TECHNICAL FIELD

This invention relates to an underwater anchoring

( structure for anchoring apparatus in a body of water. 5 Although the anchoring structure may be used in relatively shallow waters, e.g. 60m depth, the anchoring structure is primarily intended for use at greater depths, e.g. up to 500m or more, in a body of water, e.g. a sea or ocean. In particular, but not exclusively, the anchoring structure is 10 intended to be used for underwater pipelines, e.g. gas pipelines, to sink the pipelines during laying from a surface vessel and for maintaining the pipelines in a sunk position on the bed, e.g. sea bed, of the body of water.

BACKGROUND ART

15 In order to lay a gas pipeline on the sea bed, the pipeline is conventionally formed by joining together, e.g. welding, on a surface vessel, such as a barge, lengths of steel pipe clad with an anchoring structure of reinforced concrete. As the surface vessel moves forwards and lengths

20 of pipe are joined to the front of the pipeline, the formed pipeline is fed into the sea water from the rear of the surface vessel. The reinforced concrete cladding acts as a sinking structure to cause the pipeline to sink to the sea bed. Once on the sea bed, the weight of the reinforced

25 structure ensures that the pipeline remains weighed down or "anchored" in its laid position on the sea bed.

During the laying operation, the forward motion of the surface vessel and the paying out of the pipeline from the rear of the vessel cause the pipeline to adopt a generally 30 serpentine configuration as it trails downwardly from the surface vessel and adopts a generally horizontal position on the sea bed. To maintain a satisfactory curvature of the pipeline, tension in the pipeline is maintained by tensioning machines on the surface vessel. The concrete

cladding provides the pipeline with additional weight to sink and anchor the pipeline. However, although it is desirable for the concrete cladding to have sufficient weight to anchor the pipeline when it is on the sea bed, the concrete cladding should not be so heavy as to cause the pipeline to be damaged during the laying operation. For example if the weight of the cladding is too heavy for the surface vessel's tensioning capability, the pipeline may be overstressed or may fail by buckling. This failure may occur at any point along the unsupported portion of the pipeline between the surface vessel and the sea bed. Accordingly it is desirable for the pipeline, during laying, to be sufficiently heavy to sink but to be relatively light so as not to be overstressed, and when in use on the sea bed to be relatively heavy so as to remain in the laid position.

In US-A-4393901 there is described an underwater pipeline formed from pipe sections consisting of an inner steel tube and a porous cement shell filled with hollow spheres which are slowly permeable to water under pressure. The cement is filled with a sufficient volume of the hollow spheres to make the pipe only slightly heavier than sea water, providing a reduced weight during laying of the pipeline. After installation under water, the hollow spheres gradually fill with water and make the pipeline rest stably on the ocean bottom. One disadvantage of this known method is that if it is subsequently required to raise the pipeline from its submerged position the hollow spheres remain filled with water and the pipeline remains in its heavy state.

DISCLOSURE OF INVENTION

The present invention seeks to provide an improved anchoring structure for an apparatus, e.g. an underwater pipeline, which is designed to have a lighter submerged weight during positioning, e.g. pipe laying than when in its fully submerged position, e.g. on a sea bed. In pipe laying, this allows reduced tensions to be applied by the

laying vessel to achieve the desired pipe shape during the laying operation.

According to the present invention there is provided an anchoring structure applied as a layer to at least partly cover apparatus to be anchored in a body of water and including weight increasing means for increasing the weight of the anchoring structure and apparatus when they are submerged in water to at least a predetermined depth, characterised in that the weight increasing means comprises compressible means substantially impermeable to water adapted to be compressed when submerged in water to at least said predetermined depth.

Preferably the compressible means is resiliently or elastically compressible so that it will resiliently expand from a compressed condition when raised from a submerged position. In particular the compressible means expands as the surrounding water pressure decreases.

