Life extension for mobile offshore unites: How old is too old?

Posted on 30 October 2009

By Gerhard Ersdal, Arne Kvitrud, Petroleum Safety Authority Norway; Wayne Jones, Malcolm Birkinshaw, UK Health and Safety Executive

Editor’s note: This article is the second in a series of articles that will focus on the critical issue of asset integrity. The International Regulators Forum kicked off the series in the May/June 2008 issue, presenting findings from a three-year inspection programme. In this issue, UK’s HSE and PSA Norway detail how they are approaching the evaluation of offshore units operated beyond their original design life. In upcoming issues, Australia’s National Offshore Petroleum Safety

Authority and the Netherlands’ State Supervision of Mines will provide their perspectives on how industry can improve asset integrity.

A large number of mobile units in the North Sea are approaching or have exceeded their notional design life, which, for maritime systems and structural components of mobile offshore units, is typically 20 years. It is likely that many of these installations will continue to be operated for years to come. The use of installations beyond the design life requires special consideration of the effects of aging.

Aging processes can affect the structural integrity of the installation: the exposure of maritime systems and structures to conditions of stress and environment will ultimately degrade them from their initial state. Damage will accumulate, and failure becomes increasingly likely until the structures and maritime systems deteriorate to an extent that they are no longer fit for service. Loss of maritime and structural integrity can have serious consequences, as exemplified by the

Alexander Kielland accident in 1980, when 123 lives were lost.

Learning from accidents is an important aspect of improving safety and has led to the development of new technology and approaches to safety management. For instance, the Alexander Kielland incident prompted the development both in Norway and in the UK of structural fatigue design technology. However, there is a need for the offshore industry, i.e., the regulatory bodies, the classification societies, industry bodies, operators, contractors and other stakeholders, to take proactive action to ensure that the risk of accidents/incidents related to deterioration associated with aging is reduced to as low as reasonably practicable (ALARP).

At present, criteria defining acceptable age limits or how these age limits are influenced by maintenance and mitigating actions are not clearly defined. Limits will be specific to different systems, and the need to address the issue of aging mobile installations has been identified by the UK Health & Safety Executive (HSE) and the Petroleum Safety Authority Norway (PSA). It has been recognised that, with a growing and significant number of offshore units having exceeded their notional design life, there is a need to have appropriate integrity management procedures that reflect the significance of aging and hence help to demonstrate adequate performance beyond the original design life.

PSA–HSE collaboration

Due to differences in Norwegian and UK regulations for offshore installations, the issues of aging have been addressed by slightly different approaches.

The PSA introduced regulations in 2002 requiring the operator to obtain consent prior to using a facility beyond the operating life specified in the PDO application (Section 5f of the Information Duty Regulations). The regulations also stipulate that the operator must submit the application for consent for extended lifetime at least one year prior to expiration of the approved operating life. Subsequent to this, the PSA, in collaboration with the Norwegian Maritime Directorate (NMD) and Det Norske Veritas (DNV), developed a procedure for the assessment of life extension, and a circular was issued by the PSA to the industry in April 2003. A similar circular was issued in January 2007 related to jackups.

HSE’s approach is to utilise aspects of their existing regulations to address aging. Based on the principles of Regulation 8 of the Offshore Installations Design and Construction Regulations 1996, HSE expects that duty-holders periodically reassess their arrangements that are used to maintain integrity, to take account of the effects of aging processes and to ensure that any deterioration is detected in good time, particularly for installations beyond their notional design life. A Safety Notice specifically for semisubmersibles was issued in 2003.

The HSE safety notice and the identical PSA circular were issued to bring urgency to this matter and outlined appropriate measures to be considered. These include – if not already in place – fatigue life assessment to current rules/standards both in the intact and damaged condition; inspection requirements; use of information on past performance; replacement/repair of aging components; review of the effectiveness and reliability of barriers; and review of the effect of changes in knowledge concerning technology and environmental conditions that could influence existing barriers or make further measures reasonably practicable.

These measures are equally applicable to jackups. The Safety Notice was changed to Offshore Information Note 5/2007 in 2007, although the technical text is unchanged. The formal, thorough review of the installation’s Safety Case, now required on at least a five-year interval by Regulation 13 of the Offshore Installations (Safety Case) Regulations 2005, provides the means of documenting this reassessment to maintain integrity.

Aspects of aging

Operation of deteriorated, aging offshore installations requires particular consideration of hazards and threats to the structural integrity of the safety critical elements (SCEs) as a result of aging processes that can potentially limit the installation’s safe life. Aging is characterised by degradation, which, in the case of mobile units, includes fatigue, corrosion, stress corrosion cracking, embrittlement, hydrogen-induced stress cracking, erosion/corrosion and mechanical wear and tear. Degradation will also be caused by damage suffered during operation. One particular concern is the increasing probability that several components of the structure or maritime systems may fail simultaneously, leading to multiple site failures.

SCEs for semisubmersible installations include the hull, mooring system, stability (ballast and control) systems, temporary refuge (TR), escape routes and the helideck. Structural integrity is maintained mainly through good design, periodic inspection and assessment, in line with Class and other systems.   Integrity of the mooring system is usually maintained by periodic change-out of chains and ropes, according to the discard criteria. Life extension may be dependent on further change-outs. It should be noted that, for moorings, premature failures have been observed in practice, emphasizing the importance of this control measure.

