WO2011074986A1

WO2011074986A1 - Gravity anchor

Info

Publication number: WO2011074986A1
Application number: PCT/NO2010/000465
Authority: WO
Inventors: Jon Tore Lieng
Original assignee: Deep Sea Anchor
Priority date: 2009-12-15
Filing date: 2010-12-15
Publication date: 2011-06-23
Also published as: NO20093525A1; NO331713B1

Abstract

A gravity installed anchor (1) comprising an axially extending elongated main body (0) with - a nose portion (5) with a nose (51); - an intermediate stem portion (6); - a trailing portion (7); - a connection point (8) for an anchor line; - two or more tail fins (2, 19) extending laterally generally from said trailing portion (7); wherein said nose portion (5) having a larger cross-section area than a cross-section area of said trailing portion (7) of said main body (0).

Description

GRAVITY ANCHOR

Introduction The present invention relates to an anchor. More

specifically, the invention relates to a gravity-installed anchor comprising an axially extending elongated main body with a nose portion with a nose, an intermediate stem portion, and a trailing portion with two or more tail fins extending laterally generally from said trailing portion. Further, the present invention relates to improvements of the design and shape of such an anchor providing increased stability, lower manufacture costs, increased penetration depth relative to its weight and drop height above the sea-floor, and thus provides advantages during installation.

Background art and problems related thereto

EP 1 042 162 Bl (Lieng) describes embodiments of gravity- installed anchors having solid or ballasted stem portions.

While such solid stem anchors may have advantageous properties in use, they are very expensive in manufacturing, as they may not be produced from simple steel plates or pipes by simple cutting and joining processes but must be assembled from more complex shape components. Moreover, the long cylindrical external wall of the intermediate and nose portions of the stem constitutes a significant surface area, which is slowed down by friction from the sediments when penetrating the seabed. Thus an anchor according to EP 1 042 162 is relatively expensive to manufacture and does not penetrate as deep in relation to its weight as compared to the proposed

embodiments. Another gravity-installed anchor is presented in US7059263 to Zimmerman, which has a dart-shaped main body of plates with radially directed channel members with recesses, which are arranged to receive and hold plate-shaped fins both in the head and in the tail. The even thickness of the dart-shaped main body provides a long friction-subject surface both during free fall in water and when penetrating the sediments. The relatively large fins at the head portion also poses a risk of reduced directional stability when falling through the water and further reduces the penetration depth allowed, thus

requiring a high drop elevation above the sea-floor compared to the Lieng patent above.

US20050120936 Al relates to an anchor with an elongated shaft with a mooring line attachment that is rotatable around the full circumference of the anchor shaft. The lower end of the shaft has a nose section, while the upper end has a tail section. A plurality of fins are attached to the nose section and tail section, and extend radially outward from the anchor shaft. While the anchor penetrates deeper into the soil the influence of the mooring line load will cause less soil

resistance on the nose section than on the tail section, so that the anchor will rotate slightly about the attachment point of the mooring line.

Gravity-installed anchors of submerged weight of up to 75 tons are used for mooring a mobile drilling unit. Such kind of gravity-installed anchors have a generally straight

cylindrical main body, which provides a large friction surface both to the water and to the sediments during penetration downwards. The installation weight of the gravity-installed anchors is significant and it is desirable to reduce the weight of gravity-installed anchors without sacrificing their holding capacity. Further, in addition to reducing their weight, it is desirable to reduce their friction both relative to the water and also relative to the seabed when penetrating during installation. Brief summary of the invention

A solution to some of the problems related to the prior art is a gravity-installed anchor (1) comprising an axially extending elongated main body (0) with fins, said anchor comprising

- a nose portion (5) with a nose (51);

- an intermediate stem portion (6) ;

- a trailing portion (7);

- a connection point (8) for an anchor line;

The novel and distinctive features of the gravity-installed anchor according to the invention are that

- said fins formed as two or more tail fins (2, 19) extending laterally generally from said trailing portion (7);

- said nose portion (5) having a larger cross-section area than a cross-section area of said trailing portion (7) of said main body (0)

- said enlarged cross-section area of said nose portion (5) formed by one or more layers of steel (4b, 4c, 20b, 20c) , each steel layer (4b, 4c, 20b, 20c) extending from said nose (51) and successively shorter in the direction of the trailing portion (7) than an underlying layer of steel (4a, 4b, 20a, 20b) .

