Section 2 Life extension procedure

2.2.1 This sub-Section provides a brief description of the lifetime extension requirements.

2.2.4 The reassessment of the structure should be based on the latest metocean report, which would reflect any changes in the environmental data (waves, wind, current, subsidence, water level, marine growth, scour, etc.). The latest metocean report methodology should comply with the BS EN ISO 19901-1 [3] requirements.

2.2.5 The structure should be checked for the air-gap requirements based on the latest codes and standards. A more accurate and conservative approach for calculating the minimum deck elevation is presented in BS EN ISO 19902 Section A.6.3.3.2 [1]. The air-gap definition is based on the assumption that a 10000-year wave crest should not impinge the lowest deck of the platform. If API RP 2A is the governing Code then an air-gap requirement is based on a 1000-year return period; however, this is based on the assumption that the structures would be de-manned in advance of an extreme storm event such as a hurricane.

2.2.6 It should be ensured that relevant geotechnical data is available to verify the original pile capacities, including driving records and pile driving fatigue results. Piles fatigue damage caused by the driving hammer during installation should also be added to the in-service damage.

2.2.7 A revisit of the original material data is carried out to ensure that materials and welds have sufficient toughness to prevent failure from original fabrication defects.

2.2.8 The reassessment should be based on the as-is condition of the platform. The inspection reports should cover scour depths, member damage, fatigue cracks, marine growth, CP measurements and corrosion, including wall thickness measurement of critical members (e.g. members in splash zone). It is recommended that at least three consecutive water survey reports are made available at the start of the life extension process.

2.2.9 An up-to-date weight control report including latest COG location, loading and a summary highlighting all changes tracked back to the original weight report should be presented for clarity.

2.2.10 A redundancy analysis should be carried out for the up-to-date model (incorporated inspection findings, metocean data, etc.), depending on the fatigue analysis results to establish member criticality.

2.2.11 A more frequent inspection and maintenance programme may be required at areas where the fatigue damages are exceeded.

2.2.12 A ship impact analysis should be carried out if the DWT of the supply vessel has been increased or if it was never considered in the original design. A re-analysis may also be required if there is significant degradation of the platform, such as reduced wall thickness or cracked members.

2.2.13 Any changes in seismic risk category, as per the latest codes and standards, should be considered in the reassessment.

2.2.14 Any proposed future modification should be considered in the life extension model.

2.3.2 There is a checklist provided in Ch 1, 7 A sample risk matrix for the life extension process that could be used as a reference guide for identifying and alerting the common gaps.

2.4.1 The common challenges and issues related to lifetime extension studies are presented in this sub-Section.

2.4.2 Obsolescence of design codes and standards

The majority of the existing platforms undergoing LTE have been originally designed based on the API-WSD guidelines that are more applicable to US waters. However, research and development in the field of offshore engineering have led to substantial modification of these codes of practice and the introduction of new guidelines, e.g. ISO 19902. Hence, the integrity of assets which were designed based on the older versions of the design codes needs to be re-evaluated. If the structure does not pass the linear strength check based on the latest codes and standards, alternative checks including non-linear pushover analysis are to be carried out.

2.4.3 Material certificates

The design temperature requirements for the different materials have changed over the years. Although the structure would have survived to date, it is not known whether there would be adequate toughness to prevent brittle failure, if any defects grow, or if new defects develop under service conditions (i.e. by fatigue). To determine defect tolerances, a fitness-for-service assessment (i.e. ECA) could be performed, which would generate defect acceptance criteria to be used during ongoing structural inspection. In the absence of an ECA the acceptable defect tolerance would remain very conservative, requiring more invasive and frequent inspections. In cases where no documentation is available regarding the material grades of steel, then coupon tests may be used.

2.4.4 Fatigue factor of safety

A minimum design fatigue factor of safety varying from 2 to 10 is recommended by the codes based on criticality and accessibility of joints. LTE documentation should be demonstrated to meet the requirements of the Code. During installation, pile driving induces significant fatigue damage on piles. There is a concern for the relevant applicable factor of safety for pile driving. Usually this is assumed as 1,0 for pile driving damage cases, while a factor of safety of 10,0 should be assumed for pile fatigue damage due to in-place condition. For piles damage during driving, an S-N curve corresponding to air can be used.

2.4.5 Metocean data

The metocean data for most of the old platforms are outdated and not site specific. The traditional approach is to derive ‘independent’ criteria for fixed structures, e.g. 100-year winds, 100-year waves, 100-year currents, etc. More recent changes to the traditional approach take account of the joint probability of occurrence, consider a responsebased analysis and use ISO 19901-1 Metocean Design and Operating Considerations as the governing standard. ISO 19901-1 makes recommendations for new approaches, particularly with respect to deriving design criteria for waves and extreme water levels.

