Abstract

This paper discusses an on-site offshore installation concept of fully assembled 10 MW offshore wind turbines using a catamaran with a low height lifting system. The concept involves: a) loading three wind turbines vertically on the catamaran, b) transporting the turbines to the operating site, and c) installing them on top of floating spar substructures. Preliminary analysis found that the excessive horizontal relative translational response of the vessel and the substructure can hinder the installation operation. A mechanical coupling using fenders and wires is introduced and implemented between the catamaran and the spar to reduce the relative dynamic horizontal response. Global dynamic analyses are performed under various wave conditions to investigate multibody systems' critical response and assess the mechanical coupling effect. We show that the mechanical coupling reduces the relative horizontal displacement by about ten times. However, the relative motion reduction is most effective at wave periods, Tp, less than 12 s. Therefore, further improvement and optimization of the mechanical coupling method is suggested.

Introduction

Efforts to reduce greenhouse gases and prevent global warming are continuing worldwide. Various countries in Europe and Asia have actively made the budget investment and R&D plans to transition from fossil fuels to clean energy (European Commission, 2020; Government of the Republic of Korea, 2020).

In order to achieve the CO2 emission target in the Paris Agreement, IRENA (2019) suggests that wind energy must have a capacity of 1787 GW onshore wind, 228 GW offshore wind by 2030, and 5044 GW and 1000 GW onshore and offshore wind energy by 2050, respectively. Díaz and Soares (2020) show that continuous technological development will increase wind turbines' size and create wind farms in deep waters far from shore. Therefore, wind energy's global trend will shift from onshore wind to offshore wind and further to floating offshore wind.

Hywind Scotland, the world's first floating offshore wind farm (30 MW), was built by Equinor in 2017 (Equinor, 2017). Five 6 MW wind turbines are assembled and installed on floating spar substructures near the shore and the complete units are wet towed to the operation site. The installation cost increases depending on the distance and the number of wind turbines. Therefore, slow towing speed of weather sensitive tall structures can be a significant factor to consider when reducing installation costs. As of 2018, the cost of offshore wind power installation is about three times that of the onshore wind power installation cost, and the cost is suggested to be reduced by about 50% in 2030 with continuous technological development (IRENA, 2019).

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