Subsea cable installation unit

Abstract

A subsea installation tool for installing a flexible body, e.g. a subsea fiber cable on a seabed from a surface vessel, and an associated method for installing the flexible body on the seabed is provided. The installation tool includes a tensioner that may be coupled to the flexible body to actively pull it down, thus increasing tension in an upper section of the cable from the installation vessel to the installation tool. High tension in the upper part of the cable gives good control even in strong sea currents. At the same time the tensioner enables lower tension in the bottom part cable from the installation tool to the seabed, giving good control of the touchdown position on the seabed.

Claims

1. A method for installing a flexible body on a sea bed from a surface vessel, the method comprising: deploying a subsea installation tool configured to install the flexible body on the sea bed, the subsea installation tool comprising: a main body configured to be suspended from the surface vessel using an umbilical, the main body having a mass and density sufficient to create a downward force in the umbilical; a flexible body guiding element coupled to the main body; a tensioner configured to be coupled to the flexible body and configured to actively pull down the flexible body, in a controlled manner, to make the tension in an almost vertical first section of the flexible body from the surface vessel to the subsea installation tool larger than the tension in a second section of the flexible body from the subsea installation tool to a touchdown position (TDP) of the flexible body on the sea bed; and a tension control loop configured to maintain the tension in the almost vertical first section of the flexible body at or near a given value based on a control signal received from a controller on the surface vessel, wherein the tension control loop comprises a flexible body tension sensing element configured to monitor the tension in the almost vertical first section of the flexible body based on measurement of vertical and horizontal forces, wherein the tensioner is configured to support measurement of the vertical and horizontal forces by the flexible body tension sensing element based on suspension of the tensioner in the subsea installation tool for supporting measurement of the vertical and horizontal forces by cable tension sensors communicatively connected to the controller on the surface vessel; and controlling the tensioner, based on the tension control loop, to make the tension in the almost vertical first section of the flexible body from the surface vessel to the subsea installation tool larger than the tension in the second section of the flexible body from the subsea installation tool to the TDP of the flexible body on the sea bed.

2. A subsea installation system, comprising: a subsea installation tool configured to install a flexible body on a sea bed from a surface vessel, the subsea installation tool comprising: a main body configured to be suspended from the surface vessel using an umbilical, the main body having a mass and density sufficient to create a downward force in the umbilical; a flexible body guiding element coupled to the main body; a tensioner configured to be coupled to the flexible body and configured to actively pull down the flexible body, in a controlled manner, to make the tension in an almost vertical first section of the flexible body from the surface vessel to the subsea installation tool larger than the tension in a second section of the flexible body from the subsea installation tool to a touchdown position (TDP) of the flexible body on the sea bed; and a tension control loop configured to maintain the tension in the almost vertical first section of the flexible body at or near a given value based on a control signal received from a controller on the surface vessel, wherein the tension control loop comprises a flexible body tension sensing element configured to monitor the tension in the almost vertical first section of the flexible body based on measurement of vertical and horizontal forces, wherein the tensioner is configured to support measurement of the vertical and horizontal forces by the flexible body tension sensing element based on suspension of the tensioner in the subsea installation tool for supporting measurement of the vertical and horizontal forces by cable tension sensors communicatively connected to the controller on the surface vessel.

3. The subsea installation system of claim 2, wherein the tensioner is configured to be communicatively connected to the controller on the surface vessel via first power transmission and communication means, wherein the controller is configured to control the tension in the flexible body.

4. The subsea installation system of claim 2, wherein the tensioner comprises one or more wheels, wherein the subsea installation tool is further configured to move one or more wheels along a generally linear trajectory to thereby create an opening in the tensioner, the opening being dimensioned to the diameter of the flexible body for moving the flexible body into or out of at least one of the tensioner or the flexible body guiding element.

5. The subsea installation system of claim 2, wherein the tensioner comprises one or more wheels, wherein the subsea installation tool further comprises a rotation means for moving the one or more wheels in a generally circular trajectory to thereby create an opening along the flexible body guiding element in the tensioner, the opening being dimensioned to the diameter of the flexible body for moving the flexible body into or out of the tensioner.

6. The subsea installation system of claim 2, wherein the subsea installation tool comprises a closable slot in parallel with and coupled to the flexible body guiding element to thereby allow the flexible body to be moved into or out of the tensioner.

