off heat, electricity must be

flowing through the wires. That is, the system must be live.

Anytime one works on live electrical equipment, there is a risk of dangerous flashes

occurring. Fortunately, that risk can be quantified and guarded against. Recommended practices

are found inNFPA 70E, Standard for Electrical Safety Requirements for Employee Workplaces

(2000 Edition). This document is the reference for the power system analysis.

This project involves the analysis of the switch gear of an offshore power system. The

initial analysis showed that if a worst-case fault were to occur, the infrared thermographer would

be safe from incurable burns only if he were wearing a flash suit rated at 50 to 100 cal/cm. This

suit is not practical for predictive maintenance work; therefore, redesign of the power system is

needed or alternate procedures for maintenance of the power system must be established.

Research and preliminary testing with software models prove that the best solution is a main-tie-

main configuration for the main 480V switchgear. The redesign and a second analysis for

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comparison show that this new configuration leads to a dramatic decrease of available incident

energies.

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TABLE OF CONTENTS

Introduction . 1

Discussion ...3

Conclusion and Recommendations . 14

Appendices

A) Partial One-Line Diagram of Platform As Power System

B) Personal Protective Equipment (PPE) Matrix

Bibliography

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INTRODUCTION

Shell Exploration and Production Company (SEPCo) makes a dual commitment each day

it produces oil and natural gas. SEPCo is committed to increasing production and to maintaining

a safe working environment. It may seem that these two ideas contradict one another, but, in

reality, each project is arranged to find a balance between the two. This project, involving the

power system of an offshore platform, is no different. This project originated because the

Maintenance Services Team of SEPCo wishes to perform predictive maintenance on the

electrical equipment on Platform A, an offshore oil platform that is capable of generating

36 MVA of power. Before predictive maintenance can be done, a power system analysis must

be completed. Redesign of the power system, if necessary, will be performed to ensure safe

working conditions. When the commitment to safety is fulfilled, increased production will be

obtained indirectly through the predictive maintenance program.

Predictive maintenance involves testing equipment for proper operation and efficiency.

In this particular case, the work involves checking live electrical equipment for unbalanced loads

and/or loose wire connections. Both scenarios will release unwanted heat caused by power

losses. An infrared camera can detect this heat as shown in the following thermographs

(courtesy of Sierra Pacific infrared, Inc.; http://www.ir55.com/infrared_IR_camera.html).

Figure 1: (left) Loose Electrical Connections; (right) Unbalanced Loads

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An infrared thermographer looks for noticeable heat differentials in the circuit breakers and

transformers that are housed in metal-clad switchgear. The resulting images, documentation, and

report allow the platform workers to remedy the problem before a shutdown occurs because of

faulty equipment. Thus, platform production is increased because oil and natural gas can

continue to flow uninterrupted.

The thermographer must remove the back metal panels so he can view the equipment.

This is where safety precautions must be taken. If a conducting object falls within the panel

across any number of phases, a short circuit fault will occur. An arc flash will result, and

incident energy will be released. Proper personal protective equipment (PPE) must be worn to

prevent the worker from receiving incurable burns. The proper level of PPE can be determined

using a power system analysis. This includes a short circuit study, time-current curves (TCCs),

and incident energy calculations. A short circuit study calculates the available current at each

location where a fault can occur. TCCs determine the length of time it will take a protective

device to sense the fault and open to stop the flow of current. Incident energy can be calculated

using the available current, time for the protective device to trip, rated bus voltage, and distance

between the user and the equipment. Incident energy is measured in cal/cm, and PPE is

available for energies ranging from 0 to 100 cal/cm. A complete listing of the PPE requirements

for various energy levels is in Appendix B.

The Maintenance Services Team wishes to perform predictive maintenance on 480 V

switchgear. With the existing circuit breaker settings, a worst-case fault at the 480 V switchgear

would release energies ranging between 50 and 140 cal/cm. Although PPE is available for some

of these levels of energy, the flash suits that are necessary are not conducive to movement or

work (Photo of flash suit from www.aplussafety.net/).

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Figure 2: Flash Suit for Energy Levels up to 100 cal/cm

To complete predictive maintenance tests efficiently, PPE for energy levels below 50 cal/cm is

best. The goal of this project is to find the best solution for lowering the energy levels. The

change involves altering the existing equipment or installing new equipment. When the incident

energies for locations on the 480 V switchgear are below 50 cal/cm, the project will be

complete.