The anchoring structure may comprise a substantially incompressible, relatively heavy material which is permeable to water when submerged in water and which has internal volumes containing said compressible means. In this case the water acts on the compressible means after permeating through the heavy material. By using water-impermeable compressible means, water, e.g. sea water, enters into the internal volumes and compresses the compressible means into a smaller volume, the actual compressed volume being dependent on the depth of submersion and the degree of permeability to water of the heavy material.

Conveniently the anchoring structure is applied as a coating to pipe lengths destined to form an underwater pipeline. In one embodiment where the anchoring structure comprises water permeable heavy material having internal volumes containing the compressible means, the permeability to water of the heavy material is such that the surrounding water will only permeate relatively slowly, e.g. over a

number of days, through the hard material to the internal volumes. In this case, as the pipeline is laid on the bed of the body of water relatively quickly, there is insufficient time for the surrounding water to permeate through the heavy material and enter the internal volumes until the pipeline has lain in position on the sea bed for a few days. Thus the anchoring structure is relatively light during the actual pipe laying process and before the surrounding sea water enters the internal volumes. This is important since forces acting on the pipeline as it is laid on to the bed of the body of water are thereby minimised. A period of time after the pipeline is laid on the bed of the body of water, the surrounding water permeates through the heavy material and enters the internal volumes thereby increasing the submerged weight of the pipeline.

Preferably the internal volumes are filled with resiliently or elastically compressible means which is significantly lighter than sea water and which cannot permeate outwardly through the heavy material. For example the compressible means may conveniently comprise a compressible plastics material, such as a high density closed cell cross linked polyethylene foam, set in a hard matrix material, such as a concrete material. In this case, the compressible means fills the internal volumes during the manufacture of the anchoring material. In use, however, the compressible material is compressed when the anchoring structure is submerged at elevated pressures and water is then able to enter into spaces vacated in the internal volumes occasioned by the compression of the compressible means in order to increase the submerged weight of the anchoring structure. If the pipeline is subsequently brought to the surface of the body of water, the resiliently or elasticaly compressed compressible means is able to expand, forcing the water out of the internal volumes thereby decreasing the submerged weight of the anchoring structure. Other materials for the compressible means are possible. For instance, the compressible means, which must be light in weight compared with an equivalent volume of water, may comprise other compressible materials, e.g.

closed-cell sponge rubber materials, plastics materials, or internally pressurised, e.g. gas-filled, preferably air- filled, envelopes of elastically deformable material, e.g. rubber or rubber-like material.

In another embodiment of the invention, the permeability of the heavy material to water is such that the surrounding water permeates relatively quickly, e.g. as the pipeline is being laid. With such "high permeability" to water, there is a relatively rapid increase in submerged weight, although in practice there will be a time lag at any depth of submersion before the water permeates into the heavy materials and compresses the compressible means within the internal voids. If the pipeline is subsequently raised, the submerged weight will conveniently decrease relatively rapidly as the pipeline is raised.

Conveniently the heavy material may be coated with a coating which is substantially water impermeable but which dissolves or wears away over a period of time to enable surrounding water to permeate into the hard material. In this way the onset of the increased submerged weight can be delayed or controlled.

In an alternative design of the anchoring structure, the compressible means may be arranged so that water is able to act directly on, so as to directly compress, the compressible means when the anchoring structure is submerged in water. In this case the anchoring structure may be in the form of a composite layer comprising a first layer of said compressible means and a second layer, e.g. of incompressible heavy material such as concrete. Alternatively the anchoring structure may comprise the compressible material and the apparatus to be anchored may optionally include a cladding or covering layer, e.g. of incompressible, heavy material, such as concrete. The compressible means may comprise a closed cell plastics foam material such as a high density closed cell cross-linked polyethylene foam. Alternatively, however, the compressible

means may comprise a plurality of hollow spheres embedded in a matrix material.

