Technical Insight: Sour Testing

Frequently encountered in the oil and gas industry, sour service environments are those where there is enough hydrogen sulphide (H2S) present to be corrosive and cause cracking and potential failure in metallic materials.

The corrosive, flammable and toxic nature of sour environments means that equipment used for these conditions needs to be made from high-strength, sour service resistant materials capable of withstanding the effects of sulphide stress cracking (SSC) and other related cracking mechanisms.

TWI provides full scale testing of pipes, girth welds, connectors and other components in aggressive environments associated with sour service. The testing services we offer include both standard and bespoke methods, allowing us to test for a range of failure mechanisms to meet the needs of our Industrial Members.

Over the decades we have conducted a range of projects related to or involving sour service environments for the benefit of our Industrial Members and wider industry alike.

Core Research Programme (CRP)

Through the core research programme (CRP), TWI conducts projects to address challenges faced by our Industrial Members, advancing technologies and processes, and finding new solutions for industry problems. These projects are jointly funded by our Industrial Members, with the outcomes made available to all our Members.

- Behaviour of 0.5%Cr Steel in Sour Environments

This 1997 project addressed alloys such as 0.5%Cr steel that were relatively new at the time to assess hardness limits such as those stipulated in the NACE standard MR0175-96, which limited the maximum hardness of carbon and carbon-manganese steels to 22HRC (approximately equivalent to 250HV). To address this gap in the knowledge, our experts determined if the NACE hardness maximum for avoidance of sulphide stress corrosion cracking was applicable to weldments in 0.5%Cr steel.

- PWHT and Critical Hardness of Sour Service Steels

This CRP project was created in 2005 to investigate the difference between post-weld heat treated (PWHT) and as-welded weldments to assess sulphide stress cracking performance. At the time, the ISO 15156 limit for heat affected zone (HAZ) hardness was 250HV. Our experts worked to determine whether this limit could be relaxed for weldments that have undergone post-weld heat treatment that created different microstructures and levels of residual stress to as-welded components of similar hardness.

- Sour Produced Fluid Corrosion Fatigue of C-Mn Steel HAZs

Steel Catenary Risers (SCRs) used within the oil and gas industry for deep water developments are subject to both fatigue loading (from waves and tides) and corrosive environments (internal and external). Due to the financial benefit of using C-Mn steels for SCRs it was necessary to discover their limitations, with a focus on the heat affected zone (HAZ), as typically HAZ microstructures are more susceptible to cracking during exposure to sour environments, especially if they have high hardness. This 2007 project sought to establish whether corrosion fatigue in C-Mn steel in sour produced fluids displays a marked microstructural dependence, in particular for different levels of hardness of simulated HAZ microstructures. This included investigations into the effect of cyclic load frequency on the interaction between microstructure, environment and fatigue behaviour for simulated HAZ microstructures in C-Mn steel.

- Crack Depth Effects on C-Mn Pipe Steel in a Sour Environment

This 2009 project continued our investigation into sulphide stress corrosion cracking (SSCC) in carbon manganese (C-Mn) steel as used for offshore pipelines and risers. This work aimed to provide guidance on the assessment of shallow crack-like flaws in a pipeline steel operating in a sour environment.

- Fatigue Crack Growth in C-Mn Pipe Steel in Sour Environment

Also in 2009, this CRP project was created to use shallow crack test data to quantify the fatigue crack growth behaviour of C-Mn pipeline steel in a sour environment. TWI’s team sought to investigate the influence of loading conditions, crack depth, specimen geometry, pre-soak and coating configuration on the fatigue crack growth behaviour. We also evaluated test methods for generating FCGR data in a sour environment to ensure that experimental test data is appropriately conservative.

