The disclosure relates generally to a liquid storage, loading and offloading system using flexible containers and its associated applications for both fixed offshore oil production platforms and floating oil production platforms.
After crude oil is produced from an offshore oil production platform in an oil field, there are two options to deal with the produced oil: 1) export the oil out through a subsea pipeline system; or 2) store the oil in a temporary storage facility, which could be a fixed part of the platform itself or a separate storage facility, and then transport the oil out via an oil shuttle tanker. If the production of the field in terms of quantity or duration is insufficient to justify the cost of a subsea pipeline connecting the platform to the shore, a platform with a safe, reliable and low cost temporary oil storage system must be provided for the development of such a field.
The location of a temporary oil storage system is an important issue. There are only three possible options: 1) build the system above water as a fixed part of the offshore platform's topsides; 2) construct the system at water surface as a part of a floating structure; or 3) build the system under water as a part of the platform substructure or as a separate facility which is usually connected to the platform topsides via pipes for oil loading and offloading operations. Each of these options has pros and cons during its applications. With Option 1, the oil tank is usually built as a fixed part of the topsides commonly beneath the process modules and living quarters. This arrangement makes the tank independent of waves and current-induced environmental loadings. However, placing oil compartments beneath the process modules and living quarters could constitute a serious safety hazards. An even more critical issue is that this arrangement places a large mass at the platform top with a large weight variation during a loading and offloading operation, which could greatly influence the dynamic characteristics of the platform. Therefore, this option has been only used for very shallow water fixed platform applications so far in the offshore industry. With Option 2, it is easy to increase the oil storage capacity of a floating structure such as a ship-shaped FPSO vessel. However, the cost of a floating structure is usually much higher than a conventional fixed platform because the floating structure is subject to a large wind and wave induced environmental loading and it requires an expensive station keeping system to keep the floating structure in location. Therefore, floating structures are usually applied in deep water applications or some shallow water applications where a subsea pipeline system becomes too expensive to be feasible. With Option 3, it is the most preferred method for both offshore fixed platforms and offshore floating structures because the wind induced environmental loading is totally eliminated and the wave induced environmental loading should also be significantly reduced, if the top of the storage tank is sufficiently submerged under water surface. In addition, the platform's vertical center of gravity (COG) should also be lowered, which generally benefits the platform's dynamic characteristics as well as the motion responses. However, a subsea oil storage system usually faces many challenges under different types of applications. One of the most critical challenges is how to handle a large amount of buoyancy and related weight variations during the system's loading and offloading operations.
For a submerged offshore storage system, there are two common methods to store oil or other hydrocarbon liquids underwater. The first one is called “wet storage method”. Under this method, oil and seawater are stored together underwater within the same tank. Because of the density difference (crude oil density: ˜0.8 to ˜0.9), oil or other non-water solvable hydrocarbon liquids should stay at the top of the tank above the seawater and maintain a physical contact surface between the two types of liquids. During a loading operation, an amount of produced oil is imported into the tank and the equivalent amount of seawater, in the volume term, is then displaced to keep the total volume of oil and sea water constant within the tank. During an offloading operation, the process is reversed: seawater is imported into the tank to displace the oil and to keep the total volume constant inside the tank. The first advantage of the wet storage method is that the on-bottom weight at the storage tank bottom should have limited variations, generally only about 15% reduction of the maximum on-bottom weight at the tank bottom between loading and offloading operations. The second advantage of the wet storage method is that the tank does not need to be designed as a pressured vessel because the tank internal pressure is hydrostatically balanced with outside seawater throughout its service life. However, the wet storage method also possesses three major disadvantages. The first disadvantage is the environmental pollution concern. Under the dynamic motions, the contact surface between oil and seawater could produce a mixed layer to cause environmental pollution if discharged out to the sea. The second disadvantage of the wet storage method is the issue of thermal insulation. It is very hard to keep oil warm with the existence of a large contact surface between warm oil and cold seawater. The third disadvantage is that it cannot store water-soluble liquids such as methanol. Because of these critical disadvantages, especially with the environmental pollution concerns, this wet storage method has had very few field applications so far, either for fixed platforms or for floating platforms, in spite of the tremendous efforts made in the offshore industry.
