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TWI: Corrosion, welds and pipelines. (March 1999)

Stuart Bond

Published in 'Anti-Corrosion Methods & Materials', 1999, No.2, March/April

New information about TWI Corrosion Services Expertise and Failure Assessment

Updated July 2000.

TWI was established in 1946 as the BWRA, later The Welding Institute, and from its very outset was focused on the study of joining technology and its impact upon the performance and structural integrity of components. With a staff of some 450, TWI represents a unique multidisciplinary resource, which provides expertise to over 3000 companies worldwide. Welding can influence various factors in the performance of plant and equipment; naturally therefore, corrosion behaviour has been a major technology for both research and consultancy at TWI for more than five decades. This article reviews some of the issues and areas of current research at TWI appropriate to pipelines.

1. Introduction

For more than 50 years TWI has worked with industry to develop joining technologies and their subsequent application for materials in a wide range of environments. The operational requirements for pipelines and ancillary components in the energy & chemicals sector, has resulted in a large number of research programmes and provision of consultancy support to the industry. Research topics include issues such as preferential weld corrosion, stress corrosion cracking and the influence of cathodic protection upon hydrogen induced cracking resulting variously in;

  • defining the safe limits of operating environment,
  • reducing excessive conservatism to assist in economic materials and fabrication options,
  • developing weldability data for new materials and assessing performance for new environments etc.

TWI now deals with all engineering materials including composite materials (FRP or GRP) with adhesive bonding and plastics such as polyethylene, which can sometimes replace metallic components or be fitted as liners,. Additionally,the development of thermally sprayed coatings comprising corrosion resistant materials or ceramics for prevention of erosion has enabled various industries to improve the performance of plant and facilities. However, in many cases industry still needs to use metallic pipelines and ancillary equipment; TWI continues to provide a key resource to further improve knowledge leading to safe and economic application by industry.

2. Multidisciplinary approach

Naturally, the need to develop pipeline materials for new environments, or to confirm fitness-for-purpose for continued operation requires a multidisciplinary approach. To develop the necessary data in support of engineering critical assessments, TWI has extensive expertise and facilities to determine the influence of factors such as cathodic protection and H 2 S on stress corrosion cracking, fatigue and corrosion-fatigue.

Studies can range from investigation of new operating environments or defining more precisely safe operating envelopes, to assisting industry develop 'new' materials. For example, from 1972 duplex stainless steels have been subject to continuing investigation to improve weldability and enhance knowledge of stress corrosion mechanisms such as Cl - and H 2 S induced stress corrosion cracking. Presently studies are underway to provide industry with the necessary data to allow consideration of the new weldable 13Cr martensitic and supermartensitic steels as alternatives to duplex and superduplex stainless steels.

For new developments, TWI's H 2 S laboratory, which has operated for more than 25 years, is extensively equipped to handle large and small-scale specimens using ambient and elevated pressure / temperature apparatus for both static and dynamicloading of weldments, whilst full-ring specimens can also be examined. The operating field conditions can be reproduced to enable the client to confirm acceptability of the materials and weld procedures.

Conversely, existing facilities may be required to operate beyond their design life or under new conditions due to marginal field tie-in or souring of older wells. Again, TWI is able to provide full investigation of existing corrosion via appropriate NDT methods, augmented by laboratory studies if appropriate to demonstrate performance in the new environment. This information is incorporated in engineering critical assessments to provide the operator with the necessary information demonstrating that the pipeline is fit-for-purpose.

The presence of absorbed hydrogen in welded pipelines may be due to cathodic protection or sour (H 2 S) media. However, it can lead to rapid failure via stress corrosion cracking and is therefore a major concern in the definition of acceptable operating environments for pipelines and other associated equipment.

Much of TWI's work on welded materials for pipeline duty in the energy and chemicals sector, relates to the development of an understanding of the precise influence of factors such as; welding conditions, consumable and parent material composition, upon the metallurgy of the weldment and therefore its subsequent performance in the operating environment of: CO 2 , H 2 S, Cl -, under various temperatures, pressures and pH. It should be noted that TWI has developed equipment which is able to measure and control the pH in-situ at elevated pressure to ensure that data correspond to the true conditions rather than attempting to extrapolate from a reading at ambient pressure.

