Effect of environmental modelling and inspection strategy on the optimal design of floating wind turbines

John Marius Hegseth,Erin E Bachynski,Bernt J Leira

doi:10.1016/j.ress.2021.107706

Abstract

In order to reduce design conservatism and consequently the cost of energy, appropriate and cost-optimal safety factors should be derived, in light of environmental load uncertainties and lifetime costs. In the present work, a linearized dynamic model has been used together with Monte Carlo simulations and a numerical design optimization procedure to evaluate the impact of the description of wind and wave loads on the fatigue reliability and optimal design of a 10 MW spar floating wind turbine. Trade-offs between design costs and inspection costs with different design fatigue factors (DFFs) have also been assessed. The analyses have been performed for a realistic wind park site, where an environmental model has been developed based on hindcast data. Considering stochastic turbulence intensity, wind-wave misalignment, wind directional distribution, and a two-peak wave spectrum reduced the long-term fatigue damage by approximately two-thirds along the fatigue-critical part of the support structure compared to the base model. Re-designing the tower and platform with the full environmental model resulted in 11% reduction in CAPEX. However, due to the applied design optimization procedure, consistent reliability levels were achieved along the tower length, which resulted in important system side effects for the total structural reliability. Trade-offs between CAPEX and OPEX were derived based on a probabilistic fracture mechanics model and reliability updating through inspections. The necessary inspection intervals to achieve the same accumulated reliability after 20 years of operation were identified with different DFFs, and cost-optimal safety factors were computed with different OPEX costs and interest rates.

Highlights

Floating wind turbines (FWTs) are considered a promising solution for wind energy harvesting in deep waters, but are currently too expensive to compete with other energy sources
Colone et al [2] considered stochastic turbulence for an offshore wind monopile without waves, and found that using the design value resulted in a 13% increase in damage-equivalent loads (DEL) for the mudline fore-aft bending moment compared to results using Monte Carlo simulations (MCS) with stochastic turbulence intensity
Probabilistic SN and fracture mechanics (FM) models were used to evaluate the fatigue reliability of the support structure, which considered reliability updating through inspections

Summary

Introduction

Floating wind turbines (FWTs) are considered a promising solution for wind energy harvesting in deep waters, but are currently too expensive to compete with other energy sources. Wind and wave loads are highly stochastic, and due to lack of site-specific data, or to limit the computational effort during the design process, uncertainties in the environmental loads are often replaced by safety factors and deterministic design values. Such design values will typically increase the conservatism, which translates into higher costs. The actual effect of turbulence on the fatigue damage for an offshore wind turbine, depends on the relative importance of the response from wind and waves, which varies with different windwave conditions, wind turbine concepts, and location of the hotspots in the structure. Because of the different natural frequencies of a FWT compared to its bottom-fixed counterpart, the effect of the wind modelling may be altered significantly

Methods

Results

Conclusion