Arrangement for Positioning a Fall Pipe End

This application claims the benefit of priority from European Patent Application No. 24 165079.5, filed on Mar. 21, 2024, the entirety of which is incorporated by reference.

The present invention generally relates to an arrangement at a fall pipe vessel, and more specifically to a fall pipe ROV, Remotely Operated Vehicle, for positioning the end of the fall pipe during a subsea rock installation operation. In particular, a solution is presented that allows for highly qualitative and accurate subsea rock installation, while obtaining a reduced fuel consumption and increased productivity.

Fall pipe vessels or rock installation vessels are used to install rock on subsea structures, e.g. to cover pipelines or offshore cables with rock material. For this purpose, the vessel is equipped with a fall pipe, through which rock material loaded from the deck may be deposited on the desired subsea position. To allow for rock placement with high precision, a Remotely Operated Vehicle (ROV) is arranged at the bottom end of the fall pipe. Typically, the ROV is suspended from the deck by means of hoisting cables, and the fall pipe end is fixed to the ROV or arranged inside a central channel of the ROV. The ROV is self-propelled and may be operated from the deck, thereby adjusting the position of the fall pipe end. Typically, when dumping rocks according to a desired dumping trajectory, thereby covering a pipeline or subsea cable, the fall pipe vessel moves according to a direction parallel to the dumping trajectory. The vessel position is controlled e.g. by means of a Dynamic Positioning System, while the ROV position is determined relatively to the vessel, e.g. by means of acoustic positioning beacons. The fall pipe and ROV thus roughly follow the movement of the vessel, while the ROV position is adjusted accurately by means of the ROV's own propulsion means.

Prior art fall pipe ROVs are for example disclosed in WO2012002806A1 and WO2012008829A1. In these solutions, the ROV comprises thrusters, serving as the ROV's propulsion means. A thruster is provided in each of the four faces of the ROV, thereby allowing for moving the ROV according to any vector in the horizontal plane. Typically, the ROV also comprises sensors or other types of monitoring means, like e.g. disclosed in WO2012008829A1. Such monitoring means for example serve to inspect both the state of the underwater bottom or subsea structure in front of the ROV, and the state of the installed rocks behind the ROV. For this purpose, the ROV comprises two supporting arms, provided with the monitoring means, both arms mounted at opposing sides of the ROV and being in line with one another. During the rock installation operation, the arms are directed according to the dumping trajectory, thereby allowing for the envisaged inspection. A correct orientation of the supporting arms is thus required to allow for a qualitative inspection and rock installation.

Such a correct orientation of the supporting arms may be obtained by positioning the vessel with its head in a direction parallel to the dumping trajectory. However, in such a setting the orientation of the vessel may deviate from being positioned head seas, thereby leading to an increased fuel consumption of the vessel. On the other hand, if the vessel is positioned head seas, the thrusters need to be used to rotate the ROV about its own axis, thereby obtaining the correct orientation of the inspection arms. However, this reduces the thrust available for positioning the ROV, leading to a decreased accuracy of positioning the ROV. This results in a lower rock dumping precision, and may cause interruptions during the rock installation operation, thereby decreasing productivity. Finally, rotation of the ROV by means of the thrusters requires a high amount of thrust force, thus contributing to an increased power consumption.

It is an objective of the present invention to disclose a fall pipe ROV that resolves one or more of the above-described shortcomings of the prior art solutions. More particularly, it is an objective to present a solution that allows for highly qualitative and accurate subsea rock installation, while obtaining a reduced fuel consumption and increased productivity.

According to a first aspect of the present invention, the above identified objectives are realized by an arrangement suitable for positioning a fall pipe end during subsea rock installation from a vessel, defined by claim, the arrangement comprising:

Thus, the invention concerns an arrangement suitable for positioning a fall pipe end during subsea rock installation from a vessel. The vessel may be referred to as a fall pipe vessel or rock installation vessel. Rock installation refers to placing rock material at the seabed or an underwater structure. For example, rock placement is applied to cover offshore power cables and pipelines, for protection or stabilisation. It may also be used to prepare the seabed for offshore structures, or to arrange a scour protection for offshore wind turbines and platforms. Besides covering an underwater structure, the vessel may also be adapted to install rocks under a subsea structure. The rock material may comprise rocks of various sizes, stones, or other suitable aggregate material.

