US3470838A

US3470838A - Buoyant wellhead structure

Info

Publication number: US3470838A
Application number: US723013A
Authority: US
Inventors: Alan F Daniell
Original assignee: Cammell Laird & Co Shipbuilder
Current assignee: Cammell Laird & Co Shipbuilder; Cammell Laird & Co Shipbuilders & Eng Ltd
Priority date: 1967-04-27
Filing date: 1968-04-22
Publication date: 1969-10-07
Anticipated expiration: 1986-10-07
Also published as: GB1172558A; NL6805977A

Description

Oc 7, 1969 A. F. DANIELL BUOYANT WELLHEAD STRUCTURE 3 Sheets-Sheet Filed April 22, 1968 INVENTOR: ALA N F. DANIELL Oct. 7, 1969 A. F. DANIELL 3,470,838

BUOYANT WELLHBAD STRUCTURE Filed April 22, 1968 3 Sheets-Sheet 2 ALAN F. DANIELL Atvs.

INVENTOR:

Oct. 7, 1969 A. F. DANIELL 3,470,838

BUOYANT 'WELLHEAD STRUCTURE Filed April 22, 1968 3 Sheets-Sheet 3 INVENTOR;

ALAN F2 DAN/ELL BY Q f vM men's.

United States Patent 3,470,838 BUOYANT WELLHEAD STRUCTURE Alan F. Daniell, Birkenhead, England, assignor to Cammell Laird 8: Company (Shipbuilders 8: Engineers) Limited Filed Apr. 22, 1968, Ser. No. 723,013 Claims priority, application Great Britain, Apr. 27, 1967, 19,417/ 67 Int. Cl. E21b 33/035; B63b 35/00 US. Cl. 114.5 9 Claims ABSTRACT OF THE DISCLOSURE A structure for mooring over a marine oil well comprises a buoyant chamber which is ballasted to float upright. A riser pipe from the well extends up through the structure to means maintaining the pipe in tension such as a float in an open well of the structure. The structure can contain processing equipment which leaks gas into the structure to keep it buoyant.

BACKGROUND OF THE INVENTION At present wellheads of offshore oil Well installations are supported:

(a) Above the water on a structure which rests on the sea bed,

(b) Submerged below the normal draft of shipping on a lattice structure, pile or column which rests on or is embedded in the sea bed,

(0) On the sea bed using a caisson technique to gain entry to the wellhead, at atmospheric pressure, or some form of remote control for the operation and servicing of the wallhead equipment.

Both (a) and (b) involve expensive construction work at sea and method (a) can constitute a hazard to shipping, whilst method (b) involves the use of divers working externally of the well in their cumbersome diving equipment and for necessarily limited periods 0 time. Method (c), using a caisson technique, involves the use of deep sea divers and diving equipment and if based on a remote control and observation system can be complex and expensive.

SUMMARY OF THE INVENTION The present invention provides a buoyant wellhead structure to which divers can readily obtain access and in which they can work to a large extent unimpeded by equipment and the conditions which would hamper their activities it working from the outside of the wellhead structure.

According to the present invention there is provided a buoyant wellhead structure adapted to be moored over an oifshore oil well or the like installation, comprising a buoyant chamber or bell at the upper part of the structure, means for maintaining said chamber or hell under pressure so as to depress the water level therein, ballasting means at the lower part of the structure so that it is caused to float upright and an air lock providing access to said chamber or bell from below the water line.

The chamber or bell forms an enclosure in which personnel can adjust or service the wellhead equipment in comparative safety and freedom for long periods.

The required super atmospheric pressure may be maintained in the chamber or bell by permitting an escape or controlled leakage of gas from natural oil or gas processing equipment installed in the chamber or bell or in an associated part of the structure.

A riser pipe from the well to the wellhead extends up through the buoyant wellhead structure and the arrangement is such that this pipe is maintained in tension. This,

3,470,838 Patented Oct. 7, 1969 ice BRIEF DESCRIPTION OF DRAWING FIGURES FIGURE 1 is a sectional elevation of the upper and lower parts of the structure,

FIGURE 2 shows the structure in use, and

FIGURES 3 and 4 show stages in towing the wellhead structure to the well location and anchoring it thereover.

