GB2512271A

GB2512271A - Apparatus and method for installation and protection of sub sea cables

Info

Publication number: GB2512271A
Application number: GB1301113.5A
Authority: GB
Inventors: Sigurd Ramslie
Original assignee: Seatower AS
Current assignee: Seatower AS
Priority date: 2013-01-22
Filing date: 2013-01-22
Publication date: 2014-10-01
Anticipated expiration: 2033-01-22
Also published as: GB2512271B; GB201301113D0

Abstract

An apparatus for installing subsea cables comprises a pipe 1, which may be flexible or substantially stiff and may be of HDPE, arranged telescopically with a tube 2, preferably a rigid J-shaped tube. Pipe 1 and tube 2 are pre-installable in a seabed unit, such as an oil platform or offshore wind turbine foundation. Tube 2 remains substantially stationary and pipe 1 is extractable outwardly from tube 2 so as to be substantially flat on the seabed and to extend through a zone where scour protection is to be applied before passing a cable (29, Figure 9) through tube 2 and pipe 1. There may be a brake 6 for preventing unwanted extraction or retraction of pipe 1, and a handle (14, Figure 3) allowing pipe 1 to be pulled out of tube 2 using a pull-out sling wire (24, Figure 6).

Description

FIELD OF THE INVENTION

The present invention in general, relates to an apparatus for installing and protecting a cable under sea and to the methodology for doing so Particularly, the present invention relates to a technology for installation and protecting a cable in subsea conditions running from an offshore bottom-fixed structure to other offshore or onshore structures.

More particularly, the present invention relates to an apparatus according to the preamble of claim 1 and to a method according to the preamble of claim 12

TECHNICAL BACKGROUND OF THE INVENTION S. *S * . *

* * In offshore operations such as oil and gas explorations and productions, S.....

installing and protecting cables running from an offshore structure resting on seabed, to other offshore and onshore structures is quite common.

Such an offshore structure may be for example a platform/foundation resting on the seabed for supporting a wind turbine. There can be other examples as known to persons skilled in the art all of which fall within the ambit of the present invention, such as a fixed offshore oil or gas facility or an offshore transformer station. It is also known that for the sake of stability, such foundations are frequently provided with scour protection around the base.

Offshore cables are typically heavy, thick and do not bend in sharp curves.

They are also quite expensive, sensitive and costly to repair.

Close to the offshore foundation, the cable needs to be protected from for instance excessive movement, over-bending and damage from dropped objects. Excessive movement is most likely to occur during the installation phase, while the risk for over-bending is often a product of under-scouring of the cable and ensuing free span.

Dropped objects are primarily rocks dumped around the foundation to prevent scouring of the foundation, but also include non-intentionally dropped objects throughout the operational life of the cable. Hence, installation and protection of the cable between the outer edge of the scour protection and point of its entry into the foundation is of vital importance. The cables extend from the seabed to the foundation on the seabed and then to the structure supported by the foundation.

The current state of the art consists of various forms of sheathing or pipes that are installed around the cable, usually at the time of cable installation The cable is usually installed significantly later than the foundation and its scour protection. Ideally, the cable should go under the scour protection, in which case a costly operation is needed to remove the scour protection to allow the cable and then replace the scour protection to ensure stability of the foundation.

The cable conduits inside the offshore structure will typically be curved (this is always the case for offshore wind foundations)-Telescopic arrangements are conventionally thought of as comprising two straight elements which fit into each other and allow the same shape (straight tube) to be maintained throughout the adjustment of length. Thus, traditional telescopic arrangement is not possible here. A key point of the invention is that the inner tube is elastic, which allows it to assume the shape of the outer tube in retracted position and to return to an essentially straight shape after extention.

Installation and protection of subsea cables extending from, on or into the seabed, to the wind turbine or other structures supported by a foundation, with the help of tubular structures such as J-tubes, are known. Apart from the problem stated in the preceding paragraph, the requirement for divers became almost indispensable for installing such cables into the i-tubes. Even using telescopic J-tubes did not help because divers were required to lock the J-tube in place after installation.

US7438502 teaches a telescopic under-water guiding assembly for subsea elements such as cables. It teaches a telescopic assembly of an outer receiver pipe to which is slidably engaged an inner extension pipe which can undergo extraction and retraction. A cone end is secured to the inner extension pipe.