The second common method to store oil or other hydrocarbon liquids underwater is called “dry storage method”. Under this method, oil or other hydrocarbon liquids can be simply stored in an underwater storage tank without any contact with sea water. In addition, the underwater storage tank can be easily applied with a good thermal insulation protection to the stored oil. However, this simple dry storage method faces two critical challenges during its service life. The first challenge is the large buoyancy issue due to the emptied room of the tank during an oil offloading process. The second challenge is the inert gas induced environmental pollution concern. In order to prevent the evaporated gas from escaping the oil storage tank to pollute outside air, inert gas is commonly used to be injected into a closed oil storage tank's top part above the oil surface. However, the required inert gas operations such as generating, blanketing and venting could become another source of pollution hazards. To overcome the first challenge of excessive buoyancy, common practice is to use a concrete gravity based platform. The heavy weight of a concrete structure helps to offset the extra buoyancy generated by an oil offloading operation. Another solution for the first challenge is to build extra ballast tanks and to ballast water in during an offloading operation in order to compensate for the discharged oil. Due to the above concerns, so far there have been only a very limited number of field applications, mostly in the form of concrete gravity based platforms, in the offshore oil & gas industry.
One specific example of the “dry storage method” was illustrated by Wu in his U.S. Pat. No. 8,292,546. In this example, one liquid storage compartment is coupled with one water ballast compartment under a symmetrical arrangement with the existence of pressured inert gas above both liquid surface and the water surface in these two compartments. During oil loading and offloading operations, the system functions as an equal mass flow rate displacement system with a pair of coupled pumps by which a constant system mass is maintained and the COG is moved only along a vertical axis. The primary advantage of this system is that the whole system weight is constantly maintained during loading and offloading operations. However, this system gives rise to three concerns. Firstly, the oil storage volume utilization is low. More than 50% of the total system volume cannot be utilized for oil storage, but has to be left for containing seawater and inert gas. Therefore, a large amount of buoyancy will be still produced during oil loading and offloading operations (more than 50% in contrast to only about 15% in the “wet storage method” system). To overcome this buoyancy issue, a heavy concrete structure usually has to be utilized, instead of using steels which are commonly used for offshore platform constructions. Secondly, both liquid storage compartment and water compartment have to be designed as pressured vessels because of the existence of inert gas. Thirdly, the system operation is heavily dependent on a complex system using a pair of coupled pumps for oil and for water separately. This arrangement could become a safety concern during the oil storage system operation.
Therefore, there is a need for a submerged offshore storage system that combines wet storage method and dry storage method, and maintains advantages and overcomes disadvantages of these two storage methods.
A new submerged oil storage system is disclosed. The disclosed submerged oil storage system achieves the following objectives:
1. The submerged oil storage system maintains the key advantages of the “dry storage method,” such as having no physical contact between oil and water, providing good thermal insulation to ensure optimal fluidity of the stored oil, and capability of storing water-soluble liquids like methanol. At the same time, the system overcomes the disadvantages typical of the “dry storage method”, e.g., using inert gas and producing excessive buoyancy during offloading operations;
2. The submerged oil storage system maintains as a hydrostatically balanced system independent of water depth, which is similar to a “wet storage method” system. Therefore, the oil storage tank does not need to be designed as a pressured vessel. The maximum difference of the storage tank on-bottom weight difference (maximum one minus the minimum one) should be usually less than 20% of the maximum storage capacity;
3. The system storage capacity is fully utilized for oil or hydrocarbon liquids without a separate water storage tank within the system. In addition, no paired pumps are needed to deal with the loading and offloading of oil and water at the same time.
In accordance with one embodiment, the oil storage, loading and offloading system includes a submerged oil storage tank with multiple vertically placed flexible containers. The oil storage system directly connects to the topsides of an offshore oil production platform above water to assist the platform oil loading and oil offloading process. During the loading operation, oil is pumped in and stored inside each flexible container, which is expanded to displace the equivalent amount of water out; during the offloading operation, oil is pumped out from each flexible container, which contracts accordingly, and the reduced volume of each contracted container is then filled in by the equivalent amount of water from the surroundings.
One application of the disclosed oil storage system is for a shallow water marginal oil field development. With the advantages of the disclosed oil storage system, many marginal fields could be developed economically to overcome the several key challenges that commonly exist.
Another application using the disclosed oil storage system is to add oil storage, loading and offloading capabilities to deepwater floating platforms, such as a SPAR or a SEMI structure.
The drawings described herein are for illustrating purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure. For further understanding of the nature and objects of this disclosure reference should be made to the following description, taken in conjunction with the accompanying drawings in which like parts are given like reference materials, and wherein:
Before explaining the disclosure in detail, it is to be understood that the system and method is not limited to the particular embodiments and that it can be practiced or carried out in various ways. It is also to be understood that the submerged oil storage tank system and method disclosed herein may be used in any body of water. The term “oil” may comprise crude oil and other hydrocarbon liquids. The term “water” may refer to seawater or fresh water.
