GB2083788A - Arrangement in or relating to a floating body - Google Patents

Abstract

In a floating body (1) which is anchored to the sea floor (2) by means of a plurality of elongate, generally vertical tension members (3, 4), where at least some of the members are pretensioned to such a degree that the body is kept at a draught making its buoyancy greater than its weight a method for damping cyclic movements comprises that at least one of the tension members (4) is subjected to impose tension variations which are greater than the tension variations caused by the movements of the floating body (1) in the remaining tension members (3). An arrangement for performing the method comprises means (6-14) arranged to move at least one of the tension members' (4) attachment points in the body in the longitudinal direction of the member, preferably generally in step with the movements of the body and generally in the same direction as the body is moving with respect to the sea floor (2). <IMAGE>

Description

SPECIFICATION Arrangement in or relating to a floating body The present invention relates to a method and an arrangement for damping of cyclic movements in a floating body which is anchored to the sea floor by treans of a plurality elongate generally vertical tension members, wherein at least some of the members are prestressed to such an extent that the body is held at a draught which makes its buoyancy greater than its weight.

Such floating bodies have previously been suggested used as platforms for use in exploiting petroleum resources at sea and are often called tension tethered platforms. Such tension tethered platforms are meant to replace the conventional semi-submersible, catenary tethered platforms at water depths exceeding about 150 m. At water depths greater than this the tethering lines for semi-submersible platforms become very long, heavy and expensive and, furthermore, they have poorer ability of keeping the platform at rest under the influence of waves, wind and current. By comparison the tethering cables of the tension tethered platforms extend about vertically from the platform down to the seafloor and are subjected to constant pretension, so that the platform is held at a generally constant distance above the seafloor irrespective of waves and tidal water.Furthermore, the cables will be as short as possible so that they will not be loaded to any severe extent by their own weight.

However, the tension tethered platform will in use not lie entirely at rest even though the tethering cables may be pretensioned at any time. Due to their relatively great length, the tethering cables will have a certain elasticity and therefore behave as stiff tension springs. These springs will be somewhat stressed and relaxed when the platform is subjected to waves with corresponding movements of the platform. During normal circumstances, the movements of the platform will be much less than the wave heights and thus without appreciable significance, but under unfortunate circumstances, one could risk that the waves have the same frequency as the natural oscillation frequency of the platfrom, resonance being the probable result.Since the natural damping of the platform is relatively small, a resonance condition might lead to very large oscillation amplitudes which quickly could result in fatique of the tethering cables and, furthermore, necessitate very complicated and expensive motioncompensating equipment for riser pipes and other connections between the platform and well heads on the sea floor.

Thus, it is necessary to avoid the possibility of resonance. At relatively moderate depths this may be obtained by making the tethering system of the platform so stiff that the natural frequencies will lie considerably above the frequency of the waves of a certain energy to be expected.

It will be understood that at increasing depths and thereby greater lengths in the tethering cables, the total cross-sectional area of the tethering cables must be increased in order to give the desired stiffness. Such an increase of the cross-sectional area can be obtained by increasing the crosssectional area of the cables and/or increasing the number of cables. When both the length and the cross-sectional area of the cables are increased simultaneously, they quickly become so heavy that they will be difficult to handle and install. If instead one increases the number of cables, they will have to be mounted so close to each other that space problems will arise where the cables are to be attached to the platform.Furthermore, transversal vibrations which the cables will be subjected to f.inst. due to currents in the water, might make the cables impact against each other and cause damage.

Due to these circumstances, the construction and arrangement of the tethering cables will cause great problems and additional costs if the water depth is more than about 300 m.

Thus, one object of the invention is to alleviate said problems of previous suggestions for tension tethered platform structures. A further object of the invention is to provide a tension tethered platform structure which will be restrained from moving vertically in any appreciable degree when subjected to the usually occuring waves.

According to the invention, this is obtained by a method of the type mentioned by way of introduction, where the characteristic features are that at least one of the tension members is subjected to imposed tension variations which are greater than the tension variations occuring in the remaining tension members due to the motions of the floating body.

According to the invention, there is also provided an arrangement of the type mentioned by way of introduction, which is characterized in that the body is provided with means for moving at least one of the tension members' attachment point in the body in the longitudinal direction of said member, preferably generally in step with the motions of the body and generally in the same direction as the body is moving with respect to the sea floor.

In case of a threatening resonance condition, i.e.

when the prevailing waves tend to move the floating body at its own natural oscillation frequency, one can, by means of the method and arrangement according to the invention, provide periodical variations in the pretension of the tension tethering members so as to disturb the harmonic oscillating movements of the body, thereby counteracting the build-up of a resonance condition. By controlling the tension variations so caused, so that one or more of the tension members are tensioned more than the others when a wave seeks to raise the floating body and are slackened more than the others when the body is in a wave through, one can obtain variations in the pretension of the tethering members equivalent to and cancelling the forces with which the wave acts on the body, the result being that the body will be lying generally at rest.

Further advantageous features of the invention will appear from the dependent claims and from the following description of a preferred exemplifying embodiment of the invention shown schematically in the appended drawing.

The only Figure of the drawing shows a part of a floating platfrom structure 1 which is anchored to the seafloor 2 by means of generally vertical tension members in the form of cables 3 and 4. The cables are pretensioned so that the buoyancy of the platform is greater than its weight, i.e. that the water line corresponding to the water surface 5 is higher than itwould have been if the platform was floating freely.

