FIELD OF THE INVENTION The present invention relates to a buoyancy module, a clamp for the buoyancy module, use of the clamp, and a method of installing the buoyancy module on an underwater pipe as well as a system therefor.

BACKGROUND OF THE INVENTION

Offshore flexible riser systems are used routinely for transferring fluids, such as gas and oil, between a well on the sea bed and a vessel floating on the surface of the sea. Along the riser, typically, a plurality of buoyancy modules are attached, giving the riser a flow profile in which the lowest part of the riser extends relatively steeply up- wards from the anchoring point at the sea bed and wherein the upper part of the riser extends at a more flat angle towards the vessel, such that it allows a relatively large degree of lateral movement of the vessel relatively to the anchoring point at the bottom. An example of an underwater pipe with a buoyancy module as sold by the company ManuPlas is illustrated in FIG. 5a, as also published on the Internet site www.manuplas.co.uk. The buoyancy module is provided as a combination of two halves that enclose a clamp. The clamp, as illustrated in FIG. 5a and FIG. 5b is a three-piece clamp, where the three hinges between the three pieces are rotational as long as the three pairs of bolts are not tightened. The bolts provide very stable connections of the clamp and are useful due to the very high forces between the clamp and the pipe when the clamp is tightened around the pipe. Especially, the clamp has to remain safely in position without moving longitudinally along the pipe. In order to minimize influence on the routine working conditions, the buoyancy modules are designed for long term stability and tightness. However, occasionally, the buoyancy modules have to be exchanged. This can be a difficult operation under sea level, especially, if the buoyancy modules are located in an area underneath the vessel where they are inaccessible by crane-based exchange equipment carried by another vessel. Rough sea conditions may additionally complicate the operation of exchange of buoyancy modules. Disconnection of the riser from the vessel is a possibility, which, however, is a complicated and expensive solution. For this reason, it is known to exchange buoyancy modules by remotely operated vehicles (ROV) under water.

Example on subsea vehicles are disclosed in US3381485, US3621911, US4620819, US4721055, US4484838, US5039254, WO89/07071, WO2008/100943, US2012/0275864, US2014/0147217, and WO2016/184474. A floating collar is disclosed in GB2114046. A gripper is disclosed in WO2013/144601. Tube rings for pipe conduits are disclosed in US5592975 and BE1015220. A clamping device for a buoyancy body is disclosed in US7744314.

Despite using an underwater ROV, removal and mounting of clamps and buoyancy modules is a challenge, as the riser may be moving during the operation, why the op- eration time for the mounting should be kept short. Screw mechanisms for the clamps are tedious and slow. For this reason, there is a need for quicker mounting methods for clamps onto risers.

A remotely operated underwater vehicle is also disclosed in Norwegian patent N0334758 by Saltkjel for mounting and dismounting sleeves on oil and gas pipes. The ROV is provided with two opposite jaws that press two halves of the sleeve together around the pipe. The sleeves are provided with a quick coupling which locks by a male part with barbs engaging in a female cavity. Such barb connection is not easy to disassemble such that the sleeve is removed by pulling out an axle from the hinge between the two halves. The quick coupling for the sleeve in the Norwegian patent N0334758 by Saltkjel is not suitable for clamps of the type typically used for buoyancy modules because the forces on the clamp are very high and the quick coupling with barbs is likely to not safely withstand the forces. Thus, there is still a need for improvements.

DESCRIPTION / SUMMARY OF THE INVENTION

It is therefore an objective of the invention to provide an improvement in the art. Especially, it is an objective to provide a quick and stable mounting procedure for a buoyancy module with clamp onto an underwater pipe. This objective is achieved with a mounting procedure, a buoyancy module, and a clamp, as well as a system therefor, as described in more detail in the following.

The term pipe is used here as a general term for subsea risers, pipelines, cables, and umbilicals. Typical, non-limiting diameters of the pipe are in the range of 30-150 cm. The procedure for mounting a buoyancy module onto an underwater pipe while at subsea level is performed by an underwater vehicle configured for underwater operation. Firstly, a clamp is fastened around the pipe and subsequently a buoyancy body is locked around the clamp. The vehicle is configured for subsea transport of clamps, and optionally also for subsea transport of buoyancy bodies.

The vehicle comprises a propulsion and steering system for manoeuvring the vehicle to the underwater pipe at a predetermined location where the buoyancy module should be installed. For gripping and releasing the clamp, the vehicle comprises a gripper. For example, such gripper is specifically designed for the holding and mounting, and po- tentially also demounting the clamp. In order to mount the buoyancy body, the same or a different gripper is used. Either, one gripper is exchanged by another gripper on the same vehicle, or another vehicle is provided.

For example, the vehicle is provided with a coupling for mounting for different types of gripper. The gripper for the clamp and the gripper for the buoyancy body, in the following called buoy gripper, each comprises a corresponding counter-coupling for selective mount of the gripper for the clamp and the buoy gripper in order to easily switch between the grippers during an installation or de-installation procedure of the clamp and the buoyancy body on the underwater pipe. The vehicle without gripper and the two grippers are advantageously part of a system for installation and demounting of buoyancy modules. The coupling on the vehicle and the counter-coupling on the grippers are advantageously quick-couplings in which mounting and demounting of the respective gripper to and from the vehicle is done within less than 10 min, for example in less than 5 minutes.

For the mounting, the clamp is inserted into the gripper, typically near the ship from which the clamps are provided. After gripping of the clamp in the gripper, the vehicle is navigated to the location and mounts the clamp onto the pipe.

The clamp comprises an annular body forming an aperture dimensioned to fit tightly around the pipe such that sliding of the clamp along the pipe is prevented. The annular body is formed of a plurality of rigid part-annular clamp sections connected to each other by hinges. A central axis of the annular body defines a direction in which the pipe extends through the annular body. In a direction parallel to this central axis, the dimensions of the clamp are typically much less than the dimensions of the buoyancy body. Advantageously, the dimension of the clamp along the central axis is less than the diameter of the clamp, and typically even less than the diameter of the aperture in order not to be affected by bending of the pipe. In contrast thereto, the buoyancy body is much longer with a dimension along the central axis which is several times the dimension of the clamp. In order for the buoyancy body not to be affected by bending of the pipe, a clearance is provided between the pipe and the buoyancy module, the clearance preventing contact between the pipe and the buoyancy module, even when the pipe is bent. It is also for this reason that the buoyancy body is not mounted direct- ly on the pipe but instead is mounted on the clamp. For example, the buoyancy body forms a tube with an inner diameter larger than the diameter of the aperture.

For mounting of the clamp on the pipe, it is inserted into the gripper, and the vehicle is navigated with the clamp in the gripper to the location for mounting. While the clamp is in an open condition, where one specific of the hinges is disassembled and the part- annular clamp sections form jaws, providing an entrance opening for inserting the pipe between the part-annular clamp sections. The clamp is then pushed over the pipe such that the pipe is inserted between the part-annular clamp sections and the clamp is closed around the pipe by the gripper. Finally, the specific hinge is assembled for locking the clamp around the pipe.

The hinge comprises cooperating buckles with holes and a pin extending through the holes locking the knuckles to each other into a hinge with the pin forming a rotational axis. The holes and the pin of each hinge are parallel to the central axis.

For example, each part-annular clamp section comprises a first end with a central buckle, for example with only a single central buckle, and a second end with two side buckles forming a slot dimensioned for snugly fit of the central buckle of a neighbouring clamp section in the slot.

In order to drive the pin into the knuckles, the gripper comprises a pin driver. When the clamp is closed around the pipe by the gripper, the pin is driven by the pin driver in a direction parallel to the central axis into the holes. As the pin driver is part of the gripper, it is moved away from the clamp together with the vehicle. Thus, when the buoyancy body is mounted to the clamp, the pin driver is not at the location of the clamp any more, and the pin driver does not disturb the mounting of the buoyancy module. In some embodiments, the pin driver is also configured for partial or entire removal of the pin from the hinge in order to open the hinge.

Typically, after mounting of the clamp, the buoyancy module is inserted into a different gripper of the same vehicle or a different vehicle. The vehicle with this gripper is then navigated to the pipe and the buoyancy body installed onto the clamp.

For example, the clamp has a pre-mounted state, which is a state prior to mount of the clamp onto the pipe, and a post-mounted state, which is a state after mount of the clamp on the pipe. In the post mounted state, all the clamp sections are connected to each other by the hinges. In the pre-mounted states, one specific of the hinges is disas- sembled, thereby providing an entrance opening for inserting the pipe between the part-annular clamp sections. Optionally, for ease of mounting, the pin is inserted only into one of the side buckles but not into the central buckle when the clamp is in the pre-mounted state. The pin is then inserted by the pin driver into the holes of the cen- tral buckle and the hole of the second of the side buckles during transition from the pre-mounted state to the post-mounted state.

