GB2091192A - Stabilising marine vessels - Google Patents

Abstract

A vessel, for example a semi- submersible (10) is provided with means for stabilizing it against any or all of heave, roll and pitch. The stabilizing means comprises chambers (16) mounted on or in the vessel disposed to lie at least partly below the surface (A) of the water. A gyroscope, accelerometer, strain gauges or other means (18) are provided to detect the disposition of the vessel relative to a predetermined disposition, and produce an error signal in response thereto. The error signal controls means for altering the buoyancy of the chambers, for example by leading air thereinto under pressure or by allowing air to escape therefrom. <IMAGE>

Description

SPECIFICATION Vessels This invention relates to vessels having stabilizing means.

Various methods have been proposed for stabilizing vessels. One method is to provide the vessel with roll control tanks. Two interconnected tanks are provided on opposite sides of the vessel, the interconnection between the tanks including an energydissipating throttle. Pitch control may be similarly provided by means of two interconnected tanks spaced longitudinally of one another within the vessel. Such roll and pitch control devices are of limited effectiveness and, furthermore, they are incapable of providing stabilization against heave.

Another known method of attempting to stabilize against roll involves the use of movable wings at the bow end of a movable vessel. Because of the presence of these wings forward motion of the vessel generates lift, and the attitude of the wings can be changed in response to changes in the attitude of the ship with a view to providing stabilization. This method is unsatisfactory because it relies on the existence of a reasonable forward speed, and is thus inoperable when the vessel is stationary or only travelling slowly. Furthermore, such wings cannot provide stabilization against heave.

According to the present invention there is provided a vessel having stabilizing means comprising at least one chamber disposed to lie at least partly beneath the surface of waterwhen the vessel is immersed or semi-immersed in water, means for detecting the disposition of the vessel relative to a predetermined disposition and for producing an error signal in accordance with the detected disposition, and means responsive to the error signal for controlling the buoyancy of the chamber or chambers in a sense to alter the disposition of the vessel towards the predetermined disposition.

According to another aspect of the invention there is provided a vessel having stabilizing means comprising at least one chamber open to water when the vessel is immersed or semi-immersed in water such that a volume of gas is retained in each of the chambers by the water, means for detecting the disposition of the vessel relative to a predetermined disposition and for producing an error signal in accordance with the detected disposition and means, responsive to the error signal, for varying the volume of gas in the chambers in a sense to alter the disposition of the vessel towards the predetermined disposition.

Preferably there are at least two spaced apart chambers.

For the purposes of this specification the term vessel includes reference to any structure capable of floating in water or sea water whether in an immersed or semi-immersed mode, for example, ships, barges, rafts, semi-submersibles and submersibles.

The chamber may be disposed in the vessel such that the stabilizing means stabilizes the vessel against heave and/or pitch and/or roll.

The invention may be provided in either one of two basic forms. One form of the invention, referred to herein as the powered version, uses a compressor or the like to generate compressed gas which is in turn responsible for producing variations in the volume of gas in the chambers. In a second form of the invention, referred to herein as the unpowered version, no external source of power is used and the system uses instead the energy of the wave motion against which stabilization is being provided. If desired a hybrid of these two forms may be used, with the unpowered version providing part of the stabilization and a supplementary power system providing additional stabilization.

In the powered version the means for varying the volume of gas may include a low pressure reservoir and a high pressure reservoir interconnected directly or through one or more compressors.

In this latter case there may be a three way valve for each chamber, the valve ports being connected to the high pressure reservoir, the lower pressure reservoir and the buoyancy chamber gas volume.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a schematic end view of a semisubmersible having stabilizing means; Figure 2 is a schematic side view of the semisubmersible of Figure 1; Figure 3 is a schematic diagram of a powered stabilizing system for use with the semi-submersible of Figures 1 and 2; Figure 4 is a schematic end view of a semisubmersible having an alternative form of stabilizing means; Figure 5 is a schematic side view of the semisubmersible of Figure 4; Figure 6 is a more detailed view of one of the buoyancy chambers shown in Figures 4 and 5; Figure 7 is a section line Z-Z in Figure 6; Figure 8 is a schematic view of a barge or monohull provided with stabilizing means; Figure 9 is a schematic plan view of the barge or monohull of Figure 8; and Figure 10 is a schematic cross-section through the barge or monohull of Figures 8 and 9.

