Patent Description:
A poster published in <NPL> provides a comprehensive overview of the mooring technologies known in the art. That poster may be viewed online at http://www. offshore-mag. com/content/dam/offshore/print-articles/Volume%<NUM>/<NUM>/MOORING-010814REV.

Floating surface units or installations used in oil and gas production are moored to the seabed to remain in substantially the same location for many years of production. An example of such an installation is an FPSO (Floating Production, Storage and Offloading) vessel.

Some taut-leg floating production units are known but these require reinforced anchorages to be installed at the outset, which is generally achieved by injecting concrete or grout. The present invention is concerned with the more common catenary mooring lines that have generally catenary curvature when viewed in profile.

Typically, an FPSO will be held in place by a mooring system comprising catenary-shaped mooring lines arranged in groups to define semi-taut legs. When viewed from above, the mooring lines splay apart from neighbouring mooring lines in each group to define a passive spread pattern.

Usually there is one group of mooring lines at each quadrant of an FPSO, hence a total of four groups, radiating outwardly at <NUM>° intervals when viewed from above. If each of the four groups contains three mooring lines, for example, this is known in the art as a 4x3 pattern. Some surface installations, such as turret-moored vessels, are moored with three groups of mooring lines, radiating outwardly at <NUM>° intervals when viewed from above.

<FIG> show one of the mooring lines <NUM> of a conventional mooring system for an FPSO <NUM> floating at the surface <NUM>, as an example of a moored surface installation. The mooring line <NUM> hangs with catenary curvature under tension between the FPSO <NUM> and the seabed <NUM>. Whilst curved, the mooring line <NUM> lies substantially in a vertical plane.

In this example, the mooring line <NUM> is anchored by a suction anchor or suction pile <NUM>. Most of the length of the pile <NUM> is embedded in the soil of the seabed <NUM> to ensure sufficient resistance to tension in the mooring line <NUM> when the FPSO <NUM> moves back.

It will be apparent from <FIG> that the mooring line <NUM> is attached to the pile <NUM> at a level below the mid-point of the length of the pile <NUM>, typically between half and two-thirds of the way down from the top of the pile <NUM>. A similar arrangement is also shown, for example, in <CIT> to a predecessor of the Applicant. This arrangement optimises the balance of forces that act on the pile <NUM>: connecting the mooring line <NUM> at or nearer to the top would tend to cause rotation of the pile <NUM>, which would require a bigger pile to withstand a given tension in the mooring line <NUM>.

It follows that a short bottom part of the mooring line <NUM>, typically <NUM> to <NUM> long, is buried in the soil of the seabed <NUM> beside the pile <NUM>. That buried bottom part is sometimes called a 'forerunner'.

A drag anchor, also embedded in the seabed <NUM>, may be used instead of a pile <NUM> as a mooring foundation for a surface installation such as an FPSO <NUM>. Again, a short bottom part of the mooring line <NUM> will similarly be buried in the soil of the seabed <NUM> if a drag anchor is used.

In more detail, the mooring line <NUM> comprises, in sequence from bottom to top: a bottom or ground chain <NUM> attached to the pile <NUM>; a section of spiral strand wire (SSW) <NUM> attached to the ground chain <NUM>; and a top chain <NUM> that joins the SSW section <NUM> to the FPSO <NUM>.

The SSW section <NUM> will usually be of coated steel but could be of a synthetic plastics material instead: references to 'wire' in this specification are not intended to limit the meaning only to metallic wires.

The SSW section <NUM> constitutes most of the length of the mooring line <NUM> because, for a given tensile strength, wire is lighter, more compact to store and less expensive than chain. Chains <NUM>, <NUM> are used instead of wire at the bottom and top of the mooring line <NUM> to avoid damage to the wire at those vulnerable locations. As a non-limiting example, the ground chain <NUM> and the top chain <NUM> may each be about <NUM> long whereas the SSW section <NUM> may be well over <NUM> long.

