The present invention relates to a method and system for deploying an elongate member, such as a cable, underwater from a vessel.

It is necessary to deploy cables and other elongate members underwater for a variety of reasons. For instance, subsea cables are used to transmit current generated by offshore wind turbines, or in the telecommunications industry, to carry telecommunications signals across bodies of water. Subsea elongate members are also used in other industries, for example, in the oil and gas industry, such as umbilicals or risers connected to subsea wells.

The movement, especially heave, of a vessel deploying the elongate members can affect the deployment thereof. There are two main limiting factors during subsea installation especially in a conventional S-lay configuration; the minimum allowable radius of curvature and the avoidance of axial compression at seabed.

To work within acceptable parameters, it is known to provide a tensioner on the vessel to appropriately tension the elongate members as they are deployed from the vessel. The tensioners may comprise two opposing belts driven by respective rollers which capture the elongate member between the belts and direct it towards an exit chute from the vessel. Larger vessels generally provide a more stable and slower moving platform, allowing cable lay operations to continue in higher sea states than smaller vessels.

Whilst generally satisfactory, the known equipment to tension and deploy the elongate members is costly and more suited to larger vessels, such that the overall operation can incur significant expense.

An object of the present invention is to provide improved equipment/method for deployment of elongate members and/or equipment/method more suited for smaller vessels.

According to a first aspect of the invention, there is provided a method of deploying a flexible elongate member into water comprising:

(i) suspending a tensioner system underwater from a winch apparatus, the winch apparatus mounted on a deployment frame which is in turn mounted on a vessel on the water;

(ii) the winch apparatus having active heave compensation to at least in part, offset movement of the vessel in the water, and reduce the movement of the tensioner system relative to the vessel;

(iii) feeding the elongate member from above the water to under the water and into, through and out of the tensioner system, thus deploying the elongate member under the water.

Flexible elongate members include cables, risers, umbilicals. They are flexible and so are able to be stored in a continuous length of at least 100m, optionally at least 500m, coiled on a reeled storage device or in a basket.

Preferred embodiments deploy a cable.

The method may be performed in various water depths. For example, relatively shallow depths of up to 100 m, or up to 400 m. Or deeper water such as between 400 and 1000m or even deeper.

The elongate member may be deployed in an S-lay configuration.

The elongate member may be placed on or in the seabed (i.e. may be later buried).

The deployment frame may comprise a crane.

The vessel normally has a vessel chute over which the elongate member is paid out during operations.

The system may be used in combination with a remotely operated vehicle (ROV) to observe the positioning of the system so the elongate member can be deployed relative to the vessel, or for example as close as possible to the target position.

Embodiments of the present invention may be used in combination with underwater acoustic positioning and/or thrusters (which would largely negate the need for using an ROV). Ultra- short baseline underwater acoustic positioning (USBL) may be particularly useful.

An advantage of using USBL positioning is that the tensioner system does not need to stop due to poor visibility subsea when using an ROV, or a ROV losing the target. The tensioner system may be suspended between 10m above the seabed and 10m below the surface. It may be suspended at least 100m above the seabed. It may be suspended at least 100m below the surface.

According to a further aspect of the invention, there is provided a tensioner system suitable for use underwater having a tensioner comprising: an elongate member entry; an elongate member exit; drive means for driving an elongate member from said entry to said exit; a motor for driving the drive means.

The tensioner system of the further aspect of the invention can be used in the method according to the first aspect of the invention and optional steps of that method are independently optoinal steps for use with the tensioner system.

The tensioner system can also hold or clamp the elongate member to prevent movement.

The tensioner system may comprise an angle entry sensor to determine the movement and/or angle of the elongate member relative to the tensioner at the entry.

The tensioner system may comprise a movement and/or an angle exit sensor to determine the angle of the elongate member relative to the tensioner at the exit.

For certain embodiments, the sensor(s) can be in the form of camera(s) with suitable software for determining the angle of the elongate member relative to the tensioner.

The system may include an arcuate entry chute for resisting the elongate member from exceeding a minimum bend radius as it enters the elongate member entry.

Similarly, the system may include an arcuate exit chute for resisting the elongate member from exceeding a minimum bend radius as it exits the elongate member entry.

