GB2282583A

GB2282583A - Connecting buoys to vessels

Info

Publication number: GB2282583A
Application number: GB9419364A
Authority: GB
Inventors: Knut Borseth
Original assignee: MARITIME GROUP AS
Current assignee: MARITIME GROUP AS
Priority date: 1993-09-27
Filing date: 1994-09-26
Publication date: 1995-04-12
Anticipated expiration: 2014-09-26
Also published as: NO933444L; GB9419364D0; GB9702256D0; NO933444D0; GB2282583B; NO300726B1

Abstract

A method of catching a line, such as a sink line 3 lowered through a shaft 2 in a vessel, or a pick up line 8 attached to a submerged buoy 9, involves the use of a catch line 5 provided with weight means 7 and suspended in the form of a "U" eg from a derrick 6. The catch line 5 may be provided with hooks. The sink line 8 may be provided at its lower end with a hook (11, Fig. 2) or with a variable buoyancy device comprising a ballast compartment and a compressed air source which is automatically released to expel the ballast at a given depth to cause the end of the sink line to float up in front of the vessel, enabling the line 5 to be caught. In order to connect the buoy 9 to the vessel, the sink line 3 and catch line 8 are caught and hauled on deck in turn. They are connected together then the line 3 is pulled in, thus drawing the buoy to the shaft. <IMAGE>

Description

A LINE ARRANGEMENT The present invention relates to a method as recited in the preamble of the appended claim 1.

Such methods are used ie in the oil industry for connecting a submerged buoy to a vessel. However, the known methods suffer from the deficiency that they are difficult, if not impossible, to perform in bad weather.

One object of the present invention is to improve said method so that the buoy or other object may be readily connected to the vessel also during inclement weather conditions.

This is obtained in accordance with the invention as recited in the characterising clause of the appended claim 1.

Further advantageous embodiments of the invention are recited in the dependent claims.

The invention also comprises an element for use in performing the method as recited in claim 10.

Embodiments of the present invention will now be described by way of example with reference to the accompanied drawings, in which: Figure 1 is a schematic view of a body of water and parts of a vessel provided with means for performing the invention; and Figure 2 is a schematic transverse section of a floating vessel illustrating a second embodiment of the invention.

The vessel 1 in Figure 1 is provided with a shaft 2, through which a sink line has been lowered. At its free end the sink line 3 is provided with a suitable element 4 initially having negative buoyancy. The element 4 is provided with means which is released by the hydrostatic pressure at a certain depth to change the buoyancy of the element 4 from negative to positive, eg by evacuating a ballast chamber of the element or inflating a suitable float. Consequently, the element 4 will rise to the surface and assume the position indicated by 4' in Figure 1. To insure that the element 4 will reach the surface in front of the vessel 1, the vessel is reversed as indicated by the arrow on the left in Figure 1.

In its position 4' the element and the sink line 3 are caught by a catch line 5, which is deployed from a derrick 6 on the vessel. The catch line is kept substantially in the form of a U during the catching operation so as to bring the base portion 5' of the catch line into contact with the sink line 3. Once contact and a connection is established between the lines, eg by means of hooks on the sink line 3 or element 4, the sink line with the element 4 is raised to the deck of the vessel by means of the catch line 5 and the derrick 6. In order to facilitate maintaining the catch line 5 in the form of a U, it may be provided with weights 7 between the base portion and the legs of the U.

Next, the catch line 5, 5' is used to catch the pick-up line 8 attached to a submerged buoy 9, as shown on the right hand side of Figure 1. The pick-up line 8 is at its free end provided with buoyancy means 10. It will be understood that for the catching operation the vessel 1 is brought closer to the buoyancy means 10 so that it may be caught by the catch line 5 by a sweeping motion, for example made by swinging the derrick 6. Once the catch line 5 has caught the pick-up line 8 with the buoyancy means 10, the pick-up line is hoisted up to the deck of the vessel and connected to the sink line 3 or a suitable intermediary line so that the latter may be used for pulling the pick-up line up through the shaft 2 of the vessel and eventually bringing the buoy 9 into contact with the vessel.

