GB2468918A

GB2468918A - Sub sea clamping mechanism

Info

Publication number: GB2468918A
Application number: GB0905335A
Authority: GB
Inventors: David Edward Pugh; Wilfred Johnson Emmanuel Cox
Original assignee: Subsea 7 Ltd
Current assignee: Subsea 7 Ltd
Priority date: 2009-03-27
Filing date: 2009-03-27
Publication date: 2010-09-29
Anticipated expiration: 2029-03-27
Also published as: GB2468918B; GB0905335D0

Abstract

To simplify deploying of a sub-sea riser assembly, and to allow a flexible riser to be secured, there is provided a clamping device (400) for stowing a flexible riser (300) along a rigid riser (101), comprising: an elongate arm (406) having attaching means (402) at one end for attachment to a rigid riser (101); a clamping element (405) at the other end of the elongate arm (406) for clamping to a flexible riser (300); and pivot means (403) adjacent said one end; wherein: when the attaching means (402) is attached on a rigid riser (101), said pivot means (403) enables the arm (406) to move the clamping device (400) between a stowed position in which the longitudinal axis of the arm (406) extends substantially parallel to the longitudinal axis (301) of the rigid riser (101); and a deployed position in which the arm (406) extends laterally away from the longitudinal axis (301) of the rigid riser (101).

Description

SUB SEA CLAMPING MECHANISM

The present invention relates generally to the installation of sub sea riser assemblies.

The basics of riser drilling and hybrid riser systems is described in: "DIV Chikyu, Riser Operations and the future of Scientific Ocean Drilling", published in Oceanography, Vol.19, No.4, Dec. 2006; "MODU Installed Free Standing Hybrid Risers", Lustosa et al, 2H Offshore Projetos LTDA, Roveri et al, Petrobras, http://www.2hOffshore.com,technicaUibra1Y1PaPerS'2004fPaPO82.pdf, viewed on March 26, 2009; US Patent No. US-A-5,676,209 and US Publication No. US2008/0302535.

A general description of the construction of a hybrid riser system is given below.

A lower riser assembly (LRA) is first installed on a sea bed. The LRA may comprise a blow out protector, connector guide funnel and a connection pipe with a U-bend.

Connected to the lower riser assembly is a rigid riser. The rigid riser rises from the sea bed to a location that is sufficiently far below the sea surface to avoid the more severe sea and weather conditions experienced near the surface.

Installed on the top of the rigid riser is an upper riser assembly (URA). One typical design of URA comprises a nitrogen filled buoyancy can, to provide tension in the riser string, and a goose neck termination at the top of the rigid riser, for fluid take-off. A flexible riser is then connected between a support vessel on the surface of the sea and the goose neck termination on the URA. The flexible riser forms a catenary as it hangs between the support vessel and the goose neck termination on the URA.

An advantageous property of such hybrid riser systems is that the catenary allows movement of the support vessel, for example due to adverse weather conditions, to be decoupled from the rigid riser.

A standard method for constructing a hybrid riser system is described below.

In a first stage, a support vessel instafls the LRA and rigid riser.

In a second stage, a support vessel installs the URA on the rigid riser.

In a third stage, an end of a flexible riser is connected to the URA.

The fourth stage is to connect the other end of the flexible riser to a support vessel so the hybrid riser system can be operated.

In practice, the stages are not necessarily carried out in immediate succession. In particular, a different surface vessel may be used to perform each io operation; the same surface vessel may be used to perform any two or more operations or even the same vessel may be used to perform all of the operations.

For example, the fourth stage may be commenced immediately after the third stage or, alternatively, the fourth stage may be commenced a long time after the third stage has been completed.

Moreover, the operation of the hybrid riser system may be interrupted once commenced. The flexible riser is then disconnected from the support vessel, the vessel being free to leave the area. A support vessel can return at a later time, perform the fourth operation to retrieve the flexible riser and then resume operation of the hybrid riser system.

The above described flexibility is an important advantage of such hybrid riser systems. Adverse weather conditions or unavailability of a surface vessel, for example, may require a stage or operation to be suspended until a later time.

* The situation that arises between the third and fourth stages will now be considered in more detail.

After the third stage has been performed, but before the fourth stage has commenced, one end of the flexible riser pipe will be connected to the goose neck termination on the URA and the other end will be hanging down next to the rigid S 3 riser, unconnected to a support vessel. A first step of the fourth stage wili be to retrieve the unconnected end of the flexible riser and connect it to the support vessel on the surface. The operation of the hybrid riser system can then begin.

