Mooring system for a floating offshore installation

The present invention relates to a mooring system, for a floating offshore installation, such as an offshore platform of the type used in oil and gas well operations. In particular, the invention relates to a system especially adapted for quick release of the mooring lines.

Background

During operations such as in connection with oil or gas wells at the sea floor, it is known to moor a floating offshore installation, such as a floating platform or vessel from which the operations are conducted. This is done by arranging a plurality of mooring lines that extend from sea bed anchors to the installation.

When operating in cold environments, such as in oceans far north, operation can be challenged by ice ridging and icebergs. Several measures are known to overcome such problems. For instance, it is known to have icebreaking ships that relieve the contact forces between the ice and the installation.

However, when icebergs are large, it is more beneficial to move the installation away from the iceberg's path, rather than trying to alter its direction or dividing it into smaller parts. This however, results in halting the operations performed with the installation and is thus undesirable. Consequently one wants to wait moving off the moored location until the last moment, when it is clear that such a movement cannot be avoided. This motivates for solutions that reduce the time needed for disconnecting the mooring lines from the installation. Moreover, once the mooring lines are disconnected and the iceberg no longer represents any hazard, one wants to re-connect the mooring lines as quickly as possible to resume operations.

US patent publication 3,902,447 describes a mooring system for a submersible drilling platform, including a mooring ring connected to a plurality of mooring lines that extend between the sea floor and the mooring ring. The buoyancy of the mooring ring can be controlled, to lower or elevate the ring in the water. The mooring ring can be placed on

the sea floor when lowered, and be connected to a drilling platform when raised. In this way, the plurality of mooring lines is jointly connected to the platform, when connecting the ring to the platform.

Such a solution offers swift and easy disconnection and reconnection of the mooring of the platform. However, having a mooring ring below the platform, will restrain the operation possibilities below the platform, making it unnecessary complicated to perform such operations from the platform.

US patent publication 4,604,961 discloses a vessel mooring system, partly similar to the solution described in the above-mentioned US 3,902,447. The system comprises a recess in a vessel, adapted for receiving a buoyant mooring element. The mooring element is connected to mooring lines, extending between the sea floor and the mooring element. When the element is detached from the vessel, it will sink to a predetermined depth, assuming an equilibrium state between the sea floor and the water surface. From this position it can later be hoisted up into the recess, for mooring of the vessel. With this solution, the vessel is moored at one point and is allowed to revolve about this point.

European patent application publication EP 0 831 022 Al describes a mooring system for a floating construction with a quick release feature. The publication mentions the idea of groups of anchor lines, wherein each group has an independently operable disconnect means. Any explanation of how to accomplish such a solution is not described, however. On the other hand, the publication describes various embodiments in which each anchor line is provided with one disconnect means.

The invention

The present invention solves the task of providing a mooring system for mooring of a floating offshore installation, such as an offshore platform, in waters with a possible presence of icebergs, which system is adapted for quick release of mooring lines from the installation.

According to the invention, a mooring system for a floating offshore installation is provided, which system comprises a plurality of mooring lines extending between the

sea floor and the installation, wherein the system further comprises a plurality of cluster elements, which elements are adapted to each receive a plurality of mooring lines, and remotely operable cluster element locking mechanisms for releasable attachment of the cluster elements to the installation. The cluster element locking mechanisms are adapted to be controlled individually or simultaneously, as desired by the operator. With such a configuration, a quick disconnection of a plurality of mooring lines in predetermined groups can be performed by disconnection of the much fewer cluster elements from the installation.

The fact that the cluster element locking mechanisms are remotely operable will provide the operator with better control of the timing of the disconnection. Furthermore, the cluster element locking mechanisms being adapted to be controlled individually or simultaneously, as desired by the operator, gives great flexibility when an iceberg approaches, making him able to hang on to only the fewest necessary mooring line clusters before the total disconnection finally takes place.

In a preferred embodiment of the invention, the cluster elements have buoyancy which will position the elements at a pre-selected distance above the sea floor when disconnected from the installation. The weight of the mooring line will pull the cluster elements downwards, and the buoyancy will keep a predeterminable part of the mooring lines above the sea floor, when disconnected from the installation.

The above-mentioned cluster elements preferably comprise mooring line openings for receiving mooring lines. Thus, a cluster element can advantageously receive one mooring line in each mooring line opening.

The cluster elements are preferably provided with a plurality of releasable mooring line locking mechanisms for releasably locking said individual mooring lines to the cluster elements, the mooring line locking mechanisms being separately operable.