The minimum bend radius (MBR) may be from 0.5m to 5m. Optionally from 2 to 5 m, more optionally from 3 to 4m. The MBR may be relatively low for telecoms cables and larger for flexible flowlines.

The arcuate chutes together can be in the form of a semi-circular apparatus known as a quadrant.

The drive means may comprise a two-track tensioner.

The drive means may comprise a wheel-pair tensioner. The drive means may comprise opposing rollers and optionally opposing belts driven by the rollers, the belts configured to capture the elongate member therebetween.

An ejector is usually provided to separate the elongate member from the drive means (especially at least one belt) and eject it therefrom. The ejector may be retractable, and optionally in the form of a ram or lever.

The system may comprise a locking mechanism to lock the elongate member in position, rather than, at the time of locking, feeding the elongate member through the system.

The motor for the drive means may comprise a hydraulic motor or an electrical motor.

Such a hydraulic motor and optionally other any other hydraulic systems included (for example to drive a retractable ejector) usually include pressure-compensators, to function at the varying pressures at varying water depths.

The tensioner system may include electrical systems. Such electrical systems are also normally rated for subsea use.

The drive means and motor may be provided in a watertight housing (watertight at the depths to be used).

The tensioner system can comprise subsea thrusters. It may also include at least one of a camera, a light, a sonar device, a gyroscope and a compass.

The system may also include an obstacle avoidance sonar configured to adjust the tensioner system position in use in response to detected obstacles.

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying figures, in which:

Figure 1 is a schematic overview of the tensioner system of the present invention in use;

Figure 2 is an enlarged schematic view of the Figure 1 embodiment;

Figure 3 shows plotted cable compression results obtained by global dynamic analysis;

Figure 4 shows plotted cable tension comparison results obtained by global dynamic analysis;

Figure 5 shows plotted cable minimum bend radius (MBR) comparison results obtained by global dynamic analysis; Figure 6 shows plotted active heave compensation (AHC) maximum velocity results obtained by global dynamic analysis; and,

Figure 7 shows active heave compensation maximum tension results obtained by global dynamic analysis.

Figure 1 shows a tensioner system 10 according to one aspect of the present invention comprising a tensioner 16, an entry sensor 12 at a cable entry point, and an exit sensor 14 at a cable exit point. The entry/exit sensors 12/14 measure the angle at which a cable 18 being deployed enters/exits the tensioner 16 and its movement. The system 10 is powered by a motor (not shown).

The system includes arcuate entry chute 11a and arcuate exit chute 11b to support the cable as it enters and exits the tensioner 16, and resist bending past its minimum bend radius.

The chutes may be provided as part of a subsea quadrant.

The tensioner system 10 is suspended on a wire or umbilical 17 from a crane (or A-frame) 13 which in turn is mounted on a vessel 15. The crane 13 has active heave compensation (AHC) functionality meaning that the wire 17 can be automatically adjusted to compensate for surface, floating vessel movement. The movement of the suspended tensioner system relative to seabed is significantly reduced in relation to the movement of the surface, floating vessel relative to seabed. Techniques which can be used for AHC are known in the art especially for lowering equipment such as wellheads subsea and are available from, for example, Macgregor (macgregor.com) or Huisman (huismanequipment.com).

Before use of the tensioner system 10, the cable 18 to be deployed is passed through a cable entry point end in the tensioner 16 and clamped while the tensioner system 10 is on the deck of the vessel 15.

The tensioner system 10 is then deployed subsea from the surface of the body of water 19 to a predetermined position. In this way, distance from the vessel to the tensioner system in use is less than the vessel to the seabed as with known systems. Accordingly, an advantage of embodiments of the present invention is that topside axial tension is reduced, in addition to topside side wall pressure being reduced.

Figure 2 shows the tensioner system in more detail. Optional thrusters 24 are connected to the tensioner 16, which may be in the form of powered rotating blades. Suitable thrusters may be obtained from, for example, Forum Energy Technologies. The thrusters may be hydraulic thrusters, or electric drive systems. A survey sensor package 26 is connected to tensioner 16 such as the chute 11a. The survey sensor package 26 aids the remote operation of the tensioner system 10. It may include cameras, lights, sonar devices, gyroscope, compass or a combination thereof.

