Alternative Offshore Foundation Installation Methods
Authors: Spagnoli Giovanni , Weixler Leonhard 
Published: May 2013

According to the European Wind Energy Association (EWEA) in the first six months of 2012, Europe installed and fully grid connected 132 offshore wind turbines, with a combined capacity totalling 523.2 MW. Overall, 13 wind farms were under construction. Once completed these wind farms will account for 3,762 MW. Europe is therefore the industry leader since the initiation of offshore wind turbine development. Most of the projects have been limited to less than 30-m deep waters in the North and Baltic Seas. However, planned offshore wind farms in Germany will be located in water depths up to 40 m. Future offshore sites in the UK include water depths to about 65 m. Offshore wind farm systems today use three types of foundation: monopile structures, gravity structures or multi-pile structures. They are economic in relatively shallow water depths. Current monopile diameters range between 4 and 6 m, wheres as piles for jacketed structures are smaller (i.e. between 2 and 3m). They are relatively easy to install, for instance, in soft clayey soils or sandy layers. There are, however, several situations where the pile driving installation is not suitable. The department of maritime technologies of BAUER Maschinen GmbH already developed other offshore foundation installation methods based on its know-how in the onshore foundation engineering practice.

Introduction

According to the IREA report, offshore wind farms are at the beginning of their commercial deployment stage. Offshore turbines are designed to resist the more challenging wind regime offshore and require additional corrosion protection and other measures to resist the harsh marine environment. The increased capital costs are the result of higher installation costs for the foundations, towers and turbines, as well as the additional requirements to protect the installation from the offshore environment. The most obvious difference between onshore and offshore wind farms is the foundations required for offshore wind turbines. These are more complex structures, involving greater technical challenges, and must be designed to survive the harsh marine environment and the impact of large waves. All these factors and especially the additional costs of installation mean they cost significantly more than land-based systems. Besides, Moving offshore will allow the use of very large wind turbines capable of supplying typically 3.5 MW (although this will probably increase with time), installed in farms of 50 or more turbines. In contrast to typical oil and gas structures used offshore, for a wind turbine the foundation may account for up to 35% of the installed cost (Byrne and Houlsby 2003). These structures will be large; the turbine hub for a proposed 3.5 MW machine is expected to be some 90 m above the sea floor, with the rotor diameter likely to be of the order of 100 m. Initially the structures were installed in relatively shallow water (5-20 m in depth). While installing structures offshore is hardly novel, these structures are different from typical offshore structures (usually oil and gas structures) in two respects, both related to the applied loads on the structure and hence on the foundations (Byrne and Houlsby 2003). According to EWEA 270 foundations (141 or 109% more than the same period last year) were installed during the first six months of 2012 in 10 wind farms: Thornton Bank 2 (Belgium), Lincs, London Array, Sherigham Shoal, Gwynt y Môr, Teeside (UK), Anholt, Avedore 2 (Denmark), BARD Offshore 1 and Riffgat (Germany). Currently in Germany foundations depth ranges between 20 and 40 m, according to the BSH (Federal Maritime and Hydrographic Agency) at the current state. Turbine sizes range betwenn 3 and 5 MW.