Abstract:
A subsea and modular tanker-based hydrocarbon production system comprising a plurality of interlinked individual tank units which is wholly submersible, is wholly detachable from, and wholly re-attachable to, its associated subsea wellhead infrastructure. Modularity of the interlinked tank unit system allows for the processing, measurement, and storage of hydrocarbons from a wide variety of offshore hazard and water-depth related conditions and situations. In addition to being both detachable and re-attachable, the modularity of the system provides for a number of unit systems to be conjoined at surface and towed to market.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a subsea tanker hydrocarbon production system. 
     Much of the exploitation of hydrocarbon deposits is conducted in the realm of deep offshore waters. In fact some of the largest and most prolific such deposits are to be had in deep waters. So deep are these waters that existing subsea hydrocarbon extraction technology is utilised at its very operational limits. 
     Hitherto there have been two methods of hydrocarbon production offshore. The first utilises platforms raised above wave-height sitting upon concrete and/or steel towers which are themselves fixed to the seabed. Such towers are extraordinarily expensive. The second utilises subsea wellheads and clusters of tied-back subsea wellheads, production from which is conducted via a flexible riser pipe to a floating production station. Although less expensive than a fixed platform, a considerable amount of costly marine infrastructure is required for such an operation. What both of these methods have in common is their production facility, i.e. that which is placed upon the platforms, whether they be fixed or floating. Such a production facility concerns itself with a separation of the produced fluids (the various gas and oil phases, together with any produced associated water) and their subsequent measurement. It is usually the oil (and gas condensate) phases which are most prized, and are despatched to market. In the case of the fixed platforms this despatch is by seabed pipeline, and in case of the floating stations, by ship. The gas phases are often considered as less valuable, and depending on the economics of any particular project and its location, are variously disposed of, via alternative pipelines, re-injected back into the hydrocarbon reservoir rock elsewhere, or simply flared, i.e. burnt. Many authorities consider the flaring of gas phases, a common feature, as unnecessarily wasteful. Whilst such separation is a continuous process, the requirement for measurement is met by the intermittent diversion of production flows from individual wells. This is performed via a smaller and dedicated measuring separation train, and flows so measured are then rerouted back into the continuous process. Whilst the measurement of the productivity of individual wells contributes nothing to immediate economics, such measured data is essential for the extractive management of the hydrocarbon deposit (i.e. the reservoir) as a whole. Further, notwithstanding any financial or technical considerations, such processing operations are notoriously hazardous to personnel. 
     BRIEF SUMMARY OF THE INVENTION 
     According to the present invention there is provided a subsea and modular tanker-based hydrocarbon production system comprising a plurality of interlinked individual tank units which is wholly submersible, is wholly detachable from, and wholly re-attachable to, its associated subsea wellhead infrastructure. Modularity of the interlinked tank units allows for the processing, measurement, and storage of hydrocarbons from a wide variety of both offshore (i.e. hazard and water-depth related) and hydrocarbon reservoir conditions. Further, in addition to being both detachable and re-attachable, the modularity of the system provides for a number of unit systems to be conjoined at surface and towed to market. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S) 
       A specific embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which:— 
         FIG. 1  illustrates a modular series of interlinked individual submersible tanks connected to its associated subsea wellhead infrastructure. 
         FIG. 2  illustrates schematically the means by which individual submersible tanks are linked and hydraulically-connected to each other, together with the internal components of such tanks in cross-section. 
         FIG. 3  illustrates in detail the means by which the first, or lowermost, of such a series of interlinked submersible tanks is connected to its associated subsea wellhead infrastructure, details of which are similarly provided. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , a series of submerged and interlinked tanks  1  are connected by a stinger unit  2  stabbed, via a guide funnel  4 , into a dedicated anchor station  3  which is permanently grouted  5  to the seabed. Hydrocarbons are produced from one or more adjacent subsea wells tied-back to the anchor station from a subsea manifold  6 . The hydrocarbons migrate upwards from the anchor station into the series of interlinked tanks which are allowed to float vertically. Intervals of flexible high-pressure hose  7  provide the conduit for hydrocarbons between the respective interlinked tanks. Such intervals of connective high-pressure hose neither experience tension nor compression since longitudinal tension is constrained by two or more high-tensile steel chains shackled to pad-eyes  8 , and longitudinal compression similarly constrained by steel cages  9 . The uppermost of the series of interlinked tanks is connected to a crown shackle  10  and subsequently tethered by cable  11  to a marker buoy  12  which may or may not be permitted to break the surface. Gross lateral motion of the series of interlinked tanks may be restricted by additional tethering  13  anchored to one or more off-field concrete blocks  14 . 
