Testing materials for oil and gas
NPL helped develop a 4-point bend test tailored to the needs of the oil and gas industry. This will enable the reliable evaluation of materials used for extracting and transporting oil from wells.
The oil and gas industry identified the need for a way of reliably and reproducibly testing the corrosion resistant alloys used for tubulars and flow lines in extracting and transferring oil from oil wells.
The water/oil mixture in the pipes contains aggressive corrosive chemicals including dissolved hydrogen sulphide and carbon dioxide at high pressure and temperature. The combination of physical and chemical conditions in oil and gas extraction can lead to very challenging environments for materials and the possibility of cracking and fracture of the tubular or flow line. The complex interaction of the various factors is not sufficiently understood to enable a priori prediction of the likely resistance of different alloys and to guide cost-effective materials selection.
Accordingly, there is a need for robust tests that can reliably evaluate material performance in these harsh environmental conditions and reduce the risk of failure. Such failures can lead to leakage, raising concerns with health and safety, environmental protection, asset protection and economic protection.
Support from a joint venture of the National Measurement System (NMS) and oil companies allowed NPL to develop the 4-point bend test modified to meet the needs of the oil and gas industry.
The test uses four support points and the specimen is loaded placing one surface in tension. It is carried out in an autoclave for 30 days; thus, the test apparatus has to be compact. The complication comes when trying to look at what happens with the integrity of welds Small differences in test procedure could generate very different results.
NPL tailored the test to focus on the measured strains adjacent to the weld and then loading to the required strain under bending conditions as distinct from the conventional approach of using uniaxial test data. This approach was supported by finite element analysis.
The 4-point bend test developed will provide confidence for businesses across the oil and gas industry.
One example is RA Materials (RA) a UK manufacturer and exporter of duplex stainless steel. There are two choices of material for sub-sea pipes: duplex stainless steel and nickel alloy. Duplex stainless steel is half the price of nickel alloys and has been used for many years.
There have, however, been problems with duplex as the oil and gas that is coming out of the ground has become increasingly corrosive, particularly with regard to hydrogen sulphide and chlorides. The risk with all stainless steels is sulphide stress corrosion cracking and ISO 15156 sets a limit for the maximum hydrogen sulphide content that is safe for each class of alloy. However, it is possible to use alloys outside these confidence limits, provided suitable test data are available. RA has conducted testing and found that these limits can be increased under some circumstances.
The work showed that previous stressing rules for 4-point bend testing were not accurate and were non- conservative. The alternative method developed is more appropriate and gives greater confidence in the selection of materials to minimise the risk of sulphide cracking. This has enabled RA to extend the limits of use for superduplex stainless steel; increasing the alloy's competitiveness against nickel alloys, which have much greater tolerance to hydrogen sulphide.
This work was funded by the National Measurement System, the UK's national infrastructure for measurement, which delivers world-class measurement science and technology. It is overseen by the National Measurement Office (NMO), an Executive Agency of the Department for Business, Innovation and Skills and is responsible for stimulating good measurement practice and enabling business to make accurate and traceable measurements, for the benefit of the nation.
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