- Shallow Fatigue Crack Growth Behaviour in Sour Pipelines

This project continued our assessment of steel catenary risers (SCRs) used in deepwater oil and gas developments to transfer production fluids from the seabed to surface facilities. At the time of this project, in 2010, there was relatively little published fatigue crack growth rate data available for pipeline steels in sour environments, particularly at low applied stress intensity factor range (ΔK) values. Research had reported a diminished influence of the sour environment at these low ΔK values at ambient temperatures (20-30°C), although shallow cracks had been shown to grow up to an order of magnitude faster than deep cracks in a sour environment at the same (low) value of ΔK. This project sought to assess shallow crack test data to quantify the near-threshold low applied stress intensity factor range (ΔK) fatigue crack growth behaviour of a C-Mn pipeline steel exposed to a sour environment. Results would of FCGR tests were compared to fatigue endurance data from sour environments and the possible non-conservatism of predicting the behaviour of shallow flaws using test data derived under conditions of decreasing ΔK was investigated. If carbon steel could provide adequate performance under (mildly) sour conditions, then the use of more expensive corrosion resistant alloys (CRAs) and clad materials might be avoided.

- Shallow Fatigue Crack Growth in Sour Pipeline Girth Welds

This 2011 CRP project built on previous research undertaken at TWI with the aim of presenting the results of fatigue crack growth rate (FCGR) tests carried out on C-Mn steel pipeline girth welds and simulated heat affected zone (HAZ) microstructures exposed to a sour environment. This would help quantify the fatigue crack growth behaviour of C-Mn pipeline steel girth welds exposed to a sour environment.

- Evaluating Fracture Behaviour of Pipeline Steel in a Sour Environment at Ambient Temperature

At the time of this 2020 CRP project there was no adequate guidance available in existing standards on sour fracture toughness testing methods for engineering critical analysis (ECA). A fracture mechanics-based approach requires materials properties (tensile properties and fracture toughness) to be precisely defined and the fracture toughness data for the given material be generated under conditions reflecting as closely as possible the service environment. To address this gap, TWI produced a report as the outcome of experimental and analytical work associated with an ambient temperature test programme, including a brief overview of existing guidance on sour fracture toughness testing in current standards and recommended practices as well as results from testing API 5L X65 pipeline steel in a specific sour environment at ambient temperature and pressure. In addition, numerical modelling was used to elaborate the effect of loading (applied during an unloading compliance fracture toughness test) on crack tip hydrogen concentration to understand its link with experimental observations. The report also incorporated the results from assessments carried out for some pre-defined service loading for a hypothetical pipeline system to evaluate the sensitivity of flaw tolerance to the various approaches used to define fracture resistance in a sour environment (Figure 1).

Joint Industry Projects

Our joint industry projects (JIPs) allow interested parties to come together as project sponsors, pooling resources for greater research outcomes, while also being given exclusive access to the results and the opportunity to guide the direction of the projects themselves. These projects typically focus on challenges faced by specific industry sectors, providing industry-relevant information, guidance and solutions:

- Sour Corrosion Fatigue Behaviour of Girth Welded Clad Pipe

This 2006 project investigated the fatigue performance of clad risers in sour environments, determining an endurance knock-down factor under simulated service conditions.

- Factors Influencing the Sour Corrosion Fatigue Behaviour of C-Mn Steel Catenary Riser Welds

Also in 2006, this JIP investigated the effect of H2S concentration on the fatigue of steel risers at low cyclic stresses, with the tests being representative of actual riser response.

- Raising the Acceptance Level for Ni in C-Mn Steel Welds for Sour Service

This 2006 JIP investigated Ni levels in weld metal (1% to 3.5%) for sour service, including a summary of existing/historical data and standards as well as the generation of experimental data in sour and mild sour service.

- Sulphide Stress Cracking and Preferential Weld Corrosion Performance of Welds between Modified A707 and C-Mn SCR Pipe

Also in 2006, this project involved sulphide stress crack (SSC) testing in a mild sour condition and preferential weldment corrosion (PWC) tests in a sweet condition on A707 Grades 1 and 3 welded to C-Mn steel specimens. This JIP reduced uncertainty around the suitability of tapered stress joints (TSJs) in steel catenary risers (SCRs) for mildly sour service. In addition, the project addressed concerns that preferential weldment corrosion could occur in service due to the dissimilar materials in the welded joint.

- Definition of Sour Service Limits for Welded Type 316l and Cost Effective CRAs

Completed in 2008, this project defined accurate sour service limits for 316L stainless steel. The work included a summary of existing knowledge of stress corrosion cracking (SCC) limits and compared welded homogenous material with welded metallurgically clad material.

