WO2000050301A1

WO2000050301A1 - Marine drag anchor

Info

Publication number: WO2000050301A1
Application number: PCT/BR2000/000015
Authority: WO
Inventors: Déa Aparecida LOPES MASETTI; Adolfo Tsuyoshi Komura; Gustavo Adolfo Villela De Castro; Carlos Alberto Nunes Dias; Edison GONÇALVES; Philip Von Pritzelwitz
Original assignee: Rio Offshore Ltda
Priority date: 1999-02-25
Filing date: 2000-02-23
Publication date: 2000-08-31
Also published as: BR9903032A; AU2652200A

Abstract

One marine drag anchor fitted with pivotal devices (5) pivoting over the front pad eyes (2) of the anchor, the said pivotal devices (5), when pivoted as a result of a change in the direction of the pulling force actuating on the anchor, enable the change in the attack angle of the anchor, setting it in the service position.

Description

MARINE DRAG ANCHOR

Field of the Invention

The present invention refers to marine anchors intended for general application. In a more particular way, this invention refers to marine anchors embedded into seabed through a dragging operation. These anchors can withstand any type of load, from horizontal loads to vertical ones and are intended for anchoring ocean rigs and vessels . Background of the Invention

Drag anchors are a widely known technology. Basically, those anchors are embedded into the seabed through a dragging operation usually performed by a towing vessel. The drag anchor, connected to a wire rope, is lowered from the surface to the seabed at a previously selected site according to the anchoring plan.

The dragging operation consists basically of the towing of the anchor by the towing vessel in a preselected direction until the anchor reaches the designated embedment depth.

The fundamental requirement of a drag anchor is its capacity to embed into seabed when dragged, and, essentially to keep stable when subjected to pulling forces, i.e., to stay substantially on the same position when subjected to pulling forces.

Usually the drag anchors comprise one fluke fitted with a front end or embedment end and a shank which is fastened in one end to a selected part of the fluke and in the other end has means to be connected to an anchoring line .

In general, a large variety of drag anchors is employed, together with anchoring lines, to fix a vessel or a rig to the seabed. The anchoring lines connect the vessel or the rig to the anchors, and each set formed by the anchor and its respective anchoring line will operate under straining forces in order to keep the vessel or the rig on a substantially fixed position.

Anchoring lines can consist of wire ropes, chains or synthetic ropes. Those lines can operate single or multiple. This specification, merely, for reasons of simplification, will not deal with anchoring lines, as those are well known to designers and operators and also because they are not part of the scope of the invention object of the present application. Therefore, all over this specification, only the generic expression "anchoring line" will be used when referring to any of the above mentioned combinations.

Usually anchoring lines are deployed to extend from the vessel or rig until the seabed in a way that prevents significant movements of the vessel or rig in any direction. To do so, it is necessary to establish on each case, the number of anchoring lines to be installed and their arrangement around the vessel or rig, to prevent such movements .

Once decided the number of lines and their positions, the embedment operations of the drag anchors shall take place, according to usual installation procedures for this type of anchor .

After completion of the embedment operations one shall proceed to carry out the linking operation of each anchor to its respective anchoring line and, afterwards, the tightening operation of each set anchor-line, until the designed position is attained. Then, the vessel or rig will be on its operation position.

Usually, the anchors are embedded into seabed at a significant distance from the site where the vessel or rig is. Depending on the water depth on the site, such a distance can reach various hundreds of meters, resulting in a large mooring radius for the vessel or rig. This particular feature can cause some trouble when it is required to position a certain number of vessels or rigs at a site of limited dimensions.

The above circumstance occurs, for example, in the exploitation of offshore oil fields, where vessels, rigs and the like must be compulsorily, positioned very close to each other. There are cases, when potential interference among mooring lines belonging to different vessels are very likely to take place thus rendering the simultaneous operation of such vessels unfeasible with this type of anchoring system.

To cope with such a hurdle drag anchors able to withstand substantially vertical loads have been developed. The great advantage presented by those vertical load drag anchors is to provide a smaller anchorage radius for the vessel or rig. This made possible due to the high capacity of those anchors to withstand substantially vertical pulling forces. To achieve this performance it is essential that the vertical load drag anchor has a high embedment capacity during embedment operations, in such a way that the soil portion immediately above the anchor, after embedment, be sufficient to ensure the pull out capacity of the anchor, when the anchoring line is actuated, providing as a consequence, a high holding capacity of the anchor-line set. Therefore, the holding capacity of the vertical loads drag anchor is directly dependent on the depth of embedment of the anchor. The present state of the art on this subject comprises several vertical load drag anchors, which intend to comply with this requirement. One handicap presented by those anchors is related to their requiring a relatively high dragging force to be installed, implying the need of more tug boats to carry out this operation, with increasing costs . One of the existing types of drag anchor makes use of two front wire ropes, connected in one end to the front eye plates, and two rear wire ropes, connected in one end to the rear eye plates. The other ends of those ropes are put together and linked to a connecting piece, consequently forming a kind of bridle. This connecting piece is linked to the towing line used for embedment, in this case this is the anchoring line itself.

