AU591053B2

AU591053B2 - Anchor device

Info

Publication number: AU591053B2
Application number: AU35557/84A
Authority: AU
Inventors: Clas Johan Rinaldo
Original assignee: Individual
Current assignee: Individual
Priority date: 1983-10-28
Filing date: 1984-10-29
Publication date: 1989-11-30
Anticipated expiration: 2004-10-29
Also published as: JPS61500218A; EP0191765A1; IT1177049B; WO1985001925A1; IE842784L; BR8407355A; DE3469288D1; FI861754A7; FI861754L; ES293271Y; FI861754A0; DK286585D0; DK286585A; EP0191765B1; NO852530L; IE55888B1; AU3555784A; IT8423329A0; SE8305933D0; ES293271U

Description

ANCHOR DEVICE

The invention relates to an anchor device for offshore or onshore use. The anchor device is primarily intended for anchoring offshore constructions such as floating rigs or vessels for similar purposes the anchorages of which should be relatively easy to establish and flexible. Also for constructions of a more permanent character the anchor device of the invention can be used e.g. for tension leg platforms, mooring or loading buoys, plants for recovering wind, wave or heat energy, different types of seabed-connected, rigid or articu¬ lated constructions such as drilling steel jackets and loading columns. However, the anchor device can also be used onshore for staying several constructions such as towers, masts, or scaffolds.

In the conventional anchoring method, as far as floating rigs or similar constructions are concerned, a great number of anchors, 8 to 12, are distributed around the rig. The anchors which are put out from the rig by means of special anchor handling ships, then are hauled towards the rig until they engage the bottom. The anchor is of the type which is similar to the conven-t tional ship anchor and can be loaded substantially by . horizontal forces only. The anchor line therefore must have such a length that a curved shape will be imparted to it by gravity, the line being tangent to a horizontal line at the engagement location. However, the length of the line can be slightly reduced if the line adjacent the anchor is held down by means of weights. During the anchoring operation it is difficult, to predict the final position of the anchoring, i.e. the position wherein the anchor provides full anchor holding capacity. Moreover, the anchor is affected by deviations in the direction of

O PI ^YATlO§! the pull, which may make the anchor. capsize and fail . Therefore, it is necessary to have several anchors distributed around the rig.

Summarizing, a great number of anchor units with heavy winches and long lines provides a large total weight of the complete anchor system, which reduces the dead weight capacity of the rig at a predetermined available buoyancy.

FR 2,383,825 describes a prior art anchor device comprising a plate and a pole extending through said plate, for connecting an anchor chain. However, since the pole extends centrally through the plate, it is possible to arrange the line of action of the anchoring force through the centre of pressure (C.P.) of the pole only if the pole has a large diameter in relation to the Tength^' thereof, which means that it will be difficult to ram the pole into the bottom or the ground.

The anchor device of the invention comprises a plate to bear on the bottom or ground surface, and cylindrical elements extending through the plate into the bottom or ground layers. For the purpose to provide a force characteristic which is similar to that provided by a large tube of a corresponding diameter, and thus to provide a safe anchor device which can carry forces having large vertical components, the anchor device has obtained the characterizing features of claim 1.

This anchor device can be engaged and disengaged in a relatively easy manner by simple operations and low power hich can be exercised at large depths. Thus, substantial parts of the anchor device can be re-used. Since the anchor device can be loaded by vertical component forces, shorter anchor lines can be used. Because the anchor device can be dimensioned with due consideration of the geological conditions at the pre- determined locations, each anchor unit will be more reliable and thus fewer anchor units will be required. Shorter lines and fewer units provide an anchor system of a correspondingly lower weight.

The invention also relates to the use of the anchor device of the invention as defined in claim 6. In order to explain the invention in more detail reference is made to the accompanying drawings in which FIG. 1 is a diagrammatic perspective view of a rig wh.ich is anchored by means of four anchor devices of the invention,

FIG. 2 is a perspective view of the anchor device, FIG. 3 is a cross-sectional view along line a - a in FIG. 2,

FIG. 4 is a fragmentary perspective view, partly a cross-sectional view, of an attachment on the anchor device,

