WO2016099287A1 - Support sous-marin - Google Patents

Support sous-marin Download PDF

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Publication number
WO2016099287A1
WO2016099287A1 PCT/NO2015/050250 NO2015050250W WO2016099287A1 WO 2016099287 A1 WO2016099287 A1 WO 2016099287A1 NO 2015050250 W NO2015050250 W NO 2015050250W WO 2016099287 A1 WO2016099287 A1 WO 2016099287A1
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WO
WIPO (PCT)
Prior art keywords
main body
subsea
carrier
fluid
cargo
Prior art date
Application number
PCT/NO2015/050250
Other languages
English (en)
Inventor
Rainer Schramm
Original Assignee
Subhydro As
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Subhydro As filed Critical Subhydro As
Priority to US15/537,695 priority Critical patent/US20180001970A1/en
Publication of WO2016099287A1 publication Critical patent/WO2016099287A1/fr

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Classifications

    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B63—SHIPS OR OTHER WATERBORNE VESSELS; RELATED EQUIPMENT
    • B63B—SHIPS OR OTHER WATERBORNE VESSELS; EQUIPMENT FOR SHIPPING 
    • B63B21/00—Tying-up; Shifting, towing, or pushing equipment; Anchoring
    • B63B21/56—Towing or pushing equipment
    • B63B21/66—Equipment specially adapted for towing underwater objects or vessels, e.g. fairings for tow-cables
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B63—SHIPS OR OTHER WATERBORNE VESSELS; RELATED EQUIPMENT
    • B63B—SHIPS OR OTHER WATERBORNE VESSELS; EQUIPMENT FOR SHIPPING 
    • B63B21/00—Tying-up; Shifting, towing, or pushing equipment; Anchoring
    • B63B21/56—Towing or pushing equipment
    • B63B21/66—Equipment specially adapted for towing underwater objects or vessels, e.g. fairings for tow-cables
    • B63B21/663—Fairings
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B63—SHIPS OR OTHER WATERBORNE VESSELS; RELATED EQUIPMENT
    • B63B—SHIPS OR OTHER WATERBORNE VESSELS; EQUIPMENT FOR SHIPPING 
    • B63B3/00—Hulls characterised by their structure or component parts
    • B63B3/13—Hulls built to withstand hydrostatic pressure when fully submerged, e.g. submarine hulls
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B63—SHIPS OR OTHER WATERBORNE VESSELS; RELATED EQUIPMENT
    • B63G—OFFENSIVE OR DEFENSIVE ARRANGEMENTS ON VESSELS; MINE-LAYING; MINE-SWEEPING; SUBMARINES; AIRCRAFT CARRIERS
    • B63G8/00—Underwater vessels, e.g. submarines; Equipment specially adapted therefor
    • B63G8/14—Control of attitude or depth
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B63—SHIPS OR OTHER WATERBORNE VESSELS; RELATED EQUIPMENT
    • B63G—OFFENSIVE OR DEFENSIVE ARRANGEMENTS ON VESSELS; MINE-LAYING; MINE-SWEEPING; SUBMARINES; AIRCRAFT CARRIERS
    • B63G8/00—Underwater vessels, e.g. submarines; Equipment specially adapted therefor
    • B63G8/14—Control of attitude or depth
    • B63G8/22—Adjustment of buoyancy by water ballasting; Emptying equipment for ballast tanks
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B63—SHIPS OR OTHER WATERBORNE VESSELS; RELATED EQUIPMENT
    • B63G—OFFENSIVE OR DEFENSIVE ARRANGEMENTS ON VESSELS; MINE-LAYING; MINE-SWEEPING; SUBMARINES; AIRCRAFT CARRIERS
    • B63G8/00—Underwater vessels, e.g. submarines; Equipment specially adapted therefor
    • B63G8/42—Towed underwater vessels
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B63—SHIPS OR OTHER WATERBORNE VESSELS; RELATED EQUIPMENT
    • B63G—OFFENSIVE OR DEFENSIVE ARRANGEMENTS ON VESSELS; MINE-LAYING; MINE-SWEEPING; SUBMARINES; AIRCRAFT CARRIERS
    • B63G8/00—Underwater vessels, e.g. submarines; Equipment specially adapted therefor
    • B63G8/42—Towed underwater vessels
    • B63G2008/425—Towed underwater vessels for transporting cargo, e.g. submersible barges for fluid cargo

