Shallow Water STL Mooring and Riser System

Abstract

A shallow water disconnectable STL turret mooring and riser system has been developed for water depth between 30 and 50 m. This technology is based on APL’s disconnectable STL (Submerged Turret Loading) and STP (Submerged Turret Production) technologies which had been widely applied for water depth between 70 m to 2600 m for FPSOs and LNG offshore terminals. The advantage of disconnectable system is that the mooring and riser system can be designed to a preferred sea state. When the sea state is higher than design sea state (like hurricane), the vessel can be disconnected and sail away. The shallow water STL system consists of STL buoy, mooring lines, riser system and landing pad. The interface with vessel is the same as traditional STL system. The mooring and riser system are connected to the vessel through STL buoy and can be pulled into vessel by using ship winch. Unlike traditional STP and STL buoys, the shallow STL buoy has a net weight and will stay on the landing pad when disconnected from vessel. The landing pad is designed to support the impact load from STL buoy and supply enough friction for the STL buoy to stay in position during 100-year storm. The mooring system design has taken the advantage of directionality of weather when close to the shore by using different mooring line length in different directions. Further an innovative Hold-Back-Wave riser configuration has been developed for shallow water system. The riser configuration has a larger flexibility compared to traditional wave configuration and has proved to be feasible for significant wave height at least 7 m when connected to the vessel and 10+ m when disconnected from the vessel. Model test for the disconnectable shallow water turret mooring and riser system had been performed in MARINTEK, Trondheim with a LNG re-gasification vessel model at 30 m water depth. For connected system, significant wave height Hs = 6 m and 8 m has been tested. The mooring and riser system perform well, as predicted. For disconnected system (when the buoy sitting on the landing pad), significant wave height Hs = 10 m has been tested. The STL buoy is sitting on the landing pad without significant movement and the riser system performs well. SIMO program has been used to calibrate the model test results with numerical simulations. By adjusting surge, sway, yaw damping and 2nd order wave drift force, the calibrated SIMO model agrees well with model test results and can be used for similar development.