2.4.6 Vessel size

The supply vessel size has increased over the past few years. The representative velocity and size of the vessels used for impact analyses should correspond to those used in the operation and servicing of the platform (e.g. supply boats). For example, the typical vessel mass for northern North Sea platforms is in the range of 8000 MT. It should also be ensured that appropriate stiffness curves for bow, stern side impact for a bigger vessel size based on latest available Codes and standards are considered for the re-analysis.

2.4.7 Air-gap issues

Traditionally, the API requirement was to maintain an air-gap of 1,5 m in excess of the 100-year extreme wave crest. This criterion was followed for the design of most of the platforms in the world. However, the 1,5 m adopted by API does not allow for sufficient reliability in most of the geographic areas. Many shallow water Gulf platforms (de-manned) were only designed for lower reliability on the basis of economics. Recent ISO guidelines require an air-gap sufficient to clear the abnormal 10000-year crest. When older structures are reassessed against this guideline, a number of platforms suffer from wave-in-deck issue.

2.4.8 Reserve strength ratio

Based on the in-place analysis results, if it is concluded that the platform cannot satisfy the minimum factor of safety requirement for piles or primary member/joint utilisations recommended by structural codes of practice (e.g. API RP 2A WSD or ISO 19902) for the extreme environmental loading (100-year return period), a non-linear pushover analysis is recommended. The pushover analysis is employed to determine the ultimate strength of the platform against the extreme environmental load. In the ‘pushover’ approach, the environmental forces associated with 100- year extreme loads are scaled until the platform collapses.

RSR is a measure of the reserve strength in the system to withstand extreme environmental loading. RSR is defined as the ratio of collapse base shear to the design base shear. The ISO 19902 standard gives an explicit RSR value of 1,85 to be attained for manned (L1) platforms. Lower values of RSR can be acceptable for structures of L2 and L3 platforms. The RSR values have been derived from jacket load statistics based on wave loading on jackets. These RSR values are not valid for jackets subjected to wave-in-deck loads. Hence, a structure which demonstrates to have attained a Code prescribed RSR can be certified to possess the desired reliability levels when the air-gap requirements are satisfied.

2.4.9 Wave-in-deck issue

It is implied by the Code that the reserve strength ratios are applicable only if the structure is subject to jacket loading alone. For the case of a structure subject to wave-in-deck loading, a dedicated metocean hazard curve should be developed for each platform. A hazard curve represents variation of hazard load intensity for different return periods.

The hazard curve is the plot of base shear against return period in a log-linear scale. The hazard curve is developed by plotting the normalised base shear (E_RP/E₁₀₀) for different environmental return periods. E_RP corresponds to base shear for different return periods whereas E₁₀₀ corresponds to base shear for a 100-year return period. A hazard curve for a platform subjected to wave-in-deck consists of two slopes. The initial part is where the structure is subjected to wave-in-jacket only and the remaining portion includes the effect of additional wave-in-deck loads after a negative air-gap is reached by the wave crest.

Note that, assuming no resistance uncertainty, the hazard curve yields the collapse load for a given return period (i.e. probability of failure). As can be seen from the modified hazard curves, for a given return period (i.e. given probability of failure, and hence a given reliability level), the vertical line will meet the jacket-alone hazard curve (blue line) at a much lower ordinate than it will the jacket–deck load curve, see Figure 1.2.2 An example hazard curve for a platform subjected to wave-in-deck . In other words, for a structure subject to wave-in-deck loading, the RSR required to meet the same reliability levels as that of a structure subject to jacket load alone, is much higher.

Figure 1.2.2 An example hazard curve for a platform subjected to wave-in-deck

2.5.1 There are three different types of assessment method that can be used consecutively for the reassessment of the platform. A brief discussion is given below.

2.5.2 Design level analysis

This is a linear analysis, with the exception of the pile–soil interaction, based on Code design checks for in-place strength and fatigue life assessment. When the in-place strength check does not meet the acceptance criteria, alternative mitigation methods should be considered, or ultimate strength analysis should be carried out. Similarly, for the cases where fatigue life assessment results are insufficient, ultimate strength analysis with member removal can be carried out, or alternative mitigation procedures should be followed. Refined fatigue analysis, including crack modelling and propagation life calculation, can also be considered.

2.5.3 Ultimate strength analysis

This is a non-linear pushover type of analysis, which can evaluate the redundancy of the platform and calculate the reserve strength ratio (RSR). This type of analysis provides an alternative approach based on global system failure, instead of the conventional component failure. Accurate computer models and sufficient modelling details to simulate the failure modes, along with suitable software, are some of the requirements needed to successfully perform an ultimate strength analysis. For results that do not comply with the acceptance criteria, Reliability Based Assessment can be considered. Different mitigation procedures can also be considered.

2.5.4 Reliability Based Assessment

This type of assessment is based on the probabilistic modelling of load and resistance, and evaluates the reliability index, β, and the probability of failure of the system. Alternative mitigation procedures should be considered if the targeted reliability index (probability of failure) cannot be achieved.

Further details regarding the types of analyses and the corresponding requirements can be found in the latest BS EN ISO 19902 [1] Code.