The anchor of the invention has several advantages. First, the cross-section of the nose portion being larger than that of the trailing portion provides increased directional

stability during the drop phase due to the increased weight and small area of the nose portion relative to the lower weight of the trailing portion with the fins. Second, the nose portion forms a temporary channel while penetrating through the sediments. The penetration channel formed by the larger nose is wider than the cross-section of the trailing portion, thus reducing wall friction on the trailing portion, which is surrounded by water or sediment slurry. With a reduction in total wall friction, and for a given initial kinetic energy, the anchor penetrates deeper into the seabed.

Third, the larger cross-section of the nose portion (5) may also create turbulent flow about the trailing portion (7) of the main body, thus slightly decreasing the water

resistance of the trailing portion during descent through the water. This will increase the velocity potential and kinetic energy upon impact with the seabed, for penetrating to a desired depth.

Thus, with the anchor of the present invention, several tons of mass may be saved while obtaining the same penetration depth and anchoring capacity potentials as the heavier anchors of the background art such as straight-bodied gravity install anchors . Short figure captions

The invention is illustrated in the attached drawings, of which : Fig. la is a side elevation view of an embodiment of the gravity-installed anchor of the invention, showing a gravity- installed anchor built up from plates forming a main body stem comprising a lower nose portion, an intermediate portion and a trailing portion with fins. Fig. lb is a side elevation view of a detail of a preferred embodiment comprising a widening of the forward edge of the fins.

Fig. lc is an isometric view of the embodiment from Fig. la as seen from behind and to the side of a central axis of the main body.

Fig. 2a is an elevation view of an embodiment of the invention of which the fins are provided with minor ribs along the surface.

Fig. 2b shows details of such minor ribs and part of the widened forward edge of the facing fin.

Fig. 2c shows details of a transition from a forward thicker cross-section area of the axially extending elongated main body (0) illustrated by a step in the middle part of the intermediate portion.

Fig. 3a illustrates two embodiments of the invention shown in one drawing: a weight body (11) arranged in the nose portion (5) and upward facing cylindrical sleeves (10) at the rear ends of the fins (2), which is shown enlarged in Fig. 3b.

Fig. 4a shows four weight portions (11a, b) arranged for filling in the corners of a plate cross-section of the nose portion of a plate embodiment of the invention. Fig. 4b shows those four weight portions (11a, b) arranged bolted in place in the corners of the plate cross-section of the nose portion (5) . Fig. 4c is a similar embodiment of the invention in which four weight portions (11c) are arranged in place in the corners in the nose portion, either welded, bolted, or glued in place.

Fig. 5a is a rear isometric view of the embodiment shown in Fig. 3a showing the upward facing cylindrical sleeves (10) at the rear ends of the fins (2) . Fig. 5b is an enlarged image of such a cylindrical sleeve (10).

Fig. 6 is a forward isometric view of an embodiment of the invention with a nose weight with nuts and bolts arranged in exploded view.

Fig. 7 is an illustration of an anchor gravity-installed into the seabed with marker buoys on upper ends of retrieval wires attached to the rear ends of the fins of the gravity- installed anchor. The marker buoys may be used for retrieving instrument pods from the anchor.

Fig. 8a illustrates in a side elevation view an embodiment of the invention wherein the axially extending elongated main body (0) comprises a series of coaxial layers of pipes or rings (20) . Fig. 8b is an isometric view of the same.

Fig. 9a shows a ship holding an anchor according to the invention over the sea floor by an anchor line attached to a hoisting wire. Fig. 9b illustrates the same anchor after having been dropped into the sediments and with the anchor line pulled laterally by a permanently or semi-permanently moored vessel or platform.

Fig. 10a and 10b show in side elevation and isometric views an embodiment of the anchor according to the invention with a nose portion of larger diameter compared to the

intermediate and trailing portion of the main body. Fig. 10c shows the connection point for an anchor line recessed from the rear end of the main stem with access through a lateral opening between two of the fins. Description of preferred embodiments of the invention

The invention is a gravity-installed anchor (1) which comprises an axially extending elongated main body (0) with a nose portion (5) with a nose (51), an intermediate stem portion (6), and a trailing portion (7) with two or more tail fins (2, 19) extending laterally generally from said trailing portion (7) . A connection point (8) for an anchor line is arranged at the main body. The gravity-installed anchor according to the invention has a nose portion (5) with a larger cross-section area than a cross-section area of the trailing portion (7) of the main body (0) . This feature may improve the penetration capability of the gravity-installed anchor compared to similar anchors with a stem of even width, which experience larger friction.

The device according to the invention is illustrated in various embodiments in Figs, la and lc, 2a, 3a, 5a, 6, all with generally cross-shaped cross-sections of the axially extending elongated main body made of flat metal plates. In a preferred embodiment the plates are made of steel due to the advantage of high density and low price and ease of

manufacture. A different device, also according to the invention, is shown in Figs. 8a and 8b, comprising an axially extending main body built up generally from concentric metal pipes. In a preferred embodiment the pipes are made of steel.

The fins (2) may in a preferred embodiment of the invention constitute a direct lateral extension of the plates forming the trailing portion (7). The anchor of the invention has several advantages. First, the larger cross-section of the nose portion (5) than the cross-section of the trailing portion (7) provides increased directional stability during the drop phase through the water because it lowers the centre of gravity of the whole anchor structure .

Second, the larger cross-section of the nose portion (5) makes a penetration channel while penetrating through the sediments. One may expect that the penetration channel

collapses, but slower than the penetration speed of the

gravity-installed anchor. The penetration channel is generally at least slightly wider than the cross-section of the trailing portion (7), thus the friction between the sediments and the trailing portion occurs mainly along the shorter nose

portion's (5) length, with the trailing portion (7) surrounded generally by water or sediment slurry. Third, the increased cross-section of the nose portion (5) may also create turbulent water flow about the trailing

portion (7) of the main body while falling through the sea, so as to slightly decrease the water resistance of the trailing portion (7) during the fall through the water. This will increase the velocity potential and kinetic energy of the anchor upon impact with the seabed for achieving a minimum required penetration depth.

In a preferred embodiment of the invention the

intermediate stem portion (6) has a cross-section area less than the cross-section area of the nose portion (5) and larger than or equal to the cross-section area of the trailing

portion (7) . This is best illustrated in Figs, la, lc; in

Figs. 2a and 2c; Fig. 3a; Fig. 5a; and also in Figs. 8a and 8b. In the embodiments shown in Figs, la, 2a, 3a, 5a, the material thickness of the trailing portion (7) may be only one plate thickness. The intermediate stem portion (6) may be of the same width as the trailing portion (7) such as illustrated in Fig. 10, and it may also be of very short length so that the trailing portion with the fins may extend closer to the nose portion (5) .

In a preferred embodiment the axially extending elongated main body (0) with a nose portion (5) , an intermediate stem portion (6), and a trailing portion (7) with two or more tail fins (2, 19) is made from elements made in steel plates and welded, riveted or bolted together. This significantly reduces the manufacturing costs compared to forming a solid main body adapted for receiving fins of special geometry, and the weight is significantly lower, allowing important savings on

material, transport, storage, and handling. In addition, the increased velocity potential will in turn give a greater penetration potential.

Fig. 2a is an elevation view of an embodiment of the invention of which the fins are provided with minor ribs (18) along the surface. Such transverse ribs (18) may create micro turbulence over the fin (2) surface when the gravity-installed anchor falls through the water, and thus reduce the friction. Fig. 2b shows details of such minor ribs (18) and part of the widened forward edge (3) of a facing fin.

Fig. 2c shows details of a preferred embodiment of the invention in which a transition from a forward thicker cross- section area of the axially extending elongated main body (0) . This relates generally to three such steps (9) rearward along the main body. This is illustrated here by a step (9) on either side, here from a five plates thick cross section in the middle part of the intermediate portion (6) to a three plates thick cross-section in the rear part of the

intermediate portion (6) . The nose portion (5) in Fig. 2a has a seven plates thickness.

In Figs, la and 2a the increased area of the cross-section of the nose portion (5) relative to the area of the trailing portion (7) is due to an increased number of attached steel plates on the central elongate body when approaching the nose (51) . In an embodiment of the invention, in the nose portion (5) there are actually seven steel plates laminated to form the material cross section, while in the trailing portion there is only one plate in the material cross section. However, the nose portion (5) may alternatively or

additionally be provided with weights (11) made in steel or other heavy and strong metal. Fig. 4a shows four weight

portions (11a, b) arranged for filling in the corners of a plate cross-section of the nose portion of a plate embodiment of the invention. Fig. 4b shows the four weight portions (11a, b) arranged bolted in place in the corners of the plate cross- section of the nose portion (5) . Fig. 4c is a similar

