Patent Application
20140301790
Many subsea petroleum production activities require the use of chemicals or mud to be added to the active operation to properly operate. Historically, these chemical provisions have been provided through hoses, tubes or pipes bundled into “umbilicals” to supply the chemicals from nearby surface facilities to the respective points of injection. Longer offsets, remote locations and deeper water depths contribute to making umbilical solutions expensive.
Existing subsea chemical storage in use today may be used for short-term single purpose use and have relative small volumes. For example, a number of bladder style chemical storage tanks have been developed for this purpose. Existing subsea chemical storage assemblies may include single wall flexible tanks or bladders that are exposed directly to sea, which may be contained within some cage or frame for protection and transportation. However, the sizes of these storage tanks are relatively small (hundreds of gallons). Additionally, the application use subsea is typically short term (days).
In one aspect, embodiments of the present disclosure relate to a liquid storage tank that includes an outer container, wherein the outer container is rigid, and at least two inner containers disposed within the outer container, wherein the at least two inner containers include a first inner container containing seawater and a second inner container containing at least one stored liquid, wherein the at least two inner containers are flexible, wherein the at least two inner containers are pressure balanced, and wherein the volume of the outer container remains fixed, and the volumes of the at least two inner containers are variable.
In another aspect, embodiments of the present disclosure relate to a method of providing chemicals to a sea floor installation that includes providing a storage tank in a subsea environment, wherein the storage tank has an outer container and at least two inner containers disposed within the outer container, wherein the at least two inner containers include a first inner container containing seawater and a second inner container containing at least one chemical, wherein the at least two inner containers are flexible, wherein the at least two inner containers are pressure balanced, and wherein the volume of the outer container remains fixed, and the volumes of the at least two inner containers are variable.
This summary is provided to introduce a selection of concepts that are further described below in the detailed description. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in limiting the scope of the claimed subject matter.
Embodiments of the present disclosure relate to large volume storage tanks. For example, embodiments of the present disclosure may relate to liquid storage tanks that include a rigid outer container and at least two flexible inner containers disposed within the outer container, wherein the at least two inner containers include a first inner container containing seawater and a second inner container containing at least one stored liquid, wherein the at least two inner containers are pressure balanced, and wherein the volume of the outer container remains fixed, and the volumes of the at least two inner containers are variable.
Referring to
The outer container 110 may be of any shape and made of any material. For example, the outer container 110 may be a metallic construction and integrated within a larger structure. Further, the outer container 110 may be a size that is large enough to contain at least two inner containers. For example, an outer container may be large enough to contain two or more flexible inner containers that are capable of storing an amount of liquid sufficient for use for a long duration, such as between resupply operations. According to some embodiments, each of the at least two inner containers may be filled to a volume ranging up to 5,000 barrels. In such embodiments, the outer container may have a size large enough to contain the at least two inner containers. In some embodiments, each of the at least two inner containers may be filled to a volume ranging up to 3,000 barrels. In some embodiments, each of the at least two inner containers may be filled to a volume ranging up to 1,000 barrels. Further, in some embodiments, more than two flexible inner containers may be housed within a rigid outer container. For example, six or more flexible inner containers that may each be filled to a volume of up to 1,000 barrels may be housed within a rigid outer container. Other amounts of flexible inner containers, each capable of storing large amounts of liquid, may be contained within a rigid outer container. Further, at least two inner containers of the present disclosure may be capable of storing equal volumes of liquid, or at least two inner containers housed within an outer container may be capable of storing different volumes of liquid. For example, an outer container may contain at least three inner containers, wherein a first inner container is capable of storing a larger volume of liquid than the at least two other inner containers, and wherein the at least two other inner containers may be connected together in series or in parallel to achieve a total working volume. It is within the scope of this disclosure that two or more inner containers may be connected together in series or in parallel to achieve a desired working volume. Further, according to some embodiments, two or more rigid outer containers may be connected together to become part of a multi-unit structure. For example, a barge having multiple separate holds may form a multi-unit structure, wherein each hold forms a rigid outer container of the present disclosure connected to each other.