7. The subsea installation system of claim 2, wherein the subsea installation tool comprises a releasable part configured to be released to the sea bed to act as an anchor for the flexible body to be installed.

8. The subsea installation system of claim 7, wherein the releasable part comprises a gripping mechanism configured to grip the flexible body.

9. The subsea installation system of claim 8, wherein the gripping mechanism is a clamp device.

10. The subsea installation system of claim 8, wherein the gripping mechanism is a part of or all of the tensioner.

11. The subsea installation system of claim 7, wherein the subsea installation tool further comprises one or more elements configured to increase stability when deploying the releasable part to the sea bed, the one or more elements selected from the group consisting of one or more spears, one or more cross bars, and one or more mud plates.

12. The subsea installation system of claim 2, further comprising: a trencher releasably coupled to the main body of the subsea installation tool.

13. The subsea installation system of claim 12, wherein the subsea installation tool further comprises: trencher power transmission means configured to supply power to the trencher; and trencher communication means configured to support communications of the trencher.

14. The subsea installation system of claim 2, wherein the main body comprises one or more loads giving a total tension in the umbilical that is several times more than the tension pulling down the flexible body.

15. A subsea installation tool configured to install a flexible body on a sea bed, comprising: a main body configured to be suspended from a surface vessel using an umbilical, the main body configured to have a mass and density sufficient to create a downward force in the umbilical; a flexible body guiding element coupled to the main body and configured to guide the flexible body; a tensioner configured to be coupled to the flexible body and configured to actively pull down the flexible body, wherein a tension in an almost vertical first section of the flexible body from the surface vessel to the subsea installation tool can, in a controlled manner, be made larger than a tension in a second section of the flexible body from the subsea installation tool to a touchdown position (TDP) of the flexible body on the sea bed; and a tension control loop configured to maintain the tension in the almost vertical first section of the flexible body at or near a given value based on a control signal received from a controller on the surface vessel, wherein the tension control loop comprises a flexible body tension sensing element configured to monitor the tension in the almost vertical first section of the flexible body based on measurement of a force vertical and horizontal forces, wherein the tensioner is configured to support measurement of the vertical and horizontal forces by the flexible body tension sensing element based on suspension of the tensioner in the subsea installation tool for supporting measurement of the vertical and horizontal forces by cable tension sensors communicatively connected to the controller on the surface vessel.

16. The subsea installation tool of claim 15, wherein the main body is configured to give a tension in the umbilical that is greater than the tension in the almost vertical first section of the flexible body to thereby enable the subsea installation tool to remain suspended from the umbilical while a portion of the tension in the umbilical is shifted to the flexible body.

17. The subsea installation tool of claim 15, wherein the tensioner is configured to actively pull down the flexible body by shifting a portion of the weight of the main body from the umbilical to the flexible body.

18. The subsea installation tool of claim 15, wherein the tension control loop comprises: a set of motors configured to adjust the tension in the almost vertical first section of the flexible body.

19. The subsea installation tool of claim 15, wherein the flexible body tension sensing element is connected to the controller via the umbilical.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The installation tool and the method for installing a flexible body according to the invention will now be described in more detail with regard to the accompanying drawings. The drawings show one way of implementing the present invention and is not to be construed as being limiting to other possible embodiments falling within the scope of the attached claim set.

(2) FIG. 1 is a cross-sectional view of an example embodiment of the installation tool according to the invention.

(3) FIG. 2 is a front view of the installation tool according to the invention showing the tensioner when opened for flexible body release/entry subsea.

(4) FIG. 3 illustrates the separation of the first (upper) and second (bottom) parts of the installation tool for carrying out special operations.

(5) FIG. 4 illustrates an example embodiment of a method according to the invention, i.e. an installation operation making use of the installation tool according to the invention.

DETAILED DESCRIPTION OF AN EMBODIMENT

(6) FIG. 1 illustrates schematically the installation tool of the invention that enables a pulldown of the flexible body to be installed. The flexible body 1.sub.A,B, such as for example a subsea fibre optic cable is coupled, for example by clamping, to an installation tool and controlled by a tensioner 4.sub.1-2,A-B in the tool. The flexible body 1.sub.A,B is to be installed on the seabed 17 from a surface vessel 16 as indicated on FIG. 4. The installation tool is suspended from the vessel 16 using an umbilical 3. The installation tool initially has a weight sufficient to create a significant tension in the umbilical 3. The weight of the installation tool is mainly formed by a main body 2 possibly with additional loads 23, 24. By pulling down the cable 1.sub.A,B the tension in an upper part of the cable increases, thus making the upper section 1.sub.A of the cable more resistant towards shifts in position due to drag effects caused by (varying) sea currents, tidal effects, etc.

