GB2284629A

GB2284629A - Installing underwater storage tank

Info

Publication number: GB2284629A
Application number: GB9425028A
Authority: GB
Inventors: Oyvind Nedrebo; Karel Karal
Original assignee: Norwegian Contractors AS
Current assignee: Norwegian Contractors AS
Priority date: 1993-12-10
Filing date: 1994-12-09
Publication date: 1995-06-14
Anticipated expiration: 2014-12-09
Also published as: GB2284629B; GB9425028D0; NO302284B1; NO934535L; NO934535D0

Abstract

In a method of installing a storage tank 1 for petroleum products on the seabed, a liquid less dense than water (eg a light petroleum product) is used as ballast and is transferred to and from the tank via a loading buoy 23 connected to a tanker 26. Initially the tank 1 is completely filled with a light petroleum product, and air is simultaneously ventilated from the tank 1 which assumes a stable, floating position. Valves are opened in the bottom part of the tank 1 to admit seawater while the light fluid ballast is simultaneously transferred from the upper part of the tank back to the source 26 through conduit 24. Descent is controlled by devices 12, 14 until the skirts 2 of the tank penetrate the seabed 21. Further penetration is carried out by replacing the light fluid in the tank with seawater, and final penetration is effected by providing an underpressure in the volume delimited by the seabed 21, the skirts 5 and the bottom face of the tank. After installation, the buoy 23 and flow line 24 are used in the normal operation of the tank. <IMAGE>

Description

8TORAGE TANK FOR PETROLEUM PRODUCTS, AS WELL AS METHOD OF DESCENDING AND INSTALLING THE STORAGE TANK ON THE SEABED.

The present invention relates to a storage tank (buffer tank) system for petroleum products, including a method of descending the tank from the surface to the seabed and installation of the tank on the seabed. The method also can be used when removing the tank from the seabed however, the corresponding step operation will then be carried out in the reversed order. Further, the invention relates to a method of descending and installing a storage tank consisting of a selffloating tank structure divided into one or a number of volumes and constructed of concrete or the like material.

The main object of the present invention is to provide a storage system which according to the intended installation method will not be exposed to high pressure loads during the installation phase, and therefore less expensive to construct, simultaneously the method of descending and installing the tank contains new features which make this method simple and substantially cost effective in relation to the prior art methods.

The applicant's previous Norwegian patent application no.

912371, describing a method of towing a ballastable foundation structure to the installation site and installing the same, in which method a fluid lighter than water is used to adjust the weight of the structure during the installation phase. In this solution a special purpose installation vessel and equipment are used to transfer ballast fluid to the structure in the descending phase. When the structure has been installed on the seabed, the system will be disconnected, and this operation as well as possible connecting operations for the service phase must be carried out on the seabed. Accordingly, this requires comprehensive and costly installation work on the seabed.Another disadvantage or cost increasing factor is that the ballasting of the structure, i.e. transferring of oil from the installation vessel to the structure, must be carried out relatively fast (wheather window), which again involves costly installation equipment regarding the installation vessel and ballasting hoses with large dimensions etc.

Existing methods for locating structures on a seabed include the use of lifting devices like cranes etc. together with the use of buoyancy bodies. The dimensions and weigths of present storage tanks will often exclude the use of a lifting vessel. By using the buoyancy principle the volume of the structure can be utilized for buoyancy purpose. The use of air having atmospherical pressure is a well known principle to establish buoyancy, but it requires that the whole or part of the structure must be designed for an external water pressure. In deeper waters this involves substantially cost increases for a storage tank. If air is replaced by "pressure resistant" light weight fluid, a possible pressure problem for the structure will be eliminated. However, if light weight fluid is utilized, the fluid must be supplied or removed from the structure in a suitable way during the operation.The always present wheather condition problem existing for operations offshore will be an obvious problem for the safety of operation. Normally, wheather window requirements for carrying out the total operation must be satisfied. For large storage structures the use of light weight fluid by transporting the fluid through temporary hoses will involve a too time consuming operation unless costly equipment is utilized.

