DRILLING SYSTEM AND METHOD

The present invention relates to a method and system for conducting subsea operations, especially subsea drilling operations. In particular, the system and method find use in the deployment and recovery of downhole tools from a drill string during the drilling operation. The present invention relates to a system and method advantageously finding use in recovering core samples from the bore or well during the drilling operation.

The drilling of bores at underwater locations, such as on the bed of a sea or ocean (herein referred to as ‘subsea locations’), is well known and practised, for example, for the production of oil and gas from subterranean reservoirs of the same. The drilling of subterranean bores at such subsea locations is known to be a difficult and costly procedure.

There are presently three principle techniques known in the art for drilling a bore into the seabed at a subsea location. The first technique involves providing a drill at the end of a so-called ‘drill string’ deployed from a dynamically positioned vessel at the water surface and extending from the vessel to the location on the seabed. The drill string may be deployed from the vessel through open water or may be deployed through a conduit or ‘riser’ extending between the vessel and the installation at the seabed. This technique requires that the position of the vessel from which the drill string is deployed is carefully controlled, in particular to ensure the vessel remains at an appropriate position above the location of the drilling operation on the seabed to avoid unacceptable displacement of the drill string and the drill. It is also necessary to accommodate the rise and fall of the surface of the water during drilling, in order to avoid compromising the drilling operation. This may be achieved by a so-called ‘heave compensation’ facility, employing a resilient means to tension the drill string and any conduit or riser. As the depth of the water increases, the drill string and conduit or riser contribute increasing resiliency to the assembly, in turn increasing the problems of applying tension to the assembly and accommodating changes in the level of the water surface. In particular, as the depth of water increases, the drill string and riser are subject to flexing under the action of waves at the surface of the water and currents in the body of water. Strong currents can be especially problematic as a result of the phenomenon of vortex shedding. As a result, control of the drilling operation at the seabed can become difficult. In particular, to ensure the best quality core sample for analysis, it is necessary when extracting a core sample from the bore being drilled for analysis to apply a light drill pressure. Interaction between the heave compensation means and the resiliency inherent in the drill string and riser can prevent precise control of the drill pressure and rate of penetration of the drill into the seabed being achieved.

The second technique involves the use of a drill assembly that has the drill string in the form of its component lengths of drill pipe, typically stored in a carousel, deployed at the seabed from a control vessel located at the water surface. This technique can be difficult to operate reliably due to the complex process of making up a drill string when deployed remotely from the surface vessel. Servicing the drilling process with high quality drilling mud is also problematic when the mud is made up remotely at the subsea location. An advantage of using a remotely deployed drill assembly and a remotely operated vehicle is that the vessel at the water surface can be significantly smaller in size than the vessel required to handle and support a drill string and associated riser assembly. Further, the need for a heave compensation system is avoided. However, problems can arise with achieving an accurate control of the drilling operation. Further, the recovery of core samples from the bore to the surface in real time can be difficult. As a result, it is a common practice to collect core samples locally at the subsea location of the bore being drilled for later recovering once the drilling operation has stopped. It will be appreciated that such a method does not allow for an accurate monitoring of the drilling and sampling operation.

A third technique involves a drilling assembly deployed at the seabed location and operated remotely from the water surface. A conduit or riser is provided between the remotely operated drilling assembly and the surface vessel. Such a technique and system for its implementation are disclosed in WO 2011/161415. This technique offers the advantage of allowing the seabed operation to have a small footprint and allowing the conduit or riser to provide stability to the rotating drill string as the drilling operation proceeds. The drill string length is limited to the length sufficient to accommodate the expected depth of the bore hole required, typically of the order of 100 metres. This is particularly advantageous when the subsea drilling operations are being conducted at large water depths, for example 2000 metres or more. While this third technique relies on the use of a conduit or riser extending between the surface vessel and the remotely operated subsea installation, it allows for the real time recovery of core samples from the bore being drilled, in turn improving the level of control of the drilling operation that can be achieved. However, the conduit or riser must be connected to the surface vessel by means of a heave compensated system that requires a derrick or similar structure.

There is a need for an improved system and method for conducting subsea drilling operations, while allowing for the real time recovery of core samples from the bore being drilled.

EP 1 914 379 concerns an underwater seafloor drilling rig system. The system comprises a pressurised drilling capsule. The capsule is sealed and houses equipment for operating a drill string extending through an opening in the capsule. The capsule further houses a drilling fluid handling system for providing a drilling fluid feed and return between the capsule and a bore being drilled in the seabed. An umbilical flex hose bundle extends between the capsule and a surface vessel for the provision of breathable gas, electrical power, drilling fluid and fresh water to the capsule. A similar hose bundle is provided to provide a return path for fluids to the surface vessel. Optionally, wire guidelines may be provided to extend between the capsule and the surface vessel.

U.S. Pat. No. 7,380,614 discloses a remotely operated water bottom-based drilling system using a cable for auxiliary operations. The bottom-based drilling system comprises means for storing a plurality of drill rods and a plurality of core barrels. In use, the system is disposed at the seabed. The system is operated to drill a bore into the seabed, using drill rods retrieved from the drill rod storage. Core barrels are removed from the core barrel storage facility in the system and deployed down the drill rod by means of a winch on the system to retrieve a core sample from the bore. The core barrels with samples are recovered to the system and replaced in the storage facility. The system of U.S. Pat. No. 7,380,614, while allowing core samples to be obtained during the drilling operation, does not allow such samples to be recovered to the water surface until the entire bottom-based system is retrieved by the surface vessel.

