GB1576957A

GB1576957A - Method of submerging a buoyant object and apparatus therefor

Info

Publication number: GB1576957A
Application number: GB44670/77A
Authority: GB
Original assignee: COMEX
Current assignee: COMEX
Priority date: 1976-11-05
Filing date: 1977-10-27
Publication date: 1980-10-15
Also published as: FR2370131A1; NO773752L; FR2370131B1

Description

(54) METHOD OF SUBMERGING A BUOYANT OBJECT AND APPARATUS THEREFOR (71) We, COMPAGNIE MARITIME D'EXPERTISE S.A., a French Body Corporate, residing at Avenue de la Soude 13009 MARSEILLE/France do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed to be particularly described in and by the following statement: The present invention relates to a method of controlling the submergence, the stay under water and the raising to the surface of a buoyant object, as well as to apparatus for performing the same.

The technical field of the invention is that of processes and apparatus for handling buoyant objects which are submerged from surface supports.

The difficulties encountered in the ocean during the handling of objects which are to be submerged in order to be placed on the ocean bed or on an underwater structure are well known. These difficulties are in particular due to pounding action and the lack of stability of the objects resulting from the state of movement of the aquatic medium in which they are submerged.

The present invention aims at obviating these disadvantages. The object of the invention is therefore to provide a method of submerging buoyant objects and apparatus for performing the same in such a way that the pounding of the object in contact with the water is absorbed to the maximum and its stability is improved so that it can be controlled with maximum accuracy from the surface support equipped with suitable handling gear for deposition at the desired point on the ocean bed or on an underwater structure.

According to one aspect of the invention a method of controlling the submergence, the stay under water and the raising to the surface of a buoyant object from a surface support, comprises connecting at least one line from the surface support to the object, the line including a ballast member connected to the object by a flexible connection on the one hand and to a cable on the other hand and the cable extending to the surface support; and raising and lowering the cable from the support to control vertical movement of the buoyant object, the ballast member including at least one weight which tends to sink in the water whereby the movement of the cable causes or reduces slack in the line thereby increasing or decreasing the influence of the weight on said buoyant object whereby the buoyant object moves up and down.

According to another aspect of the invention apparatus for performing the method of the invention comprises at least one line for connecting the surface support to the buoyant object, the line including a cable connected to a ballast member formed of at least two weights articulated to another, one of the weights being connected to the cable and the other weight arranged to be flexibly connected to the buoyant object. Thus, the apparatus used for performing the method of the invention absorbs almost all the pounding action during the handling of the object during the submergence thereof, whilst improving its stability and permitting its gentle deposition on the sea bed or on a submerged structure.

The invention is described in greater detail hereinafter with reference to preferred, nonlimitative embodiments and with reference to the attached drawings, in which: Figure 1 is a diagrammatic view of an object shown during its handling from a barge, Figure 2 is a diagrammatic elevation of the object to which are fixed ballasting chains connected to the surface by cables, Figure 3 is a part diagrammatic view of the object, and Figure 4 is a part perspective view illustrating a floating object and it handling apparatus.

It is firstly pointed out that the term buoyant object is understood to mean any object with large dimensions whose buoyancy/ weight ratio falls within certain limits, an example of which will be given hereinafter in the present description. Thus, it includes containers, diving equipment, power modules and other such equipment.

Reference will be firstly made to Fig. 1 which illustrates one phase of the method according to the invention. It shows a buoyant object 1 handled from a surface support, for example a barge 2, by means of two lifting gears 3. Two cables 4 wound onto winches 5 are attached by their free ends to ballast members 6, in the present case formed by chains flexibly connected to the object 1.

The term chain is understood to mean any assembly composed of a plurality of elements articulated to one another, such as metal rings passed into one another or even elements which are articulated together by means of pins, or the like.

Although the drawings illustrate apparatus having articulated ballast members it is pointed out that the invention also covers other ballasting arrangements, for example weights attached to the cables 4 and attached to said buoyant object by means of flexible cables whereby said weights can be fitted in a string and can have any configuration such as spherical, cylindrical, cubic or the like. It will be appreciated that in each case the ballast members tend to sink in water.

Figure 4 shows a method for attaching the buoyant object 1 by means of four chains 6 fixed in pairs to each end of said object. Each chain 6 is fixed in an articulated manner by one end to the upper part of the buoyant object 1 and by the other end to a beam 7 which is connected to cable 4 by two steel wires 8, each of them being fixed to one of the ends of the said beam, so that the assembly of the beam 7 and the wires 8 forms a triangle.

Prior to being submerged the buoyant object 1 is, for example, located on the bridge of the barge 2.

The ballast necessary for applying the process of the invention during its submergence is attached to the object 1. When using articulated ballasts or "chains" the length of the chain which is necessary is determined as a function of the weight per metre of the latter and of the weight to be balanced. For balancing and handling reasons the length of chain 6 forming the apparatus will be the same. The other end of each chain is attached to the beam 7 which is itself connected to the cable 4.

The object 1 is firstly raised by means of the lifting gears 3 and winches 5 carried by cables 4 and chains 6 and is then lowered into the aquatic medium.

The buoyant object remains on the waves for as long as the chain 6 carried by the cables 4 and beam 7 remain raised. Its submergence is obtained in proportion to the lowering of the chains, the weight of the latter fulfilling the necessary function and making the load heavier.

Thus, the submergence of the object 1 is controlled from the surface support 2 and its lowering or raising are obtained by pulling or easing cables 4, said operation increasing or eliminating partly or wholly the weight of the ballast members attached to the object.

Hereinafter will be given as an example the method applied with a view to the determination of the necessary buoyancy of the equipment, taking account of the stresses caused by the surface support, as well as the weight and length of the chains necessary in certain special cases.

In the hypothesis that the equipment is immobilised and has a buoyancy f, the surface support being assumed to have a vertical movement and a very low speed which makes it possible to ignore forces at all times, the position of the equipment and chains is defined by: z (t) : position of the centre of gravity of the object relative to a system of co-ordinates linked with the sea bed.

x (t) : position of the ends of the chains relative to this system of co ordinates. Figure 2 shows that point x is located on a line 9 joining the ends 6a of chains 6.

zo and xo are values of z and x at equilibrium. Thus, the forces applied to the equipment are as follows: Buoyancy F Inertia (M = mass of equipment + mass of water swept along) M d2 z dt2 Weight P of chain (ballast) acting on equipment.

For simplification purposes it is assumed that the weight of chain P corresponds to the length of chain L below the equipment.

If P is the weight per metre of the chain and if two chains are joined to the object we obtain:

= pr(zzo)(xxo)] Therefore the equation of movement is:

x - xo is the swinging movement of the surface support.

xxo = A sin co t in which A is the amplitude of movement.

wt = 27r/T in which T is the natural period of the surface support.

The movement of the equipment after during the transient behaviour is: z - zo = B sinot in which

As an example the ballasting characteristics for obtaining a damping constant equal to 10 and for a period of six seconds are (where A and B are of contrary signs):

Thus, for a chain length of three metres and consequently a total deflection of six metres the equilibrium of the equipment must be obtained for a median position of the chain, i.e.

1.50 m.

Therefore, the buoyancy of the equipment must be:

1 =1.50m F =2px1. F = 3p The buoyancy/mass ratio of the equipment is: P ~ F M = 3M = 0.1 from which we gather F = 0.3 M For example: for M = 100 T F = 30KN-3T The above numerical example shows that submarine equipment whose buoyancy is low compared with its mass is subject to a very significant damping of the stresses of the surface support, for example 1/10 for a buoyancy of 3So of the mass.

WHAT WE CLAIM IS: 1. A method of controlling the submergence, the stay under water and the raising to the surface of a buoyant object from a surface support, comprising connecting at least one line from the surface support to the object, the line including a ballast member connected to the object by a flexible connection on the one hand and to a cable on the other hand and the cable extending to the surface support; and raising and lowering the cable from the support to control vertical movement of the buoyant object, the ballast member including at least one weight which tends to sink in the water whereby the movement of the cable causes or reduces slack in the line thereby increasing or decreasing the influence of the weight on said buoyant object whereby the buoyant object moves up and down.

2. Apparatus for performing the method according to claim 1 comprising at least one line for connecting the surface support to the buoyant object, the line including a cable connected to ballast member formed of at least two weights articulated to another, one of the weights being connected to the cable and the other weight arranged to be flexibly connected to the buoyant object.

3. Apparatus according to claim 2 comprising two ballast members each in the form of a chain.

4. Apparatus according to claim 3 wherein chain is attached to a cable which is raised and lowered from a winch on the surface support.

5. Apparatus according to claim 2 wherein the ballast member comprises weights regularly distributed along said line.

6. Apparatus according to claim 5 wherein said weights are each attached to the object by means of a cable and to the surface support by means of another cable operated from a winch.

7. Apparatus according to claim 6 wherein the weights are fixed in a string to a cable connected to the buoyant object at one end and to the cable operated from the winch at the other end.

8. Apparatus according to any one of claims 2 to 7 wherein lines are connected to either side of the object and the ballast member of each line is attached to a beam, the beam being connected to the cable extending to the surface support.

9. Apparatus for controlling the submergence of a buoyant object from a surface support substantially as described with reference to Fig. 1 of the drawings.

10. Apparatus for controlling the submergence of a buoyant object from a surface support substantially as described with reference to Figure 4 of the drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. The movement of the equipment after during the transient behaviour is: z - zo = B sinot in which As an example the ballasting characteristics for obtaining a damping constant equal to 10 and for a period of six seconds are (where A and B are of contrary signs): Thus, for a chain length of three metres and consequently a total deflection of six metres the equilibrium of the equipment must be obtained for a median position of the chain, i.e. 1.50 m. Therefore, the buoyancy of the equipment must be: 1 =1.50m F =2px1. F = 3p The buoyancy/mass ratio of the equipment is: P ~ F M = 3M = 0.1 from which we gather F = 0.3 M For example: for M = 100 T F = 30KN-3T The above numerical example shows that submarine equipment whose buoyancy is low compared with its mass is subject to a very significant damping of the stresses of the surface support, for example 1/10 for a buoyancy of 3So of the mass. WHAT WE CLAIM IS:

1. A method of controlling the submergence, the stay under water and the raising to the surface of a buoyant object from a surface support, comprising connecting at least one line from the surface support to the object, the line including a ballast member connected to the object by a flexible connection on the one hand and to a cable on the other hand and the cable extending to the surface support; and raising and lowering the cable from the support to control vertical movement of the buoyant object, the ballast member including at least one weight which tends to sink in the water whereby the movement of the cable causes or reduces slack in the line thereby increasing or decreasing the influence of the weight on said buoyant object whereby the buoyant object moves up and down.