The present invention is concerned with a procedure for the launch and sinking of a structure under water. The procedure according to the invention additionally permits the sinking of the structure in a controlled manner, after which the structure is positioned, oriented and fitted in the desired location. The invention is particularly concerned with a method for the launch, sinking, positioning and fitting of a protective structure over an underwater installation, such that the protective structure is to be fixed in the correct position and correct orientation in relation to the installation which it is intended to protect, that at least one guide line is attached in advance to the underwater installation by at least one attachment device provided thereon for the purpose and passes through purpose-made openings in the structure, and that the protective structure is lowered down the guide line towards the underwater installation.

The invention is further concerned with a device specially designed for the implementation of such a procedure.

A system is known in the prior art for controlling the vertical motion of a body which is sunk into water down a guide line, wherein a friction brake acts between the body and the guide line and said friction brake can be operated mechanically. Reference is made in this connection to US patent 3,465,531, in which it is shown how a process pack can be lowered along two guide lines which are attached at their top end to a boom supported by a vessel and at their lower end to an previously installed underwater structure.

The system is relatively complicated and costly because, among other things, it involves the use of braking devices with controllable and adjustable operation, requiring a complicated mechanical/hydraulic control system.

UK patent 2,234,002 discloses a lightweight protective cover provided with ballast. Figs 6a-6d of the said patent illustrate a simple sinking method using steering lines.

The method utilised involves steering lines which can be detached and reused, but the sinking technique is not claimed and is very summarily described.

As examples of other patents including procedures for

lowering large units to the sea floor one may name US patents 3,545,539 and 4,258,794. However, the equipment and method described therein are very complicated and must evidently involve the use of very costly, specialised equipment and very time-consuming procedures.

The object of the present invention is to provide a method and a device permitting the simple and inexpensive launch, sinking, positioning and orientation of a structure or of a protective cover over a structure which it is desired to protect on the sea floor. A further object of the invention is to provide a rapid lowering procedure followed by a speedy and practical positioning and orientation method, such that the sinking and positioning can be carried out in a safe and rapid manner without unnecessary loss of time or unnecessary extra cost. It is furthermore an object of the invention to achieve this result with the aid of simple, inexpensive and reliable equipment. The method can be implemented for an ordinary supply ship without a crane.

The launch takes place by sliding the structure, e.g. the cover, over the stern or side of the vessel. The sinking procedure results in the rapid lowering of objects from the surface to the sea floor without unnecessary control means, yet such that the object stops at a desired position, after which the remainder of the descent can be executed in a much more exact and precise manner so that the object engages with fixing means positioned in advance under water.

To afford a clearer understanding of the present invention, reference is made to the detailed description, below, of an example of embodiment wherein the object is a protective cover or structure, and to the accompanying drawings, where: Fig. 1 shows an example of how the launch can take place, Fig. 2 shows a protective structure after it has been lowered towards the sea floor and is located directly above an underwater installation which it is desired to protect, Figs 3-6 show different steps in the process of the exact positioning, orienting and fitting of the protective structure, and Fig. 7 shows the protective structure after fitting onto

the underwater installation.

Attention is drawn to the fact that the same reference numbers are used for the same components in all the drawings, that unnecessary details have been omitted to avoid clutter, and that the elements shown in the drawings are not necessarily drawn to the same scale.

A plurality of protective structures 1, as indicated in Fig.

1, are transported to the appropriate point on the sea surface by a suitable vessel 20, and a guide line 3 is passed through the structures 1. The protective structure 1 can be launched from the vessel onto the surface by several methods. Normally it will be preferred that the guide line 3 will be passed through a purpose-made opening 5 in the structure 1 while the guide line 3 is slack and the protective structure 1 is on the vessel's deck, after which the structure 1 is launched onto the surface, possibly over a bulge or fender fitted to the vessel.

Fig. 2 shows a protective structure 1 after it has sunk quite quickly, by the action of gravity only, down the guide line 3 and has been stopped at a suitable distance above the underwater installation 2, by butting against a stopping device 12 previously fitted to the guide line 3 at a suitable height above the underwater installation 2.

Other details apparent from Fig. 2 are an attaching means or attachment point 4 intended for the receiving and attachment of the lower end of the several guide lines 3 to the underwater installation 2; at least one opening 5 in the protective structure 1 to accommodate the several guide lines 3; additional steering lines 6 which, together with tensioning means 7, are attached to the protective structure 1; and a remotely-operated underwater vehicle (ROV) which is to perform the necessary manipulations of the equipment.

The underwater installation 2 may be mounted direct on the sea floor or on a wellhead or other underwater installation, depending on the application.