A fall pipe allows the rock material, stored at the deck of the vessel, to be dumped at the desired subsea position. The fall pipe may be made of a rigid material or a flexible material, or a combination thereof. For example, the fall pipe may comprise a series of cylindrical pipe elements made of a rigid material, while a flexible or resilient material is provided between adjacent pipe elements. The fall pipe has an elongated shape and extends between a top side and a bottom side. During use, the fall pipe extends downwards towards the seabed, wherein rock material is loaded at the top side the pipe, and the material leaves the pipe at the bottom side. For example, the fall pipe may extend through the hull of the vessel.

The arrangement according to the invention is suitable for positioning the fall pipe end during rock installation. The fall pipe end refers to some portion of the fall pipe at its bottom side. The arrangement thus allows to move the bottom end of the fall pipe relatively to the seabed, thereby adjusting the position at which the rock material will be deposited. The arrangement therefore serves as an underwater vehicle or comprises such underwater vehicle. The vehicle is typically remotely operatable, and may be referred to as a Remotely Operated Vehicle or ROV.

The arrangement comprises a submersible frame adapted to be lowered towards an underwater structure, while being suspended from the vessel. This implies that the arrangement comprises, or may cooperate with, a suspension system, that allows the frame to be suspended from the vessel, and the frame to be moved downwards and upwards. In this context, moving downwards or lowering refers to moving in a direction from the deck towards the seabed, while moving upwards or rising refers to a moving in a direction from the seabed to the deck of the vessel. Typically, the frame is suspended by means of hoisting cables, but other suspension means, e.g. comprising rigid rods or bars are possible too. During use, the frame is connected to the suspension means, typically at the top side of the frame, and the frame is submerged in the water.

The submersible frame comprises a channel structure adapted to receive the fall pipe end or to be joined to the fall pipe end. For example, the frame may comprise a channel, wherein during use, a portion of the fall pipe is arranged inside the channel. In this, the bottom side of the fall pipe may be extend beyond the bottom side of the submersible frame, or may be found inside the channel. In the latter case, the channel structure defines an extension of the fall pipe, wherein the rock material first passes through the fall pipe and next to a portion of the channel structure. In another embodiment, the bottom side of the fall pipe is joined to the channel structure, such that the channel defines an extension of the fall pipe. In any of the embodiments, the channel structure thus defines some space through which the rock material will pass during dumping. Due to the channel structure, the submersible frame has two openings, typically at the top side and bottom side of the frame, thereby allowing to receive rock material via the first opening, and direct it downwardly, towards the second opening. Remark that during use, a fall pipe portion arranged inside the channel structure is typically not connected to the frame; the fall pipe end is then loosely positioned inside the channel structure without being connected to it.

The arrangement comprises propulsion equipment adapted for moving the submersible frame in a horizontal plane. The propulsion equipment e.g. comprises thrusters or propellers, or any other means to propel the submersible frame. For example, the frame can be moved forward, backward and sideways, or a combination thereof, thus being movable according to any vector in the horizontal plane. Typically, at least four thrusters are available, of which one pair allows for the forward-backward movement and the other pair for the sideways movements. In this case, combined use of specific thrusters also allows for rotating the submersible frame about is own axis, i.e. rotating it in the horizontal plane. The horizontal plane is defined with respect to the submersible frame; during use, moving the submersible frame in the horizontal plane corresponds to moving the fame substantially parallel to the seabed or the vessel deck, but deviations due to interaction with the water, movements of the vessel, or the seabed not being completely flat may occur. The vertical direction of the submersible frame is the direction perpendicular to the horizontal plane. During use, the vertical direction of the frame will be substantially parallel to the direction of gravity, but deviations due to interaction with the water or movements of the vessel may occur. A vertical axis refers to an axis being parallel to the vertical direction. Remark that typically, the space defined by the channel structure of the submersible frame, extends in vertical direction. However, an angled direction deviating from vertical is also possible.

The propulsion equipment allows to control the position of the submersible frame independently from the vessel. The submersible frame therefore acts as a self-propelled vehicle, and, if operatable remotely, as a Remotely Operated Vehicle or ROV. The propulsion equipment, is mounted to, or connected to, a certain portion of the submersible frame. The latter portion is referred to as the first frame structure. The first frame structure is thus the portion of the submersible frame carrying the propulsion equipment. For example, the first frame structure comprises multiple frame parts, each of them carrying an individual thruster.