Figures 1 and 2 show a semi-submersible vessel generally indicated at 10. The vessel comprises a pair of parallel floats 11, four outer support legs 12 mounted at the respective ends of floats 11 and a platform 13 secured to the upper ends of the outer support legs 12 to bridge the floats 11. The platform 13 is further supported by inner support legs 14 which are spaced along the floats 11. A frame structure, a part of which is indicated at 15, extends between at least some of the adjacent pairs of legs 12,14 to provide the vessel with lateral rigidity.

Each outer support leg 12 has a chamber 16 formed in its lower portion. In Figures 1 and 2 the chambers 16 are shaded. Each chamber 16 is open at its lower end 17 such that when the vessel 10 is semi-submerged in water, as shown in Figures 1 and 2, the water is free to enter the chamber 16 through end 17. The solid line A in Figures 1 and 2 indicates the still water level. The chambers 16 are dimen sioned such that when the vessel 10 is an equilibrium in still water, as shown in Figures 1 and 2, the chambers 16 will be half filled with water, as indicated by dotted lines B.

It will be appreciated that the orientation of the vessel can be altered if the level B of the water in any given chamber 16 is changed from its equilibrium position. Thus if the water level B in the right hand pair of outer support legs 12, in Figure 2, were lowered then the platform 13 would slope downwards from right two left. Similarly, in Figure 1, if the level B in chamber 16 of outer support legs 12 mounted on the right had flat 11 were lowered then the platform would slope from right to left. Thus by altering the levels B in respective outer support legs 12, and hence the buoyancy of the vessel 10, one can aiter the orientation of the vessel 10.

Reference will now be made to Figure 3. Figure 3 shows a control system whereby the level of water in chamber 16 and hence the instantaneous buoyancy of the vessel 10 can be controlled. The control system of Figure 3 comprises means 18 for detecting the orientation of the vessel with respect to a predetermined orientation, normally the equilibrium orientation of the vessel in still water. The control system further comprises means 19 for providing gas at high or low pressure on lines 20 and 21 respectively, and means 22 for altering the pressure of gas in the chamber 16, by means of lines 23, in accordance with an error signal representing the difference between the detected orientation, detected by means 18 and the predetermined orientation.

It will be appreciated that if a chamber 16 is connected to the high pressure on line 20 water will be forced out of the open end 17 of the chamber 16 lowering the level B in that chamber 16 and increasing the localised buoyancy of the vessel 10. Conversely, if a chamber 16 is connected to the low pressure source on line 21 the water level B in that chamber will rise until such time as the gas pressure is in equilibrium with the external environmental pressure. In this case the level B will rise and the localised buoyancy of the vessel 10 will be reduced.

The source of high and low pressure 19 comprises a low pressure reservoir 24, for example at a pressure of from 1/4 to 3/4 bar, a high pressure reservoir 25, for example at a pressure of from 3 to 7 bar, and three parallel lines 27 interconnecting reservoirs 24 and 25, a compressor 28 is provided in each line 27, the compressor 28, in the top line, having a smaller capacity than that in the middle line, which in turn has a smaller capacity than the compressor in the bottom line. The provision of three compressors is optional and is designed to facilitate the mounting of compressors on floating vessels and to reduce power consumption, since at times of lower power demand one or two of the compressors can be shut down.

Means 22 comprise a valve 29 for each line 23.

Each valve 29 is a three way valve and can be moved between the first position in which it connects line 21 to line 23, to a second position in which it connects line 20 to line 23 and the third position in which it is closed. Each valve 29 is electrically or pneumatically operable. The means 22 further comprises a valve controller which operates individual valves 29 in accordance with the orientation error signal produced by the orientation detector 18. Each valve 29 is preferably an electrically operable valve capable of providing an approximately linear variation of flow rate with variation of an electrical demand signal produced by the controller.

Thus, in use, as a wave passes the vessel 10 changing the instant orientation of the vessel 10 the change in orientation is detected by detecting means 18 which produces an error signal which is fed to valve controller 30, which in accordance with the error signal operates one or more of valves 29 in such a manner as to alterthe gas pressure in the chamber 16 in a sense to return the vessel towards its predetermined or equilibrium position.

