PRE-ASSEMBLY OF A SUBSEA BASE AND PIPELINE

The present invention relates to a method for subsea hydrocarbon recovery, in which a wellbore is drilled using a base installed on the seabed, said base comprising a drilling guide, in which method at least one pipeline is to be connected to a network for storage and/ or processing operations. Such a method conventionally comprises the steps of: lowering and positioning a base on the seabed, drilling a wellbore through the drilling guide of the base, - lowering and positioning the pipeline on the seabed, to which pipeline an end termination is connected, measuring the exact distance between the end termination of the pipeline and the base, preparing a connecting element made to size - placing the connecting element manufactured to size between the wellbore and the pipeline.

Such a method is well-known and commonly applied in the art. A disadvantage of the known method is that the positioning of the pipeline and the placing of the connecting element are precise and therefore time-consuming and expensive operations.

The aim of the present invention is to provide an improved method for subsea hydrocarbon recovery. With hydrocarbon recovery all types of recovery including oil and gas recovery are intended. This is achieved by a method according to claim 1, characterized by the following steps: preparing a pre-assembly of a base and one or more pipelines, lowering and positioning the pre-assembly on the seabed, - drilling the wellbore through the drilling guide of the base,

- placing a connecting element between the wellbore and the pipeline.

One of the advantages of a pre-assembled base and one or more pipelines is that the one or more pipelines are laid at an earlier stage in the operation for subsea hydrocarbon recovery, enabling an earlier connection of the pipeline with the network for storage and/ or processing operations. Another advantage is that the placing of the connecting element is facilitated considerably, since the orientation between the pipeline end and the drilling guide for the wellbore is fixed. Normally this is done by specially- manufactured jumpers, the dimension of which has to be measured after the pipeline has been laid. Yet another advantage is that no additional PipeLine End Terminal (PLET) is required when lowering the pipeline, since the base now acts as an end terminal.

It is both possible to connect one or multiple pipelines to a single base, which will for example be dependent on the product (s) flowing through the pipeline (s). In a possible embodiment, the base is lowered first with two or more pipelines connected to it.

In a preferred embodiment, after lowering and positioning a first pre-assembly on the seabed, the base of a second pre-assembly is connected to the pipeline of the first pre-assembly. Possibly, initially a first pipeline is lowered, which can be assembled with a first base forming a first pre-assembly (but can also be connected to a network) , and subsequently connect a second base to this launched first pipeline, to which yet another second pipeline is connected. This way it is possible to install a series of bases, interconnected with pipelines, which are also connected to a network for storage and/ or processing operations.

In a preferred embodiment of the method according to the invention, an anchor is lowered and/ or positioned on the seabed prior to the pre-assembly to assist the lowering of the assembly. The weight of the base can be too little to properly and accurately lower the pipeline. Then, an anchor can assist in lowering and accurately positioning the base and the pipeline.

In a preferred embodiment, the base contains a single drilling guide. It is also possible to provide multiple drilling guides in a single base, e.g. to speed up oil recovery.

Preferably, the method for subsea oil or gas recovery according to the invention comprises the step of positioning a Christmas tree above the wellbore. After drilling the wellbore, possibly wellhead equipment is placed on top of the wellbore, and possibly well completion is performed e.g. by providing conductor pipe or casing in the wellbore. In a possible embodiment, well completion is performed and wellhead equipment is placed after drilling the wellhole. A connecting element can subsequently be placed between the wellhead equipment and the pipeline. As a connecting element, for example a jumper can be used.

In another preferred embodiment, the wellhead equipment and the connecting element are integrated in a Christmas tree. Such a Christmas tree comprises a set of valves, spools and fittings and is conventionally connected to the top of a well to direct and control the flow of fluids from the well. Possibly, the Christmas tree is placed on the seabed prior to drilling. Analogously, it is possible to place the connecting element on the seabed prior to drilling.

Possibly, on the Christmas tree, a blow-out preventer is positioned as is common in subsea oil or gas recovery operations requiring access to the wellbore in a later stage, such as workover operations .

The invention will further be explained with respect to the accompanying drawings, in which:

Figs, la-c show in perspective part of a vessel suitable for performing the method according to the invention, figs. 2a-2c show in perspective view various steps of a method for subsea oil or gas recovery according to the invention,

fig. 3 shows in perspective view a blow-out preventer positioned on a pre-assembly according to the invention.

In figs Ia-Ic part of a vessel 1 is shown, suitable for performing the method according to the present invention. The vessel 1 comprises on deck 3 a reel 2 for storage of pipeline (not shown) . On deck 3 furthermore a crane 4 is arranged, which can be used to lower an anchor (not shown) , but can also assist in lowering accessories such as a base, possibly pre-assembled with a pipeline according to the invention.

At the stern 15 of the vessel 1 a tower 5 is arranged. Tower 5 is extending upwards from the deck and can be tilted. The tower is moveable across the deck 3 parallel to the stern 15 of the vessel 1 across rails 5c. The tower is moveable for unwinding the pipeline more easily. Tower 5 is composed of an A-frame 5a in which an assembly 5b with a pipeline guide, in the shown embodiment aligner G for a pipeline (only shown partially in fig. Ib) is moveably arranged. The arrangement of tower 5 and aligner 6 shown in fig. Ia ensures easy laying of pipeline, tower 5 and aligner 6 acting as a ramp across which the pipeline enters the water. Assembly 5b with aligner 6 can be moved or rotated backwards towards the deck 3 of the vessel as is shown in figs. Ib and Ic. By retracting the assembly 5b with aligner 6, space is created in the tower 5a to introduce an accessory, such as a pre-assembled base and pipeline.

Tower 5 further comprises a working platform 7 which can be entered e.g. by welding engineers to weld a pipeline to a base in order to create a pre-assembled base and pipeline according to the invention. Of course, the working platform 7 can also be used for other operations such as inspection. In the assembly 5b arranged in the A-frame 5a of the tower 5 below the aligner 6 a straightener 8 and a pipe clamp 9 are arranged. In the embodiment shown, a working platform 10 and a pipe clamp 11 are mounted to the A-frame 5a of tower 5. Working platform 10 can be retracted away from the firing line to enable the passage of accessories such as base 12 shown in

fig. Ic by opening as is shown in fig. Ib or alternatively for example by skidding or rotating. Working platform 10 can for example be used to connect an accessory to a launched pipeline, clamped in pipe clamp 11. For reasons of clarity, pipe parts have been shown in pipe clamps 9 and 11, while no other part of the pipeline is shown.

Although it should be understood that tower 5 is appropriate for lowering any accessory, in fig. Ic a base 12 is shown. The shown base 12 is not yet assembled with a pipeline.

In figs. 2a-2d various steps of a method for subsea oil or gas recovery according to the invention are illustrated. In fig. 2a an assembly 20 according to the invention is shown launched on the seabed 21. The assembly 20 is pre-assembled from a pipeline 20a and a base 20b. Base 20b comprises a drilling guide 20c to enable drilling of a wellbore and a pipeline end terminal 2Od which can be connected to the wellbore via a connecting element. Base 20b further comprises a hook member 2Oe that can be used to connect base 20b during launch e.g. via a cable to an anchor to assist in lowering and/ positioning the assembly 20.

Positioning elements are arranged on the assembly 20 to enable an easy and precise connection between the assembly 20 and connection element 22. Preferably the connection elements are elements according to the API Standard Guide Post dimensions. In the embodiment shown in fig. 2a, columns 2Of extending from base 20b are arranged as positioning elelments to enable an easy and precise connection between the assembly 20 and connection element 22, in this case Christmas tree 22 as shown in fig. 2b. Christmas tree 22 comprises recesses 22a complementary to colums 2Of on assembly 20 to enable interconnection. Christmas tree 22 further comprises connecting members 22c and 22b to form a interconnection between the wellbore at the seabed surface in drilling guide 20c and the pipeline end terminal 2Od.

In fig. 2c the Christmas tree 22 is positioned on base and pipeline assembly 20.

Fig. 2d shows a blow-out preventer 23 positioned on pre-assembly 20 on the seabed 21.