Riser installation vessel and method of using the same

A vessel for exploration of hydrocarbons includes one or more risers extending from the vessel to the seabed, a hydrocarbon processing unit connected to the one or more risers and to a storage or transport structure for storing the processed hydrocarbons. The vessel is anchored to the seabed, the vessel includes a lifting member for lowering risers vertically towards the sea bed and for connecting a riser with a first end to a subsea hydrocarbon structure, which riser includes a connector on a second end, the vessel having a connector for attaching to the riser connector and for placing the riser in fluid connection with the processing unit.

The invention relates to a vessel for the exploration of hydrocarbons, comprising one or more risers extending from the vessel to the seabed, a hydrocarbon processing unit connected to the one or more risers and a storage or transport structure for the processed hydrocarbons. The invention also relates to a method of installing one or more steel riser pipes.

From Offshore Technology Conference OTC 11875, Houston, Tex., 1-4 May 2000 with the title “Hybrid Riser for Deepwater Offshore Africa”, Loïc des Déserts—Doris Engineering, a riser pipe for deep waters is described comprising a steel outer casing with a number of production risers, gas and water injection lines and insulation made of foam which also confers buoyancy to the riser pipe. The riser pipe is assembled on shore and towed to location where it is uprighted and connected to the foundation on the seabed. The upper part of the riser is connected to a submerged buoy. After installation of the hybrid riser pipe, the submerged buoy is connected via flexible jumpers to the surface facility such as an FPSO which may be located at a distance between 70-200 m from the buoy.

The known method has as a disadvantage that during riser installation no hydrocarbon production and/or processing can take place. Furthermore, installation requires special and dedicated installation equipment. Specialised installation vessels are designed to work in as large as possible sea states and are hence, sizeable and costly equipment.

From U.S. Pat. No. 4,182,584 it is known to attach a free-standing marine production riser for use in deepwater between a base portion and a submerged buoy. With a derrick-equipped vessel, such as a semi-sub, the riser casing is lowered through the central part of the buoy and coupled to the bottom until the rigid riser part is completed. Next, a flexible hose is attached to a surface facility for hydrocarbon production and processing.

Again, the use of separate vessels for riser installation and for hydrocarbon production/processing requires scheduling and mobilising the installation vessel to site at large day rates and the demobilisation of the installation vessel after installation of the riser.

Furthermore, in view of the large costs of the installation vessel, as many risers as possible would be installed when the installation vessel is on site, which implies capital outlay, fatigue and maintenance of the risers which are not producing.

It is hence an object of the present invention to provide a flexible system for riser installation and hydrocarbon production and/or processing, avoiding complex scheduling of the installation vessel and allowing riser installation at a suitable moment.

It is a further object of the present invention to provide a flexible riser installation method using relatively simple installation equipment.

It is another object of the present invention to provide a riser installation method in which during hydrocarbon production and/or processing additional risers can be rapidly installed.

Hereto, the vessel according to the present invention is characterised in that the vessel is anchored to the seabed, the vessel comprising a lifting means for lowering risers vertically towards the sea bed and for connecting a riser with a first end to a subsea hydrocarbon structure, which riser comprises a connector on a second end, the vessel comprising a connector for attaching to the riser connector and for placing the riser in fluid connection with the processing unit.

With the vessel of the present invention, no dedicated expensive riser installation vessels need to be used. By installing the risers from the vessel, it is possible to start hydrocarbon production and processing while at the same time installing the risers during stable weather conditions. The vessel of the present invention allows hydrocarbon production while obtaining information from the hydrocarbon field. When after start of hydrocarbon production it is required to drill and connect other nearby wells, this can be carried out simply from the installation equipment on the vessel.

The processed hydrocarbons may be stored in tanks on the vessel and transported to shore via shuttle tankers or may be transported via a pipeline from the vessel to another vessel or to an on-shore installation.

The means for lowering the risers may comprise a lifting device of the type such as described in European patent application number 02075311.7 which was filed on 25 Jan. 2002 in the name of the applicant. The lowering equipment described herein is relatively simple and takes up little deck space leaving sufficient room for hydrocarbon production and/or processing equipment.

In a preferred embodiment, the risers are extending alongside of the vessel. The vessel may comprise a derrick and a drill string extending to the seabed, such as through a moon pool in the vessel.

The risers may comprise a lower rigid (steel) part and may be connected to a submerged buoy, the upper part of the risers being made of a flexible material and extending from the buoy to the vessel. The vessel may be spread moored whereas the risers may be installed through a central shaft in the vessel according to another embodiment.