The cluster elements are advantageously provided with line guides, such as through holes, for guiding pull-in wires or messenger lines extending between winches on the installation and said mooring lines. This gives maintenance and mooring flexibility, as will be explained further below.

Preferably, the mooring lines are provided with pull-in heads, preferably comprising a sheave, for use with said pull-in wires or messenger lines in controlled release or pulling of said pull-in heads, said pull-in heads being adapted for releasably locking by said mooring line locking mechanisms.

In an especially preferred embodiment, in a moored condition adapted for quick release, cluster elements are attached to the installation by means of said cluster element locking mechanisms; - mooring lines are locked to said cluster elements with mooring line locking mechanisms; messenger lines are connected to the mooring lines in the position of the cluster elements; and the messenger lines are adapted to rupture at disengagement of the cluster elements by releasing said cluster element locking mechanisms.

With this setup, the operator will need a minimum of time to disconnect the installation from the mooring.

The mooring system preferably comprises receiving means in connection with the installation hull for receiving the cluster elements, preferably being in form of recesses in the installation hull, adapted to receive a cluster element. Thus, each cluster element can preferably be connected directly to the hull in specially adapted locations.

Furthermore, the cluster element, when in an attached position, is advantageously held in place by a hinging element on one side of the receiving means, preferably on the lower side, and at least one cluster element locking mechanism on an opposite side of the receiving means. Also, in a particularly preferred embodiment of this feature, when attached to the receiving means, a greater part of the force transmitted from a cluster element to the installation is adsorbed by the hinging element than by the cluster element locking mechanism due to the positioning of the interface between the mooring lines and the cluster element. To obtain this, the imagined vector lines representing forces of the mooring lines onto the cluster element must have a closer distance to the lunging element than to the cluster element locking mechanism. This configuration reduces the necessary dimensions of the locking mechanism.

In a preferred layout of the mooring system according to the invention, each cluster element comprises a plurality of mooring lines, such as in the range of about 4 to 10 lines, connected to the cluster element in a mutual horizontal distance and angle between each other, preferably in an angle in the range of about 4 to 12 degrees, even more preferable in the order of about 6 degrees.

In addition, the clusters of mooring lines connected to respective cluster elements extend partly horizontally out from the installation, preferably within a respective designated sector part. Thereby available space is provided in a plurality of adjacent sector parts, such as for risers. Moreover said cluster elements are advantageously connected to the installation with a mutual distance.

In a preferred embodiment, the cluster elements exhibit buoyancy and the mooring lines are provided with one or more buoy(s) at a distance from both ends of respective mooring line. As will be described in better detail below, this configuration has various advantages, such as facilitating the re-mooring the installation after a release.

Connecting means in the form of a flexible chain, wire or the like is preferably arranged between the cluster elements, encircling the installation hull and keeping the cluster elements in a predetermined configuration when released from the installation. Furthermore, connecting means pockets are advantageously attached to the spar hull between each cluster element, for storing the connection means when not in use, the pockets are preferably integrated compartments in the installation hull.

Preferably, in a moored condition one or more of the cluster elements may be controllably paid out by releasing said cluster element locking mechanisms and lowering the cluster by one or more pull-in wires from its attached position, such as for maintenance.

The mooring system preferably has at least one guiding channel ending at the receiving means, for guidance of a messenger line or a pull-in line extending between a mooring line and a mooring winch on the installation. Preferably, when in the moored position,

the mooring lines run through mooring line openings and are connected to the messenger line.

Additional features and advantages of various embodiments of the present invention will appear from the following description.

Example of embodiment

In the following, a detailed example of an embodiment of the present invention will be given with reference to the drawings, in which;

Figure 1 is an overview of an offshore platform floating in water, moored with the mooring system according to the invention;

Figure 2 is an overview of the mooring system seen from above, showing mooring lines and flexible risers expanding from the platform to the sea bottom; Figure 3 shows a cluster element attached to the platform hull, into which element a mooring line is terminated and locked with a mooring line locking mechanism;

Figure 4 shows the cluster element in figure 3, being disconnected from the platform hull;

Figure 5 shows the configuration of the mooring lines when disconnected from the platform, as well as a position of the risers;

Figure 6 shows a detail of the mooring lines of figure 5, whereby the cluster element floats above the sea floor due to its buoyancy;

Figures 7a and 7b show a cluster element with attached mooring lines being pulled into and locked to the platform hull, respectively; Figure 8a shows an embodiment wherein four cluster elements are interconnected by means of a chain, in connected position and a disconnected intermediate and a disconnected lower equilibrium position, respectively;

Figure 8b shows an alternative embodiment of the feature in figure 8a seen from above in a disconnected position; Figure 8c shows the embodiment in figure 8b in a connected position and a detail of a pocket on the spar for the interconnecting chain; and

Figure 9 shows a single mooring line being pulled into or paid out of a cluster element remaining attached to the hull.

