WO2012156742A2

WO2012156742A2 - Drilling apparatus and method

Info

Publication number: WO2012156742A2
Application number: PCT/GB2012/051103
Authority: WO
Inventors: Thomas Hasler; Harald Hufthammer
Original assignee: Ikm Cleandrill As
Priority date: 2011-05-16
Filing date: 2012-05-16
Publication date: 2012-11-22
Also published as: NO20110722A1; GB2506030A; GB201320059D0; NO339898B1; WO2012156742A3

Abstract

The invention provides an apparatus and method for handling drilling fluids during a subsea drilling operation. The method comprises providing a body at a subsea well site and receiving drilling fluid and entrained solids into the chamber from a wellbore at the subsea well site. Drilling fluid and entrained fluids are processed at the subsea well site to break up and/or fluidise the entrained solids by operating at least one cutting pump, which also functions to pump the processed drilling fluid and entrained solids from an outlet to a return conduit. The processed drilling fluid and entrained solids are conveyed to surface by pumping the drilling fluid and entrained solids through the return conduit.

Description

Drilling apparatus and method The present invention relates to apparatus and methods for use in subsea drilling operations, and in particular to apparatus and methods for drilling subsea wells for the hydrocarbon exploration and production industry. Aspects of the invention relate to apparatus and methods which minimise the discharge of contaminants to a subsea environment during drilling of tophole sections of subsea wells. Background to the invention Drilling in the seafloor for the purposes of scientific or geotechnical research or for hydrocarbon exploration and production typically involves the rotation of a drill bit on the end of a drill string (or drill pipe), which extends from a drilling platform to a drill bit. In a typical drilling system used for drilling deep sections of a subsea well, drilling fluid (referred to as drilling mud) is pumped from an offshore drilling unit down through a drill string to a drill bit. The drilling fluid fulfils a number of different functions, including providing hydrostatic pressure to control the entry of fluids from the formation into the wellbore, lubricating the drill bit, keeping the drill bit cool during drilling, and carrying particulate materials such as drill cuttings upwards and out of the well away from the drill bit. In conventional drilling, the drill string extends from the rig to the drill bit through a riser pipe, connected to the wellhead via a blowout preventer stack. The riser pipe connects the well to the rig creating a closed loop for the drilling fluid. Drilling fluid and cuttings emanating from the wellbore are carried up the annular space between the riser and the drill pipe and returned to the rig for processing, re-use, storage and/or treatment. Figure 1A shows schematically a typical drilling system 100, used for drilling the uppermost section 1 18 of the wellbore 1 10, which is referred to as the "tophole". When the tophole section 1 18 is drilled, there is no riser pipe installed between the seabed and the drilling rig 102, and as there is no conduit to return drilling fluids from the wellbore back to the surface, the drilling mud and cuttings are discharged to the subsea environment. Tophole drilling schemes of this type cause a number of commercial and environmental challenges for operators. Firstly, the discharge of drilling fluids and drill cuttings emanating from the wellbore into the environment causes pollution. The drilling fluids may for example be water-based, oil-based, or synthetic, all of which may include compounds which are detrimental to marine life and the condition of the subsea environment. In addition, the drilling cuttings themselves may be contaminated with hydrocarbons or other materials from the formation, and their presence in the marine environment is undesirable. The possible detrimental effects on the environment are increasingly a focus for regulators of the hydrocarbon exploration and production industry. Drilling operators are challenged by regulators on a global basis to improve the environmental performance of offshore drilling operations. In many cases, obtaining a licence to drill a subsea well depends on demonstrating very low or zero discharge of materials to the sea. Secondly, typical tophole drilling operations may require large volumes of drilling fluid, which is expensive to produce and difficult to transport to offshore locations. It is therefore desirable to recover drilling fluid for treatment and reuse where appropriate. Although these challenges are apparent in tophole drilling operations, for various operational and commercial reasons, drilling operators are also seeking to develop fully riserless drilling systems, in which the entire well will be drilled without a riser. In such riserless drilling systems the problems described above are exacerbated, as larger quantities of drilling fluid would be discharged into the sea. Figure 1 B is a schematic representation of a prior art riserless mud return system, which includes a drilling fluid receptacle 122, a return line 124 and a pump unit 126. Drilling fluid is pumped from the receptacle to a storage or treatment unit at surface. An example is described in US 4,149,603, and includes a mud sump connected to a submerged wellhead and supporting a quantity of mud above the wellhead. A top opening allows a drill pipe to enter the sump and pass through the sump into the well. Seawater is able to enter the upper part of the sump through the top opening, but only as far as the upper surface of the mud. A hose, separate from the drill string, is used to carry mud to the surface. A pumping means is used to pump mud through the hose back to surface, with the pump operated in dependence on the detected level of mud and cuttings supported within the sump. The system described in EP 1 ,694,941 B1 and NO 318767 B1 is said to improve upon the system of US 4,149,603 by including a filter for filtering dispersed material such as swelling clay or stones from the drilling mud. Filters in mud return flow lines are also described in US 4,410,425 and US 4,599,172. The use of filters for solid materials can mitigate problems in the mud return system, but does create a subsidiary problem of what to do with the solid materials. The system of EP 1 ,694,941 is designed to recover 80% to 90% of the drilling fluid, with large particles incapable of passing of the filter allowed to overflow through the open top of the equipment into the sea. W099/15758 describes an apparatus and method for drilling which use a pump to return drilling fluid to surface through a separate line. The document recognises that problems arise from passing the fluid with entrained solids through valves and pumps on the return segment of the circuit, and addresses these problems by using an arrangement of gravity fed filters designed to discharge to the seabed. US 6,325,159, US 2010/0147593 and US 2003/018393 describe systems for drilling which include rock crushers or pulverisers incorporated in line with mud return systems. These rock crushers are bulky pieces of equipment positioned on the seabed with dedicated inlet and outlet conduits. There is generally a need to improve environmental performance in offshore drilling operations and reduce drilling fluid waste. The above-described prior art systems do not adequately address the issue of pollutants and contaminants being discharged to a subsea environment. Such configurations are therefore limited in their application to low- or zero-discharge drilling operations. It is one aim of the invention to provide an apparatus and/or method for a drilling operation which obviates or at least mitigates one or more drawbacks of the previously proposed systems. It is a further aim of at least one embodiment of the invention to provide a subsea drilling apparatus and method, suitable for use in tophole and/or riserless drilling operations, which has improved handling of solid materials while reducing the discharge of drilling fluids and/or drilling cuttings to the environment. It is an aim of at least one embodiment of the invention to provide a drilling system which has substantially zero discharge of drilling fluids and/or drill cuttings to the environment. Other aims and objects of the invention will become apparent from a reading of the following description. Summary of the invention According to a first aspect of the invention there is provided a method of handling fluids during a subsea drilling operation, the method comprising:

providing a body at a subsea well site, the body defining a chamber for collecting drilling fluid and entrained solids;

receiving drilling fluid and entrained solids into the chamber from a wellbore at the subsea well site;

processing the drilling fluid and entrained solids at the subsea well site to break up the entrained solids;

conveying the processed drilling fluid and entrained solids to surface by pumping the drilling fluid and entrained solids through a return conduit. In the context of this invention, the term "entrained solids" should be construed broadly to means any solid materials present in or mixed with the drilling fluid, including drill cuttings, rocks, boulders, clays and other stiff materials from the wellbore and/or formation. The body may define a longitudinal throughbore having a first upper end configured to receive a drill string, and a second lower end configured to receive drilling fluid and entrained solids from the wellbore. The chamber may therefore be at least partially defined by the throughbore. Preferably, the first upper end is open to the subsea environment. In this context, "open" means that there is no fluid seal at the upper end, and seawater may to enter the chamber. Such a configuration is useful in tophole or riserless drilling applications, to avoid any effect on the pressure of fluid in the wellbore, or create interference with the movement of the drill string during drilling operations. Preferably the processing of the drilling fluid and entrained solids comprises applying a mechanical force to the entrained solids, which may comprise a force from a mechanical cutter. Preferably, the processing of the drilling fluid and entrained solids comprises cutting, breaking or fluidising the entrained solids by a rotary cutter. The rotary cutter may form a part of a cutting pump. The drilling fluid and entrained solids may be processed at, in, or adjacent a fluid outlet of the apparatus. The use of a cutting pump, which has a cutter mechanism activated along with the pump drive mechanism, has a number of advantages with respect to the prior art. The cutting pump is operable to (a) break up and/or fluidise the entrained solids; and (b) pump the processed drilling fluid and entrained solids from the chamber to a return conduit. Both functions can be achieved by the cutting pump by a single mechanism, and the pump may therefore be a compact unit which may be integrated with the body. The cutting pump may be located at, in, or adjacent the body, and the method may comprise pumping the drilling fluid and entrained solids to a subsea pump unit at a position removed from the apparatus. In preferred embodiments, the cutting pump is located between the at least one outlet to the chamber and the return conduit. The cutting pump may be located in an outlet to the body itself, so that the drilling fluid and entrained solids may be processed as they exit the chamber. This obviates the need for the unprocessed fluids to pass into a fluid conduit before processing. Inlets to the cutting pumps may form, or may be located in, sub chambers extending radially outwardly from the chamber. Outlets to the pumps may be coupled to a fluid conduit or hose. The method may comprise operating the cutting pump to control a level of drilling fluid in the chamber, and may comprise operating the cutting pump and a subsea pumping unit together to control a level of drilling fluid in the chamber. This is particularly useful in applications to tophole and riserless drilling, in which the drill string is exposed to the subsea environment, and prevents drilling fluid from overflowing into the surround sea (as well as preventing seawater from entering the return conduit) in embodiments in which the upper end of the body is open. The method may comprise comprising operating the cutting pump to generate a positive pressure sufficient to inflate a hose coupled to the at least one outlet to the chamber against hydrostatic pressure. The hose may be disposed between the outlet to the chamber and a subsea pumping unit. The method may comprise directing the flow of the drilling fluid and/or entrained solids to a fluid outlet or a means for processing the drilling fluid or entrained solids. The method may comprise actuating a flow directing means to move it between a first, inoperative, retracted position and a second operative position in which solids entrained in the drilling fluid are directed towards at least one outlet to the body or a means for processing the drilling fluid or entrained solids. The flow directing member may be an arrangement of pipes and/or valves, which may be controllable from a location remote from the apparatus. Alternatively, or in addition, the flow directing means may comprise a deflector plate. The method may comprise a first mode of operation, in which the drilling fluid and entrained solids may pass through an outlet of the apparatus, and a second mode of operation, in which he drilling fluid and entrained solids are processed to break up or reduce the size of the entrained solids. Preferably, the method comprises directing the drilling fluids and entrained solids to a first outlet during a first mode of operation, and directing the drilling fluids and entrained solids towards a second outlet during a second mode of operation, where the second outlet comprises means for processing the drilling fluids and entrained solids. Preferably the method comprises handling fluids during subsea tophole or riserless drilling operation. According to a second aspect of the present invention there is provided a method of drilling a subsea well, the method comprising:

providing a body at a subsea well site, the body having a longitudinal throughbore and defining a chamber for collecting drilling fluid and entrained solids;

providing a drill string extending from a drilling platform to the seabed through the column of water, such that at least a portion of the drill string is exposed to the subsea

environment;

passing the drill string through the body;

drilling a wellbore section while pumping drilling fluid through the drill string and collecting drilling fluid and entrained solids in the chamber;

conveying the processed drilling fluid and entrained solids to surface by pumping the drilling fluid and entrained solids through a return conduit. Preferably the method comprises drilling a tophole section of a subsea well. Embodiments of the second aspect of the invention may comprise the preferred or optional features of the first aspect of the invention and vice versa. According to a third aspect of the invention there is provided an apparatus for handling fluids during a subsea drilling operation, the apparatus comprising:

a body at a subsea well site, the body defining a chamber for collecting drilling fluid and entrained solids from a wellbore at the subsea well site;

means for processing the drilling fluid and entrained solids at the subsea well site to break up the entrained solids;

