PO082

Design & Fabrication of Post-piling Jacket Substructure for the South-West Offshore Wind Farm in South Korea

Daeyong Lee
POSCO, Incheon, Republic of Korea

Abstract

Development of three different classes of offshore wind turbines (3.0MW, 5.0MW, and 5.5MW) has been accomplished in South Korea, and 3.0MW & 5.0MW wind turbines will be installed at the Phase I (70MW) site of the South-West offshore wind farm starting May 2017. Results of design & fabrication of the post-piling jacket substructure for Doosan 3.0MW offshore wind turbine are introduced in this presentation.

Considering geotechnical and ocean-environmental conditions of the South-West offshore wind farm site, jacket is selected as the most competitive substructure system in this project. A four-leg jacket consisting of Warren-truss bracing system is chosen to reduce the total steel weight and minimize No. of welding points. And transition piece which has a structurally optimized platform stiffening detail is designed to connect wind tower to the jacket below. Slop of the jacket leg is adjusted to avoid a resonance problem among the structural components.

Usually, jacket substructures can be installed in the ocean by either pre-piling method or post-piling method. Considering ocean-environmental condition of the South-West offshore wind farm site and previous experience of the Korean construction engineers, post-piling method is selected for the installation of the designed jacket substructure.

In order to confirm safety of the jacket and transition piece, dynamic characteristic of the whole structural system, member stress levels for ULS, and member design life cycles for FLS have been checked. Also reviewed in this project are plans of fabrication for the jacket and transition piece.

Method

A post-piling four leg jacket substructure is selected for Doosan 3.0MW wind turbines which will be installed at the South-West offshore wind farm in South Korea. This presentation covers explanation of;

(1) why jacket is the most competitive substructure system at the southwest ocean in South Korea

(2) Pros and Cons of the post-piling method for jacket substructure installation

(3) plans of fabrication for the jacket and transition piece

This presentation also covers how to optimize the jacket and transition piece in terms of fabrication cost, and results of the cost analysis.

Results

(1) Considering geotechnical and ocean-environmental conditions of the South-West offshore wind farm site in South Korea, it has been found that jacket is the most competitive substructure system.

(2) Post-piling method is selected for the installation of jacket substructure mostly based on the ocean-environmental condition and previous experience of the Korean construction engineers.

(3) By adopting jacket structure consisting of Warren-truss brace, instead of X-truss brace, and transition piece which is structurally optimized, it has been found that total fabrication cost can be reduced more than 25%.

Conclusions

(1) Even though the water depth is shallow (less than 15m) at the Phase I (70MW) site of the South-West offshore wind farm in South Korea, jacket is more competitive comparing to monopile and suction bucket because of soft soil condition and lack of installation equipments.

(2) Due to lack of experience and settlement problem of the pile template plate, post-piling method is preferred for the installation of jacket substructure in South Korea.

(3) By adopting Warren-truss brace type for the jacket, No. of brace members and joint welding points can be remarkably reduced. The optimized platform stiffening detail can also minimize the total steel weight of the transition piece.

Objectives

(1) The delegates learn geotechnical and ocean-environmental site conditions of the South-West offshore wind farm in South Korea, and understand jacket substructure design/fabrication/construction environment and working practice in the same country.

(2) The delegates learn one possible way to design the jacket and transition piece that can minimize the steel weight and the fabrication cost.