Definitions

  • the present invention relates to a subsea carrier for a fluid, e.g. oil or compressed natural gas.
  • a fluid e.g. oil or compressed natural gas.
  • a mixture of hydrocarbons, water and other substances e.g. H 2 S
  • Temperature and pressure is controlled and chemicals may be added, e.g. to prevent or inhibit formation of hydrates and/or scaling.
  • An initial processing may be performed on the field, e.g. to remove some or most of the water, sand and corrosive substances like H 2 S.
  • the hydrocarbons are transported downstream toward a final destination, e.g. a refinery for cracking the mixture into individual components, a chemical factory for further processing or a consumer for heating.
  • keeping the hydrocarbons, for example NG at a pressure of a few hundred bar from production to a loading point and further during transport may save energy and costs compared to
  • transport is defined as bringing the hydrocarbons from a loading point, where all pre-processing has been performed, to an unloading point where no post-processing has commenced.
  • Transport of hydrocarbons in this sense includes transport by pipeline and transport by carrier, each with known advantages and disadvantages. If carrier transport is preferred over pipeline transport, a comparison of alternative carriers remains.
  • a first example concerns transport of natural gas.
  • LNG liquefied natural gas
  • CNG compressed natural gas
  • a CNG- carrier would need to have a little less than half the investment and operational costs compared to an LNG-carrier with the same loading volume, e.g. 150 000 m , in order to be cost competitive in transporting a given mass of NG.
  • a real comparison must also account for the speed of transport, cost of plants for cooling and evaporating LNG vs.
  • a second example concerns carrier size.
  • a Panamax-carrier able to pass through the Panama channel may be preferred for one application, whereas a VLCC (very large crude carrier) may be preferred for another.
  • surface gas carriers have different sizes for different applications.
  • subsea carriers for hydrocarbons will have different sizes depending on the cargo, the distance and other factors similar to those determining the favourable size of a surface carrier.
  • CA2028273A1 describes an unmanned subsea carrier made of concrete, wherein the external pressure from ambient water substantially compensates for an internal pressure.
  • the carrier is loaded at a subsea loading point and unloaded at an unloading point.
  • the loading and unloading points are preferably placed on solid rock due to the weight of the carrier.
  • a non-limiting example concerns a subsea carrier with a loading volume of 150 000 m .
  • This size is selected partly because it corresponds to a typical surface LNG-carrier, and partly to illustrate that subsea equipment does not scale linearly.
  • handling 200-300 bar in a 10 litre scuba tank is hardly comparable to handling 150 000 m CNG at 25 MPa (250 bar), and propelling and steering a subsea carrier of such a size is not directly comparable to propelling and steering a submarine a fraction of the size.
  • the size and required speed may render a traditional submarine design too expensive for a subsea carrier competing with a surface crude carrier or LNG-carrier.
  • the Typhoon class (Russian "Akuna” or “Shark")
  • the missile launchers are replaced with cargo holds for up to 15 000 tonnes of cargo.
  • a subsea carrier with cargo volume for 150 000 m of CNG at 24 MPa may have a displacement of 190 000 tonnes, i.e. about four times the size and mass of a Typhoon.
  • a smaller velocity can compensate for control challenges resulting from a very large mass to some extent, preferably to a level where affordable, propulsion, ballast pumps and control surfaces can handle the momentum.
  • an economical transport requires a certain minimum speed or forward velocity, e.g. about 2.5 m/s (-3-5 knots).
  • depth control, steering and changes in forward speed are changes in momentum, and hence equal to a sum of impulses, e.g. a as an integral of a varying force F(t) over a response time, or equivalently as a constant mean force F applied over the response time. Accordingly, the force required to alter momentum at a given velocity increases with increasing mass and decreasing response time. Other factors must also be considered.
  • Depth control serves as an example.
  • a submarine moving forward at 5 m/s at an upward angle of 2° This submarine would raise about 50 m in five minutes.
  • the submarine must be able to change its momentum, in particular the direction of the velocity more than 2°, in a response time of five minutes to maintain a depth range within a set depth +50 m.
  • a subsea carrier with four times the mass and the same speed than those of the submarine would have four times the momentum, and hence require four times the control force to perform the same change of velocity in the response time of five minutes. This can be done by increasing the area of fins and control surfaces, angle of attack; hydrofoil profiles etc. all of which increase the drag, and thereby cost of operation.
  • a large carrier is likely to be used for long distance transport, e.g. 5 000 -10 000 km. The extra work caused by a larger drag over a long distance requires fuel, so the added operating costs may become considerable.
  • combustion engine at a depth of 2 500 m for years might not be considered for practical reasons, and might be found too expensive even if it was considered.
  • alternatives to a combustion engine are known for a military submarine, e.g. air independent propulsion systems such as nuclear power plants, chemical batteries, fuel cells or Stirling engines, such alternative energy sources are generally not available, unsuitable and/or too expensive for a large commercial subsea carrier, and are not further described herein.
  • an objective of the present invention is to provide an improved subsea carrier solving at least one of the problems above while retaining the benefits of prior art.
  • the invention should enable transporting hydrocarbons and other fluids at a lower cost per unit than what has been previously possible.
  • the present invention relates to a subsea carrier for a fluid.
  • the subsea carrier comprises a main body for containing the fluid at a predetermined internal pressure, wherein the main body is designed to operate at a water depth where the external pressure substantially counteracts the internal pressure.
  • the subsea carrier further comprises a floating element connected to the main body by a stabilising cable, wherein the stabilising cable comprises a first rope for transmitting force, and is attached to a first connector that is movable with respect to the main body.
  • the external pressure from ambient water exceeds the internal pressure such that a compressive force acts on the main body.
  • the main body can be made of a brittle and inexpensive material, e.g. concrete as known from prior art.
  • embodiments with a somewhat positive internal differential pressure are also anticipated because handling a limited differential pressure acting on the hull structure, for example 10 bar rather than 250 bar, reduces the design requirements significantly.
  • the term "substantially counteracts the internal pressure” includes embodiments wherein the pressure difference from the inside to the outside causes an economically viable design, for example up to 10% higher than the external pressure. In that case, the hull structure must be able to withstand the tensile stress resulting from an internal pressure greater than the external pressure.
