Abstract:
A method of drilling a bore at a subsea location is provided, the method comprising providing a remotely operated drilling assembly at a subsurface location; providing a conduit extending from the surface of the water to the remotely operated drilling assembly; and drilling the bore at the subsea location using the remotely operated drilling assembly. An apparatus for conducting a drilling operation at the seabed comprises a remotely operable drilling assembly deployed at a subsea location; and a conduit extending from the drilling assembly to the surface of the water.

Description:
TECHNICAL FIELD 
       [0001]    This invention relates to subsea operations and more particularly to deep water seabed drilling operations that are required to be carried out at considerable depths, for example 2000 metres or more. In particular, the present invention relates to an apparatus and method for performing such subsea operations. The method and apparatus find particular use in subsea drilling operations, for example but not limited to, core sampling operations. 
       BACKGROUND OF THE INVENTION 
       [0002]    The drilling of bores for whatever purpose in a seabed at considerable depths is well known to be a difficult and costly process. 
         [0003]    Presently two principle methods/techniques are currently used for the handling of drills for drilling into a seabed, in particular a hard, i.e. rock, seabed. 
         [0004]    The first of these involves the provision of a drill at the end of a so-called drill string deployed from a dynamically positioned vessel. In practice, when drilling a seabed at the depths such as above mentioned it is important that the position of the vessel from which the drill string and the drill are deployed must be carefully controlled so as to avoid unacceptable displacements of the drill string and its associated drill. 
         [0005]    For example it is important to be able to accommodate the rise and fall of the surface of the water during any drilling operation involving the use of a very long and in consequence very heavy drill string suspended from a control vessel without jeopardising drilling performance. 
         [0006]    In practice, such movements are as far as possible accommodated by using a so-called heave compensation facility which employs resilient means such as spring compensation to accommodate the rise and fall (i.e., heave) of a vessel as a result of water displacements such as swell in such manner that the drill at the bottom of the drill string is not subjected to excessive undesired displacements relative to the seabed resulting in inefficient drill operation. However, problems arise in providing adequate heave compensation when drilling in deep water. In such deep water operations, the tubular core string contributes significant resilience to the assembly. In particular, the drill string and riser are subject to flexing under the action of waves and currents in the water. As a result, control of the drilling operation becomes difficult. In particular, a light drill pressure is required when drilling to extract core samples for analysis. The interaction between the resilient compensation means and the resilience of the tubular core string prevents precise control of the rate of penetration of the drill into the seabed. Accordingly, there is a need to provide a system whereby the rate of penetration of the drill head into the seabed in deep drilling operations can be controlled more accurately. 
         [0007]    The second of the presently used methods/techniques involved in deep seabed drilling involves the use of a drill assembly mounted from a remotely operable vehicle which is deployed from an appropriate control vessel. 
         [0008]    An important advantage arising from the use of remotely operable vehicles is that the associated control vessel involved can be smaller in size than the size of a vessel required to support and handle: drill, strings. Further, the need to provide for heave compensation as required when drilling from a surface vessel is avoided. However, such remote drilling operations present difficulties with accurate control of the drilling operation and do not allow for core samples to be recovered to the surface in real time. Rather, it is generally the practice that core samples are collected locally at the subsea location and only recovered to the surface at the end of the drilling operation. This does not allow for accurate monitoring of the drilling and sampling operation. 
         [0009]    There is a need to deal with the aforementioned problems and it would be advantageous if an improved method and apparatus for conducting subsea operations could be provided. 
         [0010]    Examples of known systems for conducting subsea drilling operations are as follows: 
         [0000]    EP 1094193 discloses a dual riser system for conducting drilling procedures between the deck of a dual activity drilling assembly at the surface of the water and a single well located on the sea bed.
 
WO 01/86110 discloses a subsea riser disconnect and method of using the same.
 
WO 02/066792 describes a high tensile loading top entry sub for use as an elongated wireline entry device for accessing a wellbore without disconnection of the drill string.
 
WO 2008/118680 provides a system and method for performing intervention operations using a compliant guide that extends between a surface location, such as a vessel, and a subsea installation. The system may be used to deploy tool strings into a subsea well.
 
       DISCLOSURE OF THE INVENTION 
       [0011]    The present invention addresses issues arising from the present methods of deploying deep sea drill apparatus into docking relationship with the docking station. 
