Abstract:
A method of stabilizing a floating body comprises applying to the body a vertical force so as to maintain the body in an equilibrium position which is spaced vertically from the normal floating position of the body, in which the vertical force comprises a limited vertical force applying means acting on the body in one vertical direction and a flexible tension member applying tension to the body or the force applying means in the opposite vertical direction, the tension member having a tension less than the said limited vertical force.

Description:
This invention relates to a method and apparatus for stabilizing a floating body. 
     BACKGROUND TO THE INVENTION 
     It is known to use ballast tanks in vessels for stabilization of pitch and roll motions of the vessel. It is also known to use anchor lines or tension leg systems for positioning and dynamic stabilization of vessels. Severe strain can be put on the vessel by use of such tension lines, however, if the vessel experiences high wave-induced motions and damage may result. 
     BRIEF SUMMARY OF THE INVENTION 
     According to the present invention there is provided a method of stabilizing a floating body, which comprises applying to the body a vertical force so as to maintain the body in an equilibrium position which is spaced vertically from the normal floating position of the body, in which the vertical force comprises a limited vertical force applying means acting on the body in one vertical direction and a flexible tension member applying tension to the body or the force applying means in the opposite vertical direction, the tension member having a tension less than the said limited vertical force. 
     The invention also provides apparatus for stabilizing a floating body, comprising means to apply a vertical force to the body so as to maintain the body in an equilibrium position which is spaced vertically from the normal floating position of the body, said means comprising a limited vertical force applying means acting on the body in one vertical direction and a flexible tension member applying tension to the body or the force applying means in the opposite vertical direction, the tension member having a tension less than the said limited vertical force. 
     By way of example, some embodiments of the invention will now be described with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows one form of vessel stabilization system, 
     FIG. 2 shows the stabilization system for a vessel installing a module on a platform jacket, 
     FIG. 3 shows the stabilization system for a vessel installing a module on a semi-submersible, 
     FIGS. 4 and 5 show a crane vessel carrying a module to and from a stabilized cargo barge, 
     FIGS. 6 and 7 show a modification to the stabilization system of FIG. 1, 
     FIG. 8 shows another form of vessel stabilization system, 
     FIG. 9 shows a crane vessel carrying a module to and from a stabilized cargo barge, 
     FIG. 10 shows a modification to the stabilization system of FIG. 8, 
     FIG. 11 shows a crane vessel carrying a module to and from a stabilized cargo barge, 
     FIGS. 12 and 13 show end and plan views respectively of another form of vessel stabilization system, and 
     FIGS. 14 and 15 show alternative forms of the stabilization system of FIGS. 12 and 13. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1 there is seen a floating body 1 which may, for example, be a barge. The body 1 has a positive buoyancy and may be a semi-submersible or submerged vessel. The surface of the water is indicated by 2 and the sea bed by 3. At sides or corners 4a, 4b, the body 1 is provided with stabilization control devices 5a, 5b. The control devices 5a, 5b comprise clump weights 6a, 6b which are suspended from the body 1 via flexible tension members 7a, 7b controlled by winches 8a, 8b. Clump weights 6a, 6b provide downward forces to act on the body 1. The flexible tension members 7a, 7b may be cables, wires, chains or the like i.e., be capable of transmitting a force by tension in the tension member, but not capable of transmitting on opposite force. Here, winch 8a incorporates a brake. Tension member 7b feeds from winch 8b over a pulley 9b having a stopper, e.g. chain. Clump weights 6a, 6b are engageable with surfaces remote from the body 1. Clump weight 6a is simply engageable directly with the sea bed 3. Clump weight 6b is engageable with the surface of a piled foundation or gravity base 10b on the sea bed 3. The base 10b includes a guide for lateral location of the clump weight 6b relative to the seabed. Here, the guide is in the form of an upstanding shaft 11b slidably engaging a hole 12b in the clump weight 6b. In addition, the base 10b includes snubber means 14 to absorb impacts of the clump weight 6b. Additional tugger lines and/or mooring lines 13 may be provided for the body 1. 
     The system is adjusted so that in the equilibrium position of the body 1, the clump weights 6a, 6b are partially resting on the sea bed, i.e., the tension in the tension members 7a, 7b is less than the full weight of the clump weights. Adjustment of the system may be realized by ballasting/deballasting the vessel and/or control of winches 8a, 8b. The equilibrium position of the body in this embodiment is the position at which the body floats in still waters with the clump weights partially resting on their engagement surfaces, and this is below the normal floating position of the body. 
