GB2137578A - Floating vessels - Google Patents

Abstract

A vessel comprises two or more barge form hulls (1,3) disposed apart vertically one above the other and multiple spaced connecting structural members (2) rigidly inter-connecting the hulls with a gap therebetween the upper (1) of which hulls provides the necessary buoyancy to support the vessel with the other hull or hulls flooded with sea water and/or other liquids, the arrangement being such that waves acting on the vessel may cause a water flow in the space between the hulls and the wave energy may be at least partially dissipated by the action of the water flow between the hulls and on the connecting members in order to stabilize the vessel. <IMAGE>

Description

SPECIFICATION Floating vessels This invention relates to constructions of floating vessels and to arrangements for effecting stabilisation and steadiness of such vessels.

In floating vessels, stability, steadiness and motion responses to the existing forces of the environment are a compromise between "stiffness" and the magnitude of metacentric height where motions are of importance for reasons of comport or the efficiency of process plants or processes to be carried out on board.

In barge hull vessels normal in form for the offshore oil industry, the natural tendency of the vessel to return to its upright position of equilibrium is expressed by the righting moment.

RM = GM sin H = (I/V + KB - KG) sin where: = = 2nd moment of the waterplane area V = volume displacement of the vessel KB = Height of centre of buoyance above K KG = Height of centre of gravity above K K = Reference point base of moulded mid section o = Angle of displacement As KB and KG are largely fixed quantities for any given vessel, the righting moment of any angle of tilt is determined by BM or 1/V, however, the natural periods of such a vessel are determined by its righting moments, its mass and its reluctance to respond to exciting forces tending to produce changes in attitude.

The principal resistances in this respect are created by friction, hydrodynamic drag and added virtual mass due to movements of entrained masses of sea water, and such motions may also be influenced by counter masses allowed by whatever method to progress out of phase with the motions of the vessel.

This invention provides a vessel comprising two or more vertically joined hulls with spaces between each hull intended to produce added virtual mass characteristics designed to exert a specific influence on the motion of the vessel in a seaway and with the inter-hull spaces and fastenings and the hydrodynamic coefficients relating to them disposed and designed to consume energy from the environment by converting kinetic wave energy into heat energy.

The invention also provides a vessel comprising two or more vertically joined hulls, the upper of which provides the necessary buoyancy to support the vessel with the other hull or hulls flooded with sea water and/or other liquids, cargoes or ballast.

The invention further provides a vessel comprising two or more hulls disposed apart vertically one above the other and multiple spaced connecting structural members rigidly inter-connecting the hulls with a gap therebetween the upper of which hulls provides the necessary buoyancy to support the vessel with the other hull flooded with sea water and/or other liquids, the arrangement being such that waves acting on the vessel may cause a water flow in the space between the hulls and the wave energy may be at least partially dissipated by the action of the water flow between the hulls and on the connecting members.

The connecting elements between the hulls may comprise a multiplicity of vertical and/or inclined to vertical structural members extending between the hulls, and, further fore and aft rows of inter-connecting members may be provided between the or each pair of vessel hulls.

In any of the above arrangements each hull of the vessel may be of barge form, and further, the lower hull (or hulis) may have a plurality of storagedbaliast tanks to receive sea-water or a liquid (e.g. oil) to be stored and means being provided for filiing and emptying the tanks as required. Also the upper hull may have a plurality of storage tanks and means may be provided for filling and emptying the tanks as required.

In any of the above arrangements a vertically rotatable turret may be mounted in the hulls of the vessel towards one end of the vessel and means are provided for attaching an anchor system to the turret for mooring the vessel, the turret being freely rotatable with respect to the hull of the vessel to allow the vessel to lie according to the dictates of the prevailing winds/current/waves.

More specifically the hulls may have axially aligned wells extending therethrough on the centre line adjacent one end of the vessel, the turret may extend through the wells and bearing means may be provided for mounting the turret for rotation in the well to receive said anchor system.

The invention thus provides a floating structure wherein one hull is mounted beneath another hull in such a manner that:- a) the upper of the two barge hulls remains buoyant and supplies some or all of the buoyancy required to support the assembly and any load, b) the lower of the two large hulls remains submerged and supplies some or none of the buoyancy required to support the assembly and any load, c) there is a suitable water gap between the two barge hulls into and from which water may pass with a degree of freedom dictated by the shape and disposition of members of required configuration and size dictated by the design in order to time or otherwise take advantage of the added virtual mass, velocity or turbulence created and the effect thereof on the responses of the entire vessel to the exciting forces applied by the environment, d) the lower barge is so constructed and shaped as to present a resistance to movement through the water in any direction and of any magnitude dictated by the individuai design.

