The invention relates to a system for use in offshore oil and gas production from production wells on the sea bed, comprising a submerged buoy having an outer buoyancy member which is arranged for introduction and releasable locking in a submerged, downwardly open receiving space in a vessel, and a central member rotatably mounted in the outer member and which is anchored to the sea bed and connected to at least one transfer line extending from a respective production well up to the buoy, a shaft extending between the receiving space and the deck of the vessel.

A system of the above-mentioned type, for use in buoy loading and unloading, is disclosed in the international patent application No. PCT/NO92/00054. In such systems the submerged buoy forms a collection point for one or more flexible risers and umbilical cables from e.g. a production system on the sea bed. The buoy is adapted to be raised and secured in the receiving space in the vessel in question, to establish a transport system for the petroleum product from said system to holds in the vessel.

When a buoy of the introductorily stated type is secured in the receiving space in a vessel, the vessel is rigidly fixed to the outer buoyancy member of the buoy and is pivotable about the central member of the buoy which is anchored to the sea bed by means of a suitable anchoring system. Thus, the buoy itself constitutes a rotating body or turret about which the vessel is allowed to turn under the influence of wind, waves and water currents.

This buoy structure involves a number of substantial advantages in relation to pre iously known buoy loading systems. The central member of the buoy has a small diameter and a small mass, so that there is achieved a correspondingly small diameter of the rotating body, i.e. the outer buoyancy member of the buoy, and consequently a small rotary mass and a small rotational resistance.

Connection and disconnection between vessel and buoy can be carried out in a simple and quick manner, even in bad

weather in relatively high waves. Further, the buoy may remain connected to the vessel in all weathers, a quick disconnection being able to be carried out if a weather limitation should be exceeded. In a vessel which is adapted for use with the mentioned buoy structure, the receiving space and the shaft which is arranged thereabove, suitably are arranged in the bow portion of the vessel, as shown in the above-mentioned international patent application. This enables a relatively simple and cheap rebuil- ding of existing vessels for adaptation to such a buoy loading system, for use as shuttle tankers. The combination of a submerged receiving space and a shaft which extends between the receiving space and the deck of the vessel, in addition makes possible a system giving a high security in operation and a small risk for contaminating spills.

For a further description of the buoy structure mentioned above and of a vessel of the above-mentioned type, reference can also be made to the international patent appli¬ cations Nos. PCT/NO92/00055 and PCT/NO92/00056. As regards offshore oil and gas production by means of a production vessel, different solutions are known, i.a. of the type which is based on a submerged bottom-anchored buoy and a receiving space arranged in the ship in the form of a moonpool which e.g. contains a rotating cylinder or turret for receiving and mounting of the buoy. It is common to the known systems that they are relatively expensive, mainly because of large dimensions of the rotating cylinder and the system as a whole. This results in that production vessels according to the prior art are not economic and practically usable in connection with smaller or marginal oil fields. Thus, there is a need for simple mobile production solutions making possible an economic exploitation also of smaller oil fields, especially with little gas, and with a production rate of up to 5000 m ³ oil per day.

Thus, it is a main object of the invention to provide a system for use in offshore oil and gas production which makes possible an effective and economic exploitation of e.g. smaller or marginal oil fields, when utilizing and retaining the inherent advantages of the buoy loading system mention in the introduc¬ tion.

The above-mentioned object is achieved with a system of the introductorily stated type which, according to the invention, is characterized in that it comprises a swivel unit which is arranged to be lowered to or hoisted up from a service position at the lower end of the shaft, and to be connected in the service position to a tube system on the vessel, the swivel unit comprising inner and outer mutually rotatable swivel members, and that, at the upper end of the buoy, there is arranged a coupling unit wherein the number of transfer lines in question are terminated, and which is arranged for connection to respectively release from a corresponding coupling unit at the underside of the swivel unit.

In an advantageous embodiment of the system according to the invention the swivel unit is placed on a lifting and lowering tool which is slidably mounted in a guide rail means extending between the upper and lower ends of the shaft. The swivel unit with it coupling unit thereby in a simple manner can be placed in the correct position in a coupling space at the lower end of the shaft. As the most critical components, the swivel and coupling units will be easily accessible for main¬ tenance or replacement. Connection to and disconnection from the transfer lines of the buoy can be effected as a one-step operation, with automatic shut-off valves at both sides of the coupling units. Vertical movement of the swivel unit in connec- tion with connection and disconnection suitably is absorbed by flexible tubes mounted at right angles to the axis of the swivel unit.