US 2006/0016221 discloses a method and system for deep sea drilling. The system includes a shuttle for transporting items, such as drill rods, geophysical tools and core barrels from an underwater drilling platform to a surface vessel. The shuttle is indicated to be used to allow core drilling to be carried out in deep waters without the need for a drill string extending from the underwater platform to the surface vessel. The system of US 2006/0016221 does not appear to allow for the real time taking of core samples and recovery of core barrels to the surface while the drilling operation is proceeding.

US 2009/0255728 discloses a wireline system for seabed and water bottom drilling and core sampling. The system rests on, engages with or penetrates into the seabed or water bottom. The system comprises a vertically moveable cross-beam having a winch and a drill head disposed thereon and from which a drill string is deployed and the drilling operation is performed at the seabed or water bottom. The system also comprises means for recovering a core barrel or measuring device from the bottom of the drill string without the need to reassemble the drill string. Core barrels containing core samples are recovered to the system on the seabed. Again, the system of US 2009/0255728 does not provide for real time recovery of core samples to the water surface during the drilling operation.

WO 2009/157762 describes and shows a system for the drilling a bore at a location on the seabed. The system comprises a frame assembly for placing on the seabed and means for engaging with and clamping a tube for drilling the bore. The system comprises cables for lowering the frame assembly to the seabed. There is no provision for the recovery of core samples either to the frame assembly at the seabed or to the surface.

GB 1,392,967 concerns a subsea drilling assembly and its method of operation. The assembly comprises a drilling unit which, in use, is located on the seabed. The drilling unit comprises an axially moveable drill and a facility for storing sections of drill pipe. The unit is operable to remove sections of drill pipe from the storage facility and assemble the drill string for drilling. The drill string includes a core barrel having an inner tube for receiving a core sample. The inner tube is recoverable to the surface by means of a travel body arranged for movement between the drilling unit and the water surface by means of guide wires. The travel body comprises a core-containing inner tube assembly for holding a core sample and means for retrieving a filled inner tube from the drill string. In operation, the inner tube is deployed in the drill string during the drilling operation. When the inner tube is full, the travel body is lowered to the drilling unit and the inner tube first recovered from the drill string to the travel body and then, together with the travel body, recovered to the surface, where the inner tube is emptied. The inner tube is then redeployed to the drilling unit by lowering the travel body, from where it is moved to the drill string and core sampling may continue. It is to be noted that the system of GB 1,392,967 does not permit for the drilling core to be sampled while the inner tube is being transported to the surface, emptied and returned to the drilling unit at the seabed.

The present invention provides a system for the remote drilling of a bore at the seabed, which allows for the supply of core sampling inner tubes and the recovery of core samples to the water surface in real time, in particular to allow core sampling to take place continuously while drilling is in progress.

According to a first aspect, the present invention provides a system for drilling a subterranean bore at a subsea location, the system comprising:

a drilling unit for location on the seabed and remotely operable from the water surface to drill a bore using a drill string;

a transport assembly moveable between the water surface and the drilling unit when at the location on the seabed, wherein the transport assembly comprises a receptacle for receiving a downhole tool;

first transfer means for transferring a first downhole tool from the drill string to the transport assembly; and

second transfer means for transferring a second downhole from the transport assembly to the drill string;

whereby the first and second transfer means are operable to recover from the drill string a first downhole tool and deliver to the drill string a second downhole tool in a single deployment of the transport assembly from the water surface to the drilling unit.

The present invention provides in particular a system and method for the delivery and retrieval of core sampling inner tubes to a remote drilling unit located on the seabed, in particular allowing an inner tube containing a core sample to be retrieved from the drill string and exchanged for an empty inner tube in a single deployment of the transport assembly. The present invention does not rely on and avoids the need for a conduit, such as a riser, to be provided between a surface vessel and the remote drilling unit on the seabed. The system and method will be described in general and specific terms in relation to the delivery, transport and recovery of a core sampling inner tube. However, it is to be understood that the system and method may be used for the deployment and recovery of any downhole sampling and testing equipment or instrumentation (referred to generally as ‘downhole tools’). Accordingly, references to an ‘inner tube’ are to be interpreted accordingly.

The system of the present invention comprises a drilling unit. The drilling unit is for deploying at and operation on the seabed from a vessel at the water surface. The drilling unit may be any suitable unit for remote operation from the surface vessel or another location.

The drilling unit typically comprises a support assembly, for example a support frame, for location on the seabed. The support assembly is arranged to firmly locate the drilling unit on the seabed in a stable configuration, so as to allow a drilling operation to be conducted from the drilling unit into the seabed. As discussed in more detail hereinafter, an advantageous aspect of the present invention is to employ a pre-assembled drill string. As discussed above, known remote drilling units for use on the seabed employ a facility to store and assemble lengths of drill pipe into a drill string local to the bore being drilled. Most advantageously, by allowing a pre-assembled drill string to be used, the present invention requires a drilling unit that is significantly smaller in size, offering a significantly reduced footprint to known remote drilling units, and the opportunity for being lighter in weight.

The drilling unit further comprises means for holding a drill string, that is an assembly of drill pipes connected in an end-to-end configuration and at the lower end of which is disposed a drill bit. Suitable means for holding a drill string are known in the art and include a clamp or chuck. Any suitable drill bit may be used in combination with the system of the present invention and, again, suitable drill bit configurations and designs are known in the art and are commercially available. In one preferred configuration, the drilling unit comprises a first holding means and a second holding means spaced apart from the first in the vertical direction.