The above situation also arises if the hybrid riser system was being operated but it was necessary to interrupt the operation and disconnect the flexible riser from the support vessel.

A problem therefore arises in that before the fourth stage is commenced, the flexible riser pipe is only connected to the URA. If the flexible riser is allowed to hang freely from the URA then there is a possibility that either or both of the io flexible riser and the rigid riser may be damaged. For example, the sea conditioflS may cause the flexible riser to strike the rigid riser or the flexible riser may made be unusable by wrapping round itself or the rigid riser.

Accordingly, it is necessary to ensure that such damage to either the flexible riser or the rigid riser is prevented. A known method for protecting the flexible riser, when it is only connected to an URA, and disconnected from a support vessel, is to secure the flexible riser along the rigid riser. This also ensures that the precise location of the flexible riser is known when a later retrieval operation of the flexible riser is performed.

To secure the flexible riser to the rigid riser, a series of clamps is typically provided along the length of a section of the rigid riser. The clamps provide points of attachment for the flexible riser, ensuring that it is held secure and at an appropriate spacing from the rigid riser.

The clamps are required to be orientated such that their direction protruding from the rigid riser is appropriate for the particular arrangement of the URA, and flexible riser deployed from the URA. Preferably, the clamps and flexible riser would have the same orientation so that the flexible riser can be secured into the clamps without unnecessarily introducing bends into the flexible riser.

Each clamp is typically 2m to 3m long. Other lengths of clamp are also possible, depending on the desired separation of the flexible riser and the rigid riser.

The clamps may be at depths of 400m to 600m, for example, depending On the length of the flexible riser TypicaY, 4 to 9 clamps are employed. However, any number of clamps may be employed, according to the intervals between the clamps and the length of the flexible riser.

It would be possible to install the clamps to the rigid riser having first installed the rigid riser with no clamps attached. In a subsequent operatiOfl clamps would then be installed onto the rigid riser after its deployment.

However, such installation of the clamps after the rigid riser has been deployed S very difficult. The clamps are typically required at much greater depths than 220 meters, the practical depth for divers. It is also expensive to operate with divers and there are considerable safety risks.

Alternative methods of installing the clamps after the rigid riser has already been deployed under water are also highly problematic. installing the clamps is a delicate operatiofl and to install them in an appropriately aligned condition using remotely operated vehicles (ROVS) is very diffiCUlt.

An alternative way of installing the clamps to the rigid riser is to install them onto the rigid riser before the rigid riser is deployed under water. To attach the clamps to the rigid riser above the surface overcomes the aforemeflti0fled problemS. However, a different problem is encountered.

A roller mechanism is provided on the support vessel to control the lowering operation when deploying the rigid riser. Since the roller mechanism IS adapted to also lower, for example, a Ubend or connector guide funnel of the LRA, it needs to be able to accommodate non-cylindrical objects. Accordingly, the roller mechanism has been adapted to lower a rigid riser having a protrusion.

However, the lowering of the rigid riser with rigid clamps already attached restricts the clamps to being all in the same orientation, that orientation being the same as that of the protrusion on the lower riser assembly. It is not therefore possible to flexibly pre-attach the clamps into the orientation at which they will be required for use. There is certainly no provision for pre-attachifl9 clamps in more than one orientation and it would be impossible to pre-attach clamps with, for instanCe, 1800 relative orientations.

It is therefore necessary to deploy the rigid riser with the clamps in a predetermined orientation and then perform an ROV operation to align the clamps as required This alignment operatiOfl that is conducted at considerable depth, is very awkward and impractical to perform.

In view of the aforementioned problemS embodiments of the invention provide an apparatus and method for permitting a rigid riser to be lowered through a roller mechanism with attached clamps arranged in any orientation.

According to a first aspect of the invention, there is provided a clamping device for stowing a flexible riser along a rigid riser, comprising: an elongate arm having attaching means at one end for attachment to a rigid riser; a clamping element at the other end of the elongate arm for clamping to a flexible riser; and pivot means adjacent said one end; wherein: when the attaching means is attached on a rigid riser, said pivot means enables the arm to move the clamping device between a stowed positiofl in which the longitudinal axis of the arm extends substantially parallel to the longitudinal axis of the rigid riser, and a deployed position in which the arm extends laterally away from the longitudinal axis of the rigid riser.