     Referring to  FIG. 2 , the individual modular units of the series of interlinked production tanks are at their simplest vessels for the storage of produced hydrocarbons. Whilst they may be of a single flask structure, in this particular embodiment of the invention the individual production tanks comprise a number of parallel yet separate and individual charges, and illustrated in cross-section,  20 . These charges may be constructed from standard high-pressure oil-field casing. The spacing between such charges and/or casings may be filled with protective materials such as fibres or polymers, or alternately, access may be allowed to seawater for coolant purposes. Such a multitude of storage charges are branched from the central production channel or conduit  18 . They are similarly and consequently rebranched at the alternate end of each individual tank module again to a central production conduit ( 18 ). A gas conduit  19  is provided, in parallel with the central hydrocarbon conduit  18 , although this does not serve any storage purposes. The central hydrocarbon production conduit  18  is equipped with hydraulically- and manually-activated master valves  22 , together with similarly activated wing valves  24 . Likewise, the gas conduit  19  is fitted with both master ( 22 ) and wing ( 23 ) valves. The hydrocarbon conduit  18  is connected to its corresponding feature on any adjacent interlinked tank module by an interval of flexible high-pressure hose  26 . The gas conduit  19  is similarly connected to its corresponding feature on any adjacent interlinked tank module by an interval of flexible high-pressure hose  25 . As stated above, such intervals of connective high-pressure hose neither experience tension or compression since longitudinal tension is constrained by two or more high-tensile steel chains  27  shackled to pad-eyes  8 , and longitudinal compression is constrained by steel cages  9 . The latter may comprise a circular arrangement of four or more Samson posts  28  topped with rubber bumper elements. The individual modular storage tanks themselves are not primarily load-bearing structures. Structural continuity of the interlinked series of tanks is provided for each and every unit by steel base plates  16  connected to longitudinal struts  17 , and in turn, the steel bumper cages ( 28 ) and corresponding pad-eyes for chainage. The tops and bottoms, or rather their ends, of each tank are similar. Both conduits  18  and  19 , and any subsequent branch conduits, may be fitted with flapper-type safety valves, fluid densometers, thermometers, pressure sensors, and hydraulically-activated production chokes;  29  and  30  respectively. Additionally the storage charges ( 20 ) may be fitted with baffles to aid and/or facilitate separation of oil and gas and to collect any produced sand and minor debris. 
     Referring to  FIG. 3 , the first, or lowermost, of the series of interlinked submersible tanks is equipped with a stab-in and tensile load-bearing element  2  in structural continuity with a circular plate at the base of the tank  16  which in turn is in structural continuity with two or more load-bearing elements  17  which traverse longitudinally each and every one of the interlinking tanks terminating at the shackle pad-eyes  8 . Through stab-in element  2  are bored the primary hydrocarbon conduits; a larger  31 , and a smaller  32 . The larger conduit corresponds to, and is contiguous with, the hydrocarbon conduit  18  ( FIG. 2 ). The smaller conduit corresponds to, and is contiguous with, hydrocarbon conduit  19  ( FIG. 2 ). These conduits are fitted with hydraulically- and manually-activated master valves  37 , together with similarly activated wing valves  38 . These valves may be utilised for pressure-testing purposes and to seal the tanks whilst in transit. Flapper-type fail-safe valves  40 ,  41   a  and  41   b  may also be integrated into the conduits together with hydraulically-activated chokes  41   a . In this particular embodiment of the invention stab-in element  2  enters and accesses a chamber  42  in the anchor station. It is via such a chamber that the produced hydrocarbon stream (both oil and gas) enters the primary conduit ( 31 ) and subsequently into the series of interlinked tanks above ( 1 ). The primary bore, or conduit  32 , accesses another chamber elsewhere in the anchor station  46 . The primary purpose of conduit  32  is to provide for the subsequent disposal downwards of liberated gas, if necessary, after it having traversed all interlinked tanks. Structural and hydraulic integrity between the stab-in element  2  and the anchor station is provided for by a series of hydraulically-activated pipe-ram and annular-preventer bag element units. In this particular embodiment of the invention structural integrity is provided by two pipe-ram components  43  and  46 . The rams are closed about recesses  34  and  35  in the stab-in element ( 2 ) after a no-go shoulder  33  lands out atop the anchor station  48 . Hydraulic integrity is provided also by two annular-preventer bag components  