GB2153748A - Servicing of oil rigs - Google Patents

Abstract

The invention relates to a method and a vessel for servicing oil rigs. The oil rig is raised from the sea onto the deck 30 of a semi-submersible service vessel but not transported from the operating site. Maintenance and repairs are performed at sea with the service vessel providing facilities similar to a floating dry dock. The cost of servicing may in this manner be reduced due to the fact that the rig need only be out of action for a relatively short time. Furthermore the efficiency of the maintenance and repair is improved as compared to such work being carried out under water. <IMAGE>

Description

SPECIFICATION Servicing of oil rigs The present invention relates to the servicing of oil rigs, the term including so-called semi-submersible rigs in which the platform is supported on submerged hulls and jack-up rigs in which the platform is supported on jacks resting on the sea bed.

In order to inspect or service an oil rig, the most satisfactory method is to bring the rig into dry dock. In this case, one has full access to the support structure of the rig in order to enable thorough inspection, as well as maintenance and, if necessary, repair.

The costs of dry docking are however very high.

A first reason for this is that the vessel must be towed substantial distances but there is also the factor that the vessel is necessarily out of action for a considerable time. Consequently, the maintenance has not been routinely performed in this manner and instead divers have performed inspections and repairs with the vessel in situ. This however is not satisfactory because visual inspection is not adequate under water where visibility is severely limited and furthermore such repairs as welding cannot be controlled to the full when the vessel is submerged.

In order to mitigate the foregoing disadvantages, the present invention provides a method of servicing an oil rig, which comprises raising the rig above water level aboard a carrier, performing necessary inspection, maintenance and/or repair at sea on board the carrier and lowering the rig back into the water.

There are currently available carriers designed to transport certain types of rig with the rig totally out of the water. The carriers resemble an oil tanker of which the sides have been removed to leave a flat deck.

To raise the rig out of the water, water is pumped into the carrier to submerge its deck below the platform or hulls of the rig. The water is then pumped out after the carrier has been positioned beneath the rig so that as it rises from the water it raises the rig aboard its deck.

Such carriers have hitherto been employed exclusively for transportation but the present invention proposes that servicing be performed aboard the carrier using its deck as a dry dock. Because the rig does not need to be towed into harbour, the time that the rig is out of action is reduced.

Furthermore, a full compliment of staff may be present on board the service vessel so that no time is wasted in amassing the necessary expertise once the rig is out of the water. The costs involved in servicing can therefore be reduced substantially and if serviced thoroughly at more regular intervals the life of the rig and its safety are increased.

The existing carriers have a limited width of deck and as a result the rig overhangs the deck when it has been raised from the water. This is acceptable for certain types of rigs where the hulls can withstand the stress of being supported in this manner but other rigs cannot safely be raised by exisiting carriers. If the carrier is to be used as a service vessel then, of course, an over hang cannot be tolerated.

In accordance with a second aspect of the invention, there is provided a service vessel for enabling an oil rig to be serviced out of the water while at sea, the vessel comprising a submersible hull having a flat deck for supporting the rig out of the water wherein at least one end of the vessel, that is to say the stern or the bow, is cut away level with the surface of the deck so as to present no obstruction to the sliding of an oil rig onto the surface of the deck.

It is common for rigs to be fitted with thrusters for positioning the rig. Such thrusters have propellers that project downwards beyond the hull and would thus interfere with the supporting of the rig on a flat deck.

In a preferred embodiment of the invention, the deck is provided with longitudinally extending gulleys coinciding with the locations of the thrusters so that the thrusters may be accommodated within the gulleys without removal from the oil rig. In such a case the hulls of the submersible will generally be aligned with the gulleys and in order to support the hulls shutters are inserted into the gulleys at locations not occupied by the thrusters.

If the undersurface of the hulls is to be inspected and/or repaired then conveniently blocks may be provided on the shutters so that the hulls are supported above the surface of the deck.

A problem with performing servicing at sea is that of adverse weather conditions. It is preferred that the deck of the service vessel be provided with sheets supported on a collapsible framework which may be erected rapidly to surround areas where work is in progress. Such a collapsible framework may for example be a vertically collapsible bellows.

As an alternative, the legs of a semi-submersible may serve as the support frame for sheets offering shelter to operating personnel.

In order to provide access to high parts of the rig, scaffolding may be employed but it is preferable to use a crane either mounted on the rig or on the service vessel and carrying a basket for operating personnel. As an alternative, such cranes may be arranged on self-propelled vehicles on board the service vessel.

The service vessel may itself be powered but in order to minimise manufacturing costs, the vessel need not be self propelled but may be towed onto the site and ocean going tugs may be used to achieve the necessary docking procedure.

