GB2188960A - Drill cuttings disposal system - Google Patents

Abstract

A system for the disposal of cuttings arising from the drilling of subsea wells from an offshore platform which system comprises a caisson (15) fixed to the platform, and extending from a drilling zone located on top of the platform above and wave effected zone to a depth below the wave effected zone. A pipe (17) having one end connected to the lower end of the caisson, has its other end (23) maintained above the seabed away from the base of the platform by an anchor (19) on the seabed, whereby to dispose of cuttings away from the base of the platform by discharging the cuttings down the caisson (15) and the pipe (17). <IMAGE>

Description

SPECIFICATION Disposal of waste material The invention relates to the disposal of waste material arising from offshore drilling operations.

In order to exploit subsea deposits of oil and/or gas, it is necessary to drill oil/gas wells into the seabed from platforms located offshore.

Such platforms may include fixed substructures having piled steel foundations, or steel or concrete gravity foundations; or may include 'jack-up' substructures. These platforms will be referred to as 'platforms of the type described'.

The drilling of a typical North Sea well can produce up to 1,000 tonnes of driller's cuttings. With up to 60 wells being drilled from a single platform, the accumulation of debris at the base of the platform caused by dumping the cuttings through a chute (in the drilling area of topsides), can be substantial. This accumulation has been identified as a possible problem which may effect structural performance, corrosion protection or diver safety.

Conventionally, a cutting-chute caisson is fixed within the framing of the platform's substructure, so that the caisson discharges sediment over the lower portion of the substructure, making the lowest members of that structure difficult to inspect and maintain. If the bottom of the caisson is raised, the cuttings may be dispersed over a wider area by the action of subsea currents, but may create a danger that sediment could be sucked into the inlets of sea water pumps mounted on the platform.

The combination of large quantities of cuttings and subsea currents makes it desirable to divert the cuttings a significant distance away from the base of the platform, so that the lower portion of the substructure can remain free of accumulated debris.

The invention provides a system for the disposal of material arising from the drilling of subsea wells from an offshore platform of the type described, which system comprises a cutting chute fixed to the platform, and extending from a drilling zone located above the wave effected zone to a depth below the wave effected zone, and a pipe having one end connected to the lower end of the chute, and the other end maintained above the seabed away from the base of the platform by anchoring means of the seabed, whereby to dispose of material away from the base of the platform by discharging the material down the cutting chute and the pipe.

The cutting chute may be a flexible tube, or a rigid caisson, or a flexible tube within a rigid caisson.

Preferably the pipe slopes continuously downwards from its connection to the lower end of the chute towards its said other end.

Advantageously, the pipe is flexible and the anchoring means is moveable between a plurality of positions away from the base of the platform, so that the flexible pipe can discharge material over a plurality of areas on the seabed.

The system may be used in combination with purging means to force fluid through the pipe, so to clear any material which may have restricted or obstructed the pipe.

It is preferred that there is a hopper at the connection between the chute and the pipe.

It is further preferred that there is a second flexible pipe connected to the hopper.

It is still further preferred that the purging means comprises an arrangement to supply pressurised fluid to the pipe via the hopper.

The anchoring means may consist of a conventional Danforth anchor or a clump weight, which is attached to the flexible pipe/s by either anchor chain or wire rope.

Conveniently the system is disposed beneath a flare boom extending outwardly from the platform above the wave effected zone.

Specific embodiments of the invention will now be described by way of example, with reference to the accompanying drawings, in which: Figure 1 is a side elevational view of an offshore platform to be used for drilling wells to tap subsea oil and/or gas reserves, and showing a cuttings disposal system; Figure 2 is a detailed view of a discharge nozzle for the system illustrated in Fig. 1; Figure 3 is a detailed view of an anchor illustrated in Fig. 1; Figure 4 is a detailed view of a transition from a rigid caisson to a flexible disposal chute; Figure 5 is a view on the arrow V in Fig. 4; Figure 6 is a modified arrangement of that shown in Fig. 5; Figure 7 is an end elevation of the platform shown in Fig. 1, and Figures 8 and 9 show alternative forms of the cuttings disposal system illustrated in Fig.

