the stern. When the transport (service) vessel arrives, the holdback tug will disconnect from the stern, and assist with the heading of the storage vessel. The service vessel will approach with two escort tugs, one push-pull tug at the bow and a holdback tug connected to the stern.

A full time domain model with all the DP systems and hawser connections was set up in MARIN’s software tool aNySIM. The model helped to determine the optimum layout and weather limits for this operation. To prohibit DP system interference, each support tug is station keeping to an earth fixed position set by the mooring master. The tug lines are sufficiently long to restrain the dynamics of the tanker, while giving it sufficient freedom to prevent peak line loads.

Vessel handling When a change of weather requires a significant heading change of a hose connected field vessel, all other connected vessels will need to respond with the appropriate heading and position change. This rotation around the well requires the production vessel and tug assisted storage

vessel to move along a predefined track within the bounds of the floating hose as shown in the schematic below.

Desktop numerical analysis simulated hawser loads and holding tug tow line loads for two sizes of storage and offloading tankers within a range of wind, current, wave conditions and directions. The desktop study raised concerns about the ability to coordinate the movements of five vessels - two large tugs, a stern holdback tug, the storage vessel, and the transport (service) vessel. MARIN suggested that HWCG make use of MARIN’s Houston bridge simulator. An AET Lightering mooring master then directed the operations of the tugs, storage and service vessels.

The hydrodynamic models of the storage and offloading tanker and holding tugs were then input into a bridge simulator to allow real-time simulation under various conditions. An experienced tanker offloading mooring master, DP process vessel OIM (Offshore Installation Manager) and positioning tug captains were brought in to participate. They used the bridge simulators to confirm

Predefined tracks for weather required rotations around the well

feasibility and develop procedures for posi- tioning and relocating the storage tanker using tugs.

Weather operating windows, hawser design and tug characteristics predicted by the numerical analyses were modified as a result of the bridge simulation work. The bridge simulator is now available for the training of emergency response vessel personnel. The study shows how integrating numerical analysis, bridge simulations and operator input helps solve complex operational design challenges.

report 23

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