     The control devices 5a, 5b serve as dynamic stabilizers for the body 1 by means of motion induced counter forces. When waves W induce roll or heave motions in the body 1, the corners 4a, 4b of the body move up and down. If a corner 4a, 4b of the body 1 moves down, it will be seen that the effect is to reduce the tension in the tension member 7a, 7b at that corner 4a, 4b as more of the weight of the clump weight 6a, 6b at that corner is taken by the sea bed, i.e., the downward force on the body 1 at that corner is reduced. If a corner 4a, 4b of the body 1 moves up, less of the weight of the clump weight 6a, 6b at that corner is taken by the sea bed 3 and there is more tension in the tension member 7a, 7b at that corner 4a, 4b, i.e., the downward force on the body 1 at that corner is increased. It will be seen that the tension in the tension members 7a, 7b can vary between theoretical limits of zero, when the clump weights 6a, 6b are fully supported by the sea bed, and the full weight of the clump weights 6a, 6b, when the clump weights separate from the sea bed. In practice, the size of the clump weights 6a, 6b  is desirably chosen to suit the expected amplitudes of vertical motion of the body 1 so that the clump weights are not completely lifted up or set down. The elasticity in the tension members 7a, 7b and snubber means 14 ensures that the clump weights 6a, 6b are not completely lifted up or set down for small vertical displacements of the body 1. The body 1 may be provided with only a single stabilization control device, but this would only be able to control wave induced heave motions, i.e. vertical motions of one point on the vessel. Preferably, therefore, the body 1 is provided with control devices at a number of horizontally spaced locations, e.g. at each side or corner of the body, so that wave induced roll motions as well as heave motions are stabilized. 
     In FIG. 2 there is seen a variation of the system shown in FIG. 1. Here, the clump weights 6a, 6b are engageable with surfaces on a fixed offshore structure 20, such as the platform jacket shown. The body 1 here is an installation vessel for setting down or removing a module 21 from the platform jacket 20. The clump weights 6a, 6b are located laterally by means of guides on the platform jacket 20, in similar fashion to the stabilization control device 5b described above. Here, to maintain the required tension it is necessary to be able control the length of the tension members 7a, 7b in order to compensate for ballasting/deballasting of the vessel 1 during transfer of the module 21. 
     In FIG. 3 there is seen another variation of the system shown in FIGS. 1 and 2. Here, the clump weights 6a, 6b are engageable with surfaces on a separate floating body 30, in this case a semi-submersible. The body 1 here is again an installation vessel for transferring a module 21. The clump weights 6a, 6b are again located laterally relative to the semi-submersible 30 by means of guides. Here, the guides are in the form of stabbing pins 31 which extend downwardly from the clump weights 6a, 6b and which are engageable in holes in the semi-submersible. Here again, to maintain the required tension it is necessary to be able to control the length of tension members 7a, 7b to compensate for ballasting/deballasting of the vessel 1 during transfer of the module 21. 
     FIGS. 4 and 5 show examples of the stabilization system in use on cargo barge 1 with a semi-submersible crane vessel 40 transfering a module 41. A fender 42 is interposed between the cargo barge 1 and crane vessel 40. Winches 43 control tension members 7. Clump weights 6 in FIG. 4 are engageable on the surface of a base unit 44 on the sea bed. The base unit 44 has guides for lateral location of the clump weights 6. Clump weights 6 in FIG. 5 are engageable on the surface of a purpose built bracket 45 on the crane vessel 40. Guides are preferably also included for lateral location of the clump weights 6 relative to the crane vessel 40. The winches 43 are controlled as necessary to adjust the length of the tension members 7 during transfer of the module 41 to maintain the required tension. Control of the winches 43 may be realized manually or by computer to keep the tension within required limits. 
     In FIGS. 6 and 7 there is seen a modification to the stabilization control device. Here, the tension members 50 include a section 51 which is relatively inelastic. The section 51 may for example be a riser pipe or tube, i.e. have considerably greater stiffness than a chain, cable, wire or the like 52 which makes up the remainder of the tension member 50. This increased stiffness is necessary in deeper water to avoid too high strain in the tension members 50 so that the motions of the vessel 1 are not stopped or reduced. The stiffness of the tension members must be higher than the hydrostatic stiffness of the vessel. Again, winches 53 are provided to control adjustment of the tension members 50 as necessary to maintain the required tension. In FIG. 6 there is seen a clump weight 54 which is engageable with the surface of a gravity base 55 on the sea bed, and which is located laterally by guide means 56 on the gravity base. In FIG. 7 and also in FIG. 6 there is seen an alternative, which is a weighted cone 57 at the base of the riser 51 engageable on the sea bed. 
     In FIG. 8 there is seen an alternative form of vessel stabilization system. Again, body 1 has a positive buoyancy. Here, buoyancy tanks 106a, 106b are held by flexible tension members 107a, 107b to be engageable with surfaces beneath the floating body 1. 
