Mooring Design for Directional Spar Hull VIV

Abstract

Abstract Key issues for design of Spar mooring systems in the presence of VIV are discussed. The need to consider interactions between the directional effects of the Spar mooring and VIV response, together with the directional distribution of the loop current environment, is highlighted. For instance, the combination of current speed and varying sway natural periods for environment directions between and in-line with mooring groups give rise to variations in reduced velocity. Furthermore, the Spar response itself could be directional due in part to unsymmetric coverage of the VIV suppression strakes and other appurtenances located on the outside of the Spar. Directional stiffness and drag augmentation due to VIV may also affect maximum offsets and mooring line loads. Two analysis methods are presented to determine governing mooring loads in the presence of Spar hull VIV. The first, called DRIVESIM, forces the motion of the hull in a pre - determined Figure-of-eight shape with given amplitude. Mooring loads consistent with measured drag loads are achieved by iteration on the effective hull drag coefficients. In the second method, called FORCE-IT, an applied force on the hull is used to achieve the required VIV amplitude, thereby allowing the mooring system non-linearities to achieve dynamic equilibrium. The second method shows a more realistic trajectory. Description of Spar Hull and Mooring Currently there are three Spar hull configurations that are installed or under design and construction: Classic Spar, Truss Spar and Cell Spar. The Classic and Truss Spar configurations are described in [1]. The Classic Spar has a circular cross section along its entire length. The Truss Spar hull consists of a hard tank, which has a circular cross section similar to Classic Spar, a truss structure with heave plates, and a soft tank which provides buoyancy during horizontal tow-out and holds fixed ballast in the in-place condition. The Cell Spar [2] has multiple circular cylinders joined together in contrast to a uniform circular section of the Classic and Truss Spar hull. The Classic Spar hull and the hard tank of the Truss Spar hull are fitted with helical strakes to minimize hull VIV motion due to current. The Cell Spar similarly has strakes along its length. Due to construction and transportation considerations, the strakes on Truss Spars built to date do not provide full coverage, but are truncated or have reduced height along one side, as shown in Figure 1. This allows the hull to be built close to the ground and helps with transport and float-off from the transport vessel. It also eliminates the need to rotate the hull for adding strake sections after offloading from the Heavy Lift Vessel. Therefore, the strakes, on the Truss Spars to date do not provide full coverage. On the other hand, Classic Spars built to date were rotated after offloading and the strakes completed. Furthermore, there are holes in the strakes for the mooring chains to pass through that make the strakes un-symmetric for Truss Spars and even for Classic Spars.