Revised Methodology for Assessing Fixed Offshore Structures With Wave in Deck Loading

Abstract

Wave-in-deck (WID) loading has been an issue of concern for engineers dealing with the performance of fixed offshore structures. The issue is particularly relevant for reassessment of existing structures due to subsidence or structures designed to older codes and standards, where WID was not considered. WID issues often in costly subsea remediation, impact platform occupancy levels or premature decommissioning critical structures due to safety concerns. The current in-house procedure is a simplified approach and revised to reflect the silhouette method and new industry requirements for the reliability of fixed offshore structures. The revised reassessment process shall enhance the methodology, based on widely accepted industry best practices. The platform model is calibrated based on the measured in-field platform natural frequencies using a structural health monitoring (SHM) system, so that the reliability assessment can be performed on a structural model to validate the effects of the revised wave in deck approach. The revised methodology demonstrates the potential of unlocking structural capacity of offshore structures by removing conservatism normally associated with traditional reliability assessment methods, thus significantly improving the ability to achieve target structural reliability levels in a cost-effective manner. A case study was embarked upon to demonstrate the benefit of the revised methodology for fixed offshore with insufficient air gap due to subsidence. The pilot study revealed significant improvement in ultimate strength of the structure using the revised methodology by more than 10% improvement in reserve capacity. This provides added benefits for increasing inspection intervals and optimizing the maintenance strategy due to risk reduction and avoidance of costly strengthening modification and repair (SMR). The objective of the present paper is to establish an updated methodology to determine wave-in-deck loading for ultimate strength analysis. The scope of work of the study are as follow: Perform a literature review of wave-in-deck assessment best practices, codes and standards. Develop assessment procedure based on international standards and research papers. Perform ultimate strength assessment based on revised methodology on a selected platform. Preparation of assessment methodology and case study report.