As is wet) known, such buoys are conventionally used as navigation markers, e. g. to mark deepwater channels or submerged objects. A further use for such buoys is in the extraction of oil or gas from the seabed. In such applications, the buoy may be employed for carrying equipment for use in control of sub-sea wells or other submerged equipment.

Typically such control buoys, used in offshore engineering, carry relatively small paybads, typicafiy 100 to 500 metric tonnes. The buoy may be employed to carry an umbilical for transmitting control signals or power or fluid under pressure to a sub-sea well.

Such buoys may be some 10 to 20 meters in diameter and contain within the hollow structure a diesel generator or other similar equipment whose function may be dependent, at least to some extent, on site specific requirements.

It will be apparent that a buoy in the open sea or even in inshore waters may be exposed to strong winds or currents or large waves, particularly in stormy conditions. In extreme circumstances, the buoy may even be inverted by the force of the waves, which can have serious effects on the equipment within the buoy. Moreover, the pitching motion of the buoy can cause problems with the umbilical at the point where it leaves the buoy structure, usually from the base. If the buoy does not move in harmony with the preferred motion of the umbilical, the umbilical can be subjected to large bending stresses which can result in premature wear or failure.

For atl the above reasons, it is therefore important to control and minimize the pitching motion of the buoy as it rides the waves.

According to one aspect of the invention there is provided a buoy comprising an upper float and a lower stabilizer configured and arranged to increase pitch inertia whilst having little effect on heave inertia.

Preferably the stabilizer comprises vertical plates which will be submerged when the buoy is moored in its location of use.

In a particularly preferred embodiment, the stabilizer comprises four mutually orthogonal plates. These plates may either directly abut the lower side of the float or to maximize their effectiveness, may be spaced away from the base of the float by a frame structure.

The float of the buoy may have chamfered bilges to minimize heave and to keep the natural period of heave short.

The stabilizer may be removable from the float of the buoy and may be mounted on a folding or removable frame. This assists in transporting the buoy to site and installing it.

A storm cover may be provided to avoid damage from rough seas.

Preferably the ratio of pitch period to heave period will be in the range 3 to 5, most preferably approximately 4, depending on the size of the buoy and the wave periods in the area where the buoy is to be employed.

For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which:

FIGURE 1 is a diagrammatic side elevation of a buoy according to a first embodiment of the invention ; FIGURE 2 is a cross-sectional view along the plane A-A of Figure 1; FIGURE 3 is a diagrammatic perspective view of a buoy according to a second embodiment of the invention; FIGURE 4 is a diagrammatic perspective view of a buoy according to a third embodiment of the invention; and FIGURE 5 is a diagrammatic perspective view of a buoy according to a fourth embodiment of the invention.

Throughout the various Figures, corresponding components are provided with corresponding reference numerals.

The buoy shown in Figure 1 has an upper hollow buoyant part or float 1 and a lower stabilizer 2 which depends from the base 3 of the float 1. The stabilizer comprises four triangular plates 4 which meet along the longitudinal central axis 5 of the buoy. The float 1 has chamfered bilges 6 and an upper storm cover 7 to allow waves to wash over the upper surface of the buoy. The hollow float 1 may contain a variety of equipment. If such equipment includes a diesel engine e. g. for driving an electrical generator, a vent pipe 8 is provided.

Figures 3 and 4 show two alternative embodiments of the buoy. In Figure 3, the stabilizer 2 comprises four trapizoidal plates 4 held away from the base 3 of the float 1 by a frame structure 9. The frame structure comprises four vertical members 10 each attached to an outer edge of a respective plate 4. In addition, the frame structure contains a central member 1 to which the inner edges of all four plates 4 are secured.

Figure 4 shows an embodiment in which the stabilizer comprises a plurality of plates 4 which depend directly from the base 3 of the float 1.

Figure 5 shows an alternative configuration in which the plates form an open

box-like structure. It is important for the box to be open, i. e. tubular, to avoid trapping a mass of water within, which would increase the heave inertia.

In each embodiment illustrated, the stabilizer provides relatively large vertically extending surfaces which thus provide a large resistance to lateral motion and thus generates a large pitchlroll inertia. On the other hand, as shown for example in the cross-sectional view of Figure 2, the stabilizer presents a minimal surface area when viewed in transverse section and thus provides a small resistance to vertical motion, and a low heave inertia.

It will be appreciated that the embodiment of Figure 3 provides a greatly increased pitch resistance compared with that of Figure 1, whilst making economical use of materials. The embodiment of Figure 4 will have a somewhat higher pitch resistance than the embodiment of Figure 3 but will require substantially more material and therefore is a more expensive construction.

Although not illustrated in the Figures, each plate 4 is preferably stiffened by diagonal ribs which may form a lattice structure.

All components of the buoy will in most cases be constructed of mild steel and will be treated in the conventional way to minimize corrosion from sea water. In general, the aim is to increase significantly the resistance to pitching and rolling whilst minimizing the need to add mass to the buoy. The stabilizer thus lowers the centre of added mass, lengthens the natural pitching period, increases the pitch damping but has minimal effect on the natural heave period. A typical value of the natural pitch or roll period will be 20 seconds, whereas a typical heave period will be 5 seconds. In other words, the pitch or roll period will be approximately four times the heave period. Although the preferred embodiments illustrated comprise four plates arranged in a cruciform configuration, other configurations are possible. The plates may be triangular, or quadrilateral. Moreover, a lesser or greater

number of plates could be employed. For example, three plates angled at 120° or six plates angled at 60° would be possible configurations. In the embodiment shown in Figure 5, the cruciform configuration of plates 4 is replaced by a tubular box section (as in a box kite) whilst achieving similar advantages.

The buoy is intended for use in water of depth 60 m or more, e. g. up to several thousands of metres depth.

The following claims are intended to embrace all such variants and modifications which will readily occur to those skilled in the art upon contemplation of the foregoing disclosure.