Offshore wind turbine design - Wind Power Generation and Wind Turbine Design - page 376

unique expertise need to be leveraged in order to gain better understanding of the
environmental footprint of offshore wind power plants.

5 Subcomponent design

The two key technology drivers for any offshore wind project design are:

• Achieving maximum turbine reliability to minimize the total cost of ownership;

• Designing cost-effective foundations.

These technology drivers stem from the offshore wind power plant environment
with offshore site location, extreme weather conditions and limited accessibility
throughout the year. Given these drivers, the total cost of ownership per kWh of
electricity produced is a top-level critical fi gure-of-merit. It is also constrained by
the popular demand for the placement of plants farther offshore. Signifi cant effort
on design trade-off is needed to identify the most promising confi gurations of the
following subsystems:

• Low cost foundations

• Advanced rotor design

• Advanced control, monitoring, diagnostic and repair system

• Reliable drive-train and power electric system

The critical challenges for each subsystem and component are discussed below.

5.1 Low cost foundation concepts

Controlling the cost of the offshore wind turbine foundations present unique chal-
lenges for the wind industry as coastal communities demand wind power plants
placement further offshore.
The current state of the art in the wind industry utilizes two types of foundations
depending on the site conditions and in particular the water depth:

1. Gravity base structure (GBS) type foundation is built onshore in a dry dock and
requires the transportation of a concrete caisson to the plant location either by
fl otation or by a barge. The concrete caisson is then fi lled with concrete or other
ballast materials and placed on the sea fl oor. The turbine tower is then erected
on top and the nacelle and rotor are placed on top of the tower. This foundation
type has two disadvantages: (1) it requires extensive seabed preparation and (2)
the caisson top height must exceed the sea depth so that its top surface facili-
tates the cheaper dry installation of the tower. This limits its application to the
wind industry rough defi nition of “shallow” as below 7.5 m water depth. Some
of the largest wind plant in the world are near-shore plants in shallow depths
and therefore utilized gravity base foundations.
Monopile: 2. Under the supported of the Danish government a group of two

Danish power companies and three foundation engineering fi rms studied the