GB2300209A

GB2300209A - Offshore drilling

Info

Publication number: GB2300209A
Application number: GB9608741A
Authority: GB
Inventors: John Arthur Edward Lampard
Original assignee: Kvaerner H&G Offshore Ltd
Current assignee: Kvaerner H&G Offshore Ltd
Priority date: 1995-04-27
Filing date: 1996-04-26
Publication date: 1996-10-30
Also published as: GB9508603D0; GB9608741D0

Abstract

A method of drilling subsea wells for oil and/or gas from a wellhead platform comprises the steps of positioning a jack-up rig 10 (having a drilling derrick 15 mounted on a cantilever arm 14) next to the wellhead platform, moving the cantilever arm 14 to a location over the wellhead platform, jacking the jack-up down until the cantilever arm rests on two beams (17) close to the top of the wellhead platform, disconnecting the cantilever arm from the jack-up, jacking the jack-up down again until the jack-up is clear of the cantilever arm, and then drilling a plurality of wells using the drilling derrick while that derrick is supported on the wellhead platform by the cantilever arm, and is supplied from the jack-up.

Description

OFFSHORE DRILLING The invention relates to a method of drilling subsea wells, e.g. for oil and/or gas.

More particularly the invention relates to a method of using the drilling derrick of a mobile self-elevating drilling unit to drill wells through a fixed well head platform. Mobile self elevating drilling units are commonly used to drill subsea wells in order to exploit offshore oil and/or gas deposits.

A mobile self-elevating drilling unit generally comprises a flat hull or work deck, and at least three legs disposed around the periphery of the deck. The legs are fitted with jacking mechanisms, so that the legs can be jacked down or up while the hull is floating, and the deck can be jacked up or down when "spud cans" at the bottoms of the legs are firmly founded on the seabed. In this way the deck can be elevated above the wave effected zone. Because the unit is self-elevating by jacking action, this type of unit is commonly known as a "jack-up", and will be so referred to in this specification.

In the past it has been known to install a substructure or jacket for a wellhead platform on the site of a subsea oil or gas field, and then to position a mobile drilling jackup next to the jacket. The drilling jack-up has a drilling derrick mounted on a cantilever arm. The cantilever arm can be extended so that the drilling derrick is located directly over the jacket to enable it to drill wells through the jacket. This arrangement is illustrated in Figure 4 of U.K. Patent Specification 2026573, in which the cantilever arm is 3, the drilling derrick is 6 and the jacket is 10. This is known as cantilever mode drilling.

The number of wells that can be drilled in cantilever mode from one spud-in of a jack-up is currently limited depending on the pressure rating of the wells, e.g. to a maximum of twelve. This maximum number is appropriate for only the very largest jackups, after stability, and linear and angular spud-in-tolerances of the jack-up with respect to the wellhead platform are taken into account.

It has also been known to transfer a drilling derrick from a mobile jack-up onto a fixed substructure for a wellhead platform, in order to drill through that substructure. In this case the jack-up is located next to the fixed substructure, and is raised or lowered so that skid rails on its cantilever arm are just above the level of skid rails on the fixed substructure.

The cantilever arm is then extended until skid pads on the drilling derrick overlie skid beams on the top of the substructure. Elevating beams on the substructure are raised slightly to take the weight of the drilling derrick so that the cantilever arm can be withdrawn. The elevating beams are lowered to place the drilling derrick on the skid beams of the substructure. The mobile jack-up can then have its legs raised and can be floated away from the fixed substructure. The transfer of a drilling derrick in this manner is described on pp30-31 of the publication "Offshore Engineer" for December 1983.

In a case in which the jack-up (or a semi-submersible vessel) remains adjacent to the wellhead platform and provides power and supplies for the drilling operation, drilling in this manner is known as Tender Assisted Drilling or TAD.

Some offshore oil/gas fields at exposed deepwater sites in the North Sea, and currently in development as Normally Unmanned Installations (NUls) or bridge linked satellites, have considered harsh environment jack-ups for cantilever mode drilling over weilhead platforms. These fields have had water depths and well depths that extended operation of these jack-ups to their certified operating limits. Only a few currently available jack-ups are capable of these operations. The availability of suitable harsh environment jack-ups may be a limiting factor on field development.

Year round operating availability is also a problem for cantilever node drilling from some smaller jack-ups in the severe winter weather conditions of the North Sea.