The latter is locked in a fixed position with binding blocks with set screws.

The above patent does away with the requirement for deploying divers; however it does not teach how the requirement for removal of the scour protection, during its installation can be dispensed with. Further, it does not speéifically teach that the protective apparatus can be installed along with the foundation. Furthermore, the arrangement is fairly complicated. These aspects

are true for other prior art teachings as well. ** *-* * *

* * Hence, there has been a need for a simple and efficient apparatus to act as a *..*.* conduit for installation and protection of sub sea cables, which dispenses of the requirement for removal of the scour protection of the foundation, during its installation. * St *6 *

There is also a need for a technology that provides such an apparatus, which can be installed along with the foundation, prior to providing the scour protection. This is to ensure that the apparatus extends for a pre-determined length along the seabed, through the zone in which scour protection is to be applied later, so that the cable can be installed through the scour protection since applied, without the need for removing it.

The present invention meets the above mentioned ñeeds.and other associated needs by providing an apparatus which has a first tubular member arranged to move telescopically with respect to the second tubular member, such that both can be installed together with the foundation, through the zone of possible scour protection, before it is applied.

OBJECTS OF THE INVENTION

It is the prime object of the present invention to provide an apparatus which acts as a continuous conduit for instaliation and protection of sub sea cables, which has a simple construction and does away with the requirement for removal of scour protection of the foundation during installation of the cables.

It is another object of the present invention to provide a method for installation and protection of sub sea cables, which works in a very simple and efficient manner It is a further object of the present invention to provide an apparatus for installation and protection of sub sea cables which can be installed along with :.". the foundation prior to the application of scour protection, so that the apparatus * extends to a predetermined length and forms a continuous conduit from the top

I

* * to the seabed, through the scour protection, once applied.

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* 20 All through the specification including the claims, the words "vessels", "platform/foundation", "J-tube", "protective tube", "cable", "bell mouth", "scour s". protection", "cable pull-in wire" are to be interpreted in the broadest sense of : the respective terms and includes all similar items in the field known by * *.

other terms, as may be clear to persons skilled in the art. Restriction/limitation, if any, referred to in the specification, is solely by way of example and understanding the present invention. Further, it is hereby clarified that the term "tubes" should include "pipes", "tubular members" and other similar structures as applicable.

SUMMARY OF THE INVENTION

The above mentioned objects are achieved by an apparatus for installation and protection of subsea cables from a seabed unit, comprising at least one first tubular member and at least one second tubular member, said first tubular member is arranged telescopically on the outside or the inside of said second tubular member, both tubular members being pre-installed within the seabed unit and allowing passage of at least one cable there-through, said first tubular member being adapted to be extracted outwardly from the second tubular member, while said second tubular member remain essentially stationary with respect to said seabed unit, to a position essentially flat on the seabed, in order to extend through a desired zone for scour protection.

The invention also relates to a method for installing a subsea cable from a seabed unit, said method comprises the following steps: a) pre-installing at least one second tubular member and at least one first tubular member within the seabed unit so that the first tubular member is arranged telescopically with the second tubular member; b) installing the unit in the seabed while retaining the first tubular member (1) relative to the second tubular member; c) releasing the first tubular member from the second tubular member; d) pulling the first tubular member outwardly from the second tubular member, so that the first tubular member extends outwardly essentially along the seabed to a location outside a desired scour protection zone; e) pre-installing or passing a cable pull-in line through the second tubular : .*. member and the first tubular member, so that the pull-in line with a first end extends out from the outer end of the first tubular member, and with a second end extends out from an end opposite of the first tubular member of said second tubular member; f) attaching a cable to be installed to the first end of the pull-in line; g) pulling the line pull-in line with the cable trailing; h) before or after steps f) and g), applying scour protection around the seabed unit and on a part of the first tubular member.

Further favourable features are recited in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Having described the main features of the invention above, a more detailed and non-limiting description of an exemplary embodiment will be given in the following with reference to the drawings.

