Safety first

Tom Shelley reports on some of the cutting edge developments in the oil and gas sector.

The oil and gas industry requires the use of technologies that are peculiar to its own particular needs, reducing the risk of fires and explosions from inflammable gases as much as possible, resisting corrosion and the pounding of wind and waves, and at the same time keeping costs down.

The basic technologies do not change much, but there are always new tools coming along with the potential to assist production and reduce costs, and a need to extract oil and gas from ever more difficult locations.

Peter MacAulay, managing director of Expo Technologies in Thames Ditton told us that his company has been making electrical enclosures that are continually purged with air to keep out inflammable gases for more than 25 years. However, there are always new technologies that these have to accommodate, and he made particular mention of "optical technologies" such as lasers and high-intensity lamps, mainly for analytical purposes, but also possibly for welding and cutting, in which case they could bring the same benefits to the oil and gas industries as they have on land, allowing much greater speed and precision. In such cases, Macaulay says, their task is to ensure that the light that gets out is "within certain limits".

The cabinets are purged with compressed air. Macaulay explained that the need arises because, even if the enclosures are sealed, flammable gases are liable to be sucked in during fluctuations in atmospheric pressure, in which case they tend to accumulate in the bases of the cabinets. They therefore need to be flushed out downwards before power is turned on, and then kept out by maintaining a small positive pressure relative to the atmosphere outside. When asked if it was necessary to use nitrogen or gas from bottles, we were told that it was not, because flammable gases are normally removed from the sorts of compressed air supplies used in oil and gas installations in water traps. The only alternative to purged cabinets is to house anything electrical in cast steel or plastic enclosures strong enough to withstand an explosion within them. While this is the preferred route for small enclosures, it is prohibitively expensive for large ones.

Reliable and high-quality welding leading to more reliable and cost-effective constructions are also critical requirements for the offshore industry and these are the goals of the European Union 'MintWeld' project. Led by the University of Leicester, the 11 European collaborators aim to understand welding processes better over a wide range of scales, using computer modelling that incorporates knowledge based from both laboratory and industrial experiments. According to consortium leader Dr Hong Dong: "The project is highly valuable given the potential catastrophic consequences a disaster would create, as exhibited in the 1980 Norwegian Alexander Kielland wreck in which 123 lost their lives due to a faulty 6mm weld. Failures in welded components, such as deep sea oil and gas transport systems can result in lost production valued in several billion Euros, whilst exposing the EU to increased petroleum prices and increasing EU dependency on oil and gas supplied from other regions. This project will deliver an accurate, predictive and cost-effective modelling tool that will find widespread application in the relevant European metals industry."

Another of the participants is TWI, which has particular knowledge of the subject through its ECA (Engineering Critical Assessment) work. Speaking to a meeting of the Chartered Quality Institute, Dr Isabel Hadley, manager of TWI's Integrity Section explained that ECA started off with the oil boom in the North Sea in the 1970s, to address the problem of, "What happens if you find cracks during the last stages of manufacture or during service?"

Assessment can either be based on design rules and Charpy impact energy test results, or FADs (Failure Assessment Diagrams), which assume cracks will be present, but will not grow as long as they are inside a curve that relates Kr – fracture ratio = applied stress intensity factor divided by material toughness, and Lr, or load ratio, defined as reference stress divided by 0.2% proof stress, where metal starts to yield. The trend now, is to base such curves on full fracture mechanics analysis, resulting in more cost effective designs, longer use of existing structures, and increased reliability and safety.

There is, as might be expected, a raft of standards to follow, including BS EN 7910 'Guide to methods for assessing the acceptability of flaws in metallic structures', and "API 579-1/ASME FFS-1 2007 Fitness-For-Service." These latter are mainly concerned with deciding whether existing structures can be kept in service as is and if not, what should best be done with them. Dr Hadley told us that in future, she expected to see: "Improved precision and the ability to predict the exact point of failure and margin for safety." The problem, she said, is that in order to do so, engineers needed to have "extremely detailed input data, including all likely residual stresses from welding".

Extremes of temperature are a significant factor in oil and gas applications. Trelleborg Offshore has recently secured a contract to supply Elastopipe, its synthetic, rubber-based, fire-resistant, flexible piping system for nitrogen transport in extreme temperatures, at the Sakhalin Energy Investment Company's Onshore Processing Facility. Situated in the Sea of Okhotsk which is typically ice-bound for up to six months a year, operations here are hampered by severe cold, snow, ice, icefloes, storms and even bears.

The proven advantages of Elastopipe over conventional rigid steel pipes have already made it an established solution for seawater-based deluge and sprinkler systems. This is due to its corrosion-free and fire resistant performance with minimal maintenance and lower installation costs. Elastopipe achieves constant availability with considerably less need for testing and reduced maintenance costs.

The requirement was to qualify for -45°C exterior temperatures for nitrogen gas transport around the OPF but the challenge was to prove that the Elastopipe and its couplings were gas tight, even at -45°C. Testing, which demonstrated to the client that the standard Elastopipe and standard couplings could meet the requirements of these extreme environments, was performed and accredited by the Teknisk Institutt, Norway. The tests showed that flexibility, lightweight, corrosion-free and ease of installation without the risk of welding are valuable, especially under these extreme conditions.

Another big cause of failures, particularly offshore, is corrosion. Cadmium plating is a total no-no for most onshore applications, but nobody seems to have found a satisfactory replacement for it offshore. Cadmium electroplated coatings offer a unique range of properties for which no single alternative exists. Along with the National Centre of Tribology, leading academic bodies and aerospace contractors, Poeton was a member of the DTI-sponsored RECAP project, set up to seek substitutes for cadmium electroplating, but while "careful evaluation of applications can often permit use of other coatings …This is very selective and depends much on the priority property for which Cadmium Electroplating became the original selection in the first place."

However, there are now alternatives. The French company, Chambrelan, for example, has developed a new chrome-free, white zinc plating process for their telescopic slides that offers 480h of protection against the salt spray test before the appearance of white, rust, while improving the protection against red rust from 600h to 672h. For valves for offshore use, Fisher specifies zinc-rich primer, polyamide epoxy tie coat, and polyurethane top coat.
On the subject of valves for the oil and gas industry, Swagelok has brought out a new family of valves for instrumentation and sampling applications. The design challenge was, apparently, to come up with a modular approach that would give increased functionality in smaller footprints, using both pneumatic and manual actuation.

Tom Shelley

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