Approximate methods for probability-based assessment of jacket-type offshore platforms by using incremental wave analysis

The main concepts of the proposed probability-based assessment method called Probabilistic Incremental Wave Analysis (PIWA) are investigated in the companion paper by the authors. This probability-based approach can be used for assessment of jacket-type offshore platforms against extreme sea states. In this methodology, the performance objective can be stated in terms of the mean annual frequency (MAF) of exceeding a given level of structural Demand Parameter (DP) or a limit state, e.g., the collapse prevention (CP) limit state. The CWH-based approach is suggested as a wave height-based scheme in order to estimate the CP limit state frequency. However, there are some complexities in numerical calculations associated with the PIWA framework, which can affect the practical implications of this probabilistic approach. As a result, to simplify the PIWA procedure, approximate closed-form analytical expressions are suggested herein in order to estimate the MAF of exceeding the CP limit state. The two alternative solutions are comprised of the ultimate capacity-based approach, and the Collapse Wave Height (CWH)-based approach. Hence, the proposed approximate methodologies make the PIWA framework generally suitable for being used in practical assessment of jacket-type platforms, and being implemented in guidelines and codes. Furthermore, the closed-form analytical expression regarding the CWH-based criterion provides an alternative interpretation for the design or assessment against extreme waves. It provides a simple and consistent method for probabilistic evaluation of the jacket structures. A case study jacket-type platform located in South Pars Gas Field in the Persian Gulf region is used for application of the aforementioned issues.