         [0012]    According to the present invention, in a first aspect, there is provided a method of drilling a bore at a subsea location, the method comprising:
       providing a remotely operated drilling assembly at a subsurface location;   providing a conduit extending from the surface of the water to the remotely operated drilling assembly; and   drilling the bore at the subsea location using the remotely operated drilling assembly.       
 
         [0016]    In the method of the present invention, a drilling assembly is deployed below the surface of the water, preferably at the seabed, and is operated remotely to conduct the drilling operation. However, in contrast to known remotely operated drilling systems, the drilling assembly is connected to the surface by a conduit, such as a riser. In this way, the drilling operation may be serviced from the surface through the conduit. In particular, the conduit allows drilling fluids to be provided to the remote drilling assembly, drill bits to be changed and core samples to be recovered to the surface, in turn allowing for real time monitoring of the drilling operation. Further, the conduit provides vertical stability to the drilling assembly subsea. However, unlike conventional drilling operations conducted from the surface, there is no need to provide means for heave compensation of the core drill string. Rather, heave compensation is only required for the conduit or riser. In this way, greater control over the drilling operation may be maintained, while allowing the drill module to be fully serviced from the surface. 
         [0017]    A particular advantage of the method of this aspect of the present invention is that the conduit may be used to provide drill string to the remotely operated drilling assembly. In this way, the length of drill string deployed need only be as long as required to drill the bore to the desired depth. The need to have the drill string extend to the surface, as in conventional surface-operated drilling operations, is avoided. 
         [0018]    Accordingly, in one preferred embodiment, the method of the present invention comprises the additional step of providing through the conduit to the drilling assembly at the subsea location a drill string of sufficient length to drill the bore to the required depth from the subsea location. The drill, string is of a length significantly less than the length of the conduit extending between the surface and the subsea drilling assembly and is required only to be of a length sufficient to drill the bore to the desired depth, that is of sufficient length to extend the full depth of the bore to be drilled, together with a minor additional length required for operation of the drill module. 
         [0019]    In a further aspect, the present invention provides an apparatus for conducting a drilling operation at the seabed, the apparatus comprising: 
         [0000]    a remotely operable drilling assembly deployed at a subsea location; and
 
a conduit extending from the drilling assembly to the surface of the water.
 
         [0020]    The aspects of the invention hereinbefore described are also applicable to subsea operations other than drilling that are analogous thereto, that is operations that are sensitive to changes in the level of the surface of the water and for which similar heave compensation provisions must be made. Accordingly, references to ‘drilling operations’ are to be interpreted as including such other, analogous operations, as appropriate. 
         [0021]    According to a further aspect of the invention there is provided a method of carrying out predetermined operations at a deep seabed including the step of providing at the lower end of a conduit, or tubular core string, a docking station for mounting an assembly including equipment required to perform a required operation at the seabed, deploying the core string from the surface of the water above the seabed to present the docking station to a location of the seabed at which it is intended to perform said operation, and installing said assembly at the docking station. 
         [0022]    In one embodiment, the assembly is installed at the docking station using the conduit as a means for guiding the assembly into docking relationship with the docking station. Preferably the assembly is lowered to the docking station using a controlled lowering of the assembly by means of a cable/string carrying the assembly, and a sleeve in external sliding engagement with the conduit. 
         [0023]    In a preferred method the co-operation between the sleeve and the assembly is such that once the sleeve co-operates with the assembly, the weight of the assembly is used to guide the assembly into docking relationship with the docking station. 
         [0024]    Preferably the docking station is used to guide the lower end of the assembly towards its docked position. 
         [0025]    Conveniently, during the final stages of the engagement of said lower end with the docking station the upper part of the assembly is automatically moved into its operational setting with the docking assembly. 
         [0026]    In one embodiment, the assembly is pivotably supported at its upper end to a guide moveable along the conduit, such as the aforementioned sleeve, whereby the assembly is pivoted about the guide into position within the docking station. 
         [0027]    Preferably, the guide is arranged to maintain separation between the assembly and the conduit as the assembly is moved to the docking assembly. 
         [0028]    In an alternative embodiment, the assembly is deployed using a remotely operated vehicle (ROV) and is transported to and installed in the docking station using the ROV. 
         [0029]    In accordance with a still further aspect of the invention there is provided apparatus for enabling practical operations to be carried out at a deep seabed, the apparatus comprising a support assembly for mounting equipment for carrying out a said practical operation, a support assembly docking station provided at the lower end of a conduit, or tubular core string, deployable from the surface of the water above the seabed to present the docking station to a location of the seabed at which it is required to carry out a said operation, and means for guiding the assembly to the docking station. 