at least one outlet to the body configured to be coupled to a return conduit for conveying the processed drilling fluid and entrained solids to surface. The body may define a longitudinal throughbore having a first upper end configured to receive a drill string, and a second lower end configured to receive drilling fluid and entrained solids from the wellbore. The chamber may therefore be at least partially defined by the throughbore. Preferably, the first upper end is open to the subsea environment. In this context, "open" means that there is no fluid seal at the upper end, and seawater may to enter the chamber. Such a configuration is useful in tophole or riserless drilling applications, to avoid any effect on the pressure of fluid in the wellbore, or create interference with the movement of the drill string during drilling operations. The means for processing the drilling fluid and entrained solids may comprise a device for applying a mechanical force to the entrained solids, and/or may comprise a mechanical cutter. Preferably, the device comprises a rotary cutter. The rotary cutter may form a part of a cutting pump. The means for processing may be located at, in, or adjacent a fluid outlet of the apparatus. The means for processing the drilling fluid and entrained solids may include a fluid jetting arrangement. Preferably, the fluid jetting arrangement comprises one or more jets arranged to direct fluid radially inwards of the apparatus. The fluid jetting arrangement may receive a fluid selected from: water (e.g. seawater); drilling mud; chemicals cleaning fluids; or a combination of the above. The apparatus may comprise one or more pumps. The pumps may comprise at least one low pressure rotary pump, which may be a cutting pump including at least one rotary cutter. The means for processing may be configured to break up the entrained solids to a size small enough at which all solid particles, including rocks and drill cuttings, can be passed through the return line (including valves and subsea pumps) to be conveyed to the remote location. The pump, conduit and return line may therefore be selected to accommodate the largest of solids entrained in the drilling fluid post-processing, such that no filtering means is required. The apparatus may be configured to be connected to a subsea wellhead. Alternatively, the apparatus may be configured to be mounted on the seabed, for example via a spud base. The apparatus may comprise a flow directing member disposed in the body, which may be moveable between a first, inoperative, retracted position and a second operative position in which solids entrained in the drilling fluid are directed towards the at least one outlet to the body. Preferably, the apparatus comprises a deflector plate. The flow directing member is particularly useful for diverting or deflecting stiff materials such as clay-type materials in the drilling fluid which would otherwise have a tendency to move vertically through the apparatus in the direction of the drill pipe. Embodiments of the third aspect of the invention may comprise the preferred or optional features of the first or second aspects of the invention and vice versa. According to a fourth aspect of the invention there is provided an apparatus for handling fluids during a subsea drilling operation, the apparatus comprising:

a body configured to be located at a subsea well site, the body defining chamber for collecting drilling fluid and entrained solids from a wellbore at the well site;

an outlet to the body configured to be coupled to a return conduit for conveying the drilling fluid and entrained solids to surface; and

a flow directing member disposed in the body, and the flow directing member is moveable between a first, inoperative, retracted position and a second operative position in which solids entrained in the drilling fluid are directed towards the at least one outlet to the body. Preferably, the apparatus comprises a deflector plate. The flow directing member is particularly useful for diverting or deflecting stiff materials such as clay-type materials in the drilling fluid which would otherwise have a tendency to move vertically through the apparatus in the direction of the drill string. The apparatus may comprise means for processing the drilling fluid and entrained solids at the subsea well site to break up the entrained solids. Embodiments of the fourth aspect of the invention may comprise the preferred or optional features of the first to third aspects of the invention and vice versa. According to a fifth aspect of the present invention there is provided a method of handling fluids during a subsea drilling operation, the method comprising:

providing a body at a subsea well site, the body defining a chamber for collecting drilling fluid and entrained solids, and comprising a flow directing member;

receiving drilling fluid and entrained solids into the chamber from a wellbore at the subsea well site; and

