Development of a technology type factor for jacket structures for offshore wind turbines in Rhode Island

Abstract

A marine spatial planning approach was used to locate possible sites for offshore wind development in Rhode Island. In connection with the Rhode Island ocean special area management plan, a technology development index was developed by Spaulding et al. (2010) to quantify the technical challenges of a particular site relative to its potential power production. A component of this index is the technology type (TT) factor, which quantifies the relative cost of a structure/foundation system as a function of environmental loading, water depth, and soil conditions. This paper presents the development of TT factors for jacket type support structures that is proposed for supporting the offshore wind turbines in Rhode Island Sound. TT factors were calculated by the total weight of the jacket and piles for a given water depth and soil conditions normalized by the weight of a reference structure. Jacket structure weights were determined by a frequency driven finite element analysis using the program ANSYS. The structure was subjected to hydrodynamic and quasi-static turbine loads from 50-year extreme wind and the 100-year extreme wave loading in Rhode Island Sound to determine the ultimate stresses in the structural members. Pile foundation weights were determined from an analysis of the axial capacity and the lateral capacity using commercially available pile design software. Jacket and foundation weights were calculated for water depth ranging from 30 m to 60 m and for three representative soil types. These analyses resulted in a Technology Type factor that varies with water depth according to a 2nd order polynomial, and also with soil type. The results were compared to the weights of two existing jacket structures in Europe and found to be in agreement with the upper bound estimate derived from this study. The Technology Type factor derived in this study was also in good agreement with the published actual cost (normalized) of jacket type foundations used in the United Kingdom.