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

摘要

Integrity management of mooring systems on floating structures has been gaining interest over the last fewyears, as a result of increasing interest in life extension of existing assets and the reported increase in thenumber of mooring system failures. During the second half of 2012, OMV New Zealand Ltd commencedplanning for a detailed inspection program of all underwater assets in the Maari Field, including themooring system for the FPSO Raroa. When conducting the campaign in early 2013, a series of anomalieswere found, including birdcages in wire sections of adjacent mooring legs. Through a regime ofcoordinated metocean forecasting, metocean monitoring, FPSO position monitoring, numerical modellingand ongoing inspection, OMV New Zealand was able to effectively manage an FPSO found to havemultiple adjacent degraded mooring lines through a winter season until repairs could be completed. Thedevelopment of a concurrent numerical model of the actual mooring system proved to be indispensablein 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 withdetailed site metocean data significantly enhanced the value of the numerical model. The numerical modelwas employed across a wide range of activities from forecasting performance, evaluation of hypotheticalmulti line failure scenarios, interpretation of measured performance, determination of sea state limits andinspection triggers, through to the remediation engineering of the system. Of particular importance for theMaari 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.