Some embodiments of a method according to the present invention will be described in detail with reference to the accompanying drawings, In the drawings:

FIG. 1shows a schematic view of a hydrocarbon production and/or processing vessel for carrying out the method of the present invention;

FIGS. 2-4show the sequence of horizontal extension of a riser, hook-up of the riser to a sub sea wellhead and connection to the vessel;

FIGS. 5-8show another method of riser installations according to the present invention;

FIG. 9shows an alternative method of riser installations according to the present invention employing a work vessel;

FIG. 10shows an embodiment of a vessel and a lifting device for carrying out the method of the present invention;

FIGS. 11-13show a detailed view of the lifting device ofFIG. 10;

FIGS. 14-15show a riser configuration installed by the method of the present invention;

FIGS. 16-17show a vessel having a turret moored configuration carrying risers that have been installed according to the present invention;

FIG. 18shows a spread-moored anchoring configuration of a vessel carrying risers according to the present invention;

FIGS. 19-22show a schematic view of a further embodiment of a method according to the invention;

FIG. 23shows an embodiment of the method using a separate working vessel;

FIGS. 24-30show a riser installation method using a separate work vessel; and

FIG. 31shows a method of obtaining a J-configuration of the riser using a work vessel.

FIG. 1shows a hydrocarbon production and/or processing vessel1according to the present invention. The vessel1may be connected to the seabed2via a number of anchor lines3which may be connected to a chain table4provided at the bottom of the vessel1. Alternatively, other configurations, such as a turret moored or a spread moored configuration, are also possible.

The vessel1comprises a hydrocarbon-processing unit5, such as oil and water separation systems, gas liquefaction equipment, a regassification plant, etc. Storage tanks6in the vessel1may contain crude and/or processed hydrocarbons.

From wellheads7,8on the seabed2, lower riser parts9,10extend to submerged buoys11,12. From the buoys11,12flexible risers13,14extend to connectors15,16on the vessel1. Each riser13,14comprises at its upper end a connector17,18which attaches to connectors15,16to place risers13,14in fluid connection with the processing unit5and/or hydrocarbon storage tanks6, via ducts19,20.

On deck of the vessel1, a lifting means22is provided having an upper lifting arm23and lower lifting arm24which can be moved in a vertical direction towards and away from each other. In case the vessel1is a FPDSO, on deck of the vessel1, a derrick25may be placed for drilling of a new well28, a drill string29extending through a shaft30in the vessel1.

FIG. 2shows the steel catenary riser (SCR)9being with its upper end connected to the lifting means22and extending at an angle α of 2°-10° with the vertical. The work vessel38pulls out the riser9via a winch40and chain or cable41connected to the lower end of the riser9, over the sea bed, to the well head7or to a well head connection, such as a manifold and the like. Instead of a work vessel38for pulling the riser9, it may be pulled to the well head7by a moored vessel using a winch. The chain or cable41preferably has a similar linear weight (kg/m) as the riser9in water, such that the riser9and chain or cable41follow the same catenary curve hence avoiding moments exerted on the pipe and consequent damage to the riser9. This is especially relevant for deep waters, such as water depths of 500 m and deeper, for instance between 1000 and 2000 m.

As shown inFIG. 3, the upper end of the riser9is connected to the lifting means22via a chain or cable34at the moment when the lower end of the riser9is situated near the well head7, wile the work vessel38continues to drag the riser9over the sea bed towards the well head7. When the lower riser end has reached the well head7, the riser is lowered from the work vessel38and is connected to the well head via a remotely operated vehicle (ROV), not shown in the drawing. The chain or cable41is disconnected from the riser by the ROV.

Next, as is shown inFIG. 4, the chain or cable34is pulled upward, so that the riser will have an inclination of about 15°-20° with respect to the vertical. With this angle of inclination, the riser9is connected to the vessel for transport of hydrocarbons from the well head7to the vessel.

FIGS. 5-8show the process of installing a rigid riser part32having a connector31at its lower end to stab onto a wellhead30on the seabed2. The riser32is assembled from segments33, which may be stored on the vessel1. The riser32is lowered via the lifting device22by releasing a clamping mechanism of the lower arm24and lowering upper arm23. Next, the clamping mechanism on the lower arm24is engaged with the top part of the risers32, the clamping mechanism on the upper arm23is released and the upper arm23returns to its upward position. Another segment33is clamped in the upper arm23and is connected to the riser section depending from lower arm24and the lowering cycle is repeated. A riser supporting buoy11may be stored on the vessel and is attached to the top of riser32, as is shown inFIG. 3. At the position of the lifting device22, the upper part of the buoy11may be connected to the upper arm23. As shown inFIG. 4, the buoy11is lowered from a cable34whereas a remotely operated vehicle35is operated from the vessel to attach connector31to the wellhead30. After attaching the connector31buoyancy is added to buoy11and the remotely operated vehicle35is operated to attach a flexible riser36with its lower end to the buoy11to be in fluid connection with riser32and with connector37at its upper end to connector16on the vessel1to be in fluid communication with processing unit5.

In the embodiment as shown inFIG. 9, a smaller vessel or tugboat38assists in lowering the lower end39of flexible riser36via a winch40.