The drilling unit further comprises means for rotating the drill string. The means for rotating the drill string may be hydraulically powered, for example by fluid drawn from a reservoir of pressurised hydraulic fluid in the vicinity of the drilling unit or supplied by one or more lines or umbilicals extending to a support and control vessel at the water surface. Alternatively, the means may be powered electrically, with electrical power being supplied from a local storage facility or from the surface vessel. Again, suitable means for rotating the drill string, such as power swivels, are known in the art and are commercially available items of subsea equipment.

The drilling unit further comprises means for advancing and retracting the drill string into and out of the bore. Suitable means for advancing the drill string include ram jacks, typically hydraulically powered. Such ram jacks are known in the art.

The means for holding the drill string, means for rotating the drill string and means for advancing and retracting the drill string are operated in conjunction to advance the drill string and the drill bit into the seabed, while being rotated, to drill the bore. Such operations are known in the art, for example as generally described in WO 2011/161415. In summary, the operation proceeds as follows:

The drilling operation is commenced with the lower holding means, typically a static clamp or chuck, restraining the drill string. The lower holding means is released and the upper holding means, typically a chuck, engage the drill string. The rams retract, providing downwards movement to the drill string advancing it into the bore. The drill string is rotated by the power swivel to begin drilling, with the rams continuing to retract and urge the drill string into the bore as the drill bit progresses. Drilling in this manner continues until the rams are fully retracted. At this point, the rotation of the drill string is stopped. The lower holding means is reengaged onto the drill string, the upper holding means released and the rams are fully extended. The aforedescribed procedure is then repeated. In this way, drilling continues until the bore has been drilled to the required depth.

In operation of the system, core samples are recovered into and transported in inner tubes. Such inner tubes and their use are known in the art. References to an ‘inner tube’ herein are to any container or other means that may be used to hold and transport a core sample taken during the drilling operation.

The system of the present invention further comprises a transport assembly moveable between the water surface and the drilling unit when at the location on the seabed. The transport assembly comprises a receptacle for receiving an inner tube. The transport assembly is used to deploy an empty inner tube from the surface vessel to the remote drilling unit on the seabed for delivering to the drill string and to recover an inner tube containing a core sample from the drill string to the surface vessel. The transport assembly is moveable between the water surface and the drilling unit and may be moved by any suitable means. For example, the transport assembly may be deployed and recovered through open water. More preferably, the transport assembly is moved by means of a guide assembly, in particular a tensioned guide assembly, extending between the surface vessel and the remote drilling unit, with the transport assembly being provided with guide means for engaging with the guide assembly as it moves between the water surface and the drilling unit on the seabed. The guide assembly may comprise one or, more preferably, a plurality of guides extending between the surface vessel and the remote drilling unit.

In one preferred embodiment, the guide assembly comprises one or a plurality of cables, to act as the guides for the transport assembly. A plurality of spaced apart cables is preferred, allowing the transport assembly to move therebetween when travelling between the surface and the seabed.

The transport assembly preferably comprises one or more guide members that moveably engage with the guide assembly, allowing the transport assembly to be directed and maneuvered to and from the remote drilling unit by the guide assembly as the transport assembly is moved through the water. Where the guide assembly comprises a plurality of guides, such as a plurality of cables, the transport assembly is preferably provided with a guide member to engage each guide. In one preferred embodiment, the transport assembly is provided with a plurality of guide member assemblies, with each guide member assembly comprising one or more guide members, the guide member assemblies preferably disposed along the length of the transport assembly, in particular with the transport assembly having a first guide member assembly at its upper end and a second guide member assembly at its lower end.

It is a significant advantage of the present invention that the transport assembly may be delivered to and recovered from the drilling unit without the need for a conduit, such as a riser or the like, to be provided between the surface vessel and the drilling unit. For remote drilling operations that do not require such a conduit, for example in cases where the depth of the bore being drilled is relatively shallow, such as less than 200 metres, more particularly less than 100 metres, more specifically about 50 metres, the ability to do without installing a conduit between the surface vessel or installation and the seabed is a significant advantage in both operational and cost terms. This is particularly advantageous when drilling bores in deep water, in particular when the depth of the water, and hence the length of the conduit required to reach the surface, would typically be over ten times the length of the drill string.

The transport assembly may be moved between the water surface and the drilling unit by any suitable means. A cable or wireline winch, preferably disposed on the surface vessel, for moving the transport assembly by means of a cable or wireline is one preferred embodiment.

The transport assembly comprises a receptacle for receiving and holding an inner tube. As described in more detail hereinafter, the transport assembly is used to carry an empty inner tube from the surface vessel to the drilling unit on the seabed, deliver the empty inner tube to the drill string, recover an inner tube containing a core sample from the drill string and return the inner tube to the surface vessel. The receptacle may be any suitable means for receiving and holding an inner tube in the transport assembly while being transported between the water surface and the remote drilling unit.

The receptacle may be arranged to hold a single inner tube. More preferably, the receptacle of the transport assembly is arranged to hold a first inner tube and a second inner tube. In this way, one inner tube may be recovered into the receptacle from the drill string and a second inner tube then dispensed from the receptacle to the drill string. This arrangement is particular efficient in transferring empty and full inner tubes and reduces the equipment required on the drilling unit.

In one embodiment, the receptacle comprises a housing, within which the inner tube is held, more preferably a generally cylindrical housing within which an inner tube may be retained. In a preferred embodiment, the transport assembly comprises a receptacle for inner tubes in the form of a generally cylindrical housing assembly, the housing assembly having a main housing portion and a secondary portion, with both the main and secondary housing portions being able to hold an inner tube. More preferably, the main housing assembly has an upper housing portion and a lower housing portion, with the secondary housing portion adjacent to the lower housing portion. The main and secondary housing portions may open to each other.

