Dynamic behaviour of floating wind turbines - A comparison of open water and level ice conditions

Dynamic behaviour of floating wind turbines - A comparison of open water and level ice conditions

Curfs, C.P.M.

Metrikine, A.V. (mentor)

Mechanical, Maritime and Materials Engineering

Offshore & Dredging Engineering

Offshore & Dredging Engineering

2015-01-14

In the past couple of decades, the offshore wind industry has developed from applications in shallow water to ever deeper, more remote locations with harsher environments. In these progressively larger water depths, bottom founded support structures become no longer applicable and floating support structures could provide a viable alternative. Various studies have focused on floater designs for this application and multiple prototypes have been developed. However, no research has been done on the interaction of floating support structures with ice. In this thesis, the effect of level ice conditions on the dynamics of floating offshore wind turbines is studied. The MIT-NREL TLP is used as reference floater design. Based on the structure’s motions and mooring loads, a comparison is made between ice structure interaction and wave loading in harsh open water conditions. The combined floater and turbine arrangement is modelled in a rigid body approach, with three degrees of freedom (surge, heave and pitch). The system’s non-linear restoring stiffness is defined from contributions of its mooring system and hydrostatics. Hydrodynamic characteristics from fluid-structure interaction are derived with Ansys AQWA in frequency domain and transformed to time domain for the performed simulations. Wind and wave loading are covered, taking into account irregular wind velocity and wave height. Ice structure interaction is simulated for both failure of ice in crushing and bending modes. Ice crushing failure is simulated by the application of a Matlock-Sodhi model and is applied for vertical walled geometry. Ice loads from bending failure are considered for a sloping geometry at the waterline and are modelled by application of elastic beam- and friction-theory. Simulations of open water characteristics are performed for sea states with significant wave heights ranging from 2 to 14 meters. The results are compared to ice crushing for ice thickness ranging from 0.1 to 0.7 m and ice bending for ice thickness from 0.25 to 1.5 meters. From the simulations it is found that motion and mooring load maxima from ice bending loads for ice thickness up to approximately 1 m are in line with those for very harsh open water conditions. In the case of ice crushing, structure response is found to be far larger and more volatile, even for limited ice thickness. Crushing action of level ice with 0.2 m thickness will already cause larger mooring loads than the most extreme open water condition considered (with a 14 m significant wave height). On top of that, dynamic amplification issues can arise as a result of ice loading near the structure’s natural frequencies. This mainly poses issues for the high energy loading of crushing, where motions and force variations are considerable for loads near the structure’s pitch natural frequency.

Offshore floating wind turbine dynamics

http://resolver.tudelft.nl/uuid:f339482a-8e0d-4827-85af-c8cf05b4260f

Student theses

master thesis

(c) 2015 Curfs, C.P.M.