- Fracture Mechanics Based Weld Flaw Acceptance Criteria for C-Mn Steel Pipelines in Sour Service (Phase 2) - Sour ECA

This project, which included the use of KISCC and UT flaw sizing, saw the development of a small-scale testing method and an increased understanding of / ability to predict full-scale testing behaviour.

- Safe Specification and Operation of Plastically Strained C-Mn Steel Pipelines for Sour Service (Phase 2)

This 2008 project determined safe engineering limits through an understanding of the effects of materials, pre-strain and environment as well the strains on reeling. This work also involved the development of a small-scale test for weld procedure qualifications.

- Fatigue Design Guidance for Risers in Sour Service via Full Scale Testing

This 2010 JIP project applied resonance fatigue testing of full-scale girth welds to sour service to better understand the sour performance of girth welded riser pipe. TWI developed a method for performing these tests, with the results being compared to full-scale resonance fatigue tests, strip tests in air, and strip tests in a sour environment. In addition, we undertook metallurgical examination, residual stress measurements, fatigue crack growth rate measurements and fracture mechanics assessments. This allowed us to produce validated guidance for the design of risers based on a full-scale sour service riser fatigue design curve.

- Optimisation of Weld Overlay for Productivity and Corrosion Resistance

Announced in 2010, this project was created to quantify the capabilities of a new generation of arc welding process variants for weld overlaying. This involved identifying the effect of varying dilution on corrosion and corrosion fatigue performance for a range of environments, including sever sour service. This project assessed weld overlays produced using both commonly used and more advanced techniques as well as the local variations in dilution on corrosion resistance for a range of corrosion resistant alloys (CRAs) including 316L and 904L stainless steel, alloy 625, alloy 825, and Monel alloys. This included an investigation into dilution and corrosion and corrosion fatigue resistance in different mild and aggressive environments, including severe sour service. TWI’s experts also investigated the suitability of some of the current NDT methods for ensuring the integrity and uniformity of the weld overlay.

- Fracture Toughness Testing of Steels Subject to Sour Service

The results of this 2012 research project aided the quality and safety within fracture toughness testing and engineering critical assessment (ECA) of steels in sour service, helping to fill knowledge gaps for industry and add to guidance in existing standards. TWI determined

the fracture toughness of steel components relevant for assessing flaws exposed directly or indirectly to a hydrogen source and tested whether data (normally obtained from conventional constant-load tests) in sour environments can be inferred from R curves obtained from rising load tests conducted under low strain rates in identical sour environments.

- Service Performance and Life Prediction of Polymer Lined Steel Pipe - "Polymer Lined Pipe and OCTG"

TWI determined the degree of corrosion experienced by a carbon surface that had been protected by an extruded polymer or built composite liner when exposed to a sour fluid mixture containing water and simulated hydrocarbon. The protective polymer liner material was originally chosen from the polyethylene, polyamide and polyvinylidene fluoride family before the addition of higher temperature polymeric and composite liners to the testing programme.

- Quantifying Sour Service Limits of Laser-Powder Bed Fusion Alloy 718 in ISO 15156/NACE MR0175 & API 6ACRA

By 2020, additive manufacturing technologies had advanced to the point where it was possible to routinely manufacture high-quality complex part from a range of metallic materials. However, exposure of AM parts to corrosive environments, such as sour service encountered in the oil and gas industry, were not covered by standards including ISO 15156/NACE MR0175 and API 6ACRA, meaning that AM parts were prevented from entering service. To support the acceptance of laser powder-bed fusion (L-PBF) deposited Alloy 718 material by ISO 15156/NACE MR0175 and API 6ACRA and evaluate the influence of AM material and process variables on resistance to environmental cracking, TWI applied our industry-leading expertise and experience in sour corrosion testing and AM to generate test data.

Dedicated Industrial Member Support and Other Projects

Much of the work undertaken at TWI is conducted confidentially on behalf of individual Industrial Member companies. However, there are some examples of these types of project work that we are able to share with you, as follows:

- Failure along Spiral Welds in a Gas Pipeline

The Kuwait Oil Corporation (KOC) called TWI for assistance to find the root cause of a failure in a critical high-pressure wet sour service gas pipeline in West Kuwait. KOC had discovered severe internal corrosion in the pipeline during repairs after it had suffered several leaks, including one due to a crack along a spiral weld. The pipeline was decommissioned and TWI was called to conduct a failure investigation, including extensive examination and testing of the failed pipe.

