GB2316925A

GB2316925A - Method and arrangement for connecting a submerged loading buoy to a floating vessel

Info

Publication number: GB2316925A
Application number: GB9717171A
Authority: GB
Inventors: Bjorn Morten Mikalsen; Bernt Smedal
Original assignee: Den Norske Stats Oljeselskap AS
Current assignee: Equinor ASA
Priority date: 1996-08-13
Filing date: 1997-08-13
Publication date: 1998-03-11
Anticipated expiration: 2017-08-13
Also published as: NO963359L; NO313820B1; GB2316925B; NO963359D0; GB9717171D0

Abstract

A method and an arrangement for connecting a submerged loading buoy 5 to a vessel 1, wherein a first sink line 10 is lowered through a shaft 3 in the vessel, the line 10 at its lower end being provided with a releasable weight 14 and with buoyancy means 11 and is caused to rise to the water surface alongside the vessel upon release of the weight 14 via a hydrostatic trigger 13. The end of line 10 is then connected to a further line 6 which is connected to the submerged buoy 5 and thereafter the buoy 5 is hauled up and into a submerged receiving space 4 of the vessel.

Description

Method and arrangement for connecting a submerged loading buov to a floating vessel The present invention relates to a method and an arrangement for connecting a submerged buoy to a floating vessel, for the transfer of a flowable medium to or from the vessel. The method particularly will find its application in connection with the use of a so-called STL ship, where STL stands for "Submerged Turret Loading".

STL ships are used together with a submerged, bottomanchored loading buoy for the transfer of oil to/from the ship.

The ship has a shaft which is normally located at the bow portion and extends downwards from deck level to a downwardly open, submerged receiving space at the bottom level of the ship. In this submerged space said buoy is received in connection with loading/unloading of the ship. The loading buoy is connected to a riser or hose extending downwards to the level of the sea bed where the hose communicates with oil-transferring pipes. In the inoperative condition the loading buoy is submerged in the sea.

A line which is lowered through the shaft in the ship is connected to a float-connected line extending upwards from the loading buoy, and when hauling in the lines the loading buoy is raised in the sea until it is connected in the shaft. A winch which is located over the shaft, is used for the hauling.

In order to be able to connect the line which extends downwards through the shaft to the line which extends upwards through the water from the submerged loading buoy, the two free line ends must be brought together on the ship. Traditionally, this has been done in that a line with a weight at the end is lowered through the ship shaft. An additional line is fixed to the weight, and a buoy is attached to the opposite end of this latter line. The line with the weight is lowered until the additional line with the buoy is located at some distance below the bottom of the ship. The ship is now given a lateral movement at the same time as the weight is raised again. This results in that the buoy, and the line to which it is fixed, moves upwards through the water and outwards to the side of the hull. The buoy is brought up to the surface at the ship side, and thereafter the buoy with the line end is brought onto the deck of the ship. Also the float-connected line which extends up from the submerged loading buoy, is brought onto the deck. The line from the shaft and the line from the loading buoy are connected together, and the loading buoy is hauled up for connection in the shaft by means of a winch which pulls in the shaft line and the loading buoy line. The loading buoy is hereby pulled into the submerged receiving space in the ship where the loading buoy is immediately locked.

Even if this method in principle is simple to carry out, it has been found that difficulties often arise in practice.

The weight-connected line with the buoy does not always come up at the side of the hull, but instead remains under the bottom of the ship. It has also occurred that the buoy comes up at the wrong side of the ship. Much of the reason for these difficulties probably can be attributed to the need for coordinating the lateral movement of the ship with the raising of the line-weightline buoy system in the water.

Therefore, there is a need for an improved method for connecting a loading/unloading buoy to a floating vessel. This is achieved by the present invention.

Thus, according to the invention, there is provided a method for connecting a submerged loading buoy to a floating vessel, wherein a first line is lowered through a shaft in the vessel, the line at its lower end being provided with a weight and with a buoyancy means, and the line end being caused to rise to the water surface alongside the vessel as this moves laterally, whereafter the line is picked up and another line which is connected to the buoy, is also picked up and the lines are connected together, whereafter the connected lines are pulled up through the shaft so that the loading buoy is hoisted and introduced and connected in a submerged receiving space in the vessel, wherein said weight is attached to the free end of the line by means of a hook which can be opened by a hydrostatic trigger which is set to release at a desired water pressure, the buoyancy means being constituted by at least one buoy close to the free end of the line, and wherein the line with the weight suspended nethermost is lowered until the hydrostatic trigger releases and opens the hook, so that the weight is released and disappears from the line end, and the end portion of the line rises to the water surface and can be picked up.

The invention also provides an arrangement for use in connecting a submerged loading buoy to a floating vessel, comprising a first line arranged to be lowered through a shaft in the vessel, and which at its lower end is provided with a weight and with a buoyancy means, the lower end portion of the line after the lowering being intended to float up to be interconnected with an end of a second line which is connected to the buoy, wherein said weight is attached to the free end of the line by means of a hook which is arranged to be opened by a hydrostatic trigger which can be set to release at a desired water pressure, the buoyancy means being constituted by at least one buoy close to the free end of the line.

With the present invention one does not use the buoy and the line which, according to the old system, are connected to the weight-line combination. Instead there is used only one line to which there are attached a weight as well as a buoy, or possibly several buoys. The buoy is situated close to the line end, and the weight is fixed to the line end. In the new method, the weight is attached to the line by a mechanism which is released at a certain hydrostatic pressure. The line-buoy-weight combination is lowered through the vessel shaft and into the water. The weight is released at a predetermined depth, and the buoy with the line end floats up to the surface. In order to get up the buoy with the line end in the desired manner, the ship is put in lateral motion, as in the old method. The buoy and the line are brought onto the deck, and connection to the line from the loading buoy is carried out as before. It has been found that with this new method, one is able to reduce the connecting time to half of the time used with the old method.