(1) The outer layer consists of used tires 101, chains 102, a valve 103 and two towing rings 104 with one such ring at each end. The purpose of this layer is to protect the layer beneath against friction induced wearing. Impact loading by induced friction force at the tires 101 surfaces should be passed to the towing rings 104 at both ends of the fender 100 through these chains 102;
(2) The second layer is composed of rubber material and several layers of fiber nets such as polyester nets, which are bonded together through a vulcanization process. The same vulcanization process should also bond this layer to the steel surfaces of the two towing rings to form a sealed room inside the fender 100;
(3) The third layer is composed of a thin layer of rubber or synthetic materials for the purpose of air sealing, so that the pneumatic fender 100 can maintain its designed air pressure throughout its service life with a need for only a few air pressure adjustments.
The larger a pneumatic fender 100, the more energy absorption capacity it will possess. At the time of this disclosure, a large pneumatic fender 100 can have a size of 6 meters (about 20 feet) in O.D.×12 meters (about 40 feet) in length. If the large sealed internal room of such pneumatic fender 100 is utilized as a flexible container, each one can provide a large storage capacity of more than 300 cubic meters for oil storage.
Referring now to
1) The outer layer 113 composed of rubber material and several layers of fiber nets such as polyester nets bonded together through vulcanization. It is worth pointing out that since it does not need to stand huge impact force as in the case of a pneumatic fender, thickness of the component layers of a flexible container can be significantly reduced. Such reduction will bring up two benefits: a) cost reduction; and b) increased flexibility to enable the flexible container to expand or contract more easily during loading and offloading of oil. The same vulcanization process should also bond this layer 113 to steel surfaces at these flanges 111 to form a sealed room for the flexible container 110;
2) The second layer 114 for thermal insulation; and
3) The third layer 115 composed of a thin rubber sheet or a sheet of synthetic materials for the purpose of air sealing. If needed, one more specially made thin layer could be glued to the inner surface of the third layer 115 against potential chemical corrosion by processed oil. Inside the third layer is the storage room for storing oil 116. It is worth mentioning that rubber itself is an excellent thermal insulation material. With the addition of one extra layer of thermal insulation material, the flexible container 110 should possess even better thermal insulation property.
Flange 111 connections located at both the top and the bottom of the flexible container 110 are used to connect the flexible container 110 to the steel compartment 120. The top of steel compartment 120 may be opened for repair or replacement of the flexible container 110 inside through the top flange 111. At the top of the flexible container 110, there is a curved structure 118 of the compartment 120 designed to take the pressured loading from the flexible container 110 top portion when the container 110 is full of oil 116. The interconnection piping set 130 passes through the top flange 111. There are 5 smaller pipes inside the interconnection piping set 130: one pair of longer pipes 131 for hot liquid (water or oil) circulation to keep the stored oil warm to ensure optimal fluidity; pipe 132 for oil importing and exporting; pipe 133 for air injection and release; and pipe 134 for providing sensors, such as pressure and temperature sensors inside each flexible container 110. This piping set 130 should be interconnected between the topsides and every flexible container 110 in the system. The inside wall of this connecting pipe 130 should be lined with conventional thermal insulation material 119.
One venting hole 140 at the top of each steel compartment 120 is for releasing potentially trapped air at the top of the compartment 120. During the transportation and the lowering operation, these venting holes 140 are to be closed. The flexible containers 110 should be injected with air for the sake of safety and necessary buoyancy. These venting holes 140 should also be closed prior to the submergence of the compartment 120 top surface under water. During the lowering, the tank 200 should be flooded with seawater, air should be vented through a designed venting line and air inside these flexible containers 110 should be gradually reduced accordingly. Once fully loaded with oil 116, there is only a small amount of seawater 117 left at the bottom of the steel compartment 120, as shown in
Another known existing method is to generate sodium hypochlorite in water via an electrolytic process, which can effectively prevent any marine growth. Since seawater inside the oil storage tank 200 is relatively isolated and stable, it is easy to keep an effective concentration level of sodium hypochlorite in the tank 200. This is also known as an effective, economical, and environmentally friendly method. It should be pointed out that as times goes by, even better antifouling solutions may become available.
