Fit for HPHT

Stuart Bond

June 1, 2015

Exova Group’s Stuart Bond shows how the company is working to develop testing technologies to ensure SURF components are fit for HPHT and H2S service.

Exova’s Dudley, UK, autoclave facility. Images from Exova

The behavior of materials in the presence of hydrogen sulphide (H2S) for sour service has been studied for nearly 50 years using standard tests, which can adequately predict resistance to degradation via testing in laboratory-standard conditions.

Today, technology exists to perform tests in simulated service conditions. Such tests can ensure safe materials selection while avoiding excessive conservatism in testing that could potentially eliminate good candidate materials.

During the past two decades, deepwater and ultra deepwater developments have driven the need to assess the corrosion-fatigue performance of steel catenary risers (SCRs) in mild sour conditions as well as the impact of the external seawater environment with cathodic protection. More recently, the corrosion-fatigue behavior in sweet produced fluids and the effects of lateral buckling have been addressed.

However, in these cases, the vast majority of endurance testing has been performed under ambient pressure and has been limited to maximum test temperatures of around 80°C. Some endurance testing of wires for flexibles, plus fatigue crack propagation and frequency scanning, has been undertaken at elevated temperature and pressure by Exova.

Now industry professionals need to consider how clad systems can be tested using elevated temperature and pressure. In addition, high-pressure, high-temperature (HPHT) developments could further drive corrosion-fatigue testing – even for components such as downhole tubulars and wellhead equipment.

Proactive fitness for purpose (FFP)

Exova’s Houston slow strain rate testing facility.

During the initial design phase, materials selection for subsea umbilicals, risers and flowlines (SURF) components can be predicated upon knowledge of suitability from field conditions or, in borderline cases, a desire to test materials to optimize selection. Testing is required to qualify materials and weldments for proactive fitness for purpose (FFP) for new developments, which extend the material’s application to higher pressures and temperatures. Sometimes tests are undertaken early, using non-project specific material to avoid later difficulties.

Aspects to consider include:

For existing assets, FFP must be considered to have confidence in continued integrity if a change in service or life-extension is required.

Materials sampling

A single edge notch tension in a test chamber.

In most test methods, it is necessary to sample materials to produce test specimens. For tubulars, line pipe and girth welds, the range of test methods available to assess resistance to degradation in sour service is quite wide (listed in ISO15156/NACE MR0175). Therefore, both specifiers and users of the standards must understand the ramifications of the test technique. Test techniques include reproducibility, ease of testing and material sampling, retaining the as-received surface condition and microstructure, sampling from within the wall of the product, relaxation of residual stress, relevance to service, and more. These factors influence the degree of confidence in the results and the extrapolation to consideration of FFP for the intended service duty.

Factors to consider include:

Uniaxial tensile specimens must be selected from within the body of the material (to allow the machining of threads) and are fully machined. Therefore, these cannot be used to sample the surface microstructure and condition. However, this is a reproducible geometry that provides confidence that there will be little variation between specimens influencing test performance.

The extraction of material from weldments results in a relaxation of the residual stresses, which can contribute to environmentally-assisted cracking. Due to the thermal cycle (development of the heat affected zone, HAZ), dilution of the weld metal in the root and presence of heat tint oxide, the microstructure in weldments differs from the parent material. Thus the usual recommendation is root-intact 4-point bend testing.

It is also necessary to ensure that all appropriate damage modes are assessed in materials selection, including testing when required. In the latter case, ensure the test method is suitable, e.g. stress-oriented hydrogen-induced cracking (SOHIC) for which Exova has developed a test method (as there is no standard technique, although full ring testing can induce this damage in susceptible material). Exova plans to conduct a joint-industry project for a round-robin assessment for potential adoption of the test method as a standard.

Techniques that do not require an extraction of specimens can offer potential advantages, such as:

Testing of clad components requires the isolation or removal of the carbon steel. In long-term testing for corrosion-fatigue endurance studies in particular, isolation can be difficult to guarantee via coatings, especially at elevated temperature, but the substrate must be retained.