Various known connectors <NUM> join the successive components of the mooring line <NUM>. Different types of connector <NUM>, such as chain connectors and shackles, will typically be used at different locations along the mooring line <NUM>.

It will be apparent from the detail view of <FIG> that the ground chain <NUM> is in two sections, namely a lower section <NUM> and an upper section <NUM>. Thus, in sequence from the bottom to the top of the mooring line <NUM>: the lower section <NUM> of the ground chain <NUM> is attached to a buried side wall of the pile <NUM>; the upper section <NUM> of the ground chain <NUM> is connected to the lower section <NUM> by a first connector <NUM>; the SSW section <NUM> is connected to the upper section <NUM> of the ground chain <NUM> by a second connector <NUM>; and the top chain <NUM> is connected to the SSW section <NUM> by a third connector <NUM>.

The lower section <NUM> of the ground chain <NUM> is attached to the pile <NUM> before the pile <NUM> is overboarded from a surface installation vessel and lowered to penetrate the seabed <NUM>. Thus, the lower section <NUM> of the ground chain <NUM> and the first connector <NUM> are buried under the seabed <NUM> as part of the bottom 'forerunner' part of the mooring line <NUM>.

The upper section <NUM> of the ground chain <NUM> extends from the buried first connector <NUM> beneath the seabed <NUM> to the second connector <NUM> above the seabed <NUM>, where it joins the SSW section <NUM>. Thus, a short transitional portion of the upper section <NUM> of the ground chain <NUM> lies on or close to the seabed <NUM>.

A drawback of the partially-buried ground chain <NUM> arises from inevitable movement of the ground chain <NUM>, in use, relative to the surrounding soil of the seabed <NUM>. For example, all parts of the mooring line <NUM>, including the ground chain <NUM>, will move in response to motion of the FPSO <NUM> under wind and wave action. Similarly, all parts of the mooring line <NUM> will be moved by other seawater dynamics acting on the mooring system, such as ocean currents, especially in deeper water.

The resulting movements of the ground chain <NUM> also move the adjacent soil of the seabed <NUM>. Over time, this may create a trench without soil around and above the previously-buried part of the ground chain <NUM> beside the pile, hence potentially reducing the capacity of the pile <NUM>. Similarly, a drag anchor could slip and lose its intended position. The result is that, after several years, the actual capacity of the foundation may be significantly lower than was originally intended.

In other cases, the tension applied by a mooring line to a foundation may be greater than was originally intended. For example, the floating surface installation may be enlarged or a mooring line handling system may be upgraded.

For these reasons, it may be necessary to upgrade the capacity of a subsea foundation from time to time. Upgrading capacity may involve recovering original capacity that has been lost by a foundation over time. Alternatively, or additionally, upgrading capacity may involve improving the original capacity of a foundation.

Conventionally, upgrading the capacity of a subsea foundation involves installing a new, more efficient foundation in the seabed near the old foundation. Next, part of the mooring line is disconnected from the old foundation to reconnect it to the new foundation. The old foundation is then redundant.

Disconnecting a mooring line from an existing foundation, or even slackening a mooring line to reduce tension, disadvantageously increases the load on other parts of the mooring system. It is a lengthy and costly operation that introduces a risk of the surface installation losing its position, especially if sea conditions deteriorate during the operation.

<CIT>, to a predecessor of the Applicant, discloses a frame that is used for connecting mooring line elements on the seabed. In a reverse operation, the frame can also be used to separate mooring line elements. The frame comprises pulling means to connect mooring line elements. However, the mooring line elements cannot have residual tension in them. Thus, the frame of <CIT> cannot be used on a live tensioned line; a line from the floating surface installation must first be slackened.

<CIT> discloses an alternative approach involving extra weights that sharply increase mooring resistance in the event of extreme drift. However, the possible size of such weights is limited on a permanent mooring and in any event would not provide a sufficient upgrade in tension-resisting capacity. Also, the weights apply a permanent shear stress to the mooring lines.

<CIT> relates to a twin-anchor mooring arrangement for a floating vessel, in which a first drag anchor is attached at an end of a mooring line and a second drag anchor is slidable along the mooring line to a second anchoring position.