FIG. 5 is a diagrammatic perspective view of a rig which is anchored by means of four groups each having three anchor devices of the invention, FIG. 6 illustrates the geometry of a group of three anchor devices as shown in FIG. 5, FIG. 7 is a plan view of branch lines and anchor devices when six branch lines are arranged, FIG. 8 is a plan view similar to that of FIG. 7, the anchor devices being interconnected with the branch lines forming a net structure, and FIG. 9 is a plan view similar to that of FIG. 7, when five branch lines are arranged. In FIG. 1, a rig 10 floating on the water, is anchored by means of four identical anchor devices 14 of the invention each provided with an anchor line 11. The anchor device 14 is shown in more detail in FIG. 2 and comprises a bottom plate 15 bearing on the bottom, said plate preferably being circular, and a plurality of cylindrical elements 16 which are rammed through the bottom plate into the bottom material to form a tubular, in this case cylindrical envelope as indicated by dot-and-dash lines in FIG. 2 wherein only a few elements are shown. The bottom plate 15 forms a flange at the upper end of the tube formed by the elements 16. By this arrangement there is obtained a force charac¬ teristic similar to that obtained by means of a single tube with a homogeneous curved wall constructed in the conventional manner, without the extensive work required in order to locate such a tube in the bottom layers. The elements are supported by the bottom plate which pro¬ vides a rigid connection, the bundle of elements in its entirety being kept together by the rigid bottom plate. Due to the fact that the anchor device 14 is constructed as described it is achieved that power resources which are small in relation to the holding capacity, are required in order to engage the device in a predetermined position and in 'order to disengage the device from said position, respectively. The dimensions of the bottom plate 15, the number of elements 16, and the dimensions of said elements should be adapted to the geological conditions such that the ram force required will be optimized in relation to the load capacity of the anchor device obtained. In order to increase the lateral resistance of the bundle of elements the cylindrical elements can be arranged in a so-called zigzag formation as shown in FIG. 3. In order to increase further the capacity of the device setting substances or mixtures of such substances, e.g. a mixture of cement and water, can be injected into the bottom material around the elements in order to strengthen the bottom material .

When dimensioning and constructing the anchor device, one should see to it that the line of action of the anchoring force passes through the centre of pressure and that the lateral and axial resistances of the device at the same time are sufficient in order to withstand the component forces in the corresponding directions . The position of the centre of pressure is defined not only by the dimensions and arrangement of the element bundle but also by the area of the bottom plate and the nature of the bottom layers.

When the line of action of the force passes through the centre of pressure, the force can be easily split-up into a transverse component and an axial component, and no moment acts on the anchor device. By arranging the line attachment at the edge of the anchor device this can be achieved. Then, when the anchor device is loaded, the elements will lock automatically in the bottom plate by the transfer of mome^'nt from the plate to the elements (jamming).

If the anchor device is made symmetric about an axis, the load capacity will be the same in all directions of orientation. If the attachment is made displaceable such that it can move freely along the circumferential circular edge of the plate, it is achieved that the anchor device will never be exposed to a torsional moment when the line of action of the anchoring force intercepts a normal through the centre of the bottom plate (symmetry axis) notwithstanding the direction of the anchor line.

On the bottom plate 15 there is provided a line attachment 17 for the anchor line 13 connected to the anchor device, which is mounted at the edge of the bottom plate and can be displaced freely along said edge. The line attachment is shown in FIG. 4 and embraces at a recess 21 two guide rails 19 and 20 connected by welding to the plate, the rail 19 being annular and extending along the outer edge of the bottom plate 15, and the rail 20 extending along the upper portion of the cylindrical surface of the plate. Thus, the attach¬ ment is displaceably guided by the rails.

In FIG. 5, there is shown one manner in which the anchor devices 14 preferably are used for anchoring a rig 10 floating on the water. Four identical groups each having three anchor devices 14 of the invention, are arranged and each group thereof comprises an anchor line or a main line 14 connected to the rig at one end thereof and connected to a block 12 at the other end thereof. Three branch lines 13 extend from the block 12, each of which is connected to one anchor device 14 engaged with the sea bottom. The term line relates not only to steel* wire ropes but also to ropes of synthetic fibres and chains as well as combinations thereof. The anchor. devices are distributed around the block 12, and it is necessary to have at least two devices for proper functioning of the arrangement. However, it is preferred to use at least three anchor devices for symmetry reasons. The number can also be larger than three. However, in that case special arrangements are required in order to obtain a statically defined load distribution, as will be described below.