The invention will now be described further, with example, with reference to the accompanying drawings, in which: Figure 1 is a schematic end view of a typical submersible oil rig.

Figure 2 is a plan view of a service vessel in accordance with the invention, Figure 3 is an end view of the service vessel in Figure 3, Figure 4 shows the service vessel as viewed from the stern with an oil rig arranged on its deck, and Figures 5a to 5d shows schematically the docking procedure.

The semi-submersible shown in Figure 1 comprises two hulls 10, 12. Vertical columns 14 and 16 extend upwards from the hulls 10 and 12 and support a platform 18. Thrusters 20 are mounted on the hulls and their propellers extend downwards below the lower surfice of the hulls. There are two sets of thrusters at the different ends of the hulls as shown by the solid and dotted lines and as illustrated the distances between the trusters in the respective pairs are not equal.

In use, the semi-submersible oil rig of Figure 1 lies with its hulls 10, 12 entirely submerged beneath the water. The columns 14 and 16 are partly submerged and provide part of the buoyancy for keeping the rig afloat. The rig is held above the desired drilling location by several anchors. During such use, fouling accumulates on the hulls and the submerged part of the columns and furthermore the various structural parts of the rig are subjected to very considerable stresses. It is therefore necessary to clean, service and carry out periodic checks on the oil rig and the present invention is concerned with the performing of such maintainance without the need to tow the rig into dry dock.

To raise the oil rig out of the water, a service vessel as shown in Figures 2, 3 and 4 may be employed. The vessel is not self-powered and is intended to be towed to the site of the oil rig by a carrier or by ocean going tugs. The service vessel is itself semi submersible and is designed to lift the oil rig out of the water.

As seen in Figure 4, the vessel has a generally Ushaped cross section which defines a flat deck 30.

To accommodate the thrusters, the flat deck is formed with two longtudinally extending gulleys 32. The vessels has a shallow draft to permit it to drop below the hulls of the semi-submersible oil rig even in shallow water and in order that it may have sufficient displacement to raise the oil rig from the sea it is provided with relatively wide ballast tanks 34 along its two sides. At the stern, the sides of the vessel are entirely cut away to allow unimpeded access to the flat deck so that the oil rig may slide over the surface of the deck without obstruction from the sides of the vessel.

At the bow end of the vessel, the hull is shaped as shown at 40 (see Figure 3) to facilitate towing of the vessel and also at the bow end there is an accommodation and service block 42 to provide accommodation for service personnel and work shop facilities for repairs to be carried out on the oil rig.

In order to lift the semi-submersible rig out of the water the procedure shown in Figures 5a to 5d may be employed. The anchors of the rig are first raised from the sea bed and the rig is hitched to tugs 54 which are used in the docking procedure.

The service vessel on the other hand is anchored in the desired position and water is taken into its tanks to submerge it below the level of the hulls of the oil rig. The oil rig on the other hand empties its ballast tanks so that it should rise as much as possible within the water.

Once the relative heights in the water of the service vessel and the semi-submersible oil rig have been matched to permit the oil rig to be hauled aboard, a winch line 52 shown in dotted lines is secured to the semi-submersible from the service vessel and as the winch line 52 is pulled in the tugs 54 are used to keep the line 52 taught and align the thrusters on the hulls with the gulleys 32.

Shutters 33 are arranged within the gulleys 32 along substantially their entire lengths, the shutters 33 serving the cover the gulleys 32 to provide a support surface level with the flat deck 30. However, the appropriate shutters 33 aligned with the thrusters are removed to form a well for accommodating the thrusters beneath the surface of the deck. Should it be desired to work on the under surface of the hulls then blocks such as the blocks 35 may be placed on the surface of the deck above the shutters 33 in order to support the hull clear of the surface of the deck.

With the appropriate shutters removed and the blocks 35 if necessary in place, the semi-submersible is hauled in as previously described using the winch and the tugs. When the semi-submersible is near the service vessel, it is necessary to centre its front end between the sides of the service vessel as it continues to be winched in. In order to achieve this, carriages (not shown) or so-called mules are slidable along the top of the lateral ballast tanks 34 and these each carry a winch of which the line 55 is under constant tension. Such the devices are known and, for example, the line may be wound about a drum which is motor driven through a slipping clutch so that any slack is taken-up quickly but the clutch can if necessary slip to allow line to be paid out as well as pulled in.

The lines under constant tension are secured one to each side of the semi-submersible oil rig and if their tensions are balanced then the oil rig is maintained in proper alignment. The mules move along the sides of the service vessel as the winch ine is hauled in so as to keep the semi-submersible centred during its travel. The movement of the constant tension lines 55 with the mules is shown schematically in Figures 5b and 5c.