The receptacle may be open, in which case the inner tube is exposed to ambient conditions of temperature and pressure. Alternatively, the receptacle may comprise a housing that can be sealed to hold the inner tube at a constant pressure. This arrangement is particularly advantageous when recovering core samples that are sensitive to pressure and allows such core samples to be recovered to the surface at drill string pressure. This arrangement is described in more detail below.

The receptacle may hold an empty inner tube for delivery to the drill string in the same or substantially the same position as an inner tube containing a core sample and recovered from the drill string. More preferably, the receptacle comprises a first receiving portion, in which an empty inner tube for delivery to the drill string is received, and a second receiving portion, in which an inner tube recovered from the drill string is retained. In one preferred embodiment, the transport assembly comprises a releasable retaining assembly for holding an inner tube in the first portion of the receptacle, the retaining assembly being released by an inner tube moving from the drill string into the second portion of the receptacle. In this way, the action of recovering an inner tube from the drill string to the second portion of the receptacle releases the inner tube in the first portion for delivery to the drill string. Delivery of the inner tube from the first portion of the receptacle is preferably assisted by the action of gravity.

The system of the present invention further comprises a first transfer means for transferring an inner tube from the drill string to the transport assembly, in particular an inner tube containing a core sample obtained during the drilling operation. Any suitable means may be used to move the inner tube from the drill string to the transport assembly. In one embodiment, the transfer means comprises an overshot having a grapple or similar means to grip or attach to the inner tube. Such means are known in the art.

In one preferred embodiment, the first transfer means is provided on the transport assembly. In particular, the transport assembly is provided with an overshot that is deployed from the transport assembly to attach to and withdraw an inner tube from the drill string.

The system further comprises a second transfer means for transferring an inner tube from the transport assembly to the drill string, in particular an empty inner tube. Any suitable means may be used to move the inner tube from the transport assembly to the drill string. For example, the transport assembly may be provided with a releasable retaining means for holding an inner tube, which upon release allows the inner tube to move into the drill string. In one preferred embodiment, the inner tube is moved from the transport assembly to the drill string with the assistance or under the action of gravity. More preferably, the second transfer means is provided on the transport assembly and comprises the releasable retaining means for retaining the inner tube in the transport assembly, the inner tube being allowed to fall under the action of gravity from the transport assembly into the drill string when the retaining means is released.

The movement of inner tubes between the drill string and the transport assembly may take place in any order. In a particularly preferred embodiment, the transport assembly, first transfer means and second transfer means are arranged such that an inner tube, for example an inner tube containing a core sample, is first recovered from the drill string to the transport assembly. Thereafter, an inner tube, for example an empty inner tube, is moved from the transport assembly into the drill string. Still more preferably, the second transfer means is provided on the transport assembly and comprises a releasable retaining means, as described above, with the retaining means being operated to release the inner tube from the transport assembly by the movement of an inner tube from the drill string into the transport assembly.

In an alternative embodiment, the transport assembly has a housing in which the inner tube to be delivered to the drill string is retained. The second transfer means for transferring an inner tube to the drill string comprises a means for pressurising the interior of the housing. A releasable retaining means for holding the inner tube in the housing is provided, which responds to an increase in fluid pressure within the housing, thus releasing the inner tube for delivery to the drill string. The inner tube preferably moves under the action of gravity. In particular, the means for pressuring the interior of the housing may be a supply of pressurised fluid, for example drilling mud, which pressurises the interior of the drill string and the interior of the housing of the transport assembly. This embodiment is particular advantageous where, as in many drilling operations, a pressurised stream of drilling mud is supplied to the remote drilling unit, for example through a suitable umbilical assembly extending from the surface vessel.

As described above, inner tubes are moved between the transport assembly and the drill string when the system is in operation. To achieve this, the transport assembly may engage with the drilling unit at the seabed. More preferably, the system of the invention comprises an engagement assembly for engaging the transport assembly with the drill string, in particular the upper end of the drill string. In one embodiment, the engagement assembly comprises a docking station, onto which the transport assembly is landed, the docking station allowing for the movement of an inner tube between the transport assembly and the drill string.

The arrangement in which the transport assembly engages with the upper end of the drill string, rather than with the drilling unit is advantageous, in particular when the drilling unit is being operated with a pre-assembled drill string, as described in more detail below.

Therefore, according to a further aspect, the present invention provides a system for drilling a subterranean bore at a subsea location, the system comprising:

a drilling unit for location on the seabed and remotely operable from the water surface to drill a bore using a pre-assembled drill string;

a transport assembly moveable between the water surface and the drilling unit when at the location on the seabed, wherein the transport assembly comprises a receptacle for receiving a downhole tool;

engaging means for releasably engaging the transport assembly with the upper end of the drill string; and

transfer means for transferring a downhole tool between the transport assembly and the drill string.

As indicated hereinbefore, the present invention finds particular use in the drilling of relatively shallow bores, that is bores having a depth less than about 200 metres, more preferably less than about 100 metres, for example about 50 metres in depth. The depth of the water in which such bores are drilled may be any depth that is encountered. Typically, the water depth is greater than 200 metres, more typically greater than 500 metres. The present invention may be employed with advantage in water depths in excess of 1000 metres, for example up to 2000 metres. Operation in greater water depths is also feasible.

As discussed above, systems for the remote drilling of bores at the seabed include means for storing and assembling a drill string from lengths of drill pipe retained at or local to the remote drilling unit. It has been found that the aforementioned relatively shallow bores can be drilled using a pre-assembled drill string, that is a drill string that is assembled on the surface vessel and lowered to the remote drilling unit on the seabed.