(7) The tensioner 4.sub.1-2,A-B may be coupled to the cable 1.sub.A,B so as to be able to actively pull the cable 1.sub.A,B down in a manner controlled by the tensioner using for example wheels or a caterpillar element to adjust the cable tension. The tension in a first, upper section 1.sub.A of the cable from the installation vessel 16 to the installation tool 2 is made large to eliminate drag effects and still keep the cable tension low in a second, bottom section 1.sub.B of the cable from the installation tool 2 to a cable touchdown position TDP on the seabed 17.

(8) The installation tool represents an improvement as compared to the known prior art in that the tensioner 4.sub.1-2A-B the installation tool enables an increase of the tension in the mainly vertical section of the cable 1.sub.A,B, while simultaneously ensuring a low back tension. The term back tension refers to the tension in the second, bottom section 1.sub.B of the cable running towards the seabed. The weight of the installation tool may be in the range 5-15 tons. The tensioner 4.sub.1-2,A-B may tighten the cable 1.sub.A,B, typically by shifting about 1-2 tons of the weight of the installation tool from the umbilical to the cable 1.sub.A,B, thereby obtaining a large tension in the first, upper section 1.sub.A of the cable. The large tension in this first part of the cable effectively reduces effects of drag and sea currents on the catenary. At the same time a low back tension is achieved, typically in the area of about 1-200 kg, in the second section 1.sub.B of the cable going to the sea bottom. The speed of the vessel and a Linear Cable Engine (LCE) must be synchronised to keep the back tension low. The installation tool 2 and the TDP may this way both be kept closer to a vertical line than in the known prior art solutions where a high tension is applied over the cable 1.sub.A,B as a whole, i.e. over the full length of the cable, where a high back tension tends to pull the cable away from a vertical line. The tension in the cable 1.sub.A,B, the weight of the installation tool 2 and the height of the installation tool above the sea bottom can be adjusted to minimize the expected drag effects and to optimize cable tension.

(9) The use of a tensioner 4.sub.1-2,A-B is fairly simple and reliable when mounted in the installation tool as described in this disclosure because the installation tool automatically follows/synchronizes with the movements of the vessel 16. As an alternative placement one might consider mounting the tensioner 4.sub.1-2,A-B in front of a separate ROV. However, this will increase the risks associated with the relative movements between the surface vessel and the ROV at the sea bottom. Installation using a wave compensating crane could reduce this problem somewhat however, it will require a more complex installation line on the vessel to compensate for the relative movements between the crane and the LCE.

(10) The tensioner 4.sub.1-2,A-B may comprise wheels 4.sub.N (N=1, 2) or a caterpillar structure for clamping onto the cable 1.sub.A,B, and to tension the cable. A set of one or more wheels 4.sub.N that are clampable onto the cable 1.sub.A,B is shown in FIG. 1. The wheels 4.sub.N may be run by electric or hydraulic motors 4.sub.B that are usually synchronized by torque. The tensioner 4.sub.1-2,A-B controls the tension in the cable 1.sub.A,B using the electric and/or hydraulic motors 4.sub.A,B. The desired value of cable tension is typically set by operators or a main control system on the vessel 16, and communicated from the vessel to the installation tool via the umbilical 3. The installation tool comprises a tension control loop for maintaining the tension in the cable at or close to the desired value. The tension control loop includes the motors 4.sub.A,B and a suitable cable tension sensing element for monitoring the tension in the cable. The tensioner 4.sub.1-2,A-B may be suspended in the installation tool 2 so that the vertical and horizontal forces may be measured by suitable cable tension sensors connected to the control unit 16 via the umbilical 3.