The arrangement further comprises survey equipment adapted for subsea sensing or inspection. It may also be referred to as monitoring means, inspection means, detection means, sensing means, etc. For example, the survey equipment comprises one or more sensors, wherein a sensor is a device that detects and responds to some type of input from the environment, the input e.g. being light, heat, motion, moisture, pressure or the like. For example, the survey equipment comprises one or more ultrasonic and/or optical sensors. For example, the survey equipment comprises one or more cameras, wherein a camera is a sensing device that can capture and store images. In particular, the survey equipment may be adapted to inspect the state of the underwater bottom, and/or of a subsea structure to be covered with rocks, and/or of deposited rock material. In an embodiment, the survey equipment comprises at least two inspection means, e.g. sensors, allowing for an inspection in front of the submersible frame, and an inspection behind the submersible frame, while moving the submersible frame along a dumping trajectory.

The survey equipment, is mounted to, or connected to, a certain portion of the submersible frame. The latter portion is referred to as the second frame structure. The second frame structure is thus the portion of the submersible frame carrying the survey equipment. For example, the second frame structure comprises multiple frame parts, each of them carrying an individual sensor. In an embodiment, the second frame structure may comprise one or more inspection arms, provided as elongated frame parts that each carry a sensor. For example, two such inspection arms are comprised in the second frame structure, mounted at opposing sides and being in line with one another.

The first and the second frame structure thus each refer to a particular portion of the submersible frame. Apart from the first and second frame structure, the submersible frame comprises other portions, like the channel structure, and possibly other frame parts. Typically, the channel structure is comprised in an inner structure of the submersible frame, while the first and second frame structure are comprised in an outer structure, the inner structure being adapted to surround the fall pipe and the outer structure being mounted around the inner structure. In an embodiment, the first and the second frame structure may be connected to each other, in such a way that they cannot rotate relatively to each other. The first and the second frame structure then form one unity, connected in a non-rotatable way. In such embodiment, it may be possible, however, that the second frame structure is movable or partly movable with respect to the first frame structure, e.g. due to inspection arms comprised in the second frame structure being mounted pivotably with respect to the first frame structure.

The arrangement thus comprises a self-propelled vehicle or ROV, adapted to be submerged in the water, the vehicle comprising the submersible frame, the propulsion equipment and the survey equipment. The arrangement further comprises a rotation system operable independently from the propulsion equipment. This implies that the rotation system allows to cause a rotation, but without relying on the propulsion equipment, e.g. without using the thrusters. The rotation system comprises an actuator, the actuator comprising a first and a second element adapted to mutually engage or interact. An actuator refers to a system adapted to receive energy from an energy source, and convert it into displacement, force or torque, in a controlled way. In particular, the actuator is adapted to convert an energy input into a rotation of the second element when holding the first element. The actuator may be mounted to the self-propelled vehicle, such that it is submerged during the rock installation operation, or may be positioned at the deck of the vessel.

Upon energizing the actuator, it causes a rotation of the second element relatively to the first element. For example, the second element may be a pinion, and the first element may be a curved rack provided on a slewing ring, or vice versa, the pinon and rack being adapted to engage. When holding the curved rack, the pinion may move along a trajectory along the rack, around the slewing ring, upon energizing the actuator. Energizing the actuator may e.g. happen due to a hydraulic motor comprised in the actuator, the hydraulic motor adapted to receive a pressurized fluid, and adapted to drive the pinion, i.e. rotate the pinion about its own axis. In another embodiment, another type of activation may be used, e.g. a hydraulic system wherein pistons receiving a pressurized fluid cause rotation of a slewing ring, an electrical drive, magnets, etc. The first and second element may directly engage, i.e. being in contact, or may interact without direct contact, e.g. through magnetic force, fluid jets, etc.

In any of the embodiments, the second element is the element of the actuator that is rotated upon energizing the actuator, wherein it rotates relatively to the first element. The rotation is about a vertical axis of the submersible frame. Typically, the rotation axis corresponds to the central axis of the channel structure, but another rotation axis in vertical direction is possible too. The second element of the actuator, i.e. the rotating element, is connected to the second frame structure carrying the survey equipment. On the other hand, the first element of the actuator is connected to the vessel, either directly or indirectly. In this way, in suspended condition of the submersible frame, upon energizing the actuator, the second frame structure rotates relatively to the vessel, thereby rotating the survey equipment mounted to the second frame structure. In this way, energizing the actuator allows to rotate the survey equipment about a vertical axis, relatively to the vessel or seabed, without using the propulsion equipment.

