Technical Presentations at the April 2012 Meeting

1.1         Advances in Copper Alloy Aquaculture Cages’, Carol Powell, Copper Development Association & Langley Gace, International Copper Association, New York 

Copper alloy mesh for aquaculture cages has been commercially used in Japan, Chile and Australia. Demonstration sites exist in Turkey, Korea, Hawaii and China.  This presentation gave an overview covering both the history of its development and also described the diverse test programmes which are on-going to quantify its benefits.  The latter range from laboratory and sea water immersion corrosion studies, Dome trials for copper leaching, new designs in submerged cages, and evaluations in food safety, growth performance and disease.  The original trials were carried out in Scotland  in 1976 using copper-nickel and, since that time, other alloys including proprietary brass alloys have been successfully developed.  Several different designs of cage are undergoing development trials.  The copper cages are designed as an alternative to nylon nets, which are coated with anti-fouling copper compounds.  As 50% of world fish consumption is now farmed, the potential market is high. 

Feedback to date is very positive.  It has been found that the copper cages can provide in excess of 5 years service and act to minimise macrofouling and the occurrence of diseases and parasites.  This altogether produces healthier fish and better food utilisation.  The tough mesh (generally ~4mm wire and 40mm mesh) gives superior protection against predators such as seals and sharks. 

 

1.2     Progress in the Understanding of an Umbilical Local Environment’, Thierry Cassagne, Total E&P 

Subsea umbilicals are used for control and operation of subsea oil and gas production facilities. Typically an umbilical consists of steel tubes, electrical cables, fiber optic cables, weight or strength elements and fillers. Super duplex stainless steel tube material has been used for almost 20 years within the umbilical industry.  However, a recent failure on two orbital welds of 25Cr super duplex of an umbilical installed in the South China Sea was reported.  From these results some umbilical end users recommended the use of thermoplastic coating to avoid crevice corrosion on 25Cr super duplex stainless steel above 20°C.  However, this limit is based on results obtained for super duplex stainless steel in aerated natural seawater and very little is known on the micro-environment formed by the confined seawater between metallic tubes and polymer matrix of an umbilical.  

This work reports corrosion potential and oxygen content measurements in the confined zone between the metallic tubes and the polymer matrix of an umbilical.  These measurements were performed using micro-electrodes on a 2 meter long real umbilical at 30°C in heated natural seawater.  From the measurements, it is shown that the oxygen content in the confined zone is rapidly consumed probably due to the passive current on the stainless steel tubes and then remains below 2 ppm over one year exposure period.  From the open-circuit potential measurements performed in the confined zone, it is clear that the open circuit potential remained below -150 mV/AgCl.  This translates an absence of electrochemical effect of the biofilm in the confined zone.  Visual and metallographic examinations of the tubes after exposure confirmed the results obtained by the microelectrodes and clearly indicate that no corrosion initiation occurred on superduplex stainless steel (base metal and welds) under these experimental conditions.   

The results are discussed in view of the validity of corrosion tests performed under aerated conditions and the need of further developments of new testing procedures to represent the confined situation in the umbilical design.    

Back to Minutes

1.3   Preferential Weld Corrosion in a Gas Compression System’, Dave Moore, Lloyd’s Register EMEA, Aberdeen 

A case study was considered where a pinhole leak had occurred in a weld in a 14-inch diameter process gas system line.  This was in an offshore oil and gas production facility on the UK continental shelf.  It resulted in the immediate shutdown of the gas system and subsequent shutdown of all production.  The facility shutdown lasted for more than three months while repairs were carried out.  Causes of failure were discussed, and the reasons why inspection had failed to detect the corrosion. 

Lessons learned:  Weld corrosion should be considered during design.  This case study highlights the challenge of predicting where weld corrosion will occur, and the challenge of detecting weld corrosion. Hence: the integrity management industry should accelerate cascading the most recent research findings on inspecting preferential weld corrosion to the practitioners "in the field".  Integrity providers, NDT companies and clients need to work closely together and accept that on critical systems where serious defects are located, that it should result in the deployment of advanced techniques that are capable of higher resolution with recordable results to allow for more definitive analysis of data.  Integrity engineers and inspection technicians should appreciate the practical limitations of manual UT scanning. This requires a paradigm shift in the minds of many technicians and inspection engineers who have long experience of "accurate" results from the manual UT shear wave tool, which makes them inherently resistant to appreciating its limitations.  Pressure system inspection should utilise more quantitative methods for assessing any weld defects identified by routine inspection. Essentially, this means greater use of TOFD and radiographic inspection.  

