Incorporation of Multi-Member Substructure Capabilities in FAST for Analysis of Offshore Wind Turbines

Abstract

Abstract The Fatigue, Aerodynamics, Structures, and Turbulence (FAST) code, developed bythe National Renewable Energy Laboratory (NREL), is an aero-hydro-servo-elastictool widely used for analyzing onshore and offshore wind turbines. This paperdiscusses recent modifications made to FAST to enable the examination ofoffshore wind turbines with fixed-bottom, multi-member support structures(which are commonly used in transitional-depth waters). This paper addresses the methods used for incorporating the hydrostatic andhydrodynamic loading on multimember structures in FAST through its hydronamicloading module, HydroDyn. Modeling of the hydrodynamic loads was accomplishedthrough the incorporation of Morison and buoyancy loads on the supportstructures. Issues addressed include how to model loads at the joints ofintersecting members and on tapered and tilted members of the supportstructure. Three example structures are modeled to test and verify the solutions generatedby the modifications to HydroDyn, including a monopile, tripod, and jacketstructure. Verification is achieved through comparison of the results to acomputational fluid dynamics (CFD)-derived solution using the commercialsoftware tool STAR-CCM+. Introduction Interest is growing in offshore wind turbines. Many offshore-wind-turbinesupport structures are being developed by modifying onshore wind turbinefoundations, or oil and gas (O&G) industry structures. The offshore windturbine support structures can be classified in three categories according tothe water depth, as shown in Figure 1. In shallow water, where the water depthis less than 30 meters, monopiles and gravity bases that extend to the seafloor are used. In transitional depth, where the water depth is between 30 and60 meters, new technologies are being created or adapted from the O&Gindustry, including jacket and multi-pile structures, which also extend to thesea floor. When the water depth is greater than 60 meters, a rigid structurefixed to the sea floor is no longer economical, and floating platforms may berequired. This paper is focused on analyzing the support structures intransitional water depths.