Abstract:
Disclosed is a subsea storage unit ( 15; 15′; 15 ″) including a pressure hull ( 1 ) having a cargo hold ( 5 ) configured for storing cargo ( 3, 7 ) and a base ( 2; 8 ) configured for supporting the storage unit on a seabed (B). The pressure hull has a movable hatch ( 4 ), providing access to the cargo hold. The subsea storage unit also includes suspension means ( 11   a,b,    35 ), wherein the subsea storage unit may be lifted and lowered in a body of water, and ballasting means. A seabed facility ( 30 ) configured for receiving and accommodating at least one subsea storage unit, is also provided.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention concerns a subsea storage unit and a subsea storage system, and an associated subsea storage method according to the preambles of claims  1 ,  9  and  11 . 
       BACKGROUND OF THE INVENTION 
       [0002]    Exploration and production of hydrocarbons from subsea wells require various and complex equipment, such as wellhead equipment, tie-in stations, compressors and pipelines. This subsea equipment is in frequent need of maintenance, emergency repairs, and upgrade operations. In order to perform these operations, offshore workers need various tools, spare parts, etc. Offshore units, such as floating platforms, subsea vessels and anchor-handling vessels, are in general lacking in storage space, so tools and parts are normally stored onshore until they are needed offshore. 
         [0003]    Consequently, it is necessary to be able to quickly transfer articles from land to the offshore units. As soon as the requirement for a specific article emerges at an offshore site, a request is made to an onshore supply operation. The article is then collected from the storage area and transferred by e.g. supply boats to the offshore unit, but supply boats are costly to operate and dependent on the weather. Alternatively, the subsea vessel can abort its current mission and collect articles from the onshore location, but this is also a costly and undesirable operation. 
         [0004]    Depending on the weather, the supply boats may not be capable of handing over the articles to the offshore unit within the requested time. The timing of delivering the articles is critical, and delayed delivery of maintenance equipment to the offshore unit can be both critical and costly. It is also the case that at some onshore locations, the logistics is difficult and slow. In worst case scenarios equipment for maintaining the safety of the offshore workers will not reach the offshore unit in time. 
         [0005]    Attempts have been made to overcome the problem of lack of offshore storage space. Examples include containers for storing articles on site, where the containers are buoyant and floating in the water. The floating containers are anchored to the seabed, making them unsuitable for extended storage due to exposure to wind, waves and currents, and adding the risk of the containers becoming detached from the anchor and colliding with offshore installations or vessels. 
       SUMMARY OF THE INVENTION 
       [0006]    It is therefore provided a subsea storage unit, characterized by a pressure hull having a cargo hold configured for storing cargo, and the pressure hull having a movable hatch providing access to the cargo hold; and a base configured for supporting the storage unit on a seabed. 
         [0007]    The subsea storage unit comprises in one embodiment suspension means, whereby the storage unit may be lifted and lowered in a body of water. In one embodiment, at least one ballast tank and control means are provided, whereby the storage unit buoyancy may be controlled. In one embodiment, the base comprises solid ballast. 
         [0008]    The suspension means comprises in one embodiment releasable connection means. 
         [0009]    In one embodiment, the cargo hold comprises support members configured for receiving a container, such as a standardized IMO container. 
         [0010]    In one embodiment, the subsea storage unit comprises movable, footprint-increasing plate members that are movable between retracted and deployed positions. 
         [0011]    The subsea storage unit may comprise localizing means, such as a transponder. 
         [0012]    It is also provided a subsea storage system, characterized by at least one subsea storage unit according to the invention; and a seabed facility configured for receiving and accommodating at least one subsea storage unit. 
         [0013]    It is also provided a subsea storage method, including the steps of transporting at least one subsea storage unit from an onshore location, deploying the subsea storage unit in a closed state on a seabed, locating the subsea storage unit, retrieving the subsea storage unit from the seabed to a vessel, opening the subsea storage unit in order to gain access to its cargo hold. 
         [0014]    In one embodiment the method also comprises the step of closing and deploying the subsea storage unit. 