Figure 1 shows a spar 1 floating in water. It is moored with a plurality of mooring lines 3, extending from sea bed anchors (not shown) to the spar hull 1. From the lower side of the spar 1 are flexible risers 5 arranged expanding between the sea floor 7 and the spar 1. Attached to the mooring lines 3, buoyancy elements 9 elevate a part of the mooring lines 3 and reduce the vertical forces on the spar 1 resulting from the weight of the mooring lines 3.

Figure 2 illustrates a preferred arrangement of the mooring lines 3 in relation to the risers 5. In this figure the spar 1 is seen from above. It is moored with 20 mooring lines 3, arranged in four clusters of five lines 3. The mooring line clusters are arranged in sectors displaced approximately 90 degrees in relation to the two adjacent clusters about the vertical centre axis of the spar 1. The risers are attached to a disconnectable riser buoy which is connected to the spar underneath the hull 1. This configuration avoids contact or conflicts between the risers 5 and the mooring lines 3.

In figure 2, four cluster elements 11 are indicated at the circumference of the spar hull 1. Each cluster of five mooring lines 3 ends in one cluster element 11.

The mooring lines are preferably attached to each cluster element in a horizontal spreading arrangement, side by side, wherein the horizontal distance and angle between each mooring line may be any suitable distance and angle respectively. The mooring lines are preferably individually hinged to the cluster element allowing rotation about a horizontal axis adapting to the tilting of the spar in high seas. The arrangement is preferably adapted to the number of mooring lines in each cluster and their spreading on the sea floor, such as for example of an angle between each mooring line, such as in the range of about 4 to 12 degrees, preferably in the order of about 6 degrees. The design of the cluster and/or connection means between the cluster and each mooring line may accommodate such spacing and angling, such as by angled openings or angled connecting means or even connecting means hinges for rotation about a vertical axis.

The cluster element 11 is illustrated in more detail in figure 3, showing a vertical cross section through a part of the spar hull 1 and a cluster element 11. The cluster element 11 is attached to a receiving means in the form of a recess 13 in the spar hull 1. In the lower part of the recess 13 there is arranged a hinging element 15. In the upper part, a

cluster element locking mechanism is arranged in the form of a locking pawl 17. When the cluster element 11 is positioned in the recess 13, a groove 19 faces the locking pawl 17 which locks the cluster element 11 in the recess 13 by engaging the groove 19.

The locking pawl 17 is remotely controlled by the operator, thereby enabling the operator to disconnect the entire cluster element 11 with the five mooring lines 3 in a quick and easy fashion. Preferably, the cluster element 11 has a number of grooves and locking pawls corresponding to the number of mooring lines attached to the cluster, here five grooves 19 are engaged by five locking pawls 17, in order to level and adjust to the forces and stress on the mentioned parts.

The cluster elements 11 can be located essentially anywhere on the floater, e.g. on the outside of the hull and/or high up on the hull. This feature gives great flexibility compared to previously known systems. A high position of the cluster elements is for example advantageous fore ease of maintenance. Preferably, at least two cluster elements 11 , preferably three, and more preferably four cluster elements 11 are attached to the platform hull 1. The number of cluster elements 11 can also be five or more. Furthermore, as shown in figure 2, the cluster elements 11 are located on different circumferential sites on the platform 1 , yielding rotational stability to the platform 1.

The cluster elements 11 may preferably be integrated in the recesses 13, such that no parts of the cluster elements protrude outside of the spar hull structure, but rather follow the smooth shape of the hull. This is advantageous in preventing exposure to damage on the hull such as from drift ice, smaller icebergs, ships, anchor lines etc.

Differently to the solutions proposed in US 4,604,961 and EP 0 831 022, wherein a vessel is moored by the intermediate of a turret, about which the moored vessel may swivel, the present mooring system also prevents the spar from rotating in the horizontal plane, thus exposing the moorings to additional forces such as torsion. It is therefore of outmost importance that each mooring line is connected in the correct sequence and angle in relation to the other moorings, which arrangement is secured by the cluster element. In addition, the present invention presents a mooring system which is independent from the turret comprising the risers. The latter provides the advantage of

shutting down production and disconnecting the risers, while awaiting the independent disconnection of the floater from its moorings.