         [0030]    The apparatus preferably further comprises means for controlling the lowering of the guide assembly into its operation setting with the docking station. 
         [0031]    In one embodiment, the means for guiding the assembly is displaceable lengthways of the conduit for guiding the assembly to the docking station. 
         [0032]    In a preferred arrangement the means displaceable lengthways of the conduit includes a tubular sleeve slidable externally of the conduit, and wherein the upper end of the assembly is coupled to the tubular sleeve by way of a pivotal swing arm that serves to maintain separation between the assembly and the core string during the lowering thereof and to assist in the guiding of the upper end of the assembly into its docked position within the docking station. 
         [0033]    Conveniently, the lower end of the docking station is adapted to guide this lower end into its operationally docked position under the weight of the assembly, for example by being provided with a suitable guide member. 
         [0034]    In a preferred construction the lower end of the docking station is provided with guide surfaces positioned so as to be engageable with complementary guide surfaces provided at the lower end of the assembly when the assembly is positioned adjacent to the docking station, wherein on further lowering of the assembly the docking station guide surfaces guide the assembly to its operational position relative to the docking station, and wherein said swing arm is such as to displace the upper end of the assembly into its operation position relative to the docking station. 
         [0035]    In a preferred construction jack means are provided for locking the upper end of the assembly in its operational position within the docking station. 
         [0036]    In a further preferred form of the apparatus the assembly includes an axially directed sleeve extending the length of the assembly, the longitudinal axis of the sleeve being arranged to engage in the lower end of the core string, and wherein the lower end of this axially directed sleeve is coupled to a tool required to perform a required operation. 
         [0037]    Preferably the tool is a drill head, and wherein the lower end of the docking station is such as to allow the drill to pass there through so as to be operationally engageable with the seabed. 
         [0038]    Broadly, according to a further aspect of the invention there is provided an arrangement for carrying out required operations at a deep seabed, the arrangement including a docking station for mounting equipment required to perform a required operation at the seabed at the lower end of a conduit, or tubular core string, deployed from the surface of the water above the seabed, and means for using the conduit as a guide to guide and deliver and install said equipment in the docking station. 
         [0039]    Preferably the required operation is the drilling of the seabed and the equipment includes a drilling module. 
         [0040]    As noted hereinbefore, one advantage of the apparatus and method of the present invention is that the drilling operation may be carried out remote from the surface vessel, and thus fully isolated from movements of the vessel at the surface, but yet allow the drilling assembly to be serviced. In particular, the invention allows the recovery of core samples from the drilling assembly to be made while the drilling operation is underway. 
         [0041]    In a further aspect, the present invention provides a method of recovering a core sample from a remotely controlled drilling assembly at a subsea location conducting a subsea drilling operation, the method comprising:
       providing a conduit between the drilling assembly and the surface;   providing a storage assembly for holding a container for a core sample in communication with the conduit; and   recovering a core sample to the surface by moving a container for a core sample between the storage assembly and the conduit.       
 
         [0045]    Still further, the present invention provides an assembly for recovering a core sample from a remotely operable drilling assembly deployed at a subsea location, the assembly comprising:
       a conduit extending between the surface and the drilling assembly;   a storage assembly for holding a container for a core sample in communication with the conduit, whereby a container may be moved between the storage assembly and the conduit to allow a core sample to be recovered through the conduit from the drilling assembly to the surface.       
 
         [0048]    Core samples obtained from drilling a bore are held in a container, typically a core barrel or cartridge. In the present invention, one or more containers for core samples are held in the storage assembly. The storage assembly is provided at a subsea location, preferably a location near to or adjacent the drilling assembly. In operation, containers are moved between the storage assembly and the conduit, thereby allowing an empty container to be provided through the conduit to the drilling assembly for receiving a fresh core sample and/or allowing a container containing a core sample to be recovered through the conduit to the surface. 
         [0049]    In one embodiment, a container containing a core sample is recovered from the drilling assembly and placed in the storage assembly. An empty container is deployed through the conduit to the drilling assembly, whereupon drilling may recommence. The filled container is moved from the storage assembly and recovered to the surface through the conduit while the drilling operation continues. In this way, a significant reduction in the downtime of the drilling operation due to the transporting of containers is achieved. 
