Patent Publication Number: US-2020298938-A1

Title: Buoy For The Processing Of Production Fluids

Description:
The present invention relates to an apparatus and method for mitigating over-pressurisation of gas in a buoy comprising a hydrocarbon processing facility. 
     SUMMARY 
     According to the invention there is provided a buoy for the processing of production fluids from an offshore well, the buoy comprising a hydrocarbon processing facility adapted to process production fluids received from the offshore well, the hydrocarbon processing facility being submerged below the operating waterline of the buoy, wherein the buoy has a pressure relief channel adapted to relieve pressure within a chamber below the operating waterline of the buoy in the event of over-pressurisation within the chamber, the pressure relief channel being in fluid communication with chamber and wherein the pressure relief channel is adapted to vent gas pressure above the operating waterline of the buoy. 
     Optionally the pressure relief channel has an inlet above the chamber and the pressure relief channel optionally extends upwards from the inlet, optionally in a straight line and optionally substantially vertically relative to a vertical axis of the buoy. Optionally the inlet opens into the chamber. 
     Optionally the chamber is a gas tight chamber which is adapted to be sealed off from the remainder of the buoy and wherein the pressure relief channel is the only conduit through which high pressure is exhausted from the chamber. 
     Optionally the chamber contains the hydrocarbon processing facility, but other facilities or equipment presenting an explosion risk can be disposed in the chamber. 
     Optionally the chamber contains a heater. The hydrocarbon processing facility can be dispersed in a number of different linked rooms in the chamber, and hydrocarbon processing can optionally occur in each room within the chamber. The rooms are optionally linked together by fluid conduits allowing the transmission of pressure waves between the rooms within the chamber in the event of an over-pressurisation event such as an explosion. Optionally hydrocarbons are processed, conditioned, or consumed within the chamber. Optionally the hydrocarbons processed, conditioned or consumed comprise the production fluids from the well. 
     Optionally the buoy has multiple layers of decks. Optionally the pressure relief channel is routed through sequential decks of the buoy substantially parallel to the buoy&#39;s vertical axis. 
     Optionally the pressure relief channel extends in a generally straight line substantially vertically from a space above the chamber, for example from a space containing the hydrocarbon processing facility or a space in fluid communication with the hydrocarbon processing facility, optionally to an opening on the top deck of the buoy, with minimal or no bends in the channel. Bends in the pressure relief channel are optionally restricted to a maximum bend radius of 90 degrees, optionally 45 degrees, optionally 40 degrees, or 30, 20, 10 or 5 degrees or less. A pressure relief channel that does not deviate substantially from a straight line or has minimal bends below the maximum bend radius can provide a more resilient channel for explosive gases, faster release of gas, and has less chance of sustaining blast damage in the event of an over-pressurisation event. 
     Optionally the pressure relief channel contains the effects of an over-pressurisation event, for example ignition of flammable gas within the buoy, and particularly within the chamber, and especially within the hydrocarbon processing facility. Optionally the pressure relief channel contains the over-pressurised gas within the channel, preventing or mitigating structural effects of an over-pressurisation event on the buoy, for example explosive damage. Optionally the pressure relief channel facilitates continued buoyancy of the buoy during and after over-pressurisation by containing the pressurised gas within the pressure relief channel until it is vented from the buoy, and optionally preventing extensive structural damage to the buoy. The buoy thus continues to remain upright and buoyant for recovery and repair in the event of an ignition or other explosive over-pressurisation event, rather than potentially being lost to the seabed, or becoming untethered and colliding with vessels or other marine equipment. 
     Optionally the pressure relief channel can accommodate a conduit within it, for example, an exhaust or air intake vent. Similar exhaust and air intake vents can optionally be distributed through the service trunks. 
     Optionally the upper end of the pressure relief channel comprises a cover adapted to close an upper end opening of the pressure relief channel. Optionally the cover is adapted to release during an over-pressurisation event, optionally when the pressure within the pressure relief channel rises above a threshold pressure. Optionally the cover also provides a weather-tight seal for the pressure relief channel. This prevents ingress of seawater or rain, for example, to the hydrocarbon processing facility via the pressure relief channel, with the associated risk of contamination of the production fluids and/or deterioration of the integrity of the pressure relief channel. Optionally the cover can comprise a flap, hatch or similar. Optionally the cover has a hinge to allow pivotal movement of the cover, and optionally a latch adapted to fail and allow movement of the cover to oven the upper end of the channel in the event an over-pressurisation event within the channel, for example, a rise of pressure within the channel in excess of a threshold. The cover can be located horizontally on the upper surface of an upper wall of the channel, and/or can be located on a vertical surface, for example, on the sidewall of the buoy. 
     Optionally the pressure relief channel is at least partially lined with or formed by a material comprising a metal. Optionally the pressure relief channel is at least partially lined with or formed by a composite material. Optionally the pressure relief channel is formed by reinforcing walls of the adjacent sections of the buoy so that the inner surface of the pressure relief channel is faced with the reinforced material forming the walls of the adjacent sections. 
     Optionally the buoy comprises a central access shaft, which optionally is adapted to facilitate movement of machinery, injured personnel, and buoy components between upper and lower decks of the buoy. Optionally the central access shaft extends parallel to the vertical axis of the buoy, optionally from a lower deck of the buoy, for example, the deck on which the hydrocarbon processing facility is disposed, to an upper deck of the buoy, which is optionally above the operating waterline. 
