WO2002087962A1

WO2002087962A1 - Disconnectable riser tensioning buoy

Info

Publication number: WO2002087962A1
Application number: PCT/NO2002/000152
Authority: WO
Inventors: Jon Grude Gjedebo
Original assignee: National Oilwell Norway As
Priority date: 2001-04-27
Filing date: 2002-04-23
Publication date: 2002-11-07
Also published as: NO20012079D0; NO20012079L

Abstract

A device for drilling offshore wells, where a riser (24) connected to a wellhead (20) extends up to the surface of the sea, and where the load on the riser, also during drilling operations, is taken up by a buoy (1), preferably via heave compensating means (2).

Description

DISCONNECTABLE RISER TENSIONING BUOY

This invention regards a riser bearing buoy for drilling operations offshore, which buoy is disconnectable from a drilling vessel, and where the riser in a well extends up to the surface of the sea.

When establishing a new petroleum well at great depths, a floating drilling platform or other suitable vessel equipped with the necessary drilling tools is in accordance with known techniques brought to the drilling site, where it is moored. Mention may be made of the fact that vessels using so-called dynamic positioning, where moorings are not used, are known. Dynamic positioning is relatively expensive and entails a danger of the vessel drifting under unfavourable conditions. After anchoring and rigging has been carried out, the first liners and the foundation frame are put in place.

Then a wellhead containing the necessary valves is lowered to the seabed and connected to the, foundation frame. After that, a riser is lowered, the lower end portion of which is connected to the wellhead. The upper end portion of the riser is connected to the platform via a heave compensator device, often in the form of one or more cylinders designed to maintain relatively constant tension in the riser, also during heaving of the platform.

Thus it is necessary during drilling operations according to known techniques to provide a drilling platform at the drilling site before the mooring operations can be carried out. Furthermore, during adverse weather conditions it is necessary to drain the riser of drilling fluid in order then to disconnect its lower portion from the well head. It is obvious that such a disconnection, together with the subsequent connection, is both time consuming and costly.

The object of the invention is to remedy the disadvantages of prior art.

The object is achieved in accordance with the invention by the characteristics stated in the description below and in the appended claims.

Prior to the drilling operations being initiated, a buoy is placed over the drilling site. The buoy is anchored according to techniques that are known per se, and with sufficient strength to take up the anchoring forces from the buoy and from a platform. Combinations of anchoring and dynamic positioning may also be used.

The buoy is lowered to a freeboard that enables a drilling platform to be moved in over the buoy without the tie bar of the platform conflicting with the buoy. The buoy is arranged in a space between the pontoons of the platform and two tie bars, and vertically below the drilling means of the platform. The platform is equipped with guide means that fit into vertical guides along the two end walls of the buoy in a

5 complementary manner. After the platform has been positioned over the buoy, the platform is lowered along the buoy guides until it reaches a predetermined deepdraught relative to the buoy. The buoy is then coupled to the platform, e.g. by means of air filled clamping bellows of a type that is known pero se. Thus the platform is kept in position by the buoy anchors.

The liners and the foundation frame are then placed on/in the seabed. The wellhead with the requisite valves and risers is then lowered from the platform down through a verticals through bore in the buoy, and further down to the seabed, where it is connected to the subsea template. In the upper portion, below the flexible connection of the riser, the riser is connected to the tension cylinders of the buoy designed to maintain relatively constant tension in theo riser. After the riser has been tensioned, the platform is raised to its working draught together with the buoy, whereupon well operations such as e.g. drilling, injection, production and well maintenance may be carried out.

During adverse weather conditions, when drilling has to bes interrupted, the platform and the buoy are raised to storm draught. The tension cylinders of the buoy are constructed so as to have sufficient length of movement also in this relatively high position in the sea, to take up the heaving motion of the buoy. Thus it is not necessary to disconnect0 the riser from the wellhead in bad weather, and the platform is constantly in a position to resume drilling as soon as the weather permits. In the case of a bad weather forecast, the valve on the seabed may be closed, and the platform can leave the buoy while the riser is still connected between the bottom valve and the buoy. Several production wells may be connected to the same buoy by each riser being provided with a separate tensioning system.

The following describes a non-limiting example of a preferred embodiment of the invention illustrated in the accompanying drawings, in which:

Figure 1 is a schematic side view of a platform, showing a section through a buoy, and where the platform is positioned over the buoy;

Figure 2 shows a section I-I in figure 1;

Figure 3 schematically shows the same as figure 1, but here the platform has been lowered to a suitable deepdraught for lowering of the wellhead. The wellhead is on its way down through the vertical through opening of the buoy;

Figure 4 schematically shows the same as figure 3, but here the lower end portion of the riser is connected to the wellhead, and the upper end portion is connected to the buoy via several tension cylinders;

Figure 5 schematically shows the same as figure 4, but here the platform and the buoy have been raised to the normal working draught; Figure 6 shows the platform and the buoy in figure 5 raised to a storm draught; and

Figure 7 shows the buoy in a situation where the platform is temporarily disconnected from the buoy.

In the drawings, reference number 1 denotes a buoy according to the invention, comprising tension cylinders 2, a vertical through bore 4 and guides 6. The buoy 1 is anchored via an anchor system 8, and has an adjustable deepdraught, e.g. by means of ballast control, underneath the surface of the sea 9.

A drilling platform 10 of a type that is known per se is arranged around the buoy 1 in an enclosing manner. The buoy 1 is located between the pontoons 12 and the tie bars 14 of the platform 1, so that the vertical opening 4 of the buoy 1 substantially coincides with the drilling axis 16 of the platform 10. The platform 10 is connected to the buoy 1 by means of fastening devices 18 located between the pontoons 12 and the buoy 1. The fastening devices 18 may comprise e.g. air filled clamping bellows of a type that is known per se.

When the buoy 1 has been anchored and lowered into the sea to where the freeboard is sufficiently reduced, the platform 10 is moved in over the buoy 1 to a position such that the drilling axis 16 of the platform 10 substantially coincides with the opening 4 of the buoy 1, see figures 1 and 2.

The platform 10 is then lowered into the sea along the guides 6 until it assumes the correct depth relative to the buoy 1, see figure 3, whereupon the platform 10 is connected to the buoy by means of the fastening devices 18. This depth corresponds to the preferred depth of the platform during the placement of a wellhead 20 on the seabed 22. In figure 3, the wellhead 20 is on its way down through the vertical opening 4 of the buoy 1.

After the wellhead 20 has been secured at the seabed 22, a riser 24 is lowered through the opening 4 and coupled to the wellhead 20 in the conventional manner. The tension cylinders 2 are connected to the riser 24 below the flexible link 26 of the riser, see figure 4, whereupon the riser 24 is subjected to a tensile force comprising the net weight of the riser 24 and any tensioning by the tension cylinders 2. The platform 10 and the buoy 1 are then raised to the normal working draught, see figure 5, whereby the platform is ready for the actual drilling operations.

Under adverse weather conditions, when the drilling has to be interrupted, the platform 10 and the buoy 1 are raised together to storm draught, see figure 6. The tension cylinders 2 are constructed so as to be able to take up heave motion also in this draught position. Thus it is not necessary to disconnect the 24 riser from the wellhead 20 in bad weather. The platform 10 may also leave the buoy 1 without the riser 24 having to be taken up in advance, the tension device 2 being designed to function without the platform 10 being in place over the buoy 1.

If so desired, the riser 24 and the bottom valve 20 may be disconnected from the foundation frame of the well and be moved sideways, so as allow the start-up of a new well with the riser 24 and the bottom valve 20 suspended in the sea. The device according to the invention allows more preparatory work for well drilling to be carried out before the relatively expensive drilling platform 10 has to be brought to the drilling site. A further advantageous feature of the device is that the unplanned drilling downtime caused by bad weather may be reduced considerably.

Claims

C l a i m s

1. A device for operations by offshore wells, where a riser (24) connected to a wellhead (20) extends up to the surface of the sea, c h a r a c t e r i s e d i n that the load on the riser (24), also during well operations, is taken up by a buoy (1), preferably via heave compensator means (2).

2. A device in accordance with Claim 1 , c h a r a c t e r i s e d i n that the buoy (1) is anchored to the seabed (22) via anchors (8).

3. A device in accordance with one or more of the preceding claims, c h a r a c t e r i s e d i n that the buoy (1) is constructed so as to fit in a complementary manner between the pontoons (12) of a drilling platform (10).

4. A device in accordance with one or more of the preceding claims, c h a r a c t e r i s e d i n that the anchors (8) of the buoy (1) also form the anchoring of the drilling platform (10).

5. A device in accordance with one or more of the preceding claims, c h a r a c t e r i s e d i n that the anchors ( 8 ) of the buoy ( 1 ) form the anchoring of the drilling platform (10) in combination with the dynamic positioning system of the drilling platform (10).

6. A device in accordance with one or more of the preceding claims, c h a r a c t e r i s e d i n that in its A device in accordance with one or more of the preceding claims, c h a r a c t e r i s e,d i n that in its operating position underneath the platform (10), the buoy ( 1 ) is arranged so as to make the drilling axis (16) of the platform (10) substantially coincide with the vertical opening ( 4 ) of the buoy ( 1 ) .

A device in accordance with one or more of the preceding claims, c h a r a c t e r i s e d i n that the buoy (1) is designed so that the riser (24) may be disconnected from the foundation frame of the well and be displaced sideways, so as to allow a new well to be initiated with the riser (24) and the bottom valve (20) suspended in parallel with the drill string of the new well.

A device in accordance with one or more of the preceding claims, c h a r a c t e r i s e d i n that the buoy

(1) is designed so as to be able to follow the platform (10) up to the storm draught.

A device in accordance with one or more of the preceding claims, c h a r a c t e r i s e d i n that the buoy (1) and the tension devices (2) are constructed so as to allow the platform (10) to leave the buoy (1) without the riser (24) having to be taken up.