- High Temperature Material Selection for Power Industries
Our experts in high temperature corrosion, creep resistance and coating, conducted a project to address the problems of environmental degradation in aggressive environments at high temperature to improve significantly life expectation. Materials for high temperature application have be to carefully selected and should combine good mechanical properties, creep resistance and corrosion resistance.
We undertook a study of the degradation mechanism of a 9%Cr steel in biomass environment together with various coating systems designed to improve the performance and life expectation of the material.
Laboratory testing was carried out to better understand the corrosion mechanism, with the samples being covered with a mixture of chloride and sulphate containing salts and exposed to a simulated biomass environment at 550°C for 1000 hours. The uncoated test specimen exhibited a thick corrosion layer, while no significant degradation was found on the coated test specimen (Figure 1).
- Full-Scale Sour Fatigue Testing Machine for Riser Girth Welds
TWI designed and built a unique custom rig capable of testing full-scale pipe welds in an internal sour environment, to support our Industrial Members in the oil and gas sector. Steel catenary risers are commonly used for deep water oil and gas developments, where fatigue performance is a critical factor in their overall design. Resonance fatigue testing of full-scale girth welds has become standard industry practice to demonstrate adequate performance. However, these tests alone do not account for aggressive service environments such as sour production fluids. In these instances, qualification testing is usually a two-stage process involving full-scale resonance fatigue testing to demonstrate the required performance in air, and small-scale (strip) fatigue testing (in air and in the sour environment) to determine a fatigue life reduction factor that is then applied to the base design curve. This approach accounts for geometry effects (i.e. the difference between strip and full-scale testing) and environmental effects individually, and has been adopted on many projects. However, the validity of the approach had not been demonstrated, so TWI launched a JIP with the aim of validating the approach and removing excessive conservatism, at the same time delivering cost savings and greater design flexibility. To complete this work, we designed and manufactured a rotating bending fatigue test machine capable of testing full-scale pipe welds with an internal sour environment (Figures 2 and 3).
The machine incorporated several novel features:
• Orthogonal pairs of hydraulic actuators to permit in-plane bending, rotating bending, or anything between the two, up to +/-250kNm bending moment
• Large diaphragms support the pipe at its ends, permitting angular rotation while maintaining axial alignment
• A unique internal jack/column system, allowing up to 2MN of axial preload to be applied to the pipe
• An internal annular sour cell permitting circulation of a controlled environment around the weld root region without compromising the internal column system
- Structural Integrity Assessment of Well Conductors
With many oil wells being used beyond their original design life, it is vital to ensure the structural integrity of critical components and prevent unexpected failures. In offshore wells, the outermost well casing – the conductor – protects the surface casing from the aggressive marine environment throughout its life. Conductors are subject to aging-related issues including excessive wall loss and cracking due to corrosion and many are already operating beyond their design life. Due to the high cost of replacement and severe consequences of failure, there is a growing need to closely monitor the integrity of these components throughout their lifecycle to prevent them from leaking, buckling or collapsing.
Our experts delivered a comprehensive structural integrity assessment for a major offshore oil and gas operator, providing peace of mind that around 100 oil well conductors were safe to continue service. As well as carrying out the assessment according to a custom fitness-for-service (FFS) methodology, TWI also provided an inspection strategy for the future, to ensure the most efficient use of resources and reduced inspection costs going forward.
TWI worked in consultation with the operator to determine the risk (Probability of Failure) target, with the remaining time to that target considered as the risk-based remaining life (RL) (as shown in Figure 4). This determined the point at which an inspection is recommended to be carried out to better quantify the damage state. We provided the results in terms of risk, as the product of probability and consequence of failure. This provided the company with inspection recommendations based on the strength response of the corroded conductors. These mitigation actions will determine the acceptability of the conductor to remain in service within the acceptable risk. Through this comprehensive approach to structural integrity assessment, the operator’s cost of inspection was substantially reduced.
- Solving Technical Challenges to Test in De-aerated Seawater
TWI was asked by several Industrial Members to perform mechanical tests in de-aerated brine solutions under Cathodic Protection (CP), i.e. in a 3.5% NaCl solution, at 4°C and applying a potential of -1100mV vs. Ag/AgCl. This arrangement is considered to provide conditions that are representative of those experienced by some subsea components and systems under CP. In impressed current CP systems (ICCP), gas evolution through electrolysis of brine can lead to formation of chemically aggressive species (in particular chlorine) that may lead to degradation of structural steels. The severity of the development of diffusible hydrogen in the steel can also be different in aerated and de-aerated conditions due to different cathodic reactions, and therefore testing in representative service conditions is important to generate relevant fracture and fatigue data for assessing the integrity of subsea components.
The challenge is in developing a representative de-aerated seawater test condition in the laboratory that maintains a stable pH for the full duration of environmental tests (often several weeks), while minimising the production of toxic and corrosive chlorine and by-products thereof. In service, any chlorine gas evolved is diluted within the large volume of the sea, but in the laboratory the smaller scale can mean the accumulation of chlorine to a more significant degree. The need to completely exclude dissolved oxygen from the solution also poses practical challenges. Risk of ingress of air can be mitigated by creating a fully sealed test environment, but additional measures are also required as oxygen can be also be generated in-situ during electrolysis.