According to a second aspect of the invention, there is provided a riser assembly, comprisiflg a rigid riser section and a clamping device; the clamping device comprising an elongate arm having attaching means attached at one end to the rigid riser section and a clamping element at the other end for clamping to a flexible riser portion; and the arm includes pivot means adjacent said one end; such that, the arm is movable to pivot about the pivot means from a stowed position with its longitudinal axis substantiallY parallel to the longitudinal axis of the rigid riser to a deployed position extending laterally from the longitudinal axis of the rigid riser, so that when the arm is in said deployed position1 the clamping device is arranged for holding the flexible riser portion and rigid riser section apart from each other. S 6

According to a third aspect of the invention, there is provided a method of deploying a rigid riser underwater comprising the steps of: attaching an end of a clamping device to the rigid riser, wherein the clamping device has a elongate arm that is moveable between a stowed position, in which the longitudinal axis of the arm is substantially parallel to the longitudinal axis of the rigid riser, and a deployed position in which the arm extends laterally away from the longitudinal axis of the rigid riser; configuring the clamping device such that it is in the stowed position; and lowering the rigid riser with the attached clamping device through a roller box into a submerged location.

PreferablY, a plurality f clamping devices are attached the rigid riser with different relative orientations such that when the said clamping devices are in their deployed positionS they protrude laterally from the rigid riser in a plurality of different directions. This provides a suitable clamping arrangement for the flexible riser regardless of how the flexible riser is orientated. Advantageously, it is therefore not necessary for the intended orientation of the flexible riser to be known when the rigid riser is deployed.

According to a fourth aspect of the invention, there is provided a method of stowing a flexible riser alongside a rigid riser, the rigid riser having been deployed in accordance with the method of said third aspect, wherein, for each of the one or more clamping devices, the method comprises the steps of: reconfiguring the clamping device on the rigid riser into the deployed position; and securing the flexible riser to the clamping element at the other end of the clamping device such that the flexible riser and the rigid riser are held spaced from each other by the clamping device.

For a better understanding of the invention, and to show how the same may be carried into effect, reference will now be made by way of example, to the accompanying drawings, in which: FIG. I shows a perspective view of a typical LRA; FIG. 2 shows a plan view of a roller box arrangement during the deployment of a rigid riser with a rigid clamp attached according to the prior art; FIG. 3 shows a perspective view of a rigid clamp according to the prior art; FIG. 4 shows a perspective view of a folding clamp attached to a rigid riser in position A; FIG. 5 shoWS a perspective view of a folding clamp assembly attached to a rigid riser after the folding clamp has moved from position A to position B but before the clamping element on the end of the folding arm has rotated; and FiG. 6 shows.a perspective view of a folding clamp assembly securing a flexible riser to a rigid riser.

Embodiments of the invention further provide a method for stowing a flexible riser, the flexible riser secured to a rigid riser by the said folding clamps.

FIG. I shows the lower section of a hybrid riser system, in which a lower riser assembly (LRA) 100 is installed on the sea bed. A rigid riser 101 is then attached to the LRA and rises from the sea bed to a location that is sufficiently far below the sea surface to avoid the more severe sea and weather conditions is experienced near the surface.

The LRA 100 is lowered from the surface of the sea by a support vessel.

The support vessel lowers the LRA 100 and the rigid riser sections 101 through a roller box.

FIG. 2 shows an example of a roller box 200. The roller box 200 is typical for vertical J-Iay tower systems used for installation of an LRA 100, rigid riser 101 and URA.

The typical roller box has rollers 204 to 212 for lowering a rigid riser 101. A hydraulic system 202 operates to open and close a gate 203. The gate is closed when the section of cylindrical rigid riser 101 being lowered through the roller box 200 is essentially straight and uncluttered, i.e. no substantial components are protruding from the rigid riser 101. The gate is opened when the section of rigid riser 101 being passed through the roller box is substantially curved or carries a component, such as a rigid clamp, which protrudes from the rigid riser 101. This latter scenario can be seen in FIG. 2 which shows the roller box 200 with its gate 203 open. Being passed through the roller box 101 is a section of rigid riser 101 with a rigid clamp 201 attached and protruding from the rigid riser 101 (the rigid clamp 201 is shown in FIG. 3 and discussed elsewhere Ifl the present

specification).

It is clear that the gate 203 must be open in order to permit the rigid riser 101 with its rigid clamp 201 attached to pass through the roller box 200.

Moreover, the protruding rigid clamp 201 needs to be aligned with the opening provided by the gate 203 for the rigid riser to be lowered.