DISCUSSION

An initial analysis was performed on Platform As power system. The analysis consists

of a one-line diagram, a short circuit study, time-current curves (TCCs), and an arc flash

evaluation. Together, these four components are referred to as a power system study.

The first step is to create a computerized model, or one- line diagram, of the power

system. Its major components are generators, buses, transmission lines, transformers, motors

and protective devices (fuses and circuit breakers). Once completed, various tests can be run on

the one-line diagram to simulate working conditions. A complete one-line diagram of Platform

As power system contains over 180 buses, and a portion of Platform As one line diagram is

included in Appendix A.

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The Maintenance Services Team of Shell Exploration and Production Company wants to

perform predictive maintenance on the 480 V switchgears, so this project deals mainly with the

results that concern the low voltage equipment. However, since the low voltage equipment is

dependent on the high voltage equipment (4160 V), analysis of the entire system is necessary.

The purpose of a short circuit study is to determine how much current is available during

a fault. A fault is a disruption in the normal flow of electricity, which can occur if a conducting

object falls across one or more phases of live equipment. This is known as a short circuit. When

a short circuit occurs increasing current rushes toward the location of the fault from contributing

motors and generators. High levels of current and voltage cause the air to ionize resulting in an

arc flash of electricity, and incident energy is released. PPE prevents the energies from being

harmful to anyone in the immediate vicinity of the fault. A short circuit study simulates a worst-

case (three-phase) fault at every possible location and gives the available current that results. As

an example, the available current at SWGR-4001 (480 V) during normal operating conditions is

approximately 8.3 kA. During a three-phase fault, the available current increases to 64 kA.

Calculations for a short circuit study can be performed manually; however, when a

system as large as Platform A is to be analyzed, computer software will perform the calculations

more efficiently. The following example demonstrates how one would begin to calculate

available short circuit current by hand.

A 5.9 MVA, 4.160 kV, three phase generator feeds a 2 MVA, 4.160/.48 kV, three phase

transformer. The generators reactance is 0.12 per unit on the generator base, and the

transformer impedance is 0.125 + j0.0790 per unit on the transformer base. Cable 1 and Cable 2,

located as shown in the one-line diagram below, have an impedance of 0.03 + j0.04 O each.

Determine the available short circuit current if a three-phase fault occurred at Bus 4.

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The single line diagram and Thevenin equivalent circuit are shown in Figures 3a and 3b.

.48 kV

2 MVA4.16/.48 kVDelta-Wye Grounded

4.16 kV

5.9 MVA4.16 kV3 phaseXgen = .21 pu

Generator

Bus 1

Cable 1

Bus 2

Transformer

Bus 3

Cable 2

Bus 4

Figure 3a: Single Line Diagram Figure 3b: Thevenin Equivalent Circuit

Before calculations can be made, all variables must be converted to a common base. The

software used for this project, SKMs Power Tools for Windows, uses the generator

characteristics as a common base. For this example, Sbase = 5.9 MVA, VbasePri = 4.16 kV, and

VbaseSec = .48 kV.

The generator voltage in per unit is calculated as follows.

pukV

kVVgenpu

Vbasepri

VgenVgenpu

kVVgen

=

=

=

0116.4

16.4

16.4

The per unit impedances are as follows.

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pujpuZcablepuZcable

jpuZcablepuZcable

ZbasepriZcablepuZcablepuZcable

MVA

kVZbasepri

jZcableZcable

puZgen

0136.00102.021

933.2

04.003.021

121

933.29.5

)16.4(

04.003.021

21.0

2

+==

+==

==

==

+===

The transformer impedance is given on its own base. It must be converted to the

generator base as shown below.

pujZxfmr

MVA

MVA

kV

kVpujZxfmr

pujZxfmr

2331.00367.0

2

9.5

)16.4(

)16.4()0790.00125.0(

0790.00125.0

2

2

+=

+=+=

The total Thevenin equivalent impedance is calculated below.

puZthev

pujZthev

puZcableZxfmrpuZcableZgenpuZthev

4742.0

4703.00617.0

21

=

+=+++=

Using a single mesh equation,

puIscpu

pu

pu

Zthev

VgenpuIscpu

ZthevIscpuVgenpu

109.2

4742.0

01

))((

=

==

=

To convert this value to units of Amperes, the base current at Bus 4 must be known.

kAIsc

ApuIsc

IbaseIscpuIsc

AkV

MVAIbase

965.14

)7.7141)(109.2(

)4)((

7.7141)48)(.3(

9.54

===

==

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Therefore, 15 kA is the available line current if a three-phase short circuit fault occurred at bus 4.