The cable may be fed out from a reel (not shown) on the vessel, over a vessel chute (not shown), and then take up the configuration shown in Figure 1. As the vessel 15 moves forward, the cable continues to be paid out, and rests on the entry chute 11a, enters the tensioner 16 subsea before being deployed via the exit chute 11b onto the seabed 20.

A retractable ejector (not shown) is usually provided to separate the cable from the belt and eject the cable from the tensioner 16. The ejector may be a ram or lever.

The active heave compensation of the crane allows the tensioner 16 and cable 18 between the tensioner 16 and seabed to be relatively static in the water column. An advantage of this portion of the cable 18 remaining static is that it experiences minimal change in radius and tension at seabed touchdown point. Since the system in use is suspended in water the cable weight and therefore cable tension experienced at the vessel chute and the entry chute 11a is reduced. Moreover, since the cable hangs between the vessel chute and device entry chute 11a it experiences minimal compression, irrespective of vessel motion.

By providing a means for detecting and measuring the angle or movement of the cable 18 as it enters the tensioner 16, this allows an operator, for example, on the surface floating vessel, to adjust the position of the cable 18 relative to the surface floating vessel 15 to maintain the desired entry angle of the cable 18 as it enters the tensioner 16. Similarly, by including a means for detecting the angle of the subsea cable 18 as its exits the tensioner 16, an operator may be able to adjust the speed of the tensioner 16 that is adjusted relative to the surface floating vessel 15 to maintain the desired exit angle of the cable 18 from the tensioner 16. Consequently, more control over the lay accuracy may be obtained and therefore a more accurate lay of the cable 18 may be achieved.

The tensioner may be automated such that it can be operated without human intervention. For example, the automated tensioner may comprise a sensor or a number of sensors which measure various parameters as set by an operator which allows the tensioner to function automatically. Speed, angle and/or movement sensors can be used to provide feedback to a control system, which adjusts the tensioner 16 speed and/or the speed of a reel/carousel/tensioner on the vessel 15, that is responsible for controlling the speed of cable 18 payout from the vessel. Automated cable tensioners are known in the art and can be purchased from, for example, MacArtney Underwater Technology (macartney.com). Whilst semi-circular in shape, unpowered so-called subsea quadrants are known in the art and can be purchased from, for example, Ardmore Craig Design (ardmorecraig.com).

The sensors for measuring the angle of entry and exit of the cable in the tensioner system may be an off-the-shelf sensor commercially available from, for example, Soil Machine Dynamics (smd.co.uk). A camera may be used to automatically sense the movement or angles as well.

The sensors can be used to create a feedback loop of the measured parameters. An advantage of the feedback loop is that it can be used to control the speed of the lay of the cable, among other parameters, either to an operator who manually adjusts speed to compensate, or it can be used for an automated response as described above.

The system may also comprise a cable approach sensor, which detects when the cable approaches the seabed touchdown point. Continuous feedback from the cable approach sensor can enable quicker topside response times compared with ROV monitoring seabed touchdown point and layback.

Embodiments of the present invention may also be used with obstacle avoidance sonar connected to the tensioner system. Advantageously, the vessel position may be adjusted during the lay of the cable in order to avoid any objects detected by the obstacle avoidance sonar.

The system may have the option to “lock” (for example, maintain the cable in one position) and/or “drive” (for example, move the cable using the tensioner). This may mean that the weight of a Cable Protection System (CPS) at a second end of the subsea cable can be influenced and said second end no longer has to be “weight balanced”. Advantageously, this reduces the likelihood of the cable dragging due to the weight of CPS. This may mean that an imbalance of weight between one side of the tensioner and the other could be compensated for by stopping or slowing the tensioner. An example could be where a Cable Protection System (CPS) is pulled into a Wind Turbine Generator (WTG) foundation.

Advantageously this could reduce the straight line approach distance required at foundations due to the weight imbalance causing the nearest alter course in the lay route to pull and drag on the seabed, pulling the cable out of the lay corridor.

In certain applications, the tensioner system can also be used to restrict movement, since it is clamped to the cable. This could be advantageous for an imbalance scenario described above but also for a cable repair, for example, where the vessel is stationary for days on end. The purpose of the tensioner system in this scenario would then be to limit dynamic movement of the cable whilst holding station, rather than while laying the cable on the seabed.