actuating the flow directing member to move it between a first, inoperative, retracted position and a second operative position in which solids entrained in the drilling fluid are directed towards at least one outlet to the body. Preferably the method comprises conveying the drilling fluid and entrained solids to surface by pumping the drilling fluid and entrained solids through a return conduit. The method may comprise processing the drilling fluid and entrained solids at the subsea well site to break up the entrained solids, prior to conveying the drilling fluid and entrained solids. Embodiments of the fifth aspect of the invention may comprise the preferred or optional features of the first to fourth aspects of the invention and vice versa. Brief Description of Drawings Aspects and advantages of the present invention will become apparent upon reading the following detailed description of its preferred embodiments and upon reference to the following drawings in which: Figure 1A is a schematic representation of a prior art subsea drilling system for a tophole well section; Figure 1 B is a schematic representation of a prior art subsea drilling system for a riser less drilling with a drilling fluid return conduit; Figure 2 is a schematic representation of a subsea drilling system in accordance with a first embodiment of the invention; Figures 3A, 3B and 3C are respectively sectional, top and isometric cut-away views through an apparatus used in the system of Figure 2; Figure 4A is a sectional view through an apparatus in accordance with an alternative embodiment of the invention; Figure 4B is a top view of the apparatus of Figure 4A; Figure 4C is a top view of the apparatus of Figure 4B in an operative condition (with guide funnel removed for clarity of internal components; Figure 5 is a schematic longitudinal section through an apparatus in accordance with a further alternative embodiment of the invention. Detailed Description As described above Figures 1 A and 1 B depict drilling systems according to the prior art and do not form part of the present invention. Figure 2 shows schematically a system according to a first embodiment of the invention, generally depicted at 200. The system 200 comprises an apparatus 202 disposed on a wellhead 204 on the seabed 205. A drill string 206 extends from a drilling rig 208 through the column of water to the apparatus 202, and through the apparatus 202 into the wellbore (not shown). A subsea hose 210 connects the apparatus 202 to a subsea pump unit 212, and a conduit 214 provides a return line from the subsea pump unit 212 to surface.

Control lines 218 connect the subsea pump unit 212 with the apparatus 202. Figures 2A to 2C show schematically the apparatus 202, disposed on the wellhead 204 and penetrated by the drill string 206. The apparatus 202 comprises a body 220 which defines a longitudinal throughbore 222 through the apparatus. A first, upper end 224 of the apparatus has an opening 226 (open to the subsea environment) which receives the drill string 206. A second, lower end 228 of the body 220 has a connector which couples the body 220 to the wellhead 204 by a drop-on engagement. The body has a number of fluid outlets 228 (in this case three fluid outlets 228a, 228b, 228c) located between the first and second ends, and disposed substantially radially on the body 220. The outlets 228 allow the passage of fluid radially outwards from a chamber defined by the body 220, as will be described in more detail below. The apparatus also comprises a frame 230, which supports pumps 232 located at each of the respective outlets 228a and 228b. Inlets to the pumps 232 form sub chambers 234 which extend radially outward from the main chamber in circumferentially separated locations (in this case diametrically opposed). Located within each sub chamber 234 is a cutter mechanism 236, which in this case is a rotary cutter located on the pump axis. The cutter mechanism 236 is activated along with the pump drive mechanism to mechanically break up and/or fluidise gumbo and other solids contained in the fluid returns. Outlets 237 from each of the pumps 232 are connected into the outlet 228c, which in turn is connected to the subsea hose 210. In this embodiment, the axes of the pumps 232 and the outlets 228a, 228b are inclined slightly upwards (in a direction moving away from the body) to prevent fluids and solids from accumulating in the sub chambers to the outlets at the upstream side of the pumps. Each pump is a low pressure pump capable of generating a pressure of up to