     Optionally the plant machinery and other equipment in the buoy are distributed around the central access shaft. Optionally this can result in an uneven distribution of mass of plant machinery and other equipment in the buoy. Optionally any uneven distribution of mass resulting from this distribution can be compensated for by, for example, adding ballast to the buoy. 
     Optionally the central access shaft comprises at least one closure member, optionally at each deck level of the buoy. Optionally the closure member is adapted to change configuration between an open configuration in which the closure member allows access through the central access shaft, and a closed configuration in which the closure member resists access through the central access shaft. Optionally the closure member can act as a landing site for equipment or personnel. Optionally the closure member can comprise a flap, for example a Miller flap. 
     Optionally the central access shaft comprises a ladder for movement of personnel between decks, optionally during maintenance operations. 
     Optionally the buoy comprises an air intake channel, the air intake channel being parallel to the vertical axis and parallel to the central access shaft. Optionally the air intake channel extends between a lower deck of the buoy which may be below the operating waterline, and an upper deck which may be above the operating waterline. Optionally the air intake channel has an air inlet at the upper deck. 
     Optionally the buoy comprises an exhaust shaft. Optionally the exhaust shaft is parallel to the vertical axis and optionally parallel to the central access shaft. Optionally the exhaust shaft extends between a lower deck of the buoy which may be below the operating waterline, and an upper deck which may be above the operating waterline, optionally having an exhaust gas outlet at the upper deck. 
     Optionally air intake and exhaust can take place within the same channel or channels. 
     Optionally the processing apparatus within the hydrocarbon processing facility, for example pipework, is enclosed for example, within a compartment, in a lower deck of the buoy, which may be below the waterline. 
     Keeping the processing level below the waterline minimises the extent of the hazardous area rating. It also removes the requirement for a riser to go through the wave affected zone near to the water surface, and reduces the associated risk of vessel impact with the riser, or impact with floating debris, or detrimental movement due to tidal forces. 
     Optionally the hydrocarbon processing facility is continually monitored via CCTV and or by sensors, such as pressure sensors, UV, smoke, optical, and/or heat sensors, and/or temperature sensors. Optionally the processing facility is monitored by air monitoring sensors. 
     The continual monitoring of the chamber (e.g. of the processing facility) both serves as an early alarm system in the event of an over-pressurisation or other potentially damaging event, and an additional indication as to conditions on board the buoy prior to transfer of personnel. 
     The monitors in the buoy can also optionally be used to detect parameters exceeding a predetermined threshold, and optionally alert an operator and/or activate first response systems, for example fire protection systems, or the release of the cover from the pressure relief channel, or shutdown of the buoy. The buoy may be controlled by operators from within the buoy control room. Alternatively, the operator may be in a control room within an onshore station, a host platform or similar facility, or a nearby vessel. Communication between the buoy and the control room, where this is external to the buoy, can be via satellite communication systems, or optionally line-of-sight communication systems, optionally providing real-time data to personnel. 
     Optionally the buoy is adapted to shut down automatically in the event of an extended breakdown in the communication systems. Optionally, short-term loss of communication results in the buoy commencing a troubleshooting procedure, and optionally returning to normal operations if communication is restored within a predetermined time period. 
     Optionally the processing facility is continually ventilated by a dedicated intrinsically safe HVAC (heating, ventilation &amp; air conditioning) system. Optionally the HVAC system plant (machinery room) is isolated from the rest of the buoy and contained within a separate compartment from the processing facility. Optionally the HVAC plant (machinery room) has direct ventilation from outside the buoy. 
     Optionally the HVAC system components are blast proof. Optionally they are manufactured as blast proof. Optionally the components are located within a blast proof service trunk or trunks. 
     Optionally the buoy is adapted to be a primarily unmanned installation. Optionally personnel are transferred to the buoy to carry out maintenance operations. 
     Optionally personnel are transferred from a supply vessel, or optionally a landing craft, or optionally a helicopter winch. 
     Optionally the buoy can act as a personnel refuge. Optionally the buoy has a source of potable water. Optionally the buoy contains washroom facilities. Optionally the buoy contains a crew room for short-term sheltering purposes. 
     Also according to the present invention, there is provided a fluid flowline connector interface adapted to connect a fluid flowing from a marine riser to admit production fluids from a well into a hydrocarbon processing facility contained within a buoy. 
     In one aspect the invention provides a buoy for the processing of production fluids from an offshore well, the buoy comprising a hydrocarbon processing facility adapted to process production fluids received from the offshore well, the hydrocarbon processing facility being submerged below the operating waterline of the buoy, the buoy having a fluid flowline connector interface adapted to connect a fluid flowing from a marine riser to admit production fluids from a well into the hydrocarbon processing facility. 
     Optionally the fluid flowline connector interface penetrates a wall of the buoy below the operating waterline. Optionally the fluid flowline connector penetrates the buoy at the ballast deck level. Optionally the locus of penetration is sealed to prevent ingress of seawater into the processing facility, and to prevent emission of hydrocarbons into the sea. Optionally the fluid flowline connector is an integral part of the buoy&#39;s lower decks, being typically fixed pipework integral with a bulkhead penetration through the side of the buoy installed and tested to confirm integrity prior to sail-away from the fabrication site. Optionally the fluid flowline connector is in the form of at least one tube, the tube optionally rising through the buoy. Optionally the riser is run up J-tubes externally to the buoy, to a location above the waterline for dry connection. Optionally when above the waterline the connector is tied-in to fixed pipework or cable connectors. 
