John Wintle is a consultant engineer for structural integrity and is a leader in the development of methods for the management of ageing plant at TWI. As chairman of the Pressure Systems Group he takes a keen interest in pressure equipment, and is a strong advocate of TWI's multi-disciplinary approach.
John Wintle examines how TWI is contributing towards ensuring safe reliable production from older offshore assets.
Around 90 offshore installations in the UK sector of the North Sea are now over 25 years old. Construction for the UK sector started in earnest during the seventies and continued during the following three decades. Figure 1shows the number of fixed installations placed on the UK continental shelf each year since 1966. Development of the Norwegian sector took place slightly later, but there are now over 20 installations older than 20 years. A few of these installations have been decommissioned, or are due to be decommissioned in the near future. However, there are a substantial number of installations where companies have indicated their intention to continue operation for the foreseeable future. The increased world price and demand in oil and gas has made the economics of extracting from the North Sea oil and gas fields more attractive.
Many production installations now have a continued requirement to process oil or gas, either from the original fields, or to serve as a base for neighbouring subsea completions. While some major operators have been withdrawing from the North Sea, moving the value of their assets to other parts of the world, their place has been taken by new operators keen to maximize economic production. The infrastructure is in place, and continued operation generates income and defers the costs of decommissioning.
Many structures and process plants were designed for a nominal life of about 20 to 25 years. This was both for investment appraisal purposes, and also because this formed the approximate limit of industry experience at the time of original construction. This period was therefore a reasonable basis for design assessment of fatigue and corrosion.
Over their life, the structure and topside, the process plant, the safety systems and other facilities comprising the installation have been subject to hostile operating conditions and the forces of nature. The actual life of the structure and equipment can therefore vary markedly from installation to installation depending not only on the quality of the design and fabrication, but also on the specific operation and service environment to which it has been exposed. Where it can be shown that the effects of ageing are sufficiently slow, or that design assumptions are conservative, or that ageing effects can be tolerated, there is clearly the potential for life extension beyond original design life.
Approaches to ensuring safety and technology for life management established during the many years of successful operation to date are being revised and improved to meet the challenges ahead in the life extension phase. Through TWI,oil company operators and the UK and Norwegian regulators are giving considerable attention to these issues.
Fatigue of jacket structures
Fixed steel jacket structures are most peoples' idea of 'an oil rig'. They usually comprise the main tubular legs and a complex framework of strengthening pipe-like tubular members, welded together and to the legs at nodal joints,(Fig.2). The role of the jacket structure is to carry the weight of the topside, and fix the platform to resist the varying loads of waves, tides and current and of wind and storms. In the early years of North Sea operations,there was considerable effort in inspecting nodal joints using divers or remote operated vehicles. The results showed that there were no major faults in the fabrication or fatigue design process that would limit life prematurely. Given the risks to divers and the difficulty of underwater inspection, parts of the industry have for some time used a form of monitoring called flooded member detection in preference to close visual under-water inspection.
Flooded member detection aims to detect the presence of a leaking through-wall crack by detection of sea water inside a member that would otherwise be dry. Jacket structures are designed with redundancy to be able to tolerate the reduced load carrying capacity of cracked or failed members for a limited period as load is redistributed elsewhere. The time for a through wall crack to grow from first penetration of the wall to the point of member failure is a key aspect of the safety case, since it affects the sensitivity and frequency with which monitoring for flooded members is required and the rate at which damage occurs at other locations.
In 2003 TWI carried out a review and assessment of fatigue data for offshore structural components containing through thickness cracks. During the earlier design phase TWI and other research organisations undertook extensive testing programmes to determine the fatigue life of nodal joints for validation of design methods. These tests had measured the number of fatigue cycles to first penetration of a through wall crack, and then continued to grow the crack to a size where the actuator displacement increased dramatically and the joint was close to failure and the test was stopped.
TWI was asked to assess the residual fatigue life from first penetration of a through-wall crack to when the test was stopped as a proportion of the fatigue life to first penetration. There was a spread of results depending on the joint geometry, thickness and loading mode. The residual life varied widely and could be similar to or substantially less or greater than the life to first penetration depending on the circumstances.
To confirm this result, TWI undertook some new fatigue tests on nine pipes representing brace members to determine the growth of through wall cracks in welds. The result was of considerable interest. It showed that the careful selection of an appropriate interval for monitoring flooded member detection was needed in the development of a late-life integrity management strategy for the jacket structure according to the type of joint.
Maintenance management
The management of equipment and structures on the topside deck of an offshore installation is an area where the industry is seeking to improve its performance. Maintenance is needed on a range of equipment essential for safety,production and supporting the infrastructure to sustain life offshore. It ranges from servicing major items, such as gas turbines, through to replacing a corroded hand rail. The offshore industry is well aware of the implications of a backlog or ineffective maintenance on the reliability of systems essential for ensuring safety and production, and impact of the costs and hazards of inefficient or ineffective maintenance outage.