A substantial advantage of the system according to the invention is that it gives reduced system dimensions due to the use of the special buoy which itself constitutes a rotating body. This results in weight saving and reduced equipment extent, something which gives substantially reduced costs.

The system according to the invention requires minimum rebuilding of shuttle tankers for the buoy loading system mentioned in the introduction, for transfer to production vessels. With such a production vessel seasonal operations will be possible, in addition to continuous production from marginal fields, and also test production. The vessel for example may be used for test production during the summer half in a period with

a possible excess of shuttle tankers.

In connection with, or in addition to the above- mentioned fields of use, the system according to the invention will be suitable for use in connection with water injection, water purification plants, well stimulation, pumping of oil from ship wrecks located at the sea bed, etc. The system also is suitable for use in waters with floating ice and icebergs, the system when required making possible a quick disconnection, without any risk for damage of the submerged buoy. The invention will be further described below in connection with exemplary embodiments with reference to the drawings, wherein

Fig. 1 shows a view of a vessel and a submerged buoy forming part of the system according to the invention; Fig. 2 corresponds to Fig. 1, but shows the buoy raised and introduced into the receiving space of the vessel;

Fig. 3 shows a schematic sectional side view of a bow portion of a vessel designed in accordance with the invention;

Fig. 4 shows a similar view as in Fig. 3, but with a buoy introduced into the receiving space of the vessel and with a swivel unit lowered to the lower end of the shaft above the receiving space;

Fig. 5 shows the buoy and swivel arrangement in Fig. 4 at an enlarged scale; Fig. 6 shows an enlarged section along the line VI-VI in Fig. 5;

Fig. 7 shows an enlarged sectional view along the line VII-VII in Fig. 5;

Fig. 8 shows a schematic longitudinal section of the swivel unit in Fig. 5;

Fig. 9 shows an enlarged section along the line IX-IX in Fig. 5;

Fig. 10 shows an enlarged sectional view along the line X-X in Fig. 5; Fig. 11 schematically shows a means for turning of the coupling plate of the swivel unit, arranged in the region A in Fig. 10; and

Fig. 12 corresponds to Fig. 5, but shows the coupling units of the swivel unit and the buoy in connected condition.

In the various drawing Figures c ^responding members and elements are designated by the same re. rence numerals.

In Fig. 1 there is shown a submerged buoy 1 which is anchored to the sea or ocean bed 2 by means of a suitable mooring system consisting of a number of anchor lines 3 and floating bodies 4, so that the buoy floats at a predetermined desired depth under the water surface 5, the buoyancy of the buoy then being in equilibrium with the weight of the mooring system. A flexible transfer line 6 extends between the buoy 1 and a production well 7 at the sea bed. Even if only one line 6 is shown, in practice there may be arranged several lines in the form of one or more flexible risers (for production, injection, etc. ) and an associated umbilical cable.

A vessel 8 is provided with a receiving space 9 in its bow portion and is brought into position with the receiving space located above the buoy 1. The vessel may be a shuttle tanker which is equipped as a production vessel and for this purpose will be provided with a modularized, deck-mounted processing plant and a burner means ("flare"). In preparation for production the buoy 1 is hoisted up and secured in the receiving space 9 as shown in Fig. 2.

Fig. 3 shows a schematic side view of the bow portion of a vessel 8 which is equipped to work as a production vessel in accordance with the invention. The vessel has a receiving space 9 and a shaft 10 which extends between the receiving space and the deck 11 of the vessel. A lifting and lowering tool 12 is slidably mounted in a guide rail means 13 extending between the upper and lower ends of the shaft 10, more specifically from a scaffold means 14 on the deck to a coupling space 15 between the shaft 10 and the receiving space 9. The scaffold means includes a slide rail 16 wherein there is slidably suspended a swivel unit

17, so that this can be introduced above the shaft and placed on the lifting and lowering tool 12, to be lowered into the shaft.