Two pipes 150 are equipped with one end going along the outside surface of the tank 200 to the bottom part of the tank 200 and with the other end reaching the inside of the tank 200 for intake and expelling of seawater. The reason for such arrangement is that the deeper the seawater, the less seeds of barnacles and less oxygen contents in seawater there will be. Furthermore, filter(s) can be installed at the mouth of the pipes 150 to keep seeds of barnacles or any other moving objects out and to let only seawater 117 in.
In another embodiment, at the mouth of these two pipes 150, one-way check valves, one for each, are installed to only allow the seawater to flow in one direction, whether in or out. During the oil loading process to pump water, displaced seawater should be pumped to a water monitoring/processing vessel at the topsides to ensure water's cleanness before being discharged to the sea. This oil loading process is a long process and the size of this water monitoring/processing vessel should be a reasonable one.
To sum up, there are four lines of defense against formation of sharp-edged barnacles inside a steel compartment 120 housing the flexible containers 110. First, the mouth of the water tubes 150 should be located at the lower part of the submerged oil tank 200 to take in and expel seawater, in order to decrease the chances for sucking in seeds of various barnacles. Second, these openings should be covered with filter or filters to keep out various moving objects in seawater including seeds of barnacles. Third, the steel compartment 120 and its flexible container 110 should be properly designed and built so that once full loaded, the flexible container 110 will squeeze hard against the inner surface of the compartment 120, thus creating an oxygen-free environment, which is hostile to marine life including barnacles. Fourth, the antifouling coating or chemicals may be applied. Finally, absence of sunlight inside the submerged oil tank 200 makes it a hostile environment for marine life, thus leaving very little, if any, nutritional food for barnacles to live or grow with. In actual application, any single one method or a combination of some or all listed above should be considered.
An interconnected piping system 130 is utilized to connect all the twenty-two flexible containers 110 and to pass through two of the four tubular leg members 201 to the topsides above. The reason for using two legs instead of only one is that one is used for backup for safety reasons. Multiple steel plates 202 are used to connect these compartments 120 with the tank 200 inner surface.
During the loading and offloading operations, oil 116 should be able to flow evenly into or out of all the individual flexible containers 110 simultaneously because of the existence of evenly distributed hydrostatic water pressure inside all the compartments 120.
The interior space of the compartment 120 provides an ideal place for rubber material such as the flexible container 110 because it has no exposure to sunlight and has a constantly low temperature, with both of the factors constituting a well suitable environment for the flexible container 110. It is worth noticing that absence of sunlight inside the submerged oil tank makes it a hostile environment for marine life, thus leaving very little, if any, nutritional food for barnacles to live or grow with. In addition, there is little impact loading to the flexible container 110 except for a limited number of expansions and contractions of the flexible container 110 during the oil loading and offloading operations causing some fatigue damage to the rubber material of the flexible container 110 over time. Therefore, the service life of the flexible container 110 is expected to be long.
The integrated honeycomb-like steel compartments 120 with steel plates 202 not only provide a protected room for each vertically placed flexible container 110, but also provide structural reinforcement to the circular-shaped steel tank 200. The tops of these empty compartments 203 should be covered with steel plates (not shown) in their service conditions. At the bottom of these compartments 120 structure, there should be holes to let seawater 117 flows freely from one compartment 120 to another.
One application of the disclosed oil storage system is for a shallow water marginal oil field development. A marginal oil field commonly refers to one which cannot be economically developed with conventional development methods such as using subsea pipelines. With the advantages of this disclosed system, many marginal fields could be developed economically to overcome the several key challenges that commonly exist:
(1) The number of oil wells of a marginal field platform is typically small, e.g., 3 to 8, and a wellhead platform for drilling and well completion activities should be installed several months ahead of the arrival of a production topsides; the drilling operation is usually conducted by a self-elevated drilling platform standing by the wellhead platform side;
(2) The service life is short, typically only 5-8 years, while a typical offshore steel platform service life is about 20˜30 years.
Therefore, an economically feasible platform equipped with an oil storage system should be able to achieve the following objectives:
(a) Ability to be installed at one field site easily, and also to be removed and re-install at another field site without a need for an offshore crane vessel or any other major offshore transportation vessels.
(b) The platform should be able to adjust itself for different water depths and different soil conditions of any two different oil fields.
(c) The platform should be able to have a fender system to provide a safe and reliable fending protection for offshore docking by an oil shuttle tanker.
During transportation, a control panel (not shown) may be installed at the deck of the wellhead platform 300 with soft hoses to connect individually these five suction piles 307 for air injection and release. A remote control device may be used to operate the control panel from the vessel deck of a towing tug.
