A significant and steadily increasing number of offshore installations in the UK sector of the North Sea are approaching or have exceeded their original anticipated design life. Ageing processes can affect the structural integrity of the installation, and demonstration of adequate performance beyond the original design life is a necessary requirement. The UK’s Health and Safety Executive are therefore currently undertaking a number of activities, including research, relevant to the life extension of ageing installations.
With over thirty years of oil and gas production in the UK sector of the North Sea, a significant number of platforms are approaching or have exceeded their original design life, which is specified typically as 20 or 25 years. Approximately 50% of the total population of fixed platforms on the UKCS has exceeded the original design life, and this proportion is steadily increasing with time - with a continued requirement to produce oil or gas, the majority of installations are likely to remain operational for a significant period of time in the foreseeable future. Indeed, in some cases, there are plans to extend the operational life to multiples of the design life.
The ageing infrastructure of offshore installations presents the industry with a constant and growing challenge. Ageing is characterised by deterioration, which is caused mainly by fatigue and corrosion. Deterioration has a detrimental effect on the resistance of the structure, resulting in loss of structural integrity and an increase in the probability of failure. In the extreme case, structural failure could cause the immediate total loss of an installation, with little chance of survival. Thus, there is a need to give appropriate consideration to the implications of ageing and life extension on the management of structural integrity and to demonstrate continued safe performance beyond the original design life.
The importance of ageing is also reflected in national and international standards which have been developing over recent years. Codes dealing with the assessment of existing structures, i.e. section 17 of API RP 2A, ISO 19900, section 25 of ISO 19902 and API SIM, have recently emerged, providing support for duty holders now having to address explicitly the subject of life extension. The developing codes and standards for structural integrity management generally include four main steps in the process, which are based on those included in ISO 19902, i.e.
Ÿ development of a structural integrity strategy which includes managing the approach to assessing ageing processes and the need to link inspection requirements to these;
Ÿ development of an appropriate inspection programme which takes account of life extension;
Ÿ evaluation of structural integrity taking account of the original design requirement (which may have been less onerous than modern standards), as well as the consequences of ageing processes (e.g. fatigue, corrosion), and development of any remedial actions that may be required;
Ÿ setting up and managing a system for archiving and retrieval of structural integrity management data.
Ÿ A programme of onshore and offshore inspection, implemented initially through a topic theme associated with OSD’s Key Programme 3 (KP3) and subsequently, since 2007, the Structural Integrity Management Inspection Programme (SIMIP), to establish the structural condition and the integrity management procedures in place.
As a result, HSE is undertaking a number of inter-related activities to advance its programme on ageing installations, namely:
Ÿ An ongoing programme of structural integrity research which is used to inform the offshore industry and in the development of standards and guidance. Areas of particular progress include the understanding of system performance following single and multiple member failure, the effects on fatigue life due to load redistribution and structural reliability analysis for the determination of inspection plans and evaluation of system reliability. Other areas include the latest metocean data, materials performance (e.g. grade A ship steels and high strength steels used in jack-up construction) and foundation failure criteria. A review of new technology research relevant to the assessment of ageing installations is currently underway and a review of on-line monitoring techniques has been completed.
Ÿ The development of recommendations on the structural integrity management of ageing installations – a study on fixed installations has recently been completed and an equivalent study for mobile installations has now been initiated. Some general information on ageing semi-submersibles is currently available in HSE Offshore Information Sheet 5/2007 – see http://hse.gov.uk/offshore/infosheets/is5-2007.pdf.
Ÿ A programme of onshore and offshore inspection, implemented initially through a topic theme associated with OSD’s Key Programme 3 (KP3) and subsequently, since 2007, the Structural Integrity Management Inspection Programme (SIMIP), to establish the structural condition and the integrity management procedures in place.
Ÿ Direct collaboration with PSA in Norway to enable exchange of information and experience with a view to establishing best practice in the North Sea.
HSE’s recent work on ageing installations was disseminated in a special workshop on ageing installations at the Offshore Mechanics and Arctic Engineering conference held in Estoril, Portugal in June 2008. The following papers were presented and are available in the conference proceedings:
(1) A. Stacey & M. Birkinshaw, ‘Initiatives on Structural Integrity Management of Ageing North Sea Installations’, Paper OMAE2008-57418;
(2) A. Stacey, M. Birkinshaw & J. V. Sharp, ‘Life Extension Issues For Ageing Installation’, Paper OMAE2008-57411;
(3) A. Stacey, M. Birkinshaw, J. V. Sharp & P. May, ‘Structural Integrity Management Framework for Fixed Jacket Structures’, Paper OMAE2008-57413;
(4) P. May. D. Sanderson, J. V. Sharp & A. Stacey, ‘Structural Integrity Monitoring - Review and Appraisal Of Current Technologies For Offshore Applications’, Paper OMAE2008-57425.
Further information will be published, as it becomes available, on the HSE website at http://hse.gov.uk/offshore/index.htm.
Contact:
Dr. Alexander Stacey, HSE Offshore Division
Tel: +44 (0) 20 7717 6774