HOISTING DEVICE

The invention relates to a hoisting device arranged at an upper portion of an elongated, tensionally strong body which is connected, by a lower portion, to a seabed or a subsea structure and is suspended, at an upper portion, from a floating installation, more particularly by a linear actuator being attached to an upper portion of a riser, a drill string or a constant tension wire and being provided with a gripping device arranged for releasable connection to a portion of an elongated hoisting means which extends from the floating installation down towards the seabed, and being provided with means arranged to move the gripping device in the longitudinal direction of the hoisting means at least.

By a "linear actuator" is meant, in this connection, an actuator which is arranged to make a connected object move linearly. The linear movement may be provided directly by a telescopic element being extended or contracted, for example a hydraulic cylinder, or a linear movement of an element may be provided by another element performing a rotational movement, for example a rotating screw moving a nut, a toothed wheel moving a pitch rack, a drive roller moving an endless belt, a chain wheel moving an endless chain, and a drum reeling out or pulling in a wire.

By a "riser" is meant, in this connection, any pipe resting on a base on the seabed or in a borehole and extending up

from the seabed and exhibiting a fixed length. Even though it is natural for a person skilled in the art to think of a riser ("marine riser") in the usual sense of the word within the field of petroleum production from subsea reservoirs, the term "riser" also covers, if nothing else is specified, for example a drill pipe extending, in the initial phase of the establishing of a well, from the borehole up to an installation at the sea surface without an ordinary riser having been set down.

During the construction, operation and maintenance of subsea oil and gas fields a number of hoisting operations are performed by means of hoisting devices arranged on floating installations. When objects are to be placed on the seabed or connected to objects that are already there, heave motions of the floating installation will cause a load hanging on a hoisting device, for example on a hoisting wire extending downwards from a crane or a winch, to move vertically relative to the seabed. A consequence of this is that such hoisting operations can only be carried out in very calm weather conditions, with little wave activity. With the manoeuvrability and heave compensators of today's hoisting devices, a vertical heave motion exceeding 1-1.5 m will lead to hoisting operations as described above being postponed while waiting for calmer weather.

The invention has for its object to remedy or reduce at least one of the drawbacks of the prior art .

The object is achieved through the features which are specified in the description below and in the claims that follow.

The invention provides a hoisting device which is attached to a suspended body extending between the seabed and a floating

installation, for example a riser extending from a wellhead of a well arranged for oil or gas production up to the bottom of a floating installation, a drill string extending between a borehole and a drilling machine on the floating installation, or a constant-tension wire anchored to the seabed, extending up to a constant-tension winch on the floating installation. The hoisting device includes a gripping device arranged for releasable connection to a hoisting means, for example a hoisting wire, extending from a hoisting device on a floating installation connected to the suspended, elongated body down towards the seabed and being used for hoisting objects which are to be connected to a seabed installation or removed from it. As the object is to be moved over a distance near the seabed, the hoisting operation is performed by the hoisting device according to the invention being connected to said hoisting wire which is connected to the floating installation and moving at least partially in step with the heave movement thereof created by waves on the sea surface. By the hoisting operation being carried out, in this critical phase, by means of the hoisting device which is fixed at a certain height above the seabed unaffected by wave-induced heave movement, the hoisting operation may be carried out with great precision. When the object is being lowered, the hoisting device is connected to the hoisting wire when the object has a distance to the seabed or the installation to which the object is to be connected, lying within the hoisting height of the hoisting device, typically about 5 metres, whereas, when hoisting, the connection takes place before the hoisting starts, the hoisting device according to the invention being used for hoisting over the first distance until a safe distance has been achieved, typically about 5 metres. The connection of the hoisting device and wire may happen manually on the

cellar deck of the floating installation or in some other suitable place below the hoisting device or by the remote control of a gripping device which may be remote from the installation. The device for connecting the hoisting device and hoisting wire may be a guide which surrounds the hoisting wire during the entire hoisting operation and is arranged to be releasably attached to the hoisting wire by said connection.

More specifically, the invention relates to a hoisting device at a floating installation which is connected to an upper end portion of an elongated, tensionally strong body which is connected by a lower end portion to a seabed and is suspended, at its upper end portion, from the floating installation, characterized by: - a linear actuator being secured to the body; the linear actuator being provided with a gripping device arranged for releasable connection to a portion of an elongated hoisting means which extends from the floating installation down towards the seabed; and - the linear actuator being provided with means arranged to move the gripping device in the longitudinal direction of the hoisting means at least.

The elongated, tensionally strong body may be a riser secured at a well head, a drill string extending down a borehole, or a wire anchored to the seabed.

The gripping device may be provided with a remote-controlled closing mechanism.

The gripping device may form a guiding device enclosing the cross section of the hoisting means.

The linear actuator may be a hydraulic cylinder, preferably hangingly arranged on a frame with a piston rod end facing downwards .

The linear actuator may be provided with a transmission device.