For the present purpose, reference numbers 6 and 7 will be referred to only as 'additional steering lines' and 'tensioning means'. In the embodiment shown in Fig. 1 and the following figures, the tensioning means consists of an

inflatable buoy 7 attached to the additional steering lines 6, which in turn pass over or through guiding means therefor (indicated but not numbered). The protective structure 1 is assumed to be open to the water on its underside so that the ROV can grasp the lower ends of the additional steering lines 6.

Fig. 3 illustrates a more advanced stage of the procedure.

At this point it is understood that the ROV has grasped the additional steering lines 6, drawn them down to the underwater installation 2, and attached them to suitable purpose-made attachment points 8, 9 previously disposed on the underwater installation. The attachment points 8, 9 may for example take the form of hooks, in which case the steering line may be provided with suitable loops to fit the said hooks. As soon as the additional steering lines 6 have been attached to the underwater installation 2, the ROV is moved up to the buoy 7 and pumps gas into it so as to impart to it a suitable positive buoyancy. The ROV has then finished its present task and is therefore not indicated in Fig. 3.

The situation is then such that the protective structure is suspended on the guide line 3 by means of the stopping means 12 disposed thereon, and is moreover held in the correct position and orientation by the additional steering lines 6, which are now under tension. When the additional steering lines 6 are thus tensioned, the tension on the guide line 3 can be reduced in order to lower the protective structure 1.

Before proceeding to this stage of the procedure we may mention briefly that the underwater installation 2 may appropriately be fitted with suitable fixing bars 10, 11 or similar fixing means designed to engage with matching openings or complementarily shaped fixing means in or on the protective structure 1.

Fig. 4 shows that the protective structure 1 is beginning its descent towards the underwater installation 2, during which the tension on the guide line 3 is gradually reduced.

The steering lines 6 are not provided with stopping means and hence do not impede the said descent.

In Fig. 5 the tension on the guide line 3 has been so far

reduced that the protective structure 1 has been completely lowered onto and into engagement with the underwater installation 2, and in Fig. 6 the gas has been released from the buoy 7 so that the additional steering lines 6 are also completely slack.

The ROV is used in the procedure both to ensure the correct positioning and orientation of the protective structure 1 and to release the gas from the buoy 7, after which it detaches both the guide line 3 and the additional steering lines 6 from the underwater installation 2. In the process, the stopping means 12 may also be collapsed or detached, after which the guide line 3 and the steering lines 6, together with the tensioning means 7, can be raised to the surface.

Fig. 7 shows the end result of the procedure, namely, the underwater installation 2 with the protective structure 1 fixed in the correct position and the correct orientation thereon.

The procedure described above can be modified in many ways.

For example, the operations described as being performed by an ROV may be carried out by a diver; a plurality of guide lines 3 and a greater or smaller number of steering lines 6 may be utilised; the tensioning device which has hitherto been shown only in the form of an inflatable buoy 7 may be replaced with a winch located on the surface vessel or another vessel; and the fixing bars 10, 11 may be of an entirely different design than shown.

With regard to the actual apparatus used to implement the procedure, the design principle thereof is also shown in the figures. Thus, it is necessary to use a structure having the necessary openings and steering means for both the guide line and the steering lines, but the detailing thereof is not shown, as it must evidently be adapted to the specific form of embodiment, which may be varied within wide limits.

Likewise, the attachment means provided for attaching one or more guide lines 3 and steering lines 6 to the underwater installation are not shown in detail, as these too may be designed as desired, provided only that they are adapted for operation by diver or ROV and are accessible both before and

after the protective structure 1 is in position. The stopping means 12, too, might be designed in many different ways, one of the simplest forms being a ball with an opening through which the guide line 3 passes, such that the friction between the guide line and the stop ball can be controlled or is preset to a value to ensure that it will arrest the protective structure 1 as the latter falls freely towards the sea floor, while being low enough that the guide line can be pulled through the ball by applying additional tension thereto. Other more sophisticated solutions of the stopping means 12 can be conceived, e.g. such that the stopping means is inflatable or that it can be moved laterally to an area where it can pass through the protective structure 1, or the protective structure 1 may be provided with a slit or a door such that when said slit or door is opened, the stopping means 12 can pass through the opening. In such designs the stop ball 12 may be permanently mounted on the guide line.

Further with regard to the design of the apparatus, it may be reiterated that the tensioning means 7 may be of various forms provided only that it ensures that the steering lines 6 are tightened sufficiently to steer the structure into the correct position and orientation when the guide line 3 is slackened. The material of the structure 1 may be freely chosen provided that said material is sturdy enough to withstand the expected stresses. Plastics-based composites are mentioned above as suitable materials for protective structures, and other possibilities might be e.g. laminated and sandwich structures, which may also include steel structures. Likewise it should be pointed out that the procedure may naturally be used for the sinking and positioning of all types of structures that are have to be accurately positioned on the sea floor, and not only for protective structures.