The roller box 200 needs to accommodate the lowering of the LRA 100, which may be regarded as a protrusion from the rigid riser 101. For the rigid riser base of the LRA shown in FIG. 1, for example, it will be necessarY for the roller box 200 to accommodate passage not only of the rigid riser 101 but also of the connector guide funnel 102.

To accommodate the lowering of an LRA loot the roller box 200 is configured as shown in Fig. 2. In the illustrated configuration, the roller box 200 can pass a rigid riser 101 having a protruding component1 such as the illustrated clamp or the LRA, attached thereto. During deployment of the LRA ioo through the roller box 200, the LRA must be held in a specific orientation in order to extend through the open gate. It is not possible to rotate the rigid riser ioi about its longitudinal axis 301 during its deployment.

ConsequentlY, when rigid clamps 201 are attached to the rigid riser 101, they can only be attached in an orientation which is aligned with the connector guide funnel 102 at the riser base even though the desired orientation may be different. The desired orientation may differ by as much as 180° relative to the connector guide funnel 102 at the riser base.

After the LRA and rigid riser have been installed, an upper riser assembly (URA) is installed on top of the rigid riser. A flexible riser 300 is then connected to the URA and, if the flexible riser 300 is not connected at the other end to a support vessel, then the flexible riser 300 will hang down next to the rigid riser 101. In such a situation, it is necessarY to protect the flexible riser and rigid riser from damage caused by, for example, them striking against each other. AlternatiVelY, damage may be caused in a number of other ways such as the Ilexible riser 300 wrapping around the rigid riser 101, or itself.

Accordingly, clamps 201 are provided along the rigid riser 101. The flexible riser 202 is secured by the clamps which extend from the rigid riser, so that damage is prevented.

A problem arises in how to attach appropriately aligned clamps between the rigid riser 101 and the flexible riser 300.

To attach the clamps after the rigid riser has been deployed ts time-consuming and expensive. It typically involves the difficult operation of attaching clamps to the rigid riser in the correct orientation at a much greater depth than divers can operate.

Accordingly, a known method for providing a rigid riser with clamps is to attach the clamps prior to the deployment of the rigid riser. The clamps are attached to the rigid riser before it is deployed into the sea, and the rigid riser with the clamps attached is then lowered into the sea through the roller box.

A rigid clamp 201 employed in such a known method is shown in FIG. 3.

The deployment of the rigid riser 101 through the roller box 200, with a rigid clamp 201 attached to the rigid riser 101, is shown in Fig. 2.

It is clear that in order for a rigid riser 101 with a rigid clamp 201 attached to be able to pass through the roller box 200, the clamp must be aligned with the open gate and thus with the protruding components of the LRA 100.

The clamps 201 are therefore all attached to the rigid riser 101 with essentially the same orientation. Furthermore, Since the orientation is predetermined by the orientation of the LRA 100, there is no possibility of pie-aligning the clamps 201 in any desired operational orientation.

It is therefore necessary to perform the subsequent operation of aligning the clamps 201 into their desired orientation for use. Such an operation would usually be carried out by remotely operated vehicles (ROVs).

The alignment operation entails considerable practical difficulties. An ROV is required to perform the difficult operation of releasing a rigid clamp 201, retaining the heavy clamp at the required level, rotating it to the desired orientation, and then re-fixing the rigid clamp 201. This may need to be performed at depths of up to 600m, for example. When multiple mutually-spaced clamps are provided, the task of correctly ahgning each clamp 201 in the series is very difficult for an ROV to accomplish.

To solve the aforementioned problems, embodiments of the invention provide a folding clamp 400, and a method of attaching the folding clamp 400 to a rigid riser 101 prior to the deployment of the rigid riser, with the clamp 400 already in its desired orientation.

FIGs. 4 to 6 show a folding clamp 400 according to embodiments of the invention. As described below, the clamp can move between a stowed position and a deployed position. In a practical embodiment, a series of clamps 400 will normally be provided at intervals spaced along the length of the rigid riser. The overall length of each clamp 400 will typically be 2-3 meters.

The stowed first position of the clamp 400, position A, is shown in FIG. 4.

The clamp includes an elongate arm 406 arranged to move from the stowed position to the deployed position, position B, as in FIG. 5. A clamping element 405 at the end of the elongate arm 406 is then arranged to move to the configuration shown in FIG. 6.