Our experts noted that the pipe material had been correctly selected for wet sour service and the exterior of the pipe was in a good condition, indicating effective coating and cathodic protection during service. However, visual inspection of pipe samples showed severe internal pitting at the 4-8 o’clock positions at the bottom of the pipe, with a spiral weld failure in the same area (Figures 2-3). Phased array ultrasonic testing then confirmed that the crack was longer on the inside than on the outside, revealing that the cracking originated internally. The presence of water, CO2 and H2S gases were deemed to have caused the internal corrosion, suggesting that the dehydration plant, which was the main source of internal corrosion mitigation in this pipeline, was not functioning adequately. In addition, our experts found that a small weld misalignment had resulted in slight roof-topping of the spiral weld, increasing in-service stresses in these regions and higher local tensile stress when under pressure. Added to this were a small amount of cyclic loading due to regular pipeline shutdowns. Although none of these factors were particularly severe on their own, when combined, the effect could be significant. It was determined that cracking was probably due to increased localised stress at the spiral weld, coupled with corrosion due to the severe sour service environment. As the local stress increased at the spiral weld, the cracking occurred. Following the results of this investigation, KOC replaced the pipeline with a seamless sour service pipe rated to the current level of H2S and water cut.

- Fitness-for-Service Project on Middle Eastern Pipeline

The Kuwait Oil Company (KOC) contacted TWI again when internal blistering was discovered in one of their 20" diameter cross-country pipelines. KOC despatched a 200mm long specimen of the longitudinally welded pipe to TWI for analysis along with an undamaged 200mm section from the same pipeline for comparison. The aim was to undertake a fitness-for-service assessment based on the damage and provide advice for future pipeline operation and integrity management.

TWI used several in-house disciplines for this work, including a range of destructive and non-destructive testing methods to confirm material properties, identify damage mechanisms and characterise the extent of the damage (Figures 4-5). We also reviewed the pipeline operating history and available in-line inspection data.

As a result of this investigation, we were able to recommend revalidation methodology for the pipeline based on a hydrostatic proof test as described in ASME B31.8S. The hydrostatic test was successfully completed at 1.7 times the required operating pressure before the pipeline was recommissioned. This delivered significant savings on inspection and repair works to KOC, while demonstrating the safety and integrity of the pipeline.

- Hardness Limits Relaxation in the Pipeline

In work supported by the Pipeline Research Committee of the American Gas Association (AGA), TWI was able to confirm that hardness limits for pipe welds could be relaxed without an increased risk of sulphide stress cracking (SSC), paving the way for wider use of high-productivity welding systems.

Although it was known that SSC susceptibility increases with hardness and strength, and that the rapid cooling experienced at welded joints produces hardened microstructures in the heat affected zone (HAZ), the maximum hardness limit stipulated in NACE MR 01 75 was, in principle, too conservative to be applied to all regions of pipeline since low hardnesses were achievable through reheating of root pass regions by subsequent runs. These lower hardness root pass regions could be created on the inside of the pipes, where the corrosive environment tended to be.

To investigate this theory and provide quantitative data to support the view, an experimental programme was conducted at TWI to explore the degree to which the limit could be relaxed for capping pass weld regions, with the potential to greatly reduce the cost of pipeline fabrication. The testing included for both internal and external pipe weld beads to simulate weld root and weld cap regions (Figures 6-7), with the external beads being re-examined with the sour environment on the inside of the pipe. The results showed that an increase in the allowable NACE limit was acceptable, with a similar increase deemed acceptable for BS 4515 at material thicknesses of 9.5mm or more. Not only did this work open up a route to reduced problems and costs with qualifying weld procedures but also facilitated the application of high productivity, mechanised MIG pipe welding systems.

