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This paper presents the methodology and key findings of a study aimed at comparing the reliability of a representative jacket platform with that of a representative jack-up unit. Both structures were assumed to operate at the same location under identical environmental conditions (typical of the North Sea). Each structure was designed to the limits of their relevant recommended practice (API RP2A LRFD and SNAME T&R Bulletin 5-5A for the jacket and jack-up, respectively) to ensure a consistent comparison. This was followed by component and system reliability analyses using detailed modelling for both structures. Initially, the foundation failure was suppressed to enable the structure response to be compared and then, in the final stage of the study, detailed foundation modelling was included to examine the effect of foundation reliability on the overall structural reliability. For the specific site and environmental conditions considered in the study, it is concluded that the structures do not realise substantially different structural system reliability levels when designed/assessed to their most recent practices.

This paper describes a comparitive study on the structural reliability of a sample jacket and a sample jack-up. The methodology adopted is outlined and results obtained presented with emphasis on the jack-up. Findings of relevance to the jack-up community are discussed and some comments on the SNAME RP are given. An approach for improved spudcan–soil interaction modelling to be used in the next phase of the study is presented.

The introduction of the fixed jacket Load and Resistance Factor Design code (API RP 2A-LRFD), and the Jack-up Site Assessment Recommended Practice (SNAME T&R Bulletin 5-5A), raises debates regarding differences in the reliability levels associated with the use of the two documents. In an effort to investigate whether substantial differences exist, and to understand the underlying causes of the differences, a major study has been initiated by the UK Health and Safety Executive to compare the reliabilities of these two structures when operating in similar weather conditions and in similar water depths, to the extreme weather hazard. To ensure the comparison is appropriate, both units will be assumed to be operating close to the limits of their relevant code. Both will be subject to the same analysis packages in respect to wave-current force determination, structural analysis, pushover analysis, and reliability analysis. Initially, foundation failure will be suppressed in order that structural reliabilities under extreme weather conditions can be directly compared. Foundation failure will then be introduced to quantify the effect this has on the structural safety levels. Reliabilities will be determined at both component and system level using the response surface reliability methodology. Component strength modelling will exploit the formulations currently being harmonised for the new ISO offshore code, whilst global strength will be evaluated using CAP/Seastar for both installation types, thus reducing the potential for modelling based differences occurring during the system analysis of the study.

The first major offshore oil and gas production facility went into service in the North Sea over two decades ago. Offshore structures in hostile environments like the North Sea are subjected to time-dependent local damage such as corrosion and fatigue. To ensure safe operations, the integrity of these components needs to be maintained in service. This is achieved by carrying out planned non-destructive inspection coupled with structural integrity assessments. The inspections are not an entirely reliable process, however. There are also many uncertainties involved in the fatigue integrity calculations. In order to take these uncertainties into account, reliability-based assessment can be used. This paper summarizes the recent development of reliability-based fatigue initiation and fracture mechanics (crack growth) analyses for the in-service assessment of the integrity of offshore structural components. The techniques are illustrated with two specific forms of structural connection: welded tubular joints and threaded (screw type) joints. Related practical issues, including the reliability of non-destructive inspection (NDI) and the considerations of targeting maintenance resources, are also discussed.