In stowed position A, the folding clamp 400 protrudes less from the rigid riser than a rigid clamp 201 and the cross-sectional diameter of the folding clamp 400 when attached to the rigid riser 101 is therefore far less than in the prior art method. Consequently, when the folding clamp 400 is attached to the rigid riser 101 in position A, the folding clamp can be lowered through the roller box 200 in any orientation. The clamp 400 is therefore not restricted to being in the same orientation as that adopted by the LRA 100 when passed through the roller box 200.

The clamp arm 406 is operable to move about the pivot 403 from stowed position A, as shown in FIG. 4, to deployed position B, as shown in FIGs. 4 and 5.

In deployed position B, the clamp arm 406 will typically extend substantially perpendicular to the rigid riser ioi. Although the clamp arm 406 has been described as extending substantially perpendicular to the rigid riser 101 of course it is clear that the same functiOnalitY would be obtained for a range of angles between the longitudinal axis 301 of the rigid riser 101 and the longitudinal axis of the elongate arm 406.

The clamping element 405 rotates about pivot 404 to move from the configuration shown in FIG. 5 to that in FIG. 6. This ensures that the concave clamping face of the clamping element 405 can attach flush to a flexible riser 300 that extends substantially parallel to the rigid riser 101.

When the clamp 400 is secured in the configuration shown in FIG. 6, it provides a rigid clamping device extending from the rigid riser ioi already in the desired orientation for use. The flexible riser 300 can be secured to the clamping element 405 without the need for an extra alignment step for alignment of the clamp.

The method of deploying the rigid riser comprises the steps of attaching one or more of the above described clamping devices, in stowed position A, to the rigid riser 101 prior to its deployment into the sea. The clamping device, or devices, are attached to the rigid riser 101 in their desired angular position about the longitudinal axis of the riser.

The rigid riser 101, with the clamp 400, or clamps, still attached in stowed position A, is then passed through the roller box 200 into the sea.

Once the rigid riser 101 has been deployed, each clamp is moved to deployed position B. The clamping element 405 is then rotated such that its clamping surface will be flush to a vertical flexible riser 300, or whatever orientation the flexible riser 300 will adopt. The pivoting meanS 403 and 404 of each clamp are then fixed1 or locked in position such that each clamp is secure Each clamp 400 is then ready for a flexible riser 300 to be inserted into its clamping element 405. Each clamping element will then clamp to the flexible riser and secure it in position.

Accordingly, described above is a folding clamp 400 for securing a flexible riser 300 to a rigid riser 101, and method for deploying the rigid riser 101 with one or more of the clamps already attached in their desired orientation for use.

This avoids the need to realign the clamps 201 once the rigid riser 101 has io been deployed into the sea.

The folding clamp 400 will now be described in more detail.

In FIG. 4, the clamp is in stowed position A. The clamp has an attachment means 402 at the lower end for permanent releasable attachment to the rigid riser 101. Clamp arm 406 extends substantially parallel to the rigid riser 101 from the permanent attachment means 402 to the clamping element 405. The clamp 400 comprises pivoting means 403, arranged so that the clamp arm 406 can move from position A, as shown in FIG. 4, to position B in FIG. 5. The overall length of the clamp will normally be 2-3 meters.

The clamp shown in FIG. 4 has temporary attachment means 401 for attaching the clamping element to the rigid riser so as to secure the clamp in stowed position A. A strap 401 is shown as the temporary attachment means. The strap 401 may have a burr-type attachment mechanism, for example VelcrOTM, providing a temporary attachment at one end to the clamping element 405, the other end of the strap being permanently attached to the clamping element. However, the invention is not limited to this feature and alternative ways of securing the clamp in stowed position A are possible. For example, the clamping element at the top of the clamp may comprise contractible surfaces that are operated to contract and grip the rigid riser. Alternatively, the pivoting means 403 shown at the bottom of the clamp, may be locked to prevent the clamping arm 406 from moving. Other mechanisms are possible for securing the clamp to remain in stowed position A during the deployment of the rigid riser 1 01.

The rigid riser 101 is deployed with the clamp, or clamps, aligned as required for use and stowed in position A. Once the above-described stage of deploying a rigid riser 101 with appropriately aligned clamps 400 has been completed, a subsequent stage of deploying the clamps into position B is performed. This may occur immediately after the above-described stage has been completed or, alternatively at a later and more convenient time.

The subsequent operation is to move each clamp into deployed position B, as shown in FIG. 5. Accordingly, once a rigid riser 101 has been deployed, one or more ROVs are utilized to operate on each clamp 400.

