Due to recent failures and cracking found in laboratory tests, there is some concern about the integrity of weldable martensitic stainless steel (WMSS) ~ pipelines that are currently operated.
In this paper, the susceptibility of WMSS to intergranular stress corrosion cracking (IGSCC) cracking in the heat-affected zone (HAZ) and a methodology to assess the likelihood of failure of a pipeline are discussed. This is especially relevant for pipelines where no post weld heat treatment (PWHT) has been applied after welding. The approach consists of:
1. Determine if the material is susceptible via a comparison of pipeline details with available test data and a combination of four-point bend (4PB) tests and electrochemical tests to determine sensitisation.
2. Determine if water wetting can be expected at the pipe wall. This is done via three-phase flow modelling, calculation of water wetting parameters and subsequent assessment of the likelihood of 'continuous' water wetting.
Based on the methodology discussed above, an assessment has been carried out on girth welds in rich grade WMSS line pipe, leading to the following conclusions:
· The 4PB tests showed that the material is susceptible to IGSCC at 94 °C and 100% proof stress in both simulated condensed water (pH 3.9, 100 ppm chlorides) and in simulated formation water (pH 5.4, 15,000 ppm chlorides)? Even at 25% of the proof stress, susceptibility to IGSCC was found.
· Increasing the pH reduced both the likelihood of initiation and the propagation rate of IGSCC.
· Applying local degree of sensitisation (DOS) measurements, sensitisation was detected in the zone where cracking was found in the 4PB tests.
· Investigation of the microstructure did not reveal precipitations indicative of sensitisation.
· Water wetting calculations can be used to further assess the likelihood of IGSCC in pipelines.
To mitigate the risk of IGSCC in existing pipelines, the following measures can be considered:
· Increasing the pH of the production fluids by injection of a caustic solution at the wellheads.
· Keeping water entrained in the liquid hydrocarbon stream, e.g. by injection of surfactants.
In the early nineties, weldable martensitic stainless steels were developed as an economically attractive alternative to 22Cr duplex stainless steel. Weldable martensitic stainless steels are divided into three categories: the 'lean', the 'middle' and the 'rich' grade. The lean grade has molybdenum content below 1.0 wt% and a nickel content below 2.5 wt%, the rich grade has a molybdenum content above 2.0 wt% and a nickel content above 4.5 wt%. The composition of the middle grade is in between that of the lean grade and rich grade.
Mainly due to the lower content of nickel, WMSS grades are less expensive than other corrosion resistant alloys, while their mechanical properties are superior. From a corrosion point of view, the WMSS grades are, within certain temperature and pH limits, applicable for hydrocarbon transport in sweet or mildly sour service.
Since 1996, WMSS has been used extensively as line pipe material. However, a number of cases failures occurred, which were caused by two types of unexpected cracking phenomena: an internal and an external cracking mechanism.
A few onshore, lean grade WMSS flowlines and a lean grade pipeline failed due to internal cracking at the HAZ of girth welds. Leaks occurred within one year of operation, although the operating conditions were rather mild: sweet wet gas service, temperatures of 65 - 80 °C and a partial pressure CO2 up to 1 bar. In two cases the chloride concentra