         [0015]    In one embodiment the method also comprises the step of retrieving the at least one subsea storage unit from the seabed and returning it to an onshore location. 
         [0016]    In one embodiment the subsea storage unit is deployed on a subsea facility located on the seabed, the subsea facility being configured for receiving and accommodating at least one subsea storage unit. In one embodiment, locating the subsea storage unit is provided by means of a transducer and a transponder. 
         [0017]    The foregoing and other objects, features and advantages of the disclosure will be apparent from the following more particular descriptions of exemplary embodiments of the invention as illustrated in the accompanying figures. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0018]    These and other characteristics of the invention will become clear from the following description of a preferential form of embodiment, given as a non-restrictive example, with reference to the attached schematic drawings, wherein: 
           [0019]      FIG. 1  is a perspective view of an embodiment of the invented storage unit in a closed state; 
           [0020]      FIG. 2  is a perspective view of the storage unit shown in  FIG. 1 , in an open state, showing a cargo container inside the storage unit; 
           [0021]      FIG. 3  is a perspective view of the storage unit shown in  FIG. 2 , showing also the cargo container in an open state; 
           [0022]      FIG. 4  is a perspective view of another embodiment of the invented storage unit, in an open state, showing a cargo container inside the storage unit; 
           [0023]      FIG. 5  is a front view of an embodiment of the invented storage unit in an open state, illustrating a container retaining device in the cargo hold; 
           [0024]      FIG. 6  is a schematic sectional drawing of an embodiment of the invented storage unit; illustrating an exemplary cargo conveyor system; 
           [0025]      FIG. 7  is a schematic sectional drawing of an embodiment of the invented storage unit, illustrating an exemplary ballasting system; 
           [0026]      FIG. 8  is a sectional front view of the storage unit shown in  FIG. 8 ; in a non-submerged state; 
           [0027]      FIGS. 9 and 10  are similar to  FIG. 8 , but show the storage unit in partly and fully ballasted states, respectively; 
           [0028]      FIGS. 11 and 12  illustrate a system and a method for locating a storage unit on a seabed, and retrieving the storage unit to the surface; 
           [0029]      FIG. 13  illustrates a seabed depot; 
           [0030]      FIG. 14 a    illustrates a docking device and a connector; 
           [0031]      FIG. 14 b    is an enlarged view of the region A in  FIG. 14 a   , with the connector locked in the docking device receptacle; 
           [0032]      FIG. 15  is another embodiment of the docking device and the connector; 
           [0033]      FIG. 16  illustrates an embodiment of the storage unit having deployable, footprint-increasing, plate members; and 
           [0034]      FIG. 17  illustrates a subsea template and guide wires, and a storage unit ballast system. 
           [0035]      FIG. 18  illustrates a surface vessel retrieving a subsea storage unit. 
           [0036]      FIG. 19  illustrates a subsea storage unit being transported on a surface vessel. 
           [0037]      FIG. 20  illustrates several surface vessels deploying and retrieving subsea storage units from a seabed depot. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0038]    The following description will use terms such as “horizontal”, “vertical”, “lateral”, “back and forth”, “up and down”, “upper”, “lower”, “inner”, “outer”, “forward”, “rear”, etc. These terms generally refer to the views and orientations as shown in the drawings and that are associated with a normal use of the invention. The terms are used for the reader&#39;s convenience only and shall not be limiting. 
         [0039]    Referring initially to  FIG. 1 , the invented storage unit  15  comprises a cargo housing  1  connected to a supporting structure  2 . In the illustrated embodiment, the supporting structure  2  comprises a box structure that is configured for resting on a surface B. The supporting structure  2  comprises lifting means (not shown in  FIG. 1 ), which will be described below. The box structure provides for stacking of multiple storage units. 
         [0040]    The cargo housing  1  is a pressure hull, capable of withstanding external pressures caused by e.g. great water depths. Pressure hull design parameters are well known and need therefore not be discussed in detail here. Hence, the storage unit may be used at any water depth, by appropriate design of the pressure hull. The cargo housing may be made of one or more layers (e.g. layers of steel), such as inner and outer layers with an intermediate honeycomb structure (not shown). 