Further, the volume of the outer container 110 remains fixed, and the volumes of the at least two inner containers 120, 130 are variable. For example, while the stored liquid may be added or removed from the second inner container 120 through a controlled opening 125 (and increase or decrease the respective volume of the second inner container 120) and a corresponding volume of seawater may outflow or inflow from the first inner container 130 through a controlled opening 135 (and decrease or increase the respective volume of the first inner container 130), the size and volume of the rigid outer container 110 remains fixed. A barrier fluid may be disposed between the annular space 140 between the outer container 110 and the inner containers 120, 130. The barrier fluid may be monitored for contamination, such as contamination from a leak in one of the inner containers. For example, the barrier fluid may be monitored by disposing sensors within the annular space 140 between the outer container 110 and the inner containers 120, 130, or barrier fluid samples may be periodically collected and analyzed on a periodic basis. According to embodiments of the present disclosure, a storage tank may include at least one sensor disposed in the space between the outer container and the at least two inner containers. Sensors may be used in the storage tank, for example, to monitor contamination of the barrier fluid, as discussed above, to monitor the volumes of the at least two inner containers, to monitor temperature and/or pressure conditions, or to monitor other conditions of the storage tank.
According to embodiments of the present disclosure, the active volume of fluid in each inner container may be monitored by measuring the inner container's relative location to either the topside 112 or bottom side 114 of the outer container 110. As used herein, “topside” may refer to the side of the referenced component that faces the seawater surface when the component is installed at the sea floor, and “bottom side” may refer to the side of the referenced component that faces the sea floor when the component is installed at the sea floor. In some embodiments, monitoring the active volume of each inner container may be achieved by monitoring the inflow and outflow of the respective inner containers, which may help assure integrity of the storage system as well as provide an indication of the chemical dosing performed from the storage system.
At least one inner container may be filled with a liquid including at least one of chemicals, fuel, hydrocarbons, and muds. As used herein, a “stored liquid” or a “liquid” may refer to liquids other than seawater or gases. For example, various liquids or gases that may be stored in at least one inner container of the present disclosure may include chemicals expected to be used in subsea production, such as methanol, glycol, diesel, oil, antiagglomerate hydrate inhibitors, Low Dosage hydrate Inhibitors, slops, muds and many other possible liquids or gases. Further, liquids that may be stored in the flexible inner container(s) may include those capable of functioning in deepsea hydrostatic pressure (up to 5,000 psi) and cold deepsea temperature (˜34° F.), while also maintaining the flexibility of the inner container.
Liquids stored in inner containers of the present disclosure may have a lower density than the surrounding seawater or may have a higher density than the surrounding seawater. For example, the density of a stored liquid that includes drilling mud may vary from a specific gravity of about 0.8 to about 2.0. In embodiments having a second inner container that stores a liquid with a density lower than the seawater filling a first inner container, the second inner container may be stacked on top of the first inner container. In embodiments having a second inner container that stores a liquid with a density higher than the seawater filling a first inner container, the first inner container may be stacked on top of the second inner container. For example, as shown in
According to embodiments of the present disclosure, a metering system may connect an inner container having a stored liquid therein to a subsea point of consumption. For example, as shown in
Storage tanks of the present disclosure may have at least one inner container maintained with a stored liquid. At least one inner container of a storage tank may be refilled with a liquid by refilling the tank on the seafloor from a surface vessel or by replacing the empty tank and refilling it onshore. For example, according to embodiments of the present disclosure, a method of providing liquid (e.g., chemicals) to a sea floor installation may include providing a storage tank in a subsea environment, wherein the storage tank has an outer container and at least two flexible inner containers disposed within the outer container, including a first inner container containing seawater and a second inner container containing at least one stored liquid, wherein the at least two inner containers are pressure balanced, and wherein the volume of the outer container remains fixed, and the volumes of the at least two inner containers are variable. The liquid may be, for example, injected into a wellbore through a controlled opening, such as an outflow valve, in the second inner container, provided through a downline from the seaborne vessel to the second inner container of the storage tank, or refilled into the second inner container after the storage tank has been hoisted from the sea.