DETAILED DESCRIPTION OF THE INVENTION

Figure 1 shows the general arrangement of a foundation in a bottom-fixed condition together with the J-tube and the protective pipe, according to a preferred embodiment of the present invention, with the J-tube yet to be installed on seabed.

Figures 2a, 2b, 2c, 2d show in detail the structure and working of a preferred embodiment of the stopping means according to the present invention, where figure 2a shows the stopping means out of engagement with the pipe in a elevation view, figure 2b shows the situation in figure 2a in a plan view, figure 2c shows the stopping means in engagement with the pipe in a elevation view, figure 2d shows the situation in figure 2c in a plan view.

Figure 3 shows a detail of the outer end of the pipe and the J-tube with another preferred embodiment of the stopping means of the present invention.

Figures4a and 4b show detail views of the clamp shown in figure 3.

Figure 5a is a view of the detail arrangement of the protective pipe according to * : : the present invention, prior to its pull-out.

Figure 5b is a view of the details of a hinged pull-out bar according to Figure 5a.

Figure Sc is a view of the details of a friction clamp shown in Figure 5a.

Figure 6 is a view which explains the method of pulling out the HDPE/protective pipe from the J-tube.

Figure 7 is a view of a stage when the HDPE pipe has been pulled out of J-tube, awaiting dumping of filter bags.

Figure 8 is a view which shows filter bags that have been placed over the HDPE pipe after extraction and it is ready for the scour protection system to be installed.

Figure 9 is view of a stage when scour protection has been completed and the cable is to be installed.

DETAILED DESCRIPTION OF THE INVENTION

The following descñbes a preferred embodiment of the present invention which is purely exemplary for the sake of understanding the invention and non-limiting to the protective scope.

In all the figures, like reference numerals represent like features. Further, when in the following it is referred to "top", "bottom", "upward", "downward", "above" or "below" and similar terms, this is strictly referring to an orientation with reference to the seabed, where the seabed is horizontal and at the bottom.

It should also be understood that the orientation of the various components may be otherwise than shown in the drawings, without deviating from the.

principle of the invention.

** Additionally, the present invention is explained with reference to a J-tube, within which there is a protective tube, which is telescopically movable with respect to the J-tube. This protective tube is essentially an HOPE tube, but can consist of other suitable materials. It should be understood, that the present invention embraces all such arrangements of two or more tubular members which are capable of being arranged telescopically for forming a continuous conduit for a sub sea cable to pass through.

The cables which are proposed to be installed and protected by the apparatus of the present invention, essentially extend from other offshore or on-shore structure(s), on or embedded into the seabed near a foundation resting on the seabed, then into the foundation and up to the structure supported by the foundation.

In most of the figures only one J-tube and one protective tube are shown for the sake of convenience. There can be a plurality of such J-tubes and protective tubes within the foundation according to the present invention. This is true for the various other associated components described. The J-tube and the protective tube may each be a single tube or each may be a number of tubes, attached together to form a J-tube and a protective tube of the present nvention Figure 1 is a view of the general arrangement of major components of the apparatus when the protective pipe 1 has not been extracted out to the seabed 5 from the J-tube 2, but still resides within the outer,J-tube 2. It shows the foundation 3, which in this example is a gravity based structure (GBS) resting on the seabed 5. The foundation 3 supports a topside structure (not shown) at its top end 4. The topside structure may be a windmill extending upwards from the water surface 31.

Within the foundation 3 is already pre-installed the apparatus of the present invention, which is now to be described. In this example it is a J-tube 2.

preferably made of a non-flexible material, such as steel, glass fibre reinforced plastic (GRP), carbon fibre reinforced resin or other suitable material, extending downwards from the top end 4. The J-tube has a conical portion 2b and a curved portion 2a, which has a greater diameter than an upper straight portion 2c. The protective pipe I is accommodated within the curved portion 2a of the J-tube 2, * ** between the end 7 of the J-tube and its conical portion 2b. The protective pipe 1 is slidable with respect to the J-tube 2. Thus, it is a telescopic assembly and the protective pipe I can be pulled out of the J-tube 2 beyond the end portion 7, by a horizontal pulling force.

The protective pipe 1 is generally made of a somewhat flexible material like High Density Polyethylene (HDPE) which allows it to be inserted into and bent according to the shape of the J-tube 2 and will below sometimes be referred to as HDPE pipe t Other materials, such as carbon fibre reinforced resins may also be used.