Furthermore, the cluster element 11 comprises a mooring line opening 21 for receiving a mooring line 3. Thus, in this embodiment each cluster element 11 has five mooring line openings 21, one for each mooring line 3. The end of the mooring line 3 being inserted into such a mooring line opening 21 is provided with a pull-in head 23 and a sheave 25. By means of a mooring line locking mechanism in the form of a clamp and groove assembly 27, the pull-in head 23 at the end of the mooring line 3 is releasably locked in the mooring line opening 21.

The individual locking of each mooring line 3 to the cluster element allows inspection, maintenance and replacement of single mooring lines, without the need to release the cluster element 11.

The mooring line openings 21 are preferably much closer to the hinging element 15 than to the cluster element locking mechanism, or locking pawl 17. This result in that most of the force exerted onto the spar hull 1 from the cluster element 11 is transferred through the hinging element 15. This feature provides that less force is exerted on the locking pawl 17, thereby reducing its necessary dimensions and risk of overload.

Through a channel 29 in the hull, a messenger line 31 extends between the sheave 25 and a mooring winch (not shown) on the spar 1. The sheave 25 is attached to the pull-in head 23. The messenger line 31 runs down the channel 29, turns on the sheave 25 and runs back up the channel 29 to the mooring winch. The messenger line 31 is used for pulling a stronger pull-in wire 34 which will be used for pulling a mooring line 3 into the mooring line opening 21, and pulling the cluster element 11 into the recess 13. This will be further described below, with reference to figures 7a, 7b, and figure 9.

Figure 4 illustrates the disconnection of a cluster element 11 from the spar hull I ₅ by moving the locking pawl 17 out of the groove 19. The forces from the mooring lines 3 will pull the cluster element 11 out of the recess 13, whereby the cluster element 11 « makes a rotation-like movement about the hinging element 15.

Preferably, the messenger line 31 is dimensioned to break due to the force exerted on it by the cluster element 11 through the sheave 25 when the cluster element 11 is being disconnected. The messenger line 31 can advantageously exhibit a week link where it is intended to break.

When the cluster element 11 is disconnected from the spar hull 1, the weight of the mooring lines 3 will pull it down towards the sea floor 7, as shown in figure 5. However, since the cluster element 11 exhibits buoyancy, it will carry a part of the mooring lines 3, and thus stops at an equilibrium state, for instance 5 meters above the sea floor 7. This is illustrated in figure 6. In figure 5, one sees how the buoyancy element 9 elevates a large part of the mooring line 3. The buoyancy element 9 should be arranged at an appropriately distance below the sea surface, in order to avoid contacting an iceberg. Such a distance can for instance be about 120 meters below the sea surface.

The double buoyancy arrangement of a high positioned buoyancy element 9 and a low positioned cluster element 11 provides the advantage of ease of locating and picking up the mooring line through the buoyancy element 9 while preventing damage to the cluster element 11. Preferably, the buoyancy of the cluster element provides self leveling of the cluster element, preventing the cluster element from rotating in the vertical plane and twisting the mooring lines about each other, and thus securing correct horizontal positioning for its reconnection.

When reconnecting the mooring lines 3 to the spar 1, pull-in wires 34 are drawn through the channels 29, around the sheaves 25, and back up through the channels 29. This operation will preferably be assisted by an anchor handling vessel (not shown). This is illustrated in figures 7a and 7b. In this way, by pulling the pull-in wires 34, the cluster element 11 is pulled back into the recess 13. The locking pawls 17 lock the cluster element 11 into place. However, to do this, the pull-in wires 34 must be dimensioned correspondingly. When the cluster element 11 has been locked into the recess 13, the pull-in wires 34 are again replaced with weaker messenger lines 31 that are adapted to break if the mooring shall be quick-released from the spar 1.

For a controlled pay-out or dismantling of the mooring system, one or more of the cluster elements can be lowered or released from the hull 1 in a controlled manner by

paying out the pull-in wires 34 in the opposite direction. Such a controlled release will be advantageous in case of an on-site maintenance or inspection of the cluster elements, or in case of a more thorough upgrading of the mooring system, or even for towing the floater to shore for maintenance or modification.

The cluster element 11 can be handled by an anchor handling vessel (not shown) and be connected to and pulled back to the spar 1 for re-connection.

