The invention relates to a submersible vehicle, in particular to a so-called Fall Pipe ROV (Remotedly Operated Vehicle), for manoeuvring a fall pipe for dumping rocks on or near subsea installations, such as pipelines, comprising a frame, hoisting and control cables for suspending the vehicle from a surface vessel, propulsion means, and a channel for accommodating the end of a fall pipe or forming the end of a fall pipe. The invention further relates to a kit comprising a submersible vehicle and to a method of dumping rocks on a seabed.

As explained in WO 2009/020385, rock dumping vessels are employed to dump and transport rocks of various sizes and other suitable aggregate material for offshore and coastal protection applications, including stabilization, protection and covering of cables, pipelines, and free span corrections, and filling up holes, e.g. around platforms and rigs. Other applications include seabed preparation prior to pipe laying, construction of underwater berms, thermal insulation of oil lines, protection against anchors and fishing operations and ballasting of platform and loading buoys.

Dumping can be performed with a large crane, but also by means of side rock dumping vessels and fall pipe vessels. Side rock dumping vessels sail to their destination, where shovels put the rocks overboard at a steady pace. This rock dumping method is typically used in shallow water. Fall pipe vessels are used in deep water, bringing large amounts of rock in their holds. The rocks are dumped through a long and more or less flexible fall pipe. A fall pipe ROV manoeuvres the end of the pipe to ensure accurate dumping of the rocks.

Installations on the seabed can also be protected by means of material taken from the seabed itself, e.g. by moving sand and sediment by means of so-called mass flow excavation and depositing this material over and around (live) subsea cables, pipelines et cetera. As the material does not have to be brought in, this method is usually less expensive than rock dumping. Mass flow excavation also can be used to remove sediments from the seabed, e.g. to remove sand dunes or soft top layers of the seabed in preparation of rock dumping or installation of pipelines or other equipment on the seabed. In many projects it is desirable to use both rock dumping and mass flow excavation. For instance, when installing a pipeline, some sections of the pipeline should be protected from the influence of strong currents by means of rocks, whereas other sections of the pipeline are trenched and covering by material taken from the seabed suffices. Also, it may be necessary to remove sand dunes to create an even seabed for laying the pipe on.

It is an object of the present invention to provide an improved, more versatile submersible vehicle.

To this end, the vehicle according to the present invention is characterised in that a mass flow generator is releasably mounted in the mentioned channel of the submersible vehicle. Thus, a single ROV can be used both for rock dumping and for mass flow excavation. For instance, the ROV could follow a pipeline on and in the seabed and dump rocks on and next to sections of the pipeline exposed to strong currents and, once the rocks are depleted, exchange the fall pipe with a mass flow generator and return in opposite direction to cover remaining sections, e.g. trenched sections, with material taken from the surroundings. As a result, the surface vessel needs to return to shore less frequently and/or no separate surface vessel for deploying a mass flow generator is required.

In an embodiment, the mass flow generator is connected or connectable to a power supply on the ROV. ROVs are usually connected to the surface vessel by means of an umbilical which provides data communication and electric power for propulsion. Many ROVs further comprise an hydraulic power pack to convert electric power supplied via the umbilical to hydraulic power to drive the propulsors. By using this supply of electric or hydraulic energy, the mass flow generator requires no dedicated power supply simplifying the design of the mass flow generator.

The invention further relates to a kit comprising a submersible vehicle as described above and a mass flow generator configured to be releasably mounted in said vehicle, offering the advantages discussed above. In an embodiment, the kit comprises a single mass flow generator configured to be releasably mounted in said channel. The generator may be provided with a nozzle that forms an integral whole with the generator or the nozzle may be interchangeable. In a further embodiment, the kit comprises two or more mass flow generators configured to be releasably mounted in said channel. Thus, the mass flow generator can be readily replaced in case of malfunction or two or more generators having different properties can be included to be able to select the most suitable mass flow generator based on the circumstances, in particular local circumstances such as the nature of the seabed. In a more specific embodiment, one mass flow generator comprises a relatively small diameter impeller and a relatively narrow tube or tube-like body and another or, in case of only two generators in the kit, the other mass flow generator in the kit comprises a relatively large diameter impeller and a relatively broad tube or tube-like body. The generator having the smaller impeller is more suitable for excavating and moving clay, whereas the larger is more suitable for sand.

The invention also relates to a method of dumping rock and moving sediment on or near subsea installations, such as pipelines, including the steps of mounting the end of a fall pipe or a mass flow unit to or in the vehicle, lowering a submersible vehicle from a surface vessel, manoeuvring the vehicle to respectively dump rocks or move sediment on or near a subsea installation, exchanging the end of a fall pipe for a mass flow generator or the mass flow generator for the end of a fall pipe, respectively, and manoeuvring the vehicle to respectively dump rocks or move sediment on or near a subsea installation.

