Allowable Wind Turbine Set-down Impact

Abstract

The increased awareness of the negative consequences of man-made climate change has given rise to a fast growing offshore wind industry. Wind turbines become increasingly larger, and available locations for offshore wind farms are shifting towards deeper waters, where jack-up vessels can no longer be used for installation. Heerema Marine Contractors is planning to install pre-assembled wind turbine generators (WTGs) with the semi-submersible crane vessel ‘Thialf’. During installation, the set-down stage is a critical phase in which the WTG is placed on top of the substructure. Due to uncontrollable motions in the WTG (caused by ship motions and wind loads), a significant impact can occur, which can cause considerable loads at the interface between WTG and substructure. The objective of this research is to determine the allowable impact ensuring sufficient structural integrity. The allowable impact is most conveniently expressed in the kinematic parameters impact speed Vi and oblique impact angle α.

Based on structural integrity analysis, two requirements are formulated; occurring stresses in the system shall not exceed the yield stress, and the RNA accelerations are limited to 1.5 g. In order to relate the impact kinematics (Vi and α) to the occurring stresses and accelerations, an impact model is constructed. A special case describes the collinear impact, in which α = 0°. The WTG tower and substructure are modelled as rods describing the axial motions. The applicability of the collinear case is limited, as a zero oblique angle is improbable. Yet, this collinear case provides insights in stress wave reflections and system parameter influence. The oblique case appends the classical rod with a classical beam describing transverse motions. The model simulates the oblique impact by imposing a bending moment and shear force at the interface. A contact model is created to approximate the contact stresses at the interface and the force eccentricity which determines the imposed moment.

The results obtained for the collinear case, which are successfully cross-checked with a FEM model, demonstrate that a superposition of incident waves and reflections can result in maximum stresses about three times higher than the induced stress at the interface. The induced stress at the interface is approximately linear proportional to the impact speed Vi. The oblique case shows increasing maximum stresses for increasing oblique angle, as the imposed moment results in bending stresses adding up to the axial stresses. The contact model shows significantly higher stresses than the global model. By including an elementary material damping model, considerably lower stresses are obtained, emphasizing the importance of material damping. Regarding the stress requirement, the contact model approximates an allowable impact speed Vi of 0.47 m/s for the largest possible oblique angle. It is demonstrated that the maximum RNA acceleration requirement imposes even more stringent allowable impact velocity: for the maximum oblique angle, the allowable impact speed Vi is 0.09 m/s.

The constructed models provide approximations of stresses and accelerations in the WTG and substructure. Due to several underlying assumptions, particularly the one-way coupling simplification, it is unfortunately not concluded that the oblique impact model yields conservative stress approximations. Hence, it is recommended for successive research to improve the contact model. Considering the approximated allowable impact speed Vi of 0.43 m/s limited by the yield requirement, the acceleration-limited allowable impact speed Vi of 0.09 m/s shows that RNA accelerations currently limit the impact. Hence, acceleration mitigation in the RNA could allow for significantly higher impact speeds, enlarging the operational window.