         [0050]    The storage assembly is preferably disposed adjacent the conduit. More preferably, the storage assembly and the conduit share a common opening, through which containers may be passed between the conduit and the storage assembly. 
         [0051]    In one embodiment, means are provided for moving a container between the storage assembly and the conduit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0052]    For a better understanding of the invention and to show how to carry the same into effect in relation to drilling into a seabed reference will now be made, by way of example only to the accompanying drawings in which: 
           [0053]      FIG. 1  illustrates a drill module docking unit provided at the lower end region of a core string; 
           [0054]      FIG. 2  is a face view of an embodiment of a drill module; 
           [0055]      FIG. 3  is a side view of the drill module illustrated in  FIG. 3 ; 
           [0056]      FIGS. 4 to 7  illustrate successive stages of the docking of the drill module in the docking station/unit; and 
           [0057]      FIGS. 8 to 10  illustrate successive stages in the storage of a core barrel during the recovering of a core sample from a drilling assembly. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0058]    Referring now to  FIGS. 1 and 2  of which  FIG. 1  schematically illustrates the general construction of a so called sub-frame  1  that serves as a docking stowing unit for a deep sea drill module unit (not shown in  FIG. 1 ). 
         [0059]    The upper end of the sub-frame  1  connects with the lower end of a primary tubular conduit  2  by means of which the sub-frame is deployed from an appropriate vessel (not shown). The sub-frame  1  can be conveniently regarded as an independent docking station assembly including an upper section  3  connected with the lower end region  4  of the conduit  2  and lower section  5  that is intended firmly to engage with the seabed. The upper and lower sections  3  and  4  are inter-coupled by a main frame  6 . 
         [0060]    The upper section  3  includes an upstanding tubular member  7  which is engaged by the lower end region  4  of the conduit  2 . 
         [0061]    The upper section  3  is provided with Vee shaped members  8  located along a common transverse line and positioned one to each side of the tubular member  7 . These members  8  are intended to serve as part of the arrangement to engaging with and locating the drill module (not shown but to be described hereinafter) during a docking operation. Further, the conduit  2  is provided with a longitudinal spline  8   a . Again, the spline  8   a  serves to orient the drill module relative to the sub-frame  1  as it is brought to the sub-frame along the conduit (as described hereinafter). 
         [0062]    In addition, the upper section  3  includes a so-called packer type joint  9  for ensuring that the connection between the sub-frame  1  and the conduit  2  does not in any way block the tubular interior of the conduit. 
         [0063]    A pair of side by side spaced apart inverted Vee members  10  is located adjacent to the packer type joint  9  for locating with the upper end of the drill module when the latter is docked. 
         [0064]    The lower section  5  of the sub-frame is so formed as to provide generally Vee shaped seating  11  into which the lower end of the drill module is intended to nest with one side  12  of the Vee seating  11  increased in its length to provide a guide surface extension  13  which is intended to assist in the guiding of the lower end of the drill module into the required docking position. 
         [0065]    The main frame  6  extends beneath the Vee seating  11  to provide support for a base plate  14 . The base plate  14  has a central bore that is coaxial with the bore of the main drill string  2  through which projects a tubular member  15  that is coaxial with the tubular member  7 . 
         [0066]    Four equiangularly spaced triangular plates  14 A which are intended to dig into the seabed and at the same time prevent rotation of the sub-frame  1  relative to the seabed are mounted to the underside of the base plate  14  and project outwardly of the tubular member  15 . 
         [0067]    In order to ensure that the sub-frame  1  is maintained in its required effectively anchored operational position weights  16  are provided upon the upper side of the base plate  14 . The extent of the weighting of the sub-frame  1  will be related to the environment in which it is installed. 
         [0068]    As so far described it will be noted that the upper end of the sub-frame  1  is coupled to the lower end of the conduit  2  by way of a connection that ensures that the longitudinal axis of the tubular member  15  is coaxial with the lower region of the conduit  2 . 
         [0069]    In practice, the sub-frame is deployed from the associated support control vessel (not shown) by way of the conduit  2  so that the tubular member  15  of the sub-frame bottom section  5  defines the axial position at which it is required to drill into the seabed. 
         [0070]    In conventional manner the stream of jetting water that is associated with the drilling operation is diverted at the region of the connection between the sub-frame  1  and the lower end of the core string  2  into a bypass conduit  18  whose outlet  19  is so positioned so as to be able to direct jetting material/water into the immediate vicinity of the core drilling zone immediately beneath the sub-frame lower section  5 . 