<CIT> relates to a pile anchor system for an offshore structure. The system includes an original pile attached to a mooring line, and a supplementary pile installed in the vicinity of the original pile. The supplementary pile is attached to the original pile by means of a coupling member, but is not attached to the mooring line itself.

<CIT> describes a system for creating a deep water mooring spread by successively installing independent suction piles from a floating vessel.

<CIT> discloses a system to adjust the length of the mooring lines and uses a device that can be anchored at a second subsea location.

Against this background, the invention provides a method of upgrading the capacity of a mooring system, which system comprises a pre-existing subsea foundation at a first seabed location and a mooring line, which may be catenary-shaped, connected to the pre-existing subsea foundation and extending to a moored floating unit. The method comprises: installing a supplementary subsea foundation at a second seabed location, which may be spaced across the seabed from the first seabed location; and connecting the mooring line to the supplementary subsea foundation while maintaining the connection between the mooring line and the pre-existing subsea foundation.

A portion of the mooring line may be displaced at the second seabed location to make space for installing at least part of the supplementary subsea foundation. The displaced portion of the mooring line can then be moved into connection with at least part of the supplementary subsea foundation.

For example, the displaced portion of the mooring line may be lifted away from the seabed. In that case, at least part of the supplementary subsea foundation may be moved to a position beneath the displaced portion of the mooring line, for example by effecting translational movement of that part across the seabed or by lifting that part across the seabed. Before that, at least part of the supplementary subsea foundation may be lowered to a seabed location beside the mooring line. In another approach, the displaced portion of the mooring line may be diverted around the second seabed location and laid on the seabed.

Preferably, a frame of the supplementary subsea foundation is placed on the seabed; one or more foundation elements are embedded in the seabed and coupled to the frame; and the mooring line is connected to the frame. For example, a foundation element may be coupled to the frame by installing that foundation element into or through a guide structure provided in or attached to the frame. Another approach is to place a link between the frame and the foundation element, where the foundation element is embedded in the seabed and spaced apart from the frame.

Advantageously, the mooring line is placed between first and second laterally-spaced foundation elements.

A frame of the supplementary subsea foundation may be coupled with a foundation element of the supplementary subsea foundation pre-installed at the second seabed location. For example, the frame may be lowered onto the foundation element, such as onto the top of a pile.

If the supplementary subsea foundation is a pile, the mooring line may be connected to the pile via a connection mechanism integrated with the pile.

After installation of the supplementary subsea foundation is complete, the floating unit or surface installation may continue to be moored using the existing subsea foundation and the supplementary subsea foundation in combination to resist tension in the mooring line without disconnecting the line from either foundation.

Preferably, a chain section of the mooring line is connected to the supplementary subsea foundation by clamping the chain section or by mechanically engaging links of the chain section. More generally, the mooring line is advantageously connected to the supplementary subsea foundation by embracing the mooring line.

The inventive concept extends to a related mooring system that comprises: a first subsea foundation at a first seabed location; a mooring line, which may be catenary-shaped, connected to the first subsea foundation and extending to a moored floating unit; and a supplementary subsea foundation at a second seabed location, connected to the mooring line by a remotely-operable mechanical connector that defines an upwardly-opening receptacle to receive the mooring line.

The mechanical connector preferably embraces the mooring line and may connect the supplementary subsea foundation to a chain section of the mooring line by clamping the chain section or by engaging links of the chain section.

The supplementary subsea foundation suitably comprises: a frame supporting the connector; and one or more foundation elements embedded in the seabed and coupled to the frame, for example via a guide structure provided in or attached to the frame or via a link extending between the frame and the or each foundation element, which foundation element is embedded in the seabed and spaced apart from the frame.

An interface may be provided on top of at least one foundation element and/or beneath the frame for coupling the frame to the foundation element. In another approach, the supplementary subsea foundation is a pile and the connector is integrated with the pile.

