WO2014095777A1

WO2014095777A1 - Storage system for storage within the structure of an offshore platform

Info

Publication number: WO2014095777A1
Application number: PCT/EP2013/076792
Authority: WO
Inventors: Arild K. Samuelsen
Original assignee: Kongsberg Oil & Gas Technologies As
Priority date: 2012-12-21
Filing date: 2013-12-17
Publication date: 2014-06-26

Abstract

Platform comprising a subsea gravitational storage, wherein the gravitational storage comprises a flexible bag (10) for storing crude oil arranged within a ballast tank (24) of said platform, wherein the flexible bag is equipped with at least one port (12, 16) for emptying or filling and wherein the flexible bag is adapted to be constantly surrounded by ballast water is disclosed. A method for emptying a subsea gravitational storage and a method for removing a flexible bag for storing crude oil arranged in a ballast tank of a platform is also disclosed.

Description

Storage system for storage within the structure of an off-shore platform

The present invention relates to a storage system for the storage of fluids, especially for the storage of oil within the structures of an offshore platform. The platform can be a floating/buoyant platform or a platform structure placed on the seabed. These platforms presently use storage systems based on gravitational storage making use of the differences in density between the crude oil and the seawater.

The present invention may effectively be used in floating platforms as well as platforms placed on the seabed with gravitational storage systems for crude oil.

Background

Exploitation and production of crude oil has within the last 10 years moved from shallow to deeper waters. Moving from 200-300m to great deeps such as 1500- 4000m puts higher requirements on the feasibility of the technical solutions including storage and bringing ashore the crude oil and gas.

Large depths may eliminate the possibility to employ production platforms placed on the seabed and instead demand floating platforms with high stability and high buoyant power, especially for drilling platforms or production platforms comprising crude oil production equipment such as crude oil, water and gas separators.

Exploitation now also in many cases takes place at larger distances to the nearest shore. Earlier most produced products especially gas were taken to shore via pipelines placed on the seabed. This requires considerable infrastructure at the shore terminal. A well-known problem in connection with pipeline placed on the seabed is related to the need for the pipeline to overcome all geographical obstacles on the way to the shore.

Prior art solutions to these problems include floating installations comprising storage areas for produced products on hereto connected floating transport carriers.

Where the infrastructure and pipelines are available the operator has been forced to engage in long term contracts with the owners of the terminals and the authorities in the respective country. Such contracts will influence the price of the produced products. Prior art

Prior art solutions for storing crude oil offshore have until now to a large extent been focused on ship based solutions so called Floating Production Storage Oil (FPSO), or Floating Storage Unit (FSU).

FPSO's and FSU's are often based on ship's hulls and consist of either

reengineered ships or new specially designed solutions. FSU's are employed a lot tor floating platforms such as for instance SPAR platforms without any integrated storage facilities. The crude oil is transferred from the platforms via floating pipelines to a moored FSU and is transferred from there to a tanker via a mooring system connected to the FSU. FPSO's are expensive units that additionally have some weather related limitations both during operation and offloading.

In the Gulf of Mexico SPAR platforms have been demonstrated to be very useful as production platforms at high sea depths both due to their stability and there high buoyancy load. In the light of the problems related to placing and maintenance of pipelines on great depths there has been and interest in the possibility for gravitational storage within the platforms. The authorities have been concerned about the risk of discharge of water containing oil from the gravitational storage.

The contact between the crude and water in a gravitational storage where the oil displaces water when crude oil is produced for storage and water displaces crude oil when the crude oil is transferred to a tanker results in the formation of a water oil emulsion layer that will take up a part of the available storage capacity. The emulsion layer will also contain chemicals added to enhance the separation between oil and water and limit the remixing thereof. Over time it is expected that the emulsion layer will increase taking up increasing amounts of storage space.

Concerns have also been raised that the crude oil delivered from such a storage will contain increased amounts of water including salts which may reduce its quality and marked value.

The only previous solution to remove the emulsion layer has been to pump the whole layer to a transport ship via buoy and then transfer the layer to an oil terminal or cleaning and separation facility on-shore. This is a costly process especially as this emulsion is not considered a valuable product.

Further the contact between water and oil will allow bacteria naturally present in the water to get in contact with the oil. Some bacteria such as sulphur reducing bacteria benefit for this environment in that they propagate and form colonies on the available surfaces. These bacteria convert sulphur compounds to sulphuric acid which again leads to corrosion on storage steel walls and related pipelines. The bacteria and their by-products will also be distributed via the oil stream during loading to pipelines and transfer systems on the platform and the tanker. The bacteria may also form solid particles and thereby possibly reduce the quality of the oil. In light of all these issues there is a need for an alternative storage solution that will fully overcome or limit these issues.

US4.766.836 discloses a floating storage system wherein oil is stored in a cylinder with water column communication with the surrounding water. The oil is stored in direct contact with water. The system comprises cylinders that functions as ballast tanks and which may include ballast chambers separated from the main storage volume. A platform structure may be arranged on top of the storage system.

WO2004/037681 describes a subsea located storage comprising means for anchoring, a storage section in the form of a flexible balloon of a waterproof cloth, where the storage section is arranged with in structure section which is closed in the upper part and thereby forming an upper closed volume of a size corresponding to the volume of the storage section. In the lower part of the structure section openings to the surrounding sea are arranged. The storage is illustrated arranged on the seabed. US5.243.925 discloses a modular bladder system for confining a liquid cargo in a compartment of a transport vessel. The compartment may be a ballast tank and the liquid may be crude oil, however ballast water is removed from the ballast tank prior to filling the bladders with cargo. The system does not include making use of gravitational forces during filling and emptying and it is not a gravitational storage.

Objectives of the invention

The objective of the present invention is to provide a solution for a gravitational storage avoiding the formation of an emulsion layer that would otherwise fill at least part of the storage capacity. A further objective is to provide a system that would limit the contact between the crude oil and seawater containing bacteria, thereby limiting or fully avoiding the growth of sulphur bacteria or other microbes in the crude oil.

It is also an objective to provide for use of ballast volume for storage purposes.

The present invention also aims at providing a possibility for installation of gravitational storage systems in floating platforms.

A further aim is to provide a solution that limits the environmental risks o the use of gravitational crude oil storage subsea.

The present invention also aims at providing a storage system adapted to avoid degassing of crude oil stored therein. Additional the invention aims at providing a system allowing for improved temperature control of the stored crude oil and thereby control of the viscosity of the stored crude oil.

It is also an objective to provide a solution that is independent of pipelines and pipeline terminals, thereby allowing for freely selection and exchange of supplier of oil product transport services.

It is also an objective to provide for use or additional use of ballast water pumps to empty the crude oil storage limiting the number of crude oil pumps.

The present invention provides a platform comprising a subsea gravitational storage, wherein the gravitational storage comprises at least one balloon for storing crude oil or other fluids arranged within a ballast tank of said platform, wherein the at least one balloon is equipped with at least one port for emptying or filling and wherein the at least one balloon is adapted to be surrounded by ballast water.

In one aspect the platform the ballast room comprises an upwardly closed protection structure wherein the at least one balloon is arranged.

In another aspect the ballast room comprises a ballast water inlet and a ballast water outlet, wherein the ballast water inlet is in fluid communication with a ballast water pump and wherein pressure provided by the ballast water pump is transferrable to the ballast water surrounding the balloon thereby compressing the balloon. In a further aspect of the platform is a floating platform.

Alternatively the platform is a platform adapted to be placed on the seabed.

According to one aspect the balloon and the at least one port for emptying or filling are connected to a removable and reconnectable hatch, wherein the hatch is accessible such that the balloon may be exchanged. The present invention also provides a method for emptying a subsea gravitational storage, wherein the gravitational storage comprises a balloon for storing crude oil arranged within a ballast room of a platform, wherein the balloon is equipped with at least one port for emptying or filling and is surrounded by ballast water present in the ballast room, wherein the ballast room comprises a ballast water inlet and a ballast water outlet, wherein the ballast water inlet is in fluid communication with a ballast water pump, wherein the method comprises pumping ballast water via the ballast water inlet into the ballast water room thereby compressing the balloon and emptying the balloon via the at least one port for emptying or filling. The present invention further provides a method for removing a balloon for storing crude oil arranged in a ballast tank of a platform, wherein in the method comprises releasing the balloon from the tank and from at least one pipeline for filling and emptying the balloon, connecting a wire via from a winch via a swivel block to a top section of the balloon, lifting the balloon by winding up the wire with the winch, connecting a lower part of the balloon to a drum and winding the balloon onto the drum.

The term "gravitational" as in "gravitational storage" and related terms as used here in refers to a storage wherein gravitational forces are used to assist the filling and emptying of the storage. This includes making use of the gravitational forces resulting from the density difference between crude oil and seawater.

Brief description of the drawings

The present invention is described in further details with reference to the enclosed drawings. The drawings are examples of possible implementations of the present invention; the invention can be adapted to different platform structures without departing from the main concept of the invention as defined in the claims.

Figure 1 illustrates schematically producing crude oil to an embodiment of the storage system according to the present invention. Figure 2 illustrates the same system as figure 1 but during discharge of crude oil to shuttle tanker.

Figure 3 schematically illustrates a floating platform comprising storage according to the present invention.

Figure 4 illustrates cross sectional view of the distribution system to the storage system of the floating platform of figure 3.

Figure 5 is an opened side view of a storage system according to figure 3.

Figure 6 is a schematic cross sectional view of a storage system arranged within the ballast tanks of a platform leg placed on the sea bed.

Figure 7 illustrates a system and method for replacing one balloon/bag with another balloon/bag.

Figure 8 illustrates the removal of a balloon/bag via a hatch in a system according to figure 6.

Figure 9 illustrates the removal of a balloon/bag via a hatch in a system according to figures 1 and 2. Figures 10a- l Od illustrate the lifting and arrangement of a bag/balloon on a drum prior to further transport.

Principal description of the invention

The term "platform" as used herein refers to production platforms and drilling platforms, floating/buoyant platforms with any type of anchoring such as the SPAR platform, platforms permanently placed on the seabed and platforms that can be lowered and raised by pumping ballast water in or out of the structure.

In the platforms the storage volume serves two functions: product storage and ballast room. Accordingly the storage will play an active role in sustaining the stability of the platform. To obtain this double function the oil removed from the storage is replaced with seawater and oil added to the storage displaces seawater.

The present invention provides a storage system comprising a water and oil resistant flexible material, in the form of a bag/balloon adapted to the internal structure of the available storage space.

The terms "balloon" and "bag" as used herein refers to a bag of a flexible material resistant to seawater and any compound intended to be stored within the balloon/bag such as crude oil. The density of the material for the balloon preferably is higher than the density of water which contributes to the balloon sinking into the water when it is emptied and the upper part is lifted by the oil drifting up. Thereby it is avoided that the lower part of the balloon is pressed up against the manifold and the pipe openings during emptying of the balloon.

The term "flexible" as used herein especially in connection with the bag/balloon refers to the material properties thereof allowing the bag to be inflated and deflated when it is being filed of emptied respectively.

The solution provided by the present invention is described in the enclosed claims.

The balloon is in one embodiment of the invention attached to a hatch in the upper part of the tank. Connected to the balloon and possibly also to the hatch which can be fully released is a manifold for connecting the tank to load/unloading pipes or additionally to other tanks also arranged within the platform.

As the crude oil is supplied it will fill the balloon. Due to the difference in density between the oil and the surrounding sea water the oil will first fill the upper part of the balloon. If filling with oil is continued the balloon will spread out and fill up the entire tank volume for which it is designed. The water in the ballast room on the outside of the balloon will be forced out through the established fluid connections to the surrounding sea. Due to the fact that there is no direct contact between the water and the oil this does not give rise to any environmental concerns and the fluid connections to the sea may be constantly open between the ballast room and the sea. Thereby the ballasting of the platform will at all times be within the selected regime and in balance without any danger of capsize or pollution of the surroundings. The outside of the bag/balloon is during normal use constantly surrounded by ballast water. The temperature of the crude oil being stored in the bag is normally higher than the temperature of the seawater

surrounding the platform. When warm oil is stored according to the present invention within a bag within a ballast tank heat is transferred from the oil to the surrounding ballast water via the wall of the bag. After the filling has been completed the remaining, heated ballast water surrounding the bag will function as heat insulation as there will be limited movement in the water until the bag is emptied.

During filling of the storage flashing of volatile compounds within the crude oil should preferably be avoided. This can be obtained by installing a pressure increasing valve or a choke. The water pressure on the outside is transferred to the inside of the bag due to the flexibility of the bag and the water pressure will keep the oil under constant pressure and thereby flashing is avoided. During filling the water pressure must be overcome. This can be obtained by pumping the crude oil into the bag. The outside pressure will dampen the filling process thereby smoothening the filling procedure.

The bag normally tolerates temperatures up to 80°C, which allows for supplying oil at this temperature or between 60-70°C to the storage bag. This will keep the oil within the storage over a critical temperature for a longer period of time without external heating. Critical temperature in this sense is the temperature where the crude oil obtains a viscosity that makes normal pipeline transport difficult. Should heating become necessary this can be obtained by pumping the crude oil up to the platform and via heat exchangers transfer heat to the crude oil before it is returned to the storage. The heat exchanger may make use of heat sources available from other parts of the platform processes.

Alternatively it is also possible to install heating pipes in the ballast tank. The pipes can be heated by circulating heated water or other heated fluids for instance from the processing on the platform. The heat is transferred via the water in the ballast room to the content of the bag. The heating pipes can be installed so that any direct contact with the bag is avoided if the pipes can reach a temperature above the temperature tolerated by the bag. The storage-/ballast rooms are designed and arranged in a way that limits the heat loss from the oil-bag to the surrounding water by avoiding/reducing the

displacement of the "bag-heated" water during the emptying of the oil-tank. Heated water will have lower specific weight than cold water and thus form a "barrier"' between the cold water and the bag. If there is a rupture in the bag any oil leakage will be collected in the upper closed part of the ballast room in which the bag is installed. A warning installation comprising an oil detector is arranged within this area of the ballast room and connected to the platforms control centre. If oil is detected warning is given and corrections can be made. Preferably a normally closed valve is arranged near the top of the ballast room. The valve is connected to a piping system and can be opened to pump any oil that has leakage out of the room and further to a system for environmental safe handling of oil and water mixtures. Due to gravity the oil will gather in the upper part out of the ballast room. The upper closed section of the ballast room preferably has at least the same capacity as the bag so that no oil leaking from the bag will leave the ballast room through the passages open to the sea.

A ruptured balloon/bag can be replaced by disconnecting the supply/emptying pipe from the manifold and disconnecting the hatch securing the balloon/bag within the ballast tank. The hatch and the old balloon/bag can then be hoisted up to the platform deck. Thereafter a new balloon/bag and optionally a new hatch is hoisted down into the ballast room and connected to the supply/emp tying pipes via the manifold and secured to the tank via the hatch.

In one embodiment the disconnected bag can be winded on a transport drum for more compact handling of the bag. The removed bag is transferred to a supply boat and transported to a site for environmental safe handling and/or destruction.

The main parts of the system according to the invention are:

a. Balloon/Bag b. Storage hatch c. Manifold for pipe connections for filling and emptying the bag/balloon. d. Inlet and outlet pipelines for produced/stored oil. e. Valve for closing. f. Oil leakage detection and alarm system g. Seawater channels with valves h. Optional inner manifold for heating pipes i. Branch pipe for other adjacent storage units, j. Oil pump. k. Export pipe to unloading station with insulated valves (block and bleed).

1. Yoke for bag/balloon.

The invention provides for a satisfactory combination of storage of crude oil and ballasting of floating production platforms without negative side effects such as emulsion layers or growth of bacteria. At the same time all advantages of combining storage space and ballast room is upheld.

In a gravitational storage the pressure of the oil should be kept constant or possibly be increased to avoid flashing of any more volatile compounds enclosed therein. However, should flashing occur - despite the above mentioned precautions - the gas can be bled/routed through purpose build pipelines to the common gas/oil flare on board the platform or fed back into the process system.

When emptying of the storage/unloading of the crude oil there is in the present invention no need to take into consideration a reduced storage capacity due to an over time increasing emulsion layer, or the risk of mixing seawater into the oil that is being pumped to the tank ship, which would change the specification of the crude oil product and reduce the quality and marked price thereof.

With respect to the environmental issues the present invention provides a double barrier solution as both the bag and the tank acts as a barrier against any release to the sea environment.

Embodiments of the present invention will now be described with reference to the enclosed figures.

Figure 1 and 2 illustrate an embodiment where the bags 10 for storing crude oil are arranged within a platform leg such as the leg of a semi submersible platform. The sea-level 70 is schematically illustrated. On figure 1 the production of stabilized crude oil for storage is illustrated. Ballast pump 20 is used for stabilizing the platforms balance. The oil pump 14 provides stabilized oil for storage with enough pressure to overcome the gravity forces acting on the bag. The oil is transferred via pipes 12 and 16 to the bags 10. The valve into the oil export line 18 is closed. The ballast pump 20 obtains ballast water from the ballast water inlet 21 and supplies the water to the ballast tank 24 depending on the needs to keep the platform in balance. Ballast water surrounding the bags 10 is displaced and leaves via lines 28 and an open valve through the ballast water outlet 29. On figure 2 the stored oil stored in the bags 10 is being discharged to a shuttle tanker via lines 16 and 18 stabilized oil being produced at the same time is transferred via pump 14 and line 12 directly to the shuttle tanker (not shown).

Ballast water is pumped from the inlet 21 via pump 20 and line 28 into the ballast room surrounding the bags 10. Accordingly the pumping of the seawater may aid the emptying of the bags.

Figure 3 illustrates how the present invention can be incorporated in a floating platform 160 of the SPAR-type. One or more bags 1 10 are arranges in the lower section, below the sea-level, of the platform. The bags 1 10 are arranged within the tanks/rooms normally filed with ballast water to balance the platform. The lines 1 16 illustrate the pipelines for filling and emptying the bags.

Figure 4 illustrates schematically the cross section of the platform 160 of figure 3 at the 100 m mark. It should be noted that these depth marks only illustrate one possible embodiment and that the present invention is applicable for any size of SPAR platform. On figure 4 the arrangement of four bags 110 along the

circumference. The four bags are connected to a common distribution and emptying system 1 17.

Figure 5 illustrate an opened section of the platform 160 of figure 3 below the 100m mark. Visible are three of the bags 1 10 within the ballast room 124 all connected to the common distribution system 1 17.

Figure 6 illustrated another embodiment of the present invention where the bags are arranged within the ballast tanks in the lower part of a platform leg 262 possibly arranged on the sea bed. The pipe 216 transports the crude oil to and from the bags 210 arranged in the ballast tank 224. The bags are arranged in upwardly closed sections of the ballast tank. Should a bag start leaking the oil will due to the difference in density flow to the top of the upwardly closed section from where it can be removed through a normally closed valve and pipeline leading on to the pipe 216. The surfaces surrounding the bags may include insulation material 213 to retain the temperature of the stored crude oil. A ROV hatch 240 is arranged immediately over the manifold of the bag 210. In the illustrated embodiment the hatch 240 is arranged in a closable section which may be accessible for platform personnel. Figure 8 illustrates how a bag 410 can be removed from a similar platform. When the bag is to be removed personnel can enter the room over the bag to disconnect the manifold on the bag from the line 216. Also the connection of the bag to the ballast structure may be released when the inflow of ballast water is under control. Thereafter the hatch room is sealed from the air filled sections of the leg. A ROV is employed to open the hatch. A line is connected to the top section 41 1 of the bag. By use of a winch 480 and a swivel block 482 placed on the platform above the sea-level the bag is retrieved. On the platform a drum 484 for receiving the used bag may be arranged. A new bag can be inserted using the opposite procedure.

Figure 7 illustrate another embodiment of the method for exchanging a bag 310. In the platform 362 an exchange unit 390 is employed comprising a winch 380, a swivel block 382 and a drum 384. The exchange unit 390 can be place on top of the ballast tank comprising the bag according to the present invention. A hatch 386 in the top of the ballast tank is opened and the bag retrieved by employing the winch 380.

Figure 9 illustrates how a bag exchange can be arranged in an embodiment as illustrated on figure 1 and 2. While the bag is exchanged, produced oil can be stored in the other bag. The ballast water in- and outlet to the relevant section is closed during exchange of the bag. A hatch 86 is opened and a swivel 82 and a winch 80 are employed to lift the bag 10 out of the tank. The exchange unit may also comprise a drum 84. Figures 10a to lOd illustrate the use of a drum to compact a storage bag 510 for transport. Initially the bag 510 is lifted by the winch 580 and the swivel block 582 via a line 581. Engagement means here in the form of a string 583 is arranged in the lower part of the bag 510. A drum 584 is adapted to receive the engagement means 583 and the bag 510. The string 584 is wind onto the drum and the bag will follow the string as the rotation of the drum continues. On figure l Od the winding has been completed and the line 581 can be released. The bag 510 has been compacted and has been made easily transportable.

A person skilled in the art will appreciate that the different features illustrated and discussed here can be combined to obtain other embodiments of the present invention.

Claims

1. Platform comprising a subsea gravitational storage, wherein the gravitational storage comprises a flexible bag for storing crude oil arranged within a ballast tank of said platform, wherein the flexible bag is equipped with at least one port for emptying or filling and wherein the flexible bag is adapted to be constantly surrounded by ballast water.

2. Platform according to claim 1 , wherein the ballast tank comprises an

upwardly closed protection structure wherein the flexible bag is arranged.

3. Platform according to claim 1 or 2, wherein the ballast tank comprises a ballast water inlet and a ballast water outlet, wherein the ballast water inlet is in fluid communication with a ballast water pump and wherein pressure provided by the ballast water pump is transferrable to the ballast water surrounding the flexible bag such that the flexible bag is compressed and stored crude oil emptied true the at least one port.

4. Platform according to any one of the claims 1-3, wherein the platform is a floating platform.

5. Platform according to any one of the claims 1-3, wherein the platform is a platform placeable on the seabed.

6. Platform according to any one of the claims 1 -5, wherein the flexible bag and the at least one port for emptying or filling are connected to a removable and re-connectable hatch, wherein the hatch is accessible such that the flexible bag may be exchanged.

7. Platform according to any one of the claims 1 -6, wherein the flexible bag is surrounded with ballast water with a temperature higher than the temperature of the stored crude oil.

8. Platform according to claim 7, wherein the platform comprises a heater for heating the ballast water surrounding the flexible bag wherein the heater is selected from an electrical heater in direct contact with the ballast water or a heat exchanger for indirect heat transfer.

9. Method for emptying a subsea gravitational storage, wherein the

gravitational storage comprises a flexible bag for storing crude oil arranged within a ballast tank of a platform, wherein the flexible bag is equipped with at least one port for emptying or filling and is surrounded by ballast water present in the ballast tank, wherein the ballast tank comprises a ballast water inlet and a ballast water outlet, wherein the ballast water inlet is in fluid communication with a ballast water pump,

wherein the method comprises

pumping ballast water via the ballast water inlet into the ballast water room thereby compressing the flexible bag and emptying the flexible bag via the at least one port for emptying or filling.

10. Method according to claim 9, wherein the emptying is obtained by a

combination of pumping pressure and gravitational pressure from the ballast water onto the flexible bag.

1 1. Method for removing a flexible bag for storing crude oil arranged in a ballast tank of a platform, wherein in the method comprises releasing the flexible bag from the tank and from at least one pipeline for filling and emptying the flexible bag, connecting a wire via from a winch via a swivel block to a top section of the flexible bag, lifting the flexible bag by winding up the wire with the winch, connecting a lower part of the flexible bag to a drum and winding the flexible bag onto the drum.