Accordingly, in a further aspect, the present invention provides a system for drilling a subterranean bore at a subsea location, the system comprising:

a pre-assembled drill string of a length sufficient for drilling the bore to a desired depth;

a drilling unit for location on the seabed and remotely operable from the water surface to drill a bore using the pre-assembled drill string;

a transport assembly moveable between the water surface and the drilling unit when at the location on the seabed, wherein the transport assembly comprises a receptacle for receiving a downhole tool;

engaging means for releasably engaging the transport assembly with the upper end of the drill string; and

transfer means for transferring a downhole tool between the transport assembly and the drill string.

The ability to employ a pre-assembled drill string in a remote drilling operation is advantageous. Known systems employing remote drilling units arranged to assemble a drill string from sections of drill pipe at the seabed location must employ means to rotate the drill string at a sufficient torque to assemble the sections of drill pipe. This torque is significantly higher than that required to rotate the drill string when drilling. The use of a pre-assembled drill string, for example assembled and provided from the surface vessel, reduces the complexity of the remote drilling unit, which in turn reduces the size and cost of the drilling unit. In particular, the use of a pre-assembled drill string in the present invention allows the remote drilling unit to have a smaller footprint and be smaller in size, resulting in an increase in the ease of handling, deploying and recovering the drilling unit.

In operation, the pre-assembled drill string may be deployed to the seabed together with the remote drilling unit or after the drilling unit has been installed at the subsea drilling location. In use, the drill string extends upwards from the drilling unit and is introduced downwards into the bore under the action of the drilling unit, as the drilling operation progresses. To reduce the tendency of the drill string above the drilling unit to whirl or whip, the drill string may be provided with guide members. In embodiments of the present invention in which the subsea installation is provided with a guide assembly extending between the surface and the remote drilling unit, as hereinbefore described, the drill string is advantageously provided with guide members that moveably engage with the guide assembly. These may be arranged in an analogous manner to the guide members provided on the transport assembly, as described above.

As discussed above, the transport assembly preferably engages with the upper end of the drill string. In one preferred arrangement, the upper end of the drill string comprises a guide member assembly having one or more guide members and an engagement assembly, such as a docking station, for receiving the transport assembly.

The upper end of the drill string may further comprise one or more swivels and/or valves, such as a mud swivel and valve assembly, to allow the drilling operation to be serviced, for example to provide a supply of drilling mud to the drill string. Such arrangements are also known in the art.

Alternatively, or in addition to the provision of a guide member, the drill string may be provided with tensioning means, for example buoyancy assemblies attached thereto, to tension the drill string and reduce the tendency of the drill string to whirl or whip. Suitable buoyancy assemblies are known in the art.

Core samples are taken during the drilling operation and are recovered to the surface for analysis. In the present invention, core samples are recovered to the surface while the drilling operation is proceeding, allowing the operator to analyse the core samples and obtain a real time indication of the drilling operation and its progress. It will be appreciated that core samples are taken at the pressure prevailing in the drill string. It is the case that core samples may be sensitive to changes in pressure. In particular, the significant drop in pressure arising when a core sample is recovered from the drill string to the surface can lead to damage to the sample.

Accordingly, in a further aspect, the present provides a system for drilling a subterranean bore at a subsea location and recovering a core sample, the system comprising:

a drilling unit for location on the seabed and remotely operable from the water surface to drill a bore using the pre-assembled drill string;

an inner tube transport assembly moveable between the water surface and the drilling unit when at the location on the seabed, the inner tube transport assembly comprising a receptacle for receiving an inner tube, wherein the receptacle comprises a pressure sealable housing within which the inner tube is held and maintained at a constant pressure;

engaging means for releasably engaging and sealing the inner tube transport assembly for the transfer of an inner tube from the drill string to the housing; and

transfer means for transferring an inner tube between the inner tube transport assembly and the drill string;

whereby an inner tube may be transferred from the drill string to the housing at seabed ambient pressure.

The inner tube transport assembly of this aspect of the invention is as generally described hereinbefore and comprises a receptacle having a housing. The housing is pressure sealable, whereby the housing may be sealed and the pressure within the housing maintained, in particular while the inner tube is being moved from the drilling unit at the seabed to the surface. Any suitable means may be used to seal the housing and suitable valves and seals are known in the art. In particular, the transport assembly may be provided with a valve to allow an inner tube to be moved from the drill string to the housing at drill string pressure.

A releasable engaging means, such as a docking station, is provided for the transport assembly to engage with the drilling unit or the drill string for the recovery of an inner tube at the water pressure prevailing at the end of the drill string from the drill string to the housing.

Other features of the system are as hereinbefore described. In particular, the system is preferably arranged to allow the exchange of inner tubes between the inner tube transport assembly and the drill string, as described above.

In a further aspect, the present invention provides an installation for the remote drilling of a subterranean bore at the seabed, the installation comprising:

a surface vessel disposed at the surface of the water; and

a system as hereinbefore described.

In operation of the system of the present invention, the remote drilling unit is deployed on the seabed in known manner. Core sampling is carried out during the drilling operation in known manner. The system of the present invention allows for a core sample to be recovered from the drill string to the surface vessel, for example at ambient water pressure as described above. In particular aspects of the system of the present invention, an inner tube in the drill string containing a core sample may be exchanged for an empty inner tube delivered by the transport assembly, with the inner tube and sample being recovered to the surface. Drilling may continue with use being made of the empty inner tube, while the core sample is being transported to the surface. Alternatively, the operator may elect to stop drilling until the core sample has been analysed at the surface. However, once analysis has been completed, drilling may be resumed immediately and sampling continued using the empty inner tube in place in the drill string.