Experimental

Global dynamic analysis has been performed to provide a proof of concept for a tensioner system according to the present invention.

Industry standard OrcaFlex 11 ,0e has been used to make a comparative assessment of cable lay workability for a typical scenario both with and without the deployment of a tensioner system according to the present invention.

An initial screening analysis was completed to determine suitable catenary configurations, followed by more detailed dynamic analysis.

Model setup

Global dynamic analysis models were set up in line with standard industry practices to provide a robust assessment of cable lay workability.

Cable The cable parameters, Table 2-1 , are representative of a typical subsea 3x150 mm ² cable design, selected for the analysis as the relatively low bend stiffness can often result in challenges maintaining the minimum bend radius (MBR) when laying in more severe sea states.

Wbte 2-1: CWte pemnwtsm

Vessel A cable lay barge, Table 2-2, was selected for the analysis to assess and demonstrate the improved workability offered by embodiments of the present invention when deployed from a vessel with typically limited operability in more severe sea states.

Subsea Tensioner Powered Quadrant

Tsbie 2-2: Vessel parsmssters

Ttete 2-3: SPQ and AHC parameters. The subsea tensioner system and associated active heave compensation (AHC) deck equipment parameters used for the analysis are presented in Table 2-3. The tensioner system may herein be referred to as a Subsea Powered Quadrant (SPQ).

MetOcean

Regular wave analysis has been used with wave period and directions selected to coincide with vessel response peaks. A range of regular wave heights, 3.80-5.70m, have been considered, which correspond to the Hmax of sea states with significant wave heights (Hs) 2.0-3.0m respectively. Analysed MetOcean parameters are summarised in Table 2-4.

Tsbte 2-4: psrsn’seter® OrcaFlex models

A quasi-static methodology is used, with zero vessel forward speed / cable payout. Displacement RAOs are used to model the vessel response to waves.

Comparative example -

Standard catenary (no tensioner system according to the present invention).

Modelling was carried out wherein the model was arranged with a static cable departure angle of 7.5 degrees, giving static lay tension and layback distances of 1 kN and 20m respectively.

Example - With tensioner system according to the present invention

Modelling was carried our wherein the model was arranged with a laterally offset tensioner system according to the present invention to keep the AHC connection point close to the vessel stern whilst maintaining an appropriate layback distance from vessel chute to the tensioner system in the plane of the catenary. Key parameters are presented in Table 2-5.

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Table 2-5: SPQ QreaRe model parameters

Results

Selected results are plotted in Figures 3 to 7 and discussed hereinafter.

The plotted results cover three wave directions (000, 090, 225 - see Table 2-4) and provide a direction comparison between the standard catenary with no tensioner system (solid lines), and the tensioner system of the present invention (broken lines). The results trends for the presented directions are typical of all analysed cases, with the tensioner system offering significantly improved cable loads as the wave height increases.

Cable tensions (see Figure 4), are generally quite low both with and without the tensioner system due to the reasonably steep catenary departure angles. The tensioner system being supported by an Active Heave Compensated (AHC) system means it maintains a stable vertical position in the water column, which is considered to significantly improve the compression (see Figure 3), and MBR, (see Figure 5), compared with the standard catenary having no tensioner system. Cable MBR is an important parameter for comparison, with limit breaches seen for some wave directions (e.g. 000) at wave heights less than a Hs 2m equivalent with the standard catenary. Conversely, the cable MBR is maintained well above the allowable limit for all analysed cases (up to Hs 3m equivalent) with the tensioner system of the present invention.

The AHC system reaches the pre-defined limiting velocity (2m/s) in a significant number of cases (see Figure 6); but as demonstrated by the cable results, this limited level of compensation is still sufficient to offer significant benefits.

Conclusions

Preliminary dynamic analysis has been run to provide a proof of concept for a tensioner system according to the present invention. For a typical 3x150 mm ² subsea power cable being laid from a barge, the tensioner system offers significant operability improvements

(especially up to Hs 3m equivalent) for all considered wave directions, some of which have a workable sea state limit less than Hs 2m with a standard catenary.