For each clamp, an ROV will release the clamping element 405 from the temporary attachment means 401 or other mechanism for securing the clamp in stowed position A. The detachment operation may comprise, for example, the ROV pulling the clamping element 405 from a VelcroTM attachment, cutting a temporary strap, operating a mechanism for releasing the grip of the clamping element to the rigid riser and/or releasing a locking mechanism on the pivoting means 403 at the base of the clamp. Other mechanisms for releasing the clamp from its stowed position A are possible.

Once released from position A, the clamp arm rotates about the lower pivot 403 into position B. The clamp arm may fall into position B under the weight of gravity once released, or an ROV may be used to move it into position B. As shown in FIG. 5, a concave clamping surface of the clamping element 405 that had been attached flush to the rigid riser is now aligned along an axis perpendicular to the longitudinal axis 301 of the rigid riser 101. The clamping element then rotates about the pivoting means 404 to the position shown in FIG. 6.

The clamping element may be arranged to fall into the position shown in FIG. 6 under gravity, or an ROV may operate the pivoting means 404 of the clamping element to rotate it into the desired orientation.

Once the clamp is in the configuration shown in FIG. 6, the pivoting means 403 and 404 need to be secured to prevent clamping arm 405 and clamping element 405 from moving freely.

Accordingly, pivoting means 403 comprises a locking, or fixing, mechanism that is, for example, operated by an ROV to secure the clamping arm 406 in position. Alternatively, a latch, or other mechanism may be used to automatically lock the arm 406 into position once it has moved into position B. Similarly, pivoting means 404 comprises a locking, or fixing, mechanism that is, for example, operated by an ROV to secure the clamping element 405 in position. Alternatively, a latch, or other mechanism may be used to automatically lock the arm into position once it has moved into the position shown in FIG. 6.

After the clamping arm 406 and clamping element 405 have been secured, the clamp provides a rigid structure into which a flexible riser can be secured.

The process of reconfiguring a clamp from the stowed position shown in FIG. 4 to the deployed position providing a secure clamp as shown in FIG. 6, is repeated for afl of the clamps that are to be used.

Once the above-described stages to provide rigid and appropriately aligned clamps have been completed, a flexible riser 300 can be secured into the clamping elements 405. This may occur immediately after the above-described stages have been completed or, alternatively at a later and more convenient time.

ROVs position the flexible riser 300 into each clamping element 405, and operate to secure the flexible riser into each clamping element. This latter operation may involve the application of a strap 601 across the clamp, as shown in FIG. 6, or the contraction of the clamping element 405 to grip the flexible riser.

Alternative methods of securing the rigid riser 101 into the clamping element 405 are also possible. * 15

The attachment means for attaching the flexible riser 300 to the rigid riser 101 may be the same as that previously used to attach the clamping element 405 to the rigid riser 101. AlternativelY, a different attachment means may be used.

The above describes the deployment of a rigid riser 101, with appropriately aligned folding clamps 400 pre-attached, and the subsequent operations required for securing a flexible riser 300 into the folding clamps 400.

A further stage, required for the later connection of the flexible riser to a surface vessel, is to release the flexible riser 300 from the clamps 400.

To this end, an ROV operates to release the flexible riser 300 from the io clamping means 405 securing the flexible riser 300 into each clamping element 405. Any of the possible detachment mechanisms discussed earlier for releasing the clamping element 405 from the rigid riser 101 could be similarly applied.

Before the flexible riser 300 is released from the clamps 400 by an ROy, a surface vessel may first connect to the free end of flexible riser 300 with A&R wire.

This prevents striking of the flexible riser 300 against the rigid riser when the flexible riser 300 is released.

An ROV then moves the flexible riser 300 out of each clamping element 405 so that the flexible riser hangs freely from the URA. Once this has been performed for each of the used clamping elements 405, the free end of the flexible riser 300 can be retrieved for use by a support vessel.

The embodiments of the invention described above could typically save 14 hours or longer of vessel time as compared with the previously known method of aligning rigid clamps 201 after the deployment of the rigid riser 101.

In a further embodiment, a plurality of the folding clamps 400 are attached to the rigid riser 101 in a plurality of different orientations.

The orientation of the clamps may be at equally-spaced angular positions about the longitudinal axis of the riser. For example, if clamps in four different angular orientations were attached, then the clamps would have the relative * 16 orientations of 00) 900, 1800 and 270°. For each orientation, a series of clamps 400 would be provided, vertically spaced along the rigid riser 101.