In a further aspect, the present invention provides a method for drilling a subterranean bore at a seabed location, the method comprising

providing a drilling unit on the seabed;

conducting a drilling operation using a drill string by remote operation of the drilling unit from a surface location;

recovering a core sample into a first inner tube during the drilling operation;

transporting a second inner tube from the surface to the drilling unit by means of a transport assembly;

recovering the first inner tube from the drill string to the transport assembly;

transferring the second inner tube from the transport assembly to the drill string;

transporting the first inner tube to the surface by means of the transport assembly; and

continuing the drilling operation and recovering a core sample into the second inner tube.

The method entails recovering a first inner tube containing a core sample from the drill string to the transport assembly and delivering a second inner tube from the transport assembly to the drill string. In this way, drilling and sampling may continue, with use of the second inner tube. The first and second inner tubes may be moved into and out the drill string in any order. Preferably, the first inner tube is recovered to the transport assembly before the second inner tube is deployed from the transport assembly. In one embodiment, the release of the second inner tube is triggered by the first inner tube moving into a retained position in the transport assembly. In an alternative embodiment, the release of the second inner tube is activated by an increase in pressure in the transport assembly.

The transport assembly may engage with or dock with the drilling unit. More preferably, the transport assembly engages with the drill string, in particular the upper end of the drill string. The drilling operation preferably employs a pre-assembled drill string, as described hereinbefore, with the drill string in use extending upwards from the drilling unit. In this case, the transport assembly engages with the upper end of the pre-assembled drill string.

Accordingly, a still further aspect of the present invention provides a method for drilling a subterranean bore at a seabed location, the method comprising:

providing a drilling unit on the seabed;

conducting a drilling operation using a pre-assembled drill string by remote operation of the drilling unit from a surface location;

using a downhole tool during the drilling operation;

engaging a transport assembly with the upper end of the drill string;

recovering the downhole tool from the drill string to the transport assembly; and

transporting the downhole tool to the surface by means of the transport assembly.

As described above, it can be advantageous to maintain a core sample recovered during the drilling operation at the pressure prevailing in the drill string, particularly in the case of a pressure-sensitive core sample.

Accordingly, a still further aspect of the present invention provides a method for drilling a subterranean bore at a seabed location, the method comprising

providing a drilling unit on the seabed;

conducting a drilling operation using a drill string by remote operation of the drilling unit from a surface location;

recovering a core sample into an inner tube during the drilling operation at a fluid pressure prevailing in the drill string;

recovering the inner tube from the drill string to an inner tube transport assembly at the fluid pressure prevailing in the drill string; and

transporting the first inner tube to the surface by means of the inner tube transport assembly while maintaining the inner tube at the fluid pressure prevailing in the drill string.

Embodiments of the present invention will now be described, by way of example only, having reference to the accompanying drawings, in which:

FIG. 1 is a diagrammatical representation of an installation for the remote drilling of a subterranean bore at a subsea location;

FIG. 2 is elevational view of the drilling unit and transport assembly of the installation of FIG. 1;

FIG. 3 is a partial cross-sectional view of a transport assembly for use with the installation of FIG. 1;

FIGS. 4
a to 4e are longitudinal and cross-sectional views of the housing assembly of the transport assembly of FIG. 3;

FIGS. 5
a to 5g are elevational views of the installation shown in FIG. 2 in first to seventh stages of operation; and

FIGS. 6
a and 6b are elevational views of the installation shown in FIG. 2 in use with a pre-assembled drill string at two stages in the drilling operation.

Referring to FIG. 1, there is shown a diagrammatical representation of an installation for the remote drilling of a subterranean bore at a location on the seabed. The installation, generally indicated as 2, comprises a surface vessel 4 located at the surface 6 of the sea, ocean, lake or other body of water (hereafter referred to as ‘sea’). A remote drilling unit 8 is disposed on the seabed 10 at the location for drilling a bore 12. A guide assembly 14, comprises a plurality of tensioned cables 16 extending between the drilling unit 8 and the surface vessel 4. A transport assembly 20 is movably engaged with the guide assembly 14, for movement between the surface vessel 4 and the installation on the seabed 10 by means of a wireline winch cable 22 extending from and controlled at the surface vessel 4.

Turning to FIG. 2, there is shown a vertical elevation of an installation at the seabed 10. The installation, generally indicated as 102, comprises a remotely operable drilling unit 104 located on the seabed 10 in known manner. The drilling unit 104 comprises a base 106 and a supporting frame assembly 108 extending upwards from the base. A clamp and chuck assembly 110 of known configuration is mounted substantially centrally on the base 106, through which a vertically oriented drill string 112 passes. The drill string 112 comprises a plurality of drill pipe sections 114, assembled in known manner by threaded joints 116.

In the embodiment shown in FIG. 2, the drill string 112 is pre-assembled, that is the drill pipe sections 114 have been assembled on the surface vessel to form a drill string of length equivalent to the depth of bore to be drilled. Thereafter the drill string has been lowered to the seabed. The pre-assembled drill string may be lowered to the seabed together with the drilling unit, as the drilling unit is being deployed. Alternatively, the drill string may be assembled at the surface and lowered to the drilling unit after the drilling unit is in place on the seabed, for example using a guide assembly, such as a plurality of tensioned cables described hereinafter.

