Validation of Applicability of Low Frequency Motion Analysis Theory Using Observation Data of Floating Offshore Substation

Abstract

In recent years, the social demands for the introduction of renewable energy are increasing, demonstration projects of floating offshore wind power generation are being implemented and planned around the world. In Japan, a demonstration project named Fukushima FORWARD (Fukushima Floating Offshore Wind Farm Demonstration Project) has been conducted since 2011. Fukushima FORWARD is carried out by the Ministry of Economy, Trade and Industry. The project is the world’s first floating offshore windfarm with a total capacity of 14 MW, including three floating offshore wind facilities and one floating offshore substation. In Fukushima FORWARD, Japan Marine United Corporation is in charge of floater part EPCI (Engineering, Procurement, Construction and Installation) of one floating offshore wind facility and one floating offshore substation. This floating offshore substation is installed in order to observe meteorological and oceanographic data and motion data as well as boosting the generated electric power. Since the installation in 2013 it continues to record various kinds of continuous data. The substation is an advanced spar type floater moored by four spread catenary mooring lines. In the design of the mooring system for offshore structure, the motion of the structure under environmental external force is very important. The motion of the moored floating structure is divided into wave frequency motion, which is a motion of a relatively short period, and low frequency motion caused by mooring restoring force and variable external force, both of which are important elements in the design. Among them, wave frequency motion is known to be accurately estimated by potential theory as a result of research on various types of structures. On the other hand, in addition to the existence of various calculation methods including time domain analysis, its statistical characteristic and applicability are entirely depending on the target structure. Also, observation data of low frequency motion have been very few. In this paper, long-term data observed at the floating offshore substation in Fukushima FORWARD was analyzed with focusing on low frequency motion and its statistical properties were clarified. Furthermore, we analyzed the low frequency wave force spectrum and motion by conventional low frequency motion theory using the wave drifting force calculated by the potential theory. And, we compared the calculated value with the analysis result of the observation data and validated the applicability of the simplified low frequency motion theory.

Dynamic Response Analysis of a Semi-Submersible Floating Wind Turbine in Combined Wave and Current Conditions Using Advanced Hydrodynamic Models

“…Some disagreements in the prediction of dynamic response occurred due to the strong nonlinear phenomenon, which was not included in the hydrodynamic model. The low-frequency motion of a spar-type substation was analyzed recently based on the measurement data [16] in the Fukushima demonstration project. However, the semi-submersible platform in the Fukushima demonstration project, which is much more complex than the spar-type platform, has not been studied yet.…”

A fully integrated optimization framework for designing a complex geometry offshore wind turbine spar-type floating support structure

“…To overcome the challenges that the highly promising spar-buoy floating platform type still faces, a few researchers have already worked on concepts for spar-type floating offshore wind turbine support structures, which have a reduced draft but still provide sufficient stability (Wright et al, 2019;Yoshimoto and Kamizawa, 2019;Zhu et al, 2019;Hirai et al, 2018;Yoshimoto et al, 2018;Yamanaka et al, 2017;Matsuoka and Yoshimoto, 2015;Lee, 2005). However, different approaches https://doi.org/10.5194/wes-2020-93 Preprint.…”

“…The advanced spar for the floating substation consists of three columns -or so called hulls -placed at the bottom, in the middle, and at the upper end (intersecting the water line) of the spar, so that the floating system is suitable already at around 110 m water depth, the motion performance is improved, and the cost for installation is reduced (Wright et al, 2019;Yoshimoto et al, 2018;Matsuoka and Yoshimoto, 2015). The Fukushima Hamakaze was initially using a similarly structured advanced spar, equipped with damping fins for stabilization in sway and heave direction (Main(e) International Consulting LLC, 2013; James and Ros, 2015); however, after some investigations and studies by Matsuoka and Yoshimoto (2015), finally the advanced spartype platform for the 5 MW wind turbine consists of just two large columns/hulls at the bottom and top end of the spar and, thus, is optimized with respect to the system restoring and motion performance, as well as the construction cost (Yoshimoto and Kamizawa, 2019).…”