An alternative conventional approach would be to use a Tender Assisted Drilling (TAD) unit with a skid off drilling derrick and a semi-submersible vessel. This is more complicated and expensive. A new skid base, designed to give the drilling derrick its normal skidding ability, would have to be constructed and landed on the wellhead platform before the derrick was skidded over. Moreover the whole system would rely on the weather availability of the semi-submersible vessel.

The invention provides a method of drilling subsea wells for oil and/or gas from a wellhead platform, which method comprises the steps of positioning a jack-up (having a drilling derrick mounted on a cantilever arm) next to the wellhead platform, moving the cantilever arm to a location over the wellhead platform, jacking the jack-up down until the cantilever arm rests on two beams close to the top of the well head platform, disconnecting the cantilever arm from the jack-up, jacking the jack-up down again until the jack-up is clear of the cantilever arm, and then drilling a plurality of wells using the drilling derrick while that derrick is supported on the wellhead platform by the cantilever arm and is supplied from the jack-up.

It is preferred that the jacking down of the jack-up until the cantilever arm rests on the two beams is carried out with the drilling derrick at the end of the cantilever arm away from the wellhead platform, and the drilling derrick is moved across when the cantilever arm has landed on the beams. This may only be possible for some types of jack-up.

It is further preferred that the cantilever arm is locked onto the two beams before drilling operations begin.

It is still further preferred that a multiplicity of wells are drilled at different locations on the wellhead platform by moving the drilling derrick laterally between such locations by skidding the cantilever arm on two or more beams on the wellhead platform.

The invention includes a well head platform having beams to accept a drilling derrick and cantilever arm to carry out the method described above.

A specific embodiment of the invention will now be described by way of example with reference to the accompanying drawing, in which; Figure 1 shows very diagrammatically an above waterline view of a jack-up adjacent to a wellhead jacket; Figure 2 shows a drilling cantilever arm extended over the well head jacket; Figure 3 shows the drilling cantilever arm landed on the top of the wellhead jacket; and Figure 4 shows the jack-up lowered away from beneath the drilling cantilever arm.

Figure 1 illustrates a jack-up 10 which has three legs 11 (only one of which is shown), a hull 12, a cantilever arm 14 and a drilling derrick 15. The illustration is schematic and shows only that part of the jack-up above sea level. The legs of the jackup are spudded in on the seabed close to the legs of a wellhead jacket 1 6, and the hull 1 2 has been jacked up on the legs 11 to a level where the cantilever arm 14 is above the level of the top of the wellhead jacket 1 6. This is a conventional arrangement.

In Figure 2 the cantilever arm 1 4 with the drilling derrick 1 5 has been moved outboard from the hull 1 2 to its maximum extension over the wellhead platform well bay.

In Figure 3 the jack-up 10 has been jacked down on the legs 11 to land the cantilever arm 1 4 on beams 1 7 positioned for that specific purpose above the wellhead platform weather deck. The cantilever arm 1 4 is locked onto the beams 1 7 above the weather deck of the wellhead platform, and released from the jack-up.

The jack-up 10 then jacks down further, to a level where the hull 1 2 is still above the maximum wave height, and the deck is high enough for its cranes (not shown) to perform their normal functions with respect to the piperack (also not shown) on the rear of the cantilever arm. This final configuration is shown in Figure 4.

The cantilever arm 14 can now be unlocked from the beams 17, and skidded around in all directions, limited only by the structure above the wellhead platform weather deck or the legs of the jack-up.

If the drilling derrick 1 5 is able to skid along the whole length of the cantilever arm 14, as on some jack-ups, then the initial extension and landing of the cantilever arm can be carried out with the derrick at the jack-up end of the cantilever arm. The drilling derrick can be skidded over from the jack-up to the wellhead platform after the cantilever arm has been locked to the beams 1 7 on the wellhead platform and released from the jack-up. This would reduce the overall loadings on the cantilever arm during the skidding of the drilling derrick from the jack-up 10 on to the beams 1 7 of the wellhead platform 16.

The jack-up is now isolated from the drilling derrick and is not influenced by drilling loads, so the stability of the jack-up is no longer a controlling factor in respect of drilling operations. Drilling availability will be improved, as drilling with the cantilever arm resting on the wellhead platform 1 6 should be weather insensitive.

Extra structural steel will be required in the deck and jacket of the wellhead platform (as in a conventional TAD unit), to carry the weight of the derrick, cantilever arm and drilling loads, but no new skid base will be required. The saving on elimination the new skid base should offset the cost of the extra structural steel.