approximately 30m mlc (metre liquid column or 'head'). The purpose of the pumps 232 (in addition to their breaking or fluidising function) is to cause drilling fluid and entrained solids to flow out of the chamber and along the hose 214 to the subsea pump unit 212. The pumps are capable of generating a positive pressure sufficient to inflate the hose 214 against the hydrostatic pressure and ensure that drilling fluid and entrained solids pass to the main subsea pump unit 212. The main subsea pump unit 212 functions to pump the drilling fluid and solids mixture to surface, and is capable of generating a pressure of approximately 400 mlc. Operation of the system will now be described with reference to Figures 2 to 3. During drilling, the drill string 206 is rotated while drilling fluid is pumped down the centre of the drill string, through the drill bit (not shown) and upwards in the annular space between the borehole wall and the drill string. Drilling fluid is collected in the chamber defined by the body and includes entrained solids, such as drill cuttings, rocks, boulders, clays and other stiff materials from the wellbore and/or formation. The entrained solids do not easily pass through the remainder of the return systems without processing, and the system 200 is capable of processing the mixture of the drilling fluid and entrained solids at the well site in the apparatus. The cutting pumps 232 are activated to break up and/or fluidise the entrained solids, which are then pumped through the subsea hose 214 to the subsea pump unit 212. The subsea pump 212 generates sufficient pressure to pump the processed drilling fluid and cuttings mixture to surface, where it is treated or stored for reuse or safe disposal. The pumping unit 212 and the pumps 232 (when activated) together control the level of fluid in the body 220, preventing it from overflowing into the surrounding sea and preventing seawater from entering the fluid outlets. The pumps 232 allow control of the level of fluid in the body 220 independently from the pumping unit 212. By processing the drilling fluid and entrained solids at the well site, and in apparatus located on the wellhead, it is possible to pass more (and preferably all) of the return materials from the drilling operation through the subsea pump unit and pump the return materials to surface. This mitigates problems associated with discharge of drilling waste to the seabed and avoids the inconveniences of filtration, while reducing damage to the pumping unit, valves in the return system, and the fluid return line itself. The system 200 is capable of dual mode operation. When the drilling fluid contains a low volume of entrained solids, or if the entrained solids are fine and easily handled, the pumps 232 need not be operated. Instead, an arrangement of pipes and valves directs flow to the conduit 228c (which does not have a corresponding pump) out of the body 220 and into the hose 214. When the drilling fluid contains a higher volume of entrained solids, or if the solids are large or stiff enough to cause problems in the subsea pumping unit 212 or other parts of the return system, then the pumps are activated and the valves are controlled to prevent fluid flow from the outlet 228c, with all flow directed out through the pumps 232 and outlets 228a, 228b. The presence of solids is detected by changes in suction pressure in the area of the sub chambers 234 at the inlets to the pumps 232.

Solids which may not be easily pumped to surface create partial blocking of the outlets to the chamber which results in increased suction pressure in the sub chambers 234. These changes in suction pressure are detected by pressure sensors (not shown) located in the sub chambers, which provide pressure data to surface via the control lines. The valves are controllable via the control lines to the subsea pumping unit (and ultimately from surface) but may also be capable of manual operations. One benefit of the described configuration is that there is always an available fluid outlet, even in the event of failure of the pumps, through which fluid can be circulated. Another is that the outlet 228c provides an entry point for flushing fluid through the system, for example to clean the pumps. A benefit of the dual mode of operation is that the pumps 232 need only be operated when necessary for the proper functioning and/or protection of the subsea pump 212 and return line system, as in general it may be