(11) The tensioner 4.sub.1-2,A-B may be opened so the installation tool can engage or release the cable 1.sub.A,B also when deployed in a subsea environment 18. At least one of the wheels 4.sub.N is movably mounted in the installation tool 2, so as to be able to open up space, e.g. a slot, through which the cable 1.sub.A,B may be entered into or removed from the installation tool. Advantageously, such a slot provides an opening from the chute 5 and to a front F and a bottom B of the installation tool, thereby coupling the interior of the chute 5 with the surroundings 18, and so that a near vertical cable 1.sub.A,B in a subsea position can be conveniently engaged or released by the installation tool. For this purpose, at least one of the wheels 4.sub.N can be mounted on a tilting arm, a pivot arm, or a rotatable structure, to be able to move a wheel away from its normal operating position to a free position, to allow unhindered movement of the cable through the slot-shaped opening and into the chute. When the cable 1.sub.A,B is near the inner wheel 4.sub.2, the outer wheel 4.sub.1 is moved back to its normal operating position to clamp the cable and also to close the slot. In order to close and open the installation tool for entering or removing the cable there may also be provided a movable element, e.g. a movable wall of the chute 5, that may be moved between two positions, a first closed position and a second open position, for control of the opening and closing of the chute 5 and the space for entering or removing the cable from the installation tool.

(12) For allowing a station on a cable 1.sub.A,B to pass the wheels 4.sub.N, the position of one or more of the wheels 4.sub.N are controlled by a hydraulic pressure device or alternatively a controlled spring device being a part of or mounted on the tilting arm, the pivot arm, or the rotatable structure and provided with a suitable control mechanism for maintaining a constant clamp pressure for a varying diameter of the cable, as will be the case for a cable with stations.

(13) To ensure smooth movement of the cable 1.sub.A,B and possible associated sensor station 1.sub.C through the installation tool it is required that the width of the chute 5 has to be at least as large as the diameter of sensor stations 1.sub.C along the cable 1.sub.A,B so as to allow the sensor stations to slide comfortably through the chute 5. Further, it is desirable that the cable is guided along a straight path when passing the wheels 4.sub.N of the tensioner 4.sub.1-2,A-B. For this purpose the chute 5 may include straight chute sections 5.sub.1,2 on either side of the wheels 4.sub.N to align the cable 1.sub.A,B and the sensor stations 1.sub.C and minimize the risk of cable damage when the cable station enters or leaves the wheels 4.sub.N. The cable 1.sub.A,B and stations have to move between the wheels and there should not be spaces allowing the cable/stations to be pushed to a side of the wheels. The straight chute sections 5.sub.1,2 are dimensioned so as to be able to accommodate the sensor stations 1.sub.C.

(14) A first chute opening 5.sub.A facing in the direction towards the vessel 16 will typically be wider than a second chute opening 5.sub.B facing in the direction of the TDP, in order to accommodate the larger cable fluctuations expected in the first section 1.sub.A of the cable due to waves and fluctuations in sea currents before the tension is generated during operation of the tensioner 4.sub.1-2,A-B. Some width of the first chute opening 5.sub.A may also be advantageous in that it simplifies the picking operation where the cable is taken into the chute 5 and before the installation tool clamps onto the cable 1.sub.A,B to hold the cable in a controlled position using the cable guiding means 4.sub.1-2,A-B.

(15) The installation tool may bring the end of the cable 1.sub.A,B to be installed from the vessel 16 and down to the seabed 17 during lowering. In an alternative the installation tool may grab a cable 1.sub.A,B at a subsea location, provided the cable has a vertical part which the installation tool may couple to. Such a cable 1.sub.A,B could be lowered by a separate load or by another installation unit. The installation tool may then be guided towards the cable 1.sub.A,B by the umbilical 3 and the vessel 16. The installation tool may also be equipped with thrusters 7 for improved control of the detailed subsea maneuvering as far as the umbilical 3 allows. The installation tool can fly towards the cable 1.sub.A,B, enter the cable in the chute 5, and grab the cable by moving one or more of the wheels 4.sub.N of the tensioner 4.sub.1-2,A-B, as illustrated in FIG. 2. The cable 1.sub.A,B may also be guided in between the wheels 4.sub.N of the tensioner 4.sub.1-2,A-B and via a slot provided in parallel with the chute 5 from the chute 5 and to a side face S of the installation tool. The slot only requires a width 2-3 times the diameter of the cable itself. Opening/closing of the slot may be achieved by moving/rotating the sidewall of the chute. In this case the required movement of the clamping wheels 4.sub.N is simplified as it will now only necessary to move the wheels 4.sub.N away from each other so as to allow entry of the cable between the wheels.