Referring to FIG. 6, the anchoring method principally is based on a conversion of size and direction of the forces, force transformation, by the forces being split-up into vectors such that the force in the main line 11 having a large elevation angle α, will be converted into forces in the branch lines, hav¬ ing defined smaller elevation angles α-, , _2J and α,, and conversely larger forces which are reduced, however, by the total anchoring force in the main line being split-up into several anchor forces in the branch lines. The elevation angle generally is defined as the inclina¬ tion angle of the line and the force, respect vely, to the horizontal plane at the attachment point as measured in a plane through the vertical line at the attachment point and the line/force. Since a line can transfer only forces in the longitudinal direction thereof, the force geometry, i.e. the vector geometry, will be defined by the line geometry. The lengths of the branch lines 13 thus will be defined by the direc¬ tion of the forces when the anchor devices 14 are most effective, the anchor devices at the same time being dimensioned such that the direction of the branch line will be adapted to the desired dimensions of the rest of the line configuration. If α α_n where n = 1 , 2, 3, the force directions of the anchor devices 14 will be unchanged notwithstanding other changes of the size and direction of the anchoring force.

In cases wherein more than three anchor devices 14 are required in order to limit the loads on the configuration, different embodiments can be resorted to in order to .define the forces statically. in FIG. 7, one embodiment of this kind is dis¬ closed wherein the forces from the anchor devices are reduced by the devices being interconnected in a bifurcated arrangement comprising six anchor devices 14. The number of branch lines 13 connected to the block 12, is reduced to three lines. It is achieved that all lines always are stretched and that the forces are statically defined, the arrangement at the same time including no moving parts. The configuration can be successively extended to comprise an indefinite number of anchor devices .

If the embodiment of FIG. 8 is chosen, the inter¬ connection of the branch lines, i .e. the net structure, can be made more or less complex such that a higher or lower degree of indirectness and thus force distribution will be obtained. In the embodiment of FIG. 8, the dash lines indicate lines which may be excluded. If these lines are included in the net structure, they are located in a plane below ^"the block and they are inter¬ connected independently of the block. By the anchor devices 14 being interconnected as disclosed in FIG. 9, the forces from the anchor devices will be reduced by adjacent anchor devices, e.g. 14a, 14b, and 14c, 14d, respectively, being inter¬ connected in pairs by means of a common line 13' and 13", respectively, which runs through a hawsehole or over a pulley 18 in the block 12. The resulting force then acts on the block as the force from one- line. The prerequisite for the operation of this arrangement is that the frictional forces in the hawsehole and the pulley, respectively, are small in relation to the anchor forces. If no frictional forces are present (the friction co-efficient = 0), the forces acting on the two anchor devices interconnected in pairs, will be equal . In addition to the embodiments mentioned above there is the possibility to provide the branch lines with spring elements where the number thereof is larger than three. Then, all lines will always be stretched, and if the force in one of the lines increases due to an unequal load distribution, the spring will be extended and thus the force distribution of the other anchor devices will be changed.

Claims

1. Anchor device for offshore or onshore use comprising a plate (15) to bear on the bottom or ground surface, and cylindrical elements (16) extending through the plate (15) into the bottom or ground layers, c h a r a c t e r i z e d in that several elements

(16) are arranged to form together a tubular envelope.

2. Anchor device as claimed in claim 1, c h a r a c t e r i z e d in that the elements (16) have circular cross-sectional form and are distributed in the plate (15) having circular form, symmetrically about an axis.

3. Anchor device as claimed in claim 2, c h a r a c t e r i z e d in that an attachment (17) for an anchor line (11) is arranged at the edge of the plate (15).

4. Anchor device as claimed in claim 3, c h a r a c t e r i z e d in that the attachment (17) is freely displaceable along the circular circumferential edge of the pi ate (15).

5. Anchor device as claimed in any of claims 1 to 4, c h a r a c t e r i z e d in that the elements (16) are arranged in the plate (15) in a zigzag forma¬ tion.

6. The use of the anchor device as claimed in any of claims 1 to 5, c h a r a c t e r z e d in that at least two anchor devices are fixedly interconnected by means of branch lines (13) in a junction (12) which is co^'nnected by means of a main line (11) to the object (10) to be anchored, the branch line (13) being connected to the plate (15) with the line of action of the anchor force extending substantially through the centre of pressure of the tubular envelope.

7. The use as claimed in claim 6, c h a r a c - t e r i z e d in that two adjacent anchor devices (14) are interconnected in pairs by means of a common branch line (13).

8. The use as claimed in claim 6, c h a r a c ¬ e r i z e d in that the anchor devices (14) are connected to the junction (12) by means of branch lines (13) interconnected to form a net structure.

9. The use as claimed in claim 6, c h a r a c ¬ t e r i_z e d in that two adjacent anchor devices (14a, 14b; 14c, 14d) are connected in pairs to a common branch line (13'; 13") which runs freely through the junction (12) .