beneficial not to further fluidise a drilling fluid and entrained solids mix which is already pumpable. For example, additional or excessive fluidisation of a pumpable fluid and solids mixture may cause problems during later treatment of the drilling returns to separate solids from the drilling fluid; conventional shaker-type separators may be incapable of removing very fine solids from the mixture. Figures 4A to 4C show an apparatus according to a further alternative embodiment of the invention, generally depicted at 400, which is similar to apparatus 202 and will be understood from Figures 2 and 3 and the accompanying description. The apparatus includes a body 402 with an upper end on which is located a guide funnel 408. A lower end is mounted on a wellhead 403. However, the apparatus 400 differs from the apparatus 202 in that it includes a single outlet 428, which defines a sub chamber 434, and in which is located a pump 432. The pump is a cutting pump, similar to the pump 232, and includes a cutting mechanism for breaking up and/or fluidising solids entrained in the drilling fluid returns. The apparatus 400 also includes a flow directing member in the form of a deflector plate 410. The deflector plate 410 is mounted in the body 402 of the apparatus on a pivot, and is moveable between an inoperative position in which it is accommodated in at recess in the internal wall of the body (Figure 4B) and an operative position in which it extends accords the throughbore 412 defined by the body (Figures 4A and 4C). While in the operative position, shown in Figure 4B, the deflector plate is retracted to provide substantially full or full bore access in the throughbore 412. This allows passage of drilling equipment (including collars, couplings, stabilisers and drill bits) through the apparatus and into the wellbore. In the operative position, shown in Figures 4A and 4C, the deflector plate functions to direct drilling fluids and in particular entrained solids towards the outlet 428 and pumps 432 for processing and pumping to the return line. The deflector plate may be moved to its operative position after large components of the drilling equipment have passed through the apparatus, or alternatively may be operated in response to the detection of solids in the return fluid stream (which again is by the detection of changes in suction pressure in the area of the inlet to the pump). Figure 5 is a longitudinal section through an apparatus according to a further alternative aspect of the invention, generally shown at 500. This embodiment is similar to apparatus 400 and will be understood from Figures 2 to 4 and the accompanying description. The apparatus includes a body 502 with an upper end on which is located a guide funnel 508. A single outlet 528 is provided, and defines a sub chamber 534 in which is located a cutting pump 532. A coupling 503 allows the apparatus to be connected to a wellhead (not shown). The apparatus 500 also comprises a mechanism to break up stiff materials such as clay materials or oil-based fluids and materials passing through the lower section of the apparatus. In this embodiment, the mechanism is a jetting ring 510, connected to the supply of clean drilling mud from the surface or from a subsea storage tank. A pump (not shown) injects the drilling mud at pressure through the jetting ring radially inwards towards the centre of the throughbore, creating a turbulent flow and breaking up stiff materials as they enter the apparatus. In alternative embodiments alternative fluid sources may be used for the jetting ring, for example, water can be pumped from the sea or from a hose; chemicals such as glycol may be supplied to improve the cleaning effect; or drilling fluids may be provided from the return line 214. The jetting ring 510 may be used with or without a pump and/or a cutting pump. Embodiments described above include a subsea pump unit which pumps the return fluid to surface, working in conjunction with a low pressure pump at the wellhead which causes the fluid and cuttings mixture to the subsea unit. In alternative embodiments a separate subsea pump unit can be omitted, with the pump located at the well site and integrated with the chamber providing sufficient head to pump the drilling fluid and solid mixture to surface. Such an embodiment may for example be advantageous in shallow water applications. It will be apparent to one skilled in the art that features of the above-described