In the above example, it is known that the relative orientation between a flexible riser hanging from the URA and a series of clamps will never be more than 45°. If it is known that it is acceptable to bend the flexible riser to accommodate for a clamp misalignment of 45°, then it is known that it will always be possible to attach the flexible riser 300 into clamps 400 regardless of its orientation.

Consequently, when deploying the rigid riser 101, there is no need for the orientation of the flexible riser 300 hanging from the URA to be known. So long as there are a sufficient number of clamp orientations around the rigid riser 101 to provide for the maximum clamp misalignment that the flexible riser 300 can tolerate, it will always be possible to clamp the flexible riser 300 to the rigid riser 101.

Of course, it is not necessary for all of the clamps 400 to be used. The unused clamps would typically be left attached to the rigid riser 101 in stowed position A. The flexibility provided by not needing to know the intended flexible riser orientation before first deploying the rigid riser is highly advantageous.

Various modifications of the previously described embodiments are possible.

For example, a design of clamping element 405 differing from that shown in FIGs. 4 to 6 may be used.

In one modification, the clamping element 405 would be able to attach to both the rigid riser 101 and the flexible riser 300 without needing to rotate from the configuration shown in FIG. 5 to that shown in FIG. 6. This could be achieved by, for example, providing the clamping element with two mutually perpendicular clamping surfaces. Such a clamping element would no longer require the pivoting means 404. * 17

Other modifications are possible. What is important is that each clamp should be capable of adopting either of two configurations: a first configuration in which it presents a low profile, and will readily pass through a roller box in any angular orientation, and a second configuration in which it can hold a flexible riser at a suitable position along a rigid riser and spaced therefrom. 0 18 çjmi

Claims

1. A clamping device (400) for stowing a flexible riser (300) along a rigid riser (101), comprising: an elongate arm (406) having attaching means (402) at one end for attachment to a rigid riser (101); a clamping element (405) at the other end of the elongate arm (406) for clamping to a flexible riser (300); and pivot means (403) adjacent said one end; wherein: when the attaching means (402) is attached on a rigid riser (101), said pivot means (403) enables the arm (406) to move the clamping device (400) between a stowed position in which the longitudinal axis of the arm (406) extends substantially parallel to the longitudinal axis (301) of the rigid riser (101) and a deployed position in which the arm (406) extends laterally away from the longitudinal axis (301) of the rigid riser (101).

2. The clamping device (400) of Claim i, wherein: the clamping device (400) comprises a concave clamping face for mounting flush to the surface of the rigid riser when the clamping device (400) is in the stowed position; and second pivot means (404) are provided to permit the said concave face to rotate such that, when the clamping device (400) is in the deployed position1 the clamping face can become flush to a surface of a flexible riser (300) when clamped by said clamping element (405).

3. The clamping device (400) according to any preceding claim, wherein: the clamping element (405) further comprises second attachment means (401) for attaching the clamping element (405) to the rigid riser (101) to hold the clamping device (400) in the stowed position when the attaching means (402) is attached on the rigid riser (101).

4. The clamping device (400) according to Claim 3, wherein 0 19 the said second attachment means is a strap (401).

5. The clamping device (400) according to any preceding claim, wherein the clamping element (405) is operable to grip the rigid riser (101) so as to hold the clamping device (400) in the stowed position.

6. The clamping device according to any preceding claim, wherein the clamping element (405) further comprises third attachment means (601) for attaching the clamping element (405) to a flexible riser (300).

7. The clamping device according to Claim 6, wherein the third attachment means (601) is a strap.

8. The clamping device according to any preceding claim, wherein the clamping element (405) is operable to grip the flexible riser so as to hold the flexible riser in the clamping element (405).

9. A riser assembly, comprising: a rigid riser section (101) and a clamping device (400); the clamping device (400) compriSing an elongate arm (406) having attaching means (402) attached at one end to the rigid riser section (101) and a clamping element (405) at the other end for clamping to a flexible riser portion (300); and the arm (406) includes pivot means (403) adjacent said one end; such that, the arm (406) is movable to pivot about the pivot means (403) from a stowed position with its longitudinal axis substantiallY parallel to the longitudinal axis (301) of the rigid riser (101) to a deployed position extending laterally from the longitudinal axis (301) of the rigid riser (101), so that when the arm (406) is in said deployed position, the clamping device is arranged for holding the flexible riser portion (300) and rigid riser section (101) apart from each other.