In addition, it is necessary to understand the fracture and fatigue performance of welded joints to ensure that design engineers are able to select an optimum solution without excessive conservatism and consequent increased cost.NDT is another technology necessary in the implementation of welded components and TWI is uniquely placed to deliver a multi-disciplined approach to industry's existing problems and also offer future alternative solutions. Much of TWI's work has been incorporated into International Standards and our staff continue to represent industry on 70 committees.

3. Present studies

A selection of some of the current joint industry funded research work at TWI is given below. It is possible to join the sponsor group of a project at any stage and therefore, organisations interested in possible participation should contact Stuart Bond. In addition to these joint industry projects, TWI undertakes a Core Research Programme comprising 50 projects with an annual expenditure of over £2.5million to develop underlying technology enhancing the capability of bringing solutions to industry. There are usually about 30 joint industry projects and 60 collaborative programmes in operation in a year. In addition, TWI undertakes some 600 confidential single client projects per annum addressing needs from R&D to consultancy and failure investigation.

3.1. Cathodic protection

Application of metallic components in immersed or buried conditions normally incorporates protection using cathodic protection (CP) in addition to coating and wrapping systems. However, CP can induce increased hydrogen absorption by the metal under conditions where the cathodic discharge reaction is hydrogen ion reduction. Absorbed hydrogen is in atomic form and is therefore able to diffuse through the material to sites of increased stress or to regions of increased hardness. Welding may cause regions of hard microstructure, which can be susceptible to cracking due to the presence of hydrogen even in the absence of H 2 S. Two joint industry projects addressing related issues are selected as examples of current work:

Effect of external hard zones on environmental cracking risk of C-Mn steel pipeline

Development of hardened structures at welds in carbon-manganese steels increases sensitivity to hydrogen induced stress corrosion cracking, and it is therefore common to specify maximum principal hardness levels to avoid service problems. This concern is most acute in sour H 2 S media experienced in the oil & gas industry, and, for such service, it may be necessary to restrict hardness to below 250HV 10 10 (equivalent to Rockwell 22C). The sour environment is normally within the pipe (although there are now new pipeline developments in regions of sour mud) and therefore a gradient in hydrogen concentration,and thus cracking risk, will exist through the pipe wall thickness.

Work at TWI has shown that relaxation of the maximum hardness is permissible at the outside surface and in consequence BS 4515 permits, for pipe in sour service, external hardness up to 275HV 10 and, 300HV 10 where the wall thickness is greater than 9.5mm. Even this is felt to be conservative but, without data upon the effect of external CP, industry is unable to benefit from improved safety or increased weldingproductivity. Cathodic protection may restrict hydrogen egress via diffusion to the outer surface and subsequent re-combination to molecular hydrogen, thus leading to a higher concentration of absorbed hydrogen at the external surfacethan was the case in previous studies.

The project is underway (completion in March 2000) and is defining the effect of external cathodic polarisation on the maximum permissible hardness levels at the outsides of welded joints in C-Mn steel pipe handling sour products.

Cathodic protection of ferritic-austenitic stainless steel

Ferritic-austenitic stainless steels have enjoyed considerable successful service under cathodic protection in subsea structures and pipelines. However, recent failures of subsea components due to a hydrogen-induced stress corrosion cracking mechanism have raised concern regarding the critical factors that can bring about such cracking. Whilst many may think of these materials as 'corrosion resistant alloys', CP has been applied due to the internal fluids being at high temperature, thereby leading to concern over external Cl - induced pitting corrosion.

This study will define quantitatively the material, stress and environmental parameters inducing cracking in duplex and superduplex stainless steels when cathodically polarised in seawater. Industry will gain benefit via improved reliability and confidence in duplex and superduplex stainless steels, and avoidance of significant cost resultant from a subsea failure. The project has just started and will run through to August 2000.

3.2. Effect of H 2S and acceptable limits

Safe hardness levels for C-Mn and low alloy steel welds operating in mildly sour environments

Whilst TWI has studied the behaviour of C-Mn and low alloy steels in H 2 S over many years, there continues to be demand from industry to address various aspects to facilitate use in more closely defined field conditions to avoid over-conservatism in design. A recent programme developed threshold hardness levels for weldments, which showed clear variation with H 2 S and CO 2 partial pressures. It was demonstrated that significant relaxation of hardness limits would be reasonable in many mildly sour environments up to 1psi H 2 S partial pressure.