When the linear actuator is constituted by a hydraulic cylinder hangingly arranged on a frame with a piston rod end facing downwards, the transmission device may be constituted by an elongated, tensionally strong suspension means which is secured, at a first end portion, to the hoisting device and runs uninterruptedly over multipartite sheaves arranged alternately at either end of the hydraulic cylinder and is secured, at a second end portion, to the gripping device.

The linear actuator may be rotatably suspended around a substantially vertical rotational axis.

The rotational axis of the linear actuator may substantially coincide with the centre axis of the hydraulic cylinder.

The suspension means may run over a guide arranged distantly from the centre axis of the linear actuator and movably in a plane substantially perpendicular to the longitudinal direction of the hoisting means.

The guide may be arranged on an arm projecting from the linear actuator.

The frame may bear supportingly against a portion of the hydraulic cylinder.

The frame may form a supporting bearing around the hydraulic cylinder.

The frame may be releasably attached to a riser attachment ring, alternatively to a drill pipe string.

The elongated hoisting means may be a hoisting wire extending from a hoisting drum.

In what follows is described an example of a preferred embodiment which is visualized in the accompanying drawings, in which

Figure 1 shows a principle drawing of a field installation comprising a well with a wellhead connected to a floating platform via a riser, an object being lowered towards the wellhead;

Figure 2 shows, in perspective and on a larger scale, a hoisting device according to the invention, attached to an upper end portion of the riser;

Figure 3 shows a side view of the hoisting device according to the invention; and

Figure 4 shows a principle drawing of a field installation comprising a floating platform connected to a seabed installation via a constant -tension wire extended from a constant-tension winch.

In the figures the reference numeral 1 indicates a hoisting device according to the invention.

In the figures 1, 2 and 3 the hoisting device 1 is shown arranged on a first, upper end portion 21 of a riser 2, known per se, which rests by a second end portion on an installation on the seabed 3, for example a wellhead 31. The riser extends between the seabed 3 and a floating installation 5, shown schematically here as a partially submersible platform. The floating installation 5 is provided with one or more

means for moving objects 32 between the floating installation 5 and the seabed 3 or installation 22 on the seabed 3, typically comprising a hoisting drum 41 connected to an elongated hoisting means 4, for example a wire. Hoisting operations involving objects 32 which have been placed or are to be placed at the wellhead 31, will be carried out by the hoisting means 4 moving objects near the riser 2.

The riser 2 is hung from the floating installation 5 via so-called heave compensators 24 known per se, here shown schematically, for maintaining tension on the riser 2 during vertical movements of the floating installation 5 caused by waves and varying load on board the installation 5. The upper end portion 21 of the riser 2 is provided with an attachment ring 211 for the connection of the riser 2 and heave compensator 24. From the upper end portion of the riser 2, a telescopic unit 23 extends in accordance with the prior art, known per se, up to the deck of the floating installation 5 so that a continuous pipe bore is formed between the wellhead 31 and the floating installation 5.

In figure 4 the hoisting device 1 is shown attached to a constant-tension wire 61 extending from a constant-tension winch 6 to a seabed installation 62 which may be, in its simplest form, a bottom anchor, a foundation or similar, sufficiently secured to the seabed or having sufficient weight to maintain stable anchoring for the constant-tension wire 61 as the constant-tension winch 6 maintains the prescribed tension on the constant-tension wire 61 during the heave motion of the floating installation 5 and as the hoisting device 1 is used.

The hoisting device 1 is provided with a linear actuator 11 which is arranged in a preferred embodiment by a hydraulic cylinder 112 being suspended, by a cylinder end wall 112b,

from a frame 12 which is provided with attaching means 122 for releasable attachment to attachment lugs 211a on the riser attachment ring 211. In the exemplary embodiment according to figure 4, the frame 12 is provided with means (not shown) for releasably attaching and supporting the hoisting device 1 on a portion of the constant-tension wire 61.

The hydraulic cylinder 112 is provided with several sheaves 114 arranged at the cylinder bottom end wall 112b and on a piston rod end 112a projecting downwards. An elongated suspension means 113 in the form of a wire is attached to the hydraulic cylinder 112 and is passed uninterruptedly over alternately upper and lower sheaves 114, thereby forming a transmission device as a multi-groove pulley. In figure 2 the hoisting device 1 is shown with a fourfold passing of the suspension means 113 around the hydraulic cylinder 112. Thereby is achieved a quadrupling of the vertical movement of the free end portion of the suspension means 113 in relation to the longitudinal movement of the hydraulic cylinder 112.

To the free end portion of the suspension means 113 is attached a gripping device 111 which is arranged to be releasably connected to the elongated hoisting means 4 by gripping around an arbitrary portion of the hoisting means 4. Even though the gripping device 111 is only shown schematically here, it is obvious to a person skilled in the art to arrange the gripping device 111 for remote control. As it is shown in figure 1, the gripping device 111 could also be operated manually by personnel positioned on a so-called cellar deck 51 of the floating installation 5.