A supplementary subsea foundation for use in the mooring system of the invention comprises: a frame; a remotely-operable mechanical connector that defines an upwardly-opening receptacle to receive the mooring line and is supported by the frame, which connector is arranged to connect with a mooring line; and at least one foundation element attachable to or integrated with the frame, arranged to be embedded in seabed soil.

The invention therefore provides a back-up or auxiliary mooring system that reinforces an existing mooring system without having to disconnect the mooring line. The invention embodies the principle of installing a subsea structure on the seabed near the existing mooring line, which is then lifted and connected to the structure. The subsea structure may be integrated with or attached directly to an anchoring foundation, such as a suction pile, or may be installed separately and anchored by driven piles to form a hybrid foundation anchor.

In summary, the invention provides a method and apparatus for upgrading the capacity of a catenary mooring line under tension, without disconnecting the line from its existing subsea foundation. This saves time and cost and reduces the risk of the anchored vessel drifting. The existing foundation remains connected to the mooring line during the upgrading operation and so remains available to restrain motion of the vessel.

The use of a supporting frame rather than a simple mechanical connection advantageously limits lateral motion of the part of the mooring line that is embedded in the seabed soil. The reduced lateral motion of the buried line portion reduces the trenching effect and so delays any additional loss of resistance to tension in the line.

The apparatus of the invention is designed to be installed and actuated in deep water from the surface, using known and readily-available means such as cranes and ROVs.

Embodiments of the invention provides a method to upgrade the resistance capacity of a mooring system. The method comprises: vertically lifting the mooring line near an initial foundation; inserting a supporting frame below a lifted section of the mooring line; releasing the vertical lift on the mooring line and mechanically connecting the frame to the mooring line; and coupling at least one additional foundation to the frame.

Embodiments of the invention also provide a device to upgrade the resistance capacity of a mooring system. The device comprises: a supporting frame inserted between the mooring line and the seabed; a remotely-actuated mechanical connector for connecting the mooring line to the supporting frame; and at least one additional foundation coupled to the supporting frame.

The additional foundation may comprise one or more piles, such as suction piles, gravity piles, pin piles or driven piles. Other forms of additional foundation are possible, preferably embedded foundations such as drag anchors.

At least one pair of additional foundations may be used, one foundation on each side of the mooring line. Thus, one foundation of each pair is preferably on a respective side of the mooring line.

Additional foundations such as piles may be inserted in slots, openings or guide tubes of, in or attached to the supporting frame. In another approach, additional foundations may be coupled to the frame by mooring line elements such as chains or wires, for example spiral strand wire.

Where an additional foundation is a suction pile, the top of the suction pile may serve as the supporting frame. The mooring line is suitably lifted aside during installation of the frame/pile. Mechanical connection between the mooring line and the frame/pile may be effected via a recess in the frame/pile.

The supporting frame may be arranged to lie on the seabed, for example taking the form of a generally flat mat or mudmat. Such a mudmat may have a peripheral skirt to engage with the seabed soil or may have sled formations designed to slide over the seabed, at least in an installation direction intersecting the general plane of the mooring line.

The additional foundation can be installed in or on the seabed before or after the frame. However it is preferred to install the additional foundation after the frame because it may be more challenging to position and couple the frame to a pre-installed foundation.

Mechanical connection between the frame and the mooring line may, for example, be effected by a chain stopper or by a clamp.

To illustrate the prior art background, reference has already been made to <FIG> of the accompanying drawings, in which:.

In order that the invention may be more readily understood, reference will now be made, by way of example, to the remainder of the drawings in which:.

Referring firstly to <FIG>, this largely corresponds to <FIG> and therefore like numerals are used for like parts. Here, a supplementary foundation <NUM> that supports the mooring line <NUM> in accordance with the invention is shown interposed between the FPSO <NUM> and the suction pile <NUM>. The supplementary foundation <NUM> engages the ground chain <NUM> of the mooring line, where the ground chain <NUM> crosses or lies on the seabed <NUM>.

