GB2435316A

GB2435316A - Method and apparatus for offshore pipe installation

Info

Publication number: GB2435316A
Application number: GB0602630A
Authority: GB
Inventors: Jean-Baptiste Pose
Original assignee: Individual
Current assignee: Individual
Priority date: 2006-02-10
Filing date: 2006-02-10
Publication date: 2007-08-22
Also published as: GB0602630D0

Abstract

A method for lowering or recovering in a controlled manner a submarine article 3 comprises attaching elements 19 to the article, elements whose role is to support the submarine article and whose net buoyancy is remotely adjustable from the surface at any stage of the operation. Preferably, the net buoyancy is changed using compressed gas, which is supplied to, or ejected from, the elements 19.

Description

1 2435316 1 METHOD AND APPARATUS FOR OFFSHORE PIPE INSTALLATION

4 Field of the invention

6 The present invention relates to pipe-laying apparatus and a method of laying pipelines, 7 and in particular to a method of laying pipelines in offshore locations.

9 This invention is to be used in the development of subsea offshore oil and gas fields. It constitutes a new pipe laying method which will be particularly adapted for deep waters 11 where current laying methods are becoming limited, but can also be effective in shallow 12 waters.

14 Background to the invention

16 The most common laying methods used today are shown in outline in Figures 1 and 2.

17 Figure 1 shows a system for laying pipe on the seafloor known in the art as the "J-Lay" 18 method. A similar method is the "S-Lay" method as depicted in Figure 2. Both method 19 names originate from the shape the pipeline 3 takes as it is lowered from the surface of *.

the ocean (Z) to the seabed (X).

22 These systems have a number of drawbacks. Both systems include a device 1 for 23 gripping or holding the pipe in tension during the deployment operation. For example, to 24 lay pipe in approximately 1000 m water depth, both the systems of Figure 1 and 2 require S S...

the laying vessels A to be able to hold approximately 500 tons of tension. This requires *.

26 heavy deck equipment and specially commissioned large payload ships with 27 displacement above 60,000 tons.

29 Recently, prospecting, developing and exploiting deeper fields has become economically viable and orders are being passed for drilling rigs able to operate 3000 meters water 31 depth. Installing a pipeline at such depths requires top tension capabilities in the order of 32 around 1000 tons, which exceeds the capabilities of most equipment currently available.

34 It is an aim of the invention to obviate or at least mitigate the drawbacks and disadvantages associated with prior art apparatus and methods.

37 It is an object of the invention to provide a method of laying pipeline that obviates the 38 need to hold the full suspended weight of the pipeline from a surface vessel.

1 It is an object of at least an embodiment of the invention to provide pipe-laying apparatus 2 using remotely adjustable underwater floats to control pipeline immersion depth and 3 elevation profile.

It is a further object of at least an embodiment of the invention to provide a method of 6 laying pipeline where pipeline or pipeline sections are fabricated onshore or offshore, 7 launched into water, towed across seas or oceans regardless of distance to the 8 deployment site, and then progressively lowered to its final location.

Other aims and objects of the invention will become apparent from the description below.

12 Summary of the invention

14 According to a first aspect of the invention, there is provided a method of installing a subsea pipeline, the method comprising the steps of: 16 -at least partially supporting a pipeline by one or more adjustable floatation 17 means; 18 -lowering the pipeline by adjusting the buoyancy of the adjustable floatation * 19 means.

21 Preferably, the pipeline is supported by a plurality of adjustable flotation means spatially 22 separated along the length of the pipeline. More preferably, the method includes the step 23 of lowering the pipeline by lowering successive adjustable flotation means.

Preferably, the method includes the step of lowering successive portions of the pipeline.. : 26 by sequentially adjusting the buoyancy of successive adjustable flotation means.

28 In this way, the method allows the controlled lowering of a pipeline from a suspended 29 height to its installation depth, for example at the seabed. A first or distal end of the pipeline may be lowered to its installation depth and secured to the seabed by an 31 anchoring device. Lowering of successive adjustable floatation means allows the pipeline 32 to be lowered along its length.

34 Preferably, a proximal end of the pipeline is partially supported by a vessel. More preferably, the method includes the additional step of manoeuvring the pipeline using the 36 vessel.

38 Preferably, the buoyancy of the adjustable flotation means is adjusted by controlling the 39 volume of gas, for example air, present in a chamber of the adjustable flotation means.

1 More preferably, the method includes the step of releasing a compressed gas source into 2 the chamber. The method may include the step of venting gas from the chamber into the 3 sea.

The compressed air may be provided to the chamber via an umbilical from the surface.

6 The umbilical may be connected to a compressed gas storage container.

8 The method may comprise the additional step of compressing air at the surface.

Alternatively, a compressed gas source may be provided on the adjustable floatation 11 means or within the pipeline itself.

13 The method may comprise the additional step of adjusting the lateral position of the 14 pipeline using propulsion means provided on one or more adjustable floatation means.

16 The method may include the additional steps of: 17 -monitoring the depth of adjustable floatation means using a depth sensor, and; :...:.

18 -regulating the depth of the adjustable floatation means by adjusting the buoyancy 19 in response to a signal from the depth sensor.

21 According to a second aspect of the invention, there is provided apparatus for installation 22 of a subsea pipeline, the apparatus comprising: : ..

23 -one or more adjustable floatation means adapted to support a pipeline; 24 -means for controllably lowering the adjustable floatation means.

26 Preferably, apparatus comprises a plurality of adjustable floatation means. More 27 preferably, the apparatus comprises a plurality of adjustable floatation means adapted to 28 be spatially separated along the length of the pipeline in use. More preferably, the 29 apparatus is adapted to lower successive adjustable flotation means.

31 Preferably, the apparatus includes means for partially supporting a proximal end of the 32 pipeline from a vessel.

34 Preferably, the adjustable flotation means comprises a chamber, and the means for controlling the volume of gas, for example air, present in a chamber.

37 The apparatus may include a compressed gas source. The method may include a valve 38 arrangement adapted to vent gas from the chamber into the sea.

1 The apparatus may include an umbilical connecting a gas source to the chamber. The 2 umbilical may be connected to a compressed gas storage container. Alternatively, the 3 umbilical may be connected to a gas compressor at the surface.

Alternatively, a compressed gas source may be provided on the adjustable floatation 6 means.

8 The adjustable floatation means may be provided with propulsion means.

Preferably, the adjustable floatation means is provided with a depth sensor and the 11 apparatus comprises a depth control system for regulating the depth of the adjustable 12 floatation means in response to a signal from the depth sensor.

14 In one embodiment of the invention: 1) The majority of the pipeline weight is supported using underwater floats.

16 2) The generated net buoyancy of the floats may be remotely controlled.

17 3) A control system is designed and made available to monitor and adjust pipeline 18 water depth and profile in the vertical plan. :. . : 19 4) A surface vessel is used to tow the pipeline and position it correctly in the a horizontal plan.

21 5) The floats are being fed in compressed air through an umbilical.

23 Depending upon the available surface vessel winch capacity, it is possible that the winch ** a 24 allow of the pipeline weight to be carried which in turn will reduce the quantity of required a asa floats.

:..DTD: 27 Brief description of the drawings

29 There will now be described, by way of example only, embodiments of the invention with reference to the following drawings, of which: 32 Figure 1 is a schematic representation of a prior art "J-Iay" pipe-laying system; 34 Figure 2 is a schematic representation of an alternative prior art "S-lay" pipe-laying system; 37 Figure 3 represents a method of laying a pipe in accordance with an embodiment of the 38 invention. The appropriate air and control / command umbilical has not been represented 39 for sake of clarity; 1 Figure 4 is a plan view of the embodiment of Figure 3. The appropriate floats and 2 umbilical have not been represented for sake of clarity; 4 Figure 5 is a schematic representation of an underwater float design (rigid bell) according to an embodiment of the invention; 7 Figure 6 is a schematic representation of an alternative underwater float design (flexible 8 parachute) according to an embodiment of the invention; Figure 7 is a schematic representation illustrating the basic regulation principle of an 11 underwater float according to an embodiment of the invention; 13 Figure 8 is a schematic representation of the pipeline fabrication and launching process 14 from an onshore base according to an embodiment of the invention; 16 Figure 9 is a schematic representation of a control command system according to an 17 embodiment of the invention; and S.....

19 Figure 10 is a schematic representation of propelled floats according to an embodiment * of the invention.

22 Detailed description of the invention.:*

24 With reference firstly to Figure 3, a method of laying pipeline 3 is generally depicted. S... "S S *S 5

26 An installation vessel (A) carries a winch (2) with a stored wire length (5) approximately 27 1.3 times the maximum water depth at a pipeline installation site and a sufficient tonnage 28 capacity. It carries all equipment necessary for the floats system i.e. in the present 29 embodiment an air compressor and the float system control command. In addition, it may carry what is called an ROV (Remotely Operated Vehicle) which is an unmanned robot 31 used extensively in the offshore installation industry and able to perform inspection or 32 simple tasks at depth.

34 The towing configuration (a) is used. A pipeline towing depth is selected and the float control system controls the pipeline depth to prevent the pipeline from floating on the 36 surface or dragging on the seabed or over stressing the winch wire. The depth regulation 37 principle will be described further below. As the pipeline is towed below the upper layer 38 of the ocean, subject to swell, the pipeline is exposed to a very limited amount of fatigue.

I When the installation vessel A has reached its destination, pipeline lowering can 2 commence. The far end of the pipeline 3 is lowered from (a) to (b), by reducing the 3 buoyancy of the floats located toward the end of the pipeline. Note that all floats are 4 separately adjustable to always be able to get the right pipeline profile in the water. Once the pipeline end is close to the seabed in position (b), the connection of the loose end to 6 the seabed is achieved via an anchor (14) and an initiation wire (15) for initiating the 7 laying of the pipe (an assisting vessel will be required). Once initiated, the laying can 8 continue by successively activating the floats from the far end toward the near end of the 9 pipeline.

II During this phase of laying the pipe, the initiation vessel (A) manoeuvres to land the 12 pipeline within its horizontal corridor (16) as depicted in Figure 4. A section of the 13 pipeline is already on the seabed (3b), whereas (3c) represents a section clear off the 14 seabed and still to be lowered. As can be seen, (A) is pulling the pipeline sideways to make the necessary horizontal adjustments and required directional changes.

17 It is advised that all vessels involved in the operation be dynamically positioned, and able 18 to keep precise position and move with an equivalent amount of thrust in any horizontal 19 direction.

21 Once the entire pipeline section is lowered and laid, the winch wire is disconnected and 22 the floats released using the ROy. The floats are then collected at the ocean surface. s em..

24 With reference now to Figures 5 and 6, there are depicted adjustable floatation devices. :::: 25.. : 26 It will be appreciated that other types of adjustable floatation could be employed within 27 the scope of the present invention. In particular, the present invention is not limited to 28 one type of floatation device, but any floatation device remotely controlled and adjustable 29 net buoyancy and suitable for using in the method described above in relation to Figure 3.

Figure 5 shows a rigid device and Figure 6 shows a flexible device for handling 3 1 underwater weights. The rigid float is quite similar to an antique diving bell, whereas the 32 flexible float is close to underwater parachutes used by divers to bring weights back to 33 the surface.

Each float is composed of an upper part made of permanently buoyant material (6) which 36 guarantees: 37 o the float will remain in a vertical position, avoiding accidental air loss through 38 steep vertical misalignment.

39 o the float will get back to the surface once disconnected from the pipeline.

I Below this permanent buoyancy section is a skirt [cylinder closed at the top but opened at 2 the bottom] (17), whether rigid (figure 5) or flexible (figure 6) which can be filled with air or 3 water. The bottom of the skirt remains opened and there is constant pressure equilibrium 4 between the inner chamber of the float and the water outside which allows the design of non pressure resistant skirt walls.

7 By varying the water level (12) inside each float's skirt, each of them can provide a 8 variable amount of net buoyancy. The depth information of each float being sent to the 9 control system, it is then possible to fully control and adjust the profile of the pipeline.

11 Each float is supplied with high pressure compressed air via an umbilical line 7 12 comprising armoured small diameter flexible pipe bundled with power and 13 instrumentation cables. This umbilical is connected to an air compressor or to stored 14 compressed air on the installation vessel or in the pipeline. The umbilical is itself fully buoyant. The compressor's capacity should be above the maximum water pressure at 16 depth, the required flow should be evaluated and will be a function of the required 17 quantity of underwater floats, water depth, etc., etc. : . ..:. * *

19 The air connection to the float is done through a valve (8) which can be operated remotely via the umbilical and can be set in the following states: 21 6) Shut: no air is to come in or out of the enclosed capacity.

22 7) Open towards the sea: the air is purged and buoyancy of the float decreases. : ..

23 8) Open towards the air umbilical and air is forced within the float increasing its net 24 buoyancy. *S.. * S. 4

26 Each valve from each single float may be operated separately.

28 The floats should be equipped with the following sensors: 29 o pressure sensor to indicate water depth.

o (10) load cell to determine carried weight and to know whether the pipeline has 3 1 been lowered or not.

32 o As an option for the rigid skirt type, an inner water level (9) to determine available 33 range of buoyancy still available.

34 o Some critical floats (extremities or singularities in the pipeline -a T) could be equipped with a video camera.

37 The valves are controlled by commands that are transmitted to the valve via cable (18) 38 included in the air umbilical and is connected to a surface control command system. Air 39 is supplied to the floats via the umbilical for controlling inflation and buoyancy of the floats.

2 The control system is operated from the surface vessel. The control command system is 3 computerised and appropriate software is used to drive the system and is user friendly.

Use of depth sensors, and appropriate control command software, various kinds of 6 automation is possible, such as: 7 o Regulation upon depth of sensors; 8 o Regulation upon speed of pipeline ascent I descent; 9 o Maintaining the pipeline within a depth corridor; and o Providing a laying mode where driving the buoyancy of one float drives 11 automatically the others in order to obtain the ideal profile of the pipeline in the 12 water for landing the pipeline.

14 Floats are tied around the pipeline using soft slings which are easily removable and will not damage the pipeline's external coating. The appropriate number of floats is installed 16 at a specific spacing as required and the floats are driven according to the following 17 parameters: ***...

18 -net buoyancy per element 19 -linear weight of the submerged pipeline..: -the required safety factor 21 -pipeline resistance to buckling due to freespan [length of unsupported section].

23 Depth regulation of the floats is illustrated in Figure 7, which describes the basic 24 regulation principle. Two depths are represented dO and dl. For example, Let us * S..

consider a float in position A between dO and dl and very slowly sinking. As it sinks, the.. : 26 pressure increases and the trapped air volume reduces thus reducing the uplift generated 27 by Archimedes force. The floats sinking speed increases.

28 When it reaches dl at stage B, the control system orders the valve to fill the inner skirt 29 with more air. The volume of air increases, stopping the descent and generating a slow upward movement (C). As the float is going up, the trapped air, seeing a reduction in 31 water pressure, expands and the Archimedes force increases thus increasing the ascent 32 speed. This movement is stopped in D through the command system ordering the release 33 of some air directly to the sea. Following this action, the float is in the E situation similar 34 to the original A situation and an entire cycle of regulation has been run.

36 Various compressed air sources may be used. In the above embodiment, air is supplied 37 from an air compressor located on a surface vessel.

1 In an alternative embodiment, the compressed air could be stored in the pipeline itself. In 2 such a case, the pipeline would be pressurized before being launched and the air 3 umbilical would be connected to the pipeline via a valve.

To provide system redundancy in case of equipment failure, the pipe-laying system may 6 include both sources installed with an umbilical being fed from the surface or from the 7 compressed air kept within the pipeline.

9 The survival of the pipeline relies upon the integrity of the umbilical. Various safety devices might be used to cope with the accidental loss of an umbilical such as: 11 -pre installation of spare umbilical quickly connectable.

12 -additional weights that could be released bringing the pipeline back into surface.

13 -capacity of compressed air being towed with the pipeline and acting separately from 14 the main system if a certain depth is reached.

16 In a further embodiment, the system could include propulsion devices to assist with 17 buoyancy. This would assist with manoeuvrability in the horizontal plan to rotate the 18 section of pipeline, to counter the effects of current and to help for a pinpoint landing 19 within pipeline corridor. * 21 Figure 8 describes a possible onshore fabrication line. (S) represents the storage of pipe 22 elements which are being welded together in the assembly line (f). A tug boat provides a: ..

23 permanent pull at the line extremity allowing the launch of the pipeline in water as its 24 fabrication progresses. Floats are being attached progressively and activated to the S...

correct value of net buoyancy in the launching zone (I). .. : 27 Note: Often a pipeline contains in-line structures like Ts, manifolds or valves. These 28 structures are to be inserted within the line onshore and might require their own buoyancy 29 modules to guarantee they are going to land upright and zeroing the excess of submerged weight generated.

32 Once the pipeline section (which can measure above 3km in length) is fully launched it is 33 towed towards deeper waters and the level of air in each float is adjusted to sink it to the 34 required towing depth. From this moment, accurate depth regulation is required as over the entire tow duration various conditions of sea water densities (temperature, salinity) 36 will be encountered, changing the equilibrium of vertical forces (gravity against 37 Archimedes).

39 Various modifications and improvements may be made to the invention herein described without departing from the scope of the invention.

Claims

Claims What is claimed is: 1. A method for lowering or recovering or

stabilising around a given depth or supporting in a body of water a submarine structure. The method comprising attaching elements to the structure, elements which role is to control the immersed depth and or profile of the submarine structure and which net buoyancy is remotely adjustable from the surface whilst the element is in operation.

2, The method of claim I wherein the submarine structure is a subsea pipeline which is towed from its fabrication area within a given water depth range and subsequently lowered to the seabed at its final installation location in a controlled manner.

3. An apparatus for towing a submarine structure within a controlled depth range and comprising: * tow means attached to the structure; and * at least one underwater float which buoyancy is adjustable and may be piloted from the surface.

4. The apparatus of claim 3 wherein the float: a is fed in compressed gas or gas mixture from the surface via a flexible pipe to increase net buoyancy; and a the gas may be vented from the float into the sea to reduce net buoyancy.

5. The apparatus of claim 3 wherein a control system monitors the depth of each float and allows to modify each float net buoyancy independently or not as deemed necessary by the depth or vertical speed of individual or a group of floats.

6 The apparatus of claim 3 wherein the float is fed in compressed gas or gas mixture from a capacity containing compressed gas and immersed with the submarine structure.

7. The apparatus of claim 3 wherein the float is fed In compressed gas or gas mixture stored within the submarine structure.

8. The apparatus of claim 3 wherein the gas is being compressed at the surface from a vessel equipped with appropriate compressor means and compressed gas storage capability.

9. The apparatus of claim 3 wherein the floats are controlled via a control command umbilical linking the float to the surface.

10. The apparatus of claim 3 wherein the floats are wirelessly operated using underwater acoustics.

11. The apparatus of claims 4 and 8 wherein a single multi-purpose multi-product umbilical incorporates two or more of the three below functions: * gas delivery to depth, * control command of the float (s), * towing line.

12. The apparatus of claims 5, 8 and 11 wherein the main towing vessel is carrying the compressing capabilities, the control command system and the umbilical.

13 An apparatus for lowering or recovering to or from the seabed a submarine structure in a controlled manner and comprising: * tow means to allow correct positioning within a generally horizontal plane (seabed plane) supported by surface vessels; and * at least one underwater float which buoyancy is adjustable and may be piloted from the surface.

14. The apparatus of claim 13 wherein the float: * is fed in compressed gas or gas mixture from the surface via a flexible pipe to increase net buoyancy; and * the gas may be vented from the float into the sea to reduce net buoyancy.

15. The apparatus of claim 13 wherein a control system monitors the depth of each float and aVows to modify each float net buoyancy independently or not as deemed necessary by the depth or vertical speed of individual or a group of floats.

16. The apparatus of claim 13 wherein the float is fed in compressed gas or gas mixture from a capacity containing compressed gas and immersed with the submarine structure.

17. The apparatus of claim 13 wherein the float is fed in compressed gas or gas mixture stored within the submarine structure.

18. The apparatus of claim 13 wherein the gas is being compressed at the surface from a vessel equipped with appropriate compressor means and compressed gas storage capability.

19, The apparatus of claim 13 wherein the floats are controlled via a control command umbilical linking the float to the surface, 20. The apparatus of claim 13 wherein the floats are wirelessly operated using underwater acoustics.

21. The apparatus of claims 14 and 18 wherein a single multi-purpose multi-product umbilical incorporates two or more of the three below functions: * gas delivery to depth, * control command of the float (s), * towing line.

22, The apparatus of claims 15, 18 and 21 wherein the main holding vessel is carrying the compressing capabilities, the control command system and the umbilical.

23. The apparatus of claims 12 & 22 wherein: * it is the same vessel which is used for towing to site and subsequent deployment to depth; or * it is the same vessel which is used for recovery from depth and subsequent towing to another location.

24 The apparatus of claims 3 to 23 wherein the submarine structure is a subsea pipeline to be installed or recovered and moved.