This example uses the true characteristics of Platform As power system. The generator

is the same as generator 1, ZAN-701, on the Platform As one- line diagram. Bus 1 represents the

high voltage switchgear, SWGR-4000, and Bus 4 represents the low voltage switchgear, SWGR-

4001. The transformer is XFMR4001, and Buses 2 and 3 are the primary and secondary buses of

the transformer, XFMR 4001-P and XFMR 4001-S, respectively. Note that in Platform As

power system, SWGR-4001 feeds many downstream loads. These were omitted from the

example for simplicity.

The manually calculated short circuit current, 15 kA, is smaller than the actual available

short circuit current at that location because SWGR-4001 is actually fed by six operating

generators through two radial connections that are connected in parallel. Each connection has a

transformer and cables l and 2 as described in the example. The actual short circuit current is

higher because it takes all of the systems generators, transformers, and loads into account. See

Appendix A for a diagram of Platform As actual layout.

The time-current curves (TCCs) are part of the next analysis. An example is below.

Figure 4: Sample Time-Current Curve

23 kA

0.5 sec

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TCCs are used to determine the amount of time it takes for each protective device to sense a

fault. Note that current is the independent variable on the x-axis and time, measured in seconds,

is on the y-axis. The graph is logarithmic, and current is measured in 10^1 amps. Each

protective device has its own curve. The L-shaped curve shown in Figure 4 models 52-401

SG4001, a molded-case circuit breaker that is immediately upstream of SWGR-4001. It is

parallel with 52-409 SG4001, another circuit breaker with identical characteristics and an

identical curve. Together these breakers will sense the overload of current from the short circuit

fault. Each breaker will be exposed to 23 kA of current, and together they will stop the fault in

0.5 seconds.

The last step of the analysis for this project uses data that is collected from each of the

previous parts. The arc flash evaluation uses this data to calculate incident energy, which is

information used in determining the safety conditions of the environment. Actual available

incident energies are displayed in Figure 5. The low voltage buses are labeled on the x-axis, and

the energies are labeled on the y-axis in units of cal/cm. Note that the energies where work is to

be done exceed 50 cal/cm (shown by the horizontal line).

Incident Energies with Present Design

0

40

80

120

SWGR-4001 MCC-4002 MCC-4001 JB-4002 SWGR-5000 SWGR-5000B ZAN-708 BUS

Energy[cal/cm2]

Figure 5: Incident energies for the present system are above threshold range of 50 cal/cm.

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Incident energies, E, are calculated using formulas that are dependent on (1) bus voltage, kV,

(2) distance from the fault, D, (3) time for the breaker to sense the fault and open, t, and (4)

available short circuit current, FB. Different formulas exist for different bus voltages. When bus

voltages are greater than 1 kV, incident energy is calculated with the following equation.

2

***793

D

tFkVE B= [cal/cm]

The first variable, bus voltage, is fixed and cannot be changed to lower the energies. The

distance from the fault is generally assumed to be eighteen inches (the length of the forearm)

because it is the minimum distance the thermographer should ever be from the possible fault

locations. This exposure is present when the metal-clad switchgear is opened. A worker who is

only taking pictures with an infrared camera can stand approximately 3 feet away, so

calculations can be done using 36 inches for the distance. Still, this does not drastically lower

the available energies.

The third variable, time for the breaker to sense the fault and open, can significantly

affect the amount of incident energy released. Altering the settings on the protective devices can

cause a breaker to sense the fault and open faster. This will lower the incident energy released

and is one possible design solution.

Protective devices, such as circuit breakers, trip in the range of 0.01 seconds to 10

seconds. They are sensitive devices with specific time delays, and they are connected to each

other in both parallel and series configurations. When a fault occurs, the breakers should be set

so that the one immediately upstream of the fault location will trip first. If another breaker that is

further upstream senses the fault prior to the first upstream breaker, an additional portion of the

system will be put out of service. This is the result of protective devices that are not coordinated.

Changes that enable breakers to trip faster can increase the probability of an unnecessary

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blackout because the settings will be so close to one another that it would be very difficult to

predict which would sense the fault first. This tactic, however, may be valuable as a temporary

solution for already existing platforms because it lowers incident energies without causing loss

of production. That is, no additional equipment needs to be installed.

Immediately prior to predictive maintenance work, a skilled technician could change the

settings of the circuit breakers that are upstream of the switchgear where work is to be

performed. The change would set the breakers to trip at the fastest time interval to ensure a

decrease of incident energy should a fault occur. After the work is completed, the technician

could return the breakers to their proper settings, which would place the switchgear back into its

normal configuration.

For example, if the work needed to be done on SWGR-4001, upstream breakers (located

in switchgear requiring less PPE) could be changed to the fastest setting. This could cause the

shutdown of an additional portion of the system because a breaker other than the one

immediately upstream of the fault would sense it. However, the breakers that are immediately

upstream of SWGR-4001 are located inside it and cannot be safely altered. In this case, the

relays that trip the breaker on the primary side of the transformers that feed SWGR-4001 should

be set to the lowest setting. Changing the settings forces the relay to sense an overload of current

before the immediately upstream breaker, and a worst-case fault only releases 49.41 cal/cm of

incident energy. Without changing the relay settings, as much as 140.14 cal/cm could be

released (See Appendix B for a complete listing of the PPE required for each category).

The most energy released during a fault is at SWGR-4001. This is expected because it

has the most available current. Once SWGR-4001 is in an acceptable PPE range, its immediate

downstream breakers (which are located inside the switchgear) can be lowered. This will put the

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immediate downstream switchgear in an acceptable range as well. The procedure can continue

until all the low voltage switchgears are within the acceptable range. Then, predictive

maintenance can be performed as planned. When the maintenance work is complete, the

breakers and relays can be changed to their original settings in reverse order. That is, the furthest

downstream breaker should be reset first.

This task, as with all others, has a price. A skilled technician would cost approximately

$1,000 per day, and infrared thermography predictive maintenance work will generally take

between five to seven days. At a maximum cost approaching $7,000, this procedure is

comparatively inexpensive. However, predictive maintenance is a routine occurrence, and

Platform A may need to be checked annually for the rest of its productive lifespan. This total

could easily continue to increase as long as Platform A is operating.

Another solution for the problem at hand is to replace a portion of SWGR-4001 to

implement a main-tie-main configuration. This setup splits SWGR-4001 into two parts and

separates them with a circuit breaker that is open under ordinary working conditions. The

breaker is placed in the switchgear between the two connections to the step-down transformers as

seen in Figure 6.

Figure 6: Main-tie-main configuration on SWGR-4001.

XFMR XFMR

SWGR-4001BSWGR-4001A

LoadLoad

LoadLoad

closed breaker closed breaker

open breaker

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If a short circuit fault were to occur, the main-tie-main breaker would remain open. This

configuration blocks one path that current could flow through to contribute to each fault. That is,

if the fault is on the left side of the system, the current contribution from the generators will only

feed the fault through the left transformer instead of flowing through both transformers. This

breaker also inhibits some of the back flow contributions from the downstream motors. If a fault

were to occur during predictive maintenance, the reduction of available current would reduce the

amount of incident energy.

The main- tie- main breaker should always remain open unless purposely closed by a

technician. This would be necessary if one of the radial feeds from the high voltage switchgear

to the low voltage switchgear (SWGR-4001) is put out of service. If this were to happen, the

main-tie-main breaker could be closed so that electricity would continue to flow to the

downstream loads.

In many of the larger platforms, a main-tie-main configuration is part of the original

design. While it was not included for the low voltage (480 V) switchgear, SWGR-4001, there is

a main-tie-main breaker on SWGR-5001. Using this as an example, a breaker dividing SWGR-

4001 was added into the model, and the power system analysis was completed again. The

resulting arc flash evaluation shows decreases in incident energies; see Figure 7 below.

Incident Energies with Main-Tie-Main Design

0

40

80

120

SWGR-4001 SWGR-4001A MCC-4002 MCC-4001 JB-4002 SWGR-5000 SWGR-5000B

Energies[cal/cm2]

Figure 7: Incident energies with main-tie-main configuration are below the threshold range.

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Although it is clear that the energies are reduced to acceptable levels, the solution is

difficult to implement into an already active power system. Platform A would need to be shut

down for a time to install a new section of switchgear. The oil and natural gas that is not

produced during the shutdown would result in a great deal of money lost. Several factors need to

be assessed before a decision can be made.

The first point of interest is the cost of the new switchgear section. Switchgear sections

are designed to be interchangeable, so only one piece of equipment would need to be purchased.

480 V switchgear sections can contain one to four breakers of various sizes; thus, the price will

vary according to the specifications. For a vertical section of two breakers, the price is

approximately $15,000 to $19,000.

There is also the cost of lost production. Annual summaries show that in 2001 Platform

A produced approximately 50,000 barrels of oil and 75 million standard cubic feet (mscf) of

natural gas per day. The cost of oil in 2001 was on average $21.91 per barrel, and the cost of gas

was $4.30 per thousand standard cubic feet (Platform A 1). The total daily production loss in

monetary terms adds up to $1,418,000. Assuming that a new section of switchgear cannot be

installed in a single day, a more accurate result is this value multiplied by two or three. After

including the additional money that is lost during the time it takes for the equipment to ramp up

to its maximum flow rate, this value can increase to as much as five million dollars. The cost of

equipment is very small in comparison to the overall cost.

Now one must determine if a design change is economically feasible for the company and

the platform in particular. Financial advisors should review the lifespan expectancy of Platform

As reserves in addition to the frequency of the necessity of predictive maintenance work. Both

design solutions will fulfill the requirements of lowering incident energy, yet the economics of

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each solution is a limiting factor. Either design change would only be economically justifiable if

the predictive maintenance is considered necessary work. Predictive maintenance, specifically

infrared thermography, is only useful if work reveals a loose connection or unbalanced load that

would normally have caused a blackout of the system if left undetected. If this were to happen,

many days of lost production would result while the workers located the problem area, obtained

the proper equipment to fix it, and actually completed the necessary repairs. Lost production

leads to less revenue. For this reason alone, predictive maintenance should be routine work. If it

is performed annually, Platform A is likely to continue to produce oil and natural gas efficiently.

CONCLUSION AND RECOMMENDATIONS

Predictive maintenance is a necessary factor for efficient production of Platform A. The

present design allows this work to be completed, but the personal protective equipment needed to

maintain a safe working environment is not conducive to normal movement associated with

predictive maintenance. Redesign of the system would decrease the incident energies during a

fault so that fewer PPE items would be necessary to maintain a safe working environment. For

the already producing Platform A, there are two solutions: adjusting the upstream breakers

during maintenance or modifying the existing switchgear.

Adjusting the circuit breakers demands that the settings be set to the fastest trip time

before predictive maintenance work is begun. The original settings would be reset after the work

is completed. This temporarily lowers the incident energies so that a safer working environment

is achieved. This solution is less expensive in the short term than the second solution below;

however, this work would be necessary each time predictive maintenance is to be performed.

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The second solution requires equipment replacement. The center section of SWGR-

4001, the primary low voltage (480 V) switchgear, would need to be replaced with a section that

contains an open circuit breaker. Installing this piece of equipment would divide SWGR-4001

and reduce the available fault current. This solution lowers the incident energies, but the cost is

greater than the prior solution. A shutdown of the platform would be necessary to implement the

change, and it would result in lost production. Lost production would lead to large profit loss, so

this solution is not ideal for an already existing platform.

For the project at hand, it is best to adjust the upstream breakers. Changing the breaker

settings before and after predictive maintenance work ensures quick trip times and lower

energies. This solution does not interrupt the flow of production like the installation of new

equipment would. Additionally, Platform A will continue to produce for many years but not

long enough to allow this procedure to cost as much as the main-tie-main solution.

For any future platforms with power systems that are as large as Platform A, a main-tie-

main configuration should be included with the original design. It increases the initial cost, but it

ensures the safety of all workers when the platform is energized. Safety should be a primary

concern in the design of any project. The results of this analysis and design show that a main-

tie- main configuration is an effective method of reducing energies, and it should not be

overlooked

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APPENDIX A

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BIBLIOGRAPHY

A+ Electrical Safety Equipment. URL: http://www.aplussafety.net/2002.

Infrared Thermal Imaging Camera. Sierra Pacific infrared, Inc. URL:

http://www.ir55.com/infrared_IR_camera.html 2002.

NFPA 70E, Standard for Electrical Safety Requirements for Employee Workplaces (2000

Edition). National Fire Protection Agency. 2001.

Platform A Well Scores Gulf of Mexico Production Record. URL: http://www.countonshell.com/

October 6, 1999.

Power Tools for Windows. SKM System Analysis, Inc. Computer Software. Version

4.0.2.6. 2002.