It will be appreciated that as a wave passes the vessel it will be continually acting in a sense to displace the vessel but in a continuously changing manner. Because the control system is provided with low pressure and high pressure reservoirs it is possible to vary the gas pressure in any one of the chambers 16 extremely quickly and therefore very rapid stabilization can occur and the gas pressure in chamber 16 can be constantly changed throughout the passage of the wave. This is controlled by the valve controller 30 which operates as follows.

The controller 30 incorporates a control algorithm which receives vessel motion information from heave, roll and pitch sensors. It may also receive information on buoyancy chamber internal gas pressures and internal chamber water levels from sensors inside each buoyancy chamber.

The control algorithm then constructs a full order mathematical model of the heave, roll and pitch dynamics of the floating vessel and of the dynamics of the water columns inside each buoyancy chamber. This mathematical model is used to reconstitute those vessel system state variables which cannot be directly measured. The wave exciting forces and moments on the floating vessel are such state variables. The mathematical model also uses the measured variables to deduce the properties of the wave spectrum exciting the vessel into heave, roll and pitch motions.

Linear combinations of the reconstituted states are used as control signals to control valve operation for each buoyancy chamber such as to control the gas flow rate into or out of the chamber. Both the design of the optimal observer mathematical model and the parameters in the state feedback controller are chosen using the techniques of linear optimal control theory, described in the book entitled "Linear Optimal Control Systems" by H. Kwakernaak and R.

Sivan, Wiley Interscience, 1972, ISBN 0-471-51110-2.

The controller is designed to minimise a performance index J where J = E(x R x + ú S u) with the expected valve operation denoted by the function E( ) whereas x denotes the vector of vessel displacements in the controlled mode and u represents the vector of control signals; the ""'symbol denoting the transposed vector. The matrices R and S are chosen to specify the amount of control input effort which is deemed worthy to suppress the motions x.

Both the designs of the observer mathematical model and of the controller depend on the spectrum of exciting forces generated by the sea. This spectrum is computed by a separate algorithm in the controller. The observer and controller algorithms are automatically altered to account for changes in the exciting force spectrum or a change in control strategy as demanded by the vessel master. The computation of the exciting force spectrum can be excluded from the control algorithm in which case the controller performance will remain good but will not be optimal.

In high seas the controller will occasionally demand flow rates into or out of the chambers which are too high. In such an event, the valves will be commanded to remain open to their maximum extent when the demanded flow is above the maximum available. The controller parameters are preferably designed to make this a rare occurrence within the specified performance of the stabilization system.

It will be appreciated that by providing for buoyancy chambers disposed as shown in Figures 1 and 2 the vessel can be stabilized against the effects of heave, pitch and roll.

If the stabilization apparatus is used in a conventional ship, it may only be possible to have a pair of open ended chambers spaced lengthwise in the ship. In this case one could protect against heave and pitch but not against roll. Alternatively, one could have a pair of side by side chambers which could counteract roll and heave but not pitch. Yet a further possibility is to have just one buoyancy chamber rather than a plurality thereof, though the range of conditions in respect of which stabilizing can then be provided is even more limited.

In a sumbersible an array of open ended chambers in conjunction with a control system of the type shown in Figure 3 can be used to maintain the trim of the vessel in a constant and active manner. Thus, if the submersible is working on the leg of an oil rig it can be arranged to maintain a particular orientation with respect to that rig.

It will be appreciated that as the gas system is a closed loop system gas bled from chamber 16 into low pressure vessel 24 is compressed by one or more of compressors 28 to maintain the pressure in high pressure reservoir 25 at the required pressure whilst maintaining the pressure in reservoir 24 at the required low pressure.

The orientation detector 18 may include one or more accelerometers for detecting the motion of the vessel 10 in particular directions, alternatively it may include a gyroscope. A still further possibility is for strain gauges to be mounted on particular elements of the vessel 10 so that variations in the strain on these elements due to the passage of the wave is detected. Any combination of strain gauges, accelerometers or gyroscopes may be used if circumstances warrant it.

The pressure of gas in the chamber 16 may be varied mechanically, by respective pistons in the chamber 16 or alternatively, the bottom of the chambers could be closed and water pumped in and out of the chambers 16, each chamber 16 having a direct vent to the atmosphere.

Generally the gas used in chamber 16 will be air, but if for reasons of safety or any other requirements it is desired to use some other gas, such as an inert gas, this can be done. In this case the closed loop system shown in Figure 3 would be particularly suitable.

The pressure in the chambers 16 and the internal water surface levels B may be monitored to provide additional inputs into the valve control means 30.

In the unpowered version of the invention the compressors 28 and interconnecting lines 27 are omitted and the gas reservoirs 24 and 25 are treated as one and interconnected by opening valve 31.

What the control means 30 then does is to control opening and closing of the valve 29 in response to the vessel orientation detected by the means 18. In this case the valves 29 may be large annulus butterfly or gate valves capable of being fully open or fully closed in response to an open or shut command issued by the controller 30. If a given valve 29 is open and the water level in the corresponding chamber 16 is forced to rise by wave action, then the air trapped in the upper part of the chamber may freely leave the chamber through the corresponding line 23 into reservoirs 24 and 25, so that the rise in water level in the chamber 16 is substantially unimpeded. On the other hand, if the water level in a chamber 16 is forced to fall by wave action, interconnection by valve 29 to lines 20 or 21 will allow the water level in chamber 16 to fall without substantial impediment.If, however, a given valve 29 is shut then if the water level in corresponding chamber 16 attempts to rise or fall any such rise or fall can only be affected against a counteracting force produced by compression or expansion of the trapped air within the upper part of the chamber. Thus, in this case the rise or fall in water level within the chamber 16 is impeded. In this way operation of the valves 29 controls the buoyancy variations in the chambers 16 and provides corresponding stabilization. Reservoirs 24 and 25 may be regarded as temporary storage for pneumatic energy leaving and entering the chambers 16. Alternatively, an appropriate design may dispense with the reservoirs 24 and 25 and replace them with the atmosphere as a "reservoir" of pneumatic energy.

In the unpowered system, the algorithm of the controller 30 is designed to compute the peak frequency and the spectra of the vessel heave, roll and pitch motions that are occurring. A present 'look up' table is then referred to within the controller algorithm to read the optimum valve fully open or fully shut configuration for maximum motion alleviation. This configuration is transmitted to the valves as commands for valve operation. A manual override and valve configuration monitor facilities may be provided by the controller to allow the vessel master a measure of authority over selected valve configuration.

Like the powered system controller, the unpo wered system controller can be implemented automatically, although a manual system will provide a reduced motion stabilization performance.

The unpowered version is capable of providing buoyancy changes sufficient to produce partial alleviation (by up to approximately 60%) of wave induced motions. If it is desired to achieve a greater degree of stabilization than this whilst still not employing a fully powered version, powered stabilization may be used to supplement unpowered stabilization to the required extent.

Instead of using simple chambers 16 it is possible instead to use chambers in which a piston is movable up and down, the piston separating air above it from water below it. Yet another possibility is to use a chamber whose internal shape may be altered to control the stabilization. For example, an inflatable toroidal bag could be provided on the inside wall of the chamber, inflation and deflation of the bag serving to alter the internal cross-section of the chamber at a given region along its height, thereby affecting in a predetermined way the manner in which the buoyancy of the chamber will vary with varying water level in the chamber.

In the modified embodiment shown in Figures 4 and 5, in addition to locating the buoyancy chambers 16within the support legs 12, buoyancy chambers 116 are placed outside the vessel, for example as concentric circular collars on the vertical cylindrical legs. Any appropriate number of such external buoyancy chambers may be present, with eight chambers being used for the embodiment of Figures 4 and 5. It is to be understood that the external chambers could be used instead of, rather than in addition to, the internal buoyancy chambers. A commercial advantage exists in placing the buoyancy chambers outside the vessel since currently operating vessels can be retro-fitted with stabilizing means according to the invention. A concentric circular collar is only one possibility for buoyancy chamber placement outside the vessel hull.Very many other alternative external buoyancy chamber positions can be used. A secondary effect of certain external buoyancy chamber attachments is to allow a substantial increase in deck payload over and above the payload of the unmodified vessel.

It may be advantageous to have the water level B in the buoyancy chamber 116 to be at the same vertical elevation as the external still water level, for which purpose the chambers will only be partially disposed beneath the surface of the water. It is to be pointed out that although the drawings show internal buoyancy chambers completely disposed below the water level and external buoyancy chambers partially disposed below the water level, it is also within the scope of the present invention to provide external buoyancy chambers completely disposed below the water level and/or internal buoyancy chambers partially disposed below the water level.

For the unpowered version, the valves connecting the buoyancy chambers to pneumatic reservoirs or the atmosphere can remain open or shut to change the state of the vessel throughout a storm rather than by operating so as to open or shut within individual crest/trough wave cycles.

Figures 6 and 7 show in more detail the construction of one of the chambers 116. The chamber is open to the sea at its lower end 117. The chamber is divided into three compartments by partitions 140 which extend vertically dowwards from a horizontal bulkhead 141 to the lower end 117. The partitions are not essential but assist in stabilization control because the annular chamber 116 can be treated as three separate compartments. Avalve 129 (corresponding in principle to valve 29 of Figure 3) controls gasflowthrough a duct 142 communicating with the top of the compartment. The duct 142 leads through valve 129 to a pair of pipes (shown schematically by 143) which communicate with the reservoirs 24 and 25 respectively, (see Figure 3). Where the atmosphere is being used as the "reservoir", the duct 142 leads to a pipe 144 through valve 129.The detail shown in circle M is repeated for each of the compartments.

It is noted at this point that although the division of a buoyancy chamber is shown in Figures 6 and 7 in connection with an external buoyancy chamber it is equally applicable to an internal buoyancy chamber.

Figures 8, 9 and 10 show a barge or monohull shape with a layout of internal buoyancy chambers.

A chamber 16a is provided at the bow end and a chamber 16b is provided at the stern end, each chamber being divided into four compartments.

These serve to provide heave and pitch alleviation in head seas. Two further chambers 1 6c at the sides of the vessel provide heave and roll alleviation in beam seas. Although not shown, partitions to divide these chambers may also be provided. In Figures 8 to 10 the normal internal water level is indicated by T and can be seen to be the same as the external water level A. All the chambers are open at their lower ends 17.

Claims (17)

1. Avessel having stabilizing means comprising at least one chamber disposed to lie at least partly beneath the surface of the water when the vessel is immersed or semi-immersed in water, means for detecting the disposition of the vessel relative to a predetermined disposition and for producing an error signal in accordance with the detected disposition, and means responsive to the error signal for controlling the buoyancy of the chamber or chambers in a sense to alterthe disposition of the vessel towards the predetermined disposition.

2. A vessel according to claim 1, wherein a plurality of chambers is provided spaced longitudinally from one another with respect to the length of the vessel to provide pitch and heave correction.

3. A vessel according to either preceding claim, wherein a plurality of chambers is provided spaced laterally from one another with respect to the length of the vessel to provide roll and heave correction.

4. Avessel according to any preceding claim wherein the or each chamber is open to allow water to enter.

5. Avessel according to any preceding claim, wherein the means for altering the buoyancy comprises means for varying the volume of gas in the chamber or chambers

6. A vessel according to claim 5, wherein for each chamber the means for altering the buoyancy comprises at least one valve, the valve operating in response to the said error signal to open to allow gas to enter or leave the chamber or to close to prevent gas entering or leaving the chamber.

7. A vessel according to claim 5 or 6, comprising a source of low gas pressure and a source of high gas pressure, and wherein for each chamber the means for altering the buoyancy comprises valve means for selectively connecting the chamber to either pressure source.

8. A vessel according to claim 7, wherein the said valve means is also closable to prevent gas entering or leaving the chamber.

9. A vessel according to claim 7 or 8, wherein the sources of high and low pressure are fed by a plurality of compressors of different sizes, one or a plurality of the compressors being usable at a given time to provide a given power.

10. A vessel according to any one of claims 5 to 9, wherein the said gas is air.

11. A vessel according to any one of claims 5 to 9 wherein the said gas is an inert gas.

12. A vessel according to claim 5, wherein the or each buoyancy chamber is connected to a gas reservoir.

13. A vessel according to claim 5, wherein the or each buoyancy chamber is connected to atmosphere.

14. Avessel according to any preceding claim, wherein the or each chamber is mounted externally in the vessel.

15. A vessel according to any one of claims 1 to 13 wherein the or each chamber is mounted internally within the vessel.

16. A vessel according to any one of claims 1 to 13, wherein there are a plurality of chambers at least one of which is mounted externally on the vessel and at least one of which is mounted internally within the vessel.

17. Avessel according to any preceding claim, wherein the chamber or at least one of the chambers is internally partitioned into a plurality of com part- ments.