         [0041]    The cargo housing  1  comprises a hatch  4 , connected to the housing via hinges  6  and comprising conventional locking and sealing means (not shown) for providing a sealed connection between the hatch and housing when closed. The hatch may thus be opened and closed in an manner which is known in the art. 
         [0042]      FIG. 2  shows the cargo housing  1  with the hatch  4  in an open position, providing access to an internal cargo hold  5 . In the illustrated embodiment, the cargo hold  5  comprises support members  9  configured for supporting a cargo container  3 . The support members  9  comprise rollers and locking means (not shown) that per se are known, facilitating easy insertion and retraction of the container  3 . The support members  9  are configured to suit the shape of the container, e.g. a standard IMO (International Maritime Organization) container. 
         [0043]      FIG. 3  shows the cargo container  3  in an open state, illustrating individual cargo items  7 . 
         [0044]    The cargo items  7  may require certain environmental criteria, for example regarding pressure, humidity and salinity. The storage unit may thus be fitted with equipment (not shown) for sensing, monitoring and controlling environmental parameters within the cargo hold, e.g. in order to creating a non-corrosive environment. Such control equipment may comprise pressurized Nitrogen systems, which are known in the art, responding to sensed parameters and predetermined values. 
         [0045]      FIG. 4  illustrates an embodiment of the storage unit  15 ′ where a lifting frame  11   a  is connected to the supporting structure  2  via releasable locking means  11   b.  A lifting chain  10  is connected to the lifting frame  11   a.  A footing  8  is connected to the lower portion of the supporting structure  2  and provides a landing structure for the storage unit. The footing may be dimensioned so as to distribute the load in order to avoid substantial soil penetration on the seabed B. The footing  8  comprises in the illustrated embodiment a ballast material in the form of a concrete slab. 
         [0046]      FIG. 5  shows an alternative embodiment of the internal supporting member  9 ′, where releasable retaining members  19  secure the cargo container  3  in place. The retaining members  19  serve to secure cargo containers in the space provided by the supporting member  9 ′, and may comprise hydraulic or pneumatic dampers, which are known in the art. 
         [0047]    Referring now to  FIG. 6 , the cargo housing comprises in an alternative embodiment a loading/unloading system for the cargo items  7 . A board  20 , having collapsible wheels  22 , is slidably arranged on rails  21  in the housing  1 . A winch  23  may be used to pull the board out of the housing  1 . 
         [0048]      FIGS. 7-10  illustrate a ballasting system for the storage unit (only the cargo housing  1  is illustrated, not its supporting structure). This system may be used together with or without the concrete ballast described above with reference to  FIG. 4 . The cargo housing (pressure hull)  1  comprises a number of ballast compartments  24  and ballasting pumps  25 . The ballasting pumps  25  is in the illustrated embodiment powered by on-board batteries  26  (although not illustrated, the skilled person understands that the batteries are kept in a dry environment, e.g. in a watertight casing). The ballasting pumps are fluidly connected to inlet/outlet ports  27  (optionally with remotely controlled valves; not shown), whereby the ballast compartments  24  may be filled and emptied in a controlled manner. The ballasting pumps are controlled in a manner which per se is known in the art. 
         [0049]    In  FIG. 8 , the cargo housing  1  is floating in the water surface S, and the ballast compartment  24  is virtually empty. In the illustrated embodiment, the ballast compartment  24  is enclosing the dry cargo hold  5 .  FIG. 9  shows and intermediate ballasting state, where the ballast compartment  24  has been partly filled with seawater W, through the ports  27 . Although not illustrated, it should be understood that the ballast compartment comprises one or more ventilation valves (e.g. check valves), preferably in the upper portion of the compartment, whereby air may be evacuated as water is flowing into the compartment. In  FIG. 10 , the ballast compartment  24  is full. The storage unit may thus be selectively ballasted and de-ballasted by means of the ballast compartments. 
         [0050]    Although  FIGS. 7-10  illustrate the cargo housing  1  having a number of cargo items  7  in its hold  5 , it should be understood that the ballasting system may also be used in the embodiment where e.g. an IMO container is arranged in the hold  5 . 