- Full-scale Sour Fatigue Testing Machine for Riser Girth Welds

To provide greater support for our Industrial Members operating in the oil and gas sector, TWI designed and built a unique custom rig capable of testing full-scale pipe welds with an internal sour environment (Figures 8-9). The use of this custom, rotating bending fatigue test machine was created to help eliminate excessive conservatism and provide significant cost savings and improved design flexibility, as well as greater confidence in likely material behaviour in service as a result of quantified material performance. In turn, the improved understanding of fatigue in sour environments led to enhanced safety by reducing the risk of corrosion fatigue-related failure.

- Ensuring the Structural Integrity of Deepwater Risers

TWI was called upon to perform a series of detailed engineering critical assessment (ECAs) for a set of deepwater steel catenary risers (Figure 10) prior to their installation in an oil and gas field. The ECAs were to define the maximum initial allowable flaw sizes, ensuring the integrity of the risers through a complex cycle of installation and operation, while also taking into account the possibility of fatigue loading, sour service and internal corrosion during the operational period. The structural integrity requirements of risers has increased as ever-deeper offshore oil and gas reserves are exploited. These risers are subjected to a variety of static and cyclic loads as well as environmental degradation and environmentally assisted cracking (particularly in the presence of sour fluids). The structural integrity of girth (circumferential) welds (Figure 11) is of particular significance, since they are subjected to cyclic (fatigue) loading and are typically uninspectable once installed.

- Fitness-for-Purpose of Duplex Stainless Steel Risers

Risers in sour service was also the subject of this project, which was related to an offshore floating production, storage and offloading (FPSO) vessel (Figure 12). Single Buoy Moorings (SBM) Monaco were commissioned to design and build the vessel, which was to be used in the Wanaea and Cossack fields off NW Australia, but unexpectedly high hardness values were discovered in the weld zones during pipework welding where duplex stainless steel had been used for the rigid arm and riser piping. Since these weld areas were giving hardness measurement values outside the design requirements, SBM sent examples of the welds to TWI for evaluation. We undertook simulated environment tests to prove that, aside from the high hardness values, the welds were perfectly normal. These tests included slow strain rate (SSR) tensile testing and four-point bent beam tests at elevated temperature and pressure in a hot, sour brine solution, with the H2S and CO2 contents used to simulate field conditions. The high hardness was determined to be related to the strain induced by welding and the welds were passed as fit for purpose, providing reassurance to SBM.

- Frequency Scanning Testing of Armour Wire

TWI Industrial Members, Exxon Mobil asked TWI to develop a testing programme to determine the effect of loading frequency on the fatigue crack growth rate (FCGR) of armour wire in a corrosive environment. To solve the challenge, our expert team developed a frequency scanning test. Single-edge notch bend specimens with a rectangular cross section of 4mm by 11mm were machined from armour wires before FCGR tests in a simulated sour environment were carried out under three-point bending using TWI’s specialist facilities (Figure 13). The environmental conditions were controlled to match those found in-service, allowing TWI to determine the effect of loading frequency on FCGR in the sour environment for the armour wire specimens (Figure 14). The work produced control data that allows for enhanced product design for offshore applications.

- New Test Assesses Cracking of Corrosion Resistant Alloys

Slow strain rate testing (SSRT) has been used for the assessment of environmentally-assisted cracking of corrosion resistant alloys (CRAs) in the oil and gas industry for environments containing chloride ions and hydrogen sulphide (H₂S). However, because this process was not only deemed too severe but also unrealistic in terms of applied stress, a modified SSRT method was developed involving cyclical loading below 0.2% proof stress in order to reduce the severity of the process. This new SSRT test method was further developed to create the ‘ripple load test’ (RLT).The RLT applies slow and repetitive tensile load cycles on a test specimen while exposing it to worst-case operating conditions to accelerate sulphide stress cracking or stress corrosion cracking in susceptible materials. This creates a relatively short test duration that is ideal for assessing the resistance of CRAs to environmentally-assisted cracking. As a result of this work, TWI modified SSRT rigs and developed RLT capabilities (Figures 15-17) that can be used by Industrial Members, including tests to assess stress corrosion cracking resistance on stainless steels in sour environments.

These are just some examples of the many sour testing-related projects undertaken at TWI over the decades. You can find out more about the sour testing support available at TWI, here:

https://www.twi-global.com/what-we-do/services-and-support/sour-testing