     Optionally connection of the riser to the fluid flowline connector interface can be performed by an ROV or by divers. 
     Optionally the riser is a dynamic flexible riser. Optionally the riser connects directly to the wellhead. Optionally the riser connects to a manifold. Optionally the riser connects to another subsea apparatus adapted to separate the production fluids. Optionally the riser adopts a “lazy S”, or mid-water arch configuration. Optionally the riser is supported and restrained from significant movement by a riser support disposed on a subsea anchor or other local subsea apparatus. 
     Optionally locating the hydrocarbon processing facility, and optionally the fluid flowline connector interface, below the waterline and below the keel of passing vessels can optionally facilitate the attachment of protective coverings for the interface fluid flowline connector interface, optionally over the highest risk elements of the interface. Optionally the fluid flowline connector interface is at least partially protected from damage, for example by dropped objects from overhead, by a protective covering which may be disposed on the buoy. Optionally the protective covering encases part of the connector. Optionally the protective covering is in the form of a panel disposed above the connector. 
     The proximity to the buoy, with service vessels, transfer of components, and maintenance going on overhead, puts the site of penetration of the riser into the processing facility at particularly high risk of damage from objects dropped from above if the processing facility is sited below the waterline. Any piercing or severance of the pipework at this location would lead to emission of production fluids from the riser into the sea, and potentially ingress of seawater into the buoy. As the buoy is optionally asymmetrically weighted and ballasted, optionally to compensate for the internal distribution of weight in the buoy, ingress of seawater into the processing facility could lead to destabilisation, capsizing, and sinking of the buoy, with the associated risk to nearby vessels, equipment and personnel. 
     Optionally the buoy comprises a pump system to pump out fluid in the event of a flooding incident. Optionally the pump system discharges above the waterline. Optionally the pump system includes pumps which are individually routed from each deck of the buoy to prevent vertical flooding. Optionally each deck comprises drains equipped with isolation valves, which can be actuated to isolate each deck during flooding. Optionally the pump system is powered at least partially by a high pressure saltwater system. Optionally the high pressure saltwater system can pump fluid both in and out of the buoy. Optionally the high pressure saltwater system can also be used to provide water ballast, and/or a supply of water for firefighting equipment. 
     Optionally the decks in the submerged section of the buoy comprise watertight hatches and doors. Optionally all external doors are weather-proof and security controlled. Optionally all HVAC and electrical conduit routes passing through watertight decks or bulkheads are optionally fitted with seals, or optionally watertight glands, or optionally another means of protecting the conduits from water in the event of flooding. Optionally, pipe routes that pass through watertight decks or bulkheads are sealed such that watertight integrity is preserved. 
     Also according to the present invention, there is provided a method of venting gas from an offshore production buoy in the event of over-pressurisation, the method comprising channelling the gas through a pressure relief channel, wherein the pressure relief channel is in fluid communication with a submerged chamber, optionally containing a hydrocarbon processing facility, located below the operating waterline of the buoy, and venting the gas to atmosphere from the pressure relief channel above the operating waterline of the buoy. 
     The method optionally includes releasing a closure member covering an external opening of the pressure relief channel when the pressure within the pressure relief channel rises above a threshold pressure. Optionally the closure member is actively opened as a result of a trigger signal received from the buoy, for example by a hydraulic hinge or pivot system. Optionally the closure member opens passively under the force of the released gas. 
     The method optionally includes locating the hydrocarbon processing facility and apparatus in one location and directing the pressure relief channel vertically upwards from this one location. Adopting a vertical, straight path of the pressure relief channel, optionally coupled with the grouping of the processing apparatus at one end of the pressure relief channel improves the constraint of the over-pressurisation force within the buoy, as the force is channelled directly upwards. 
     The method optionally includes distributing plant and other equipment necessary for functionality of the buoy around a central access shaft, wherein the central access shaft optionally extends vertically from the a lower deck of the buoy below the operating waterline to an upper deck disposed above the operating waterline. Optionally ballast is added to the buoy, optionally in tanks within the buoy, optionally in the form of weights or tanks external to the buoy. Optionally the ballast compensates for uneven distribution of mass in the buoy due to the central access shaft. 
     In an alternative embodiment of the invention, the consumption of diesel oil for power generation can be reduced by deploying a wind turbine or other alternative renewable energy generator on the buoy. 
     The various aspects of the present invention can be practiced alone or in combination with one or more of the other aspects, as will be appreciated by those skilled in the relevant arts. The various aspects of the invention can optionally be provided in combination with one or more of the optional features of the other aspects of the invention. Also, optional features described in relation to one aspect can typically be combined alone or together with other features in different aspects of the invention. Any subject matter described in this specification can be combined with any other subject matter in the specification to form a novel combination. 