Thus, the pile <NUM> and the supplementary foundation <NUM> cooperate with each other to resist tension in the mooring line <NUM>. In other words, the supplementary foundation <NUM> adds to the capacity of the pile <NUM> to upgrade the overall capacity of the mooring system. Importantly, this upgrade is achieved without disconnecting the mooring line <NUM> from the pile <NUM>. Movement of the FPSO <NUM> is restrained continuously by the mooring line <NUM> throughout.

Advantageously, the lowermost portion of the mooring line <NUM> between the pile <NUM> and the supplementary foundation <NUM>, including the portion that is buried in the soil of the seabed <NUM>, is restrained against movement by the supplementary foundation <NUM>. The supplementary foundation <NUM> also isolates that portion of the mooring line <NUM> from movement of the remainder of the mooring line <NUM>. Thus, the capacity of the pile <NUM> will no longer degrade significantly due to trenching effects. The upper portion of the mooring line <NUM> between the supplementary foundation <NUM> and the FPSO <NUM> can, of course, continue to move but its amplitude of lateral motion is restricted. In any event, movement of the upper portion of the mooring line <NUM> takes place largely above the seabed <NUM>, where it cannot give rise to a trenching effect.

<FIG> illustrate one way in which the supplementary foundation <NUM> of <FIG> may be installed in accordance with the invention.

<FIG> shows a portion of the ground chain <NUM> of the mooring line <NUM> lying on the seabed <NUM>. Part of the catenary curvature of the mooring line <NUM> between the FPSO <NUM> and the seabed <NUM> is evident on the right side of <FIG>, where the ground chain <NUM> rises above the seabed <NUM>. The pile <NUM> to which the ground chain <NUM> is attached is not shown but will lie to the left of the portion of the ground chain <NUM> shown in <FIG>, at the end of a further portion of the ground chain <NUM> buried in the seabed <NUM>.

<FIG> shows a mid-point of the ground chain <NUM> lifted from and held above the seabed <NUM> by a wire <NUM> that hangs from a surface support vessel (not shown). The wire <NUM> suitably hangs from a winch on the vessel, such as an abandonment and recovery (A&R) winch, but could instead hang from a crane on the vessel.

In <FIG>, the surface support vessel has also lowered a frame <NUM> onto the seabed <NUM> in a holding location beside the raised portion of the ground chain <NUM>. The frame <NUM> carries a connection mechanism <NUM> that is arranged to grip or engage the ground chain <NUM>. The connection mechanism <NUM> may, for example, employ the principle of a clamp or a chain stopper as disclosed in <CIT>. In this example, the frame <NUM> also carries one or more guide tubes <NUM> to receive pin piles, as will be described more fully with reference to <FIG> of the drawings.

The frame <NUM> is suitably lowered from a crane on the surface support vessel but could instead hang from a winch on the vessel. An ROV <NUM> is shown in attendance to monitor the operation and to disconnect slings <NUM> that support the frame <NUM>.

In <FIG>, the ground chain <NUM> is still held above the seabed <NUM> by the wire <NUM> but the frame <NUM> has now been moved across the seabed <NUM> to lie under the raised portion of the ground chain <NUM>. The direction of movement of the frame <NUM> is transverse to, preferably substantially orthogonal to, the general plane of the mooring line <NUM> so as to intersect that plane.

This translational, generally horizontal movement of the frame <NUM> could be achieved by using the crane on the surface support vessel to lift the frame <NUM> from the holding location on the seabed <NUM>. In that case, the ROV <NUM> shown in <FIG> can, if needs be, disconnect and reconnect slings <NUM> to avoid the slings <NUM> clashing with the raised portion of the ground chain <NUM>. Another approach is to slide the frame <NUM> across the seabed <NUM> like a sled into its final position under the ground chain <NUM>.

The enlarged view of <FIG> shows the ground chain <NUM> now lowered into, and engaged with, with the connection mechanism <NUM> on the frame <NUM>. The connection mechanism <NUM> defines an upwardly-opening receptacle for this purpose. The connection mechanism <NUM> embraces and receives the ground chain <NUM>.