As it appears from the above, in contrast to the prior art, the hinges are screwless. For example, the hinges are held together only by the single pin in each hinge. Optionally, all the hinges are identical.

In some embodiments, the part-annular clamp sections are spanning identical angular sections of a circle. For example, the plurality of clamp sections, such as three, four, five or six clamp sections, are identical, although this is not strictly necessary.

In some embodiments, the two side buckles at the second end of the part-annular clamp section have a convex semi-circular shape; and on either side of the central buckle at the first end of the part-annular clamp section, there are provided semi- circular recesses taking up the semi-circular side buckles, configured for rotational sliding of the side buckles in the recesses.

Although the clamp has been explained above of having two side buckles and one central buckle, it is implicit that the clamp has further central buckles and further side buckles, for example to or three central buckles and, correspondingly, three or four side buckles, two adjacent side buckles forming a slot for the central buckle.

SHORT DESCRIPTION OF THE DRAWINGS The invention will be explained in more detail with reference to the drawing, where FIG. 1 shows a ship connected by a riser line to a subsea station;

FIG. 2 illustrates operation of a ROV underneath the ship;

FIG. 3 illustrates an underwater vehicle in the process of mounting a buoyancy module to a riser in a) a first perspective, b) second perspective;

FIG. 4 illustrates a clamp in enlarged view, in a) pre-mounted state, b) transition state, and c) post-mounted state;

FIG. 5 is a reproduction of parts of the internet page www.manuplas.co.uk, where a) is an explanatory sketch of a pipe with clamp and buoy, and b) is a detailed image of a three-piece clamp. DETAILED DESCRIPTION / PREFERRED EMBODIMENT

FIG. 1 illustrates a riser pipe 2 between a station 3 at the sea bed 4 and a ship 5 at the surface 6 of the sea. The pipe 2 extends relatively steeply from an anchoring point 7 at the sea bed 4 up to a floating part 8 of the pipe 2. The floating part 8 comprises a plurality of buoyancy modules 1 attached to the pipe 2. As an alternative to the ship 5, the riser could also extend to a stationary surface construction, for example a platform. FIG. 2 illustrates a system for removal and attachment of buoyancy modules 1 to the riser pipe 2. The example of the pipe being a riser pipe is only for illustration, and the term pips is used herein for an underwater pipe in general, for example pipelines, cables, and umbilicals. The system comprises an underwater operating vehicle 10 for transportation of buoyancy modules 1 under sea level and for removal and attachments of the buoyancy modules 1 to the pipe 2. The underwater vehicle 10 comprises a propulsion system, for example propellers or jet thrusters, as well as a steering system for in cooperation moving the vehicle 10 safely controlled under water. The vehicle 10 is unmanned and steered by remote control by operational control signals transmitted to the remotely operated vehicle (ROV) from a control vessel 13 through an umbilical 14 connecting a control centre 12 on the control vessel 11 with the vehicle 10.

Alternatively, the vehicle 10 is operating autonomously and not connected to the con- trol vessel 13. In this case, a computer of the vehicle 10 is programmed to steer the vehicle 10 autonomously to a certain underwater location and perform the necessary operation in accordance with the programming and supported by evaluation of the video recording taken continuously by a camera system of the vehicle 10. FIG. 3a and 3b is an illustration of a sub-sea operation showing the remotely operated vehicle (ROV) 10 in the process of holding the buoy gripper 18 around a buoyancy body 9, where two buoyancy elements 9a, 9b form halves of the buoyancy body 9 enclosing the pipe 2. For attachment and detachment, the two buoyancy elements 9a, 9b have to be separated a distance sufficient for the pipe 2 to fit into the central hollow part of the buoyancy body 9. Two buoyancy body halves 9a, 9b are secured to either of two jaws of the buoy gripper 18, and the opening action of the jaws, once engagement has been made with the buoyancy body 9 by the buoy gripper 18, the jaws can be used to force the two buoyancy body halves 9a, 9b apart for release from the pipe 2 or for attachment thereto. The buoyancy body 9 is locked around a clamp 13 and secured by a fastener 24, for example a metal band, to form a buoyancy module 1 comprising the buoyancy body 9 in combination with the clamp 13.