In the embodiment in Fig. 3 there is taken as starting point a shuttle tanker of the design shown in the aforeirentioned international patent application No. PCT/N092/00055. THUS, the space 15 ^! s shown to contain a coupling unit 18 which is associated with a loading line (not shown) on the vessel, and which is intended for interconnection with a suitable buoy when

the vessel is operated as a shuttle tanker. Further, the space 15 is connected to a drainage line for emptying the shaft of water, e.g. for inspection and maintenance purposes, either when a buoy is placed in a sealing manner in the receiving space 9 or when the space 15 is shut off from the sea by means of a shutter (not shown). An additional drainage line (not shown) may be provided for drainage of possible oil leakage or the like to a collection tank on the vessel.

Further, the shaft 10 is connected to a conduit or line 20 leading to the inert gas and ventilation system of the vessel, so that the shaft when required can be filled with inert gas, as a safety precaution, a suitable closing means being provided at the upper end of the shaft. When producing, the space for coupling against underwater buoy and multicourse swivel will be a critical region with potential ^" danger of gas and explosion. This region preferably will be separated by a cofferdam, in addition to the fact that the space as mentioned may be filled with inert gas, or with water, in order to reduce the explosion hazard. In Fig. 4 a buoy 1 is shown introduced in the receiving space and mechanically locked by locking means 21. Further, the lifting and lowering tool 12 with the swivel unit 17 is shown to be lowered (by a winch means, not illustrated) to a lower position in the coupling space 15. In this position the tool 12 is blocked by means of an end stop 22 at the lower end of the guide rail means 13.

The buoy and swivel arrangement is shown more in detail in Fig. 5.

The buoy 1 comprises an outer buoyancy member 30 which is arranged for releasable fastening in the receiving space 9 by means of the locking means 21, and a central member 31 which is rotatably mounted in the outer member 30 and is anchored to the sea bed by means of the anchor lines 3. Apart from the riser system in the central member 31 of the buoy, the basic structure and manner of operation of the buoy essentially corresponds to what is disclosed in the aforementioned international patent application No. PCT/NO92/00056, and reference is made to this application for a further description of the general buoy structure. Generally, the buoy will be adapted to the field in

question with respect to pressure, capacity and the number of courses for the riser system.

In the illustrated embodiment, three flexible transfer lines are connected to the central member 31 of the buoy through buoy stiffeners 32 having a relative orientation appearing from the cross-section in Fig. 6. The lines consist of a pair of flexible risers 33, e.g. 6" tubes, wherein one is intended for production and the other for injection, and of a service line or umbilical cable 34 which i.a. contains hydraulic and signal lines. The risers and the service line are brought up through the central member of the buoy and at the upper end thereof are terminated in a coupling unit 35.

A plan view of the coupling unit 35 is shown in Fig. 7. The unit consists of a coupling plate 36 in which the risers 33 are terminated by means of respective connectors 37 and the lines of the umbilical cable 34 are terminated in a connector 38. There is also shown a connector 39 for a chemical line. These connec¬ tors are female connectors for possibly lockable interconnection with suitable male connectors in a coupling unit which is arranged in connection with the swivel unit 1 , as described below. The coupling plate 36 further is provided with guide pins 40 for guiding in connection with said interconnection. The connectors 37 for the risers will be provided with so-called "dry break" valves which open and close automatically in connecting and disconnecting, respectively.

The lines may be provided with an additional shut off valve 41, as shown in Fig. 5. This will be able to close if the dry break valve fails. In addition the coupling unit 35 can be dismounted above the valves 41 and pulled up together with connectors and valves of the coupling unit of the swivel unit (to be described later), in order thereafter to be able to put down equipment for pigging or well maintenance which cannot pass through the "dry break" coupling and the swivel.

The swivel unit 17 consists of a swivel stack consti- tuted by a pair of high-pressure production swivels 50, 51, e.g. one 6" swivel for production and one 6" swivel for injection, a hydraulic swivel 52 and a service line swivel 53. Each swivel of the swivel assembly is of conventional construction and consists of inner and outer mutually rotatable swivel members. The

fundamental construction appears from the schematic longitudinal section in Fig. 8 and the cross-sectional view of the high- pressure swivel 50 in Fig. 9. As appears from Fig. 9, and as applies to the fluid-transferring swivels, the inner and outer swivel members 50', 50" communicate with each other through an annulus 54, the inner swivel member having a tube connection 55 communicating with the annulus 54, and the outer swivel member having a tube connection 56 communicating with the annulus 54 and having a flange 57 for connection to the tube system of the vessel. The sliding surfaces between the inner and outer swivel members are sealed in relation to each other by means of gaskets. With a triple gasket system the swivel unit will be instrumented to register leakages via primary and secondary gaskets before a leakage into the coupling space occurs. A backup swivel stack will also be stored on board the vessel.