When the semi-submersible oil rig is on board the service vessel, two further such mules with constant tension lines are connected to the rear of the oil rig so that the oil rig may remain centred on the deck after the tug boats 54 have been released.

The mules and the self-tensioning lines 55 may be used to ensure correct positioning of the semi-submersible in relation to the gulleys for accommodating the thrusters and the blocks for supporting the hulls above the deck.

With the oil rig now correct!y aligned with the service vessel, the service vessel is raised out of the water by pumping water out of its ballast tanks simultaneously raising the oil rig aboard its deck.

The sides of the semi-submersible may further carry movable cranes for raising and lowering the shutters 33 in the gulleys 32 and for performing other service functions during the docking of the semi-submersible.

The accommodation block 42 remains out of the water even during the docking procedure and preferably the docking operation is controlled from a bridge on top of the accommodation block 42.

The optimum time for removing any fouling on the hulls is as the hulls are being withdrawn from the water. It is therefore preferred to provide selfpropelled units which are movable along the hulls as the latter are being withdrawn from the water and which carry a series of scrubbing brushes to dislodge as much of the fouling as possible whilst the hulls remain wet. Once the semi-submersible is entirely out of water it should therefore already have been cleaned and personnel may work on the oil rig from the service vessel in much the same as working ould be carried out in a floating dry dock.

If desired once the oil rig has been hauled aboard the service vessel, a gate at the stern of the vessel may be raised or closed and this would provide additional safety and weather protection. By the analogy with a floating dock, it may be possible if the gate is designed to be fully watertight to work with the deck below water level so that the service vessel with the oil rig on board may sit lower in the water for additional stability.

Various cranes may be provided at strategic positions both on the sides of the service vessel and on its deck to enable access .o different parts of the structure of the oil rig. Access may further be assisted by self-propelled vehicles carrying working baskets on crane arms or by means of cranes suspended from the surface of the oil rig itself.

In order to provide weather protection, self supporting frames may be anchored at suitable places to the deck and raised in the manner of a bellows to provide shelter for operating personnel. As an alternative, weather protection sheets may be secured to the legs of the semi-submersible.

A primary advantage of the use of a service vessel as proposed by the invention is that it may bring dry dock facilities to the site of the oil rig so that the rig need only be out of action for the minimum time that is necessary to raise it from the water, perform the necessary servicing and return it to the water without the need for it to be towed away from the oil field.

The speed of servicing is assisted by the fact that it is then possible and cost effective to arrange for all the necessary expertise to be available on the service vessel so that once the oil rig is out of the water all the operating skills are available to attend to the necessary servicing and repairs. Naturally, the servicing may be performed to a better standard than has hitherto has been possible by underwater inspections and repairs.

It is preferable to provide services to the oil rig such as water for the cooling of the generators so that while resting on board the service vessel life on the oil rig may continue unchanged with power and heat still available for the personnel aboard.

This can mitigate another serious problem in repair of oil rigs namely that of maintaining and providing accommodation for a full crew on dry land while the necessary repairs were being undertaken in dry dock.

Claims (10)

1. A method of servicing an oil rig, which comprises raising the rig above water level aboard a carrier, performing necessary inspection, maintenance and/or repair at sea on board the carrier and lowering the rig back into the water.

2. A service vessel for enabling an oil rig to be serviced out of the water while at sea, the vessel comprising a submersible hull having a flat deck for supporting the rig out of the water wherein at least one end of the vessel, that is to say the stern or the bow, is cut away level with the surface of the deck so as to present no obstruction to the sliding of an oil rig onto the surface of the deck.

3. A service vessel as claimed in claim 2, wherein the deck is provided with longitudinally extending gulleys whereby thrusters on the oil rig may be accommodated within the gulleys without removal from the oil rig.

4. A service vessel as claimed in claim 3, wherein removable shutters are arranged within the gulleys to provide a support surface level with the deck surface.

5. A service vessel as claimed in claim 3 or 4, wherein to enable access to the underside of the hulls of the oil rig, blocks are provided for supporting the hulls above the deck surface.

6. A service vessel as claimed in any preceding claim, wherein the deck of the service vessel is provided with weather protection sheeting supported on a collapsible framework to be erected to surround areas where work is in progress.

7. A service vessel as claimed in claim 6 wherein the collapsible framework is formed as bellows.

8. A service vessel as claimed in any preceding claim, wherein the vessel is not self propelled.

9. A method of servicing an oil rig substantially as herein described with reference to and as illustrated in the accompanying drawings.

10. A service vessel substantially as herein described with reference to and as illustrated in the accompanying drawings.