DETAILED DESCRIPTION OF INVENTION The wellhead structure shown on the drawings comprises a hull 1 of elongated cylindrical bell-like form, constructed with inner and outer skins or shells forming toroidal watertight compartments. The upper part of the hull forms a bell-shaped enclosure 2, and the lower part is increased in both its external and internal diameters. At the bottom the space between the inner and outer skins is filled with heavy fixed ballast 3. y

The hull (see FIGURE 2) is moored by at least three mooring ropes or chains 4, disposed symmetrically in plan, and connected to mooring anchors or heavy sinkers 5. The mooring ropes or chains are passed through cylindrical trunks 6 attached externally to the hull, and of divergent form at the bottom 7. The upper ends of the mooring ropes or chains are attached at the upper part of the hull to windlasses or Winches 8.

The top of the hull is filled with a removable hatch or cover 9, and part of the way down the hull an air lock 10 is provided, for the entry and exit of personnel.

A riser pipe 11 from the oil well 12 passes through a hollow toroidal float 13, and is attached thereto. The riser pipe 11 terminates with a valve assembly 14, which is connected by flexible pipes 15 to pipes 16 which are rigidly attached to the hull.

The pipes 16 connect to the internal spaces between the inner and outer skins of the hull 1, and these spaces may be utilised for processing of the product of the well and in particular for separation of gas from oil, or of liquid from gas.

Pipes 18, communicating with the internal spaces of the hull, are connected to flow lines 17 which carry away the products of the well. The flow lines 17 pass from the sea bed into trunks 6a attached externally to the hull. The trunks 6a are of divergent form at the bottom 7a.

A vent 19 connects the bell 2 with the external sea.

In normal operation, a small quantity of gas is permitted to escape from the processing equipment (e.g., via the valve assembly 14) into the bell 2, depressing the water level in the open-bottomed well or compartment 20 and finally escaping at the vent 19, The water level within the well 20 will thus be maintained slightly below the level of the vent, and since the upper part of the lock 10 is in free communication with the bell 2, the water level within the lock will be maintained at the same level.

Certain of the compartments between the inner and outer skins of the hull are provided with sea cocks and vents (not shown) to permit the admission of a controlled amount of water ballast. By adjustment of the amount of ballast, the hull is maintained in a slightly buoyant condition, with the centre of gravity below the centre of buoyancy, so that it will take up a vertical attitude straining against the mooring chains. The length of these chains is adjusted so that the top of the hull is maintained at a depth adequate to give submergence at all states of tide and sea, and if necessary adequate to avoid dangers to navigation, but yet not so deep as to be beyond the practical limit of normal diving operations.

Currents and waves will cause some motion of the hull, but this motion will be very small at the lower end of the hull, where the moorings are attached or held. Horizontal motion of the upper part of the hull will be communicated to the float 13 and thence to the riser pipe 11. The buoyancy of the float 13 is made adequate to ensure that the axial sress in the riser pipe 11 is tensile, over its whole length, and thus the danger of buckling of the slender riser pipe is avoided and some degree of flexure is permissible.

Adjustment of the valves 14 and other equipment at the wellhead may be carried out by divers entering through the lock 10. Once within the bell 2, the divers will be working in dry conditions but will require breathing apparatus if the atmosphere within the bell consists of the leakage gas from the well together with any residual gas from the atmosphere at the time of installation. To avoid explosion hazard, the bell may be purged with nitrogen or other inert gas at the time of installation.

A floor or the like is provided in the bell over which the personnel can walk.

To install the device at an existing well, the anchors 5 (see FIGURE 4) are first laid in a symmetrical disposition around the wellhead, and to each anchor there is attached a length of chain cable 4, the end of which is supported by a buoy B. The hull 1 containing the float 13, temporarily clampel at the upper end of the well or compartment 20, is fitted with an extension tube 21, see FIGURES 3 and 4. Messenger ropes, in the form of continuous loops, are passed through each of the trunks 6, and a further similar loop is passed through the tube 21, and through the whole length of the well 20 via the hole or tunnel in the float 13. By adjustment of water ballast in the compartments between the inner and outer skins of the hull, the latter is caused to float horizontally, as shown in FIGURE 3.