FIG. 10shows the vessel1comprising the lifting device22according to the present invention. The lifting device22comprises a vertical frame23′ carrying the cable34having at spaced-apart locations support members in the form of broadened parts45,46,47. At the end of the cable34, a connector44is provided that is attached to the buoy11, which is being lowered via the cable34. The cable34is stored in a looped configuration in a storage compartment or hawse-hole40, substantially without being tensioned. From the storage compartment40, the cable34is guided via a sheave52to an upper lifting structure53and a lower lifting structure54. The upper lifting structure can travel up and down along the vertical frame23′ and can releasably engage with the broadened parts45,46,47on the cable34. Stationary lower lifting structure54can also engage and be disengaged with the broadened parts45,46,47. By the lower releasing lifting structure54and lowering the cable suspended from upper lifting structure53, the buoy11is lowered. After lowering of the upper lifting structure53by a certain amount, the lower lifting structure54is engaged with one of the broadened parts of the cable34, whereas the upper lifting structure53disengages from the cable and is returned to its upper position. In this way, the buoy11can be successively lowered until it reaches its desired depth. The buoy11may, prior to being lowered from lifting device22, be placed overboard by a crane57, which is thereafter disengaged such that the buoy can be lowered from cable34.

As an alternative to the looped configuration, the cable34may also be stored in the compartment50in a coiled from, for instance around a conical raised bottom part of compartment50, or be stored on a drum or, again, alternatively as separate line sections.

FIG. 11shows a detailed view of upper and lower lifting structures53,54. The upper lifting structure53comprises two parallel cylinders60,60′, which are powered by hydraulic pump62. Each cylinder comprises a sleeve63,63′ and a rod64,64′ moveably received within the sleeve63,63′. At the end of each rod64,64′ a pulley65,65′ is connected. Both pulleys are interconnected via a frame66. A moveable clamp67,67′ is slidably connected along each rod64,64′ and along each sleeve63,63′. The clamps67,67′ are connected to the end part of a cable having a first cable section68,68′ extending from the pulley65,65′ to the respective clamp67,67′ and a second cable section69,69′ extending along the rod64,64′ and sleeve63,63′ to a fixed position71,71′. As shown inFIG. 11, the lower lifting structure54is clampingly engaged via clamps55,55′ with a part of the cable34. An upper cable section72is engaged with clamps67,67′ of upper lifting structure53, such as shown inFIG. 12. Thereafter, the clamps55,55′ of the lower lifting structure34are opened, while upper clamps67,67′ remain engaged with the broadened part on the upper cable section72. Under control of the hydraulic pump62, the rods64,64′ are under the weight of the cable34and buoy11and riser32, pulled into sleeves63,63′ such that the clamps67,67′ descend along the sleeves63,63′. In the lowered position, such as shown inFIG. 13, the clamps55,55′ engage with a broadened part of cable section34. Hereby, the whole cable weight and the weight of the buoy11and riser32are again supported from the lower lifting structure54. The clamps67,67′ are then disengaged and the pulleys65,65′ are returned to their upper position as shown inFIG. 11. In an advantageous embodiment, the lifting structures53,54are formed as an integral unit in a frame, which is suspended from the crane57. In this way, heavy loads can be handled at large water depths from vessels having a standard crane by the lifting device of the present invention.

Instead of a lifting means as shown inFIGS. 11-13, a heave-compensated winch could be used.

FIG. 14shows a riser configuration installed according to the present invention in which a connector base80is attached to vertical riser32, which connector base80is connected to a wellhead81via a horizontal bottom pipe82. In the embodiment ofFIG. 15, the riser32is connected via a J-curved section84to the wellhead81and is being tensioned to the seabed via tensioning base83. Alternatively, the riser10may be connected to a connector base85in a J-curved manner, similar to riser32. After completion of the drilling operation, the drilling riser29and the derrick25may be removed.

FIG. 16shows a turret moored vessel90in which anchor lines91,92are with one end attached to a turret98and with another end to the seabed. Risers93,94are connected to a hydrocarbon processing unit on the vessel. Through a lifting means96, riser sections97are lowered from the deck of the vessel90through a central shaft95in the turret98to be attached to a subsea wellhead.

In the embodiment shown inFIG. 17, a turret-moored barge is shown in which the lifting device96is placed at the side of the vessel. Via a small tug97, the riser installation may be assisted.

FIG. 18shows a spread-moored configuration in which the risers93,93′,94,94′ are placed at the regularly spaced positions on the sides of the vessel, and are after lowering from crane96and attached to a buoy, towed in position by a mall tugboat.

FIGS. 19-22show a method of riser installation in which first a connector base80is lowered onto the seabed. Via ROV35a first end of riser82is connected to wellhead81whereafter the vessel1is moved towards connector base80for connection of the other end of riser82, such that bottom pipe82extends substantially horizontally on the seabed2. In the next steps, a vertical riser part, a buoyancy member and flexible riser such as shown inFIG. 14may be installed.

In the embodiment ofFIG. 23, riser sections83and buoyancy member84are supplied from a separate supply vessel38.

In the embodiment ofFIGS. 24-30, the small working vessel38is employed to lower the riser32and buoy11and to operate ROV35for connection of the risers32to a wellhead and for installing flexible riser13.

Finally, in the embodiment ofFIG. 31the working vessel38places the horizontal riser section84on the seabed in an upwardly extending J-configuration to obtain the riser configuration as shown at the left-hand side ofFIG. 15.