To reduce the above disadvantages relating to the installation and later removal of subsea storage tanks, the present invention propose that the intended permanent export flowline is used for transporting the light weight fluid during descending and installation of the storage tank on the seabed. Further, the export flowline is connected to a tank vessel (tanker) by a loading buoy. The advantage of this system, in relation to the prior art, is that the export flowline from the loading buoy is connected to the storage tank at the sea surface. Normally, the transport line would be connected to the storage tank after it is located on the seabed, and costly and time consuming installation work on the seabed will therefore also be avoided according to the installation method of the present invention.Due to efficiency requirements in the operation phase the export flow line from the loading buoy is designed for a large transporting capasity of fluid and are in addition robust, which makes it very suited and effective for utilizing light weight fluid as buoyancy medium for the tank structure during installation and removal. The large transfer capasity of the export flow line makes it possible to transfer light weight to the tank at a heigh rate, which again is very important in the installation phase regarding the wheather window etc. When the tank is installed on the seabed the connection between this and the loading buoy will therefore be ready for permanent operation, and the loading buoy can for example also in a simple way be used for ballasting the tank in case of removal of the tank from the installation site at a later state.

The above objects are achieved by an invented method of descending an installing a storage tank for petroleum products on the seabed according to claim 1, and also by a storage tank device according to claim 6.

At present it is two basic principles for continuous production with loading to a tank vessel. Continuous offshore loading of crude oil can be accomplished either by a dual offloading system with overlapping shuttle tanker hook up or by a single off loading system routed through a buffer crude oil storage. The buffer crude oil storage can be accomplished either by using a stationary tanker anchored to the seabed or by installing a subsea storage tank for the particulare purpose.

The applicants subsea buffer storage alternative which is based on a concrete tank located on the sea floor for the required production period, can be utilized for all relevant storage capasities, installation depths and seabed (soil) conditions.

Several storage structures can be clustered to the same import/export flow lines allowing cost efficient buffer sizing in line with future changes in production rates.

The storage tank structure is based on the open storage principle with oil exposed to the external sea water pressure.

This concept is approved by the Norwegian Petroleum Directorate and the Pollution Board and is currently in service for the Draugen plattform. The storage is filled by pumping oil from the production unit causing a discharge of sea water. Oil flows from the storage to the off loading tanker by simple gravity flow. This concept involves a very simple and robust mechanical outfitting resulting in low investments and operating cost.

The concrete storage tank can be completed in dry dock and towed directly to the installation site. The skirts are penetrated into the soil to provide sufficient stability.

The storage tank is retrievable, and the component selected for the storage will provide a life of service which most likely will exceed the service life required at the first operation site. Therefore, the possibility of retrieval and reinstallation at a new service site proves and added value for this products compared to alternatives of a more stationary nature.

The subsea oil storage allows for continuous oil production with periodic loading from shuttle tankers.

For off loading systems including buffer storage the production rates, vessel sizes and shipping businesses will define the optimal number of shuttle tankers to be used for transport. The buffer storage will also allow for continuous production during storm conditions when tanker hook up and loading may prove difficulty due to waves and wind conditions.

The main components in an offshore off loading system of this type is: - An oil import flow line from the production facility to the oil storage A A storage tank - An off loading system with flexible export riser such as the "Submerged Turret Loading" (STL) system, Ugland Kongsberg Offshore Loading (UKOLS) and similar systems - A control system based on umbilical between a production platform and oil storage - The oil storage allows for high speed loading of shuttle tanker, and during production the oil storage installation will allows for: - crude oil to be produced to the storage for intermediate storage prior to offloading to tanker, - the crude oil storage to be exported to tanker by gravity flow, - crude oil to be exported to tanker directly from the production platform by passing the oil storage import flow line.

The mechanical system is simple and easy to operate, and dimensioned to correspond with the design values for import and export rates. Bypassing the storage require that a ESD (Emergency Shut Down) valve is closed. All valves and the offloading connector are located at a steel support between two cell compartments just above seabed. Operation control is through the umbilical. The umbilical includes a water sample (test) hose from the inlet/outlet, an instrument cable and a hydraulic line to operate the ESD valve. The umbilical is assumed to be attached to the transport line between the production unit and the storage tank.