  • the floating element can be located at any depth above the main body, and helps orienting the main body through the stabilising cable. More particularly, the stabilising cable exerts an upward force on an upper part of the main body to reduce roll, i.e. rotation about a longitudinal axis pointing in the direction of travel.
  • the first rope transmits the force acting upward on the main body, and can be a synthetic rope or a steel wire capable of transmitting the buoyancy force from the floating element.
  • the first rope preferably has close-to-neutral buoyancy to avoid undesired vertical forces on rope and the floating element, and would thus be provided with buoyancy elements if a steel wire is selected as the rope.
  • the first connector is primarily movable in a longitudinal direction in order to adjust the pitch of the main body.
  • the pitch should be adjusted such that the main body presents a minimum area perpendicular to the direction of travel at all times in order to minimise drag and thus operational costs. Equally important, e.g. a positive pitch causes the main body to move upward. This is undesired because the external pressure should exceed the internal pressure in order to compress the structure.
  • the first connector may also be movable in a lateral direction in order to control roll, in particular at loading and unloading points.
  • the length of the stabilising cable exceeds the water depth.
  • the floating element floats on the surface, so that if the main body starts rising, the floating element rises out of the water. This reduces the buoyancy and thereby causes the main body to sink due to gravity. Opposite, if the main body starts sinking, an increased volume of the floating element is submerged. This increases the buoyancy and causes the main body to rise due to buoyancy transferred through the stabilising cable.
  • a preferred embodiment comprises a bridle for distributing a towing and lifting force.
  • the bridle is preferably a length of steel wire running over two pulleys attached to the main body at two separate attachment points, e.g. one behind the other.
  • a connector attached to the steel wire can be moved along the main body by turning the pulleys in order to adjust the pitch.
  • Similar pulleys can be employed to reduce roll of desired.
  • any other actuator may be employed to change the geometry of the bridle and to introduce forces to control pitch or roll of the main body.
  • the main body comprises a ballast element connected under a tank element.
  • a suitable ballast material could be, for example, gravel of magnetite, i.e. iron ore, due to its relatively low cost and high density. In some applications, such ore could also be used in the walls of the main body.
  • the main body comprises a first cargo compartment separated from a second cargo compartment by a movable, gas-tight sealing element.
  • the main purpose of the gas-tight sealing element is to prevent gas in the first cargo compartment from dissolving in water within the second cargo compartment.
  • the gas-tight sealing element is a flexible membrane.
  • other sealing elements are anticipated.
  • one or more pipes could be placed within the tank volume, each pipe having one end open to the cargo compartment, the opposite end open to the ballast compartment and an axially movable piston.
  • Several steel tanks known in the art, e.g. of a kind used in surface crude carriers, enclosed in a shell of concrete might still be less expensive than a surface carrier.
  • the subsea carrier preferably has at least one ballast tank for trimming and depth control.
  • the buoyancy of the main body is close to neutral, i.e. that a transport mass is close to the displacement of the main body.
  • Ballast water is pumped into or out of the ballast tanks to adjust the transport mass to the actual displacement as known in the art.
  • the ballast tanks can e.g.be made of relatively inexpensive steel pipes, and they can be oriented parallel to a length axis though the main body to help control roll. Separate ballast tanks at a front end and a rear end of the main body can be used to control pitch and depth.
  • the main body comprises a control surface and/or a thruster for causing rotation of the main body about at least one of tree mutually perpendicular axes of rotation.
  • Control surfaces and/or thrusters can replace or supplement the ballast tanks for trimming and depth control, and they need not be used actively for the entire journey. For example, the bottom must be down at the loading and unloading points, but roll does not matter during a journey.
  • ballast tanks can be used for depth control if the main body halts for some reason, whereas relatively small control surfaces can keep the main body within an allowed range of depths as long as the main body moves through the water. The actual choice of means for placing and orienting the main body within a body of water depends on the mass, response time etc. as discussed above.
  • the main body comprises at least one thruster for causing translation of the main body along at least one of tree mutually perpendicular axes.
  • the main body does not need towing, but can be towing the floating element.
  • Such thrusters can also lift the main body and/or shift the main body to one side with or without a rotation in any plane.
  • the invention concerns a system for transporting a fluid comprising a subsea carrier as disclosed above, further comprising: a communication line for conveying control related signals between a controller at the surface and the main body and a power line supplying electrical power from an electric generator at the surface to the main body.
  • the communication line facilitates transmitting sensor data from the main body to a controller at the surface, and appropriate steering commands in the opposite direction.
  • the communication line is preferably an electric conductor or a fibre-optic cable, as acoustic signalling through, for example, 2 500 m of water with varying salinity is expected to attenuate and/or distort the acoustic signal to a degree that the communication would be unreliable or inefficient due to low signal/noise ratios.
  • the power line enables an electric generator powered by a combustion engine at the surface, where air is readily available from the atmosphere, and one or more electric motors for driving steering devices on the main body, where the orientation of the main body is controlled most efficiently.
  • the power line preferably comprises an electrically insulated metal wire, e.g. made of aluminium for low loss and low cost, provided with floating elements to make the power line neutrally buoyant.
  • Such a power line may be similar to dynamic cables used in the subsea oil & gas industry and known in the art.
  • the system further comprises a surface vessel with the controller and the electric generator, and a control cable with the communication line and the power line.
  • the control cable is preferably provided with a fairing in order to reduce drag.
  • An embodiment without a towing cable would require a thruster for propulsion on the main body.
  • the control cable in the first embodiment of the system also comprises a second rope for transmitting force, i.e. such that the surface vessel tows the subsea carrier.
  • the second rope can be, for example, a synthetic rope or a steel wire like the first rope transferring forces between the main body and the floating element.