The compressible means, if arranged so as to directly contact the surrounding water when submerged, may be in block or sheet form. If the compressible means is in block form, it may be arcuate being affixed, e.g. by strapping, closely to the outer surface of the pipeline at spaced apart locations along its length. If the compressible means is in sheet form, it may be wound, e.g. helically, around the pipeline either with or without overlapping of adjacent turns. Thus the compressible means may totally encapsulate or only partially cover the outer surface of the pipeline, or the clad pipeline if the latter is coated or clad, as is conventional, with an incompressible, relatively heavy material. Alternatively in the case of a clad pipeline having said composite layer, the first layer of compressible means and the incompressible cladding of heavy material may be installed adjacent to each other so that both surfaces are exposed directly to surrounding water when submerged.

The compressible means may suitably be attached to said apparatus, e.g. a pipeline, or to a coating layer, e.g. of incompressible, relatively heavy material applied to a pipeline, by chemical bonding or, as previously mentioned, by strapping. The compressible means may be attached during the normal coating or cladding process of the pipe or may be attached when on the pipe laying vessel. In the latter case the compressible means may be attached after the pipe has passed through tensioners.

The compressible means may comprise one or more fluid, e.g. gas or air, filled envelopes or "cells" and an important preferred embodiment of the invention is obtained if the compressible fluid in the envelopes or "cells" is, at sea level, at an elevated pressure, i.e. the fluid within the closed cells is raised to a pressure greater than atmospheric pressure. In particular, in the case where the compressible means comprises a closed cell plastics or

rubber material, these cells are pressurised during the manufacture of the compressible means. This pre-pressuring during manufacture may be achieved by injecting pressurised gas, e.g. nitrogen, into the plastics material and allowing the foam to expand in a pressurised chamber so that when the foam material "sets" the gas within the cells will be at an elevated pressure in excess of 1 bar, e.g. at least 1.5 bar and preferably at least 4 bar, e.g. 6 bar depending on the depth of the water in which the apparatus is to be submerged. In this manner the compressible properties of the closed cell plastics material are influenced. Alternatively, the "closed envelopes" may be in the form of a tube or tubes arranged, e.g. helically wound, around the pipeline or other apparatus to be anchored and preferablfy containing pressurised compressible fluid. In this case, during laying of a pipeline, the compressible material will only start significantly to compress when the sea water pressure exceeds the pre-pressurisation of the gas in the closed cells or envelopes.

BRIEF DESCRIPTION OF DRAWINGS

Embodiments of the invention will now be described, by way of example, with particular reference to the accompanying drawings in which:

Figure 1 is a schematic view of a gas pipeline comprising joined together lengths of pipe as it is laid from a surface vessel onto the sea bed.

Figure 2 is a sectional view, on an enlarged scale, through the gas pipeline shown in Figure 1,

Figures 3A and 3B are sectional details, on enlarged scales, of part of the gas pipeline shown in Figure 2 when on the surface and when submerged at depth, respectively,

Figure 4 is a schematic sectional view of a gas

pipeline clad with incompressible heavy material with an arcuate block of compressible material strapped thereto;

Figures 5 and 6 are schematic side views showing strip of compressible material being wound on to lengths of pipeline in overlapping relationship and non- overlapping relationship, respectively;

Figure 7 is a longitudinal sectional view through a pipeline clad with incompressible heavy material and having compressible material arranged in channels formed in the heavy material, and

Figures 8 and 9 are a schematic side view and an end view, respectively, of another embodiment of a pipeline clad with compressible material.

BEST MODES OF CARRYING OUT THE INVENTION

Figure 1 schematically shows a pipeline 1, e.g. for conveying gas or other fluids, being laid from a barge 2, or other surface vessel, onto the sea bed 3. The pipeline 1 is made by welding together pipe lengths (not shown) on the barge 2 and feeding the pipeline, as it is formed, into the sea water from the rear of the barge 2, which has anchoring cables 10 attached at its front to an anchoring machine (not shown) . The various pipe lengths each comprise an inner steel tube 4 (see Figure 2) through which the fluid is intended to be conveyed and a cladding 5 of concrete material acting as a ballasting or anchoring structure. As is conventional with such pipelines, the purpose of the cladding 5 is to add weight to the pipeline 1 so that it remains on the sea bed in use with little or no movement. The need to "anchor" the pipeline is particularly pronounced when the pipeline is used to convey gas when the submerged weight of the pipe filled with gas is lighter than if the pipe were full of liquid.