         [0071]    From the forging it will be apparent that the upper part of the sub-frame lower section is adapted so as firstly to provide a seating  11  for a sub-sea deep drilling module (not shown in  FIG. 1 ) and a means for assisting in the guiding of the lower end of the drill module into its required position in the sub-frame  1 . 
         [0072]    Also from the forgoing it will be seen that the upper section  3  of the sub-frame  1  is required to afford two functions: a first to provide means for initially engaging with the drill module during its presentation to the sub-frame  1 ; and a second function in which it provides means for assisting in the setting of the upper end of the drill module when present in the sub-frame into its operational alignment within the sub-frame. The means associated with these functions will be considered in more detail hereinafter. 
         [0073]    Referring now to  FIGS. 2 ,  3  and  4  these Figures schematically illustrate the main constructional features of an embodiment of a deep sea drill module  20 . 
         [0074]    The module  20  includes a main frame  21  comprising two parallel elongate elements  22  that are, in practice, several metres in length. The ends  23  of these, elements  22  that are at the lower end of the module  20  are interconnected by a rectangular cross piece  24  which is itself connected with a further cross piece  25  having inclined surfaces  26  defining locating and locking surfaces complimentary to the seating  11  for the purpose of the docking of the module in the sub-frame  1 . The surfaces  26  are intended to engage with the inclined surfaces  12  of the seating  11  of the sub-frame lower section when the module  20  is in the docked position in the sub-frame  1 . 
         [0075]    A central bore  27  is provided in the cross piece  24  and in the cross piece  25 . The longitudinal axis of this bore  27  can be regarded as the axis of the module  20  and is coaxial with the core string  2  and the tubular member  7 . 
         [0076]    The other ends  28  of the elements  22  are bridged by a cross piece  29  which piece is parallel to the cross pieces  24  and  25  and which has a central bore  30  coaxial with the axis of the module  20 . 
         [0077]    The crosspiece  29  is used to mount hydraulic jacks  31  located one to either side of the longitudinal axis of the module  20 . These jacks  31  are intended to engage in the inverted Vee members  10  of the upper section  3  of the sub-frame  1  when the module  20  is in its docked position in the sub-frame  1  thereby to lock the module  20  in the docking position in the docking sub-frame. 
         [0078]    The upper ends of the frame elements  22  mount one to each side a module suspension system ( FIGS. 2 and 4 ) including a swing arm  32  of a generally semicircular shape of which one end  33  of the arm is pivotally connected to a sleeve  34  by way of stub arms  35 . The sleeve  34  is moveable lengthways of the core string  2  and is so engaged with the core string  2  that when the sleeve  34  is at its lowermost part of its travel the stub arms  35  for the ends  33  of the swing arm  32  engage in the Vee shaped members  8  of the upper section  3  of the sub-frame  1 . 
         [0079]    The other ends  36  of the swing arms  32  are pivotally connected to the upper ends  37  of the elements  22  of the main frame  21  of the frame of the module  20  such that both swing arms  32  are able simultaneously to pivot with respect to the longitudinal axis of the drill module. 
         [0080]    The end  36  of each swing arm  32  also connects with an end of an associated stub arm  38 . The other ends  39  of the stub arms  38  are interconnected by a cross bar  40  whose central part is connected to the end of a cable  41  by means of which the drill module  20  may be raised or lowered relative to the seabed using the conduit  2  as a directional guide. 
         [0081]    Thus by reason of the engagement of the sleeve  44  with the conduit  2 , the drill module  20  is constrained to follow the conduit during the raising and lowering of the module by its cable  41 . As has been mentioned the longitudinal axis of the conduit  2  is coaxial with the longitudinal axis of the member  27  at the lower section of the module main frame. 
         [0082]    Referring now to  FIG. 4  this Figure can be regarded as illustrating a first phase in the docking of the drill module  20  into the sub-frame  1 . At the position shown in the  FIG. 4  it will be noted that the sleeve  34  is moving along the conduit  2 . 
         [0083]    As a result of the positioning of the swing arm  32  and in conjunction with the weight of the drill module  20  the latter will hang vertically and will be positioned at a short distance to the side of the conduit  2 . 
         [0084]    As the drill module  20  approaches the sub-frame  1 , the sleeve  34  engages with the spline  8   a  which acts to rotate the sleeve and the drill module into the correct orientation with the sub-frame. 
