Pipe-laying barges of the S-lay and J-lay type that are currently available have sufficient tensioner capacity to lay down pipelines at great depths down to about 1000 to 2000 meters. The specific depth depends to a great extent on the diameter, wall thickness and weight of the pipeline that is to be laid. In order to increase the maximum depth at which today's conventional equipment is able to lay pipelines, it is necessary to reduce weight, add buoyancy or both. The alternative is to increase vessel sizes and tensioner capacity, but this is not considered commercially viable.

Another aspect of deepwater production is the high pressure differential between the surface and the seabed. The hydro- static pressure exerted on the production fluid at this depth may under some circumstances contribute to the forma- tion of hydrates. This makes hydrate formation a far greater challenge than at more normal depths. Since the formation of hydrates depends on pressure and temperature, the detrimental effect of the increased pressure may be offset by supplementary thermal insulation. Presently, dif- ferent kinds of insulation materials are in use, i. e. insu- lating syntactic foam or rubber, for pipelines at great depths. However, these materials have so far proven to be expensive.

The use of lightweight concrete has previously been de- scribed in US patent no 4 393 901. Here the lightweight concrete, which by nature is porous, is provided as a sur- face layer directly exposed to the surrounding seawater.

Water is gradually absorbed and as a consequence buoyancy and thermal insulation will be lost as water is absorbed.

The depth at which this can be used will be limited to the concrete's uni-axial compressive strength and the capacity

of the pipe to support external water pressure without buckling or imploding. This design derives no significant strength from the surrounding concrete.

The use of pipe-in-pipe designs where the annulus between the pipes is filled with insulating materials such as min- eral fibers is well known in the industry. Norwegian patent no 304662 and UK patent 2 271 410 describes the use of mi- cro-spheres which as a solid will provide strength and re- strain the external pipe from collapsing under external pressure. Such particles (micro-spheres) will have point- wise contact and high stress will be set up in these con- tact points. This will severely limit the compressive strength of the micro-spheres and the external hydrostatic pressure which can be supported. The injection of epoxy or cement based adhesive bonding agents to improve shear strength is also described. This will, however, not have significant impact on compressive strength as the micro- spheres will still make point contact through the much softer adhesive. The capacity of this pipe to sustain ex- ternal pressure will not be significantly different from the pipe on its own.

It is the aim of the present invention to provide a deepwa- ter pipeline that solves the two aforementioned problems in a cheap and reliable manner, which does not require modifi- cations of existing pipe-laying equipment or special auxil- iary pipe-laying equipment. The characterizing features of the invention are given in claim 1. Preferable features and embodiments are given in the dependent claims.

The present invention is described in reference to the at- tached drawings, of which: Fig. 1 shows a general overview of one possible pipe-laying technique, and

Fig. 2 shows a schematic section of a preferred embodiment according to the invention.

Fig. 1 shows a general overview of a pipe-laying technique of the S-lay type. The free-span portion 1 of the pipeline 2 has to be carried by the pipe-laying barge 3. In order to prevent excessive sagging of the free-span portion 1, the pipeline is held in tension by the barge 3. The deeper the seabed, the heavier the load is on the barge 3 and tension equipment. The tension is exerted by tension equipment on board of the barge and is mainly dependent on friction forces between the tension equipment and the pipeline 2.

Obviously, the pipeline 2 may be exposed to such high clamping forces that it may be crushed and/or damaged.

Fig. 2 shows a schematic view of one embodiment according to the present invention. The applicant has found that a lightweight concrete mantel layer 5 around the internal thick walled pipeline 6 may provide both buoyancy and insu- lation to a pipeline 2 that is to be laid down in great wa- ter depths.

As noted lightweight concrete has a modest strength. Ac- cording to the present invention it is provided with an ex- ternal thin-walled tube or skin 7, preferably of metal but possibly of any other suitable material, which reinforces the lightweight concrete mantel layer 5 against the pres- sure applied from the tensioning equipment and hydrostatic pressure. Such tri-axial confinement causes a two to three- fold increases of the compressive stress that can be sus- tained. The invention also includes the use of high strength concrete technology pioneered in Norway. The tech- nology of producing high strength concrete involves the use of very carefully graded particles which pack together in such a way that contact stresses between particles become very uniform. This gives a several-fold increase of com- pressive strength.

For deepwater applications the external hydrostatic pres- sure which can be supported is a primary consideration.

Sufficient compressive strength of the concrete is one im- portant feature. The second aspect is to prevent the unsta- ble collapse of the pipe under external pressure. The high strength lightweight concrete according to this invention will develop extensive composite action between the two pipes, which will vastly increase the collapse strength.

This makes it possible to reduce the wall thickness of the steel pipeline itself considerably.

As also noted, lightweight concrete also tends to absorb water. In order to achieve the most favorable heat conduc- tivity rates, it needs to be kept dry. The external thin- walled tube or skin 7 therefore also prevents moisture from penetrating into the concrete mantel layer 5 and thereby contributes to the insulating capability of the concrete mantel layer 5. Finally, the external thin walled tube or skin 7 may act as formwork for casting of the concrete man- tel layer 5. In one embodiment, the internal thick walled pipeline 6 is centrally positioned within the external thin walled tube or skin 7, whereupon lightweight concrete is poured into the annulus formed between the internal thick walled pipeline and the external thin walled tube or skin 5. The lightweight concrete mantel layer 5 is thereafter allowed to settle and cure. The ends of the annulus may fi- nally be covered by suitable, annually shaped covers or by other suitable means.

In summary it is seen that high strength lightweight con- crete with a confining carrier pipe gives the following significant advantages: The lightweight aggregate concrete provides permanent buoyancy and insulation.

The external carrier pipe is supported by the concrete which has a tenfold greater compressive strength than the aggregate would have as particles with or without a

cement past or other form of adhesive coating. This makes it possible to use a very thin walled carrier pipe.

The external carrier pipe prevents ingress of water that otherwise would reduce the buoyancy and increase the thermal conductivity of the concrete.

The composite action of the concrete and the two steel pipes increases the crushing strength of concrete through confinement. It also dramatically increases the external pressure that can be sustained by. the pipeline without buckling, as for deep-water applications buck- ling of the pipe under external hydrostatic pressure will determine wall thickness. This reduces the total amount of steel required in deepwater applications.

Non-limiting examples of suitable, lightweight concrete for enhancement according to the present invention are given in the following table: Table 1 Lightweight concrete Dry density Heat conductivity type Kg/l W/m K Exfoliated vermiculite 0.40-0.80 0.11-0.22 Air entrained cement 0.35-0.80 0.10-0.20 Foamed slag 0. 65-1.00 0.12-0.50 Pumice 0.70-1.00 0.22-0.33 Pipe-laying with concrete weight coating is a well- established technique. The techniques used according to the present invention will include many of the same elements, but the technical challenges will differ. Developing suffi- cient strength of the external thin walled tube or skin 7 in combination with the sufficient crushing strength of the concrete mantel layer 5 must be balanced against obtaining the desired heat conductivity or density. It is envisaged that double joints with lightweight concrete surrounded by an outer steel tube or skin 7 will be supplied to a conven- tional lay barge from an onshore facility. Field jointing techniques will be similar to weight coated pipelines.