The same vehicle 10 or a different underwater vehicle is used for mounting and de- mounting clamps 13, for which the vehicle 10 in this case is equipped with a different gripper, specially adapted for mounting clamps, however, working according to a similar principle such that the procedure with the opening and closing jaws is similar to the procedure for mounting the buoyancy body 9. In the case, where the same vehicle 10 is use, it is provided with a coupling 17 onto which various tools and grippers can be mounted, such as one gripper 18 for the clamp and the buoy gripper 18' for the buoyancy body 9.

A possible embodiment of a buoy gripper 18' is disclosed and described in more detail in International patent application WO2016/184474.

A clamp 13 is illustrated in FIG. 4a, 4b, and 4c. The clamp 13 has an annular shape comprising three clamp sections 13', each spanning a third of a circle and connected to the neighbouring section 13' by a hinge 21. The hinge 21 comprises a centre knuckle 16a, provided on one end of the clamp section 13 ', and two narrower side knuckles 16b at the opposite end of a neighbouring clamp section 13', such that the centre knuckle 16a fits into the slot 25 formed between the two side knuckles 16b of a neighbouring clamp section 13' . The side knuckles 16b are accommodated in semi-circular recesses 23 configured for rotational sliding of the side buckles 16a in the recesses 23. When the three clamp sections 13' are brought into a circle, as illustrated in FIG. 4b, the centre knuckles 16a of all three clamp sections 13' are inserted into the space between the side knuckles 16b of the corresponding neighbouring clamp section 13' . For mounting, the three clamp sections 13' are connected by two hinges 21, and one hinge 21 with two clamp sections 13' forming open jaws such that the clamp 13 can be brought into position around the pipe 2. As a next step, the clamp 13 is closed, as illustrated in FIG. 4b, and a pin 19 is inserted into the aligned holes 20 in the centre knuckle 16a and the side knuckles 16b, which is illustrated in FIG. 4c. The clamp 13 is brought into position around the pipe 2 by the underwater vehicle 10, typically a ROV, which has a gripper system 18 adapted to close the clamp sections 13' around the pipe 2. The gripper system 18 is also equipped with a suitable pin driver to press the pin 19 into the correct position such that it extends through the transverse holes 20 of all knuckles 16a, 16b. For ease of mounting, the pin 19 would nor- mally already be prepositioned inside the hole 20 of one of the side knuckles 16b, as illustrate in FIG. 4a, such that it has only to be pushed further into the aligned holes 20 of the centre knuckle 16a and the hole of the second side knuckle 16b.

Shown are two side knuckles 16b and one centre knuckle 16a, however, a different number of knuckles is possible, for example two centre knuckles and three side knuckles forming two slots for accommodation of the two centre knuckles. Also, a different number of clamp sections are possible. Whereas two or three clamp sections 13' result in a stiff circular structure, more than three clamp sections would make the clamp flexible for adjustment to the pipe 2 in case that this gets deformed, for example due to bending. The latter can be ad advantage in some cases. However, the hinge principle and the mounting principles with the insertion of a pre-inserted pin apply equally well.

FIG. 6 illustrates a clamp 13 on a pipe 2 prior to mounting a buoyancy body 9 on the clamp 13.

FIG. 7a-c illustrate the mounting procedure and the gripper 18 for the clamp 13 in more detail. As seen in FIG. 7a, vehicle 10 with the gripper 18 has approached the pipe 2 and with the jaws of the gripper 18 open, the vehicle 10 manoeuvres the clamp 13 to the pipe 2. The knuckles 16a, 16b of the clamp are spaced apart and then closed, as illustrated in FIG. 7b and 7c, once the clamp 13 is in place. A pin driver 26 pushes a pin 19, as illustrated in FIG. 4a, into the clamp section 13 ' in order to secure it. Typically, the pin driver 26 is hydraulic. Typically, it is also used for pressing the pin 19 out of the clamp section 13' when demounting the clamp 13 from the pipe 2. Reference Numbers

1 buoyancy module

2 underwater pipe

3 station at sea bed 4

4 sea bed

5 ship

6 surface

7 anchoring point

8 floating part

9 buoyancy body

9a, 9b buoyancy elements

10 vehicle

11 control vessel

12 control centre on control vessel

13 clamp

13' clamp section

14 umbilical

15 hollow interior of buoyancy body 9

16a centre knuckle

16b side knuckle

17 gripper coupling on vehicle

17' counter coupling on gripper

18 clamp gripper for clamp 13

18' buoy gripper for buoyancy body 9

19 pin

20 hole

21 hinge

22 central axis of clamp

23 recess for side buckles 16b

24 fastener for buoyancy elements 9a, 9b

25 slot between side buckles 16b

26 pin driver