Improved FPSO Offloading Strategy Based on Analysis Including DP System

Direct offloading operations from turret moored FPSOs are traditionally performed with dynamic positioned (DP) shuttle tankers with a bow loading hose and hawser line connected to the stern of the production facility. The hawser line restricts the mobility of the shuttle tanker which is often forced to point towards the FPSO. The inherent increased risk of collision in case of a forward drive-off incident has motivated a study for a hawser-less tandem offloading operation. The structural limits of the flexible loading hose allows for larger operational sector and heading flexibility which enables the shuttle tanker to point away from the FPSO and hence reduces the probability of collision. An analysis model with two representative vessels for offloading operations in the North Sea has been developed in order to investigate the new concept. The model includes a compiled version of the DP system core algorithm, extracted from the real time system of both vessels. The new offloading strategy, with heading offset away from the FPSO, has been implemented in an updated control algorithm for the shuttle tanker. Time-domain analysis of the complete tandem offloading system has been simulated with measured wind, waves and current from a HindCast data base. Heading and relative motion between FPSO and shuttle tanker has been extracted and statistically evaluated in order to determine the time for which the shuttle tanker is directed towards the FPSO. The paper demonstrates how a marine simulation model, including core algorithm of the actual DP system and numerous simulations based on historical measured environment, can be used to document the feasibility of a new offloading operation control strategy.