The figure 1 also shows the end portion (bell mouth 30) of the retracted HDPE pipe 1, which is seen coinciding with the end portion 7 of the J-tube 2. A stopping means 6 (explained in detail below) can also be seen which prevents the HDPE pipe from being inadvertently extracted, or retracted once it is installed on the seabed 5. This stopping means 6 is provided in close proximity to the end portion 7 The conical section 2b forms a transition between the curved portion 2a and the straight portion 2c, with a smaller diameter than the curved portion 2a, of the J-tube2.

Figures 2a, 2b, 2c and 2d are detailed views of the end portion 7 of the J-tube 2, showing a preferred embodiment of the stopping means 6. The stopping means 6 prevents extraction or retraction of the protective tube 1 beyond a certain pre-defined limit. It comprises a plate 8a provided near said point of exit, which can slide along vertical stands 8b for engagement with complementary grooves 9 in the outer surface of the protective tube. The grooves 9 are provided at specific points on the protective pipe 1 where it is desired to retain it with respect to the J-tube 2. When the pipe I has been pulled out the desired length, the sliding plate 8a, whose rounded profile matches with the diameter of the groove 9, can be inserted into the appropriate groove 9 and lock the movement of the pipe 1. * *

* The sliding plate 8 may fall into the groove 9 by gravity or by spring force or may be operated manually by an ROV or an actuator (not shown). With this mechanism, the pipe I is locked in position with the i-tube 2 and can neither * *. move forward or backward. This would be particularly clear from the front views 2a and 2c which shows two consecutive stages, in the first stage the sliding plate 8 is approaching the grooves 9 but is yet to become engaged. In figure 2c the two are in engagement with each other. This would be dear from figure 2d as well, of which figure 2c is a front view. It shows clearly the engagement between the grooves 9 and the sliding plate 8. Disposition of grooves 9 along various lengths of the pipe 1 provides the option to selectively predetemline the length of the pipe 1 that will be extracted out of the i-tube. In a separate detail view in figure 2c, the plate Ba is shown separated from the rest of the stopping mechanism.

Figure 3 shows another embodiment of the stopping means 6. This comprises a restraining wire 10 is attached to a ring 35 attached to the protective pipe 1.

The ring is in turn connected to a cover 18 by a connection wire 36* The cover 18 prevents entry of unwanted material like soil or small rocks into the pipe 1

ID

during installation. The restraining wire 10 is at its opposite end secured to a pad eye 19 fixed to the foundation 3. The restraining wire ID restrains the protective pipe 1 from extracting out of the J-tube 2. A safety wire 12 is also connected between the ring 36 and the pad eye 19 The safety wire has a length corresponding with the desired extraction length of the pipe 1.

Figure 3 also shows a clamp 11 near the protective pipe 1 for clamping the same in order to prevent the pipe from retracting into the J-tube when it has been extracted to the desired length. Figure 4a is an enlarged view of the clamp 11 which is of the hinged type, having a hinge 16. Figure 4b is an enlarged view of the clamp 11 in operation which shows a T-bar 17a for engagement by an ROy, a right hand thread bar 1 Tb, a stopper plate 1 7c welded to the thread bar 1 7b, a plate lid with an oblong hole (not shown) to allow the thread bar to pass through, and a left hand thread bar lie.

Referring back to figure 3, the cover 18 is equipped with a handle 14 for attachment by a pull-out line (to be explained later) for pulling out the protective pipe 1. Figure 3 also shows a pull-in line 13, which is used to pull in a cable : inside the fully extracted protective pipe 1, as will be explained later, and a monkey fist ROV grab 15 attached to the outer end of the pull in line 13.

* .* Figures 5a and Sb show another alternative embodiment of the stopping means as well as a different embodiment of the means for extracting the pipe 1. Here a friction clamp 20 is preinstalled at the outer end 7 of the J-tube and adapted to clamp the pipe 1, to ensure that after the pipe 1 has been extracted to a desired length, it does not retract back inside J-tube, particularly when the cable 29 (shown in figure 9) is pulled in through the protective pipe 1. The clamp 20 is explained in detail in figure 5c.