Whilst the majority of work was at 25°C and 500psi total pressure, it was observed that at higher temperatures CO 2 caused pitting corrosion, but reduced risk of stress corrosion cracking; whereas, higher pressure apparently demonstrated that lower hardness limits were necessary to avoid cracking. Therefore this project is determining the effects of temperature and total system pressure on safe hardness limits for operation of C-Mn and low alloy steel equipment in mildly sour conditions.

Through a relaxation of hardness limits, more economic welding procedures may be adopted. Similarly, existing pipelines may be demonstrated as fit-for-purpose in mildly sour conditions even if the hardness levels are in excess of currently accepted limits for sour service, thereby saving the cost of laying a replacement pipeline.

Definition of H 2 S limits for ferritic-austenitic stainless steels in sour service

The duplex and superduplex stainless steels are increasingly being used for oil and gas duty, and whilst known to be susceptible to cracking in H 2 S, limiting H 2 S partial pressures have been defined for a number of grades e.g. NACE MR0175-97. However, these limits have been derived from tests of typically 720 hours duration and longer tests are needed which recognise the potentially adverse effects of: sustained high load and associated room temperature creep, and loss of corrosion resistance due to intermetallic precipitation.

This project is aimed at increasing confidence in H 2 S limits for long-term service for these materials. Again, reduced conservatism in design will lead to lower fabrication costs and the results from this programme will lead to enhanced reliability of thesematerials in service, potentially reducing the incidence of failure.

Effect of intermetallic phases on corrosion resistance of ferritic-austenitic stainless steel weldments

This project somewhat complements that above, insofar as duplex and superduplex stainless steels are susceptible to formation of intermetallic phases when heated to 700-1000°C. This can occur during welding and therefore, to avoid problems from loss of properties, it is necessary to minimise exposure to the damaging temperature range. This results in expense due to reduced productivity.

There is uncertainty whether intermetallics significantly impair corrosion resistance of these materials. Therefore, this project is currently investigating the influence on corrosion resistance in chlorinated seawater (at up to60°C), brine with CO 2 and some O 2 , and a sour environment with H 2 S and CO 2 .

3.3. New metallic materials and alternative products

Optimising the properties of weldments in weldable martensitic and supermartensitic stainless steels

A range of 'weldable' martensitic stainless steels has been developed with about 11-13%Cr and low carbon giving improved resistance to fabrication hydrogen cracking compared with traditional martensitic stainless steel grades. Some of the recent types have additional alloying additions (Mo and Cu) to give improved resistance to chloride ions, CO 2 and H 2 S, the term 'supermartensitic' being applied. These various grades have been marketed but industry requires further data to quantify both fabrication hydrogen cracking risk, and the resistance to corrosion and cracking in a range of media.

The principal properties of these weldable grades are their high strength and good corrosion resistance for predominantly sweet CO 2 -containing environments, in addition to substantial cost savings that can be realised over the competing duplex stainless steels. However, available information indicates that the weldable martensitic grades have considerably lower resistance to H 2 S and chloride containing environments than the latter class of material. Also, localised hardening and microstructural changes in the weld zone are likely to reduce the environmental resistance below that of the parent steel. It is therefore essential that the environmental limitations of welded structures be quantified to allow confident selection. Previous TWI work has assessed the corrosion behaviour for welds in a number of commercial of steels, but further data are required to define limitations in a wide range of service media.

The project has just started and is addressing: corrosion in CO 2 / chloride media; sulphide stress corrosion cracking; stress corrosion cracking in the absence of H 2 S and hydrogen induced cracking under cathodic protection. Resistance to fabrication hydrogen cracking and the effect of pipe reeling strain will be examined, as will HAZ phase transformations. The project will thereby identify service conditions under which the weldable martensitic stainless steels may be exploited and optimum welding procedures for their safe implementation selected. The project is scheduled to complete in the autumn of2000.

ERW/HFI welded line pipe for sour service applications

Electric resistance welded (ERW) / high frequency induction (HFI) line pipe has been successfully used for many years in a number of high integrity onshore and offshore applications. Its use is being limited, however, due to a lack of confidence arising from historical problems. In particular, there is reluctance on the part of many operators to use it for sour oil and gas. This is despite the very high quality which is now possible with modern ERW/HFI line pipe,and potentially very significant savings from its use.