The tragedy of Pipe Alpha was, in part, due to ineffective maintenance procedures, and more recently there have been other serious incidents relating to maintenance and repair activities. Since 2000, the industry has been focusing on reducing hydrocarbon leaks, many of which were due to poor maintenance. HSE's Key Programme 3 (KP3) launched in 2003 was designed to assess the state and integrity of equipment and structures from targeted inspections across a wide range of installations and operators.
Most attention was paid to the state of the 'safety critical elements'. These are the systems whose purpose is to protect against a major accident hazard occurring, such as release of inflammable hydrocarbons. Other SCEs are systems provided to mitigate the impact should a major accident hazard occur, by limiting its magnitude and escalation to further hazards, such as a fire. These include the gas, smoke and fire detection systems, emergency shutdown and isolation, and the active and passive fire protection systems.
KP3 introduced a traffic light system and found room for improvement of one or more SCEs on a high proportion of the installations inspected. The quality and level of maintenance of the SCEs was highlighted as a major issue. For the operator, maintenance in an offshore production environment is a substantial management task requiring significant planning, complex logistics of personnel and materials offshore, and the competencies and systems needed to ensure quality and effectiveness within a restricted time window.
Many people are involved. In today's world of outsourcing maintenance and specialist contractors, there are often many companies in the supply chain. While companies want to improve their maintenance management, it was recognised that the industry needed a way to benchmark performance and identify improvement steps.
Capability maturity modelling
The Energy Institute, in association with a consortium of oil companies and the HSE, invited TWI to partner with experts from Cranfield University, Poseidon and Sauf Consulting to develop a Capability Maturity Model for maintenance management.
The concept of the Capability Maturity Model (CMM) was founded in the software industry as a means for reducing a complex management operation into its constituent processes. Some of the processes were core to the day to day activity, while others were supportive yet essential for success over a longer term. The model was structured on a defined five tier assessment system of how well the organisation managed all its processes. The tiers range from an initial (learner) level to optimised best practice. CMMs generally have five levels of maturity. These are described generically in Table 1.
Table 1 Generic maturity levels
| Maturity level | Description |
| Optimised | The Organisation is 'best practice', capable of learning and adapting itself. It not only uses experience to correct any problems, but also to change the nature of the way it operates. Evidence that the organisation uses feedback not only to improve the process but also to improve the way it operates. At this level it would expect feedback from global level being used for improvement. |
| Managed | The Organisation can control what it does in the way of processes. It lays down requirements and ensures that these are met through feedback. Evidence is required that feedback is used to improve the process. Training is also used to improve competency. |
| Defined | The Organisation can say what it does and how it goes about it. Formal up-to-date documentation of the process available. |
| Repeatable | The Organisation can repeat what it has done before, but not necessarily define what it does. Basic or limited documentation is based on previous experience only. |
| Initial | Lowest level, the Organisation is at the learner stage, without the characteristics of the higher levels. Ad-hoc, no or limited documentation. |
The CMM concept is becoming more widely used to measure organisational maturity in other industries. It has been applied to several different management activities in the offshore industry, including design safety, the achievement of reliability in subsea equipment and structural integrity management. ISO 9004:2000 defines a similar set of levels as a measure of maturity in quality assurance.
The output from application of a CMM to an activity yields important information about how well the organisation is performing the activity and its readiness to change in response to its experience and external factors. The model is essentially about the extent of organisational learning and ability to adapt its practices and procedures, people and management, and those of its suppliers and stakeholders to new circumstances. The ethos of a highly mature organisation is willingness to question the effectiveness of its processes and to sustain a culture of self-improvement driven by senior management.
CMM for maintenance management
TWI's consortium with Cranfield University and offshore industry consultants applied the principles of capability maturity modelling to the management of maintenance on offshore installations. The main activity was broken down into six core processes, (Fig.3). Alongside these were complementary processes which may only apply in some circumstances, and supporting processes that take place over a longer period of time and underwrite the effectiveness by which core processes can be carried out.
For each process, five descriptions were prepared describing the characteristics of how an organisation at each of five maturity levels would be performing. In addition, improvement steps were provided suggesting ways in which an organisation could move to a higher level of maturity.
As an initial guide, maturity levels were described for the main activity of maintenance management considered as a whole. These are shown in Table 2. As the maturity level rises it shows the organisation progressing from one that is copying what it has done before to an organisation that is starting to think for itself, learning from experience and adapting its procedures, its people and its relationships with its suppliers and customers. Ageing and life extension may require a significant change in how the maintenance regime is managed over time and a more responsive organisational culture.
Table 2 - Maturity levels for maintenance management as a whole activity
| Level 1 | There is no real understanding of asset management and how to achieve it through maintenance.
Maintenance may range from following manufacturers' guidelines to responding to faults only, without regard to production or safety implications.
Maintenance is left to general subcontractors or to plant operators, generally without specific training and unsupervised.