(16) During operation the installation unit is coupled to the installing vessel 16 by the umbilical 3. First power and signal transmission means including transmission lines are integrated in the umbilical 3 ensures delivery of power and communication to the installation tool.

(17) There is no need for further vertical lines than the umbilical 3 and the cable 1.sub.A,B to be installed, thus minimizing the risk of entanglement of the vertical lines. The lower end of the umbilical 3 may be clamped to a stiff steel messenger 11 on the installation tool. The steel messenger could be arranged to rotate around a pivot point. If the pivot point includes a torque motor the tilt of the installation tool can be optimised for catching the cable 1.sub.A,B and for other operations.

(18) The tensioner 4.sub.1-2,A-B and the thrusters 7 are supplied with power and control signals via first power and communication means being an integrated part of the umbilical 3. The installation tool may also be provided with navigation units as well as camera and lighting equipment. Such equipment may be accommodated in sealed modules 21,22, for example in the upper part 2.sub.A of the installation tool, and/or in a free space provided above a bottom load 23 in a lower part 2.sub.B of the installation tool. When referring to upper 2.sub.A and lower 2.sub.B parts of the installation tool in the context of this application the word upper and lower refer to the relative positions of the first 2.sub.A and second parts 2.sub.B of the installation tool when suspended in its operative state.

(19) The second chute opening 5.sub.B at the bottom of the installation tool may be narrower than the first chute opening 5.sub.A, because the direction of the cable 1.sub.A,B is defined by the TDP. A certain width is required in the second chute opening 5.sub.B due to the same reasons as for the first chute opening 5.sub.A. In addition there will be some vertical cable movements due to the waves lifting the vessel 16 and the installation tool, thereby changing the cable catenary and touchdown position periodically.

(20) The weight of the installation tool must be adjusted so as to handle the level of sea currents to be expected, the drag effects on both the cable 1 to be installed as well as the umbilical 3, and the vertical tension in the cable 1.sub.A,B. The overall weight can be divided between the upper and lower parts 2.sub.A,B of the installation tool. The lower part 2.sub.B could carry a bottom load 23, and the upper part 2.sub.B may have an upper load 24. The loads 23, 24 could be formed as solid metal or cement blocks, so as to form a large weight in the installation tool.

(21) The installation tool can be manufactured so that the upper and lower parts 2.sub.A,B may be split apart during operation. The lower part 2.sub.B of the installation tool may be placed at the sea bottom and released and used as an anchor to maintain the tension in the cable 1.sub.A,B to be installed. If the installation tool is to carry out other operations during installation of the cable 1.sub.A,B, the lower part 2.sub.B of the installation tool may be clamped to the cable 1.sub.A,B and located on the sea bottom 17.

(22) The upper part 2.sub.A of the installation tool may as explained above be released and has an operating range defined by the risk for entanglement between the cable to be installed and the umbilical. The installation tool may then also be retrieved separately, and the upper and lower parts 2.sub.A,B may be re-coupled subsea by use of guide pins, Hydraulically activated guide pins 25 will lock the upper and lower parts 2.sub.A,B together. To allow for separation the installation should have wet mate connectors/stab plates 26.

(23) During such a separating operation the lower part 2.sub.B must include a gripping mechanism for holding the cable 1.sub.A,B. The gripping mechanism can be the tensioner 4.sub.1-2,A-B itself which is already adapted to grip the cable 1.sub.A,B. A hydraulic tensioner may grip for some time by use of accumulators. Anchoring for a long time may require more permanent locking pins activated by hydraulic or electric actuators. The locking pins will be activated when the upper and lower parts 2.sub.A,B of the installation tool are decoupled and the tensioner does not have energy available (fail-safe concepts).

(24) The lower part 2.sub.B of the installation tool can include one or more spears 31, as illustrated on FIG. 3, to be injected or pushed into the seabed for more stable and fixed location. Alternatively or in addition the lower part 2.sub.B may also include cross bars or mud hats at the bottom to get a stable operation depending on the soil conditions. The first, wide, chute opening 5.sub.A follows the upper part 2.sub.A of the installation tool when split to remove the drag forces resulting from that part of the chute 5, by moving it away from the lower part 2.sub.B of the installation tool. Only a minor chute 6 is required to ensure a reliable operation when the lower part 2.sub.B is on the seabed with a clamped cable 1.sub.A,B. This minor chute 6 will be outside of the lower end of the top chute 5.sub.A when the upper and lower parts 2.sub.A,B of the installation tool are coupled together.

