Operating Performance Forecasting and Evaluation of an FPSO with Degrading Mooring System Components Available to Purchase

Integrity management of mooring systems on floating structures has been gaining interest over the last few years, as a result of increasing interest in life extension of existing assets and the reported increase in the number of mooring system failures. During the second half of 2012, OMV New Zealand Ltd commenced planning for a detailed inspection program of all underwater assets in the Maari Field, including the mooring system for the FPSO Raroa. When conducting the campaign in early 2013, a series of anomalies were found, including birdcages in wire sections of adjacent mooring legs. Through a regime of coordinated metocean forecasting, metocean monitoring, FPSO position monitoring, numerical modelling and ongoing inspection, OMV New Zealand was able to effectively manage an FPSO found to have multiple adjacent degraded mooring lines through a winter season until repairs could be completed. The development of a concurrent numerical model of the actual mooring system proved to be indispensable in achieving this objective and in facilitating the day-to-day operation of the facility over this period. Being able to verify numerical predictions against actual vessel motion response data in combination with detailed site metocean data significantly enhanced the value of the numerical model. The numerical model was employed across a wide range of activities from forecasting performance, evaluation of hypothetical multi line failure scenarios, interpretation of measured performance, determination of sea state limits and inspection triggers, through to the remediation engineering of the system. Of particular importance for the Maari situation was the early detection of potential line failure on the degraded leeward mooring legs, which was heavily reliant on comparisons of numerical predictions against observed system behavior.