The method need not necessarily be carried out in the order specified above. For instance, it is possible to first lower the vehicle and then mount the mass flow generator, e.g. by employing divers. Within the framework of the present invention, the term "rock" includes natural rocks, stones, and larger pebbles as well as artificial, e.g. concrete, blocks in various shapes. Further, the term "channel" refers to any space in the submersible vehicle suitable for accommodating a mass flow generator. Figure 1 is cross-section of an example of a fall pipe ROV according to the present invention.

Figure 2 is a perspective view of a mass flow generator for use in the vehicle shown in Figure 1.

Figure 3 is side view of the mass flow generator shown in Figure 2.

Figures 1 and 2 show a submersible remotely operated vehicle (ROV) 1 for

manoeuvring a fall pipe (not shown) for dumping rocks on or near subsea installations, such as pipelines, comprising a frame 2, hoisting cables 3 connected to the frame 2 for suspending the ROV 1 from a surface vessel (not shown) and controlling the position of the ROV 1 in the vertical direction. The ROV 1 comprises a plurality of hydraulically driven propulsors, in this example two sets of two propulsors 4 each, a first set for propelling the ROV 1 in a first horizontal direction and a second set for propelling the ROV 1 in a horizontal direction perpendicular to the first direction. Hydraulic power is provided by means of an hydraulic power pack 5 which receives electric power from the surface vessel by means of an umbilical 6, integrated in one of the hoisting cables 3, and which supplies hydraulic power to a so-called common rail 7. Compensators 8 are provided to reduce, in a manner known in itself, the pressure difference over the seals in the various hydraulic devices.

The ROV 1 comprises a dynamic positioning system 9, connected to the surface vessel via the umbilical. In this embodiment, the ROV 1 is arranged as the master, whereas the surface vessel is arranged as the slave, i.e. the ROV 1 is operated or programmed to follow a predetermined path and the surface vessel follows the submerged ROV 1.

The ROV 1 further comprises a channel 10 providing a means to releasably

accommodate the end of a fall pipe (not shown). The channel 10 extends through the middle and, in this example, through the centre of gravity of the ROV 1. The channel 10 comprises an upper section 10A converging downwards and a lower section 10B converging upwards, the sections together defining a waist. At this waist, the channel 10 is provided with one or more friction elements, e.g. a plurality of the resilient blocks 11 that can be moved radially inwards and outwards by means of hydraulic cylinders 12 mounted about the outer wall of the channel 10. In accordance with the present invention, a mass flow generator 15, shown in isolation in Figures 2 and 3, is releasably mounted inside the channel 10. The mass flow generator 15 comprises a tube-like body 16 having a large diameter upper rim 16A, an inlet opening, a cylindrical middle portion 16B, and a discharge nozzle 16C. The nozzle is exchangeable. The nozzle 16C shown in the Figures converges in the flow direction (downwards in the Figures), but can be readily exchanged for another nozzle, such as a straight nozzle or a diverging nozzle, which would be more suitable for a low velocity, high discharge flow of water through the mass flow generator. An impeller 17 and a hydraulic motor 18 for driving the impeller are mounted inside the tube-like body 16 at the transition of the large diameter inlet opening and the cylindrical middle portion.

During operation, the end of a fall pipe is fitted inside the channel and the ROV is lowered while the fall pipe is gradually assembled in a manner similar to assembling a drill string, i.e. by adding segments to the top end of the fall pipe. When the fall pipe is completed, the submerged ROV directs the end of the fall pipe to the location where the rocks are to be dumped and, once this position is reached, dumping is initiated. The ROV can, for instance, follow a pipeline on and in the seabed and dump rocks next to and/or onto only those sections of the pipeline that are exposed to strong currents or other hazards.

When mass flow excavation is required or when the rocks are depleted, the end of the fall pipe is removed, the mass flow generator is inserted into the channel of the ROV until its upper rim rests on the ROV and then bolted in place and also secured by the friction elements. The ROV manoeuvres the mass flow excavator just as it moves the fall pipe, thus allowing accurate control of the excavation. The mass flow generator can be used for excavation, for trenching or for covering areas of the seabed, the latter making use of ambient water current for transport of the seabed material that is loosened by the mass flow excavator. For each section of a subsea installation, in this example a pipeline, a suitable material, e.g. rocks or sand, is selected and that material is dumped or deposited onto that section. Thus, rocks are used efficiently and the surface or length of an installation that can be covered with one payload of the surface vessel increases significantly. Furthermore, as power supply, deployment system, propulsion, torque compensation, and general controls are provided by an ROV that is already present, i.e. the ROV for manoeuvring the end of a fall pipe, the design of the mass flow generator can be kept relatively straightforward.

The invention is not restricted to the above-described embodiments, which can be varied in a number of ways within the scope of the claims. For instance, the vehicle may be used in excavation projects where no rock dumping is required of where excavation is required in preparation of rock dumping.