The invention will be further described below in connection with an exemplary embodiment with reference to the accompanying drawings, wherein Fig. 1 shows a vertical section through the longitudinal axis of the hull of a ship; Fig. 2 shows a vertical cross-section through the hull and a hull shaft; Fig. 3 shows a vertical cross-section through the hull and the hull shaft, with connection equipment lowered down and out from the shaft; Fig. 4 shows a vertical cross-section through the hull and the hull shaft, wherein elements of the connecting equipment are separated from the remaining equipment; Fig. 5 shows a vertical cross-section through the hull and the shaft, wherein the connecting equipment is brought up to the sea surface at the side of the ship; Fig. 6 shows a vertical section through the longitudinal axis of the ship hull, wherein a connection is effected between the ship and a submerged buoy; and Fig. 7 shows a vertical section through the longitudinal axis of the ship hull, with the submerged buoy brought up to the hull shaft.

As appears from Fig. 1, the system comprises a floating vessel 1 and a buoy 5 which is to be connected to the vessel in a downwardly open, submerged receiving space 4 arranged in the vessel. The vessel is a tanker, for example a so-called shuttle tanker, and the buoy is a loading/unloading buoy for the transfer of a flowable medium to or from the vessel.

The loading buoy is anchored to the sea bed by means of a suitable number of mooring lines 8 extending in a catenary fashion between the loading buoy and the sea bed. At its underside the buoy is coupled to a transfer line in the form of a flexible riser extending between the buoy and pipes/hoses at the bottom, where the pipes/hoses are to deliver a flowable medium to, or receive a flowable medium from, the riser.

As appears from Fig. 1, the submerged receiving space 4 is arranged in a lower part of the bow of the vessel. The receiving space 4 is connected to the deck of the vessel via an access or service shaft 3. In the deck region of the ship there is arranged a hoisting means in the form of a winch 2 having a line which can be lowered via the shaft 3 and the receiving space 4 and be connected to the buoy 5, so that the buoy can be hoisted up and moved in place in the receiving space 4.

In order for the ship to be able to connect itself to the submerged loading buoy 5, this is connected to a line 6 which in turn is connected to a float 7 floating at the water surface.

The method for buoy connection according to the invention will now be described below with reference to Figs. 2 7.

The ship moves towards the loading buoy 5 and takes up a position not far from the float 7. The ship or the bow of the ship is now given a lateral movement of the order of 1,5 knots, cf. Figs. 2 and 3. The lateral or sideways motion is achieved by the use of bow trusters with which such ships are provided. A line 10 is simultaneously lowered through the shaft 3. In the illustrated embodiment, a pair of buoyancy buoys 11 are attached to the line 10, and below said buoys there is a hook 12 which is connected to a hydrostatic trigger 13. A weight 14 is suspended from the hook 12. The weight may for example be in the form of a bag filled with 75 kilos of gravel.

The hydrostatic trigger 13 is set or adjusted to open at a certain water pressure, for example a pressure corresponding to a depth of 50 meters. When a sufficient length of line has been let out from the winch 2 so that the hydrostatic trigger opens, the weight 14 loosens from the trigger 13 and disappears in the water, cf. Fig. 4.

The lateral movement of the ship and the buoyancy of the buoys 11 cause the line 10 to move upwards in the water and outwards to the side of the hull side, cf. Fig. 5. The line end is picked up onto the deck. The ship thereafter moves into position above the subsea loading buoy and picks up the float 7 which is connected to the end of the line 6 connected to the loading buoy 5, cf. Fig. 6. The lines 10 and 6 are interconnected, and the lines 10, 6 are pulled in with the winch 2. With continued pulling-in of the interconnected lines 10 and 6, the loading buoy 5 is hoisted in a known manner through the body of water until it comes to rest in and is locked in the receiving space 4 located at the lower end of the shaft 3 in the vessel, cf. Fig. 7. When the loading buoy 5 is connected to the vessel, the load transfer can start.

Claims

Patent claims

1. A method for connecting a submerged loading buoy to a floating vessel wherein a first line is lowered through a shaft in the vessel, the line at its lower end being provided with a weight and with a buoyancy means and the line end being caused to rise to the water surface alongside the vessel as this moves laterally, whereafter the line is picked up and another line which is connected to the buoy is also picked up and the lines are connected together, whereafter the connected lines are pulled up through the shaft so that the loading buoy is hoisted up and introduced and connected in a submerged receiving space in the vessel, CHARACTERIZED IN that said weight is attached to the free end of the line by means of a hook which can be opened by a hydrostatic trigger which is set to release at a desired water pressure, the buoyancy means being constituted by at least one buoy close to the free end of the line and that the line with the weight suspended nethermost is lowered until the hydrostatic trigger releases and opens the hook so that the weight is released and disappears from the line end, and the end portion of the line rises to the water surface and can be picked up.

2. An arrangement for use in connecting a submerged loading buoy to a floating vessel, comprising a first line arranged to be lowered through a shaft in the vessel which line at its lower end is provided with a weight and with a buoyancy means the lower end portion of the line after the lowering being intended to float up to be interconnected with an end of a second line which is connected to the buoy CHARACTERIZED IN that said weight is attached to the free end of the line by means of a hook which is arranged to be opened by a hydrostatic trigger which can be set to release at a desired water pressure, the buoyancy means being constituted by at least one buoy close to the free end of the line.