Referring now to
For example, a downline 220 may be provided from a seaborne vessel 230 to the second inner container, wherein the downline includes a refill nozzle 240 connecting the downline 220 to the storage tank 200 and a pressure control valve positioned at the refill nozzle 240. The pressure control valve may control the downline outlet pressure to a maximum differential over the ambient hydrostatic pressure from the surrounding subsea environment. By controlling the downline outlet pressure to a maximum differential over the ambient hydrostatic pressure, the pressure control valve may prevent over-pressurization of the storage tank during refill operations. For example, the pressure control valve may control the downline outlet pressure to a differential pressure of less than about 20 psi, and less than 10 psi in some embodiments.
Referring still to
According to embodiments of the present disclosure, a method of providing a storage tank to the sea floor may include lowering the storage tank to the sea floor using at least one variable buoyancy chamber disposed along at least one wall of the storage tank. For example, referring to
Further, storage tanks of the present disclosure may be floated at the surface of the sea for towing to and from the shore. For example, according to embodiments of the present disclosure, a storage tank may be larger than 3,000 barrels, larger than 5,000 barrels in some embodiments, and larger than 8,000 barrels in yet other embodiments. The storage tank may contain volumes in the disclosed ranges using either a single flexible second inner container (for holding the stored liquid), or multiple flexible second inner containers connected together in series or in parallel to achieve the desired total working volume. Further, as described above, a storage tank of the present disclosure may include one rigid outer container (holding at least two flexible inner containers) or multiple rigid outer containers (each holding at least two flexible inner containers) connected to each other. The total volume of the storage tank (including the rigid outer container and at least two flexible inner containers) may range from greater than 3,000 barrels to a volume small enough to fit under a hoisting device and/or small enough for ROVs to maneuver the structure into its desired location on the seafloor. Such storage tanks may also have a high weight, and thus, support vessels may have inadequate crane capacity to lift the storage tank into or from the water. According to embodiments of the present disclosure, a storage tank may be hoisted towards the surface of the sea from the sea floor by releasing the water from the buoyancy chambers to float the storage tank.
According to embodiments of the present disclosure, a storage tank may be shaped to act as a barge or other seaborne vessel with an internal cargo hold containing at least two flexible inner containers. The storage tank may include a bow for towing and/or double-sided walls to minimize consequences if a collision occurs during towing. Double-sided walls of a storage tank may also be used for buoyancy in floating the storage tank during towing and transit, which may subsequently be flooded when the tank is fully submersed. Further, in some embodiments, a storage tank shaped as a seaborne vessel may be subdivided into smaller compartments for containing and segregating multiple flexible inner containers filled with at least one type of chemical or for greater chemical storage volume.
The amount of rigging used to transition from a storage tank towing bridle to a rigging used to lower the storage tank to the seafloor on an active heave compensated lift line may be minimized. For example, a hinged towing bridle may be used at the bow of a storage tank. In some embodiments, a post may be braced at the center of a storage tank wherein the post has a connection profile on top of the post (at the end most distal from the storage tank) for a rapid connect/ROV release connector for attachment of the lifting line suspended from a workboat crane. A towing vessel may pull the storage tank alongside the crane equipped workboat (i.e., a two vessel operation), wherein the crane is attached to the top of the post for tank submergence and lowering to the seafloor.