Despite the technical challenges raised during these tests, TWI successfully developed new capability and expertise in fully controlled environmental testing in de-aerated seawater under ICCP and galvanic anode CP for tensile, fracture and fatigue tests (Figures 5 and 6).
- Industrial Coatings for Mechanically Aggressive Uses
Low surface energy (or non-stick) coatings are currently used in a range of sectors including aerospace, power generation and land transportation. Low energy coatings are primarily required in applications where it is necessary to form a temporary mating of surfaces, such as mould-releases, or seals. However, they are also successfully used in applications where surface contamination will reduce performance, such as heat exchangers, porous construction materials, display devices and textiles.
The large number of products available makes it complex to assess the different types and select a technically appropriate coating within a reasonable time-frame. With no guidance on how to choose between different coatings, to suit the needs of a particular application, there is a need for an assessment methodology that allows low surface energy coatings to be quantitatively compared.
We launched a project to develop a methodology and its application for the assessment of a range of low surface energy commercial coating chemistries on a commonly used substrate to establish baseline performance in mechanically aggressive environments.
- Integrity in Aggressive Environments
Pipelines and risers for oil production are increasingly required to carry sour fluids, i.e. those containing significant levels of H2S. Depending on the precise chemistry of the environment, there may be a significant effect on the fatigue and stress corrosion resistance of girth welds in the pipe and risers. These effects must be taken into account in the design stage and when establishing flaw acceptance criteria for welding quality standards based on engineering critical assessment (ECA). For ECA, reliable fatigue crack growth rate (FCGR) data are required and, generally, the data are obtained using test specimens where the crack tip is exposed to an aqueous solution saturated with H2S gas. There are, however, operational conditions where nominally dry H2S gas is exported and this environment will be different and potentially less onerous than that for bulk aqueous sour systems. There is limited data in the open literature showing the effect of low moisture contents in H2S gas on FCGRs in pipeline steel, which led us to conduct a 2022 project to investigate the effect of different moisture contents in a 7% H2S/bal.N2 gas mixture on FCGR behaviour of API X65 carbon manganese pipeline steel. A non-contact moisture analyser (NCMA) was used to measure both H2O and H2S content during the FCGR tests. The study also compared the FCGR data in the dry gas mixtures with data obtained in air and in an aqueous sour environment (Figure 7).
- Fracture Toughness in Aggressive Environments: Effect of Crack Monitoring Techniques on Test Results
This project was created to address the industry requirement to generate accurate fracture toughness data in aggressive environments. Fracture toughness is the resistance of materials to a crack propagating under load, potentially leading to catastrophic failure. In aggressive environments, the fracture toughness of metals can be significantly reduced. Quantifying this reduction in fracture toughness is very important to industries that operate structures in aggressive environments. For instance, CMn based materials that are subjected to seawater are prone to corrosion, unless some form of protection is implemented. Offshore oil and gas pipelines and related structures can be economically and consistently protected from corrosion by applying cathodic protection (CP). Using active CP systems typically involves applying a small direct electrical charge to the material. However, one of the main drawbacks of this method is that the fracture toughness of the material deteriorates. The main mechanism of such deterioration is the diffusion of hydrogen atoms from the surrounding seawater towards the main body of the CMn material. The metal thus becomes susceptible to hydrogen embrittlement.
Understanding the gravity of hydrogen embrittlement in the reduction of fracture toughness is typically done by extracting small specimens from the material and testing under laboratory conditions that simulate the real-life environmental conditions as accurately as possible. During those tests, either tensile or bending forces are applied to a specimen that has a pre-manufactured sharp crack. The crack is extended and this crack growth is monitored in relation to the applied loads. To monitor the crack mouth opening and the extension of the crack under load, while the whole specimen is submerged in the aggressive environment, clip gauges can be used. Clip gauges are displacement transducers attached to the crack mouth. They convert the physical displacement of the crack mouth sides into electrical signal, which is recorded by a data logger and used for the crack related calculations. The use of clip gauges is limited in most aggressive environments, because it can be a great challenge to design them to survive the environmental conditions. For that reason, the use of direct current potential drop (DCPD) has been considered, as an alternative method. Using DCPD involves applying a direct current through the body of the specimen and measuring the voltage at its ends. When there is crack propagation in the specimen, the cross sectional area is reduced, increasing the resistance of the material, and causing a change in the current that passes through. Using DCPD for crack measurement can potentially overcome the challenges of environmental impact on the measuring device of fracture toughness testing. Clip gauges and DCPD were used as crack measuring methods in this project (Figures 8, 9 and 10).
These projects are just some examples of the aggressive environment testing work undertaken by our experts at TWI over the years, to find out more about our work in this area, please see here:
https://www.twi-global.com/who-we-are/who-we-work-with/industry-sectors/oil-and-gas/testing-in-aggressive-environments