The drilling unit 104 further comprises a pair of hydraulic feed rams 118 extending vertically from the base 106 and supporting a powered swivel 120. In operation, the clamp and chuck assembly 110, the feed rams 118, and the powered swivel cooperate to rotate and advance the drill string 112 into the bore in known manner, such as described in WO 2011/161415.

A drill bit of conventional design is mounted on the lower end of the drill string 112 for drilling the bore. During drilling, core samples are taken using an inner tube within the drill string in known manner.

A plurality of spaced apart guide cables 122 are connected at their lowered ends to the frame assembly 108 of the drilling unit 104 and extend generally vertically to the surface vessel, as shown in FIG. 1. The guide cables 122 are tensioned in known manner from the surface vessel and provide a guide assembly for guiding a transport assembly, as described hereinbelow.

The drilling unit 104 is provided adjacent the upper portion of the frame assembly 108 with a funnel-shaped bit guide 124, for guiding a bit and drill string lowered from the surface vessel into the power swivel 120 and the clamp and chuck assembly 110.

The installation employs the drill string 112 to drill the bore. In operation, as shown in FIG. 2, the drill string extends vertically through the drilling unit and projects upwards from the drilling unit. The drill string 112 comprises at its upper end a mud swivel 126 of conventional configuration. An umbilical hose 128 extends through the open water from the surface vessel to the drill string and is connected to the mud swivel. Drilling muds are provided to the bore via the umbilical hose 128 and the drill string 112, as required for the drilling operation, again in known manner.

The upper end of the drill string 112 is further provided with a valve 130, providing a means to seal the upper end of the drill string.

The drill string 112 is subject to whirl and whipping, for example as a result of being rotated during the drilling operation and under the action of currents in the water. Accordingly, to stabilise the drill string 112, there are provided both an upper drill string guide assembly 132 at the upper end of the drill string and a lower drill string guide assembly 134 in the region of the bit guide 124 adjacent the frame assembly 108 of the drilling unit 104. Each of the drill string guide assemblies 132, 134 comprises a lateral arm 136 extending radially outwards to each of the guide cables 122 and terminating in a generally tubular guide 138 extending around the respective cable and allowing movement of the drill string guide and, hence, the drill string, along the cables.

The upper drill string guide 132 is provided with a docking station 140 thereon, to allow for the docking of a transport assembly, as described below, and to provide access, through the valve 130 to the interior of the drill string 112.

The mud swivel 126 serves to rotationally isolate the portion of the drill string below it from the assemblies above it, such that the portion of the drill string below the mud swivel is free to be rotated by the power swivel 120 in the drilling unit, while the drill string valve 130, upper drill string guide assembly 132 and the docking station remain stationary.

A transport assembly, generally indicated as 202 is shown in FIG. 2 and in more detail in FIG. 3. The transport assembly 202 comprises a receptacle for inner tubes in the form of a generally cylindrical housing assembly 204. The housing assembly 204 has a main housing 206 having an upper housing portion 208 and a lower housing portion 210. The assembly 204 further comprises a secondary housing 212 adjacent but open to the lower housing portion 210. This arrangement of the housing assembly 204 is shown more clearly in FIGS. 4a to 4e. As can be seen in FIGS. 4a to 4e, the upper housing portion 208 is of a size to accommodate an inner tube. Similarly, the secondary housing 212 is sized to accommodate an inner tube. The main and secondary housings 206, 212 are open to each other. A lateral cross-section through the upper housing portion 208 of the main housing 206 along the line A-A and a lateral cross-section through the lower housing portion 210 and the secondary housing 212 along the line B-B are shown in FIG. 4e.

The secondary housing 212 is provided with a sleeve 214 at its upper portion, to locate and retain the upper end of an inner tube. The lower end portion of the secondary housing 212 is provided with a releasable latch assembly 216. As shown in FIG. 4a, the latch assembly 216 is in a first position for holding the base of an inner tube in the secondary housing 212. The latch assembly 216 is released, for example by an inner tube moving upwards within the main housing 206 activating a trigger mechanism or by a pressure-activated trigger, allowing an inner tube to fall under the action of gravity from the secondary housing 212. A pressure activated trigger may be operated, for example, by pumping mud into the drill string with the interior of the housing assembly being exposed to fluid pressure within the drill string by appropriate positioning of the valves.

A ramped wall portion 218 at the lower end of the secondary housing 212 guides the inner tube as it falls into the main housing 206, from where it exits the housing assembly. This sequence of operation is described in more detail below.

Turning again to FIG. 3, the transport assembly 202 further comprises a lower guide assembly 220 at its lower end and an upper guide assembly 222 at its upper end. Each of the guide assemblies 220, 222 comprises a lateral arm 224 extending radially outwards to each of the guide cables 122 and terminating in a generally tubular guide 226 extending around the respective cable. This allows movement of the transport assembly 202 along and between the cables between the surface vessel and the drilling unit at the seabed.

The transport assembly 202 is supported and moved by a wireline winch cable 230 extending from the surface vessel. The wireline winch cable 230 extends through the upper guide assembly 222 and into the housing assembly 204. An overshot 232 is mounted on the lower end of the wireline winch cable within the housing assembly 204.

The transport assembly 202 may be arranged to have the housing assembly 204, and consequently any inner tubes therein, exposed to ambient fluid pressure. As described above, the pressure prevailing in the housing assembly 204 will change with changes in the hydrostatic pressure, as the transport assembly is moved between the surface vessel and the drilling unit. For pressure sensitive core samples, the transport assembly 202 may be arranged to allow the housing assembly to be sealed. In this arrangement, seals in a stuffing box 234 are provided in the upper guide assembly 222 around the wireline winch cable to provide a fluid-tight seal at the upper end of the housing assembly 204. The seals should allow for the wireline winch cable 230 to move therethrough, to allow the overshot to be deployed and used, as described below. A valve (not shown for clarity) is provided to seal the lower end of the housing assembly 204. The valve should allow for the passage of the wireline winch cable, overshot and an inner tube therethrough, as also described below.