(25) When deploying a seismic cable 1.sub.A,B with seismic stations 1.sub.C, it may be required to trench the stations to some degree for improving the acoustic coupling to the ground and/or for protection purposes. The trencher may be provided with handling means 8.sub.A adapted for applying a pressing/pushing or wiggling of the seismic station into the seabed.

(26) In some situations it may only be the seismic stations that must be trenched. Such a point trenching leaves more time for the trenching while the installation tool continues installation of the part of cable 1.sub.A,B between stations. The trencher 8 must, however, keep up with the installation tool operation to be ready for the trenching of the next station.

(27) The trencher 8 can be clamped onto the installation tool during deployment and retrieval as seen on FIG. 1. There may be a separate trencher locking mechanism 26 on the installation tool or the trencher can have manipulators to perform this function. The installation tool may include a hinged docking base 27 to rotate the trencher from horizontal to vertical during installation and retrieval. This will reduce the overall drag forces. The trencher 8 can include a reel 28 for the required trencher umbilical 9 from the installation tool to the trencher 8. The reel 28 will control the length of umbilical required for each operation/positioning of the trencher 8.

(28) The main structural parts of the installation tool can be produced from steel bars to reduce the overall drag effects. Even the top chute 5.sub.A can be made from steel tubing to reduce drag. Pressure vessels for instrumentation etc will be located so as to add a minimum of additional drag.

(29) The trencher umbilical 9 forms a part of second power transmission and communication means for supplying power to the trencher 8 and for communicating with devices and units on the trencher, e.g. from the surface vessel via the installation tool. The trencher may also include other means known to a person skilled in the art for assisting in moving around and performing its operations, such as e.g. trencher thrusters 12, a camera, lighting equipment, and/or navigation devices 12. The installation tool umbilical 3 may be designed to have capacity for supplying other units with power and signal capacity. Wet mate connectors may be implemented on suitable locations for mating subsea. The trencher 8 may be connected to the upper part 2.sub.A or the lower part 2.sub.B of the installation tool, or both in parallel. In this way it can be chosen whether the trencher 8 is going with the upper part 2.sub.A of the installation tool to carry out other operations or land close to the parked lower part 2.sub.B while the upper part 2.sub.A is carrying out another operation.

(30) The installation tool may be towed without the product (the tensioner 4.sub.1-2,A-B and the trencher 8) at higher speeds for pre-surveying and post-surveying operations. It is for this reason most useful to have surveying instruments mounted on the upper part 2.sub.A of the installation tool.

(31) The installation tool can include a manipulator arm to operate subsea shackles, mate connectors, remove items etc. The arm may be useful for repair operations. It can be used to locate and inspect a failed part, either directly or by use of an attached ROV. The installation tool or an ROV can handle cable cutting tools and cable grippers. Installation tool may also be provided with equipment for coupling the cable end to e.g. a hub at the seabed. The installation tool can assist during installation of the loop of excess cable usually required in a cable repair.

(32) FIG. 4 is a sketch of a typical application of the installation tool according to first aspect of the present invention. FIG. 4 also illustrates a method according to the second aspect of the present invention for installing a flexible body, such as a subsea optical fibre cable, at a sea bed location by guiding the body during deployment or retrieval with an installation tool according to the first aspect of the invention.

(33) An installation vessel 16 is placed on the sea surface almost straight above the point of installation. Then the umbilical 3 with the installation tool at the end is lowered into the sea. From a separate position on the deck of the vessel 16 a cable 1.sub.A,B is also lowered into the sea. The umbilical 3 with the installation tool will deploy in a generally vertical direction down towards the bottom and keep the installation tool about 10-20 meters above the seabed 17. The high weight of the umbilical 3 and the weight of the installation tool keep the umbilical 3 in an almost vertical line even in strong sea currents. The umbilical is heavy due to the required strength and all armouring (steel cables). The cable 1.sub.A,B to be installed is almost parallel to the umbilical but will usually have more curved catenary due to drag effects. The detailed shape of the catenary of the cable 1.sub.A,B will depend on the cable design, sea current, and the pull down tension from the tensioner 4.sub.1-2,A-B in the installation device when this is set in operation.