During storage operation the elevation of crude oil/water interface is monitored. The sonar base instrument required for this purpose will continuously give information about the volume of oil stored in the storage tank. The sampling hose is used to monitor water discharge to verify no pollution effect. By also measuring the temperatur and pressure of stored oil, the interface measurement system may also be used for fiscal metering of off loaded oil to shuttle tanker.

For offshore oil production platforms with shuttle tanker transport of oil to terminals, fiscal metering has been performed using a meter station located on a platform. For this conventional concept the oil is metered when pumped through the meter station during transfer from the storage unit to the shuttle tanker. The subsea sonar based measurement system will be utilized to obtain fiscal metering of oil without having the oil passing through a meter station when transferred from the storage unit to the shuttle tanker. The advantage is cost savings obtained by not needing platform (loading tower or buoy) to house a conventional metering package.

To perform fiscal metering, the instrument system located subsea on the storage tank is used in combinationn with a small size conventional metering station located on the production unit. The metering station located on the production platform registers the quantity of oil exported to the storage tank or directly to the shuttle tanker. The law continuity implies the volume of oil exported from the production platform to equal the volume of oil stored in the tank and the volume of oil transferred over time to the shuttle tankers.

The mechanical system will safe guard against accidental oil spill caused by damage to the off loading system by closing the ESD valve. The safe guarding -system will automatically close the ESD valve if the oil/water interface level exceeds the predefined acceptable limit.

The storage is based on recognized technology from 20 years of experience with offshore concrete structures. No maintenance work is anticipated for the service life of the storage. The sonar instrument used for oil/water interface monitoring is exchangeable through remote operated vessel (ROV) intervention.

Future changes in oil storage demands can be accomodated with several storage units utilizing one import/export flow line. A manifold centre with remote operated valves will make it possible to produce to or of flood from any of the units at any time. All storage units are then connected to and controlled from this manifold centre. In principle, many tankers can then be clustered.

The storage tank is designed from the assumption that it will not be exposed to high pressure loads or large concentrated loads. With the construction carried from the surface to the seabed by the light weight crude oil inside the storage, and "even" pressure will be active on the upper domes of the storage. This load condition is equivalent to the load conditions guiding design for the operation phase. Thus, the installation will from a constructional point of view not add significant cost to the product.

The density of crude oil to be used for buoyancy purposes requires the storage to be constructed using for example light weight concrete to obtain a net positive buoyancy during installation.

The skirts of the storage tank will be penetrated by suction to reach a safe foundation condition for the storage.

The storage transport and installation is a simple and fully controlled operation. The operation can be interrupted and reversed at any time and conducted in a speed determined by the operator.

Descend to the seabed and acsend to surface for retrievel and re-use is based on a passive system. The storage is made neutrally buoyant by filling light weight fluid into the storage compartments. In order to inititate and maintain descending to the seabed, light weight fluid (crude oil) must be discharged from the storage into a tanker assisting the operation, simultaneously water will flow into the storage tank. Upon removal crude oil must be pumped into the storage to establish the required buoyancy for ascend to the surface.

When the active discharge/filling of light weight fluid stops, stops also the motion of the storage, i.e. the system is "passive".

After descent to seabed and positioning, the storage tank is fixed to the seabed by means of skirts penetrated into the seabed. The penetration is accomplished by water pressure reduction in the skirt compartments which in this phase communicate with the storage itself. The gravity flow discharge of oil from the storage provides sufficient suction to accomplish required penetration of the skirts.

In the following the application will be further explained with reference to an embodiment of the invention, including figures wherein: fig. 1 shows the storage tank connected to a preinstalled transport flow line and loading buoy at an installation site, fig. 2 shows a first phase of the descent to the seabed in which light weight fluid from a tanker is pumped trough the transport line to the storage tank, fig. 3 shows the descending phase of the storage tank in which light weight fluid is transported back to the tanker, fig. 4 shows the storage tank located on the seabed, and light weight fluid is returned back to the storage tank, and fig. 5 shows a system where the storage tank is in a operational phase, and the tank functions as a buffer storage between a production unit and a tanker.