In summary it was found, that the “ingredients” for failure were present: Carbon steel (welded)/ Carbon dioxide/ Water.  Degree of wetness.  Design of the gas system: carbon steel with no facility for corrosion inhibition.  The inspection technique used to screen for weld root corrosion was manual ultrasonic testing (UT) using a zero degree compression wave probe, through the weld cap. This technique is ineffective at reliably detecting this type of defect.  Manual shear wave UT has limited reliability in detecting and quantifying weld corrosion.  The preferred technique for weld corrosion inspection is ultrasonic time-of-flight diffraction (TOFD), providing the component falls within its capabilities. The preferred alternative technique is tangential radiography (HOIS Report).

 

3.1     Integrating Mathematical Modelling with ROV Survey Data to Improve Interpretation, and Provide Input to Ongoing CP Strategies’, Tim Froome, BEASY 

Marine structures are frequently monitored or surveyed as part of an ongoing integrity management program to obtain data on corrosion potentials, anode consumption and in some cases field gradients. One objective is to determine how effective the CP system is at providing protection to the structure and another is to verify that sacrificial anodes are being consumed at a rate consistent with design assumptions. However it is not economic, or in many cases even feasible, to measure potentials on a complete structure or to collect data from all anodes, so the CP engineer is required to infer the condition of the overall structure from the available data. 

For structures with complex geometry, the accuracy of data collected may not be reliable (eg field gradient measurements being affected by other nearby metal surfaces), and problems with access can make reliable orientation and placement of the probe on the structure/anode difficult to achieve. These issues require the CP engineer to use his judgment to determine which data to use and which to discard. 

Computer modelling has been used in the design of CP systems firstly to optimize the design, and secondly to provide assurance to the operator that requirements for anode life are met, and that the system will sustain the potentials required to protect the structure (ie it determines the distribution of potentials across all surfaces).  

A case study was presented in which computer modelling was used to evaluate data from a single survey, to enhance the data to provide a view of potentials over the complete structure, to help identification of anomalies, and to provide a view of consumption of all anodes.  [TFroome@beasy.com]

 

3.2   

QA/QC Tools to Ensure the Quality of Duplex Stainless Steel Components’, Roger Francis, Rolled Alloys 

It has long been understood that the heat treatment of duplex and superduplex stainless steels is critical to obtain the optimum structure and the desired properties.  Over the last twenty years there have been a number of cases where inadequately heat treated components have been delivered by the manufacturer and then subsequently identified as defective further down the supply chain. In some cases the problem was identified and resolved prior to fabrication and installation, while in others fittings have leaked in service due to poor microstructure from incorrect heat treatment.  

Common to all these cases is that the cast and batch production test certificate indicated that the goods met specification requirements in all respects. Hence the similitude between cast and batch specific test pieces and the production parts has been called into question.  The use of additional testing when specifying these alloys is common but there is no agreement on what these tests should be. 

There has been extensive discussion on how best to test individual components non-destructively to detect unsatisfactory material. Some have suggested that magnetic measurement of the ferrite content is adequate, whilst others believe the test to be insufficiently discerning, resulting in too many good parts falsely being identified as “suspect” and causing unnecessary remedial action.   

The present paper describes the procurement specifications used by the authors’ company to ensure adequate properties in service.  The paper addresses the strengths and limitations of magnetic ferrite measurements and shows how the readings are affected by manufacturing route, product form, surface roughness and radius of curvature.  The paper goes on to show how the test can be used to identify material that may contain sigma phase and that in-situ metallography is then required on these suspect areas to either release the part or condemn the part to remedial heat treatment.  The results of five years successful experience with this combination of tests are discussed. 

This is an expanded version of a paper presented at the NACE Conference in Salt Lake City, March 2012.  [rfrancis@rolledalloys.com]

 Back to Minutes