  • the second rope tows the main body in addition to the floating element, so the second rope must be able to sustain greater tension than the first rope.
  • a steel wire might be preferred over a synthetic rope for the second cable because the synthetic rope is more elastic than the steel wire, and thus would apply a towing force on the main body varying over a greater range of elastic strain than a steel wire.
  • the second rope is preferably included in a fairing together with the communication line and the power line.
  • control cable further comprises a fuel line from the main body to the surface vessel.
  • hydrocarbons from the main body may feed a combustion engine driving the electric generator, and possibly the propulsion motor for the surface vessel.
  • a thruster for propulsion on the main body is mandatory.
  • the floating element comprises the controller and the electric generator, and the stabilising cable comprises the communication line and the power line.
  • the stabilising cable is preferably equipped similar to the control cable of the first embodiment, in particular provided with a fairing and floating elements to make it neutrally buoyant.
  • the stabilising cable may comprise a fuel line from the main body to the floating element.
  • the invention concerns a method for operating a subsea carrier with a first cargo compartment and a second cargo compartment.
  • the method comprises the steps of: filling the first cargo compartment at a subsea loading point with a cargo fluid at the predetermined internal pressure; transporting the cargo fluid from the loading point to a subsea unloading point; expelling the cargo fluid from the first cargo compartment; filling the first cargo compartment at the unloading point with a return fluid at a second internal pressure; transporting the return fluid from the unloading point to the loading point.
  • the method is distinguished in that expelling the cargo fluid involves allowing ambient water to flow into the second cargo compartment; and filling the first cargo compartment at the unloading point with a return fluid involves pumping water out from the second cargo compartment.
  • the cargo fluid can be any fluid defined in the introduction, e.g. CNG from a subterranean reservoir.
  • the return fluid can be payload, e.g. N 2 or C0 2 intended for injection into a subterranean structure, e.g. for pressure support of the reservoir or for deposit in an aquifer or depleted reservoir.
  • the water pump can be relatively inexpensive.
  • the cargo fluid is CNG at a predetermined internal pressure 240 MPa
  • the external pressure is 250 bar
  • the return fluid is dry air.
  • a certain volume of CNG e.g. 150 000 m
  • the first cargo compartment should be completely void of CNG before air is loaded for the return journey.
  • water at 250 bar flows into the second cargo
  • the dry air for the return journey should also have a mass of 33 900 tonnes in order to achieve close to neutral buoyancy. This means the air must have a second internal pressure of 206 MPa. A pressure of 206 bar cannot expel water at 250 bar, so a pump handling a pressure difference of 44 bar is required. The pump(s) must also have a throughput that displaces 150 000 m , of water in a reasonable time.
  • the subsea loading point and/or the subsea unloading point comprises a subsea platform serving as a foundation for the main body during loading and/or unloading.
  • the step of expelling the cargo fluid involves allowing the water to flow from a storage tank in the platform, and the step of filling the first cargo compartment with a return fluid involves pumping the water back into the storage tank.
  • a main body arrives with near neutral buoyancy and settles on the platform.
  • the combined mass of the main body and platform does not change during loading or unloading.
  • the main body leaves the platform with near neutral buoyancy.
  • the weight working downward from the platform changes very little during loading and unloading.
  • the platform can be provided with near neutral buoyancy when no main body is present, thereby further reducing the ground pressure. It can even hover above the seabed, e,g, over soft soil or rough surfaces.
  • a further benefit of the closed loop for water is that contaminated water is not released into the environment at the loading and/or unloading point.
  • Fig. 1 illustrates a first embodiment and some important forces acting on the system
  • Fig. 2 illustrates a second embodiment
  • Fig. 3 illustrates a third embodiment
  • Fig. 4 illustrates a fairing with different components comprised in a cable
  • Fig. 5 illustrates the fluid connections to a main body
  • Fig. 6 illustrates schematically a segment of the main body
  • Figs 7a - c illustrate a first method of ballasting
  • Figs 8a - c illustrate a second method of ballasting
  • Fig. 9 illustrates the main rotational axes of the main body
  • Fig. 10 illustrates loading and unloading at a subsea platform
  • Fig. l la-c illustrates loading and unloading at a subsea platform in greater detail.
  • An important aim of the invention is to carry a cargo of fluid, e.g. hydrocarbons, from a loading point to an unloading point along a predefined path as inexpensively as possible.
  • a cargo of fluid e.g. hydrocarbons
  • this implies keeping an elongated main body 101 aligned with a predetermined path in three dimensions fixed to the Earth, e.g. longitude, latitude and depth, by controlling local coordinates fixed to the main body 101, e.g. roll, pitch and yaw as illustrated with reference to figure 9.
  • Figure 1 illustrates a first embodiment of a system according to the invention, comprising a subsea carrier 100 partly submerged in a body of water 1 having a surface 2.
  • a towing vessel 3 on the surface 2 tows the subsea carrier 100 by means of a towing cable 32, in the claims expressed as a control cable with a second rope.
  • the subsea carrier 100 comprises a main body 101 connected to a floating element 102 by a stabilising cable 132.
  • the floating element 102 is at the surface 2 for passive depth control of the main body 101.
  • the floating element 102 starts to come out of the water 1, which immediately decreases a buoyancy B acting on the floating element 102, and hence decreases the upward pull from cable 132 on the main body 101.
  • the reduced pull upward on the main body 101 causes a net vertical force acting downward, such that the main body 101 sinks.
  • an increasing part of the floating element 102 comes below the surface 2, which immediately increases the buoyancy B such that the main body 101 moves toward the surface 2 of the body of water 1.
  • a main body 101 could have a loading volume of 150 000 m and an internal pressure suitable for CNG, e.g. in the order of 250 bar or 25 MPa.
  • the external pressure acting on the main body 101 should be somewhat larger in order to ensure a limited, compressive force on the walls of the main body 101.
  • 250 bar corresponds to a depth of water of 2 500 m., such that a suitable towing depth would be 2 500 m + 50 m if the differential pressure should be kept within limits 10 bar (-100 m of water depth) apart.
  • the stabilising cable 132 might be 2.5-3 km long and the towing cable 32 somewhat longer.
  • FIG. 1 Some important forces working on the subsea carrier 100 are shown on figure 1. These are a towing force T, gravity or weight G, a resistive drag R acting on the main body 101, a drag L acting on the stabilising cable 132 and the buoyancy B. For simplicity, a towing force acting on the main body from the stabilising cable 132 and a drag acting on the towing cable 32 are not shown.