SCEs for a typical jackup installation are the legs, rack and jacking system, foundations (spud cans, etc), deck, TR and escape system. The legs and rack are often made from high-strength steels (greater than 600 MPa), which are more vulnerable to hydrogen embrittlement from cathodic protection than typical offshore steels. Erosion and wear can also affect the rack and jacking system, but these can be detected by visual inspection. The spud cans have suffered from hydrogen-induced cracking in the past due to a build-up of anaerobic bacteria. However, nowadays this can be controlled by adding biocides.

Apart from the technical aspect of aging, human and organisational factors may also play an important role. These include lack of relevant documentation for maritime systems and structures and procedures that are lost. In addition, spare parts for aging equipment may not always be easily obtainable. An aging unit may also have worker-related health, safety and environmental aspects that need to be evaluated. The following may be significant:

• Noise, vibration and bad ergonomic design of old equipment.
• Reduced accessibility for inspection of safety-critical structures, systems and components.
• Design of living quarters fail to provide the necessary rest and restitution for workers.

Life extension process

The main objective for the PSA and the HSE has been to ensure that aging facilities are still robust and that their safety does not differ from that of younger units.

To achieve this objective, the following elements are recommended when evaluating a life extension for an existing unit.

1. Evaluate history

Evaluate the degradation history of the unit and its components. An important aspect is whether degradation has developed slowly over time or increased rapidly in the recent past. The history of incidents and accidents on the unit should also be evaluated, along with their influence on such aspects as the strength of the structure. Statistics on actual incidents and accidents will influence the risk analysis, which provides the basis of design accident specifications for the unit.

Any successful performance history (such as the absence of cracks during inspections) will also be important documentation for reducing uncertainty about the unit’s status. Without such information on performance history, uncertainty will be a major challenge, and life extension will need to be based on significantly higher safety factors.

2. Evaluate the as-is condition

The assessment of the unit must be based on its as-is condition, updated drawings, and computer models and analysis of the as-is condition to ensure that all changes to the as-built condition and any new technology are incorporated in the new analysis. This includes the impact of any damage to and degradation of the unit.

3. Evaluate the future for the aging unit and potential life extension

Evaluate which aspects of aging may reduce safety. This will include identifying possible aging-related failure modes that may affect the unit. The main safety barriers are expected to be identified, and the aging mechanisms which may lead to degradation of the barriers should be identified. Performance of the barriers is expected to be monitored and maintained to ensure that the unit is fit for purpose at any given time.

Looking ahead, the evaluation must also include a risk analysis that covers future operation and is updated with a relevant incident and accident history of the unit in question. Finally, planned changes and modifications to the unit during the period of extended life must be analysed.

Challenges related to the aging of units can be mitigated by taking technical or operational action, or by making modifications to the unit itself. Mitigations may include additional inspection and maintenance, operational procedures such as reduced staffing in severe storms, operational limitations, removal of equipment, strengthening of structures and changes to maritime systems. A broad evaluation of possible mitigating actions and modifications is necessary.

4. Assessment

Finally, the unit’s integrity must be evaluated – taking into account its as-is condition, its assumed future performance and use, and proposed modifications and mitigations.

5. Make decisions

Ultimately, the question must be whether to continue using the unit or to decommission it. The answer depends on whether a combination of mitigating actions and modifications will be sufficient to comply with national safety requirements. In addition, further mitigation to ensure continuous improvement needs to be evaluated.

Documentation

An assessment of life extension will normally result in documentation of the following elements:

1. Technical and operational integrity of the unit, including how the integrity of maritime systems and structures relates to regulations and standards.
2. Plans for implementing technical and operational mitigations and modifications in order to ensure sufficient integrity in the future.
3. Maintenance plans for ensuring sufficient integrity, including ways to monitor and identify degradation and aging, as well as the relevant mitigation.
4. Plans for ensuring the availability of sufficient expertise to operate and maintain the unit.
5. Plans to ensure that necessary spare parts will be available.

Challenges

The recognition that a large and increasing number of mobile offshore units are approaching or have exceeded their design life has prompted regulatory authorities in the UK and Norway to seek specific action to justify continued operations and to seek discussion on the need for guidance on the subject.

There is a need for the development of industry practice on the structural integrity management of aging and deterioration of installations. HSE recently initiated work in this area, the results of which will be publicly available in 2009. In Norway, a draft NORSOK-standard for structures is under development, giving requirements to assessments for life extension. This standard is expected to be published in 2009. During OMAE 2008, a joint HSE–PSA session on aging and life extension was arranged. Further, PSA plans to continue its R&D project on aging and life extension until 2010.

HSE and PSA will continue to collaborate on this topic for the foreseeable future.

Due to the error of Drilling Contractor, an article discussing the regulator/industry collaboration on the IADC HSE Case Guidelines was published in the July/August 2008 issue as part of the Asset Integrity series in place of the correct article, which is published here. Drilling Contractor apologizes for this error.

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