     Various aspects of the invention will now be described in detail with reference to the accompanying figures. Still other aspects, features, and advantages of the present invention are readily apparent from the entire description thereof, including the figures, which illustrates a number of exemplary aspects and implementations. The invention is also capable of other and different examples and aspects, and its several details can be modified in various respects, all without departing from the spirit and scope of the present invention. Accordingly, each example herein should be understood to have broad application, and is meant to illustrate one possible way of carrying out the invention, without intending to suggest that the scope of this disclosure, including the claims, is limited to that example. Furthermore, the terminology and phraseology used herein is solely used for descriptive purposes and should not be construed as limiting in scope. Language such as “including”, “comprising”, “having”, “containing”, or “involving” and variations thereof, is intended to be broad and encompass the subject matter listed thereafter, equivalents, and additional subject matter not recited, and is not intended to exclude other additives, components, integers or steps. Likewise, the term “comprising” is considered synonymous with the terms “including” or “containing” for applicable legal purposes. Thus, throughout the specification and claims unless the context requires otherwise, the word “comprise” or variations thereof such as “comprises” or “comprising” will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers. 
     Any discussion of documents, acts, materials, devices, articles and the like is included in the specification solely for the purpose of providing a context for the present invention. It is not suggested or represented that any or all of these matters formed part of the prior art base or were common general knowledge in the field relevant to the present invention. 
     In this disclosure, whenever a composition, an element or a group of elements is preceded with the transitional phrase “comprising”, it is understood that we also contemplate the same composition, element or group of elements with transitional phrases “consisting essentially of”, “consisting”, “selected from the group of consisting of”, “including”, or “is” preceding the recitation of the composition, element or group of elements and vice versa. In this disclosure, the words “typically” or “optionally” are to be understood as being intended to indicate optional or non-essential features of the invention which are present in certain examples but which can be omitted in others without departing from the scope of the invention. 
     All numerical values in this disclosure are understood as being modified by “about”. All singular forms of elements, or any other components described herein are understood to include plural forms thereof and vice versa. References to directional and positional descriptions such as upper and lower and directions e.g. “up”, “down” etc. are to be interpreted by a skilled reader in the context of the examples described to refer to the orientation of features shown in the drawings, and are not to be interpreted as limiting the invention to the literal interpretation of the term, but instead should be as understood by the skilled addressee. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIG. 1 a    shows a schematic view of the internal layout of the buoy; 
         FIGS. 1 b  and 1 c    show section views of the  FIG. 1  arrangement in different aspects, notably through section lines A-A and B-B of  FIG. 3 ; 
         FIG. 2  shows an external side view of the buoy; 
         FIG. 3  shows a plan view of the top deck of the buoy; 
         FIG. 4  shows a plan view of 1 deck of the buoy; 
         FIG. 5  shows a plan view of 2 deck of the buoy; 
         FIG. 6  shows a plan view of 3 deck of the buoy; 
         FIG. 7  shows a plan view of 4 deck of the buoy; 
         FIG. 8  shows a plan view of 5 deck of the buoy; 
         FIG. 9  shows a plan view of 6 deck of the buoy; 
         FIG. 10  shows a plan view of 7 deck of the buoy; 
         FIG. 11  shows a plan view of 8 deck of the buoy; 
         FIG. 12  shows a side view of the buoy with the fluid interface connector; and 
         FIG. 13  is a close up view of the fluid interface connector of  FIG. 12 . 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1 a - c    show a buoy  10  for the processing of production fluids from an offshore well. The buoy  10  has a vertical axis x-x and a hydrocarbon processing facility adapted to process production fluids received from the offshore well. The hydrocarbon processing facility in this example comprises an oil processing room  20 . The oil processing room  20  is constructed in a blast sealed chamber  35  with a seabed separator and storage cell (SSSC) heating room  21  immediately above it. Both the oil processing room  20  and the heating room  21  are disposed on lower decks of the buoy  10 , submerged below the operating waterline WL and both are contained within the chamber  35 . The chamber  35  is sealed off from the remainder of the buoy, through pressure-resistant decks, walls, and doors etc., forming the external boundaries of the oil processing room  20  and the heating room  21 . Doors in the external walls of the chamber  35  are normally closed, and are gas tight, hence the two rooms  20 ,  21  together form a unitary sealed chamber with a common volume within the buoy. Gases can flow between the two rooms  20 ,  21  through fluid conduits in the floor, or through stairwells, or through other channels. Hence, an over-pressurisation event in one of the rooms  20 ,  21  transmits pressure into the other with substantially no resistance within the combined chamber  35 . The oil processing room  20  comprises various items of processing equipment suitable for processing production fluids received from the well into the oil processing room  20 . As the untreated production fluids are received within the oil processing room, they optionally pass through a multi-functional degasser, a surge drum, and optionally a flare knock-out drum if continuous flaring is required. Other kinds of processing equipment can be located or used in the oil processing room  20 . In this example, the design is based on intermittent or emergency cold venting as the preferred means of venting in the event of non-explosive spikes in gas pressure. 
     Above the oil processing room  20 , the heating room  21  is also formed within the same gas tight chamber  35 . The heating room  21  optionally also incorporates some hydrocarbon processing equipment, in this example a gas conditioner to remove corrosive gases (such as sulphides) from the production fluids, optionally following the de-sanding and de-gassing processing carried out in the oil processing room  20 . In the heating room  21  the gases are consumed by heaters which heat fluids in a fluid circuit as will be described later. The external walls of the heating room  21  and the oil processing room  20 , defining the chamber  35  in this example, are reinforced to resist over-pressurisation events, and are gas tight to resist gas flow out of the rooms, but the boundaries between the rooms  20 ,  21  are porous to gas, and in this example incorporate fluid flowpaths, vents in the floors, common unsealed stairwells, etc. allowing and indeed facilitating free flow of gas between the two rooms  20 ,  21 . Hence, an over-pressurisation event in one of the rooms  20 ,  21  causes gas to expand in each room, and pressure waves are transferred between the two rooms  20 ,  21  (within the chamber  35 ) without substantial resistance. 