  • the towing force T acts along the towing cable 32, and may be decomposed into a vertical component T v and a horizontal component 7 3 ⁇ 4 .
  • the vector sum of forces in the vertical direction may be varied around zero to provide controllable depth adjustments.
  • the cables 32, 132 should preferably have approximately neutral buoyancy to minimize their effect on the vertical force balance.
  • FIG. 1 A dynamic depth control using, for example, thrusters, ballast tanks and/or fins with control surfaces is anticipated. However, use of active elements should be kept at a minimum to minimise operational costs.
  • the embodiment in Fig. 1 provides an inexpensive, passive depth control. It also provides a fail-safe behaviour in the case of power failure, preventing the main body from uncontrolled sinking or rising without electrical controls.
  • the force L acting on the cables 32, 132 generally have different directions and magnitudes at different depths due to their own motion during towing and different currents in the body of water 1.
  • the horizontal component T h of the towing force must overcome the resistance or drag R on the main body and a horizontal drag component of the force L acting on the cables 32 and 132.
  • the drag R on the main body 101 can be modelled as a sum of pressure drag and friction drag, both of which increases with the towing speed. At an assumed towing speed around 5 knots or 2.5 m/s and a substantially cylindrical form, the friction drag is likely to dominate.
  • the drag component of L acting on the towing cable 32 and the stabilizing cable 132 can be estimated in a similar manner. Due to the length of the cables, their combined cross section and surface areas can be significant compared to the corresponding areas of the main body. Fairings and other techniques known in the art can reduce the drag from the cables.
  • the towing cable 32 is attached at an attachment point 34.
  • the attachment point 34 for a towing cable 32 is preferably close to the centre of gravity of the main body 101, as a long arm from the centre of gravity to the attachment point 34 might provide an undesired permanent pitch or trim.
  • the vertical towing component T v could easily lift the forward end of main body 101, and thus provide a larger cross sectional area during towing. This will also introduce induced lift and induced drag similar to an aircraft wing. In addition, it will create interference drag from vortices forming behind the misaligned body. To compensate, extra trimming ballast in front and/or extra trimming buoyancy aft would be required to keep the main body 101 substantially parallel to the direction of travel.
  • the towing cable 32 is preferably attached to a bridle 340.
  • the bridle 340 comprises a length of rope, e.g. steel wire, and adjusts itself to variations in the actual towing force T.
  • the bridle 340 may also be dynamically adjusted to move the attachment point 34 back and forth along the main body 101.
  • the bridle 340 contributes to avoid a permanent and unwanted pitch as discussed above.
  • the bridle 340 can be attached at four points on the main body 101, and thereby decouple the towing force from rotations around the x -axis in addition to the rotation about the y-axis, i.e. to facilitate control of roll in addition to pitch.
  • the floating element 102 is a vertically elongated buoy at the surface 2, preferably with a height of a few tens of meters. While not shown in the drawings, two or more physical floating elements may be provided. One purpose of the floating element is to provide a stabilising force through the stabilising cable 132 and the attachment point 134.
  • a floating element 102 on the surface 2 is exposed to oscillating forces from surface waves.
  • the shorter wave components rise the sea surface with respect to the floating element 102, which is connected to the heavy main body 101.
  • the shorter waves are transformed to long wave components with smaller amplitudes and longer periods than the original waves.
  • the long wave components are more easily handled by a dynamic control system if required.
  • the stabilising cable 132 is connected to the main body 101 through a connector 134.
  • the connector 134 is similar to the connector 34 for the towing cable. Likewise it is movable on the main body, e.g. to compensate for vertical component of towing force induced when the floating element 102 and cable 132 is towed through the water 1.
  • the connector 134 may also be connected through a bridle (not shown).
  • the floating element 102 can be disposed anywhere between the main body 101 and the surface 2.
  • the main advantage of submersing the floating element 102 is a shorter stabilising cable 132, and hence less and more predictable drag L on the stabilising cable 132.
  • the advantage of better control of orientation, in particular pitch, provided by a movable point of attachment and/or a bridle is retained in this embodiment.
  • sensors and steering means on the main body 101 for depth control are required in this embodiment.
  • the main body 101 is propelled in its longitudinal direction by a thruster 103.
  • the cable 32 is assumed to provide power to the thruster 103, but not to provide a significant towing force. If the cable 32 does not impose any significant force on the main body 101, its attachment point 34 can be placed anywhere on the main body 101, including in the front as shown, without adverse consequences.
  • Fig. 3 shows yet another embodiment, wherein the subsea carrier 100 is self- propelled and controlled by means of a radio link 105.
  • a floating element 102 near the surface 2 contains a conventional generator 4 comprising a combustion motor coupled to an electric generator.
  • the fuel required in the combustion motor e.g. natural gas
  • the oxygen required for combustion can be supplied as ambient air through a snorkel 104.
  • Thruster 103 receives electrical power through the stabilizing cable 132, and propels the main body 101 in a forward direction, i.e. provides a force F acting on the main body 101 in the longitudinal direction termed x in figure 9.
  • the embodiment on figure 3 may be suitable for the same or different sized carriers than the embodiments on figures 1 or 2.
  • sensors recording position, direction and orientation of the main body 101 at certain points in time transmit the relevant data by means of the radio link 105 to a remote controller (not shown).
  • the controller (not shown) compares the data to a planned course and depth, and returns appropriate steering commands though the radio link 105.
  • These steering commands are used to adjust the speed, course and/or depth of the main body 101 using suitable steering means.
  • An autonomous control without a radio link, based on a pre-programmed trajectory, is also possible for some or all parts of the operation.
  • the bridle 134 limits a misalignment of the main body 101 that would increase the drag and the risk for an undesired ascent.
  • the reduced changes of momentum, i.e. directional deviation from a desired trajectory can be handled by relatively small dynamic control surfaces, further limiting drag and providing other benefits such as an ability to dampen control oscillations.
  • the need for an active control system can be further reduced by using a passive system for depth control, e.g. a buoy floating on the surface.
