Section 3 Brief overview of codes and standards life extension requirements

3.1.1 To demonstrate fitness-for-service, ISO offers a range of qualitative and quantitative assessment methods for existing platforms which are summarised in ISO 19901-9, Table 2. The platform is usually categorised as L1, L2 or L3 based on life-safety, environmental and economic consequence. Assessment of an existing structure shall be designed for the full compliance with ISO 19902 and ISO 19901-9. If the design criterion is not fully met, alternative measures such as using platform specific data (e.g. mill certificates) or further justification based on risk mitigation measures could be followed. There is also a provision to consider reduced partial action factors based on the exposure category of the platform.

3.1.2 Verification of all limit state criteria (ULS, ALS, SLS and FLS) may also be established quantitatively by performing a Design Level Assessment of the platform on a component basis using the explicit or implied probabilities within ISO 19902. It is recognised that for reassessment the platform may be in a degraded state and verification of all of the limit state criteria may not be achieved; the assessment may be re-performed with a more complex but less conservative analysis method. Ultimate strength methods may be performed using static or dynamic analysis techniques that include material nonlinearity and geometric nonlinearity to determine the maximum actions/vectors the structural system can sustain within the relevant limit state. The concern with an ultimate strength assessment is that the global behaviour of the platform and localised failure of structural members, joints or foundations is inconsequential if the structure’s capacity achieves the required limit state. ISO 19901-9 provides requirements for existing structures including the use of the system strength approach.

3.1.3 Structural reliability analyses are also permitted to assess the effects of uncertainties in the actions, resistances and modelling. The inherent or physical randomness in the basic variables (known as aleatory randomness) should be represented together with epistemic uncertainties originating from lack of sufficient data and shortcomings in the model’s representation of reality.

3.2.1 Reassessment of existing fixed offshore steel framed platforms is addressed in API RP 2SIM. Like ISO simplified qualitative methods are allowed provided an appropriate level of conservatism (or safety) can be demonstrated. Engineering judgement, experience and knowledge of key structural aspects such as type and function of platform, year designed, number of legs, bracing scheme, water depth and present condition are prerequisites.

3.2.2 A design level component assessment, the same procedure for the design of a new platform, in accordance with API RP 2A WSD 22nd edition, is permitted. However, for reassessment there is scope for reducing jacket member effective lengths (Section 8.5.3.2.2).

3.2.3 API RP 2SIM also offers an alternative to a non-linear assessment such as pushover analysis. The platform’s ultimate strength may be assessed by way of equivalent linear or non-linear methods.

3.2.4 An equivalent simplified linear assessment would be to perform a Design Level Assessment but with the API 2A safety factors removed. This can provide a first pass estimate of the ultimate strength. The other known source of conservatism, namely the use of mean yield strength, which compared to nominal yield strength may also be taken into account for the linear analysis. The structure passes ultimate strength assessment if no elements have exceeded their yield strength. A small number of overloaded members and/or joints may be deemed acceptable on a rational basis and providing the platform does not collapse.

3.2.5 Ultimate strength can also be assessed using large deformation non-linear methods to demonstrate that the platform has adequate strength and stability to withstand the ultimate strength loading. Mean yield strength may be used rather than nominal values. Increased strength due to strain hardening may also be used if the structural section is sufficiently compact, but not for rate effects beyond the normal (fast) mill tension tests.

3.2.6 API RP 2SIM also recommends different performance criteria for platforms located in the US Gulf of Mexico. API standards evolved primarily based on platform performance and economic risks in the Gulf of Mexico region. Reasonable caution should be exercised when using these codes in regions with limited knowledge of platform performance, as using a Code developed for a different region will provide an unknown level of reliability.

3.3.1 Reassessment of existing structures is addressed in N-006, the scope of which includes fixed and floating structures constructed from steel, concrete and aluminium. In contrast to the principles for life extension of existing structures given by API and by ISO, the recommendations in the NORSOK standard aim to ensure the same safety level for personnel as is required for new platforms based on N001 requirements.

3.3.2 NORSOK N-006 (2nd edition,) has been developed as a companion document to the existing suite of NORSOK offshore standards (N-001, N-003, N-004) for the purpose of providing specific guidance for the reassessment of ageing/existing structures including fixed and floating structures constructed from steel, concrete and aluminium. N-006 was developed to align with International Codes and Standards; for example, when considering the resistance of damaged tubular members Chapter 8.8 refers to ISO 19902. However, whereas ISO and API are broadly aligned there are key differences in the NORSOK approach. In contrast to ISO 19902, where load and resistance factor is considered unity for pushover analysis, the NORSOK N-006 standard recommends the use of partial factors in the non-linear analysis.

3.3.3 The NORSOK standard permits use of advanced non-linear methods (quasi-static/pushover or dynamic time-domain analysis) to determine ultimate global capacity. Whereas ISO and API permit the use of mean yield stress in ultimate capacity analyses, N-006 stipulates adopting characteristic resistance whereby the SMYS is reduced by N-001 partial material factors. The standard also does not provide any provision for structural reliability analysis.

3.3.4 For life extension purposes NORSOK N-006 is premised on maintaining the same safety level for personnel as for new platforms. If the structure does not meet the ULS and ALS requirement related to environmental action then an alternative route such as de-manning could be adopted, provided the structure can meet the requirements of unmanned facilities once the shutdown procedure is implemented. It is also important to ensure that storms do not affect the platform's main security function or risk causing significant pollution. Lower safety factors are provided for unmanned platforms, so in effect the platform may need to be de-rated under certain environmental circumstances in order to justify continued use.

3.3.5 For ULS and ALS reassessment checks, the effects of corrosion and/or mechanical damage are to be properly considered/modelled. The member checks and joint capacity checks are to be carried out in accordance with the principles of N-001/003/004. In addition, for existing platforms, where the joints are subjected to cyclic loads beyond the limits of linear behaviour, there is a requirement provided in the standard to take account of the accumulative effects of low cycle fatigue.