embodiments of the invention may be combined in ways other than those expressly described. For example, the various components of the apparatus may be used interchangeably, and features specific to particular systems 200, 400 and 500 may be incorporated in other systems described herein. For example, although the described embodiments of the invention are mounted on wellheads, it will be apparent that all embodiments of the invention can be fitted to either a subsea wellhead or a spud base with or without a seabed penetrating skirt. The invention provides an apparatus and method for handling drilling fluids during a subsea drilling operation. The method comprises providing a body at a subsea well site and receiving drilling fluid and entrained solids into the chamber from a wellbore at the subsea well site. Drilling fluid and entrained fluids are processed at the subsea well site to break up and/or fluidise the entrained solids by operating at least one cutting pump, which also functions to pump the processed drilling fluid and entrained solids from an outlet to a return conduit. The processed drilling fluid and entrained solids are conveyed to surface by pumping the drilling fluid and entrained solids through the return conduit. The present invention addresses the need to improve solids handling in offshore drilling operations while maintaining high standards of environmental performance and reducing drilling fluid waste. Embodiments of the invention recover all or substantially all of the contaminated materials from a drilling operation, and lend themselves to zero discharge drilling operations. Embodiments of the invention do not rely on rock crushers or pulverisers incorporated in line with the mud return systems having dedicated inlet and outlet conduits. The invention has particular application to riserless drilling operations, such as tophole drilling and full riserless drilling. The foregoing description of the invention has been presented for the purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise form disclosed. The described embodiments were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilise the invention in various embodiments and with various modifications as are suited to the particular use contemplated. Therefore, further modifications or improvements may be incorporated without departing from the scope of the invention herein intended.

Claims

Claims 1 . A method of handling fluids during a subsea tophole or riserless drilling operation, the method comprising:

providing a body at a subsea well site, the body comprising a longitudinal throughbore having a first upper end configured to receive a drill string, and a second lower end configured to receive drilling fluid and entrained solids from a wellbore, and a chamber at least partially defined by the throughbore for collecting drilling fluid and entrained solids;

processing the drilling fluid and entrained solids at the subsea well site by operating at least one cutting pump to break up and/or fluidise the entrained solids;

pumping the processed drilling fluid and entrained solids from at least one outlet to the chamber to a return conduit by operating the at least one cutter pump; and conveying the processed drilling fluid and entrained solids to surface by pumping the processed drilling fluid and entrained solids through the return conduit.

2. The method as claimed in claim 1 comprising breaking up and/or fluidising the

entrained solids by applying a force to the entrained solids from a rotary mechanical cutter of the cutting pump.