  • a control system is required to limit the changes of momentum, and thereby the force and time required to amend the deviations.
  • accurate and inexpensive sensors e.g. three-dimensional MEMS accelerometers
  • controllers e.g. FPGA or microprocessor based embedded controllers.
  • the actuators e.g. motors for the bridle 134 and dynamic surfaces, the controlling surfaces themselves and any ballast pumps, are important components of investment and operational costs.
  • Fig. 4 schematically illustrates a cable with all the functions of the embodiments in figs 1 - 3. It is understood that a practical cable 32 or 132 may include some or all of the components 320-324 shown in fig. 4.
  • reference numeral 320 refers to a fairing 320 to minimise the effects of drag force L (fig. 1) on the cable. Contrary to the main body 101, there is no need to maximize the volume of fairing 320, which accordingly is shown with a streamlined cross section.
  • a fuel line 323 may convey fuel from the main body lOlto an engine-generator unit 4 as discussed with reference to figure 3.
  • a fuel line 323 may also be present in the cable 32 in figures 1 or 2 if it is desirable to fuel the propulsion system of the vessel 3 with
  • a design using pistons may replace the flexible membrane 115 provided that the sealing element is gas-tight and capable of emptying the first cargo compartment 110 by filling the second cargo compartment 120 with water.
  • a "gas-tight" membrane or sealing element should be construed as having sufficiently low permeability to prevent undesired dissolving of gas in water.
  • a fluid or granular material floateing on the water might provide a barrier between water and gas.
  • Permanent ballast 151 provides close to neutral buoyancy. That is, the permanent ballast 151 has a mass to make the transport mass, i.e. the mass of the main body 101 when fully loaded with cargo fluid, approximately equal to the displacement, i.e. the mass of water displaced by the main body 101. Ore is relatively inexpensive and has a relatively high density, and may thus be a suitable ballast material.
  • the ballast tanks 152 are adapted to contain more or less water to equalize variations in buoyancy, e.g. caused by local variations in density of the ambient water.
  • Elements 510-541 belong to an external network, and are not part of the main body 101. Similar valves, pipes, connections etc. are required to seal off the cargo compartments 110 and 120 during transport. These valves etc. within the main body 101 are not shown for clarity.
  • the first cargo compartment 110 still contains dry air from a return trip, and should be loaded with CNG.
  • the air contains oxygen, and should not be mixed with CNG for safety reasons.
  • all air should be expelled from the first cargo compartment 110 before it is loaded with CNG.
  • the ambient pressure is greater than the internal pressure within the first cargo compartment 110, so the air is conveniently expelled by opening valve 521 until the entire tank volume is filled with water, i.e. such that the membrane 115 engages the upper tank wall and all air is expelled through the outlet 540 for return fluid at the top of tank 140.
  • the mass of the main body is greater than the transport mass defined above.
  • the added mass may be considerable. This is further discussed with reference to figure 10.
  • the main body 101 When the first cargo compartment 110 occupies the entire tank volume and contains a cargo fluid at a predetermined internal pressure, here CNG at 240 bar, the main body 101 has a transport mass close to its displacement as explained above. In this state, the cargo fluid is transported, preferably at depths providing a compressive force on the main body, to an unloading point similar to the generic loading and unloading point shown in figure 5.
  • the next task is to fill the first cargo compartment 110 with dry air for the return journey.
  • air has a different density than CNG, the air pressure must be 206 bar to achieve the transport mass for the return journey in this example.
  • water is pumped out of the second cargo compartment 120 by means of pump 232 while the return fluid, here dry air, is supplied through line 510 or 540 from the surface.
  • lines 510 and 540 can be different lines or combined in a variety of ways.
  • line 510 can receive a cargo fluid, e.g. CNG, in one interval and supply return cargo, e.g. dry air, N 2 or C0 2 in a second interval.
  • line 540 can be a simple outlet for return fluid such as dry air or a line to the surface.
  • the transport can be symmetrical in the sense that the return fluid in one loop is the cargo fluid in an opposite loop using the same two loading and unloading points.
  • a loading point at a platform producing CNG can be the unloading point for C0 2 , e.g. for pressure support of the CNG-producing geologic formation.
  • Fig. 6 is a perspective view of a segment of the main body 101.
  • the main body 101 comprises several such segments mounted end to end, and is closed at each end by a rounded end segment as shown in figs 1-3.
  • the tank 140 is made of precast rings with a wall thickness AD
  • the walls and bottom 153 of the cradle 150 is made of precast concrete elements, e.g. such as the precast elements used for floors in a building.
  • the invention does not exclude a double skin or sandwich structure, e.g. two concentric shells of steel with reinforcing, radial ribs and/or a concrete fill between them.
  • a tank segment made of concrete has substantially lower manufacturing costs, and concrete is therefore used as much as possible in a preferred embodiment.
  • the wall thickness AD has a minimum value required for strength that depends on the particular type of concrete in the wall, for example about 1 m for fibre reinforced ultra-high performance concrete (UHPC).
  • UHPC ultra-high performance concrete
  • the walls may intentionally be made thicker than this minimum value. Concrete walls are brittle in the sense that they break more easily when exposed to shear or tensile stress than when exposed to compressive stress.
  • a main body made of concrete would preferably be operated at a depth where the external water pressure is greater than the internal pressure, e.g. from CNG.
  • the brittleness can be counteracted by known means.
  • the entire main body 101 can be made of reinforced concrete on an assembly site.
  • a structure in the order of 25 m in diameter and 350 m in length is well within the limits of conventional techniques. If a somewhat greater flexibility is desired, several segments such as the one shown in fig. 6 can be joined with elastomeric gaskets in between or sealed permanently with grouted joints , and held together using conventional post tensioning techniques, e.g. with tensioned steel wire in tubing, which may later be filled with mortar or concrete for corrosion protection.
  • the concrete can be reinforced by a conventional steel rebar, or by fibres of steel, polypropylene or any other material as known in the art.
  • the cradle 150 with ballasting elements 151 comprises largely permanent ballast with density larger than the ambient water.
  • a high density material e.g. magnetite or another ore, may be preferred to limit the ballast volume, cross section and skin surface of the main body and hence the drag R according to equation (1).
  • This permanent ballast is provided at the lower part of the segment or main body 101 to lower the centre of gravity, and hence facilitate orientation of the main body 101, in particular at the loading and unloading points where it is important that the cradle 150 is below the tank 140 in order to support its weight.