Drag anchors to be embedment require that their ropes be shorter than the rear ones, so that an attack angle is formed between the towing rope and the fluke. This angle is one of the features that define the embedment capacity of the anchor into seabed.

After the anchor is embedment into seabed it is necessary that the attack angle be modified to make the actuating forces vertical to the fluke. Shear pins can be used to achieve this modification as they collapse under a previously calculated force resulting in 4 (four) ropes with identical length. After this the direction of the force actuating on the anchor will be substantially vertical and the anchor is ready to operate.

The choice of the shear pins is dependent on the type of soil and the designed embedment depth. This is a relatively empirical process, demanding a great deal of data on the characteristics of the seabed and, consequently, it is rather costly. Besides, in case the shear pins collapse before the anchor reaches the designed embedment depth, it will be necessary to retrieve the anchor and to re-set it for a new embedment operation, which contributes to increase the cost of the operation. Another means of modifying the attack angle is the use of two installation ropes, being one intended exclusively for the embedment and the other to the anchoring system. Both ropes are connected to the anchor bridle. This bridle comprises a specially designed device, which enables the change in the angle. However, this alternative has the inconvenience of always requiring two ropes and, at least, two towing vessels, for the installation of the anchor, thus contributing to the cost increase.

These and other problems are solved or circumvented by the object of the present invention, as can be realized by reading the detailed description to follow. Summary of the Invention

This invention represents one type of vertical load drag anchor, which permits a more effective embedment. One advantage of the invention is the use of only one mooring line for both the embedment and the changing of angle of attack operations. Also, the modification of that angle is activated only at will.

There are two pad eyes in the rear, to which the lower ends of the rear ropes are connected. There are also two twin pad eyes in the frnt, on which two pivotal devices turn. Each pivotal device is fitted with a slot in which slides the lower end of one front rope. The front ropes are shorter than the rear ones and the upper ends of those ropes are linked to a connecting plate, forming a bridle for the anchor. To the top end of this connecting plate a mooring cable is connected.

When the anchor is in position for embedment, the lower ends of the front ropes are located at the closer ends of the slots. When the anchor is to be placed in the operational position, there will be an inversion of the direction of pull, resulting in a shifting of the lower ends of the front ropes towards the farther ends of the slots. This will cause the pivotal devices to turn over the front pad eyes . The length of the slot added to the length of the front rope is substantially the same of the rear rope .

The front pad eyes can be, optionally, open in their rear ends, enabling the retrieval of the anchor by means of a pulling action of the rear ropes. Brief Description of the Drawings

The invention will now be described in more detail together with the attached drawings, where:

Figure 1 is a side view of the first materialization of the anchor, according to the invention, ready to start its embedment operation;

Figure 2 is a top view of the anchor already shown in Fig.l; O 00/50301 _g PCT/BRO0/O0O15

Figure 3 is a side view of the anchor in Fig. 1 after modification of the attack angle;

Figure 4 is a side view of a second materialization of the anchor, according to the present invention, ready to start its embedment operation;

Figure 5 is a side view of the anchor in Fig. 4 being retrieved by means of its rear ropes. Detailed Description of the Invention

Figure 1 is a side view of an anchor 20 according to the invention. One can see one fluke 1 fitted at its backside with a retrieving pad eye 4, to which one of the ends of a retrieving line 10 is connected. One can see one rear pad eye 3, to which the lower end of a rear rope 7 is connected, and one front pad eye 2 to which a pivotal device 5 is connected.

The pivotal device 5 is fitted with a slot 11, which is provided with one end 11A situated next to the pivoting pin between the pivotal device 5 and the front pad eye 2, and one end 11B situated at the opposite end of the slot 11.

The lower end of one front rope 6 is given a free sliding movement through the slot 11 in the pivotal device 5. In the Figure 1 the lower end of the front rope 6 is situated at the end 11A. As one can see on Figure 2, the anchor 20 is provided with two rear pad eyes 3, two rear ropes 7, two front pad eyes 2, fitted with respective pivotal devices 5, and two front ropes 6, all symmetrically disposed in relation to symmetry axis Z-Z of the fluke 1.

The upper ends of the two rear ropes 7 and of the two front ropes 6 are connected to a connecting plate 8, thus forming the anchor bridle, and to the upper corner of this connecting plate a mooring line 9 is connected. As can be observed in Figures 1 and 2, the front ropes 6 are shorter than the rear ropes 7, and as a consequence the anchor 20 is shown in Figures 1 and 2 in its embedment mode, i.e., its attack angle is such that pull-our forces frm the mooring line 9 will cause the embedment of the anchor into sea bed.