     The buoy  10  has a pressure relief channel  30  adapted to relieve pressure within the chamber  35  containing the oil processing room  20  and the heating room  21  in the event of over-pressurisation in either, which will be discussed in detail below. The pressure relief channel  30  is in fluid communication with the heating room  21 , opening into a vent in the ceiling of the heating room  21 , and extends vertically upwards, relative to the axis of the buoy  10 . Since the heating room  21  and the oil processing room  20  are connected by gas-porous channels which present substantially no resistance to gas flow between the two rooms, an over-pressurisation event in either room  20 ,  21  vents through the pressure relief channel  30 , rather than through the structural walls, floors or ceilings of the two rooms  20 ,  21 . The pressure relief channel  30  is adapted to vent gas pressure above the operating waterline WL of the buoy  10 . Hence, the buoy  10  is more able to cope with unexpected over-pressurisation events, which could be caused by unexpected slugs or surges of gas phase fluids in the production fluids, leaks in the production fluids (liquid or gas) and/or explosions and/or ignitions of hydrocarbons in the production fluids. 
     The buoy  10  shown in  FIGS. 1 and 2  is deployed at the surface next to or directly above an offshore oil or gas well, and is equipped to receive production fluids produced from the well through a riser  85 , which will be discussed in detail below. The riser  85  carries production fluids from the wellhead or other manifold of the offshore well and delivers the production fluids to the buoy  10  for processing within the oil processing room  20 . Typical processing within the buoy  10  can include at least one or more of heating, degassing and de-sanding operations within the oil processing room  20  to remove at least one of sand or gas from the imported production fluids received via the riser  85 , and to at least partially stabilise the production fluids within the buoy, but other types of processing can optionally be deployed. Optionally the production fluids are treated first in the processing room to remove sand and optionally gas, which can be flared or used for fuel for other generators on the buoy, and is optionally also heated to undergo at least partial temperature based stabilisation of the oil (removal of water) before being exported from the buoy  10 . 
     The riser  85  penetrates the buoy  10  leading, in this example, directly into the compartment of the buoy housing the oil processing room  20  and the heating room  21  in a sealed and enclosed chamber. The oil processing room  20  is located on decks  7  and  8  at the base of the buoy  10 , as is best shown in  FIG. 1 . All items of hydrocarbon processing equipment in this example are contained within the oil processing room  20  and the heating room  21 , and in that sealed chamber formed by the two rooms  20 ,  21 , the oil is processed (to remove gas and sand etc.) and is optionally heated to remove water via a temperature based stabilisation process. The reinforced and sealed chamber formed by the oil processing room  20  and the heating room  21  are isolated in a stack within a particular quadrant of the decks  7  and  8 , as is best shown in  FIGS. 10 and 11 , so that the processing equipment in the processing room  20  and the heating room  21  is contained on all sides in an eccentric and asymmetrically arranged chamber  35  that is offset with respect to the vertical axis x-x of the buoy  10 . 
       FIG. 2  shows an external view of the buoy. A set of external stairs is shown extending from the crew landing area  90  with a door into the control room at the top of the stairs, and a door into the lobby from the crew landing area  90 . Several air vents are disposed around the exterior of the section of the buoy  10  that is above the waterline WL. The submerged section  10   s  is shown lying beneath the waterline WL. 
     The buoy  10  has multiple layers of decks. In this example as shown in  FIG. 1 , the buoy  10  has 8 decks plus a top deck T. The top deck T and decks  1 - 4  are above the waterline WL of the buoy  10 . Decks  5 - 8  of the buoy  1  are submerged. The upper and lower deck structures are connected by a central access shaft  40 , the pressure relief channel  30 , and by service trunks  50  and  60  at disposed opposite sides of the central access shaft  40 ; the service trunks  50  and  60  optionally accommodate air intake and exhaust ducts. The pressure relief channel  30  is routed through sequential decks (8-T) of the buoy  10  parallel to the buoy&#39;s  10  vertical axis x-x. The floor plan of each deck is shown in  FIGS. 3-11 . 
     The pressure relief channel  30  contains the effects of an over-pressurisation event, for example ignition of flammable gas, by containing the force of the ignition within the channel  30 . In doing so, any damage resulting from an over-pressurisation event is contained within the pressure relief channel and is not transmitted to the rest of the buoy  10 . The buoy  10  thus continues to remain upright and buoyant for recovery and repair in the event of an ignition or other explosive over-pressurisation event, rather than potentially being lost to the seabed, or becoming untethered and colliding with vessels or other marine equipment, or sinking. 
     The pressure relief channel  30  extends vertically upwards from the chamber formed by the oil processing room  20  and heating room  21 , the channel  30  starting on 5 deck ( FIG. 8 ) and extending to an opening on the top deck T of the buoy  10 , with few or advantageously no bends or deviation from a vertical pathway. Any bends in the pressure relief channel are (in this example) not greater than 45 degrees. 
