Global Response Analysis of Wind Turbine Installation Vessels in Semi-submerged Condition. A Modified Quasi-Static Approach

Abstract

Wind Turbine Installation Vessels (WTIV) have a significant importance for the installation and maintenance of offshore wind farms. For a range of water depth, wind turbines will require a fixed foundation which can be installed with a jack up vessel. Jack up leg and jacking system design had been traditionally governed around consideration of the fully elevated condition, called also as survival condition.

Industry, nonetheless, had seen various leg and jacking system damages happening in transient phases (more often during semi-submerged condition) whether elevating or lowering legs for installation or maintenance duties at the required locations. The semi-submerged condition is a geometrical configuration where legs are attached to the seabed with a specific leg penetration and with the hull partially immersed. During this condition the exposure to wave loads, current loads, wind, and soil-leg interaction exposes the structure to high non-linear effects which worsen with large hull drifts, structure flexibility and dynamic effects.

Such conditions are not commonly covered by class societies or well acknowledged by designers. This transient phase is important for the vessel operation and depending on the environmental conditions it will be possible to become the governing design load case for some elements of the leg, jack up system, or other vessel components.

Knowing the global behave of the structure and the stress distribution during such conditions will allow the preparation of better designs, so analysis should be dedicated and delicately performed. To demonstrate the importance and risk associated, a study for the global response of WTIV in semi-submerged condition is performed characterizing the global behave of the structure and comparing it against the common design load case, the survival condition in extreme response.

A simplified structural model of a WTIV is used. Subsequently a CFD Hydrodynamic analysis coupled with a FE solver is performed to verify load increments during exposure to different sea states in the conditions mentioned. The analysis proposed is a Deterministic Linear QuasiStatic analysis where dynamic effects are modelled by modifying the FE model with additional inertial loads acting on the hull center.

Inertial wave loads acting on the body are determined by Potential Flow Theory solving the diffractive potential. Pressure distribution around the hull is calculated applying Panel Method with direct numerical integration over the surface. Additionally, Morison formulation is used to determine drag forces acting on the legs and later applied and solved on the FE model. Current profile is modelled as a constant of 5 m/s along the sea bed, used for drag linearization.

Different geometrical configurations of the semi-submerged condition are investigated by varying the water depth. The structure response is characterized: verifying the dynamic effects of the structure for the semi-submerged condition, comparing the structure response against the survival condition, varying the percentage of weight carried by the legs during the transient phase. Cases are evaluated and reported where hull displacements, base shear, vertical forces and overturning moment are reported for every case.

The study resulted in some interesting findings and discernments. It is uncovered that the semisubmersible condition represents a high risk operational condition that should be considered during the vessel design to reduce operational risk.