The cable it attached to the platform via two hydraulic cyiinders 6 and 7. The hydraulic cylinder 6 is single acting and is denoted pretensioning cylinder. Its pressure side 8 is via a partition means 9 connected to a gas volume 10 which is sufficiently large with respect to the stroke volume of the pretensioning cylinder that the pressure on the pressure side 8 of the cylinder 6 is generally the same irrespective ofthe position ofthe piston 15.

The pretensioning cylinder 6 will therefore provide a relatively constant pretensioning force in the cable 4 even if the platform 1 should perform heave motions in heavy sea.

The other hydraulic cylinder 7 is double acting. Its two sides are connected with a controllable pump 11 which is reversible so as to pump hydraulic fluid in both directions. The pump 11 can furthermore act as a hydraulic motor when the hydraulic cylinder 7 acts as a pump. The pump 11 is driven by an electric motor 12, which also can act as a generator when the pump 11 acts as a motor. The pump-motor unit 11, 12 is controlled buy a control unit 13 receiving a signal from an accelerometer 14. The control unit 13 is arranged to intergrate the signal from the accelerometer 14 so that the pump-motor unit 11, 12 can be controlled both bythe velocity and position of the platform 1.

If the unit 11, 12 is controlled by the velocity of the platform, i.e. so that the cylinder 7 exerts a force on the platform 1 which is proportional to the velocity of the platform and directed in the opposite direction, one in reality increases the viscose damping of the platform. In this case, the combined pump and motor 11 will all the time be acting as a motor, whiie the combined motor and generator 12 will act as a generator, the gas reservoir 10 acting as a temporary storage for some of the energy taken out of the system. Such amplification of the natural damping will in many cases be sufficient to prevent that, in a resonance condition, sufficient energy is transmitted to the platform to cause dangerously high motion amplitudes.

If, on the other hand the pump-motor unit 11, 12 is controlled by the platform's motion or position with respect to the equilibrium position, itwill be possible to have the hydraulic cylinder 7 exerting a force on the platform 1 being at any time approximately equal and opposite to the excitation force from the waves. In this case, the sum of the external forces acting on the platform will be about zero, so that the platform will not move in any appreciable degree.

Since the platform is not moving, no energy can be transmitted to it from the waves, and therefore no resonance condition can occur.

In practice one would probably combine the two control modes for waves over a certain size in order to keep dimensions and capacity of the various components 7, 11 and 12 at a reasonable level. It will be seen that by suitable control one can obtain a net power output from the system.

The invention may be varyed in a number of ways within the frame of the following claims. Thus, the pretensioning cylinder 6 may be omitted and the cylinder 7 instead be sized in order to give both the average pretensioning force and the necessary variations in this force. However, this will mean larger capacity in the pump-motor unit 11, 12 and therefore this simplification will probably entail a more expensive system. The drawing shows only one cable being provided with an arrangement according to the invention. However, there is no reason why several cables could not be provided with such arrangements, and where the anchoring cables are grouped at different locations on the floating body, f.inst. at the corners of the platform structure, it would be advantageous to have an arrangement according to the invention for each group. Each of these arrangements may have their own control unit, but these may again be subjected to a superimposed common control unit for improved control of the rolling motions of the platform, possibly also its lurching motions.

It will be understood that the effect of the method and arrangement according to the invention will be highest when the tension variations to which the cable 4 is subjected occurs at the frequency of the platform's motions. However, these tension variations may also have a frequency, which departs from the motion frequency of the platform and still provide interruptions of the harmonic oscillations of the platform so that resonance is counteracted.

Claims (9)

1. A method for damping cyclic movements in a floating body (1) which is anchored to the sea floor (2) by means of a plurality of elongate, generally vertical tension members (3, 4), where at least some of the members are pretensioned to such a degree that the body is kept at a draught making its buoyancy greater than its weight, characterized in that at least one of the tension members (4) is subjected to imposed tension variations which are greater than the tension variations caused by the movements of the floating body (1) in the remaining tension members (3).

2. A method according to claim 1, characterized in that said imposed tension variations are exerted in step with the movements of the body (1).

3. A method according to claim 1 or 2, characterized in that said imposed tension variations are exerted generally in phase with the position variations of the body.

4. An arrangement in or relating to a floating body (1) which is anchored to the sea floor (2) by means of a plurality of elongate, generally vertical tension members (3, 4) where at least some of the tension members are pretensioned to such a degree that the floating body is held at a draught making its buoyancy generally greater than its weight, characterized in that the body (1) is provided with means (6-14) arranged to move at least one of the tension members' (4) attachment point in the body in the longitudinal direction of the member, preferably generally in step with the movements of the body and generally in the same direction as the body is moving with respect to the sea floor (2).

5. An arrangement according to claim 4, char actecized in that said means comprises a double acting hydraulic cylinder (7) which is connected to a reversible pump-motor-unit (11) which is driven, resp, drives a combined electrical motor-generatorunit (12).

6. An arrangement according to claim 5, characterized in that said units (11, 12) are connected with a control unit (13) which receives a signal from an accelerometer (14).

7. A device according to claim 4, 5 or 6, characterized in that it comprises a single acting hydraulic cylinder (6) the pressure side (8) of which being in communication with an accumulator means (9, 10) comprising a gas velum (10).

8. A method according to claim 1 substantially as hereinbefore described with reference to, and as shown in the accompanying drawings.

9. An arrangement according to claim 4 and substantially as hereinbefore described with reference to, and as shown in the accompanying drawing.