     The upper end of the pressure relief channel  30  in this example comprises an optional cover  32   a  and  32   b  adapted to close one or more openings at an upper end opening of the pressure relief channel  30  ( FIG. 3 ) within the pressure relief channel  30 . The covers  32   a, b  are adapted to release during an over-pressurisation event, when the pressure within the pressure relief channel  30  rises above a threshold pressure. The covers  32   a, b  also provide a weather-tight seal for the pressure relief channel  30 , and thus prevents ingress of seawater or rain, for example, to the oil processing room  20  via the pressure relief channel  30 . 
     The pressure relief channel  30  in this example is at least partially lined with a material comprising a metal, or at least partially lined with a composite material. In this example, the walls of the pressure relief channel  30  are formed by reinforced portions such as discrete panels of the walls of the adjacent structures, which can be reinforced in their entirety, or simply the individual panel sections forming the inner walls of the pressure relief channel  30  can be reinforced in comparison to the other walls of the adjacent sections. Thus the pressure relief channel  30  is lined with a reinforced material and is generally more resistant to damage as a result. In some examples, the inner walls of the pressure relief channel can be blast proof. In some examples, and in some sections of the present example, the pressure relief channel  30  can be formed by a duct which can be reinforced. The pressure relief channel  30  can optionally have a rectilinear or arcuate cross section. 
     The central access shaft  40  allows access between decks for movement of machinery, injured personnel, and buoy components. The central access shaft  40  extends parallel to the vertical axis x-x of the buoy  10 , from 8 deck ( FIG. 11 ), on which the oil processing room  20  is disposed, to 3 deck ( FIG. 6 ), above the waterline WL. 
     The central access shaft  40  comprises at least one closure member, for example a Miller flap, optionally at each deck level of the buoy. The closure member is adapted to change configuration between an open configuration in which the closure member allows access through the central access shaft  40 , and a closed configuration in which the closure member resists access through the central access shaft  40 . The closure member, when in the closed configuration, acts as a landing site for machinery or personnel, and limits the fall height of dropped loads. 
     The central access shaft  40  optionally has a ladder (not shown) extending between 3 deck and 8 deck for movement of personnel between decks, optionally during maintenance operations. 
     The air intake channel  50  is parallel to the vertical axis x-x and parallel to the central access shaft  40 , and extends between a lower deck of the buoy below the operating waterline WL, and an upper deck above the operating waterline WL. The air intake channel  50  has an air inlet at an upper deck, for example, on the top deck T. 
     The service trunk  60  is also parallel to the vertical axis x-x and optionally parallel to the central access shaft  40 , and in this example, extends between a lower deck of the buoy below the operating waterline WL, and an upper deck above the operating waterline WL, having an exhaust gas outlet at the upper deck, again in this example, at the top deck T. The service trunk optionally houses individual ducts which can be engine exhausts and/or air intakes, optionally in the same service trunk  60  or  50 . The service trunks can be subdivided into separate sectors for performance of different functions in each trunk  50 ,  60 , as for example shown in  FIG. 4  as an optional feature. 
     The plant machinery and other equipment in the buoy  10  are located in the rooms and compartments around the central access shaft  40 , in an asymmetric arrangement, as seen in  FIGS. 6-11 . This can result in an uneven distribution of mass which must be compensated for by, for example, balancing the distribution by adding ballast to the buoy  10 . In this example the buoy  10  has multiple water ballast (WB) tanks  23  distributed around the perimeter of the submerged section  10   s , the plan layout of which is shown in  FIGS. 8-11 . The perimeter of the submerged section  10   s  also contains a plurality of other tanks, comprising diesel oil (DO) storage tanks  24 , diesel oil (DO) service tanks  25 , lubrication oil (LO) tanks  26 , and chemical (chem) storage tanks  27 . 
     3 deck comprises a deck connection for refuelling of the diesel oil (DO) storage tanks  24  in the submerged portion  10   s  of the buoy  10  as required. This can be in the form of, for example, a distribution manifold that directs the flow of the fuel into the tanks  24 . The tanks  24  also have a fuel transfer system for transferring diesel between different storage tanks  24  as required, or to and from service tanks  25 . The machinery that requires diesel fuel, for example generator machinery, is serviced by a service system that optionally gravity feeds the diesel from the service tanks  25 . Other means of transferring the diesel fuel from the service tanks  25  to the machinery that requires diesel fuel can be used. 
     The buoy also has a bilge or oily water tank, located below 8 deck, which is connected to a bilge system that carries deck water away from within the buoy  10  to the bilge tank. The bilge system feeds from the deck drains (not shown), located on each deck, and gravity feeds into the bilge tank. The lubrication oil drain lines from the power generators within the buoy  10  also feed into the bilge/oily water tank. All liquids from the bilge/oily water tank are automatically pumped to the degasser/desander when the tank fills to a predetermined percentage volume, for example 70-90%. Liquids are pumped from the bilge/oily water tank by at least one transfer pump. The oily water is then exported and processed with the production fluids. 