1.

As shown in Fig. 1, a drilling rig 10 stands on a deck 11 of an offshore platform 12. This deck is supported on an eight-legged substructure 14. (Only legs B1 to 84 of the substructure - commonly referred to as a jacket - are visible in Fig. 1.) A rigid cutting-chute caisson 15 extends from a drilling zone at the foot of the rig 10 (significantly above the wave-effected zone) to a transition 16 (significantly below the wave-effected zone). At the transition 16, the lower end of the rigid caisson 15 is connected to one end of a flexible pipe 17. A discharge nozzle 23 at the other end of the flexible pipe 17 is maintained above the seabed away from the base of the platform 12 by a chain 18 connected to a Danforth anchor 19.

The connection of the flexible pipe 17 to the anchor chain 18 is shown in Fig. 2, and a lifting arrangement for the Danforth anchor is shown in Fig. 3.

A conventional anchor (with or without a clump weight) is preferred to a concrete block, because it is easier to recover. However, the laying of the anchor 19 must be carefully supervised, to ensure that its flukes are adequately buried.

The anchor 19 is advantageously located under a flare boom 20, which extends outwardly from the deck 11. This position minimises interference from approaching vessels and their anchoring patterns, (because vessels may not be permitted to unload onto the platform from beneath the flare boom 20), and is also clear of riser and flowline routings.

The rigid caisson 15 is formed of a typically 600mm - 800mm dia. casing, which is run down the substructure to a transition level. At this level, there is adequate backing structure for the caisson support to react catenary forces from the flexible pipe 17 and the anchor system 18/19, and sufficient height above the mudline to allow the flexible pipe to adopt a suitable trajectory.

As shown in Figs. 4 and 5, a reducing hopper 21 is fitted to the lower end of the casing 15 at elevation -77.0m. (Fig. 6 shows an alternative hopper arrangement.) At the base of this hopper 21 there is a flange 22 to connect with one or more flexible pipes 17 having a maximum inside diameter (ID) of 250mm; these are pre-installed on the jacket while it is being fabricated onshore, seafastened, transported to the offshore site and then pulled to the desired dump location and held by one or more anchors. The end(s) of the flexible pipe(s) are connected to the anchor(s) by the chain 18 or a wire rope.

The main objectives of the scheme are to achieve low installation costs and a long maintenance-free service life. The scheme has the following advantages: it is simple to install and maintain; the outfall location can be varied by changing anchor location and/or flexible pipe length; hazards to shipping are reduced by the use of anchor-chasing systems; and if required, the hopper can be unbolted and a conventional cutting disposal system reinstated.

Installation of the system is carried out in the following way.

The flexible pipe(s) 17 and the anchor system 18/19 are fully rigged and temporarily fixed to Frame 1 of the substructure or jacket by a suitable lashing system.

The complete system can be installed and tested onshore, and fixed to the jacket prior to loadout. The final arrangement of the points which need to be accessible for offshore installation activities should be chosen to suit crane access, work-boat operation and the release of temporary lashings by divers.

The temporary fixing of the flexible pipe(s) 17, anchoring system 18/19, and rigging has to be designed to resist the loads which arise during launch of the jacket from the barge used to carry it to its offshore location. Wave slam and buoyancy forces should be considered. A temporary sea inlet could be fitted at the base of the casing, if required, to assist flooding; this would also provide additional access for flushing or rodding.

As shown in Fig. 7, the flexible pipe(s) 17 could be fixed to a member at the transition 16 of the cutting chute caisson 15, and run up Leg B1 to E1.+10.000m. An alternate route (indicated with a dashed line) would be up the main diagonal bracking to Leg Al and then up to E1.+10.000m. The anchor chain 18, anchor 19 and discharge nozzle 23, seafastened at E1.+10.000m or (in part) stored on a Heavy Lift Vessel (HLV) (not shown), can be passed directly to an anchor-handling work-boat (also not shown) using the HLV crane or a platform crane.