The straining of the mooring line 9 is effected by means of an auxiliary vessel (not shown on the Figures), which runs over a preselected course towing the anchor 20 towards its front end, until it is embedment to the designed depth.

After the designed depth is reached the attack angle shall be made to be modified so that the pullout force on the anchor is substantially vertical. To achieve this the auxiliary boat is to reverse its course so that the mooring line pulls the front ropes towards the rear end of the fluke. This movement, however, is limited and just to make the lower ends of the front ropes 6 slide over the whole length of the slots 11 on the pivotal devices 5, shifting from the ends 11A towards ends 11B. As the pivotal devices 5 turn over front pad eyes 2, they can adjust thenselves in reaction to the pulling out forces from the front ropes 6 originated in the movement of the mooring line 9 towards the back portion of the anchor 20. In the end of this movement the pivotal devices 5 will be in the position shown on Figure 3. The span of the slots 11 between ends 11A and 11B is such that when added to the length of the front ropes these will be substantially equal to the rear ropes 7 and, as a consequence, the anchor 20 wiil be in the proper position to withstand vertical loads.

The anchor 20 to be retrieved will require only a pullout action by means of the retrieving line 10. This operation is quite familiar to operators reason why it will not be assessed here for the sake of simplification of the text.

It must be observed that the anchor 20 can be embedment with the aid of only one dragging line 9, which enables this line to be also the mooring line itself, connecting the anchor 20 to the floating unit. Figure 4 shows an alternative materialization of the anchor conceived by this invention, identified by the number 30. This anchor 30 is identical to the anchor 20 but for the front pad eyes and for the non-existence of the retrieval pad eye 4. In this configuration there are open type front pad eyes 12 instead of the front pad eyes 2 of the previously described anchor 20. Such pad eyes 12 are open in their rear portion in order to make the retrival of the anchor possible with thge use of the mooring line itself, as it will be assessed hereinafter.

The anchor 30 is embedment in the same way of the anchor 20, and so is the change of the attack angle. The big difference between anchor 30 and anchor 20 is how it is retrieved after use: it is only required that an auxiliary vessel pulls the mooring line towards the rear of the anchor until the hinge pins fastening teh pivotal devices 5 to the open type front pad eyes 12 slide off them, thus releasing the pivotal devices and the front ropes 6, as shown in Figure 5. Now the anchor 30 will be bound only to the rear ropes 7 and the continuing of the pulling action in the mooring line will cause the easy retrieval of the anchor 30 frm the seabed. As can be grasped by the description herein, the anchor object of this invention is very simple, and the operation for its embedment does not present difficulties and is less costly than those for the embedment of anchors requiring two installation lines. The anchor object of this invention is likely to be improved and modified, without disrupting teh essence of the concept and scope of the invention, which is limited to the scope of attached claims .

Claims

1. Marine drag anchor comprising one fluke (1), one retrieval pad eye (4) rigidly fastened to the back part of the said anchor (1), one retrieval line (10) connected to the said retrieval pad eyes (4), one pair of rear pad eyes (3) rigidly fastened to the said fluke (1), one pair of front pad eyes (2) rigidly fastened to the said fluke (1), one pair of rear ropes (7), which lower ends are connected to the said rear pad eyes (3), one pair fo front ropes (6), which lower ends are linlked to the said rear ropes (7) and front ropes (6) linked to a connecting plate (8) forming one anchor bridle; one actuation line is linked to the upper corner of the said connecting plate (8); said anchor characterized by the following additional features: one pair of pivotal devices (5) being connected to the said front pad eyes (2); the said pivotal devices (5) being each provided with a slot (11) with one near end (11A) and one far end (11B); the said front ropes (6) being shorter than the said rear ropes (7), the lower ends of the said front ropes (6) being fitted to the slots (11), initially positioned on the near ends (11A) while the anchor is being embedment and moving towards the far ends (11B) when the anchor is to be set in the service positions; the said movement of the lower ends of the said front ropes causing the said pivotal devices (5) to turn over the said front pad eyes (2) so that the sum of the length of each front rope (6) to the length of the slot (11) is substantially identical to the length of each rear rope (7), when the anchor is in the service position.

2. Anchor according to Claim 1 characterized by the said actuation line being the mooring line itself.

3. Anchor according to Claim 1 or 2 characterized by the said front pad eyes being "open type" front pad eyes (12), which are provided with openings in their back portions, what enables the said pivotal devices to be removed from their hinges in the said open front pad eyes

(12) when the actuation line (9) is pulled towards the rear of the anchor, thus permitting the anchor to be retrieved by means of its rear ropes.