The housing assembly 204 is further provided with a ball valve 236 and a non-return valve 238 for pressure control and release.

The operation of the installation of this embodiment of the present invention is shown in FIGS. 5a to 5g. In particular, FIGS. 5a to 5g show seven key stages in the operation, as follows:

Referring to FIG. 5a, in a first stage, the drilling operation is shown underway, with the drilling unit 104 operating to rotate and progress the drill string 112 and drill the bore, in known manner. The valve 130 in the upper end of the drill string 112 is closed. Drilling mud is provided to the drill string 112 through the mud swivel 126, again in accordance with conventional drilling practices. While drilling is underway, the transport assembly 202 is lowered from the surface vessel along the guide cables 122 by means of the wireline winch cable 230. The transport assembly 202 has an empty inner tube 300 retained in the secondary housing 212.

Referring to FIG. 5b, in a second stage, the transport assembly 202 has been landed on the docking station 140 at the upper end of the drill string 112. A core sample is being collected from the bore in an inner tube 302 disposed in the drill string 112, in conventional manner.

Referring to FIG. 5c, in a third stage, a core sample has been collected in the inner tube 302 in the drill string. Drilling is stopped. The overshot 232 at the end of the wireline winch cable 230 is deployed by lowering the cable from within the housing assembly 204 into the drill string. The overshot 232 is shown in FIG. 5c engaged with the upper end of the inner tube 302.

Referring to FIG. 5d, in a fourth stage, the inner tube 302 with the core sample is recovered to the housing assembly 204 of the transport assembly 202. This is achieved by raising the wireline winch cable 230 and the overshot 232, together with the inner tube 302. As shown in FIG. 5d, the inner tube 302 is passing upwards within the main housing 206 and passed the empty inner tube 300 held within the secondary housing 212.

Referring to FIG. 5e, in a fifth stage, the inner tube 302 with the sample has been fully recovered into the upper housing portion 208 of the main housing 206. This position is shown in more detail in FIG. 4a.

Referring to FIG. 5f, in a sixth stage, the empty inner tube 300 is released and allowed to fall under gravity from the secondary housing 212, from where it enters the upper end of the drill string 112. The sequence of movements of the inner tube 302 containing the core sample and the empty inner tube 300 to and from the housing assembly 204 of the transport assembly 202 is shown in detail in FIGS. 4a to 4d.

Referring to FIG. 5g, in a seventh stage, the empty inner tube 300 is moved to the lower end of the drill string 112 within the bore. This may be under the action of gravity and/or by means of pumping with pressurised drilling mud. As soon as the empty inner tube 300 has left the housing assembly 204 of the transport assembly 202, the valve 130 at the upper end of the drill string 112 is shut. The housing assembly 204 is sealed, as necessary, and the transport assembly 202 is lifted from the docking station 140 by the wireline winch cable 230 and recovered to the surface vessel.

Drilling may recommence as soon as the empty inner tube 300 has reached the lower end of the drill string 112. Alternatively, should the operator need to analyse the core sample before proceeding, drilling may be suspended until the core sample is recovered to the surface. However, the installation is in a condition to recommence drilling as soon as the operator has completed the core sample analysis and without the need to return an inner tube to the drilling unit.

If required, the transfer assembly 202 can be removed from the wireline winch cable and a fishing tool or the like attached. Such tools are known in the art. This tool may then be deployed to the seabed using the guide cables 122 to latch onto the upper end of the drill string 112. With the seabed frame clamp released, the drill string 112 together with the power swivel 120, clamp and chuck assembly 110, and the feed frame can be recovered to the surface for maintenance and service. Alternatively with the seabed frame clamp engaged and the clamp and chuck assembly 110 released, just the drill string 112 can be recovered to surface, for example to change the drilling bit. In such cases, the frame of the drilling unit 104 remains in position on the seabed, allowing the bore already drilled to be quickly re-entered.

As discussed above, the system of the present invention is particularly suitable for use with a pre-assembled drill string. Referring to FIGS. 6a and 6b, there is shown the installation in operation both at the start of a drilling operation using a pre-assembled drill string and at the end. In operation, a drill string is assembled on the surface vessel to a length corresponding to the depth of the bore to be drilled. In the embodiments shown in the figures, a preassembled drill string of 50 metres in length is being used. It will be appreciated that other lengths of drill string may be employed up to the maximum length that can be handled and assembled on the surface vessel.

In particular, turning to FIG. 6a, the drilling operation is ready to commence. The pre-assembled drill string 112 extends vertically upwards from the drilling unit 104 on the seabed. As shown, the system of the present invention is operable as the transport assembly 202 lands on the docking station 140 on the upper end of the drill string. Turning to FIG. 6b, drilling has been completed, with the drill string 112 fully extended into the bore in the seabed 10. Throughout the drilling operation, the upper end of the drill string 112 moves closer to the drilling unit 104, with the transport assembly 202 travelling further along the guide cables 122 to engage with the docking station 140. As will be appreciated, the system of the present invention, as illustrated in FIGS. 6a and 6b, is able to accommodate a wide range of lengths of pre-assembled drill string and, hence, a wide variety of drilling and core sampling operations.

DRILLING SYSTEM AND METHOD

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CPC

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