Abstract Mooring integrity is a critical element in the safe and reliable operation of deepwater Floating Production Storage and Offloading (FPSO) systems. This abstract presents a case study detailing the inspection, evaluation, and replacement of mooring chains following the detection of severe localized pitting during an Underwater Inspection in Lieu of Drydocking (UWILD). The objective of this abstract was to assess the structural integrity of the mooring system and determine the necessity of chain replacement to maintain the FPSO's station-keeping capability and overall operational safety, in alignment with the requirements set forth by the International Association of Classification Societies (IACS) and applicable industry standards. The inspection methodology followed a structured, phased approach. During the initial UWILD, divers conducted visual inspection and manual gauging to measure pit depths and estimate wall thickness loss on critical chain links. Finite Element Analysis (FEA) was then performed using these manual gauging results to evaluate the residual strength of the degraded links under maximum anticipated mooring tensions, considering environmental loading and dynamic behavior typical of deepwater operations. In a subsequent UWILD cycle, selected links underwent high-resolution three-dimensional (3D) scanning to obtain a more accurate surface profile and volumetric assessment. A second round of FEA was conducted using the 3D scan data. The results of the 3D scan-based FEA aligned closely with the earlier manual gauging-based FEA, providing strong validation of the original assessment and strengthening confidence in the decision-making process. Observations from both manual gauging and 3D scanning indicated that certain links exhibited material loss up to 35% of their original cross-sectional area, particularly concentrated in the crown and curvature zones, areas known for high stress concentration. The FEA results consistently showed that the effective minimum breaking strength of the compromised links had deteriorated below the minimum safety factors required by classification and regulatory standards. Furthermore, localized sharp pitting was identified as a fatigue risk factor, with potential for accelerated crack initiation under cyclic loading. The novelty of this work lies in the staged application of manual gauging and 3D scanning, combined with localized FEA stress analysis, to provide a progressively refined and conservative evaluation of mooring chain integrity. This multi-phase approach surpasses traditional assessment methods, which typically rely on generalized corrosion allowances and visual estimates. Additionally, the study proposes a practical workflow that allows operators to make timely, data-driven intervention decisions, optimize inspection resources, and minimize downtime while avoiding unnecessary full system drydocking. The findings of this abstract underscore the importance of integrating advanced inspection technologies with robust structural analysis methods for effective deepwater mooring integrity management. The methodology and lessons learned are highly relevant for operators managing aging offshore assets worldwide, where maintaining mooring system reliability is critical for continuous safe operations and environmental protection.
Mohamad et al. (Mon,) studied this question.
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