The conventional solution to anchoring the submerged pipeline on the sea bed 3 is to make the cladding sufficiently heavy to enable the pipeline to withstand wave and current loadings. The cladding of concrete material is also conventionally reinforced with reinforced material, e.g. wire mesh. However problems may arise during laying of the pipeline if the cladding 3 is made too heavy. In particular the pipeline 1 adopts a generally serpentine configuration in the sea water between its upper end where it trails downwardly and rearwardly from the advancing barge 2, and its lower end on the sea bed. The parts of the pipeline 1, indicated in Figure 1 by the arrows X and Y, where the pipeline curves onto the sea bed and where the pipeline leaves the pipelaying vessel, are particularly subjected to strain and are liable to fail if the weight of the pipeline is too heavy.

In order to overcome the problem outlined above the present invention is concerned with providing an anchoring structure which has a submerged weight which is greater when the pipeline is in position on the sea bed than during the pipeline laying operation. To this end, the cladding 5 is provided with internal volumes 7 (see Figures 3A and 3B) containing resiliently or elastically compressible means 8 which is substantially impermeable to water, e.g. sea water. The compressible means 8 typically comprises a compressible plastics material such as a high density closed cell cross- linked polyethylene foam, or a compressible closed cell rubber foam material. The internal volumes 7 may be of any convenient size.

The cladding 5 is applied to the inner steel tube 4 by forming a mix of wet concrete or cement material and particles of the compressible means 8 and, before the mix has set, applying it to the tube 4 and any surrounding reinforcing mesh or the like. The volumetric ratio of the concrete material and the compressible means 8 may be varied as required for a particular application, but typically may be 1:1. The concrete material sets as a hard, substantially

incompressible material around the particles of water impervious compressible means 8. Although the set matrix of concrete material is hard and substantially incompressible, it is porous to water. This porosity may be controlled to control the compression of the compressible means.

In use, a pipeline 1 having a cladding 5 in accordance with the invention will initially have its internal volumes 7 completely filled with the compressible means 8. In this condition, the pipeline should weigh about the same or slightly greater than the specific gravity of the sea water in which it is to be laid. The compressible means 8 are lighter than equal volumes of sea water and the cladding 5 will thus be relatively light during laying of the pipeline 1 on the sea bed. As soon as the pipeline 1 enters into the sea water during pipe laying, the surrounding water will begin to permeate into the concrete material at a rate dependent on the degree of porosity of the concrete material. As the sea water reaches an internal volume 7, the pressure of the sea water, because of the depth of submersion of the cladding 5, will cause the compressible means to be compressed within its internal volume so allowing the sea water to permeate into vacated parts of the internal volume. The internal volumes 7 remain substantially constant in volume whatever the submersion depth because of the incompressibility of the concrete material. However the reduction in volume of the compressible means 8 and the subsequent ingress of heavier sea water into the space vacated in each internal volume 7 by the compressible means 8 results in the submerged weight of the cladding 5 increasing. The degree of compression of the compressible means, and thus the amount of water which can enter the internal volumes, increase with water pressure (i.e. with the depth of submersion).