Offshore installation consists of releasing the temporary lashings, assembling the anchor 19, anchor chain 18, flexible pipe(s) 17 and wire rope; and then moving the system out by work-boat and placing the anchor 19 at its predetermined location.

The installation of the system can be carried out during the jacket installation period immediately following upending, or after module placement, and can be completed within a 12hour day. The critical path involves the release of the subsea temporary lashings. A detailed description of the procedure is given in the following paragraph.

If the jacket installation contractor is used to install the system, then the anchor 19, anchor chain 18 and wire rope are stored on the HLV prior to installation. These items are then transferred to the work-boat or jacket for assembly. After the release of the flexible pipe(s) 17 from the jacket, the free end is transferred to the work-boat by the HLV crane, and connected to the anchor chain 18 at the discharge nozzle 23. The work-boat is then run out to the anchor location, paying out sufficient anchor chain as it goes. The bollard-pull of the work-boat must be capable of pulling the discharge nozzle 23 to its predetermined location. The wire used to lower the anchor must be specially prepared to place the anchor fluke directly into the seabed (see Fig. 3), and also apply a tension. Once the anchor is installed, the wire rope can be dumped overboard. This should only be done when it has been confirmed that the anchor is correctly installed.

The use of high-quality materials minimises maintenance requirements. Using Coflexip, or similar pipe, a service life of 15 to 20 years, or longer, would be expected. The service life for this pipe in pressurised pipeline usage is quoted at 10 to 15 years.

Inspection/replacement of a flexible pipe 17 is effected as follows.

A recovery (anchor-chasing) system is incorporated to allow inspection and replacement operations and, if necessary, movement of the outfall to a clear area adjacent to its original position.

The retrieval system allows inspection and maintenance of the flexible pipe 17, discharge nozzle 23, anchor chain 18 and anchor 19, if required.

Each pipe 17 has two chasing rings 24 and 25. The larger ring (24) is designed to pass over the discharge nozzle 23 at the end of the flexible pipe, and is used to pick up the anchor 19, chain 18 and flexible pipe 17. The smaller chasing ring (25) is designed to fit the discharge nozzle, and is used to lift the end of the flexible pipe 17 to the surface of the sea in the event of failure of the anchor system 18/19. The two rings 24 and 25 could be tied off at the surface or at the hopper.

The upper end of the flexible pipe 17 has a system of padeyes 26, which are designed to facilitate its underwater removal and re-connection at the hopper 21.

To replace (or inspect) the flexible pipe, a wire is connected to the larger chain-chaser 24 and the other end is passed to the work boat. The work boat lifts the anchor 19 and heads slowly towards the driller's outfall; the weight of the flexible pipe 1 7 is initially taken by the hopper connection. By the use of padeyes 26 and a rope hoist, the weight of the flexible pipe 17 is then taken off the flange 22.

A wire rope is attached to the padeye 26 on the top flange of the flexible pipe, and the other end of the rope is passed to the workboat, which has now moved in as close as possible to the substructure. After taking up slack in the wire rope, the flange of the flexible pipe is released and the brake on the rope hoist is eased off, until the wire rope goes slack. The weight is now on the work-boat.

The rope hoist is disconnected at the flange of the flexible pipe; the work-boat pulls the anchor chain 18 into a chain locker and feeds the flexible pipe 17 onto a drum. The sequence is reversed for the fitting of the new flexible pipe.

In the event of a failure in the anchor system 18/19, the smaller chaser ring 25 is used to lift the discharge nozzle 23 onto the workboat. The discharge nozzle is then reconnected to an anchor system. As the large chaser ring 24 is fitted over the smaller ring 25, they must be lashed together.

Once the anchor 19 has been connected and rigged with the wire rope, it can be lowered and tensioned as previously described.

The chaser rings 24 and 25 should be run back to the hopper 21 and tied-off.

A flushing system may be incorporated, to ensure that the driller's cuttings remain in suspension, particularly at the change in crosssectional area of the hopper 21 adjacent to the transition level. Consideration might be given to reducing the size of the rigid caisson if a flushing system is introduced, or to running the piping for the flushing system inside the 762mm casing. Alternatively the rigid caisson could be replaced by a continuous run of one or more smaller-diameter flexible casings all the way up to the cuttings collection hopper below the drilling base.