         [0051]      FIG. 11  illustrates a storage unit  15  arranged on a seabed B. In this configuration, the storage unit may have been ballasted by one or more of the means described above. The storage unit  15  is equipped with a transponder  53 , which is well known in the art. A surface vessel  14 , equipped with a crane  29 , is emitting sonar signals from a transducer  13  in order to locate the storage unit  15 . A lifting wire  10 ′ is provided with a transponder  34 . Each storage unit is assigned a unique identification code, whereby the surface vessel operator is able to pick the desired storage unit. The identification code may comprise information about the individual cargo items. In  FIG. 12 , the lifting wire  10 ′ has been connected to the storage unit  15 , by means of an ROV (Remotely Operated Vehicle) and the storage unit is being hoisted to the surface by means of the crane  29 . In a deployment operation, the sequence is reversed: the ROV releases the lifting wire when the storage unit has been placed on the seabed. 
         [0052]      FIG. 13  illustrates a seabed depot  30 , which may be dimensioned for accommodating one or more storage units  15  on a foundation  31 . Trawl deflectors  32  protect the storage units from dragged objects. A removable roof (not shown) may also be provided. A transponder  33  on the seabed depot  30  facilitates localizing, e.g. by the surface-borne sonar  13 . The seabed depot transponder  33  may be configured to emit unique identification codes, specific to the seabed depot or/and its contents. 
         [0053]    Information regarding the content articles in the storage units may be transmitted from the storage unit. Other information, such as operational parameters for the cargo housing may also be requested and transmitted. 
         [0054]      FIGS. 14 a,b    and  15  illustrate an automated connection system that obviates the need for ROV or diver assisted connection and disconnection. A docking device  35  is connected to the storage unit  15  and comprises a receptacle  38  and proximity sensors  37 . The receptacle comprises a plurality of spring-loaded pegs  40  and that are configured to interlock with corresponding sockets  39  in a connector  36  attached to the lifting wire  10 ′. The spring-loaded pegs may thus automatically interlock with the connector when it is lowered into the receptacle.  FIG. 14 b    shows the connector  36  in the locked position in the receptacle. The proximity sensors  37  may also be configured to sense the presence of the connector  36 , and operate the pegs accordingly. The docking device may also be remotely operated, e.g. via the above-mentioned transponders. 
         [0055]      FIG. 15  illustrates an alternative embodiment of the connection system, where a threaded connector  36 ′ (attached to the lifting wire  10 ′) has been connected to the docking device  35  via corresponding threads in the receptacle  38 ′. An umbilical  41 , extending along the lifting wire from the surface vessel, provides power and control signals to an electric motor (not shown) inside the connector  36 ′, whereby the connector may be rotated and screwed into (and out of) the threaded receptacle  38 ′. 
         [0056]      FIG. 16  illustrates a variant of the invention where the storage unit  15  is furnished with plate members  43 . Each plate member is hingably connected to the storage unit and is rotatable between retracted (dotted lines) and a deployed positions. Operation of the plate members is performed by actuators (e.g. hydraulic or electrical), and the plate members may be remotely controlled or configured to operate based on local parameters (ambient pressure, seabed proximity, etc.). In a deployed position, the plate members  43  increase the storage unit lower surface area, which may tend to stabilise the storage unit as it is lowered towards the seabed, and also increase the storage unit footprint on the seabed B, thus preventing the storage unit form sinking into the seabed. 
         [0057]      FIG. 17 a    illustrates another embodiment for lowering the storage unit  15 ″ to the seabed B. An ROV places guide wires  46  connected to guide posts  48  on a subsea template  47 . The storage unit  15 ″ comprises in the illustrated embodiment ballast tanks  50  with associated pumps and control systems  51 , and an inlet/outlet manifold  52 . The ballast tanks  50  comprise internal compartments  24 ′ (see  FIG. 17 b   ) having interconnecting valves  49 . 
