Technical Presentations at the October 2012 Meeting

1.1   Full Ring Corrosion Test to Evaluate H2S Corrosion Susceptibility of a Subsea Pipeline in External Sour Environment’, Tanvir Sumon, Saipem Ltd

Full ring corrosion test was conducted in accordance with HSE OTI 95 635 to evaluate H2S corrosion susceptibility of a 48” carbon steel pipeline exposed to mild sour seabed environment. The seabed is characterised by the presence of SRB and anaerobic conditions, which might expose the pipelines to mild sour conditions externally. A study of the seabed conditions defined the external environment as “mild sour” within SSC Region 1 of NACE MR0175/ISO 15156.

Full ring test was utilised as sample size allowed full circumferential ring to be tested, suitable for pipeline girth weld qualification. The test specimens (approx. 1244 mm long x 1215 mm OD x 30.9 mm WT) containing girth welds were exposed to test environment containing a H2S concentration and a pH simulating the seabed environment. The specimens were exposed to test environment for 30 days. At the end of the test, the samples were evaluated by visual inspection, MPI, and UT. No evidence of SSC, HIC, and SOHIC was identified. A few localised high hardness readings were observed at the edge of cap repair and HAZ of one sample, exceeding 300 HV10 at a depth of 0.2 – 1 mm from the repair. Temper bead technique was utilised to restrict the hardness to a max. of 300 HV10.

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1.2  Effect of Process Variables on Corrosion Behaviour of Duplex Grades’, Janardhan Saithala, GL Noble Denton

Super duplex stainless steel (SDSS) products are widely used in the oil and gas industry for various process equipment such as pipes, fittings, tubes, topside pipe-work, flow lines, seawater handling systems and subsea umbilical’s etc.  The SDSS microstructure consists of approximately 50% austenite (γ) and 50% ferrite (α) phases.  However if incorrectly heat treated, these materials suffer from intermetallic phase (IP) precipitation, such as sigma (σ) and chi, that will degrade the mechanical and corrosion properties.  Therefore correct heat treatment of SDSS is critical to guarantee the product integrity.  This particular study was conducted to investigate potential susceptibility to SCC of sigmatized SDSS exposed to simulated North Sea platform production brine.  Samples containing varied levels of σ-phase are tested in mineral oil and North Sea brine to understand the failure behaviour of SDSS in the presence of σ-phase.  

The study found that s-phase forms rapidly even for short exposure times at temperature at 850 oC. Low levels of s-phase (< 2%) did not appear to suffer environmental cracking in H2S brine environments.  However, it is still quite important to detect these deleterious phases as mechanical toughness properties can go down even at very low vol % of s-phases which can lead to catastrophic failures in service.  Therefore, caution is needed by duplex SS product manufacturers to maintain and evaluate s standards carefully by non-destructive examination (NDE).  The kinetics of s-phase precipitation and its influence on corrosion resistance is a complex function of the actual chemical composition of the alloy, therefore each SDSS needs to be studied separately.  The metallurgical changes that occur in SDSS during heat treatment depend on a number of manufacturing process variables, such as welding, cold work, chemical composition, heat treatment temperature etc that will have a significant impact on the final microstructure.  Therefore, it is of utmost importance to ensure high quality of SDSS products, especially during heat treatment. 

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  4.1  ‘Mathematical Modelling of Cathodic Protection Anode Currents - comparison with classical equations, and the impact of interference effects for anode sledsJohn Baynham & Tim Froome, BEASY  

John Baynham presented results of investigations into resistance to ground of various anodes and groups of anodes. He compared resistance values determined using analytic and approximate formulae and using simulation. Single anodes considered included cylindrical ground rods, cuboid shapes and cylindrical anodes with large diameter. Combinations of anodes included some for which published results are available, and others typical of anode sleds used by the industry. Assessment of results demonstrated the appropriateness of the formulae and tabulated data regularly used by CP Designers.

4.2  Corrosion Issues in the Offshore Oil and Gas Industry’, Mike Dale, MACAW Engineering Ltd   

Examination of an Ageing Pipeline:  Mike Dale of MACAW Engineering presented an offshore case study involving a pipeline intelligent pig inspection, its analysis and the aftermath of finding minor corrosion within the pipeline. 

The presentation reported on the findings of an in-line inspection of an oil production pipeline whose original analysis indicated potential wall corrosion integrity issues. However a second inspection and a re-analysis of data demonstrated the pipeline was Fit For Purpose and the newly identified corrosion was not a threat to the pipeline integrity. 

Whilst the pipeline engineer was satisfied there was no problem Mike raised the larger question of why was corrosion taking place when the oil fluids were correctly treated with a corrosion inhibitor. He demonstrated that the corrosion was in fact the result of active bacteria known as Sulphate Reducing Bacteria who generate hydrogen sulphide which can cause several forms of corrosion, some rapid.  

Through a series of logical questions and further field investigations Mike was able to demonstrate that the minor corrosion identified within the pipeline was but the tip of a much larger problem involving the system wide contamination by bacteria which could cause integrity issues for all up-stream process vessels, platform pipe-work and infield pipelines between the platform and the numerous reservoir wellheads. 

A plan was demonstrated for the sequential treatment of the bacteria which involved complex offshore operations utilising divers to biocide treat at the source of contamination and then on down-stream throughout the whole infield pipeline system, the oil processing platform and the export pipeline first identified as having minor corrosion. Mike reported that all these activities were successfully conducted without incident. 

Whilst this was an interesting insight to the offshore corrosion and pipeline integrity issues Mike’s main point was the importance of following up all integrity issues, no matter how apparently minor, to understand the causative agents and their impact on the whole oil production system and thereby develop a mitigation plan, if required, which will deal with all issues and ensure system integrity.

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