A review is also recommended of existing knowledge of the discharge and flow of sediments such as driller's cuttings. This would ensure that minimal sediment reaches the substructure area, as well as minimising material costs and handling problems.

An alternative, shown in Fig. 8, is the same in all respects as the scheme illustrated in Fig.

1, except that a concrete block 27 is used instead of an anchor to keep the flexible pipe in the desired location, and only one chaser is required. Onshore work consists of lashing the flexible pipe to the substructure. The concrete block and anchor chain are best stored on the work-boat. Installation consists of releasing the temporary lashings, connecting the flexible pipe to the anchor chain, and then running it out to a predetermined anchor-block location.

The block is then lowered into position. The tolerance on the anchor-block location would be high because of the low sensitivity of the discharge nozzle. Also, the concrete block would be more difficult to recover with an anchor-chasing system than would a conventional anchor. Thus, maintenance, inspection and possible relocation of the discharge would be more difficult than with a conventional anchor.

The flexible casing can be fitted to an existing cutting chute caisson by installing a connection fitting with a hopper to the bottom of the caisson, and then actuating the flexible casing. The rigid chute may have to be cut back to a revised termination elevation, in order to obtain the desired distance from the platform for discharge, or to obtain an adequate location for reaction of the catenary forces.

Alternatively, the flexible casing can be pulled through an existing caisson (trimmed up to the desired level), by fitting a pulling shoe to the bottom of the rigid caisson, sending a messenger line down the caisson for connection to the flexible casing, and pulling the flexible casing up to the main collection hopper under the drill rig base.

A second alternative, shown in Fig. 9, consists of a pipe bridge 28 of triangular cross section, which is supported at its outer end by two mudmats 29 and at the substructure end by a sliding vertical support 31. Installation would consist of lifting the pipe bridge into place, and connecting the bottom of the 762mm-diameter casing to the top boom of the bridge with a flexible pipe. This system, while offering a similar remote discharge capability, would incur higher initial costs and would not provide flexibility of discharge location.

Claims (13)

1. A system for the disposal of material arising from the drilling of subsea wells from an offshore platform of the type described, which system comprises a cutting chute fixed to the platform, and extending from a drilling zone located above the wave effected zone to a depth below the wave effected zone, and a pipe having one end connected to the lower end of the chute, and the other end maintained above the seabed away from the base of the platform by anchoring means on the seabed, whereby to dispose of material away from the base of the platform by discharging the material down the cutting chute and the pipe.

2. A System as claimed in claim 1 in which the cutting chute is a rigid caisson.

3. A System as claimed in claim 1 in which the cutting chute is a flexible tube within a rigid caisson.

4. A System as claimed in any one of the preceding claims in which the pipe slopes continuously downwards from its connection to the lower end of the chute towards its said other end.

5. A System as claimed in any one of the proceeding claims in which the pipe is flexible and the anchoring means is moveable between a plurality of positions away from the base of the platform, so that the flexible pipe can discharge material over a plurality of areas on the sea bed.

6. A System as claimed in any one of the preceding claims in combination with purging means to force fluid through the pipe, so to clear any material which may have restricted or obstructed the pipe.

7. A System as claimed in any one of the preceding claims in which there is a hopper at the connection between the chute and the pipe.

8. A System as claimed in claim 7 in which there is a second flexible pipe connected to the hopper.

9. A System as claimed in claim 7 or claim 8 as dependent on claim 6 in which the purging means comprises an arrangement to supply pressurised fluid to the pipe via the hopper.

10. A System as claimed in any one of the preceding claims in which the anchoring means is a conventional Danforth anchor.

11. A System as claimed in any one of claims 1 to 9 in which the anchoring means is a clump weight.

12. A System as claimed in any one of the preceding claims, disposed beneath a flare boom extending outwardly from the platform above the wave effected zone.

13. A System substantially as hereinbefore described with reference to and as shown in Figs. 1 to 7, or Fig. 8, or Fig. 9 of the accompanying drawings.