     The processing apparatus within the oil processing room  20  and optionally in the heating room  21 , for example heaters, de-sanders, de-gassers and associated pipework, is enclosed in a lower deck of the buoy as can be seen in  FIG. 1  and  FIGS. 10 and 11 . The cavity containing the hydrocarbon processing facility in this example spans decks  6 ,  7  and  8  at or near to the base of the buoy  10 . Submerging the hydrocarbon processing facilities in the oil processing room  20  and heating room  21  and locating it as near the bottom of the buoy as possible has the beneficial effects of removing the requirement for a riser passing through the water air transition zone and according to the present example, the riser connection to the buoy is therefore at lower risk of impact damage caused by surface vessels or floating debris. Submerging the processing facilities also minimises the hazardous area rating. 
     The chamber  35  containing the oil processing room  20  and optionally the heating room  21  in this example are continually monitored via CCTV (not shown) either from within the control room of the buoy  10  or onshore, or on a vessel. The chamber  35  containing the oil processing room  20  and heating rooms  21  in this example is also monitored by air monitoring sensors, and optionally temperature and pressure sensors. The monitoring of the oil processing room  20  and the heating room  21  both serves as an early alarm system in the event of an over-pressurisation or other potentially damaging event, and an additional indication as to conditions on board the buoy  10  prior to transfer of personnel. 
     The chamber  35  containing the oil processing room  20  and the heating room  21  in this example is optionally continually ventilated by a dedicated intrinsically safe HVAC (heating, ventilation and air-conditioning) system, which is contained in isolation from the rest of the buoy  10  ( FIG. 5 ) in the HVAC machinery room  22 . 
     The HVAC plant in this example has direct ventilation from outside the buoy  10  to ensure a reliable supply of clean air to the oil processing room  20  and the heating room  21 . The HVAC system components can be blast proof, either through design and manufacture of the components, or by locating them within a blast proof compartment. 
     The buoy  10  in this example is primarily an unmanned installation, with personnel being transferred from vessels to perform maintenance operations. Personnel can be transferred to 3 deck&#39;s extensive crew landing area  90 , as best seen in  FIG. 2 or 6 , by supply vessels via a “walk to work” system, or landing crafts with the use of V-ladders. Alternatively, personnel can be landed on the crew landing area  90  using a helicopter winch. In this example the buoy  10  comprises three entry points, with access being available via the main entry door into the control room, crew landing area  90 , or access from the roof into the control room. 
     The buoy  10  in this example has provisions to temporarily house personnel in the event that they cannot be safely retrieved. In this example the buoy  10  comprises a crew room, washroom facilities, and a source of potable water from freshwater (FW) tank  29 , offering a short-term refuge environment where transfer and egress to a supply vessel may not be safe. 
     The buoy  10  in this example also has a fluid flowline connector interface  80  ( FIGS. 12 and 13 ), which connects to a marine riser to admit production fluids from a well into the oil processing room  20 . The fluid flowline connector interface  80  penetrates the wall of the buoy  10  near the base of the buoy  10 , within the submerged section  10   s  at the ballast deck level, leading into the oil processing room  20 , and optionally directly into the same. Optionally the fluid flowline connector  80  is an integral part of the buoy&#39;s lower decks, being typically fixed pipework integral with a bulkhead penetration through the side of the buoy installed and tested to confirm integrity prior to sail-away from the fabrication site. Optionally the fluid flowline connector  80  is in the form of at least one tube, the tube optionally rising through the buoy  10 , or optionally externally to the buoy  10 , to a location above the waterline WL. Optionally when above the waterline WL the connector is tied-in to fixed pipework. 
     The fluid flowline connector interface  80  is protected from damage, for example from dropped objects, by a protective covering  83  disposed on the buoy  10 . The protective covering  83  is a panel that extends from the buoy  10 . The panel can be made from metal. Siting the fluid flowline connector interface  80  and the oil processing room  20  near the base of the buoy  10  means that the protective cover can safely be appended to the exterior of the buoy  10 , while reducing or removing the risk of vessel impact with the protruding cover  83 , or impact with floating debris, or the cover being adversely affected by tidal forces in the wave affected zone, for example. 
     In the event of over-pressurisation of the gas, for example atmospheric gas within the chamber  35  formed by the heating room  21  and the oil processing room  20 , the over-pressurised gas is vented through the pressure relief channel  30 , immediately above the heating room  21 , which is in fluid communication through ceiling vents or other gas-porous vents etc. with the oil processing room  20 . In particular, in the event of sudden pressure changes arising from explosions in either the heating room  21  or the oil processing room  20 , pressurised gas within the chamber is vented through the pressure relief channel  30  in a vertical direction along the pressure relief channel  30 , substantially parallel to the vertical axis x-x of the buoy  10 , and is released through the opening of the cover  32  on the top deck T of the buoy  10 . Since the pressure relief channel  30  has few or no bends or other deviation from a substantially straight line, the over-pressurised gas is not restricted within any space in the buoy  10 , and hence can vent to atmosphere in a relatively harmless manner. The cover  32  on the top deck T is optionally normally closed and is optionally secured in the closed position via a latching device, which optionally releases at least one of the covers  32   a  and/or  32   b  at a relatively low pressure threshold in response to the over-pressurisation of the pressure relief channel  30 . Accordingly, destructive over-pressurisation events such as explosions occurring within the oil processing room  20  or heating room  21  immediately above it are guided by the pressure relief channel  30  to atmosphere, and the effects of the over-pressurisation event on the remaining infrastructure of the buoy  10  are limited. 