If, by design, sea water is only able to permeate into the incompressible concrete material at a controlled, relatively slow rate, it will take a length of time for the sea water to permeate into the internal volumes and.

accordingly, the submerged weight may not increase immediately but only after a period of. time. This could be of advantage if it is necessary, during pipeline laying, to temporarily lower the pipeline onto the sea bed. For instance if weather conditions deteriorate, it is normal practice to temporarily lower the front or upper end of the pipeline on to the sea bed before raising it again when the weather conditions improve. Provided that sea water has not entered into the internal volumes, the submerged weight of the pipeline part to be raised will not have significantly changed. Even if water has entered into the internal volumes 7, the resiliently compressed compressible means 8 will expand again as the pipeline is raised and the pressure within the internal volumes will decrease thereby exhibiting the "reversibility" of the process. It will be appreciated that if the permeability of the concrete is low then the reversability of the process will also be slow. Therefore the degree to which the permeability of the concrete is controlled will depend upon the operational requirements of the pipe layers - i.e. high permeability results in rapid increase in submerged weight and similarly rapid reversal rate whereas low permeability results in a slow increase in submerged weight and similarly a slow reversal rate. By design, the rate of permeability of the concrete material to sea water can be selected to suit the pipe layers' needs. Ideally the uncompressed compressible means 8 will occupy from 10-50%, e.g. about 30%, of the volume of the cladding layer at atmospheric pressure. The compression of the compressible means 8 is dependent on the depth of submersion, but typically at the maximum designed submersion depth, the compressible means 8 will only occupy about 10% of the cladding layer volume.

Although it is preferred for the compressible means 8 to comprise a resiliently compressible plastics or rubber material, such as a closed-cell foamed plastics material, e.g. polyethylene foam, or a closed-cell foamed rubber material, other compressible means 8 can be provided. For example, the compressible means 8 may comprise air-filled or

gas-filled water imperveous envelopes. In this case it is preferred that the envelopes are made of elastically deformable material and are filled with pre-pressurised air or gas.

A particularly preferred embodiment is obtained if the compressible means is formed as a closed cell rubber material in which the "cells" contain a pre-pressurised gas, i.e. gas or air at pressures above atmospheric pressure at sea level. In this case, the initial compression of the compressible means does not commence until the pipeline has been submerged a certain distance. If the pipeline is subsequently raised from its submerged position, the compressible means is encouraged to expand to at least almost its original size before it reaches the surface of the water.

The matrix of the cladding material preferably comprises reinforced concrete material, but other hard, substantially incompressible materials could be used provided they are at least partially porous to water.

The rate of permeability to water could be controlled by applying a water impermeable layer to the outside of the concrete matrix which dissolves or decays over a period of time. Thus the onset of the increase in submerged weight can be delayed or controlled.

A typical specification for a pipeline to be laid on a sea bed at a depth of 150m below sea level is as follows:

Steel pipe (4) internal diameter - l

Steel pipe (4) wall thickness - 2.5cm

Density of steel - 7850 kgm"*^ Thickness of cladding (5) - 14.3 cm

Ratio of foam (compressible means) to concrete in cladding - 1:1

Average submerged weight of the pipeline between water surface and sea bed - 211 kg/m

Submerged weight of pipe on sea bed - 328 kg/m

Figures 4 to 7 illustrate alternative embodiments of the invention showing pipelines 20, 30, 40 and 50, respectively, for conveying gas or other fluids and for laying on a sea bed. Each of the pipelines 20, 30, 40 and 50 is formed by welding together lengths of steel pipe and have an anchoring structure applied as a layer thereto.

In Figure 4, the pipeline 20 has a layer of hard, incompressible material 21, e.g. of concrete, surrounding the steel pipe 22. Secured by strapping 23 to the clad pipe 22 is a layer in the form of an arcuate block 24 of resilient compressible material which is impermeable to water and which partly covers the material 21. In use, the compressible block 24 is able to contact sea water directly when the pipeline is submerged in water.

The pipelines 30 and 40 show alternative forms of anchoring structures in which strips of resiliently compressible material 31 and 41, respectively, are applied in overlapping and non-overlapping relationship to the pipelines which are clad with a non-compressible heavy anchoring material, e.g. concrete. In those embodiments, water is able to contact directly the compressible materials when the pipelines are submerged in water.