         [0085]    In practice, the lowermost end of the drill module  20  is so positioned that on further lowering of the module the apex of the pair of inclined surfaces  26  automatically engages with the extended inclined surface  13  of the Vee formation at the lower section  5  of the sub frame as is shown in  FIG. 5 . 
         [0086]    Further lowering of the module  20  in conjunction with the inter-engagement of the inclined surfaces  25  and  13  allows the lower end of the module  20  to slide further and further into the Vee seating  11  at the lower end of the sub-frame  1  and at the same time the module is caused to tilt slightly to the vertical. This situation is generally illustrated in  FIG. 6 . 
         [0087]    Further lowering of the module  20  causes the bottom of the module fully to nest into the Vee shaped seating  11  of the sub-frame  1 . 
         [0088]    By reason of the nesting of the lower end of the module into the seating lithe module  20  is automatically set to a vertical position in the sub-frame. 
         [0089]    Once this position has been reached the hydraulic jacks  31  are operated so as to cause them to engage the locating Vee&#39;s  10  in the upper section  3  of the sub-frame  1  thereby locking the drill module  20  in its fully docked position. 
         [0090]    Once the drill module has been so fully docked the longitudinal axis of the module  20  is coaxial with the longitudinal axis of the sub-frame  1  and that of the conduit  2 . 
         [0091]    The drill module  20  can now be regarded as being ready to carry out a required drilling operation. 
         [0092]    The features concerned with the actual drilling will be considered in more detail in relation to  FIGS. 2 and 3 . 
         [0093]    The actual drill head (not shown) is provided at the lower end of a drill string  43  ( FIG. 2 ) which is lowered from the support (not shown) by means of an appropriate cable (not shown) located within the conduit  2  which now essentially serves as guide tube or conduit for the drill string  43 . 
         [0094]    The drill string  43  and its drill head (not shown) are clearly sufficiently smaller external diameter relative to the internal diameter of the conduit  2  as to permit easy passage of the drill string  43  through the conduit  2  and through the tubular member  7  of the sub-frame  1 . 
         [0095]    The lowering of the drill string  43  is continued until the drill head thereof contacts the seabed. In other words the drill string  43  extends the full length of the drill module  20 . 
         [0096]    The drilling arrangement for the module includes a pair of hydraulic rams  44  having pistons  45  are mounted so as to extend lengthways of the module main frame  21  and are located one to either side of the longitudinal axis of the module. The free ends  46  of the pistons  45  connect with a power unit  47  mounting a rotatable clutch unit  48  so arranged that the latter can be reciprocated by the hydraulic rams  44  lengthways of the drill module  20  so as to enable the rotary clutch unit  48  to be displaced in the axial direction of the module frame and thus the secondary core string  43 . As will be seen from  FIG. 2  the rotary chuck unit  48  is located substantially midway along the length of the drill module. 
         [0097]    The drill module is provided with a second clutch unit  49  adjacent the lower end of the module  20 . In addition the module incorporates a hydraulic power unit  50  together with control equipment which enables the drill module  20  to be operated and remotely controlled from the support vessel or from other form of control station/base. 
         [0098]    The length of the drill string  43  is very much shorter than that of the conduit  2  and is of such length that it is able to accommodate the expected depth of the desired drilling operation plus the length required to extend through the drill module  20  into the conduit  2 . It is to be noted that, in this way, it is not required to have the drill string  43  extend to the surface, as is required in conventional drilling operations. 
         [0099]    A drill operation, assuming that the drill head is in contact with the seabed can be carried out as follows. The power unit  50  is operated to cause the two clutch units  48  and  49  to be set to their open positions. The jacks pistons are operated to in such sense as to advance the rotary clutch  48  upwards lengthways of the module  20  by a distance set by the stroke of the rams. At this point the rotary clutch  48  is caused to clamp onto the secondary core string  43  whilst the fixed clutch  49  remains open. 
         [0100]    The drill string  43  is then rotated by the action of the power unit  50  under the control of its associated control arrangements. At the same time the drill string  43  is moved by the jacks  44  in the downward direction at a rate compatible with drilling conditions. These conditions are monitored by the equipment provided on the drill module. 
         [0101]    When the jacks  44  have moved downwards through their stroke taking the rotating drill string  43  with them rotation of the drill string  43  is stopped. The rotary clutch  48  is released from its gripping position and the other clutch  49  at the bottom of the module is operated so that it gripped with the drill string to lock it in position relative to the drill module  20  and thus the sub-frame  1 . With this arrangement in the event of movements such as heave the drill module  20  and the drill string  43  displace as a unit. 
