Fatigue load estimation of a spar-type floating offshore wind turbine considering wave-current interactions

Wave-current interaction effects on structural responses of floating offshore wind turbines

The design and analysis of offshore wind turbines is a difficult task compared to onshore, due to the numerous load cases that have to be considered and the calculation work involved. Not only the amount of sea states to deal with at a specific site, but also the differences between sites and even between locations within a wind park should be taken into account for offshore wind energy. To ease this process, the use of the linearized frequency domain tool ECN TURBU, which is very fast compared to the commonly used nonlinear time domain tools, is investigated. TURBU is a fast fully integrated wind turbine design and analysis tool, which deals with aerodynamics, hydrodynamics, structural dynamics and control of modern three bladed wind turbines. The linearized wind turbine model is derived from geometric and material properties and site conditions. The system is then transformed to the frequency domain, useful for load calculation, stability analysis and control design. The wind, gravity, wave and water current loading are applied as input spectra to the wind turbine model. The results from load calculations can be analyzed both in the frequency domain (output spectra) and the time domain (simulations). Although frequency domain methods are common practice in offshore industry, the use of a fully integrated wind turbine design and analysis tool in the frequency domain is new to the wind energy sector. It requires a different design approach than conventional time domain tools, but creates opportunities as well due to quick feedback on the results during the design process. In this report it is shown how TURBU can aid the design of (offshore) wind turbines. A footprint of the wind turbine visualizing the effect of important design choices is constructed from output load spectra obtained with TURBU. Optimization using parameter variation can assist the designer in finding optimal settings for critical design parameters. Also the analysis of an offshore wind turbine with TURBU is addressed. It is used to identify the sea states contributing to fatigue, which reduces the number of sea states to analyse with nonlinear time domain tools. The results are compared to those obtained with a nonlinear time domain analysis tool (ECN PHATAS). Special attention is paid to the influence of the (linearized) hydrodynamics and the calculation method on the results.

Aero-elastic simulation of offshore wind turbines in the frequency domain

In the design of an offshore wind turbine the natural frequencies of the structure are of importance. In the design of fixed offshore wind turbine support structures it cannot be avoided that the first eigenmode of the structure lies in the frequency band of wave excitation. This study indicates that wave-current interaction should be taken into account for support structure design load calculations. Wave-current interaction changes the shape of the wave spectrum and the energy content in the wave frequency range of 0.2 – 0.35Hz. This is in the range of natural frequencies fixed offshore wind turbine structures are designed for. The waves are affected by the current in two ways. First there is a frequency shift, Doppler effect, for the fixed observer when the wave travels on a current. Second the shape of the wave is modified in case the wave travels from an area without current into an area with current. Due to wave-current interaction the wave height and wave length change. For waves on an opposing current the wave energy content increases, while for wave on a following current the wave energy content slightly reduces. Simulations of normal production cases between cut-in and cut-out wind speed are performed for a 5MW wind turbine in 20m water depth including waves with 1) a following current, 2) an opposing current and 3) no current present. In case of waves having an opposing current, the 1Hz equivalent fore-aft tower bending moment at the seabed is about 10% higher compared to load cases with waves only.

https://iopscience.iop.org/article/10.1088/1742-6596/524/1/012093/pdf

Fatigue loading on a 5MW offshore wind turbine due to the combined action of waves and current

The offshore wind energy industry is turning out ever larger numbers of offshore wind turbines every year. One way to achieve a costeffective design is to have a better understanding of the dynamic response of offshore structures. That is why it is getting more and more important to understand the dynamic behavior of soil and interaction between soil and piles. To avert damage to offshore foundation, it becomes necessary to identify and quantify the soil-structure interaction and the related damping effects on the system. In this study, a single pile is investigated by means of boundary integral equations. The pile is modeled as a solid or hollow cylinder and the dynamic excitation is applied vertically. The surface along the entire interface is considered rough and with full contact between the soil and the structure. Somigliana’s identity, Betti’s reciprocal theorem and Green’s function are employed to derive the dynamic stiffness of pile, assuming that the soil is a linear viscoelastic medium. The dynamic stiffness is compared for solid and hollow cylinders by considering different values of material properties including the material damping. Modes of resonance and antiresonance are identified and presented. It is observed that the absolute value of normalized dynamic stiffness is independent of Young’s modulus and Poisson’s ratio, whereas it is dependent on the soil’s damping.

Stiffness and Damping related to steady state soil-structure Interaction of monopiles

A simple pile platform was installed in 1982 approximately 60km offshore Hook of Holland, at the entrance of the Rotterdam harbour channel, in 32m water depth. The platform is being used for the continuous measurement of oceanographic and meteorological data and is normally unmanned. In the autumn of 1986 a long term measuring program of the structure was carried out in order to determine horizontal accelerations at deck level. Analysis of the data showed a strong relation between the magnitude of the horizontal accelerations and tidal variations, in particular when tidal current coincided with the direction of wave propagation. The obvious explanation of this phenomenon is that the change on frequency of encounter of the waves due to tidal current leads to an appreciable increase in wave energy input at the natural frequency of the platform. As a consequence, the fatigue life of the platform may be affected. Calculation of the fatigue life of the platform by means of a simplified spectral fatigue analysis confirmed the findings. The paper describes the relevant results of the in-situ measurements (including the estimate of the total damping), the simplified spectral calculation procedure (stress transfer function determined in the time domain in regular waves, linearisation, combination with scatter diagram) and the resulting fatigue life estimate. The effects of current are illustrated and discussed.

H Boonstra

Papers

Fatigue loading on a single pile platform due to combined action of waves and currents