embodiment of the invention in which four weight portions (11c) are arranged in place in the corners in the nose

portion, either welded or bolted or riveted in place. The generally circular cross-section of the weight (11) in filled nose portion (5) may contribute to the penetration ability of the gravity-installed anchor through the sediments as it may significantly reduce wall friction on the middle and

trailing portions (6, 7) of the stem. The same may provide an improved vertical acceleration through water after being released . Fig. 5a is a rear isometric view of the embodiment shown in Fig. 3a showing the upward facing cylindrical sleeves (10) at the rear ends of the fins (2) . In a preferred embodiment of the invention, those sleeves are arranged for holding

instrument pods (12a) at the lower ends of retrieval wires (12b) which may be provided with buoyancy elements or marker buoys (15) in their opposite, upper ends. The marker buoys (15) may be provided with loops (15a) for connecting to a wire for pulling out and retrieving the instrument pods (12a) from the sleeves (10), thus retrieving the instrument pods to the surface. Fig. 7 illustrates an anchor gravity-installed in the seabed with marker buoys (15) on upper ends of the retrieval wires (12b) attached to sleeves (10) at the rear ends of the fins of the drop anchor. Here, an anchor line is tightened laterally through the seabed toward a lateral direction and inclined upwards between the rear end of the anchor and some anchored structure at the sea surface, such as a drilling platform, a petroleum production barge, an oil transfer buoy, or the like. Fig. 6 is a forward isometric view of an

embodiment of the invention with a nose weight (11a, b) with nuts (14b) and bolts (14a) arranged in exploded view.

One advantage of the embodiment of the anchors made by flat plate-shaped elements is the fact that they may be easily manufactured by cutting out and welding together the anchors of the embodiment of the invention. Another advantage is that they use much less steel, are easier to transport, store and handle. In addition, there is an increase in the pullout capacity to weight ratio compared to previous solutions.

Fig. 8a illustrates in a side elevation view an embodiment of the invention wherein the axially extending elongated main body (0) comprises a series of coaxial layers of steel in the form of pipes or rings (20a, 20b, 20c, 20d) . Fig. 8b is an isometric view of the same. The nose portion (5) of the main body (0) essentially has a larger outer diameter than the intermediate portion and the trailing portion (6, 7) . The fins extend radially from the trailing portion (7) .

The pipe-shaped main body (0) according to the embodiment of Fig. 8 may be formed in different ways: The nose portion (5) may be formed from a pipe section (20c) having larger outer diameter than the outer diameter of the intermediate pipe section (20b) further having a larger outer diameter than the trailing pipe section (20a) . The inner diameter of all pipe sections may be the same for all pipe sections (20, 20c, 20b, 20a) or decrease from the nose portion towards the trailing end. A very short, externally wider pipe section, rather called a ring (20d) forms the ultimate nose portion of this pipe embodiment. Thus the forepart of the main body has both a larger material cross section than the trailing part. In the embodiment shown in Fig. 8a and b rearward directed steps (29) are formed by the transitions in outer diameter.

In a preferred embodiment of the invention, as shown by broken lines indicating a longitudinal section of the pipes of the main body (0) , the inner diameter of the nose portion pipe section (20c) may further have a lesser inner diameter than an inner diameter of the intermediate pipe section (20b) , which further may have a lesser inner diameter than an inner

diameter of the trailing pipe section (20a) . Internal steps corresponding to the external steps are formed. The nose portion which may be formed in this embodiment by the ring (20d) may also have a lesser internal diameter compared to the internal diameter of the rest of the nose portion (20c) . In this embodiment the friction between the inner wall of the pipe portions behind the nose portion and the sediments within the pipe is reduced. The nose portion pipe (20c) , possibly with the nose ring (20d) will penetrate the sediments to form a channel of

slightly wider cross-section than the rest of the elongated main body, thus reducing friction by sediments. Likewise, rearward steps (29) formed by the rear end of pipes (20c) ,

(20b) may form turbulence when the drop anchor sinks through the water and contribute to a reduction in water friction and thus increase the acceleration of the drop anchor when

dropped. In addition, the increased velocity · potential will in turn give a greater penetration potential.

Fig. 9a shows a ship holding an anchor (1) according to the invention over the seafloor (16) by an anchor line

attached to a hoisting wire. The release from the hoisting wire may be remotely controlled using a disconnecting link controlled by e.g. an ROV. Fig. 9b illustrates the same anchor (1) after having been dropped into the sediments and with the anchor line pulled laterally by a permanently or semipermanently moored vessel or platform.

