The prediction of ultimate and fatigue demands for the design of offshore wind turbines (OWTs) requires accurate simulation of the dynamic response of OWTs subject to time-varying wind and wave loads. The magnitude of damping in an OWT system significantly influences the dynamic response, however, some sources of damping, such as foundation damping, are not explicitly considered in design guidelines and may increase damping significantly compared to commonly assumed values in design. Experimental and analytical studies have estimated the magnitude of foundation damping to be between 0.17% and 1.5% of critical, and this paper investigates how increased damping within this range affects load maxima and fatigue damage for a hypothetical 5MW OWT subjected to a variety of wind, wave, and operational conditions. The paper shows that increased damping effects the greatest percentage reduction of ultimate moment demands and fatigue damage when the OWT rotor is parked and feathered. In such cases, the aerodynamic damping is relatively low, allowing for additional damping from the foundation to account for a relatively larger proportion of the total system damping. Incorporating foundation damping in design guidelines may lead to more efficient structures, which is a crucial factor in overcoming the high cost barrier associated with offshore wind development.
Floating offshore wind energy has seen significant progress, evidenced by multiple demonstration projects and the first floating wind farm (Hywind Scotland). However, the high capital cost associated with floating wind development remains one of the primary hurdles in the industry's growth. In efforts to lower this cost, this paper investigates a novel shared anchor concept to reduce the total number of anchors and installations. Two different multiline geometries are studied-a 3-line anchor system and a 6-line anchor system. Simulations of the anchor forces are generated using National Renewable Energy Laboratory's OC4-DeepCwind semisubmersible floating system and 5-MW wind turbine, and the anchor forces of the 2 different multiline geometries are compared to those of a conventional single-line anchor geometry.Multiline anchor net force is calculated by vector summing the contributing tensions of the lines connected to the anchor. Results show that the behavior of the multiline anchor net force is governed by the connected line contributing the largest tension.Mean and maximum anchor forces are decreased in the 3-line anchor system and increased in the 6-line anchor system, relative to the single-line system. The average direction of multiline anchor net force is aligned with environmental load direction, and a wider range of multiline anchor net force directions are exhibited for wavedominated load cases. Direction reversal of the multiline anchor net force under constant wind, wave, and current direction is small and infrequent. These force direction results reveal that a multiline anchor must have axisymmetric strength.
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