As discussed above, high pressure buoyancy may be disposed along the topside of a storage tank according to embodiments of the present disclosure. By adding buoyancy chambers along the topside of a storage tank, the buoyancy may be provided above the center of gravity of the storage tank, and thus, the load may be stable when suspended from a lift line. The buoyancy chambers may reduce the submerged weight of the storage tank system such that a readily available crane or winch on a workboat with an ROV may be capable of lowering the tank to the seafloor, positioning and hooking up the storage tank system. The crane or winch used to maneuver the storage tank may be actively motion compensated to minimize the added mass loads due to the support vessel heaving. Buoyancy chambers may be provided in various forms. For example, fixed solid buoyancy rated for the working depth or a composite pipe capped and securely racked at the top of the storage tank may be used. The buoyancy pipe may be sized (diameter and wall thickness) to appropriately resist collapse pressures at the storage tank's operating depth while also providing the required amount of buoyancy. A buoyancy pipe may also be used as a compressed air storage volume. For example, once a storage tank is lifted from the seafloor to a near-surface location (e.g., during a storage tank replacement operation) the air from the buoyancy pipe may be released into the variable buoyancy spaces within the structure of the storage tank to deballast these spaces and prepare the storage tank for surface towing. Using a fixed buoyancy pipe as compressed air storage may eliminate the need to connect an air hose or a water pump to deballast the sidewall tanks upon its return to surface.
Further, a storage tank may be fitted with piping and compartments to house and protect the chemical injection pump and meter components that route the chemicals (or other liquid other than seawater) through high pressure hoses or tubes to their injection points. In some embodiments, the injection pump and related components may be returned with the storage tank, and thus may be routinely maintained along with the storage tank. In some embodiments, the injection pump and metering components may be separately located on a module that is independently maintained.
Depending upon the chemical dosing rate and the application, both the piping and injection pump may be appropriately sized, or if the chemical (or other liquid) injection is into a sub-hydrostatic environment, then a throttling valve and metering system may also be used. A control pod may control injection pumps and to monitor any sensors monitoring the operation of the storage tank and the metering system. The control pod may interface into the production control system using standard protocols. Further, a flying lead for power, data and command communications may be deployed from the storage tank to the subsea electrical connection point. The control pod, pump and metering system may be located onboard the storage tank or it may be separately positioned in the production system. Lockers for flying leads (both electrical and chemical) may be located on the storage tank, which may manage the flying leads during tank deployment and recovery. A locker may be optimized for ROV operation. A flying lead deployment mechanism may also facilitate the efficient recovery of flying leads in the event the storage tank is changed out.
Storage tanks of the present disclosure may be ballasted to sink below the surface of the sea, which in some cases, may include submersing the storage tank below waves at the sea surface. According to some embodiments, columns may be attached to each corner of a storage tank. Columns may vary in size and shape, but may include, for example a height ranging from 10 to 15 feet. The columns may provide semisubmersible performance and motion control during ballast down operations until the tops of the columns submerge, which may also provide for storage tank stability in the near surface wave environment.
Seafloor environments may vary, for example, the seafloor may be firm and compacted (on which a storage tank may be directly placed), or the seafloor may be soft (on which a storage tank may be placed on an intermediate foundation placed on the seafloor, such as a concrete mudmat). According to embodiments of the present disclosure, a suction pile foundation may be installed on the seafloor and then a storage tank of the present disclosure may be placed on the suction pile foundation. A suction pile foundation may provide hard spot landing points that are suitably reinforced to support the weight of the storage tank system. A foundation may also feature alignment posts (e.g., having at least two different heights) to capture matching funnels and sleeves built into a storage tank. The posts and funnels may assure proper location, alignment and orientation of the storage tank with respect to the rest of the subsea production system and equipment. A storage tank of the present disclosure may be maneuvered using a combination of the surface vessel positioning and the monitoring and maneuvering provided by at least one ROV. Further, there may be some constraints imposed by higher seafloor currents (and available ROV power), and thus, landing the storage tank may depend upon performing the operation during the cyclic low current time periods.
According to some embodiments, a skirt may be added to the bottom side of the storage tank to prevent its shifting. The skirt may be segregated into sections with piping to the topside of the storage tank, which may enable an ROV to dock and pump water into the skirt spaces under the storage tank to minimize any suction loads as the storage tank is lifted from the seafloor during a change-out operation.
Although only a few example embodiments have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the example embodiments without materially departing from embodiments disclosed herein. Accordingly, all such modifications are intended to be included within the scope of this disclosure as defined in the following claims.