44  and  45 . Additionally these two annular-preventer bag components serve to divide the upcoming and down going hydrocarbon streams. Shear-ram elements  47  provide for the possibility to disconnect the series of interlinked tanks above it by severing the stab-in element ( 2 ) should either of the pipe-rams fail closed. Whilst the use of these elements here is not that for which they were originally intended, the function- and pressure-testing of such is well-known in the industry and need not be discussed here. Hydrocarbons are produced, via subsea manifold, from one or more adjacent and tied-back subsea wells enter the anchor station at  50 . Similarly, gas may exit the anchor station  49  for subsequent distribution. In addition, within this the first of the series of interlinked production tank modules ( 1 ), a number of compressed-air cylinders  51  are provided to allow for recharge of the corresponding compressed-air cylinders used to function all ram- and annular-preventer elements, which may be performed via the conduits  32  prior to the commencement of any hydrocarbon production operations. Similarly, such tanks ( 51 ) may also serve, via a cross-over tee  52 , to flush the hydrocarbon production chamber ( 42 ) of the anchor station of any residual hydrocarbons prior to disconnection of the stab-in element. Item  48 , the top of the anchor station ( 3 ) may, optimally, be fitted with a hydraulically activated ‘trash’ cover. Inductive coupling may also be provided between items  33  and  43  to recharge any batteries associated with any electrically-controlled functions and monitoring functions of the anchor station. It should be noted that conduits  18  and  19  on the ultimate (uppermost) of the interlinked series of tanks need be mated, or otherwise closed. 
     Prior to any production operations the anchor station is positioned and grouted to the sea-bed at or about the time any operations are conducted to tie-back any subsea wellheads and subsea wellhead hubs. The modular series of interlinked subsea tanks is towed into approximate position on the surface by a suitable tendering vessel. Such a vessel need be equipped with a remote vehicle (ROV) facility to monitor operations subsea, a winch unit, and a pump unit. All mechanical and hydraulic connections between the individual interlinked storage tank modules should be established and pressure-tested. Hydraulically-operated recharge tanks ( 51 / FIG. 3 ) should be charged. The entire series of interlinked tanks may be placed on vacuum (i.e. eliminated of air). Although not strictly essential, vacuum here serves two purposes; i) the tanks become more negatively buoyant and become thus freer to sink, and ii) the tanks may be filled with hydrocarbons without any subsequent requirement to vent. Under guidance from tether lines the interlinked tanks are then allowed to sink toward the anchor station. As the station is approached by the tanks, fine positioning may be facilitated by making adjustments to the winch and the position of the tendering vessel. The stab-in element is allowed to enter the anchor station until it lands out, as described above. Concerning specifically the anchor station; the lower set of pipe rams is then closed and the chamber below it pressure-tested and the entire system subsequently load (tensile) tested. Likewise, the remaining pipe rams and annular-preventer bag elements are individually functioned and pressure-tested. Finally, and returning again specifically to the stab-in production unit and its modular series of interlinked tanks, all master valves are ascertained as open, and the many individual production chokes set as appropriate (i.e. as defined by the reservoir operations management team) which of course may be varied throughout the course of production. Once filled, the reverse of the above described sequence may be initiated. Production should cease from the subsea hubs feeding the anchor station, and the small chamber within the chamber through which production is facilitated should be flushed of hydrocarbons; by air and/or seawater. The gas conduit may be utilised to recharge the hydraulic units of the anchor station associated with pipe-ram and annular-preventer elements. All valves on the individual interlinked tank units should be activated to the closed position, and the pipe-ram and annular-preventer elements of the anchor station subsequently opened. By virtue of its hydrocarbon charge, the series of tanks should be positively buoyant. Movement may (again) be controlled by the winch on the surface tendering vessel. At surface, a replacement series of interlinked tanks may be connected to the (primary) stab-in element and subsequently re-positioned. The remainder may be conjoined and towed to market. By such a system, field development may be considered as wholly contingent on what each and every well alone may produce without reference to complex and costly surface infrastructure. There need be no limit to the size or number of said units interlinked save the strength of the materials used in their manufacture.