10. The riser assembly of Claim 9, wherein: * 20 the clamping device (400) comprises a concave clamping face for mounting flush to the surface of the rigid riser (101) in the stowed position; and second pivot means (404) are provided to permit said concave face to rotate such that, when the clamping device (400) is in the deployed position said clamping face is flush to the surface of the flexible riser, when clamped by the clamping element (405).

11. The riser assembly according to Claim 9 or 10, wherein: the clamping element (405) further comprises second attachment means (401) for attaching the clamping element (405) to the rigid riser (101) to hold the clamping device (400) in the stowed position.

12. The riser assembly according to Claim 11, wherein the second attachment means is a strap (401).

13. The riser assembly according to any of Claims 9 to 12, wherein: when the clamping device (400) is in the stowed position, the clamping element (405) grips the rigid riser (101) so as to hold the clamping device (400) in the stowed position.

14. The riser assembly according to any of Claims 9 to 13, wherein: the clamping element (405) further comprises third attachment means (601) for attaching the clamping element (405) to a flexible riser when a flexible riser (300) is positioned in the clamping element (405) and the clamping device (400) is in the deployed position.

15. The riser assembly according to Claim 14, wherein the third attachment means (601) is a strap.

16. The riser assembly according to any of Claims 9 to 15 wherein: when a flexible riser (300) is clamped by said clamping element (405) and the clamping device (400) is in the deployed position, the clamping element (405) is S 21 operated to grip the flexible riser (300) so as to hold the flexible riser (300) into the clamping element (405).

17. The riser assembly according to any one of claims 9 to 16, further comprising a flexible riser (300) clamped by said clamping element (405).

18. A method of deploying a rigid riser (101) underwater comprising the steps of: attaching an end of a clamping device (400) to the rigid riser (101), wherein the clamping device (400) has a elongate arm (406) that is moveable between a stowed position, in which the longitudinal axis of the arm (406) is substantially parallel to the longitudinal axis (301) of the rigid riser (101), and a deployed position in which the arm (406) extends laterally away from the longitudinal axis (301) of the rigid riser (101); configuring the clamping device (400) such that it is in the stowed position; and lowering the rigid riser (101) with the attached clamping device (400) through a roller box (200) into a submerged location.

19. A method according to Claim 18, wherein a plurality of said clamping devices (400) are attached to the rigid riser (101) prior to the deployment of the rigid riser (101) into the water, the plurality of clamping devices alt being attached in the stowed position.

20. A method according to Claim 18 or 19, wherein: at least two of the said clamping devices are attached to the rigid riser with different relative orientations such that when the said clamping devices are in their deployed positions they protrude laterally from the rigid riser in different directions.

21. The method according to Claim 20, wherein, when the clamping devices (400) are in their deployed positions, a plurality of the clamping devices are orientated to protrude from the rigid riser at various angular positions. O 22

22. A method according to any of Claims 18 to 21 wherein the clamping device is the clamping device of any of Claims I to 8.

23. A method of stowing a flexible riser (300) alongside a rigid riser (101), the rigid riser having been deployed in accordance with any of Claims 18-22, wherein, for each of the one or more clamping devices (400), the method comprises the steps of: reconfiguring the clamping device (400) on the rigid riser (101) into the deployed position; and securing the flexible riser (300) to the clamping element (405) at the other end of the clamping device (400) such that the flexible riser (300) and the rigid riser (101) are held spaced from each other by the clamping device (400).

24. The method of Claim 23 wherein, at least one ROV is used to reconfigure the clamping device (400) to the deployed position.

25. The method of Claims 23 or 24 wherein, at least one ROV is used to position the flexible riser (300) into the clamping element (405).

26. A method according to any of Claims 22 to 25, wherein: an ROV is used to detach a clamping element (405) on the end of the clamping device (400) from the rigid riser (101) to release the clamping device (400) from the stowed position.

27. A method according to any of Claims 22 to 26, wherein: an ROV operates to rotate the clamping element such that a clamping face of the clamping element becomes flush to the flexible riser (300) when the flexible riser (300) is attached to the clamping element (405).

28. A clamping device substantially as hereinbefore described with reference to FIGs. 4-6 of the accompanying drawings. O 23

29. A riser assembly substantiallY as hereinbefOre described with reference to FIGs. 4-6 of the accompanying drawings.

30. A method of deploying a rigid riser under water substantially as hereinbefore described with reference to FiGs. 4-6 of the accompanying drawings.

31. A method of stowing a flexible riser alongside a rigid riser substantially as hereinbefore described with reference to FIGs. 4-6 of the accompanying drawings.