The various tube and conduit connections for the swivel assembly 17, such as the tube connection 55, is connected to and terminated in a coupling unit 60 corresponding to the coupling unit 35 in the buoy 1. Thus, the coupling unit consists of a coupling plate 61 having male connectors 62 for possible lockable connection with the female connectors 37 in the coupling unit 35, and additional male connectors (not shown) for connection with the remaining female connectors 38, 39 i the coupling unit 35. The coupling plate 61 is also provided with sockets 63 for reception of the guide pins 40 on the coupling plate 36.

Also the connectors 62 for the tube connections of the swivel units for the high-pressure swivels will be provided with dry break valves which open and close automatically in connecting and disconnecting, respectively. In the position illustrated in Figs. 4 and 5, the swivel unit 17 is located in a stand-by position above the buoy 1, with a distance of ca. 250 mm between the coupling units 35 and 60. In interconnection of the coupling units, the swivel unit is lowered said distance in relation to the lifting and lowering tool 12, so that the connectors of the coupling units are brought into correct engagement with each other. In order to absorb vertical movement of the swivel unit and occurring hull-induced movements between the swivel unit and the tube system of the vessel, the high-pressure swivels 50 and 51 are connected to

respective lines 64 and 65, one for production and one for injection, by means of flexible high-pressure tubes 66 and 67 of a length of 3-4 meters and which are mounted at right angles to the axis of the swivel unit. For achieving said lowering movement in connection, and a corresponding lifting movement in disconnection, the swivel unit 17 is supported on the lifting and lowering tool 12 by means of three hydraulic cylinders 68, for example as suggested in Fig. 10. The cylinders constitute an integral part of the lifting tool. For support of the swivel stack on said cylinders, there may be provided an especially adapted lifting or carrying frame, as suggested at 69 in Fig. 10.

The described equipment for handling of the swivel stack will also be able to be utilized for pulling-up and setting-down of bearings and coupling elements in connection with repair and maintenance activities when the vessel is operated as a shuttle tanker. The lifting tool 12 possibly may be furnished with dedicated carrying frames which may be adapted to the main dimensions of the different components. When a buoy is introduced into and locked in place in the receiving space in a vessel, one must see to it that the coupling unit 60 of the swivel stack is in a correct position in relation to the coupling unit of the buoy before interconnection is carried out. For this purpose there is provided for a turning means for turning of the swivel stack coupling unit 60 which is rigidly connected to the rotatable inner swivel member 50' . An embodiment of such a means is shown in Fig. 11. The means 75 consists of an e.g. hydraulic motor 76 which is mounted on the lifting and lowering tool 12 and via a conical gear 77 is in driving connection with an encircling gear rim 78 which is arranged on the coupling unit 60 concentrically with the central axis of rotation of the swivel member.

When the coupling units 35 and 60 are brought into correct relative position by means of the motor 76, something which is suitably achieved by means of remote control and possibly supervision by means of video cameras, the swivel stack is lowered by contraction of the hydraulic cylinders 68, preferably to a mechanically locked position. A possible disalignment between swivel stack and buoy is absorbed by means

of the cylinder support, so that the connectors of the coupling unit are interconnected in a safe and correct manner. This situation is shown in Fig. 12. The interconnection movement will open the shut off valves which are built into the coupling units, and disconnection will automatically close the same valves, the valves in open position resting against a spring in tension. When the swivel unit and the couplings are in this position and the different line connections are tested with respect to pressure and function, the system is ready for transfer of the flow of medium from the well at the sea bed.

In a normal disconnection, valves at the wellhead will shut off the production line in order to make possible pressure relief in the flexible risers and in the processing plant on the vessel. With activation of the hydraulic cylinders 68 the swivel stack with its coupling unit is thereafter lifted clear of the buoy at the same time as the spring-loaded valves are closed. The buoy thereafter may be released from the receiving space by disengagement of the locking means, so that the buoy sinks down clear of the vessel. In case of an emergency disconnection the locking means may be released directly, so that the buoy immediately sinks down clear of the vessel. The swivel stack with its coupling unit which stands with pressure against the buoy, will remain hanging in the lifting tool. The spring-loaded valves in the coupling points automatically close. The valve springs will work against a hydraulic damping (a time delay), but with a very limited oil spill as a result.