Analysis of the Installation of a Series of Piles for Offshore Wind Turbine Foundations

Abstract

The offshore wind farm industry is evolving towards a developed stage, resulting in increasing competition between offshore contractors. The tendency of larger offshore wind farms in deeper waters challenges the cost effectiveness of offshore wind turbine support structures. Jackets and tripods are appropriate for deep waters. They are fixed to the seabed by foundation piles. Pre-piling is a technique that installs the foundation piles before the installation of the substructures, which realizes a cost reduction. The present research focuses on the offshore operations of pre-piling. Controlling the cycle time is essential to finish a project successfully. An analysis of reported times of the offshore activities is carried out by means of SPC charts. The data for five pre-piling projects are collected and analysed. The production time is divided into seven activity groups: (1) jack-up platform positioning, (2) piling template handling, (3) offshore pile transfer, (4) upending and stabbing of piles, (5) pile driving, (6) top of pile dredging and (7) survey of piles. The primary objective regarding this approach is to define behaviours, trends and bottlenecks, of which further investigation leads to a better understanding, control and possible improvement. Offshore operations are sensitive to weather conditions. Those have a major impact on the completion date of the project, which has on its turn a big influence on the overall cost of the project. A suitable weather window is defined by the weather limits and duration of the planned activity. The waiting on weather between activities is simulated by means of a monte carlo simulation to calculate the effects on the completion date. The monte carlo simulation is based on the cumulative frequency distributions of the weather window and downtime persistence. The ultimate goal is to increase production by making use of shorter weather windows. The combination of the analysis of the cycle time and the simulation of the waiting on weather provides a tool to quantify the benefits and costs of different methodologies in a benchmark analysis on production efficiency. Concrete examples are presented and discussed.