Standards

A full ring test with axial loading.

It is imperative that the industry has appropriate standards to ensure commonality of materials, processes and testing protocols. However, it is essential that users and specifiers understand the limitations of techniques (e.g., impact of sampling or ability to reproduce the service conditions), particularly for elevated temperature and pressure. This can be quite difficult and needs discussion between the client, engineering contractor and testing organization to ensure agreement in advance.

Additionally, as the offshore sector places increasing demands on materials to handle more aggressive conditions, test techniques need to be reviewed, modified or developed to allow laboratory testing to provide confidence in materials performance and qualification.

Exova supports international standards development and maintenance via leadership and representation on committees such as NACE MR0175/ISO15156, NACE TM0177 and TM0284; support of the European Federation of Corrosion Working Party 13 and the two sour service guidance documents (EFC16 & EF17).

Testing

Originally, test methods for sour service such as NACE TM0177 (SSC) and NACE TM0284 (HIC) were developed to provide QA/QC methods using laboratory test environments, which were formed to aggressively identify materials susceptible to damage. Now, the industry seeks to reduce the excessive conservatism this approach imposes and the assessment of corrosion resistant alloys has increased. Therefore, more application-specific testing has been undertaken to simulate operating conditions.

Corrosion resistant alloys require testing at elevated temperature and pressure using autoclaves and 4-point bend (4pb) specimens in constant deflection (particularly for weldments). When ranking tests are required to compare materials, slow strain rate testing (SSRT) (or ripple strain rate testing (RSRT) is used.

Similar considerations apply to the fracture toughness testing of materials, such as carbon steels in which the damage mode is related to the presence of absorbed hydrogen interacting with the plastic zone of the crack tip. If the material is charged under cathodic polarization, the hydrogen concentration in the material is considerably less than that induced from sour service.

If the material is charged and then tested in air – after it has been, for example, stored in liquid nitrogen to reduce losses from hydrogen diffusion – the rate of loading, temperature of testing, and the loss of hydrogen from diffusion must be considered. Such techniques can be suitable to assess the impact of an embedded flaw in hydrogen-charged material. However, these techniques do not consider the behavior of a surface-breaking flaw in the production environment, where hydrogen is generated near the crack tip due to the corrosion of the freshly exposed metal resulting from the crack extension (to derive critical stress-intensity factor). Loading modes in fracture-mechanics testing must also consider the replication of in-service conditions; single-edge notch tension (SENT) geometry is suitable for installation assessment but single-edge notched bend (SENB) geometry might be better suited to assess fracture response in operation.

New test methods from Exova

Exova has developed a proprietary technique for full ring tests with axial loading that can impart the desired stress for evaluation, even as high as 90% actual yield stress (AYS) of the parent material for conservative assessment. The technique can be used with loadings suitable for design or operating conditions.

The axial loading technique is currently being assessed for modification to deliver a new proprietary approach to corrosion-fatigue endurance testing, allowing testing of carbon steels in sour conditions above ambient pressure, and corrosion resistant allows (including clad products) at elevated temperature and pressure. Unlike the traditional segment (strip) testing for corrosion-fatigue testing in production environments, which might not sample the most susceptible region in the joint, this method will test the entire weldment (including start-stop location) under very realistic loading.

Exova believes that this new concept will allow industry to undertake the testing necessary to support research in HPHT materials performance without compromise in test environments (up to about 200°C) or in sampling of material (SURF and wellhead components including clad items). Feedback from the sector worldwide has been positive in terms of its potential to help them meet challenges now and in the future.

Stuart Bond is group corrosion business development manager, of Exova, global testing, calibration and advisory services provider, Exova. Bond has more than 27 years of experience in consultancy and industrial applied research, with a particular focus on the oil and gas sector. Prior to joining Exova, he managed TWI’s Materials Performance and Ferrous Alloys Section. Before joining TWI, he was a corrosion engineer at British Non-Ferrous Metals Technology Centre. Bond is a professional member of the Institute of Corrosion and recently took over the post of vice chairman of NACE STG32, a two-year post prior to two years as chairman.