The design of any offshore base, especially offshore wind turbine bases, is determined by a number of factors including: bed/geological conditions, water depth, imposed environmental loads (wind, wave, currents and ice), construction methodology and accidental loading.

When erecting offshore wind turbines and related structures, it is essential that they be securely anchored to the seabed to prevent toppling or unwanted movement.

A typical offshore wind turbine has a rotary diameter turbine on a tower. Conventionally, anchoring offshore wind turbines is achieved in two ways ; mono-pile and gravity foundations.

There is a need for alternative bases for anchoring offshore structures like wind turbines or bouys. It would be preferable if one such alternative combined low design and manufacture costs with reliable hydrodynamic properties at least equal to those of the currently available offshore bases.

According to one aspect of the present invention, there is provided an offshore base having two or more legs radiating from a central support structure, wherein the legs are wholly or substantially formed from plate steel.

Plate steel is easily workable and conjoinable, and so provides a much simpler method of construction compared to, e. g. tubular steel.

The central support structure may also be wholly or substantially formed from steel plate.

Alternatively, the central support structure is rolled or otherwise tubular as known in the art.

Although steel is considered to be the most-suitable material for construction, it is understood that'any other material with suitable properties could be used. Plate steel, as used herein, is. a common term in the trade and refers to pre-formed steel in a sheet-like form.

Preferably the central support structure is suitably designed to support an above water means such as a

tower. One such tower is preferably constructed so as to accommodate a rotary diameter wind turbine.

Preferably the offshore base has three legs or more, preferably five radial legs.

The legs may have any suitable size, shape or design. Preferably the legs have a box structure.

The legs may also be provided with stiffeners or other stiffening. The stiffeners may be internal and/or external of the legs, including inter-leg pieces. In one arrangement, stiffeners can be added between the distal ends of the legs to form a radial band.

Preferably the legs outwardly taper to their distal ends. Any taper need not be constant.

Preferably the legs are provided with piles that anchor the base to the foundation on which the base rests. Pile loads generally decrease with an increase in the length of leg and number of piles.

However the percentage decrease in load between consecutive numbers of legs reduces as the number of legs increases. For a typical 80m tower it is preferred to use not less than five legs and pile array diameters greater than or equal to 20m.

A large number of different combinations of legs/pile spacings are possible. If good ground conditions are available then a four leg base may be possible between 16-24m piles spacing.

Piles when used, could be located at any suitable point along the length of the leg. If necessary or desired an additional pile could be positioned beneath the central support structure. Such a central pile reduces deflection and stresses in the zone beneath the centre of the base.

The base may separately or additionally be anchored to the seabed by ballast, chains or other suitable means known in the art.

Generally, the pieces of plate steel are conjoined by being welded together, although other conjoining means knowing in the art could be used.

According to another aspect of the present invention, there is provided an offshore base having a central support structure and a radial circular or conical surround structure, the surround structure being wholly or substantially formed from plate steel, wherein the base includes one or more anchoring piles.

The piles provide the extra anchoring to create a stable structure.

Embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings in which: Fig. 1 is an elevated perspective view of a first offshore base according to the present invention;

Fig. la is a cross sectional view of the radial leg of Fig. 1 ; Fig. 1b shows an internalised view of a radial leg of the base in Fig. 1; Fig. 2 is an elevated perspective view of a second offshore base according to the present invention ; Fig. 2a is an internalised view of the radial legs of the offshore base in Fig. 2; Fig. 2b is a front view of the offshore base in Fig.

2 revealing surface details thereof; and Fig. 3 is an elevated perspective view of a third offshore base according to the present invention.

Referring to the drawings, Fig. 1 shows an offshore base (10) having a central support column, being a tower (12), from which project five radial box legs (14). Each leg (14) is spaced at an angle of approximately 72° from its neighbouring legs. The legs (14) are shaped such that their lower surfaces all lie on a plane and are anchored to the sea bed (not shown) by piles (16).

The legs (14) are formed from sections (20) of plate steel welded together to form a box section structure (15) shown in Fig la. Plate steel pieces (20) are easy to weld together, lowering manufacturing costs significantly. Corner

stiffening pieces (22) are added to increase the.. rigidity of box section (15).

Fig. 1b shows internalised views of two of the radial box legs (14), showing the plate steel plates (20) of the box legs with a series of vertical stiffening plates (24). The combination provides a very strong rigid shape for each leg (14) purely from plate steel pieces (20).

Dimensionally, the legs (14) are approximately 4m deep at the tower (12), tapering to 2m deep at the pile end. They are also approximately 2m wide. The plate (20) thickness in the legs (14) will be in the order of 15-25mm. Force concentrations may occur at the pile/leg and leg/tower interfaces. Stiffening pieces (26) have been added at the latter locations.

Piles (16) are attached to the legs (14) and provide tensile support for the base (10). The piles (16) may be attached to the legs (14) by welding, or alternatively the piles (16) may pass though a pile sleeve in each leg (14), thereby effectively pinning the base (10) to the seabed. The pile/leg interface is generally completed on site and as such will require underwater working. This connection must be capable of resisting bending moments, and there are several ways to achieve this. They include concrete casing of the pile within the leg structure, swaging of the leg structure around the pile, and welding/steelwork between the pile and the leg.

The piles (16) are sunk into the seabed using conventional means known in the art (e. g. drilling or driving). Further support, if required, may be provided by the use of ground anchors or ballast; indeed this may be preferable in fast or deep water.

Tensile safe working loads of 1500-3000kN can be achieved with ground anchors.

A second offshore base (30) is shown in Figures 2 and 2a. Again, the base (30) comprises a central support column (31) and five radial box legs (32).

The angle of decline of the legs (32) from the column (31) has a three staged decline, shown diagrammatically as angles X, Y and Z.

The internal structure of the legs (32.) is similar to those legs shown in Fig. 1b and lc, i. e. a basic box structure made of sheet steel with internal stiffeners. In addition, the second base (30) has connecting box arms (34) connecting the radial legs (32). The box arms (34) are also made of plate steel and have the same internal structure as the legs (32), with internal stiffeners (35) as shown in Fig. 2a.

Fig. 2b shows the external bolt projections (36) on the surface of the offshore base (30) of the internal stiffening (35), and anchoring piles (38).

A third offshore base is shown in Fig. 3. The third base (40) comprises a central support structure (44) and a radial conical surround structure (46). The

base (40) has five anchoring piles (48). The surround structure (46) is made from layers of sheet steel.

The embodiment of the current invention shown in Fig. 1 has been studied to provide a suitable base for an offshore wind turbine using an 8m significant wave height, water depth up to 15m and current speed of around 2m. The embodiment uses common jack-up, lifting and piling equipment, keeping pile diameters and length generally less than Im and 25m respectively. Generally the base will have five legs and pile array diameters around 20m in order to ensure that pile/ground capacities are not exceeded.

The present invention extends to using ground anchors in instances where the base diameter must be reduced and tension capacities cannot be achieved by tension piles. With the use of ground anchors and appropriate ground conditions, the base size could be reduced to 4 legs (16m diameter) or 5 legs (14m diameter).

The present invention provides a simpler form of construction for an offshore base, replacing the currently used tubular steel pieces, which have difficult welding arrangements and subsequent weak stresses, with a far simpler straight welding arrangement, with stronger. stress points, and/or more easily reducible stress points by the use of stiffeners in a manner well known to those skilled in the art.