Offshore Wind

A Comprehensive Guide to Successful Offshore Wind Farm Installation

Kurt Thomsen

• Publisher: Elsevier
• Year: 2011
• ISBN:
  9780123859372
• Topic: Technology & Engineering
• Subtopic: Renewable Power Resources
• Index: Technology & Engineering

This book is the first-ever roadmap to successful offshore wind installation. It provides a ready reference for wind project managers, teaching them how to deal with complications on-site, as well as for financers, who can utilize the text as an easy guide to asking the pivotal questions of petitioning wind project developers. These developers' planning stages will be improved by the book's expert advice on how to avoid wasting money by scoping out and mitigating potential problems up-front. Wind turbine manufacturers will benefit from insights into design optimization to support cheaper installation and hauling, thereby incurring lower project costs, and helping developers establish a quicker route to profitability. The book sheds light not just on HOW to solve a particular installation difficulty, but delves into WHY the problem may best be solved in that way.

• Enables all stakeholders to achieve cheaper, faster and safer offshore wind projectrealization
• Explains the different approaches to executing on- and offshore projects, highlighting theeconomic impacts of the various financial and operational choices
• Provides practical, proven advice on how tough challenges can be overcome, using real-life examples from the author's experiences to illustrate key issues

Chapter One. What Is an Offshore Wind Farm?

Before we go any further in this book, it is probably a good idea to describe what exactly makes up an offshore wind farm. It will also be helpful to have a quick overview of many, if not all, of the technical terms used in this book so it will be easier to understand the descriptions.

Some people may think this superfluous; however, the year after installation of the Kentish Flats a letter to the editor of The Times, sent by an elderly couple, demonstrated to me very clearly that not everyone understands the concept. This couple was probably a bit extreme, but in essence they complained that they could not walk the beach anymore simply because the wind generated by the turbines made it impossible for them to keep their balance. So, just to make it clear—a turbine does not generate wind by using electricity; it generates electricity by using the wind as the energy source to turn the rotors. This in essence is what the editor wrote back, as kindly as possible, to the couple.

Anyway, now to get serious and down to the basics of wind energy. In this book, we are looking at the offshore wind farm installation and the offshore wind farm—or a wind farm in general; it can be described in the following manner.

A wind farm is made up of a number of wind turbines. Any wind farm will look more or less like another; Figure 1.1 shows a typical one. The wind farm is situated in an area with relatively shallow water not too far from the coastline and, of course, in an area where the mean wind speed is favorable.

Figure 1.1 Aerial photograph of an offshore wind farm.

A wind turbine consists of three main components—basically what can be seen from the outside:

• The tower, which is two or more steel tubes bolted together

• The nacelle, or the generator house, which is fitted on top of the tower

• The rotor, which consists of three blades connected to a central hub on the nacelle

Of course, the turbines cannot stand on the water (or the ground if onshore, for that matter). They need a foundation, as shown in Figure 1.2. It can just barely be seen sticking out of the water.

Figure 1.2 Components of a wind turbine.

When a turbine is onshore, this is easy because the foundation is a concrete slab that is heavy enough to create sufficient moment and holding force to withstand the movements and bending moments of the wind acting on the turbines. When a turbine is offshore, the outcome is the same, but there are four additional factors to consider when designing the foundation:

• Water depth: The foundation must have an additional free-standing column.

• Wave load: The waves induce more loads and bending moments on the foundation than the turbine itself.

• Ground conditions: The foundation will not necessarily be fixed to the seabed immediately, but it may easily require additional depth before the ground has any bearing capacity due to the composition of the seabed.

• Turbine-induced frequencies: The turbine acts and counteracts the wave load, giving new and possibly higher loads to the foundation when the waves are added, which must be taken into consideration.

The design of offshore foundations for wind turbines is an entire science unto itself and is not covered in this book. However, the subdivision of the four most commonly used offshore wind turbine foundations are described in the following sections, along with the pros and cons of each type.

Monopile

A steel tube of a large diameter (4–8 m) that is driven into the seabed using a large hydraulic hammer is known as a monopile. It is able to stand upright because of the friction of the seabed on the sides and not having any vertical ground pressure on it. A monopile is commonly used in hard to semihard seabed conditions up to a water depth of around 25 m (Figure 1.3).

Figure 1.3 A monopile being lifted out of the transport vessel. Courtesy of A2SEA.

Gravity Base

A gravity-based foundation is a very heavy displacement structure usually made of concrete (Figure 1.4). The gravity base, which applies vertical pressure to the area below, stands on the seabed. The base is usually 15 to 25 m in diameter, and all of the forces and bending moments are transported through the base of the foundation. Typically, a gravity base is used on semihard, uniform seabed conditions and at shallower water depths, although a couple of projects (Thornton Bank, for example) have been installed in much deeper water using a gravity-based foundation.

Figure 1.4 Principle of the gravity-based foundation.

The size and weight of the foundations (from 1500–4500 tons) make transport and installation cumbersome, and it is worth noting that the seabed must be prepared by dredging and backfilling material in order to install the foundation. So while concrete is cheap to build, it is extremely expensive and time consuming to install. Therefore, the gravity-based foundation is not the preferred solution.

Tripod

A tripod is a steel tube that protrudes out of the ocean surface (Figure 1.5). Under water there is a three-legged foundation; each “leg” ends in a pile sleeve, where an anchor pile is driven into the seabed to hold the foundation in place. The advantage of the tripod is that while the area penetrating the wave zone is as small as a monopile, because it is a single tube, it is spread out like a camera tripod on the seabed. Thereby it provides enormous stability against bending moments.

Figure 1...

Table of contents

• Cover image
• Table of Contents
• Front matter
• Copyright
• Dedication
• Preface
• Acknowledgments
• About the Authors
• Chapter One. What Is an Offshore Wind Farm?
• Chapter Two. Obtaining Permits for Wind Farms
• Chapter Three. Project Planning
• Chapter Four. The Basic Organization
• Chapter Five. Project Preparation
• Chapter Six. Project Execution
• Chapter Seven. Interface Management
• Chapter Eight. Health, Safety, and Environmental Management
• Chapter Nine. Work Vessel Coordination
• Chapter Ten. Logistics Solutions
• Chapter Eleven. Commonly Used Installation Methods
• Chapter Twelve. Vessels and Transport to Offshore Installations
• Chapter Thirteen. Basic Information about Ports
• Chapter Fourteen. Project Criteria
• Chapter Fifteen. Transporting Wind Turbines
• Chapter Sixteen. Deployment Strategies
• Chapter Seventeen. Repairing Offshore Wind Farms
• Chapter Eighteen. Environmental and Other Issues
• Conclusions About O&M Transport Systems
• Final Thoughts
• Index