The radial wellbay (RAW) Spar is, for the most part, based on conventional Truss Spar technology. The primary difference between the RAW Spar and the conventional Truss Spar is the wellbay arrangement.

An integral structural component of the RAW Spar is the ABCD (Adjustable Buoyancy Centerwell Device) located in the lower portion of the centerwell. The ABCD is connected to the interior of the hard tank using a number of shear plates. The ABCD captures the buoyancy in the open centerwell and contributes to the total buoyancy of the hard tank. As an option, the ABCD can be used for in-hull storage. On the RAW drilling Spar, the ABCD can be used to support the setback and pipe racking system.

Design applications using the ABCD raise an additional requirement in the design of the hard tank, specifically, an accurate estimate of the load in the structure that connects the ABCD to the hard tank. Contributions to this load come from the inertia effect of the Spar motions, hydrostatic responses affected by ballast conditions in the device, and hydrodynamic forces affected by pressure in the riser slot gap. Connection loads are required to design the structure connecting the ABCD to the interior walls of the hard tank.

An ABAQUS™ based time domain semi-empirical model was developed to predict the local and global loads on the ABCD. A series of 1:50 scale model tests on the RAW Spar were carried out at the OTRC basin in College Station, Texas. The model’s ABCD, mounted on a dynamometer system inside the hard tank, was used to measure the six-degree-of-freedom forces between the hard tank and the ABCD. The paper focuses on a comparison of the predicted and measured loads.

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