Finally, as shown in <FIG>, pin piles <NUM> are inserted into the guide tubes <NUM> on the frame <NUM> to secure the frame <NUM> to the seabed <NUM>. This completes the supplementary foundation <NUM> as also shown in <FIG>.

The connection mechanism <NUM> is operable remotely to connect the ground chain <NUM> to the supplementary foundation <NUM>. The connection mechanism <NUM> clamps the ground chain <NUM> or engages links of the ground chain <NUM> in the manner of a chain stopper.

The FPSO <NUM> remains moored thereafter using the pile foundation <NUM> and the supplementary foundation <NUM> in combination to resist tension in the mooring line <NUM> without disconnecting the line <NUM> from either foundation <NUM>, <NUM>. The line <NUM> remains connected to both foundations <NUM>, <NUM> thereafter and may remain so connected at least until the whole mooring system is eventually decommissioned. This may therefore be for longer than a moored FPSO <NUM> or other surface installation remains on station, which could be for more than twenty years.

Turning next to <FIG>, this shows a supplementary foundation <NUM> in a second embodiment of the invention, being a variant of the first embodiment shown in <FIG>. In this variant, a frame <NUM> supports a connection mechanism <NUM> as before but the guide tubes <NUM> are omitted. Instead, the frame <NUM> is anchored by remote foundation elements that are exemplified here as piles <NUM> but could take other forms such as drag anchors. The frame <NUM> is connected to the piles <NUM> by respective tensile links <NUM>, which could be of chain or wire.

Preferably, as shown in <FIG>, the ground chain <NUM> is placed between laterally-spaced foundation elements such as the piles <NUM>. This balances forces on the system while leaving space for installing the piles <NUM> without having to divert the ground chain <NUM> laterally as shown in the next embodiments.

<FIG> show a supplementary foundation <NUM> in a third embodiment of the invention, in which a connection mechanism <NUM> is integrated with a foundation member. The foundation member is exemplified here as a suction pile <NUM>. In effect, the top of the pile <NUM> is equivalent to the frame <NUM> of the preceding embodiments.

<FIG> shows the ground chain <NUM> pulled laterally across the seabed <NUM> to allow space for installation of the pile <NUM> along the normal route of the ground chain <NUM>. Once the pile <NUM> has been installed in the seabed <NUM>, the ground chain <NUM> is lifted into engagement with the connection mechanism <NUM> as shown in <FIG>.

Finally, <FIG> shows a supplementary foundation <NUM> in a fourth embodiment of the invention, in which a frame <NUM> that supports a connection mechanism <NUM> can be mounted to a pre-installed pile <NUM> via an interface structure <NUM>. Like <FIG>, the ground chain <NUM> extending across the seabed <NUM> is shown here diverted to allow space for installation of the pile <NUM> along the normal route of the ground chain <NUM>.

As in the first embodiment, the frame <NUM> is suitably lowered from a crane on a surface support vessel but could instead hang from a winch on the vessel. An ROV like that shown in <FIG> can monitor the operation and then disconnect slings <NUM> that support the frame <NUM>.

Once the frame <NUM> has been mounted onto the pile <NUM> via the interface structure <NUM>, the ground chain <NUM> is lifted into engagement with the connection mechanism <NUM> in a similar way to the arrangement shown in <FIG>.

Claim 1:
A method of upgrading the capacity of a mooring system that comprises a pre-existing subsea foundation (<NUM>) at a first seabed location and a mooring line (<NUM>) connected to the pre-existing subsea foundation (<NUM>) and extending to a moored floating unit (<NUM>), the method comprising:
installing a supplementary subsea foundation (<NUM>; <NUM>; <NUM>; <NUM>) at a second seabed location; and
connecting the mooring line (<NUM>) to the supplementary subsea foundation (<NUM>; <NUM>; <NUM>; <NUM>) while maintaining the connection between the mooring line (<NUM>) and the pre-existing subsea foundation (<NUM>).