  • Ballast tanks 152 extend along the outside of the tank 140.
  • the ballast tanks 152 work in a conventional manner, and are essentially used to control buoyancy by pumping water in or out.
  • the pitch may be controlled using ballast tanks in the front and rear ends of the main body 101.
  • ballast tanks 152 can also control roll. However, roll hardly matters during the transport and return journey, so no energy should be spent on pumping water for controlling roll, except perhaps at the loading and unloading points. A similar argument applies to thrusters (not shown) for controlling roll.
  • Figures 7a-c illustrates transport of a fluid with a main body 101 and equipment 521- 541 similar to those on figure 5.
  • Figs. 7a-c illustrate the method referred to in connection with Fig. 5.
  • valves and other equipment within the main body 101 are not shown.
  • no valves appear on figure 7a and 7c, which illustrate transport, and the valves in figure 7b correspond to the external valves shown in figure 5.
  • Seawater has typically a density about 1 025 kg/m 3.
  • a water density 1 000 kg/m 3 is used in some of the examples.
  • Other inaccuracies in the numbers are likely to be bigger than those caused by this approximation.
  • FIG. 7c illustrates the ballasted return journey.
  • the first cargo compartment 110 now contains return fluid 703, and the complete mass of the main body 101 is close to the transport mass.
  • the mass of return fluid 703 should be
  • Fig. 8a illustrates a transport where the first cargo compartment 110 contains cargo fluid 701 at the predetermined internal pressure, e.g. CNG at 240 bar, and the second cargo compartment 120 contains water 703.
  • 150 000 m of dry air at 240 bar has a mass of 46 400 tonnes.
  • the first cargo compartment 110 can contain 30 000 tonnes of CNG at 240 bar
  • the second cargo compartment 120 can contain 16 400 tonnes of water.
  • the external water pressure is 250 bar as before, yielding a net differential pressure at 10 bar working on the outer surface area of the tank during transport.
  • the cargo fluid 701, e.g. CNG is expelled through valve 541 as water 702 enters through valve 511 as explained in connection with figure 7b.
  • FIG. 8c the tank is fully loaded with return fluid 701. That is, the first cargo compartment 110 occupies the entire tank volume.
  • the second internal pressure in Fig. 8c is by presupposition equal to the predetermined internal pressure in Fig. 8a.
  • the predetermined internal pressure and the second internal pressure are 240 bar, and the cargo has a mass 46 400 tonnes in both directions.
  • Figs. 8a-c limits the compressive force acting on the tank. However, about 11% of the tank volume in Fig. 8a is occupied by water 702, and thus unavailable for cargo fluid 701. Furthermore, a slight change in pitch might cause the water 702 to flow toward one end of the tank volume. Thus, the second cargo compartment 120 would require internal bulkheads or the like to maintain the distribution of water 702. In contrast, the method in Figs. 7a-c only involves water in figure 7b, where the main body 101 rests on a horizontal fundament and no significant amount of water can collect in either end.
  • Figure 9 shows a main body 101 with the origin of an imaginary Cartesian coordinate system at its centre of gravity.
  • the x, y and z-axes are fixed with respect to the main body 101, and the terms roll, pitch and yaw are used in their usual meaning, i.e. as rotations about the x, y and z-axis, respectively.
  • the main body can also be translated along these axes, e.g. by means of thrusters.
  • the thruster 103 in Figs. 2 and 3 provides an example of translation along the x-axis.
  • the benefits of active controls e.g. dynamic control surfaces and ballast tanks, are discussed above. Further details regarding submarine control and towing are known in the respective arts.
  • the subsea platform 200 is comprised of a plurality of rectangular tank elements 201, each having an inner cylinder similar to the main body 101.
  • the platform 200 in figure 10 has passive buoyancy control in the form of a plurality of heavy chains 210 hanging from the platform and resting partly on the seafloor 4. If the platform 200 starts rising, chains are lifted from the seafloor and adds weight. If the platform starts sinking, a larger part of the chains comes to rest on the seafloor 4, thereby reducing weight. In both cases, the length of the chains that rest on the seafloor 4 counteract undesired vertical motion.
  • the platform 200 is connected to the surface through a line 220.
  • This line 220 represents the lines 510 and 540 described with reference to figure 5.
  • a connection unit 230 represents the valves, pumps and lines required for the loading and unloading explained with reference to figure 5, 7 and 8.
  • the main body 101 has a near neutral buoyancy during transport, and an increased weight when filled with water to expel cargo fluid or return fluid.
  • the water can be supplied from storage tanks 201 in the platform 200 and be pumped back into the storage tanks 201 in a closed loop.
  • the combined weight of the platform 200 and main body 101 remains unchanged during loading or unloading.
  • the platform 200 supports the extra weight, but provides no extra downward force, e.g. on the ground.
  • this extra weight may be considerable for a large cargo volume, e.g. 120 000 tonnes as provided in an example above.
  • Handling ballast water in a closed loop has the added benefit that the buoyancy and/or flow of water into or out of the main body 101 does not have to be closely monitored during loading or unloading.
  • a further benefit is that any contamination of the water, e.g. due to a rupture in the membrane 115 described above, is kept within the closed loop, and not released into the environment at the loading and unloading points.
  • Figures 1 la-c show an alternative embodiment of the main body 101 and the platform 202.
  • the main body 101 is made of a concrete with higher density than the previous embodiment.
  • the higher density can be achieved by adding a suitable material to the cement, e.g. iron ore.
  • the increased density reduces or eliminates the need for external ballast 151, and hence the nose drag on the main body 101.
  • Fig 11a shows a main body 101 loaded with cargo fluid 701, e.g. CNG.
  • cargo fluid 701 e.g. CNG.
  • Storage tanks 20 in the platform 200 are filled with seawater 702.
  • the tanks 20 are understood to be part of the tank elements 201 referred to above.
  • Fig. 1 lb illustrates pumping seawater 702 from the storage tanks 20 to the main body 101.
  • This displaces cargo fluid 701, which is piped to a region above the water 702 in storage the tanks 20.
  • a membrane or other seal separate the cargo fluid 701 from the water 702 for reasons explained above.
  • the pumping of water 702 continues until the water 702 fills the main body 101 and the cargo fluid 701 fills the storage tanks 20. There is no change in total mass of the main body 101, platform 200, cargo fluid 701 and water 702 during this exchange of cargo fluid 701 and water 702. Hence, the load 41 on the seabed 4 does not change due to this step.
  • Loading the main body 101 with a cargo fluid on another terminal may be performed in a similar manner, i.e. using the water 702 as a piston.
  • the main body 101 and storage tanks 20 should each be completely filled with water 702 before a new fluid 701, 703 is let into the space above the water 702.