     In operation, the buoy  10  is optionally fully commissioned at the quayside prior to deployment, and is towed out and moored in position above, or in close proximity to, a subsea oil or gas field. The buoy  10  is connected to the riser  85  via the fluid flowline interface connection  80  for the delivery of production fluids from the subsea well to the buoy  10 . The riser  85  can be connected to the interface  80  by, for example, an ROV or a diver. Initially, the buoy  10  can be deployed in position above the oil field while relatively high in the water, and can be ballasted or tethered in a manner such that it adopts the operating position shown in  FIGS. 1 and 2 , in which the buoy  10  is relatively low in the water, and the lower four decks  5 - 8  of the buoy  10 , comprising section  10   s , are submerged some way below the water line WL. In this example, a substantial body of the buoy  10  is submerged below the waterline WL, for example up to 5 to 35 metres of the vertical height of the buoy  10  extends below the waterline WL, and approximately the same above. The lower four decks  5 - 8  of the buoy  10  contain relatively heavy components, including the main generator rooms, auxiliary machinery and processing plant room, as well as the oil processing machinery in the oil processing room  20 , and the heating equipment in the heating room  21  immediately above it. Accordingly, the lower section of the buoy  10  has a relatively large mass in comparison to the upper section of the buoy  10  above the waterline  10 , which assists in the stability of the buoy  10  in the water as the centre of gravity is lowered. In this example, the buoy  10  is tethered to an anchor point on the seabed, optionally a subsea storage tank, to which processed oil is delivered from the buoy  10  to the subsea storage tank for further storage, and optionally for further processing. The tethers connecting the buoy  10  to the seabed are advantageously held in tension in order to increase the stability of the buoy  10  within the water. 
     Production fluids flow from the well through the riser  85  and into the oil processing room  20 . Here, the production fluids are initially stabilised by heating to a predetermined temperature, for example 60-100′C, and in this example, around 80′C, to flash off light ends and gas. The gas flashed from the production fluids is burned in either the buoy&#39;s  10  engines, for power generation, or boilers, for heat generation, subsidising a portion of the diesel fuel initially used for this purpose and supplied from the diesel oil (DO) tanks around the perimeter wings of the submerged portion  10   s  of the buoy  10 . The mixture of gas and diesel eventually comprises up to 70% gas and 30% diesel. Excess gas produced from the degassing process, for example if there is a spike in gas production, can be cold vented through the flare and cold vent  10   f  on the top deck T (see  FIGS. 1 and 2 ). 
     The degasser/desander vessel collects sand or particulates that are entrained in the production fluids. Once the level of sand reaches a pre-set value, the sand is fluidised by a fluidisation device and transferred as a dirty slurry to the sand cleaning apparatus. The degasser/desander optionally contains a heating coil to maintain the temperature of the fluids at around 80′C during the de-gassing and de-sanding process. Alternatively or additionally, heating of the production fluids can also be achieved by a heat exchanger reclaiming heat energy from the exhaust gas from the engines. Sand is removed from the degasser/desander, cleaned, and offloaded into a supply ship as required. The oil processing room  20  can also contain equipment to heat the heavy oil fractions prior to storage. 
     Fluids can be transferred to a subsea storage tank through appropriate outlets, which may conveniently be sited in oil processing room  20 . Therefore, production fluids entering the buoy  10  at the oil processing room  20  can optionally be treated and re-routed within the same chamber formed by the oil processing room  20  and the heating room  21 , optionally leaving the buoy  10  when processing is complete. Accordingly, processed fluids are only transiently stored within the buoy  10  during the processing phase, which increases the safety margin of the operations of the buoy  10 . 
     In the present example, de-gassed and de-sanded fluids treated in the oil processing room  20  are routed from there to the heating room  21  immediately above the oil processing room, the two rooms  20 ,  21  together forming a sealed chamber  35 , where the treated fluids are further conditioned to remove corrosive gases before being exported. 
     Further stabilisation and separation can take place in the storage tank, remote from the buoy optionally thermal stabilisation, or optionally pressure stabilisation. The storage tank can be heated to stabilise the production fluids further (removing water content and light ends from the production fluids in the tank) using heated glycol fluid pumped in a heat exchange loop from boilers located in the heating room  21 , which can be fuelled by the flashed off gases obtained from the production fluids. 
     The standard operating pressure of the degasser/desander unit in the buoy  10  is selected to exceed the static head of the fluid column, to facilitate flow of the degassed fluids under gravity to the storage tank. Water separated from the oil can be pumped from the storage cell back to the buoy  10  for final polishing (to less than 30 ppm), metering, monitoring, and disposal at sea as necessary. Oil extracted in the polishing process can be routed back to the storage tank. Separated oil is offloaded from the storage tank to a tanker. 
     In the event of surges of gas in the production fluids leading to uncontrolled escape of gas from the processing equipment in the oil treatment room  20  or the heating room  21 , any ignition of gas in that sealed chamber  35  containing the oil treatment room  20  and the heating room is vented through the pressure relief channel in a vertical direction from the location of the ignition below the waterline WL to the top of the pressure relief channel  30 , where at least one of the covers  32   a  and  32   b  is released from its respective opening to allow venting of the gas pressure wave into the atmosphere with reduced damage to the fabric of the buoy  10 .