With references to the drawings the invention will firstly be described by the following main steps: 1) At the installation site a pre-installed transport flow line 24 is connected to the storage tank 1 by means of a permanent coupling located either in the lower part or in the upper part, depending on the conditions at the specific site.

The storage tank 1, which in this phase is without ballast, will by means of a connecting device/descending control device 12 be connected to an installation vessel 11, and this connection will be maintained until the installation of the tank on the seabed 21 is completed.

2) A petroleum product/light weight fluid is pumped from the tanker 26 through a loading buoy 23 and transport flow line 24 into the storage tank 1, and the tank is filled up as required while air is ventilated out. When sufficient light weight fluid is supplied, one or more of the valves in the tank 1 is opened for the inlet of water. This will stabilize the pressure differencial which the storage tank 1 must be designed (dimensioned) for.

3) Light weight fluid is pumped back to the tanker 26 through the same transport flow line 24. Due to heavier sea water replacing light weight fluid, the weight of the storage tank increases and the tank will descend. Required descending control is achieved by a connection device 12 between the tank 1 and the installation vessel 11 and a floating device 14, 15 connected to the tank 1. The control devices for descending the tank 1 will be explained in the following.

4) Descending of the intermediate (buffer) storage tank 1 stops at a predetermined level above the seabed 21 and the tank 1 is positioned on the seabed 21 in a known way per se.

5) Light weight fluid is pumped out of the buffer storage 1 simultaneously the light weigt fluid is replaced by water, and the tank penetrates the seabed 21.

6) Further penetration of the seabed 21 is carried out by applying a suction in the skirt compartments, and the tank 1 is emptied for remaining light weight fluid, and thus the installation of the tank 1 itself is carried out. Thereafter, the tank 1 will be connected to a production unit 32 via an import flow line 31.

The descent of the tank is controlled by the amount of light weight fluid which at any time is kept in the tank 1.

As a result of a certain compressability of the light weight fluid the method will involve that the buoyancy of the tank will change from the sea surface 20 to the seabed 21.

Therefore, in the proposed method a "lift" or "push force" is also provided in addition to the set of forces which are active from the weight/buoyancy of the tank 1 including light weight fluid.

In order to provide a "lifting force" buoyancy bodies 14 and 15 can be utilized. The system consists of a number of smaller buoyancy units 14 arranged such that these are gradually mobilized when added buoyancy for the system is required. A larger ballastable buoyancy unit 15 is fixed to the upper end (termination) of the installation system string.

This buoyancy unit 15 is operational in the last phase of the descent in which the storage tank 1 must be moved into a position above the seabed 21 before it is finally positioned on the seabed. As an alternative a surface vessel installation vessel 11 including winch - or crane lifting capasity alone or in combination with a surface buoy can be used. By combining winch or buoy as a controlling system, a large lifting force can be achieved without the heave motions of the vessel inducing push and pull forces with subsequent possible collaps of the wire system. By this method the wire system runs from the winch on the vessel down to the sheaves on the storage tank 1, and back to the surface buoy which have a small water plane area.In that the applied lifting force is higher than the loss of buoyancy which is a result of the compressability of the light fluid, the system will be stable during the descending operation from the surface to the sea bottom.

In order to control descent by use of "push force" or a ascent by "lifting force", a distributed ballast weight, for example a ballast chain can be included in the system. One or several units of the ballast chain are mounted on the storage tank structure 1 before the operation starts and is handled by a surface vessel 11, such that weight is added or removed from the storage tank 1 as required during the operation.

After completing the descent of the storage tank 1 to the seabed 21, the tank 1 will be fixed (anchored) to the seabed 21. The first penetration phase of the tank 1 into the seabed will be achieved by the increasing submerged weight of the storage tank 1 when light weight fluid is removed and transported to the tanker 26. Further penetration is achieved by applying additional water pressure difference. The proposed method requires that valve(s) in the bottom part of the storage tank 1 is opened by means of a remote operated vessel (ROV). Filling of water for replacing light weight fluid which is removed will therefore result in a water pressure difference for further penetration of the tank 1 in the seabed 21 until the required penetration depth is achieved.