Figures 5a and Sb also shows a hinged pull-out bar 21 that is attached to the outer end of the pipe 1 and is to be used for pulling out pipe 1. The hinged pull-out bar 11 is attached to the pipe 1 in close proximity to the end portion 7, and just behind the bell mouth 30. It also shows the cable pull-in wire 13, which in this embodiment is attached to the hinged pull-out bar 21 by a sacrificial wire sling 23. The overall length of the pull-in wire 13, which is, e.g., 20 to 25 meters, may be arranged on a bracket reel (not shown) located at the upper end of the J-tube.

A restraining wire sling 10 is connected to the end portion 7 of the protective pipe 1 to prevent the pipe 1 it Irom accidentally sliding out during transport and installation of the structure 3. A safety wire 12 is secured to the pad eye 19 fixed to the foundation 3 and is also connected to the pull-out bar 21 The safety wire has a length corresponding with the desired extraction length of the pipe 1.

Once fully extracted, through the possible zone of scouring for the foundation 3, the protective pipe 1 cannot retract into the J-tube 2 as it is arrested by the friction clamp 20.

Figure 5c is a close view of the friction clamp 20 that is used to arrest the motion of the pipe 1 after it has been pulled out. This is actually a tong like arrangement with friction linings on the inner faces. The two arms 20a and 20b are hinged at one end 20c and at the other end threaded bar and nut : arrangement 20d are provided for closing the two arms 20a, 20b. This threaded bar and nut arrangement 20d can be operated by an ROy. On activation, the device firmly clamps the pipe 1, without damaging it.

Figure 6 shows an exemplary arrangement to facilitate, the pull-out of the * ** protective pipe 1 from the J-tube 2. It shows two sets of J-tubes 2 and protective pipe 1 within the foundation. In practice, there can be several J-pipes, as * explained above. A sling wire 24 is attached to the hinged pull-out bar 21 (shown in figure 5a and Sb) or to the handle 14 of the cover 18, which is fixed to the pipe 1 near the outer end thereof, and is passed through a pulley held near the seabed with a clump weight 25.

The pulling wire 24 is attached to a constant tension winch 26 placed on a vessel 27 on the water surface 31.. The pulling wire 24 can thus provide a substantially horizontal pull to the hinged bar 21 and thus to the pipe I for smooth extraction from the J-tube 2.

Figure 7 is a view of a stage when the protective pipe 1 has been pulled out of the J-tube 2 awaiting placement of sand bags, filter bags or other types of relatively soft material that is able to protect the pipe 1 from the rocks of the scour protection.

Figure 8 is a view of the stage subsequent to what is shown in Fig 7. Filter bags 27 have been placed over the protective pipe 1 and is now awaiting application of scour protection. It also shows the bell opening 30 of the fully extracted protective pipe I which is outside the possible zone of scour protection. Now scour protection for the foundation 3 is next to be applied.

Figure 9 is a view of a stage when scour protection 28 has been applied and the cable 29 is about to be pulled into the bell mouth 30 with the help of the cable pull-in wire 13 (shown in figures 3 and 5a).

The above figures are again referred to now for the purpose of explaining the operation of installation of the apparatus and the cables so that the functioning of each component, as described hereinbefore is understood.

At the first stage on-shore preparation of foundation 3 for offshore installation is *0** done. First, the protective pipe 1 is inserted into the J-tube 2 and placed within the foundation 3. In this configuration, the protective device 1 is itself protected 0e**e * and does not get in the way of marine operations. This arrangement is shown in figure 1. The components shown in figure 1 have been explained in detail before with reference to figure 1 and are not repeated. * * -* . * *.

The foundation 3 with the J-tube 2 and protective pipe I so installed are now towed to the offshore location. The sacrificial hold back sling 10, connected to the end portion of the protective pipe 1 shown in figures 3 or 5a, ensures that the protective pipe 1 does not accidentally slide out of the J-tube 2 during towing. The foundation 3 is thereafter suitably installed on the seabed 5. The installation of the foundation will not be described in detail here, as this procedure is known to persons skilled in the art.