The buoyancy-changing element 4 may have a general structure like an ordinary hydraulic accumulator, ie a metallic pressure vessel having an internal cavity divided into two compartments, one for pressurising gas and one for pressurized liquid, by a flexible membrane such as a rubber bladder. In order to give the element a negative buoyancy before lowering it into the water, the pressure vessel is filled with a sufficient amount of water to make the total weight exceed its buoyancy by a suitable amount. Next, the water compartment is closed and the gas compartment is pressurized, for instance by compressed air or nitrogen. If the pressure vessel is two thirds full of water and the element is to change its buoyancy from negative to positive at a depth of 50 meters, an air pressure of 20 bar will be sufficient in the gas compartment.The water compartment of the pressure vessel is provided with a discharge valve set to open at a predetermined external water pressure, eg 5 bar corresponding to a depth of about 50 meters. It will be readily apparent to the skilled person how such a valve may be constructed to perform this function. As an example, one might use a valve element subjected equally to the internal and external water pressures and additionally being biased by a spring force compensating for the 15 bar differential pressure at the trigger depth. Another possibility would be to use a pilot operated valve.

It will be understood that when this discharge valve opens, the 20 bar air charge will expand and expel all the water from the opposite compartment without the pressure of the air becoming less than 5 bar. With the water emptied, the buoyancy of the element will become positive and the element will rise to the surface bringing the sink line along. In order to facilitate recovery of the element at night time, it may be provided with a beacon. Furthermore, the pressure vessel may be provided with a casing of fendering material, which also may serve as buoyancy material in case the pressure vessel itself or appurtenant grappling hooks etc on the element prove to be too heavy for the available air volume.

Figure 2 illustrates a modification of the present invention where the free end of the sink line 3 is provided with a grappling hook 11 or the like. Before lowering the sink line 3, the catch line 5 is placed in a position behind the shaft 2, ie to the left of the shaft as seen in Figure 1. After lowering the sink line 3 with the grappling hook 11 to the position shown in Figure 2, the catch line 5 is moved forwards so that its base portion 5' will catch the sink line 3 with the hook. The base portion 5' may be provided with buoyancy elements 12 so that it will not sink if going slack, or the base portion may be made of a material that has neutral or slightly positive buoyancy. At the bow of the ship the catch line 5 is hoisted up to the deck of the ship along with the sink line 3, for instance by using the derrick 6.

It will be understood that the invention is not limited to the exemplifying embodiments outlined above.

Claims

1 A method for catching from a vessel (1) a first line such as a sink line (3) lowered through a shaft (2) in the vessel, or a pick-up line (8) attached to a submerged object (9) and provided with buoyancy means (10) remote from said object, said method comprising the steps of: - lowering from the vessel a catch line (5) provided with sinking means (7), - moving the catch line into contact with said first line (3,8) to provide a connection therebetween, and - raising said first line to the vessel by means of the catch line, the method being c h a r a c t e r i z e d b y keeping the catch line (5) substantially in the form of a U during the catching operation and bringing the base portion (5') of the U into said contact with said first line (3,8).

2 A method according to claim 1, characterized in that the catch line (5) is provided with weight means (7) at the ends of the base portions (5').

3 A method according to claim 1 or 2, characterised in that the catch line (5) is moved, with the base portion (5') extending transversally of the vessel (1), from a position behind the shaft (2) forwards to a position at the bow of the vessel before it is raised.

4 A method according to one of the preceding claims, characterized in that the catch line (5) is swung in front of the bow of the vessel (1) transversally of its longitudinal direction in order to establish said connection with said first line (3,8).

5 A method according to claim 4, characterized in that the catch line (5) is swung by means of a derrick (6).

6 A method according to one of the preceding claims, characterized in that the sink line (3), prior to lowering through the shaft (2), is provided with an element (4) having negative buoyancy, said element (4) being arranged to change its buoyancy from negative to positive at a predetermined depth.

7 A method according to claim 6, characterized in that the change from negative to positive buoyancy takes place by evacuating a ballast chamber.

8 A method according to claim 6, characterized in that the change from negative to positive buoyancy takes place by inflating a float.

9 A method according to one of claim 6 to 8, characterized in that in order to change the buoyancy of the element (4) at a predetermined depth, means are used which are released by the hydrostatic pressure at said depth.

10 An element for use in a method according to one or more of the preceding claims, said element (4) comprising a pressure vessel having a first compartment containing a gas and an attachment for a line (3,8), characterized in that the pressure vessel comprises a second compartment separated from said first compartment by a flexible membrane, means for introducing a ballast liquid, eg water, into said second compartment, means for introducing a gas under pressure into said first compartment, and means responsive to external pressure for opening said second compartment to the ambient for permitting said gas under pressure to expel said ballast liquid when said element has reached a predetermined water depth.