Apart from the actuator, the rotation system may comprise other components to implement the rotation, including a chain of components for providing energy from an energy source at the vessel towards the actuator. If the actuator is found at deck level, typically all components of the rotation system are positioned at the vessel, thus not being submerged during the rock installation operation. If the actuator is mounted to the submersible frame, typically not all of the components of the rotation system are mounted to the submersible frame; in this case, some components of the rotation system are submerged during the rock installation while other components may be found at the deck. For example, the rotation system may comprise a pump for feeding the actuator with pressurized fluid, the pump mounted to the submersible frame, an electrical motor for driving that pump, the electrical motor mounted to the submersible frame, an umbilical for supplying electrical energy towards the electrical motor, and a power supply at the vessel. Moreover, the rotation system may comprise bearings for enabling a relative rotation between the second and first element.

The invention goes along with multiple advantages. First, by rotating the survey equipment, the inspection or sensing means may be brought into the correct orientation during the rock installation operation, thereby allowing for a qualitative inspection and rock installation. In this, the rotation of the survey equipment may be done without using the propulsion equipment; whereas in prior art solutions four thrusters need to be energized to obtain a rotation of the ROV about its own axis, the invention allows to rotate the survey equipment by energizing the actuator, without relying on the thrusters. Consequently, the thrust available from the propulsion equipment is fully available for positioning the ROV, thereby obtaining positioning with an increased accuracy. This contributes to rock dumping at higher precision, and avoids interruptions during the rock installation operation, thereby leading to an increased productivity. Moreover, as no thrust force is required for rotating the survey equipment, this contributes to a reduced power consumption. Finally, the correct orientation of the survey equipment may be obtained even if there is a misalignment between the vessel and the submerged vehicle. Thus, the vessel may be positioned head seas during the rock installation operation, thereby contributing to a reduced fuel consumption of the vessel.

Optionally, the first element of the actuator is adapted to be connected to the vessel, via a hoisting system for suspending the submersible frame, or due to the first element being fixed directly to the deck of the vessel, thereby allowing for a rotation of the second element relatively to the first element upon energizing the actuator. In an embodiment, the first element of the actuator is connected to the vessel indirectly, e.g. due to one or more hoisting cables connecting the submersible frame to the vessel, and the first element being connected to the submersible frame. In another embodiment, the first element may be directly connected to the vessel, e.g. by being mounted at deck level. Due to the direct or indirect connection with the vessel, the first element may remain static, such that only the second element rotates upon energizing the actuator. It is also possible that the first element would slightly rotate too, e.g. due to the hoisting cables being non-rigid; in that case the second element will be rotated to a larger extent, such that still the second element rotates relatively to the first element. Due to the first element being connected to the vessel, while rotating the second element, the second frame structure carrying the survey equipment may be rotated without winding up the hoisting cables. This contributes to a more stable orientation of the survey equipment, and less required adjustments during the rock installation operation.

Optionally, the first element of the actuator is connected to the vessel via a hoisting system, and the second element of the actuator is connected to the submersible frame, such that during use, the actuator is submerged into the water together with the submersible frame. This means that the first element is indirectly connected to the vessel, e.g. by means of one or more hoisting cables. During the rock installation operation, the actuator, mounted to the submersible frame, is submerged in the water together with the frame. In other words, all the elements of the actuator make part of the self-propelled vehicle found in the water during the rock installation operation. This allows for a light, convenient construction, opposed to a rotation system wherein the first element would be found at deck level, the latter possibly leading to a more heavy and cumbersome construction.

In an embodiment, the complete channel structure may be connected to the first element of the actuator, such that the channel structure does not rotate upon rotating the survey equipment, or only to a less extent. In another embodiment, a portion of the channel structure is connected to the first element of the actuator and the other portion of the channel structure is connected to the second frame structure, such that a portion of the channel structure rotates upon rotating the survey equipment. In yet another embodiment, the complete channel structure is connected to the second frame structure, such that the complete channel structure rotates upon rotating the survey equipment.

Optionally, according to a first group of embodiments, the first frame structure carrying the propulsion equipment and the second frame structure carrying the survey equipment are joined, such that by energizing the actuator, the survey equipment and propulsion equipment are rotated together about the vertical axis. In particular, the first frame structure and the second frame structure are joined in such a way that they cannot rotate relatively to each other. In an embodiment, the first and frame structure are rigidly connected, such that both form one unity wherein not any movement between the first and second frame structure is possible. In another embodiment, the first and second frame structure may be joined, not allowing for a rotation between both structures, but still allowing that the second frame structure is moved or partly moved with respect to the first frame structure. For example, the second frame structure may comprise foldable arms, such that the arms may be pivoted with respect to the first frame structure.