In such an arrangement two advantages may be achieved over a conventional monohull barge, and these may be varied in magnitude to produce desirable modifications in overall response amplitude operators of the vessel. These two advantages are consequent upon the lower hull having its resistance unaltered by its depth of submergence whilst it is beneath the wave exciting forces of the surface of the water and its wave conditions to an extent dictated by its design of submergence, and by the degree to which the available power of the environmental exciting force is consumed bythe movements and turbulence of entrained water masses and therefore made unavailable to act upon the vessel and produce motions.

Because the decrease in lateral plane movements of the lower vessel parts in comparison to the upper hull will lead to an inverted pendulum effect on the whole vessel dependent on the magnitude of the lateral exciting forces applied to the upper hull and its cargo, and the overall height of the metacentre (GM) of the whole vessel, this pendulum effect will produce pitch, roll, sway and surge movements proportional to the restraining effect of the lower barge or hull and its appurtenances. In cases where these added lateral rotations and horizontal increments are undesirable, they may be mitigated or diminished by use of passive or active systems. Such anti-motion systems are known in the art.

The following is a description of some specific embodiments of the invention, reference being made to the accompanying drawings in which: Figure lisa side elevation of a vessel; Figure 2 is an end elevation of the vessel; Figure 3 is a plan view of the vessel; Figure 4 is a perspective view of a vessel according to another embodiment of the invention; Figure 5 is a side elevation view of the vessel of Figure 4; Figure 6 is a plan view of the vessel; Figure 7 is a stern view of the vessel; Figure 8 is a plan view taken between the upper and lower hulls of the vessel of Figure 5; Figure 9 is a section on the line 9-9 on Figure 8; Figure 10 is a section along the line 10-10 of Figure 8; Figure 11 is a section on the line 11-11 on Figure 8;; Figure 12 is a diagrammatic section of a forward part of the vessel showing an anchor system and riser attachment mounting on the vessel; Figure 13 is a diagrammatic view of the vessel moored to the anchor system; Figure 14 is a plan view of an alternative mooring arrangement; Figures 15 and 16 are detailed views of the attachment of the anchor chains to the vessel in the system of Figure 14; Figure 17 is a graph showing "heave" plotted against "wave period" for a conventional mono-hull and the vessel according to the invention; Figure 18 is a diagrammatic view of a motion suppression system fitted to the vessel; Figure 19 is a graph showing "roll" plotted against "wave period" showing the reduction in "roll" obtained using the motion suppression system as compared with the roll of a conventional mono-hull.

Referring firstly to Figures 1 to 3 of the drawings, one is an upper water buoyant hull connected through support and bracing members 2 with a lower hull 3 which may supply none or some of the buoyancy of the total structure. The lower hull is anchored at connections 4. In such an arrangement two advantages are achieved over a conventional mono-hull. These are consequent upon the lower hull having its resistance unaltered by its depth of submergence whilst it is beneath the wave exciting forces of the surface of the water and its wave conditions to an extent dictated by its depth of submergence and by the degree to which the available power of the environmental exciting force is consumed by the movements and turbulence of entrained water masses and therefore made unavailable to act upon the vessel and produce motions.

Because the decrease in lateral plane movements of the lower barge hull or vessel parts in comparison to the upper barge hull will lead to an inverted pendulum effect on the whole vessel dependent on the magnitude of the lateral exciting forces applied to the upper barge hull and its cargo, and the overall height of the meta-centre (GM) of the whole vessel, this pendulum effect will produce pitch, roll, sway and surge movements proportional to the restraining effect of the lower barge hull and its appurtences. In cases where these added lateral rotations and horizontal increments are undesirable, they may be mitigated ot diminished by use of passive or active systems applied to the upper hull such as may create mass movements out of phase to said motions.

Having described an embodiment of the invention in broad outline form, reference will now be made to Figures 4to 11 in which a more detailed embodiment of the invention is illustrated. The vessel which is intended as a floating production unit for a sea-bed oil well comprises an upper "barge form" hull 20 which may measure 400ft by 1 00ft by 25ft deep and is attached by an intermediate braced steel structure indicated generally at 21 to a lower barge form hull 22 somewhat smaller than the upper hull and for example measuring 300ft by 90ft by 20ft deep and spaced below the upper hull by said structure.

The upper hull has a deck superstructure 23 at the stern thereof which houses all the required accommodation, offices, workshops and process control room and other such services and over which extends the helicoptor landing deck 24 for which associated fire and damage control stations and re-fuelling/de-fuelling facilities are provided. A ballast control centre is provided within the accommodation unit which houses controls for levels in ballast tanks for both the upper and lower hulls.

Within the body of the upper hull 20 there are the following compartments; i. trim and ballast tanks.

ii. ballast manifold and pump.

iii. chain lockers and fairleads.

iv. motion suppression tanks.

v. platform utilities.

vi. power generation and distribution.

vii. water injection pump.

viii. oil surge tanks.

ix. produced water settiing tanks.

The lower hull 22 contains ballast piping which connects through to the ballast manifold and pumps located in the upper hull and also has ballast tanks.

Figure 5 of the drawings shows the vessel inside elevation with the upper hull 20 floating to its normal waterline and the lower hull 22 flooded with sea water and fully submerged.

The structure connecting the upper and lower hulls is shown in detail in Figures 8 to 11 and comprises a row of upright columns 23 extending between the upper and lower hulls along the centreline of the hulls and further rows of columns 24 extending along either side of the hulls. Alternate columns along the centre line denoted 23' extend from the keel of the lower hull, up through that hull, across the gap between the hulls and up through the upper hull to the deck of the upper hull whereas the remaining columns interposed alternately between the columns 23' extend between the upper deck of the lower hull and the underside of the upper hull.Likewise, in the rows of columns 24 along either side of the vessel, alternate columns marked 24' extend from the keel of the lower hull to the deck of the upper hull whereas the remaining columns extend between the deck of the lower hull to the underside of the upper hull. At the stern of the vessel, all the columns extend from the keel of the lower hull to the deck of the upper hull. In addition, in each line of columns, the base of each column 24' is connected to the upper end of the intermediate column 24 and likewise the base of each column 23' is connected by bracing members 26 to the upper end of the intermediate column 23.Across the vessel, the base of each outer column 24 is connected to the upper end of the column 23 in line with the columns 24 by bracing members 27 further, in the rearward part of the vessel the lower ends of the outer columns 24 are connected by bracing members 28 to the upper end of the column 23 one step forward along the centre line of the vessel. Likewise the lower ends of the columns 24 in the forward part of the vessel are connected by bracing members 29 to the upper ends of the centre line columns 23 one step to the rear in each case. The arrangement thus provides a multiplicity of vertical columns braced by inclined members extending between the upper and lower hulls to form an integral structure between the upper and lower hulls and also to achieve certain characteristics in the water flow between the hulls.In particular, the columns and bracing members are shaped and located to promote conflicting current and vortices in the water entrained between the hulls. Additional honeycomb or slotted plate structure such as indicated at 30 on Figures 9 and 10 may be provided between the hulls and connected to the columns and/or bracing members and/or the hulls to act on water moving between the hulls. Thus a very large power sink is created in the gap between the hulls which consumed wave energy which would otherwise cause pitch, heave and roll motion in the vessel. In other words the pumping action and turbulence of the entrained water mass between the two hulls in a seaway consumes a part of the energy from the sea which would otherwise impart motions to the upper hull.The flat surfaces of the hulls produce resistance to movement through the water whilst they are not fully subjected to the exciting forces causes by waves. Although the lower hull is below the maximum wave action, it is still subject to some degree of pressure change and water particle movement. Because the lower hull resists movement better than the upper hull and is not subjected to the same amplitude of horizontal plane forces, the unit reacts to an extent as a damped upside down pendulum as well as a normal pendulum suspended from the surface. The lower hull can be ballasted and trimmed to vary the centres of gravity of the whole unit, and therefore the metacentric height in pitch and roll, until the upward pendulum and downward pendulum forces are almost equal in magnitude and nearly opposite in phase.Residual roll and pitch of the unit can be damped using a known proprietary system which will be described in brief later.

The structure 21 inter-connecting the hulls as illustrated in Figures 5 and 7 is somewhat simplified for the sake of clarity.

The upper hull of the vessel incorporates an atmospheric pressure, crude oil surge tank from which pumps deliver crude oil via a metering unit to a tanker loading terminal. Surge tank capacity is selected to allow sufficient buffer storage for normal tanker turnaround. The lower hull can however be designed to provide a further oil storage if required. The low centre of gravity of the vessel allows production equipment to be stacked in multiple levels or to be enclosed for weather protection in severe environments. The vessel can thus accommodate oil and gas production systems together with associated water injection and gas conditioning and compression systems. All production operations and marine systems are controlled and monitored from a central control room in the accommodation block.

The ballasting control systems also permit full de-ballasting of the lower hull in suitable weather conditions to make it possible to raise the vessel for dry access to the whole of the upper hull and inter-hull structure. The vessel can thus be readily inspected for re-certification survey requirements on site without the necessity for dry-docking. The de-ballasting facility also enables the vessel to be floated out from its construction site with a minimum draft before ballasting on site to its full draft.

Towards the forward end of the vessel the upper and lower hulls are formed with axially aligned cylindrical wells 40, 41 respectively in which a rotary turret structure indicated at 42 is mounted in upper and lower bearings indicated at 43 and 44. The table 43 carries the upper end of a marine riser indicated generally at 53 to which a pipeline or pipelines are connected from below the surface to provide the appropriate feeds and supplies. Six anchor chains 54 are secured at their upper ends to the table 43 by fastenings 55 and the chains extend downwardly through the turret and out through hawse pipes 56 mounted in the lower part of the turret to extend in catenary manner to anchors on the sea bed. The anchoring arrangement is illustrated in Figure 14.The vessel is thus free to rotate around its anchorage in accordance with the prevailing conditions of wind, current and wave so that the vessel will automatically weather vane in adverse conditions thus minimising the effect of those conditions on the motion of the vessel. The vessel is thus above to be maintained in operation in severe conditions.

The vessel can also be moored to an eight point catenary anchoring system from the bow and stern of the vessel as illustrated in Figure 14 in which case anchor chains 60 are connected to the vessel through side fair leads 61 on the lower hull, fair leads 62 at the deck edge of the upper hull, chain stoppers 63 and electric powered winches 64, each winch being provided with drums 65 at either end of its power shaft to receive pairs of chains 60.

The vessel may also be provided with a proprietary roll motion suppression system as indicated diagrammatically in Figure 18 for that purpose, the upper hull 20 is provided with downwardly open side tanks 70 having control valves 71 at the upper ends thereof for venting the tank. The valves can be closed to maintain the water level in the side tank 70 or open to allow the water level to rise and fall as the vessel moves Tests using a model of the vessel of Figure 4 indicate a reduction in roll movement in seas as compared with a conventional mono-hull vessel of the same displacement as indicated in the graph of Figure 9.

Claims (16)

1. A vessel comprising two or more vertically joined hulls with spaces between each hull intended to produce added virtual mass characteristics designed to exert a specific influence on the motion of the vessel in a seaway and with the inter-hull spaces and fastenings and the hydrodynamic coefficients related to them disposed and designed to consume energy from the environment by converting kinetic wave energy into heat energy.

2. A vessel comprising two or more vertically joined hulls, the upper of which provides the necessary buoyancy to support the vessel with the other hull or hulls flooded with sea water and/or other liquid.

3. A vessel comprising two or more hulls disposed apart vertically one above the other and multiple spaced connecting structural members rigidly inter-connecting the hulls with a gap therebetween the upper of which hulls provides the necessary buoyancy to support the vessel with the other hull or hulls flooded with sea water and/or other liquids, the arrangement being such that waves acting on the vessel may cause a water flow in the space between the hulls and the wave energy may be at least partially dissipated by the action of the water flow between the hulls and on the connecting members.

4. A vessel as claimed in claim 3 wherein the connecting elements between the hulls comprise a multiplicity of vertical and/or inclined to vertical structural members extending between the hulls.

5. A vessel as claimed in claim 4 wherein fore and aft rows of inter-connecting members are provided between the or each pair of vessel hulls.

6. A vessel as claimed in claim 5 wherein rows of said structural members are provided between the or each pair of hulls adjacent the sides of the hulls and also along the centreline of the hulls.

7. A vessel as claimed in any of claims 4 to 6 wherein lattice members are also provided inter-connecting the lower ends of the structural members to the upper ends of adjacent structural members on the hull above.

8. A vessel as claimed in any of the preceding claims wherein each hull of the vessel is of barge form.

9. A vessel as claimed in any of the preceding claims wherein the lower hull (or hulls) has a plurality of storage/ballast tanks to receive sea-water or a liquid (e.g.oil) to be stored and means are provided for filling and emptying the tanks as required.

10. A vessel as claimed in any of the preceding claims wherein the upper hull has a plurality of storage tanks and means are provided for filling and emptying the tanks as required.

11. A vessel as claimed in any of the preceding claims wherein a vertically rotatable turret is mounted in the hulls of the vessel towards one end of the vessel and means are provided for attaching an anchor system to the turret for mooring the vessel, the turret being freely rotatable with respect to the hull of the vessel to allow the vessel to lie according to the dictates of the prevailing winds/current/wave direction.

12. A vessel as claimed in claim 11 wherein the hulls have axially aligned wells extending therethrough on the centre line adjacent one end of the vessel, the turret extends through the wells and bearing means are provided for mounting the turret for rotation in the wells to receive said anchor system.

13. A vessel as claimed in claim 11 or claim 12 wherein a catenary anchor system is attached to the turret.

14. A vessel as claimed in any of the preceding claims wherein a thruster or thrusters are provided on the or each of the hulls for driving/manoeuvring the vessel.

15. A floating vessel substantially as described with reference to and as illustrated in Figures 1 to 3 of the accompanying drawings.

16. A floating vessel substantially as described with reference to and as illustrated in Figures 4 to 12 of the accompanying drawings.