The pipeline 50 shows a further modified design in which steel pipe 51 is clad with non-compressible heavy material 52, e.g. concrete, having circumferential channels 53 formed therein. Annular rings 54 of compressible material are received in the channels 53. The heavy material 52 and rings 54 form a composite layer surrounding the pipe 51. The rings 54 are able to contact water directly on submersion of the pipeline 50 in water.

In all the embodiments shown in Figures 4 - 7, the compressible material is substantially non-permeable to

water and typically is elastically or resiliently deformable. Suitably the compressible material comprises a foam material, e.g. a closed cell plastics foam material such as a high density closed cell cross-linked polyethylene foam. Alternatively, however, the compressible material could comprise a plurality of air or other gas filled hollow spheres embedded in a matrix material. The compressible material may be bonded or strapped to the pipelines as required.

Instead of "solid" strips of compressible material being applied to the pipeline, one or more gas-filled, e.g. air-filled, compressible "tubes" may be arranged around the pipeline. The "tubes" may be arranged to extend along the length, e.g. axially, of the pipeline but are conveniently arranged in turns around the pipeline. The turns may be in the form of concentric rings (in which case many separate tubes would be required) but are preferably formed as helical turns in the form of a helical winding. Gaps may or may not be provided between adjacent tube lengths or turns. As previously described, the gas within the tubes is preferably at an elevated pressure in excess of 1 bar, e.g. at least 1.5 bar, preferably at least 4 bar, e.g. 6 bar, at sea level. The internal tube pressurisation will, to a large extent, be dependent on the depth of water to which the pipeline will be submerged, greater pressurisation being required at greater submersion depths. A particularly preferred design is shown in Figures 8 and 9 in which a hose 60 in a collapsed or flattened form is helically wound around a steel pipeline 61. High tensile strength webbing 62 is wound in a different helical direction to retain the hose 60 on the pipeline 61. Subsequently the hose 60 is internally pressurised so that the helically wound hose 60 adopts the form shown in Figures 8 and 9. A tough outer protective layer 64 which conveniently may allow water to pass therethrough is arranged around the tube-covered pipeline for protecting the latter during installation, e.g. when being fed from an installation barge. In Figure 8 the protective coating or layer 64 is only shown schematically.

The hose 60 may be of any convenient material but should preferably collapse flat when not inflated or internally pressurised with fluid. Suitable hose is made of a rubber or rubber-like material, e.g. of the type used for irrigating fields or the like. An end plate 63 may be provided to close each end of the wound tube assembly.

In use, on submersion of a pipeline in sea water, the surrounding sea water will compress the compressible material. If the compressible material contains pre- pressurised gas- or air-filled voids, e.g. the closed cells of a foam material, this compression may not occur until the pipeline is submerged to a predetermined depth. On compression of the compressible material, the volume of the pipeline is reduced thereby reducing the volume of water displaced by the pipeline and reducing the upthrust on the pipeline. Thus the submerged weight of the pipeline effectively increases. By using resiliently or elastically compressible material, the volume of the pipeline will increase, and the effective weight of the pipeline will decrease, if the pipeline is subsequently raised from its submerged position.

According to another aspect of the present invention there is provided a closed cell foamed plastics material in which the gas within the closed cells has a pressure greater than atmospheric pressure when the ambient pressure is atmospheric. The gas pressure within the closed cells should be sufficient to affect the compressible mechanical properties of the material and will conveniently exceed 1.5 bar and preferably will exceed 4 bar, e.g. 6 bar. A particular, but not necessarily exclusive, application of the plastics material is as the compressible means of the anchoring structure of the invention.

INDUSTRIAL APPLICABILITY

The invention is applicable in assisting the underwater

anchoring of apparatus, in particular, but not exclusively, in the anchoring of underwater pipelines on the sea bed at shallow depth, e.g. 60m, or greater depths, e.g. up to 500m or more.