After installation of the foundation 3 on the seabed 5, the hold back sling 10 is cut (preferably by an ROy). Now one end of the sling wire 24 is attached to the hinged pull-out bar 21 or the handle 14. The bar 21 or the handle 14 is fixed to the pipe 1 (as shown in figures 3 and Sa) close to the end thereof. The other end of the sling wire 24 is passed through a pulley held near the seabed with a clump weight 25 and attached to a constant tension winch 26 placed on a vessel 27 on the water surface 31. This provides a substantially horizontal pull to the pipe 1, so that the pipe 1 is smoothly extracted from J-tube 2 As shown in figure 7. the pipe 1 is pulled out in the direction of the arrowhead along the seabed 5. Figure 8 shows the bell mouth 30 of the fully extracted protective pipe 1 The bell mouth 30 is now outside the application zone of scour protection. Further, it would be clear from figure 7 and also from figures 8 and 9, that the J-tube 2 and the fully extracted protective pipe 1 form a continuous conduit for cables from the top portion 4 of the foundation 3 to the seabed, through the zone of scour protection. For that purpose, it is vital that the length of extraction of the protective pipe 1 is predetermined accurately.

On reaching the desired length of pull-out, the protective pipe 1 is locked in position by suitable stopping means 6, as explained with reference to figures 2a, 2b, 2c, 2d, 5a and 5c. This stopping means 6 ensures that once the maximum length of extraction of the protective pipe I is reached, it neither retracts back, say during cable pull-in through the extracted protective pipe 1 * ** and J-tube 2, nor accidentally slides further out of the J-tube. * . 0* S

It needs to be explained further with reference to the pull-out of the protective pipe 1 as explained in the preceding paragraphs, that while the pipe 1 is pulled beyond the intended scour protection area on the seabed 5, it simultaneously pulls the cable pull-in wire 13 along with it. The cable pull-in wire 13 is attached to the hinged pull-out bar 21 with a sacrificial sling 23 as shown in figure 5a or is prevented to escape into the pipe 1 by the cover 18, as shown in figure 3 The cable pull-in wire 13 thereby passes through the J-tube 2 and protective pipe 1.

Actually prior to pull-out of the pipe 1, this wire 13 is made to pass through the J-tube 2 and the pipe 1 such that one end of this wire extends out of the outer end of the pipe 1 while the other end extends out of the J-tube at its upper end.

After the scour protection 28 is applied, the cable 29, which may already be placed near the bell mouth 30, may be inserted into the bell mouth 30. For that purpose, the pull-in wire 13 is attached to the cable 29 and pulled in through the bell mouth 30 through the extracted protective pipe 1 and up the J-tube 2, pulling the cable 29 with it.

Thus cable installation is achieved after the scour protection has been applied.

The need for removal of scour protection is effectively eliminated. This is possible because the protective tube can be pulled out of the J-tube to a pre-determined length. This length is so adapted that the end with the bell mouth 30 of the extracted pipe I is beyond the scour protection 28. At this length the pipe 1 will be locked relative to the J-tube.

Thus it is always ensured that the pull-out of the pipe 1 is always unidirectional and maintains the desired, pre-calculated length. Further, as the HDPE pipe 1 is pre-installed in the J-tube 2 and both are installed on the seabed 5 along with the foundation 3, the entire operation is very weather robust. e

The present invention has been described with reference to a preferred embodiment and some drawings for the sake of understanding only and it should be clear to persons skilled in the art that the present invention includes all legitimate modifications within the ambit of what has been * *. 20 described hereinbefore and claimed in the appended claims. ** . ** * * *

* As an alternative to the relatively flexible pipe I, the pipe may also be stiff but curved to correspond with the curvature of the curved portion 2a of the J-tube 2.

This means that the pipe 1 will curve upwards when it has been extracted.

However, after full extraction, the pipe may be rotated through 90° so as to lay flat on the seabed. This will require a somewhat longer pipe 1, as the pipe will extend in a curve through the scour protection zone.

It is also conceivable to have the pipe 1 telescopically received on the outside of the J-tube 2 Instead of on the inside.

It is also evident that other means for preventing the pipe 1 from sliding inadvertently out of or into the J-tube 2 may be used. Instead of ROV operated means, remotely operated actuators may be attached to the foundation, pipe I and J-tube 2. * . .

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