The valve settings are changed from installation state to operational state by means of a ROV. Also the required temporary instrumentation is carried out by a ROV. The instruments of the ROV will measure the depth, inclination and assimuth orientation. The ROV camera is used to observe the penetration of skirt 2 by reading a penetration scale.

By reading a flow meter, the operator on the oil tanker 26 can control the rates and volumes of the supplied or discharged light weight fluid. An instrument for reading the oil level manually from the top of the tank 1 during filling of crude oil is also used for control purpose.

Claims

PATENT CLAIMS

1. A method of descending and installing a storage tank (1) for petroleum products on the seabed, wherein the tank (1) at the installation site via a descending control device (12) is connected to an installation vessel (11) and a buoyancy device (14, 15) and via a conduit (flow line) (24) to a light weight fluid source (26), and wherein the tank (1) in a first descending phase is completely filled with a light weight fluid (petroleum product), simultaneously air is ventilated out of the tank (1) and thus assume a stable, floating position, whereafter descent of the tank (1) is carried out by opening of valves in the bottom part of the tank (1) such that sea water is let into the tank (1) simultaneously light weight fluid is removed from the upper part of the tank and back to the source (26) through the conduit (24), and the descending is controlled by a descending control device (12 and 14) until the tank (1) is located and penetrates the seabed (21), whereby further penetration of the downward directing skirts (2) of the tank (1) is carried out by replacing the light weight fluid in the tank with sea water, and the last part of the penetration of the skirts (2) is performed by providing a underpressure in the skirt compartment, i.e. the volume of the compartment delimited by the seabed (21), the skirts (5) and the bottom face of the tank, characterized in that a flexible transport conduit (24) from a preinstallated loading buoy (23) which can be connected to a tanker (26), is connected to the storage tank (1) at the sea surface (21) and applied for filling and removing light weight fluid in the tank (1) during descent and installation, as well as for the subsequent permanent operation, and eventual for later removal of the tank (1).

2. A method according to claim 1, characterized in that the underpressure in the skirt compartment is provided by penetrating the skirts (2) of the tank (1) into the seabed (21) and thereby closing the skirt compartment(s) towards surrounding sea, simultaneously valves in the bottom of the tank (1) towards the skirt compartment(s) are opened and light weight fluid in the upper area of the tank (1) is forced out through the transport conduit (24) and back to the tanker (26).

3. A method according to claim 1 and 2, characterized in that the descending control device (12) between the tank (1) and the installation vessel (11) is provided in the form of a chain body, and in that the required length of the chain body is placed on the top of the tank (1) as variable ballast for the tank (1) during the descending phase.

4. A method according to claim 1 and 2, characterized in that the descending control device (12) between a tank (1) and the installation vessel (11) is provided in a well known way per se by a crane winch system on the installation vessel (11).

5. A method according to claim 1 and 2, charcterized in that the buoyancy of the tank (1) during descent is controlled by buoyancy bodies (14, 15) which gradually are mobilized as required.

6. A storage tank (buffer tank) (1) for petroleum products for providing continuous production from a production unit (30) and periodic loading of the products to a tanker (26), characterized in that the storage tank (1) is located on the seabed in proximity to the production unit (30) and is connected to this via a transport conduit (31), and the storage tank (1) is further connected to a loading buoy (23) via a flexible transport conduit (24) for transferring the petroleum product to a tanker (1), whereby the petroleum product is pumped from the production unit (30) at the surface (20) via the transport conduit (31) and into the upper part of the intermediate (buffer) storage tank simultaneously seawater is discharged through valves in the lower part of the tank (1) and the petroleum product will further freely flow through the flexible transport conduit (24) and up to the loading buoy (23) at the sea surface (20).

7. Storage tank (1) according to claim 6, characterized in that the tank (1) is provided with a measurement device including a sonar based measurement system for monitoring the oil/water interface level during operation and also an instrumentation system for measuring the temperature and pressure of stored oil in the tank (1).

8. Measurement device according to claim 7, characterized in that the device in combination with a small size metering station on the production platform is utilized to obtain fiscal metering of off loaded oil to the shuttle tanker.