Due to the first and second frame structure being joined, the survey equipment and propulsion equipment are rotated together about the vertical axis upon energizing the actuator. This has the advantage that no additional measurements faults are introduced. Indeed, typically acoustic positioning beacons are mounted to the submersible frame to know the position and orientation of the submerged vehicle relative to the vessel. As the first frame structure carrying the propulsion equipment is joined to the second frame structure carrying the survey equipment, the acoustic position beacons allow to know the position and orientation of each thruster as well as of the survey equipment. No supplementary measurement is thus needed to know the orientation of the inspection arms, in view of determining the required rotation. Conversely, in a solution wherein the second frame structure carrying the survey equipment would be rotatable with respect to the first frame structure, and the latter being rotatable on its own by means of the thrusters, such a supplementary measurement would be needed, thereby requiring additional equipment and/or introducing additional measurements faults.

Optionally, according to a another group of embodiments, the second frame structure carrying the survey equipment is mounted rotatably with respect to the first frame structure carrying the propulsion equipment, such that by energizing the actuator, the survey equipment is rotated about the vertical axis without rotating the propulsion equipment. For example, the second frame structure carrying the survey equipment may be rotated around the rest of the submersible frame, in particular around the first frame structure carrying the propulsion equipment. In this way, a double rotation option is obtained, wherein the first frame structure may be rotated on its own by means of the propulsion equipment, and the second frame structure may be rotated around the first frame structure by means of the rotation system. This has the advantage that the design of the first frame structure, channel structure and propulsion equipment may remain similar as in a prior art ROV, while the actuator and second frame structure are added as an outer structure around the existing design.

In an example within this group of embodiments, the first element of the actuator is provided as a slewing ring with toothed rack, and the second element of the actuator is provided as a pinion. In this case, the first element is connected indirectly to the vessel via the hoisting system, and upon energizing the actuator, the pinion rotates around the slewing ring. In another example, the first element of the actuator is provided as a pinion, and the second element of the actuator is provided as a slewing ring with toothed rack. In this case, the first element is connected indirectly to the vessel via the hoisting system, and upon energizing the actuator, the pinion drives the slewing ring such that the latter rotates relatively to the pinion.

Optionally, according to yet a another group of embodiments, the first element of the actuator is fixed directly to the deck of the vessel, and the second element of the actuator is connected to the submersible frame via a hoisting system, such that during use, the actuator is found at deck level and is not submerged into the water. This means that during the rock installation operation, the frame carrying the propulsion equipment and survey equipment is submerged into the water, while the actuator for enabling the rotation of the survey equipment is not submerged but is installed on the deck of the vessel. Upon energizing the actuator, the submerged frame rotates, together with the second element found at the deck. For example, the submerged frame may be fixed to the fall pipe, such that the fall pipe rotates as well. In another embodiment, a complete fall pipe system or tower may be provided, positioned on a turntable on the deck. In the latter case, the fall pipe rotates together with other components of the dumping system found at the deck.

Optionally, the second frame structure comprises one or more elongated arms, the survey equipment being mounted to the one or more elongated arms. Typically, means for inspection are provided at the bottom side of the elongated arms, thus being directed towards the seabed during the rock installation operation. In an embodiment, the one or more elongated arms are rigidly connected to the rest of the second frame structure. In another embodiment, the one or more elongated arms are collapsible with respect to the rest of the second frame structure. For example, an elongated arm, or a portion thereof, may be pivotable, thereby allowing to fold and unfold the arm with respect to the rest of the frame. Having elongated inspection arms has the advantage that during the rock installation operation, a substantial inspection range is obtained. Moreover, having collapsible or foldable inspection arms allow for a compact device in folded condition of the arms, thereby contributing to a smooth lowering and rising of the device in and out of the water, and allowing for a compact storage at the deck.

Optionally, the second frame structure comprises a set of two elongated arms, each of the arms mounted at opposite sides of the submersible frame, wherein the two elongated arms are rigidly connected to the rest of the second frame structure and are in line, or the two elongated arms are collapsible with respect to the rest of the second frame structure, and are in line in unfolded condition. This has the advantage that inspection may be done according to a straight line, thereby allowing for inspection in front of the submerged frame and behind the submerged frame. For example, the state of a subsea structure like a pipeline or cable may be inspected before dumping rocks, and the state of the deposited material may be inspected after dumping.