The hoisting device 1 is provided with an arm 116 which projects from the hydraulic cylinder 112 and is provided with a guide 115 for the suspension means 113 in the form of a

further sheave. In addition, the hydraulic cylinder 112 is rotatably suspended around a vertical axis, whereby, in addition to its vertical movement, the gripping device 111 may be moved through a sector to enable, thereby, the object 32 which is being hoisted to be placed with greater precision in the horizontal plane by the linear actuator 11 being rotated.

To support the elongated linear actuator 11 in the form of a hydraulic cylinder 112, the frame 12 is provided with a supporting bearing 121 surrounding a portion of the hydraulic cylinder 112. The frame 12 is also provided with an adjustable abutment 123 for supporting the frame 12 against the riser 21.

When the hoisting device 1 according to the invention is used in an operation in which the object 32 is to be set down on the wellhead 31 or seabed 3 or some other area which does not move in step with the heave motion of the floating installation 5, the object is lowered to a safe vertical distance from the receiving point. The safe distance must lie within the vertical range of movement of the hoisting device 1. With the gripping device 111 pulled all the way up, the gripping device 111 is connected to the hoisting means 4. By further paying out of the hoisting means 4 from the hoisting drum 41, the hoisting means 4 with the hoisted object 32 is hung off in the hoisting device 1. With the hoisting drum 41 disengaged, the hoisted object 32 is then lowered to the receiving point 3, 31 without being influenced by the heave motion of the installation 5, the hoisting device 1 being supported on the seabed via the riser 2.

When an object 32 is hoisted from the seabed 3 or wellhead 31 or some other area which does not move in step with the heave motion of the floating installation 5, so much of the

hoisting means 4 is paid out by the hoisting drum 41 that the heave motion of the floating installation 5 does not cause any movement of the hoisted object 32, but the slack of the hoisting means 4 is taken up to a safety limit determined on the basis of the prevailing heave motion. Next, the gripping device 111 is lowered before it is connected to the hoisting means 4. Then the hoisted object 32 is pulled up from the surface 3, 31 to a safe distance before the hoisting drum 41 takes over the hoisting operation and the gripping device 111 is disconnected from the hoisting means 4.

Even though a linear actuator in the form of a hydraulic cylinder 112 is shown and described here, it is obvious that any form of linear actuator having sufficient manoeuvring speed and hoisting power could be used within the scope of the invention. For example, a winch drum could replace the hydraulic cylinder 112.

When the hoisting device 1 according to the invention is arranged on the constant -tension wire 61 as it is shown in figure 4, lowering of the hoisted object 32 onto the seabed installation 62 may be carried out in the following way (it is assumed in the example that the hoisting height of the hoisting device 1 is minimum 5 metres) :

1. The constant-tension wire 61 is lowered until its end is 5 m above the seabed installation 62.

2. The hoisting device 1 is connected to the constant-tension wire 61 at a suitable height above the cellar deck 51.

3. The hoisted object 32 which is to be landed is lowered by the hoisting means 4 to 10 m above the final landing level.

4. When the gripping device 111 of the hoisting device 1 is

in its lowermost position, it is connected to the hoisting means 4 and the weight of the hoisted object 32 is transferred to the hoisting device 1 and constant -tension wire 61. The hoisting means 4 between the gripping device 111 and hoisting drum 41 is kept unloaded. The hoisting device 1 pulls the hoisted object 32 up into the uppermost position of the gripping device 111.

5. The constant-tension winch 6 pays out the constant-tension wire 61, which is secured to the seabed installation 62. The hoisted object 32 is now 5 m above the landing level .

6. The constant-tension wire 61 is tautened by the constant -tension winch 6 compensating, in a manner known per se, for the heave motion of the floating installation 5.

7. The hoisting device 1 lowers the hoisted object 32 while the hoisting drum 41 keeps the hoisting means 4 slack above the gripping device 111.

The hoisting of a hoisted object 32 may be carried out like this:

1. The constant-tension wire 61 is lowered to 5 m above the seabed installation 62.

2. The hoisting device 1 is connected to the constant- tension wire 61 at a suitable height above the cellar deck 51.

3. The hoisting drum 41 lowers the hoisting means 4 to 10 m above the seabed installation 62.

4. The gripping device 111 of the hoisting device 1 is connected to the hoisting means 4, and the hoisting device 1 carries the part of the hoisting means 4 extending downwards

from the gripping device 111. The hoisting means 4 between the gripping device 111 and the hoisting drum 41 is kept unloaded.

5. The constant -tension winch 6 pays out the constant-tension wire 61 which is secured to the seabed installation 62. The end of the hoisting means 4 is now 5 m above the seabed installation 62.

6. The constant -tension wire 61 is tautened by the constant-tension winch 6 compensating, in a manner known per se, for the heave motion of the floating installation 5.

7. The gripping device 111 lowers the hoisting means for connection to the hoisted object 32. The hoisting device 1 is activated and pulled in. The hoisted object 32 is lifted without being influenced by the heave motion of the floating installation 5.

8. The constant-tension wire 61 is slackened sufficiently for the hoisting drum 41 to take over the load. The gripping device 111 is disconnected from the hoisting means 4 and the hoisted object 32 is lifted to the surface.