As an oilfield is exploited there is a tendency for the produced fluids, the liquids and gasses that are brought up the wells and into the surface production, to become sour; that is, to contain quantities of hydrogen sulphide. Sweet crudes, generally regarded as having less than half a per cent of sulphur, attract a higher price than sour crudes, but the implications on the production system can be a lot more significant.
Most of the platforms that are now regarded as ageing were, when designed, expected to have a life of 25 or 30 years and to extract maybe 30 per cent of the oil originally in place. As production technology has improved, life extension has become common, to the great benefit of the whole oil industry (except perhaps the decommissioning sector). Process systems have correspondingly become more complicated, to handle increased water content and sourer fluids.
Operating in harsher environments
Beginning in the 1990s in the North Sea, there has been a tendency to exploit deeper reservoirs, which, due to their greater depths, involve higher pressures and temperatures than shallower fields. As the temperatures and pressures increase, the demands on components naturally become more onerous. This has been noted at TWI, as the conditions for which we are asked to test components has increased from 5000psi, through 7500, 10,000, 15,000 and now to 20,000psi. These high-pressure, high-temperature (HPHT) fields also tend to have more sour fluids than traditional oilfields, and so the industry is interested in the performance of materials in such fluids and at HPHT conditions.
TWI has developed uniquely specialised testing facilities to meet the needs of our oil and gas Members, building a laboratory dedicated to carrying out dynamic tests on parts in HPHT and sour conditions. Much of our equipment we designed and constructed in-house, to ensure we can simulate real-world conditions as closely as possible. Our bespoke equipment also enable tests to be carried out on full-scale parts.
With the oil industry still enduring its longest ever price reduction, now is not an obvious time to research development of materials and increase understanding of harsh environment conditions. However, as the industry adapts to what could be a new long-term reality of price, confidence seems to be increasing – indicating that there will be a return to new developments. Whatever happens, costs have to be better controlled, and part of this includes smarter use of materials and techniques.
Adapting to the challenges of deepwater extraction
There are factors to consider when establishing the durability of materials in aggressive environments, such as sour fluids in HPHT conditions. Not only do the parent materials have to withstand the conditions, but they have to be joinable and the joints also have to be able to withstand the conditions. Whereas selecting the correct materials (often duplex and super duplex stainless steels) can be relatively straightforward, welds in such materials are quite complex and they need not only to be able to withstand the static forces and the aggressive environments, but often to have considerable fatigue resistance as well.
The use of duplex and super duplex steels represents one set of solutions to HPHT aggressive environments; using flexible pipes as flowlines and risers is another. These are generally multilayer constructions of stainless steel interlocking elements (a bit like a shower hose): wires for strength and polymer layers for pressure containment. These too have to be tested and TWI facilities include flow loops to more accurately replicate real-world conditions.
To talk to us about your aggressive environment testing needs, please email contactus@twi.co.uk.