3. The method as claimed in claim 1 or claim 2 wherein the at least one cutting pump is located at, in, or adjacent the at least one outlet of the apparatus.

4. The method as claimed in claim 3 wherein the at least one cutting pump is located between the at least one outlet to the chamber and the return conduit.

5. The method as claimed in any preceding claim comprising operating the at least one cutting pump to pump the processed drilling fluid and entrained solids to a subsea pumping unit at a position removed from the body, and pumping the processed drilling fluid and entrained solids to surface using the subsea pumping unit.

6. The method as claimed in any preceding claim comprising operating the at least one cutting pump to control a level of drilling fluid in the chamber.

7. The method as claimed in claim 6 when dependent on claim 5, comprising operating the at least one cutting pump and the subsea pumping unit together to control a level of drilling fluid in the chamber.

8. The method as claimed in any preceding claim comprising operating the at least one cutting pump to generate a positive pressure sufficient to inflate a hose coupled to the at least one outlet to the chamber against hydrostatic pressure.

9. The method as claimed in claim 8 wherein the hose is disposed between the at least one cutter pump and a subsea pumping unit.

10. The method as claimed in any preceding claim comprising actuating a flow directing means to move it between a first, inoperative, retracted position and a second operative position in which solids entrained in the drilling fluid are directed towards at least one outlet to the chamber.

1 1 . The method as claimed in any preceding claim comprising directing the drilling fluids and entrained solids to a first outlet to the chamber during a first mode of operation, and directing the drilling fluids and entrained solids towards a second outlet to the chamber during a second mode of operation, wherein the at least one cutting pump is located at the second outlet.

12. A method of drilling a tophole section of a subsea well, the method comprising:

providing a body at a subsea well site, the body having a longitudinal throughbore and defining a chamber for collecting drilling fluid and entrained solids and an open upper end;

providing a drill string extending from a drilling platform to the seabed through the column of water, such that at least a portion of the drill string is exposed to the subsea environment;

passing the drill string through the body;

processing the drilling fluid and entrained solids at the subsea well site by operating at least one cutting pump to break up and/or fluidise the entrained solids; pumping the processed drilling fluid and entrained solids from at least one outlet to the chamber to a return conduit by operating the at least one cutter pump; and conveying the processed drilling fluid and entrained solids to surface by pumping the processed drilling fluid and entrained solids through the return conduit.

13. The method as claimed in claim 12 comprising handling fluids according to the

method of any of claims 1 to 1 1 .

14. An apparatus for handling fluids during a subsea tophole or riserless drilling

operation, the apparatus comprising:

a body at a subsea well site, the body comprising a longitudinal throughbore having a first upper end configured to receive a drill string, and a second lower end configured to receive drilling fluid and entrained solids from a wellbore, and a chamber at least partially defined by the throughbore for collecting drilling fluid and entrained solids from the wellbore;

processing means for processing the drilling fluid and entrained solids at the subsea well site; and

at least one outlet to the chamber configured to be coupled to a return conduit for conveying processed drilling fluid and entrained solids to surface;

wherein the processing means comprising at least one cutting pump operable to break up and/or fluidise entrained solids, and pump drilling fluid and pump the processed drilling fluid and entrained solids from the at least one outlet of the chamber to the return conduit.

15. The apparatus as claimed in claim 14, wherein the at least one cutting pump

comprises a rotary mechanical cutter.

16. The apparatus as claimed in claim 15, wherein the rotary mechanical cutter is

activated by a pump drive mechanism.

17. The apparatus as claimed in any of claims 14 to 16, wherein the at least one cutting pump is located between the at least one outlet to the chamber and the return conduit.

18. The apparatus as claimed in any of claims 14 to 17, wherein the processing means includes a fluid jetting arrangement.

19. The apparatus as claimed in any of claims 14 to 18, further comprising a flow

directing member disposed in the body, which is moveable between a first, inoperative, retracted position and a second operative position in which solids entrained in the drilling fluid are directed towards the at least one outlet to the body.

20. The apparatus as claimed in any of claims 14 to 19, configured to direct the drilling fluids and entrained solids to a first outlet to the chamber during a first mode of operation, and directing the drilling fluids and entrained solids towards a second outlet to the chamber during a second mode of operation, wherein the at least one cutting pump is located at the second outlet.