The number of wells that can be drilled can be catered for by design of the specially constructed beams 1 7 on the wellhead platform deck, and is not limited by cantilever drilling restrictions.

The beams 17, on which the cantilever arm 14 is to land, will need to be high enough to allow the use of the drilling BOP. It is normal to have a gap of 3 to 5 metres above the wellhead platform weather deck to allow for relative motion between the jackup and the wellhead platform in cantilever drilling mode. This should be unnecessary with the arrangement now described, and incorporation of the beams 1 7 should allow the jack-up to work at lower than its normal operating height for any particular field.

Modifications to the jack-up are confined to making the sliding guides for the cantilever arm removable, and possibly some changes to the drag link. The cantilever arm may require extra stiffening if it is to be skidded transversely to its normal skidding direction on the jack-up skid beams.

The jack-up will be operating at a height less than that required to drill in the cantilever mode, and in an unloaded condition, since the deadload of the cantilever arm 14 and drilling derrick 1 5 (and live drilling loads) will be supported directly on the wellhead platform 1 6.

The hull of the jack-up should not have to reach a height any greater than that if it were in a pure cantilever mode drilling for the same field.

Spud-in setting tolerances should not affect well slot availability, as the cantilever arm itself can be skidded around as with a normal on-platform drilling unit.

In summary, mounting the cantilever arm 1 4 on the beams 1 7 of the well head platform 1 6 both enlarges the drilling footprint, and also reduces the loads on the jack-up.

The consequent possibility of using smaller jack-ups implies lower day rates, which would offset any extra cost over a pure cantilever drilling mode. Additionally, water depths greater than those considered for present jack-up drilled developments may be possible for larger jack-ups.

A wide variety of jack-ups should be able to operate in this mode, their leg length being the only normal restriction.

Claims

1. A method of drilling subsea wells for oil and/or gas from a well head platform, which method comprises the steps of positioning a jack-up (having a drilling derrick mounted on a cantilever arm) next to the wellhead platform, moving the cantilever arm to a location over the wellhead platform, jacking the jack-up down until the cantilever arm rests on two beams close to the top of the wellhead platform, disconnecting the cantilever arm from the jack-up, jacking the jack-up down again until the jack-up is clear of the cantilever arm, and then drilling a plurality of wells using the drilling derrick while that derrick is supported on the wellhead platform by the cantilever arm, and is supplied from the jack-up.

2. A method as claimed in claim 1 in which the jacking down of the jack-up until the cantilever arm rests on the two beams is carried out with the drilling derrick at the end of the cantilever arm away from the wellhead platform, and the drilling derrick is moved across when the cantilever arm has landed on the beams.

3. A method as claimed in claim 1 or claim 2 in which the cantilever arm is locked onto the two beams before drilling operations begin.

4. A method as claimed in any one of the preceding claims in which a multiplicity of wells are drilled at different locations on the wellhead platform by moving the drilling derrick laterally between such locations by skidding the cantilever arm on two or more beams on the wellhead platform.

5. A method substantially as described with reference to the accompanying drawing.

6. A well head platform having beams to accept a drilling derrick and cantilever arm to carry out the method of any one of the preceding claims.

6. A well head platform having beams to accept a drilling derrick and cantilever arm to carry out the method of anyone of the preceding claims.

Amendments to the claims have been filed as follows 1. A method of drilling subsea wells for oil and/or gas from a weihead platform, which method comprises the steps of positioning a jack-up (having a drilling derrick mounted on a cantilever arm) next to the wellhead platform, locking the drilling derrick to the cantilever arm, moving the drilling derrick and the cantilever arm horizontally from a location on the jack-up to a location over the wellhead platform, jacking the jack-up down until the cantilever arm rests on two beams close to the top of the well head platform, disconnecting the cantilever arm from the jack-up, jacking the jack-up down again until the jack-up is clear of the cantilever arm, unlocking the drilling derrick from the cantilever arm so that the drilling derrick can move laterally on the cantilever arm, and then drilling a plurality of wells using the drilling derrick while that derrick is supported on the wellhead platform by the cantilever arm, and is supplied from the jack-up.

3. A method as claimed in claim 1 or claim 2 in which the cantilever arm is connected to the two beams before drilling operations begin.

4. ' A method as claimed in any one of the preceding claims in which a multiplicity of wells are drilled at different locations on the wellhead platform by moving the drilling derrick laterally between such locations by skidding the cantilever arm on two or more beams on the wellhead platform.