Offshore Power Plant Foundation

The reason why the whole wind power production is not yet offshore is the price of:

– The concrete caisson foundation or alternatively The mono pile type rammed into the seabed

– The offshore crane day rate for lifting the tower, engine room and rotor – The long distance cabling into the seabed

– The operation and maintenance

The construct selection started from many possibilities from a quite small foundation using the weight or pressure of water. In practise the foundation will be fixed watertight to the sea bottom. This is possible on a rocky or a concrete area and smooth bottom so that the foundation can be fastened like a suction pad to a window. This was rejected because the sea bottom seldom has the required conditions and because of the need for watertight control.

A second idea was to raise the ready assembled wind power station with the help of a barge and levers as well as wires. This is complicated technically and therefore economically impossible.

A further idea was to fill the tower with water to get more load to keep the turbine vertical with a smaller and cheaper foundation. The negative point is that the centre of gravity rises and on a sandy or weak bottom there is a greater possibility that the turbine will fall down (this possibility is also in Appendix 1).

Figure 32. The selected idea for offshore wind turbine and foundation (Satagrafia 2001).

Figure 32 shows the selected offshore turbine and foundation (Figure 24a). The con-struct optimises the given 20 parameters preserving the features, and minimises the cost by maintaining:

– A sufficient bending / holding moment – A sufficient floating features

In this construct the foundation material is steel. This facilitates:

– building the foundations round the world and transporting them with barges (small picture top right corner)

– floating the foundations to the assembly place, for example a yard, harbour, etc.

– lifting the tower, engine room and rotor onto the top of the tower by crane on land (next picture down)

– floating the ready-assembled wind power plant to the site (next picture down) – sinking the foundation and anchoring the power plant to the sea bottom at the site

with assistance ropes or wires at the top and bottom of the turbine keeping the turbine upright during the sinking procedure (bottom right corner)

– in the case of bigger service or repairs, the possibility of towing the power plant back to the harbour.

The selected idea must be very simple for it to be economically viable. The idea is presented in Appendix 1 and Figure 33 (patent FI 107 184). Another development is a collar to protect against possible ice. Inside the foundation bottom is ballast, e.g.

concrete. To minimise the amount of steel, the ballast is used as part of the stiffening element.

Figure 33. The selected idea for offshore wind turbine and foundation (a patent principle drawing).

In Figure 33, Fig.2 (number 2 presents foundation, 3 the tower, 6 and 7 are ballast tanks) is presented the assembly work in the harbour. It is possible that the turbine stays on the bottom of the harbour during the work (in practice a mobile crane is needed to reach about 100 m lifting height). An example of alternative technical details are also presented in Figure 40. In Fig. 3, 4 and 7 (number 4 presents transport vessel, 5 gripping device) there is a second alternative: a gripping device keeps the turbine upright during the transportation and sinking operation. The foundation keeps it floating and the gripping device allows movement up and down. In Fig. 5 and 6 (numbers 4, 11 present modified barge and 5 gripping device) there is a third alternative: a barge with a link keeps the foundation fixed to barge. In the sinking operation the steering device keeps the turbine upright.

The main benefit is to avoid using an offshore crane and assistance fleet on all three alternatives. The foundation cost is about three offshore crane days. The crane can operate only in summer time and on those days only 50 % of the time (Statement of Håkans 2002). The day rate will be charged for all days. The second benefit is that all the work will be carried out on land and not at sea. The negative point is for the first alternative the growth of the foundation diameter required to stay upright during the towing operation. In a alternatives II and III the foundation diameter is only the diameter needed to stay on the sea bottom. The negative points are the need for a special gripping device for the ship or special barge for transportation from harbour to site.

Oil rig foundation development is perhaps 20–30 years ahead of wind power plant foundations. The first oil was drilled on land, then the drilling happened in shallow water, then in deeper waters over 100 meters deep. After this come floating devices over waters of several hundred meters in depht. Today the drilling water depth is up to 2–3 kilometres. The foundations on the sea bottom are of the same 3 main principle foundations as in chapter 2.6.1. An alternative could be the “jack up” type with floating hull and jacking devices to lift the hull above the sea surface. There seems to be in the future also the possibility of floating wind mill foundations.

A mono pile alternative (Figure 24c) was rejected because the cost of foundation and assembly 0.7 M / 1.5 MW (Table 8 Bockstigen) and no possibility to move after installation without heavy cranes. The mono pile type only suits a certain type of sea bed.