         [0102]    When the requisite depth of drilling has taken place the two clutches  48  and  49  are released so that the drill string  43  is free to be moved relative to the module  20  and also relative to the conduit  2 . 
         [0103]    At this point the drill string  43  can be raised to the surface through the conduit  2  and recovered to the surface. 
         [0104]    Since the drill string has a very short length as compared with the length of the conduit  2  the withdrawal of the drill head is a relatively rapid process as compared with the time required to raise the conduit  2 . 
         [0105]    When it is required to bring the drill module to the surface it is merely necessary to raise the drill string  43  back into the conduit  2 , release the jacks locking the module to the sub-frame  1  and then to retract the cable attached to the drill module  20 . The lifting action exerted on the swing arms  32  will cause the upper end of the module to move away from its packed position back to the free hanging position as shown in  FIG. 1 . 
         [0106]    Turning to  FIGS. 8 to 10 , there is shown an embodiment of an assembly for the deployment and recovery of core barrels during the drilling operation described hereinbefore. The assembly comprises the conduit  2 , as hereinbefore described, extending between the surface at its upper end and the drill module  20  at its lower end. A core barrel storage assembly  102  is provided adjacent the conduit  2  at a subsea location near to the drill module  20 . 
         [0107]    The storage assembly  102  comprises a generally cylindrical housing  104  having a capacity sufficient to hold one or more core barrels  106 . The interior of the housing  104  is in communication with the interior of the conduit  2 , such that core barrels  106  may be moved between the storage assembly  102  and the conduit  2 . In one preferred embodiment, the conduit  2  and the housing  104  share a common opening through which the core barrels may pass in operation. 
         [0108]    Means for moving core barrels between the storage assembly  102  and the conduit  2  comprises a generally tubular guide  108  and upper and lower arms  110  and  112 . The arms  110 ,  112  are pivotably mounted so as to move the guide  108  between a first position, shown in  FIG. 8 , in which it lies within the conduit  2  and a second position, shown in  FIG. 9 , in which it lies within the housing  104  of the storage assembly  102 . 
         [0109]    Alternative means for moving the core barrel  106  between the storage assembly  102  and the conduit  2  include diverter valves, flaps and the like. 
         [0110]    In operation, an empty core barrel  106  is deployed from the surface within the conduit  2  in known manner, as shown in  FIG. 8 . As the core barrel  106  reaches the portion of the conduit adjacent the storage assembly  102  it is received by the guide  108  in the first position. The arms  110 ,  112  pivot to move the guide  108  and the core barrel  106  upwards into the housing  104  of the storage assembly  102 , as shown in  FIG. 9 . It will be noted that the bore of the conduit  2  is clear of obstruction with the core barrel in the second position shown in  FIG. 9 . In this way, access to the drill module  20  through the conduit  2  is available. 
         [0111]    A core barrel containing a core sample may be removed from the drill module through the conduit  2  in conventional manner and recovered to the surface. During this operation, the core barrel  106  in the storage assembly  102  is returned to the conduit  2  by downwards movement of the arms  110 ,  112 . A guide member  114  in the housing  104  of the storage assembly urges the lower end of the core barrel  106  out of the guide member  114  through a longitudinal opening in the side of the guide member and into the conduit  2 . The core barrel  106  is then lowered into the drill module  20  and the drilling operation continued. It will be noted that the recovering of the core sample to the surface can be conducted with the drilling operation resumed. 
         [0112]    In this way, the time that the drill module is not operating is kept to a minimum, in turn significantly reducing the overall time taken to drill a given bore. 
         [0113]    In one embodiment, the storage assembly  102  as shown in  FIGS. 8 to 10  may be arranged to also act as a guide member and perform the function of the spline  8   a  described hereinbefore in orienting the drill module when being lowered to the sub-frame  1 . 
         [0114]    It should be noted that whilst the aforementioned general and specific description has specifically discussed the positioning and use of a drill for a sub-sea drilling operation the concepts of the invention can be applied to other predetermined operations at a deep seabed requiring operatively positioning equipment/devices at the seabed in such manner that the operations being carried out at the seabed are not jeopardised by the rise and fall of the sea levels resulting in unacceptable displacements of the equipment while in use. Accordingly, references herein to drilling operations are to be interpreted as including such other subsea operations.