For connecting the anchor line an ordinary shackle (81) may be used. The connection point (8) may in a preferred embodiment of the invention be in the form of reinforced eyes (84) at the rear portion of the steel structure of the rear part of the trailing portion (7). The reinforced eyes may be provided with a transverse axle (85) of a link plate (83) with an eye with a shackle bolt (82) and a shackle (81), such as shown in Fig. lb and Fig. 8b. In an embodiment of the invention the connecting point (8) is not at the immediate rear end of the stem, but somewhat retracted from the end, please see Fig. 10c. In Fig. 10a and Fig. 10b is shown a simple, generally straight cylindrical main body embodiment of the gravity- installed anchor according to the invention. The embodiment comprises an axially extending elongated cylindrical main body (0) with a nose portion (5) with a nose (51), an intermediate stem portion (6), and a trailing portion (7) with four tail fins (2), extending laterally generally from the trailing portion (7) . The gravity-installed anchor according to this embodiment has a rounded nose (51) on a nose portion (5), which is forward tapering off. The rear part of the nose portion (59) ends up in a circular edge, which forms a step transition to the lower-diameter intermediate stem portion (6) . Thus the nose portion (5) immediately ahead of the circular edge has a larger cross-section area than a cross- section area of the intermediate portion (6) and the trailing portion (7) of the main body (0). One or more such rearward steps (59) may be formed for reducing the external diameter of each subsequent section relative to the preceding section may be formed.

Claims

1. A gravity-installed anchor (1) comprising an axially extending elongated main body (0) fins, said anchor

comprising

- a nose portion (5) with a nose (51) ;

- an intermediate stem portion (6) ;

- a trailing portion (7);

- a connection point (8) for an anchor line;

characterized by

2. The gravity-installed anchor of claim 1,

said intermediate stem portion (6) having a cross-section area less than the cross-section area of said nose portion (5) and larger than the cross-section area of said trailing portion (7) .

3. The gravity-installed anchor (1) of claim 1,

said axially elongated main body (0) made in steel plates (4a, 4b, ... ) forming a successively thicker material cross-section as counted from the trailing portion (7) towards the nose portion (5) .

4. The gravity-installed anchor (1) of claim 1,

said tail fins (2, 19) made in steel plates (4a, 2) .

5. The gravity-installed anchor (1) of claim 1,

said steel layers (4) of said main body (0) comprising first steel plates (4a) extending along a major portion of said main body (0) from near said nose (51) along said nose portion (5) and said trailing portion (7), said material cross-section of said nose portion (5) thickened by one or more layers of steel plates (4b, 4c, ...) each steel plate (4b, 4c, ...) extending from near said nose (51) and successively shorter in the direction of the trailing portion (7) than an underlying steel plate (4a, 4b, ...), so as to form one or more rearward decreasing transverse steps (9) on the surface of said main body (0) .

6. The gravity-installed anchor of claim 1,

a frontal edge (3) of said tail fins (2, 19) having a larger cross-section than the general plate thickness of said tail fins (2) .

7. The gravity-installed anchor of claim 5 or 6,

said nose portion (5) provided with one or more solid weights (11) attached to said nose portion (5) and filling in corners formed by said steel plates (4a, 4b, ...) near said nose (51).

8. The gravity-installed anchor of claim 7, wherein said fins (2) are provided with minor transverse ribs (18) along the fin surface .

9. The gravity-installed anchor (1) of claim 1,

said main body (0) built of a first steel pipe section (20a) forming said trailing portion (7), and two or more steel pipe sections (20b, 20c) of larger diameter forming said nose portion.

10. The gravity-installed anchor (1) of claim 1,

said main body further comprising an intermediate steel pipe section (10b) forming said intermediate portion (6), said intermediate steel pipe section (10b) being of lesser diameter compared to the diameter of said nose portion (5) and of larger or equal diameter compared to the diameter of said first steel pipe section (20a) of said trailing portion (7) .

11. The gravity-installed anchor of claim 1,

said connection point (8) comprising one or more reinforced eyes (84) at the trailing portion (7).

12 The gravity-installed anchor of claim 1, said reinforced eyes (84) may be provided with a transverse axle (85) of a link plate (83) with an eye with a shackle bolt (82) and a shackle (81) .