In an especially preferred embodiment of the present invention, the cluster elements 11 are interconnected with chains 33 that encircle the spar hull 1 when the mooring is disconnected. That is, each cluster element 11 is connected to its two adjacent cluster elements 1 1. This solution is shown in figure 8a in three stages, from a disconnected state, via an intermediate state and to a connected state. This makes the process of reconnecting the cluster elements 1 1 to the spar hull 1 easier and faster, as the spatial encircling arrangement of the clusters is maintained and the position of each cluster element is kept closer to the future position when connected to the spar.

Figure 8b shows in detail an alternative embodiment of the interconnecting chains or wires 33 in a released configuration of the four clusters 11 viewed from above. The weight of the mooring lines 3 may pull the clusters away form the spar 1 in the centre of the encircling configuration obtained, before the spar 1 is towed or has drifted away from its initial moored position.

Figure 8c shows an optional feature of a pocket or compartment 40, attached to the spar buoy for the storage of the interconnecting chain 33. Figure 8c shows the configuration of the pockets 40 positioned around the spar from above when the clusters 11 are in a connected configuration on the spar 1. The figure also illustrates, in an enlargement, the actual pocket 40 wherein in the excess length of the interconnecting chain 33 (in dotted line) is contained inside the pocket 40, (the relative proportions are not correct and for illustration only). The pockets 40 are positioned in between each cluster unit 11 on the spar hull, for colleting and storing the interconnecting chain or wire 33 connecting the clusters to each other. The pockets 40 are preferably integrated compartments in the spar hull and follow its smooth surface in order to prevent exposure to damage. The pockets 40 shield the interconnecting chains 33 and the spar hull from wear or damage

such as by abrasion between the hull and chains in high sea and drifting ice, and also protects the chain or wire 33 itself from tangling into drift ice, smaller icebergs, ships, anchor lines etc. The interconnecting chains 33 may for example be hoisted by winch on board the spar and into the pockets 40 after the use. The proportions of the elements in figure 8b and 8c are not comparable and are for illustration only.

Figure 9 illustrates the possibility of disconnecting an individual mooring line 3 by a controlled lowering through the cluster element 11 by means of the pull-in wire 34 handled by the mooring winch (not shown) on the mooring deck. The clamp and groove assembly 27 is then in an unlocked state, whereby the pull-in head 23 will run free in the mooring line opening 21. The pull-in wire 34 then runs through the channel 29 and the mooring line opening 21, and is attached to the sheave 25 at the end of each mooring line 3. The controlled disconnection of each separate mooring line 3 will be required for inspection, maintenance and repair. The pull-in wires 34 for each mooring line may be the same wires as used for pulling in the cluster, or the separate independent pull-in lines may be used for the cluster, as the number of pull-in lines for the cluster may be fewer than the number of mooring lines connected thereto. Preferably, only a selection of the pull-in wires for the mooring lines are also used as pull-in wires for the cluster.

The total disconnection time for disconnecting the spar 1 from the risers 5 and mooring lines 3 can be expected to be approximately between 30 and 90 minutes. The disconnection time for the mooring lines 3 alone can be expected to be from one to five minutes, and advantageously about two minutes. This is thus a short time in comparison to the total disconnection process. The disconnection of the mooring lines is the last disconnection to be performed before the platform floats freely.

This separate disconnection of risers and mooring allows for delaying the mooring disconnection to a time where e.g. collision with an iceberg is considered unavoidable or highly likely. This will reduce the number of disconnection events as disconnection at a lower risk of collision is avoided. With longer disconnection time, one needs to start the disconnection when an iceberg is in the vicinity of the floating installation. With a quick release system however, the operator can hang on to the mooring until a collision is much more likely.

As mentioned above, the cluster element locking mechanisms 17 are preferably adapted to be released individually or simultaneously, as desired by the operator. Releasing only one cluster element 11, will result in that the weight of the oppositely arranged mooring lines 3 will pull the platform 1 in their direction, thus moving it away from the originally moored position. By releasing two opposite arranged cluster elements 11, this is avoided. However, when releasing two oppositely arranged cluster elements 11, forces exerted on the platform 1 from currents and wind may off course affect the position of the platform more significantly.

A sequenced disconnection is also envisaged wherein the operator may release two or three cluster elements (out of a total of for example four or five cluster elements) in order to let the spar move out the icebergs path, but without letting the platfoπn float freely, thus limiting the distance for repositioning and reconnection of the spar to the remaining moorings and the risers.

The improved ease and speed of disconnection and reconnection described above, will of course contribute to an increase in production and improved safety.

The described mooring system according to the invention is also applicable to a plurality of other types of floating offshore installations, for instance a semi submersible or a monohull (ship shaped) floater.