Tomorrow's energy sources ... addressing the materials issues
TWI Bulletin, May - June 2008
As energy conservation issues take hold of our future the materials associated with alternative energy production are beginning to dominate our thinking.
Paul Woollin joined TWI in 1992 after obtaining MA and PhD degrees from the University of Cambridge. He is an Associate Director and manages the Metallurgy, Corrosion and Surfacing Technology Group. His work at TWI has concentrated on the performance of welded stainless steel and included failure investigations, consultancy and research and development programmes.
Stephen Birch gained his BEng degree in Materials Science and Technology and his MPhil degree in Metallurgy from The University of Birmingham. He joined TWI as a Project Leader in the Metallurgy and Materials Department in 1997, where he developed his knowledge of ferritic materials in a wide range of applications. In 2000 he transferred to the company's Marketing Department to use his knowledge and experience to promote TWI's services to industry.
On almost a daily basis we hear well-respected commentators expressing their views on energy supplies; how competition for dwindling oil and gas sources drive countries into damaging conflicts, how delays in the development of alternative renewable energy threatens security of supply and introduces a risk of power cuts, how the nuclear option is expensive and risky, or how turning land over to the production of crops for biofuels could lead to food shortages. On top of this, continued growth in CO 2 emissions threatens continued global warming. Paul Woollin and Stephen Birch report.
However, there are opportunities to reverse these trends and two specific alternatives exist for powering transportation; continued extraction and use of oil and gas through the exploitation of harder-to-reach hydrocarbon reserves,combined with carbon sequestration, and the development of alternative 'carbon-free' energy sources, such as hydrogen fuel cells.
There are enormous technical challenges that need to be overcome and TWI is making significant contributions to the global efforts to develop solutions to the problems.
There is no doubt that there are still large reserves of hydrocarbons available in the world. Whilst these will eventually run out and hence must be used wisely, the primary challenge for many countries is the fact that the easy to reach and cheap to extract supplies of petrochemical fuels are either running low or inaccessible due to political barriers. Much of the hydrocarbon reserve that is not the subject of geopolitical conflict, however, is technically challenging to extract and it is these technological barriers that we are concerned with here.
TWI's contribution to overcoming the technical challenges of oil and gas production
Courtesy of Maritime Pusnes AS
Many problems arise from the complex interactions that take place between the materials used in the oil and gas extraction process and both the non-hydrocarbon fluids that are produced and the external environments in which they operate. Oil reserves can be found either on land or offshore but most easy onshore reserves have already been exploited. In recent years increasing efforts have been put into extraction from increasingly deep offshore locations.External temperatures can range from over 40°C in deserts to -40°C in arctic wastelands and internal fluid temperatures can reach well in excess of 100°C, with enormous well pressures. Applied loading may be simply from the internal fluid pressure, eg in pipelines on land. But unpredictable cyclic loads are experienced in oceanic environments, eg in steel catenary risers (SCRs) which are long pipes bringing fluids from deep sea oil and gas fields to the surface.
Much of the oil and gas produced is accompanied by water, salts and gases. These 'non-hydrocarbon fluids' have the most significant influence on the materials required for the extraction activities. In the extraction of the least contaminated hydrocarbon products and at the lower temperatures, it is possible to use standard carbon steel pipeline materials, but the water present is typically acidified by the presence of carbon dioxide (CO 2) or hydrogen sulphide (H 2S), which causes corrosion and, in the latter case, sulphide stress cracking of carbon steel.
Many oil fields become increasingly sour ( ie produce increasing quantities of H 2S) with age, making this an increasing industry problem. This then leads to the consideration of alternative corrosion resistant alloys (CRAs), including stainless steels and nickel-based alloys. Even here, there are complications, since CRAs all have limitations to their corrosion and sulphide stress cracking resistance and the more exotic, very corrosion resistant materials may be prohibitively expensive.
Sour corrosion fatigue data showing dependence of crack growth rate on hardness and frequency
The addition of welds to fabricate structures adds further complications, since welding operations typically introduce a different material at the joint, alter the mechanical properties and corrosion resistance of the heat affected material adjacent to the weld and introduce residual stress. The resistance of the heat affected zone and the weld metal to corrosion and cracking in corrosive environments may be quite different from, and typically worse than, those of the parent material. Control of the welding operations to maximise performance of the fabricated structure, therefore, is frequently essential.
It can be seen from this summary that there is a wealth of potential problems that face oil and gas industry engineers. A number of TWI's on-going research projects addressing these pressing needs are discussed here.