No written procedures are in place; outputs from processes are inconsistent. |
| Level 2 | A belief that if previous practice is followed, the asset on the whole will achieve satisfactory and consistent performance through maintenance.
Maintenance is organised and controlled at the local working level (eg plant supervisor), who may rely a lot on previous experience of the equipment.
Problems are not usually reported unless they significantly affect production or cost.
Basic procedures are in place (but may not be documented); asset maintenance is not specifically managed or quality controlled. |
| Level 3 | There is an understanding of what the basic requirements to maintain the asset are, and whether they have been achieved in the past, but little use if made of any data collected to effect change and improvement.
Written procedures, which define the maintenance process to achieve consistent good performance (particularly for safety critical elements, SCEs), are available and integrated into the safety management (SMS) and the quality assurance (QA) systems. These may not cope well with changing conditions (eg ageing, changing process conditions).
Maintenance is beginning to be integrated with other functions such as operations, health and safety management etc, but there is very limited feedback of lessons from maintenance and service failures and limited understanding of how to predict future asset performance.
Training in maintenance management and procedures is encouraged. |
| Level 4 | The organisation has an understanding of how to set targets for asset maintenance, how to improve maintenance from experience, research and development (R&D), and how to include the supply chain.
Different maintenance strategies may be implemented for equipment at different stages of life or where appropriate. Experience is made available across the organisation and to the supply chain.
Maintenance is fully integrated with other functions such as operations, health and safety management etc.
Processes are adapted and performance improved in response to observed failures in maintenance and service.
Training and appropriate competence is a requirement throughout the asset maintenance function, including within the supply chain. |
| Level 5 | The organisation has developed a good understanding of how to improve asset maintenance continuously (particularly for SCE), based on knowledge, world wide experience, and appropriate R&D.
Processes, procedures and targets (including supply chain performance) are regularly monitored and researched and discussed. Communications and learning are optimised across the organisation to develop and disseminate best practice.
Procurement, equipment, operations and maintenance management are modified in response to experience and best practice is defined.
Organisations including supply chain adapt both management processes and maintenance processes to sustain long-term asset performance under changing conditions.
Training and appropriate competence is optimised, including the development of training methods and materials across the organisation and the supply chain. |
The maintenance management CMM model has been applied at a leading oil company to determine the maturity of its processes and identify where improvements in effectiveness and efficiency may be made. Interviews were held with the maintenance manager and a range of personnel responsible for implementing the different processes, both on and off-shore. A questionnaire was developed to enable those applying the model to investigate the processes and determine their maturity level. The model and questionnaire are now available in the final report available through the Energy Institute.
Requirements for life extension
As many older and ageing installations are reaching the life extension phase, TWI and its partners have been assisting the safety regulators in the UK and Norway to determine what is required to gain assurance that continued operation beyond original design life is safe. The UK Offshore Safety Case Regulations (2005) highlight life extension as a reason to revise the safety case, while in Norway offshore operators need to apply for consent to operate beyond original design life or current agreed life span. There is considerable discussion about how to interpret these requirements and determine what actions are necessary as end of original design life is approached.
TWI's work has centred on collating current approaches to life extension being practised by the offshore industry and other industrial sectors. Key aspects that have been addressed are the identification of integrity and performance indicators for the structure and safety critical topside equipment. The process, conditions and obstacles for life extension to proceed are considered within a generic framework.
As North Sea and other oil fields mature there is a tendency for the oil and gas to contain increasing amounts of water and acidic gases like carbon dioxide and hydrogen sulphide. Product in this condition is said to be sour. It can create a significant challenge to the integrity to the materials and welds of the pipes and vessels in terms of increasing stress corrosion susceptibility and rates of corrosion fatigue. Assuring the adequacy of the materials and welds under these conditions is of major importance.
In response to this challenge, TWI has launched several new Group Sponsored Projects to investigate the effects of sour service on materials and welds under different conditions. Experimental facilities for testing under sour conditions have been expanded within the Trevor Gooch laboratory, and innovative techniques developed for the testing of full sized sections of pipe, (Fig.4). The aim of these programmes is to generate a substantial database that the industry can use for assessing the fitness-for-service and life of its equipment.
In other current work TWI is leading a project to develop a model for the management of ageing safety critical elements using performance indicators. Leading indicators of different types over different timescales can help senior management, asset managers and verifiers to gain assurance that management and control systems, like maintenance, underwriting integrity and performance are being effectively upheld, and to provide forewarning of potential trouble ahead when these measures fall behind so that timely action may be taken.
Conclusion
This brief review has highlighted some of TWI's recent work to sustain oil industry operations using older assets in the North Sea and elsewhere. With the high price of oil, these operations could continue for many years to come. As installations are eventually decommissioned there will be the opportunity to gain information from examinations and testing of the materials and the welds. TWI continues to take a strong interest in the integrity of North Sea installations and pipelines, and is likely to be busy for some time.