On arrival at the site of the well, water is admitted to compartments at the lower end of the hull, so causing it to take up a vertical attitude, with the extension tube 21 and the upper part of the hull projecting above the surface, as shown in FIGURE 4. The outer ends of the messenger ropes through the mooring trunks 6 are passed out to the corresponding buoys B, and connected to the chain cables 4. The buoys are then detached, permitting the chain cables to fall to the sea bed. The messenger ropes are hauled in until the ends of the chains 4 reach the top of the trunks 6. The chains 4 are then attached to the chain windlasses 8. At this time, water ballast is admitted to reduce the buoyancy of the structure, and the chains 4 are hauled in on the windlasses 8 until the top of the hull is submerged to the required depth, with the extension tube 21 still projecting above the water surface. The plane or lateral positioning of the hull may be adjusted to coincide with the oil well location by individual adjustment of each mooring chain length.

The outer part of the messenger rope, passing through the centre of the hull, is attached to buoyed guide lines previously fixed at the oil well template, and these guide lines are hauled through the centre of the hull, the float 13, and the extension tube 21. With the assistance of the guide lines, the riser pipe 11 is lowered through the extension tube 21 and latched t0 the well at the sea bed. The extension tube is then removed, the hatch cover 9 fixed in position, and the level of water within the bell 2 lowered by the admission of gas under pressure. The float 13 is then disconnected from the wall of the well 20 and attached to the riser pipe .11, and the valves and control equipment 14 and flexible pipes 15 installed. The oil or gas outflow lines 17 are then connected.

In the case of a new production well, the wellhead structure is preferably installed as previously described before the well is drilled. In this case, the extension tube 21 may be connected to the top of the hull by a flexible joint and the top of the extension tube flexibly connected to the mobile drilling ship. The well is then drilled through the central well 20 in the buoyant wellhead before the float 13 and pipe 11 are installed. This arrangement has the advantage that the casing is largely protected from wave and current action, and temporary abandonment and recovery of the well, if this should be necessary, is much facilitated.

If, at a later date, workover operations on the well become necessary, the bell 2 can be flooded, and the hatch 9 removed and the extension tube 21 reconnected by divers. By the use of suitable connecting devices at the wellhead 14, workover operations can then be carried out from the surface, and on completion the extension tube can be removed, the hatch replaced, and the structure reinstated in its previous form.

The air lock 10 may be provided with doors as shown in FIGURE 1 but this not essential.

I claim:

1. A buoyant wellhead structure adapted to be moored over an offshore well comprising a chamber at the upper part of the structure, means for maintaining said chamber under pressure so as to depress the water level therein when the wellhead structure is moored over an offshore well, ballasting means at the lower part of the structure so that it is caused to float upright, an air lock providing access to said chamber from below the water line, a riser pipe for connection to the well and which extends up through the wellhead structure, and means for attaching said riser pipe to the wellhead structure to maintain said riser pipe in tension when said pipe is connected to the well.

2. A wellhead structure as claimed in claim 1 wherein means is provided for permitting a controlled leakage of gas into the chamber from processing equipment installed in the structure thereby to maintain the chamber under pressure and depress the water level therein.

3. A wellhead structure as claimed in claim 1 wherein said riser pipe attaching means comprises a float positioned within an open well of the wellhead structure whereby said riser pipe is maintained in tension.

4. A wellhead structure as claimed in claim 1 including a hull comprising inner and outer skins which provide watertight compartments, the lower part of the space between said inner and outer skins containing heavy ballast.

5. A wellhead structure as claimed in claim 1 wherein the riser piper is connected to internal spaces of the wellhead structure, which internal spaces contain means for processing from the wellhead.

6. A wellhead structure as claimed in claim 5 wherein valve means is provided in said connection between the riser pipe and internal spaces of the wellhead structure.

7. A Wellhead structure as claimed in claim 1 and having on its exterior trunk means for pipe lines which carry away the products from the wellhead.

8. A wellhead structure as claimed in claim 1 and having a vent for gas for depressing the water level in the chamber of the structure, positioned to maintain said water level slightly below the level of said vent.

9. A wellhead structure as claimed in claim 1 wherein the riser pipe terminates at its upper part in a flexible pipe connection.

References Cited UNITED STATES PATENTS 962,019 6/1910 Flood et al 61-81 X 2,519,453 8/1950 Goodman 61-69 3,036,438 5/1962 Sims 61-82 TRYGVE M. BLIX, Primary Examiner