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  • Engineering & Computer Science (AREA)
  • Mechanical Engineering (AREA)
  • Chemical & Material Sciences (AREA)
  • Combustion & Propulsion (AREA)
  • Ocean & Marine Engineering (AREA)
  • Aviation & Aerospace Engineering (AREA)
  • Earth Drilling (AREA)
  • Surgical Instruments (AREA)
  • Glass Compositions (AREA)
  • Disintegrating Or Milling (AREA)

Abstract

L'invention concerne un support sous-marin (100) pour transporter un fluide, par exemple CNG ou brut, qui comprend un corps principal (101) pour contenir le fluide à une pression interne prédéfinie, le corps principal (101) étant de préférence constitué de béton et conçu pour fonctionner à une profondeur d'eau à laquelle la pression externe agit sensiblement à l'encontre de la pression interne. Le support sous-marin comprend un élément flottant (102) relié au corps principal (101) par un câble de stabilisation (132), le câble de stabilisation (132) comprenant un premier cordon (321) pour transmettre une force et étant fixé à un premier raccord (134) qui est mobile par rapport au corps principal (101). L'invention concerne également un système dans lequel le support sous-marin est remorqué par un navire de surface (3) ou est auto-propulsé et commandé à distance. Le support sous-marin (100) réduit les coûts de fonctionnement par rapport à des supports sous-marins à l'aide de surfaces de commande classiques et de systèmes de lestage ayant de grands volumes de cargaison, par exemple 150 000 m3 ou plus.
PCT/NO2015/050250 2014-12-20 2015-12-17 Support sous-marin WO2016099287A1 (fr)

Priority Applications (1)

Application Number Priority Date Filing Date Title
US15/537,695 US20180001970A1 (en) 2014-12-20 2015-12-17 Subsea carrier

Applications Claiming Priority (2)

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NO20141549 2014-12-20
NO20141549A NO340274B1 (no) 2014-12-20 2014-12-20 Undervannstanker

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WO2016099287A1 true WO2016099287A1 (fr) 2016-06-23

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NO (1) NO340274B1 (fr)
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Cited By (2)

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WO2020226507A1 (fr) * 2019-05-07 2020-11-12 Equinor Energy As Stockage immergé de fluides hydrocarbures
WO2023287302A1 (fr) * 2021-07-15 2023-01-19 Equinor Energy As Procédé de commande de flottabilité

Families Citing this family (4)

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AU2020389081A1 (en) 2019-11-20 2022-06-16 Transoceanic Llc Ultra-large marine submersible transport boats and arrangements for transportation of aqueous bulk liquids, including fresh water
GB2585488B (en) * 2020-05-22 2021-08-04 Equinor Energy As Shuttle loading system
CN113086094B (zh) * 2021-04-21 2022-04-19 鹏城实验室 无人潜航器回收系统以及回收方法
WO2022221924A1 (fr) * 2021-04-22 2022-10-27 Christopher Colin Stephen Système de transport et de stockage de gaz

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GB1067703A (en) * 1966-01-17 1967-05-03 Arthur Paul Pedrick Submarine cargo trains, with arrangements for the use of obsolete or surplus nuclearsubmarubmarines
FR2423393A1 (fr) * 1978-04-19 1979-11-16 Entrepose Gtm Etpm Procede et dispositif pour le remorquage pres de la surface de l'eau de longs troncons de pipeline
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FR1159028A (fr) * 1956-10-08 1958-06-23 Procédé et dispositif pour le transport de fret, en particulier de charges payantes liquides ou pulvérulentes, par voie d'eau et plus spécialement par mer
GB1067703A (en) * 1966-01-17 1967-05-03 Arthur Paul Pedrick Submarine cargo trains, with arrangements for the use of obsolete or surplus nuclearsubmarubmarines
FR2423393A1 (fr) * 1978-04-19 1979-11-16 Entrepose Gtm Etpm Procede et dispositif pour le remorquage pres de la surface de l'eau de longs troncons de pipeline
WO1981003475A1 (fr) * 1980-05-30 1981-12-10 W Boyce Vehicule sous-marin remorque a commandes laterale et verticale

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WO2020226507A1 (fr) * 2019-05-07 2020-11-12 Equinor Energy As Stockage immergé de fluides hydrocarbures
WO2023287302A1 (fr) * 2021-07-15 2023-01-19 Equinor Energy As Procédé de commande de flottabilité

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US20180001970A1 (en) 2018-01-04
NO340274B1 (no) 2017-03-27
NO20141549A1 (no) 2016-06-21

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