         [0058]    In operation, the storage unit  15 ″ is lowered by one or more lifting wires  10 ″ from the derrick  44 , through the moon-pool  45 , along the guide wires  46 . The lowering may be assisted by a controlled operation of the ballast control system (distributing the ballast water within the compartments  24 ′), or be accomplished solely by the weight of the storage unit itself (and, optionally, cargo). Storage unit retrieval is also performed by the lifting wire  10 ″ and a winch (not shown) in the derrick, through the moon-pool. 
         [0059]    Although the invention has been described with reference to a cylindrical cargo housing with domed ends, it should be understood that the cargo housing may have other shapes. The shape of the cargo housing may thus deviate from a circular shape, depending on the applicable ambient water pressure. 
         [0060]      FIG. 18  illustrates a surface vessel  14  with a movable crane  54  which retrieves a storage unit  15  by means of a lifting wire  10 . Prior to the storage unit  15  being retrieved, the storage unit  15  has been identified and connected to the wire  10  by means previously described with reference to  FIG. 11 . The lifting wire can also be automatically connected to the storage unit  15  as described with reference to  FIG. 14   a,b.  The movable crane  54  can be any kind of mechanism able to reel in the wire  10  and the storage unit  15 , or in other ways being able to retrieve the storage unit  15 . The movable crane  54  can, after retrieving, also be used to transport the storage unit  15  to a desired location on the vessel  14 . The surface vessel  14  can have a vast number of storage units  15  on board, depending on the area of application. The storage units  15  may either be full of equipment, or near empty, ready to be filled with used equipment. The transponder  53  can, in addition to provide the position of the storage unit  15  and the connection means between the wire  10  and the storage unit  15 , also provide information on what the storage unit  15  contains. 
         [0061]      FIG. 19  illustrates the storage unit  15 , on board the vessel  14 , being transported to a desired location by means of the movable crane  54 . A logistics system on board the surface vessel  14  keeps track of which storage units contains what cargo, such a system is commonly known in the art and in handling containers on and off shore. In the figure, there is available space for the storage unit  15  below deck, so the crane  54  must transport the storage unit  15  to an opening  55  between the two decks. The device  54  which transports and distributes the storage units on the vessel  14  need not be the same device as the crane  54  which retrieves the storage units from the sea bottom, this is dependent on the logistics preferred on the vessel. While the storage unit  15  is transported to the desired location on the vessel  14 , the vessel  14  can navigate to the next desired position for either retrieving or deploying more storage units. As explained above with reference to  FIG. 11 , the transducer  13  indicates when the vessel  14  has reached the correct position, i.e. above the next storage unit to be retrieved, or above an empty slot on the sea bed where a storage unit is to be deployed. 
         [0062]      FIG. 20  illustrates how several surface vessels  14  can deploy and retrieve storage units  15  to and from a seabed depot  30 ′ on the seabed B. Such a seabed depot  30 ′ can simply be a designated area on the seabed B, a concrete slab or similar to facilitate storing of several storage units  15  thereupon, or a seabed depot as described with reference to  FIG. 13  with trawl protection, optional roof, etc. The seabed depot  30 ′ can cover a relatively large area, in order to allow several surface vessels  14  to operate simultaneously. The storage units  15  can either be arranged such that units with a certain content is located at one specific area on the seabed B, or the transponders  53 ′ mounted on the storage units can provide information on what the storage units contain, as explained above with reference to  FIG. 18 . Systems where several storage containers are connected together, in order to allow for more efficient retrieving and deploying, is also possible. Surface vessels with different objectives can collect their desired storage unit(s) from the seabed, and return the storage unit(s) when they are done. 
         [0063]    In order to provide efficient transport and utilization of the equipment in the storage units  15 , one or more subsea supply vessels  14  can transport storage units with new or serviced equipment from an onshore location to the seabed depot  30 ′. When other vessels have identified and located the storage units  15 , the vessels can retrieve the storage units and the content of the storage units can be utilized. If expedient, the content of the subsea storage unit can be replaced by used or damaged equipment, and the subsea storage unit  15  can be transported back to an onshore location. If the storage unit is not transported directly back to the onshore location, it can be deployed and stored on the seabed facility ( 30 ′) until a vessel hauls it and transports it back to the onshore location.