There can be significant problems with using carbon steels for transporting unprocessed hydrocarbons, due to the presence of sour, acidified water, whilst stainless steels may be prohibitively expensive. Solutions to this problem include use of (i) 'low cost' corrosion resistant alloys, typically with moderate corrosion resistance due to the fact that elements that act to increase corrosion resistance, eg Mo and Ni, are expensive and (ii) clad products, eg which may comprise an outer carbon steel pipe with a thin layer of more corrosion resistant, and hence more expensive, corrosion resistant alloy on the inside. Establishing the limitations of such cost effective solutions to the handling of aggressive fluids, so that they can be selected with confidence, is vital to the commercial viability of exploiting the more challenging oil and gas fields.
Previous work at TWI has demonstrated that welded carbon-manganese steel pipes used for SCRs exhibit significantly reduced fatigue performance in the presence of sour acidic water. This work is on-going in the form of two joint industry projects (JIPs) and being extended to give a comprehensive understanding of the effects of fatigue in sour liquids on both carbon manganese steel and CRA clad pipes, which is directly supporting development of huge,technically challenging oil and gas fields in the Gulf of Mexico and offshore West Africa.
More recently it has become apparent that even water that is acidified only by CO 2 (so-called 'sweet' conditions) can also have a significant effect on the fatigue performance of such components. TWI has used its knowledge of this complex process, which involves an understanding of the interaction of corrosive liquids and fatigue loading with a weld microstructure, to develop world-leading test facilities and to establish the first quantitative understanding of the fatigue properties of welded SCRs in sweet conditions.
It was identified that the key factor in corrosion fatigue of welded C-Mn steel SCRs in sweet environments is the initiation of the crack, rather than the propagation. Therefore, testing regimes concentrated on how the cracks initiated, making it inappropriate to run accelerated tests, since corrosion is a time-based phenomenon, and if the fatigue loading frequency is too high, testing will not allow for synergistic effects to be manifested. This work is currently being extended as a further JIP to generate the data necessary to allow safe design of SCRs for cutting edge, deep water, sweet oil and gas field developments.
Section through corrosion fatigue specimen after test, showing cracking at the weld
Another activity that TWI has been involved in relates to defining the sour service limits in which particular stainless steels should be allowed to operate, under nominally static loading conditions, and in particular when welded.Accurate definition of these environmental limits gives designers the option to be less conservative in their material selection, and give them confidence that the materials they select will perform adequately in the anticipated environment, even if they select a low cost option. These limits are embodied in NACE MR0175/ISO 15156, which is used throughout the world to guide materials selection. TWI is actively involved in the development of this standard and the data required to support the limits set within it.
Recent improvements to this standard have removed previous uncertainties relating to some CRA materials, leading to areas of conservatism, which some end users believe to be excessive.
The most recent work at TWI in support of this activity has involved tests on girth welded 316L stainless steel pipe and C-Mn steel pipe with internal 316L cladding.
As part of a JIP, cross-weld samples have been tested in environments containing 0.1-10bar H 2S, chloride concentrations between 50ppm and 100,000ppm, at temperatures ranging from 60°C and 120°C, to give guidance appropriate to design of wellhead, flowlines and topside components. The data produced is already available to the sponsors to allow their designers more confidently to specify 316L stainless steel components in aggressive sour service in future. The ultimate aim of the project is to enable TWI to submit this data to the International Organization for Standardization so that the guidance used by the worldwide oil and gas industry can be changed to reflect this more accurate definition of operating limits.
TWI's contribution to the developing hydrogen economy
Corrosion fatigue test machine
Whilst oil and gas continues to dominate the world energy market, other potentially renewable fuel sources with lower environmental impact exist. Looking to the future, the potential for using hydrogen as a fuel is enormous,particularly for automotive applications, as it is not only renewable but it produces no CO 2. The technical and economic barriers to widespread use of hydrogen are equally enormous but the economic barrier is being gradually eroded by the spiralling oil price.
One particular technical challenge is the fact that hydrogen has the capacity to cause embrittlement or cracking in virtually all engineering materials. Furthermore, welding induces microstructural changes that typically make the weld more susceptible to hydrogen effects than parent materials. Hence, affordable storage of hydrogen is a particular technical challenge, especially when it is recognised that putting sufficient hydrogen into a vehicle to give it a useful range requires storage at very high pressure and very low temperature.
Corrosion fatigue test specimen
In response to the increasing demand for knowledge about the embrittlement behaviour of candidate materials for high pressure, low temperature hydrogen storage in cars, TWI has built unique facilities able to perform tensile,fatigue and fracture toughness testing of materials in an atmosphere of high purity hydrogen up to a pressure of 1,000barg, at temperatures between -150°C and 85°C. These facilities are being commissioned currently and are scheduled to begin testing in earnest in the middle of 2008, as part of a collaborative project, to pave the way for mass production of hydrogen powered cars in coming years. Combined with TWI's expertise in the relationships between microstructure and hydrogen embrittlement, this provides a facility that is unique in Europe, if not the world.
To learn more, contact materials@twi.co.uk