     Buoyancy tanks 106a provide upward forces to act on body 1. The tension members 107a, 107b may again be cables, lines, chains or the like as described earlier and be anchored to the sea bed by a gravity base 110, for example, or a piled anchor 111. Snubber means 108 are preferably provided to absorb impacts between the body 1 and buoyancy tanks 106a, 106b. Again, additional tugger lines and/or mooring lines 13 may be provided for the body 1. Here, no winches are needed to control the tension members 107a, 107b. Instead, the necessary tension is maintained by controlling as necessary the draft of the body 1, i.e. by ballasting and deballasting the body. It will be seen that the upthrust from the buoyancy tanks 106a, 106b on the body 1 can be adjusted between theoretical limits of zero, when the body 1 separates from the buoyancy tanks and the full upthrust of the buoyancy tanks, when there is zero tension in the tension members 107a, 107b. The system is arranged so that in the equilibrium position of the body, the upthrust on the body from the buoyancy tanks is less than the buoyancy force of the tanks. Here, the equilibrium position of the body will be spaced above the normal floating position of the body. As in the previous system, in practice the buoyancy tanks are preferably arranged to be able to compensate for wave induced body motions without reaching the limits of tension. A single control device can stabilize vertical motions of only one point on the body and therefore a number of such devices are preferably provided at various locations, e.g. at each side or corner of the body. In this way, wave induced pitch and roll motions of the body as well as heave motions can be stabilized. It will be appreciated that tension members 107a, 107b may be anchored to a fixed structure, instead of the sea bed as shown in FIG. 8. 
     In FIG. 9 there is seen a modification of the alternative form of stabilization system of FIG. 8. Here, the buoyancy tank 106 is held by tension member 107 to a purpose built bracket 112 on another floating body 113, which in this case is a crane vessel for transferring a module 116. The tension member 107 passes under a pulley 114 on the bracket 112 and to a winch 115. The necessary tension in the tension member 107 to stabilize body 1 is maintained by adjusting the winch 115 and/or ballasting/deballasting the body 1. A fender 117 is interposed between the body 1, which here is a cargo barge, and the crane vessel 113. 
     In FIG. 10 there is seen another modification in which a single float 120 is used. The float 120 is anchored by tension members 121 to a base unit 122 which is fixed to the sea bed by piles 123. Snubber means 124 are interposed between the body 1 and float 120 to absorb impact forces. Openings 125 are preferably provided in the float 120 to reduce wave loading. The required tension in the tension members 121, i.e. giving the necessary degree of upthrust on the body 1, is controlled by adjusting the draft of the body 1 by ballasting/deballasting the body. 
     In FIG. 11 there is seen a crane vessel 200 transfering a module 201 to or from a stabilized cargo barge 202. The cargo barge 202 is stabilized by means of a clump weight 206 which is engageable with the deck of the barge. Stabbing pins 231 on the clump weight 206 are engageable in suitable holes on the barge to provide lateral location for the clump weight relative to the barge. The clump weight 206 is suspended by a tension member 207, e.g. chain, cable, wire etc. passing over a pulley 209 to a winch 208. The magnitude of the downward force exerted on the barge 202 by the clump weight 206 is controlled by adjusting winch 208 and/or ballasting/deballasting barge 208 during lifting or setting of module 201. As in the previously described stabilization systems, it will be seen here that upward or downward movement of the barge 202 automatically gives rise to an opposed, counterbalancing force from the clump weight, the magnitude of which force is governed by the tension in the tension member. 
     In FIGS. 12 and 13 there is seen a stabilization system for a crane vessel 300 using an assisting vessel 301. Fenders 302 are interposed between the vessels 300 and 301. The assisting vessel 301 has water ballast 303 and counter ballast 304. Clump weights 305 are hung by tension members 306, e.g. chains, from jibs, booms, derricks or the like 307 on the crane vessel 300. FIG. 13 shows the crane vessel 300 being used to transfer a module 308 using shearlegs 309. Additional tugger lines and/or mooring lines 301 may be provided for the crane vessel 300. Tension in the tension members 306 may be adjusted by means of winches and/or by ballasting/deballasting of the assist vessel 301. The stabilization system is able to control vessel motions induced by beam waves, indicated by arrow 311, because there is an effective broadening of the beam of the crane vessel 300. 
     The Assisting vessel 301 may be positioned in head waves with its stern connected to the vessel 300, as is shown in dotted lines in FIG. 13, for reduction of vessel motions. 
     In FIG. 14, the same principle is used with an assisting vessel 401 which is anchored by tension members 402 to the sea bed. 
     In FIG. 15, the same principle is again used, but here the &#34;assisting vessel&#34; is a fixed structure 501 on the sea bed.