The illustrated configuration for this wellhead platform 300 makes it self-installable without a need for a crane vessel or a transport vessel. The wellhead platform 300 could be installed several months prior to the arrival of the storage tank and the production topsides, because a drilling and well completion program could be performed first by a self-elevated drilling platform from one side of the installed wellhead platform 300.
Prior to the arrival of the oil storage tank 200, drilling and well completion activities should have been finished at the wellhead platform with conductors 307 installed.
Referring now to
Combining the oil storage tank 200 and the topsides 400 together will provide many benefits, as follows:
(1) The oil storage tank 200 itself can be transported and self-installed with the topsides 400, there is no need to employ a crane vessel or a transport barge;
(2) The topsides 400 provides a sufficient weight to the submerged oil storage tank 200 so that the on-bottom weight at the tank 200 bottom is greater than the required minimum on-bottom weight during the loading and offloading operations;
(3) Two out of the four tubular leg members 201 and the tank 200 outer shell will be utilized to provide a strong support for the fender system 404.
For clarity purpose,
To reduce the buoyancy of the floating tank 200, some of these compartments 120 and empty compartments 203 will be ballasted and flooded in order to increase the draft of the floating tank 200. Other compartments 120 will be used as buoyant compartments with their associated flexible containers 110 being injected with air. In one embodiment, before the top of the tank 200 is submerged, all venting holes 140 at the tank 200 top should be open. During the lowering process after the top of the tank 200 is submerged below water surface 206, air from these selected containers 110 should be released in a controlled manner to lower the floating tank 200 slowly until the pads 405 sit on the tops of pads 306. After the installation, air should be totally released from these containers 110, with some suction action if necessary, before commencing oil storage activities.
For the removal of the oil storage tank 200 for a new oil field development, after oil 116 are pumped out of all of the flexible containers 110, air will be injected into the flexible containers 110 to displace the water 117 out of these steel compartments 120, as a result, lifting the tank 200 to the water surface 206. With additional de-ballasting action from these empty compartments 203 and the proper utilization of tides, the tank 200 can be easily moved out of the site with the help of tugs.
The whole platform is divided into two substructures: the wellhead platform 300 and the combined structure of the oil storage tank 200 and the production topsides 400. One of the primary reasons for such division is for each structure to perform its own function. For the wellhead platform 300, it is an adjustable structure to suit different water depths and different site soil conditions. For the production topsides 400, it could be a standardized structure configuration suitable for the whole region applications as long as the wave conditions are identical throughout the region. Some modifications or replacements might be needed for the topsides equipment, if required by next installation.
There should be sufficient on-bottom weight at the tank 200 bottom. The combined structure with the tank 200 and the topsides 400 functions as a gravity based structure sitting on the foundation provided by the wellhead platform 300, without a need for welded connection between the two structures.
Selection of the distance (D2) between water surface 206 and the tank 200 top is dependent on potentially encountered maximum wave height (Hmax) during the platform service life in the region. Typically, the minimum value of D2 should be larger than half of the value of Hmax in order to minimize the wave induced loading to the combined structure.
The site removal operation of both wellhead platform 300 and oil storage tank 200 should be the reverse of the installation of these structures to make them self-removable structures without a need for a crane vessel or any major transportation vessels. After the site removal operations, these structures could be moved to a dry dock for required modifications to suit the next marginal field development.
To summarize, the disclosed oil storage system can be utilized for the application of a shallow water marginal field development. The oil storage tank 200 provides the function as a submerged oil storage system to minimize the wave induced loading. The oil storage tank 200 may also be transported independently and self-installed to eliminate the need for a crane vessel or a transport vessel during the installation. These characteristics of the oil storage tank make the combined structure of the oil storage tank and production topsides self-installable and self-removable, with the flexibilities to suit the potential modification requirements for different marginal field developments. The method disclosed herein also provides a way for easy installation, easy removal and easy re-installation of the system for multiple consecutive applications, which best suits multiple developments of marginal oil fields in the same region.
The disclosed oil storage system may be easily implemented to deepwater floating structures such as a conventional SPAR 500, a truss SPAR 510 or a SEMI 600 with little inference to the basic characteristics of these floating structures.
Although preferred embodiments of a submerged oil storage system in accordance with the present invention has been described herein, those skilled in the art will recognize that various substitutions and modifications may be made to the specific features described without departing from the scope and spirit of the invention as recited in the appended claims.
This Application is a divisional of application Ser. No. 14/979,448, filed 27 Dec. 2015.
Number | Date | Country | |
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Parent | 14979448 | Dec 2015 | US |
Child | 15697419 | US |