This project is just starting and aims to address issues currently preventing the widespread use of modern ERW/HFI line pipe in both sweet and sour environments. The programme will cover pipe manufacturers' data, experience in fabrication and pipe laying, performance in service and cost benefit analyses. Depending upon the outcome, it is expected that a focused experimental test programme will be undertaken. The first phase of work will be complete at the end of 1999.

3.4. Corrosion fatigue

Corrosion fatigue crack growth in sour media

Whilst industry has spent many years, and continues, developing an understanding of the mechanisms of stress corrosion cracking in sour media for a range of materials, there are virtually no data on the potential for these cracking mechanisms to accelerate crack growth under cyclic or variable loading conditions. In many applications, design must account for fatigue loading and this includes pipelines, which may be subject to variation in load from operation,product flow or external influences such as water currents or vibration due to pumps and compressors.

Crack growth rate under fatigue conditions can be significantly increased by the presence of a corrosive environment and the threshold stress intensity for initiation of cracking can be reduced. Despite the increasing developments in situations where materials exposed to sour media may be subject also to fatigue loading, there is a paucity of data.

This project has just been launched and the objective is to generate quantitative information upon the corrosion fatigue crack growth rate for a range of metallic materials (ferritic steels and corrosion resistant alloys) in H 2 S environments. Parent material and welds will be addressed. The programme is of two years duration and at the time of preparation of this article TWI was discussing the details of the work scope with industry.The benefits will include improved knowledge of performance of welded pipeline, amongst other components, and therefore better design for future applications. This will assist in reducing the risk of unforeseen failure, the consequent cost of which subsea may be of the order £10million in downtime and remedial work in addition to environmental impact.

3.5. Related projects

Many other projects are either running or planned at TWI funded via the joint industry project route. Related topics include:

  • fatigue performance of girth welds made from one side;
  • interfacing NDT with engineering critical assessments;
  • development of a reliability assessment procedure for containment vessels subject to low temperatures;
  • technical and economic feasibility of using titanium alloys for risers.

4. NDT and corrosion assessment

The results of TWI's research within the Core Research Programme can lead to direct application in the field or modification of assessments of fitness-for-purpose, etc. Alternatively the data may generate sufficient information to give confidence that further development funded by industry will provide significant returns on their investment in joint industry or single client projects. An example of this is the development of the long-range ultrasonic tool,Teletest®, which can be combined with TWI's expertise in corrosion and engineering critical assessment (ECA). Following R&D in association with industry and a series of field trials, this new technology is about to be provided as a proven technique worldwide.

TWI staff frequently advise industry on the interpretation of NDT results, procedures for NDT application and devise special methods to address difficult cases. The development of Teletest® allows the rapid surveying of many metres of pipe without removing insulation and allows inspection under road-crossings, suspended pipe etc. It is capable of providing information on wall loss due to both internal and external corrosion. This allows selection of regions for closer examination e.g. manual UT to pinpoint wastage or automated UT for mapping or data for ECA.

Assessment may include general corrosion, wall loss due to erosion, localised attack such as pitting, or cracking problems such as that induced by hydrogen e.g. HIC or sulphide SCC types. Assessment, using fracture mechanics to BSPD 6493 (as developed by TWI and now being revised & upgraded to BS 7910), ASME B31G or, detailed assessment using the RSTRENG approach etc, may enable TWI to provide data to allow the continued operation of a pipeline, saving significant cost or deferring outage until a planned maintenance period.

TWI has established a wholly-owned subsidiary (Plant Integrity Ltd) to provide field services involving NDT (including the new Teletest® technology), whilst TWI continues to further develop techniques and undertakes the ECA.

5. Summary

TWI continues to provide industry with a very valuable resource to enable the application of new materials for pipelines and to ensure that these assets provide a maximum life in safe conditions even if the operating environment is subject to change. Corrosion has been, and continues, to play a significant role in the development of technology that enables industry to understand the influence of the environment upon both the parent material and weld regions of pipelines. TWI is proud of its history and current activities in providing answers to the problems of today and solutions for the future.

For further information please contact
Stuart Bond
Business Development - Corrosion & Materials
TWI, Cambridge

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