Optionally, the vertical rotation axis corresponds to the central axis of the channel structure, such that the survey equipment can be rotated about the central axis of the channel structure.

Optionally, the first or second element comprises a ring having a central axis extending in vertical direction. In an embodiment, the first element comprises the ring, and the actuator is adapted to move the second element along a ring-shaped trajectory coaxially with the ring, such that the second element is rotated about the central axis of the ring. For example, the ring may be provided with a toothed rack, and upon rotating a pinion about its own axis, the pinion may move along a circular trajectory around the ring, due to the pinion engaging with the toothed rack. In another embodiment, the second element comprises the ring, and the actuator is adapted to rotate the ring about its central axis. For example, the ring may be provided with a toothed rack, and upon rotating a pinion about its own axis, the ring is driven by the pinion, such that the ring rotates.

Optionally, the first or second element respectively is a slewing ring comprising a toothed rack along its circumference, and the second or first element respectively is a pinion adapted to engage with the toothed rack.

Optionally, the first element or the second element respectively comprises a slewing ring having a central axis corresponding to the vertical rotation axis, and the rotation system comprises multiple bearing assemblies distributed over the circumference of the slewing ring, each of the bearing assemblies adapted to retain the slewing ring in vertical direction with respect to the submersible frame, while allowing for a rotation of the slewing ring relatively to the second or first frame structure respectively. One bearing assembly may comprise multiple bearings, e.g. each of the bearing assemblies is a set of three bearings, namely an upper, lower and side bearing. Multiple bearing assemblies are provided along the circumference of the slewing ring. This means that instead of providing a single ring-shaped bearing that extends over the complete circumference of the slewing ring, multiple separate bearing assemblies are provided along the circumference. This has the advantage of being less prone to underwater conditions, especially due to salt water, thereby allowing for proper rotation when the actuator is submerged in the water. Also remark that such separated bearing assemblies suffice, as the occurring rotations are over an angle that is rather small; e.g. typically no rotation over 360° will occur.

Optionally, each of the bearing assemblies comprises three individual bearings, of which the first bearing engages with the upper surface of the slewing ring, the second bearing engages with the bottom surface of the slewing ring, and the third bearing engages with the side surface of the slewing ring.

Optionally, the first and second bearing each comprise a sliding interface, and the third bearing comprises a rolling element.

Optionally, the first and third bearing are integrated in a bearing block, the bearing block being tiltable with respect to the slewing ring, such that in tilted condition, the first and the third bearing do not longer engage with the upper surface respectively side surface of the slewing ring. Upon tilting the bearing blocks, a passage is created, thereby allow to mount or remove the slewing ring, and allowing for easy maintenance.

Optionally, the propulsion equipment comprises multiple thrusters, distributed along the circumference of the first frame structure.

Optionally, the thrusters are adapted to move the submersible frame according to any vector in the horizontal plane.

Optionally, the propulsion equipment comprises six thrusters, evenly distributed along the circumference of the first frame structure.

Optionally, the arrangement comprises one or more acoustic positioning beacons, mounted to the submersible frame, adapted to determine the position of the submersible frame with respect to the vessel.

Optionally, the arrangement comprises one or more hoisting cables, adapted for suspending the submersible frame from the vessel.

Optionally, the actuator comprises a hydraulic motor mounted to the submersible frame, the hydraulic motor being adapted to receive a pressurized fluid, and to drive the first or second element.

Optionally, the pressurized fluid for feeding the hydraulic motor is supplied by a pump mounted to the submersible frame, the pump being driven by an electric motor mounted to the submersible frame.

Optionally, the rotation system comprises an umbilical power cable, for supplying electric power from the vessel to the electric motor at the submersible frame.

Optionally, the umbilical power cable is integrated within a hoisting cable.

Optionally, the rotation system comprises an umbilical power cable, of which one end is fixed with respect to the second frame structure. For example, an electric motor may be connected to the second frame structure, and the end of the umbilical power cable is fixed to the electric motor. This implies that, upon rotating the second frame structure with survey equipment, the end of the power cable also rotates. This has the advantage over a dragging contact that an easier implementation and a more robust solution are obtained. On the other hand, the cable length may limit the possible range of rotation.

According to a second aspect of the present invention, there is provided a method for positioning a fall pipe end during subsea rock installation from a vessel, the method comprising: