Emergency Pipeline Repair Systems (EPRS) Specific ... · PDF file1st December 2011 Emergency Pipeline Repair Systems (EPRS) Specific Corrosion Assessment for Sour Service Use Subsea

1st December 2011

Emergency Pipeline Repair Systems (EPRS) SpecificCorrosion Assessment for Sour Service Use

Subsea Pipelines Integrity Management 2011

© Det Norske Veritas AS. All rights reserved.

Discussion

Findings

Approach

Scenario

Contents

Conclusions

Introduction


Introduction

Given the consequences of any pipeline failure many operators are taking a proactive approach to planning for an emergency pipeline repair

There are several EPRS on the market which largely fall into 4 categories:• Couplings• Clamps• Hot Tap Tees• Isolation Plugs

Most have only been used on sweet systems


In reviewing the various designs for mechanical connectors available on the market, QG had concerns that internal spaces of the connector could be subject to corrosion.

Introduction

DNV were asked to evaluate whether the potential for corrosion damage in the occluded space of a connector would adversely impact a mechanical connector installed on; either a carbon steel pipeline or a corrosion resistant alloy (CRA) lined pipeline in wet sour service thereby making it unsuitable for use.

Fluid composition CO2 and H2S content was 3.9 mole% and 2.4 mole% respectively (worst case). Modelling had shown very high uninhibited corrosion rates were possible without effective chemical mitigation.


Scenario

Standard Mechanical Connector Details & Potential Corrosion Path


Approach

Comparative review for the mechanisms active in the occluded space when compared to the pipeline itself.

Utilised worst case client approved input data from previous corrosion modelling studies for the pipelines.

Preliminary computational fluid dynamics (CFD) analysis was carried out to provide an indication of flow behaviour within the occluded space to add further confidence to the conclusions reached.

Based on the review, evaluation of the feasibility for use of a mechanical connector as a suitable EPRS was undertaken for carbon steel and CRA lined carbon steel pipelines.


Approach

• General: corrosion models, scale formation• Occluded space: mass transport, active

mechanisms• Cut end: galvanic corrosion• Connector end: galvanic corrosion

• General• Configurations• Pressure fluctuations• Corrosion considerations

• Structural integrity• Functionality• Inspection

Corrosion considerations

Computation fluid dynamics

Mechanical connector


Findings: Corrosion

Defined operating practices should preserve scales. Testing had shown Mackinawite should dominate. Low reactivation of scales within the occluded space.

Scale

Models used based on CO2 corrosion rate with a pitting factor added to account for H2S. Conservative and only applicable to flowing conditions.

Corrosion model

Fixed small volume and low refreshment “normally”Finite corrosion action even during pressure surgesAny scale/deposits formed would further limit corrosion

Mass Transport

Driving force, electrolyte and corrosive speciesSulphide scales “short circuit” galvanic effectsLow conductivity fluids so connector/CRA unlikely

Galvanic Effects


Findings: CFD

Ovality / MisalignmentPressure fluctuationsCirculating flow % versus axial distanceCorrosion considerations

What?


Findings: CFD

Standard Configuration Misalignment

Gap

Annulus


Findings: CFD

What?Configuration

Corrected Positive Mass Flow Through Gap [kg/s]

(Full Size Pipe)

Gap Positive Mass Flow / Overall Pipe Mass Flow

Standard Gap = 10mm / Annulus = 10mm 4.536E-02 0.0146%

ID Misalignment

Gap = 3mm / Annulus = 3mm 6.696E-03 0.0022%



Gap = 30mm / Annulus = 30mm 1.318E+00 0.4231%


Findings: CFD

What?

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.00E+00 2.00E-01 4.00E-01 6.00E-01 8.00E-01 1.00E+00 1.20E+00 1.40E+00

Mass flow (kg/s)

Volu

me

(m3 )

Total Mobile

Volume versus mass flow


Findings: CFD

What?

Circulating flow versus axial distance


Findings: CFD

What?

Pressure fluctuations (10bar) during normal operations


Findings: CFD

Flow induced localised corrosion (FILC)Hydrodynamic effects versus scale fracture stress

10% pressure fluctuation gives <0.75m/s

15MPa scale fracture stress calculated

94-150MPa measured

Corrosion considerations


Findings: Connector

Corrosion at the cut ends may reduce axial load capacity

Under seal corrosion is unlikely but could reduce radial stiffnessSeal location and material are key

Inspection of the annulus is a challengeAnnual test port pressure test may give early indications of issues

Considerations


Discussion


Conclusions

The probability of any significant corrosion damage to the external wall of the cut pipe and/or the inner surface of the EPRS connector is considered to be low.

Some corrosion in the annulus must be accounted for; incorporating a corrosion allowance comparable to the pipe wall. Corrosion damage to the outer surface of the cut pipe is not an immediate concern as the connector is the pressure retaining surface.

The potential galvanic interactions in the CRA lined pipeline are considered to be mitigated by scale formation on the carbon steel surfaces, the presence of low conductivity fluids and low mass transport into the occluded space, unlikely to disrupt the scale formed.


Conclusions

The high contact pressure between the seal and the seal surfaces will prevent ingress of corrosive fluid and mitigate the probability of under seal corrosion. Based on the CFD analysis undertaken placing the primary seal at least 200mm from the gap would further mitigate the risk of under seal corrosion because the area would not be exposed to fluid refreshment.

The volume of annulus should be minimised as far as practicable to mitigate mass transfer particularly during operational pressure fluctuations.

As with all components within the pipeline systems, to ensure integrity, monitoring and inspection is required to confirm that the assumptions made on the degradation mechanisms remain valid. The mechanical connector modifications implemented by QG including the proposed monitoring and mitigation measures are considered a prudent means to ensure long term integrity.


Conclusions

DNV see no reason why a carbon steel connector installed on a carbon steel or CRA lined pipeline could not be designed for a service life commensurate with the design life of the pipelines.

All elements of the mechanical connector should be reviewed including seal materials and their compatibility with all fluids (product and chemicals) and their positioning.


DNV would like to thank Qatargas for allowing publication of this information and Oilstates for use of their standard drawing

Acknowledgements


Safeguarding life, property and the environment

www.dnv.com

Documents

Emergency Pipeline Repair Systems (EPRS) Specific ... · PDF file1st December 2011 Emergency Pipeline Repair Systems (EPRS) Specific Corrosion Assessment for Sour Service Use Subsea