(34) The umbilical 3 and the cable 1.sub.A,B may be deployed from opposite sides of the vessel 16 to increase their separation. This will reduce the risk of entanglement. The curve followed by the cable 1.sub.A,B will bulge in the direction of the sea current.

(35) The vessel 16 may be rotated to a position that will maximize the separation between the cable 1.sub.A,B and the umbilical 3. In order to reduce vertical movement due to the sea waves, the cable 1.sub.A,B and umbilical 3 may be deployed from the centre of the vessel 16. A trencher 8 may be a fixed part of the installation tool during deployment and retrieval of the installation tool. The vessel typically moves sideways during installation, as indicated by the arrow A in FIG. 4.

(36) Following the lowering of the umbilical 3 and the cable 1.sub.A,B the installation tool is moved so that the cable 1.sub.A,B enters the tensioner, to be clamped in position by the wheels 4.sub.N or a caterpillar element of the tensioner 4.sub.1-2,A-B. Then the tensioner is activated to pull down the cable so as to create a high tension (typically in the range of 1-2 tons) in the first part 1.sub.A of the cable 1.sub.A,B. A low tension (typically in the range of 1-200 kg) in the second part 1.sub.B of the cable 1.sub.A,B can be achieved by proper position of the vessel with respect to the TDP.

(37) The catenary of the cable 1.sub.A,B will touch down on the seabed at a certain distance behind the installation tool. The touchdown position (TDP) is fairly close (typically 100m) to the vessel due to the weight of the cable, the low cable tension and the high height of the installation tool.

(38) After touchdown the trencher 8 can start trenching of the sensor stations only or alternatively the whole cable. If the whole cable is trenched the trencher 8 must move along the cable with the speed of the vessel in a continuous manner.

(39) Alternatively, if only the stations are to be trenched, there is more time for the trenching at each station. The trencher 8 can maintain its position at a station for some time to do the trenching while the vessel continues installation at a constant speed, and when finished trenching one station the trencher 8 can catch up with the vessel by moving swiftly to the next trenching position (the next station). In other words, the trencher 8 is maintained in position while placing a station in a trench. In such a point trenching scenario more attention must be paid to the trencher umbilical to avoid this from getting snagged.

(40) The operating range of the trencher 8 is defined by the length of the trencher umbilical. However, it will be obvious to a person skilled in the art when reading this disclosure that the handling of the trencher umbilical from the installation tool is simpler than having another umbilical down from the vessel 16 that could possibly tangle with the other vertical lines. In applications in deep waters the distance between the lines (umbilical/cables) is almost negligible compared to the depth (20-50 m separation for a depth of 1-2000 m).

(41) In summary, there is provided subsea installation tool for installing a flexible body, e.g. a subsea fibre cable 1.sub.A,B on a seabed from a surface vessel, and an associated method for installing the flexible body on the seabed is provided. The installation tool includes a tensioner 4.sub.1-2,A-B that may be coupled to the flexible body 1.sub.A,B to actively pull it down, thus increasing tension in the flexible body in an upper section 1.sub.A of the cable from the installation vessel 16 to the installation tool. High tension in the upper part of the cable gives good control even in strong sea currents. At the same time the tensioner enables lower tension in the flexible body in the bottom part cable from the installation tool to the seabed, giving good control of the touchdown position on the seabed.

(42) The overall performance of a subsea cable deployment will not depend on the installation tool alone, as the overall performance will depend on the cooperative effort of the vessel, the installation tool and the monitoring, control and communication system tying the various components together so as to handle the cable, the umbilical and the installation tool during varying conditions of the sea and the sea bed.

(43) Although the present invention has been described in connection with the specified embodiments, it should not be construed as being in any way limited to the presented examples. The scope of the present invention is set out by the accompanying claim set. In the context of the claims, the terms comprising or comprises do not exclude other possible elements or steps. Also, the mentioning of references such as a or an etc. should not be construed as excluding a plurality. The use of reference signs in the claims with respect to elements indicated in the figures shall also not be construed as limiting the scope of the invention. Furthermore, individual features mentioned in different claims, may possibly be advantageously combined, and the mentioning of these features in different claims does not exclude that a combination of features is not possible and advantageous.