The invention firstly relates to a subsea pipe-in pipe (PIP) system for the thermally controlled transport of fluids, comprising an inner pipe for conveying said fluid and an outer pipe surrounding the inner pipe and separated there from by a thermally insulating space positioned between the inner pipe and outer pipe.

Such PIP systems are known for the transport of, for example, hot or cold fluids. These PIP systems are subject to thermally induced deformations, specifically expansions and contractions. Thus measures are required for avoiding high stresses in the PIP system (especially when the PIP system is connected to a stationary structure) . The state of the art provides several manners in which such high stresses may be avoided, such as the use of a thermal expansion loop or the use of flexible joints in the PIP system.

An expansion loop generally uses straight pipe sec tions connected by elbows welded at joints for providing a flexibility in a U-bend. However, a PIP system of the present type due to its construction and inherent stiffness (among others as a result of the double wall of the PIP system and the provision of connectors between the walls of the inner pipe and outer pipe for limiting stresses due to differential expansions and contractions) requires very large dimensions of such an expansion loop, which in many cases is impractical. Further such an expansion loop will rest on or is buried in the seabed and when the soil generates high friction the re quired deformation is hindered.

Examples of flexible joints are metal bellows expan sion joints (also referred to as compensators) or sliding joints with appropriate sealing gland packings. However, the use of these flexible joint especially may be problematic for the inner pipe of a PIP system which is subjected to the most extreme temperatures.

A more general disadvantage of the state of the art methods for reducing stresses in PIP systems is the limited capability of a load transfer between the inner and outer pipe. The load transfer is typically limited as the insulating space between both pipes is typically used to reduce heat transfer between the medium in the inner pipe and the environ ment (specifically seawater) outside the outer pipe. Due to the thermal insulation provided by the insulating space the inner and outer pipes will have a different temperature. This temperature difference as well as potential differences in ma terial properties between both pipes will generate different expansions/contractions of these pipes. Thus, too high inter nal thermal stresses in PIP systems should typically be avoid ed by the provision of connections between the inner and outer pipes .

It is an object of the present invention to provide an improved PIP system.

In accordance with the present invention the subsea pipe-in-pipe system is characterized in that over at least a length section of the length of the PIP system the outer pipe is absent, wherein the inner pipe at said length section is surrounded by a closed stationary compartment from which sea water is evacuated in at least such an amount that the section of the inner pipe within said closed compartment is not in contact with the seawater, wherein the outer pipe in a sealed and movable manner is connected to a wall part surrounding a hole in a wall of the compartment through which the PIP system extends, and wherein the section of the inner pipe within said closed compartment extends in a pattern for defining at least one expansion loop.

As a result of such a construction a part of the PIP system only comprises the inner pipe and not the outer pipe. Said part of the PIP system is more flexible and thus may be used for defining an expansion loop (of which the dimensions may be kept within reasonable limits) . This expansion loop is located within the closed stationary compartment (and thus ba- sically in a dry space as far as the position of the expansion loop within the compartment concerns) out of contact with the seawater and as a result a large heat transfer between the in ner pipe and seawater can be avoided notwithstanding the ab sence of the outer pipe at the expansion loop. The provision of a thermal insulation in the dry space is conceivable too.

The sealing (water tight) and movable connection of the outer pipe with the said wall part allows deformations of the outer pipe while maintaining the required closed configuration of the compartment within which the expansion loop is located.

It is noted that the pattern also may define more than one expansion loop. The expansion loop may be of any ap propriate type and shape.

In one embodiment the expansion loop extends substan tially free from any part of the compartment. This means that the part of the PIP system defining the expansion loop within the closed compartment (thus the part of the PIP system where only the inner pipe is present) does not (or only in a very limited way) engage the closed compartment and thus is free to move as required for reducing stresses.

In one embodiment of the PIP system the closed com partment is part of a stationary offshore structure resting on the seabed, wherein the section of the inner pipe within said closed compartment is an end section of the inner pipe which connects to a respective piping of the offshore structure.

In such an embodiment the closed compartment is de fined by a specially designed part of the offshore construc tion (for example an internal space made watertight) and the PIP system ends in such offshore construction (generally mean ing that the PIP system only has to traverse a single wall of the compartment where the outer pipe is connected to a respec tive wall part in the previously defined manner) .

In an alternative embodiment of the PIP system ac cording to the present invention the closed compartment is de fined in a separate structure resting on the seabed and com prises two holes in its wall, wherein the outer pipe has a first outer pipe section which in a sealed and movable manner is connected to a wall part surrounding a first one of said holes in the wall of the compartment and a second outer pipe section which in a sealed and movable manner is connected to a wall part surrounding a second one of said holes in the wall of the compartment, wherein the section of the PIP system com prising the inner pipe surrounded by the second outer pipe section of the outer pipe is intended to be connected to a piping of, for example, a stationary offshore structure rest ing on the seabed.

In such an embodiment the PIP system extends into and next again out of the closed compartment before, for example, reaching the stationary offshore structure. This means that the PIP system has to traverse two walls of the compartment where the outer pipe is connected to a respective wall part in the previously defined manner.

In one embodiment the seawater is completely evacuat ed from said closed compartment.

In another embodiment the outer pipe defines an outer pipe end facing the closed compartment and wherein said outer pipe end in a sealing manner is connected to the inner pipe.

The outer pipe end facing the inner compartment means the end of the outer pipe which is located in the vicinity of the re spective wall part of the compartment. The sealing allows to improve the insulating effect of the space between the inner and outer pipes.

It is conceivable that said outer pipe end is welded to the inner pipe.

In one embodiment the thermally insulating space po sitioned between the inner pipe and outer pipe defines a vacu um space. The amount of vacuum may be varied.

In one embodiment of the PIP system according to the present invention the outer pipe defines an outer pipe end facing the closed compartment, which outer pipe end ex-tends through the respective hole into said compartment beyond the respective wall part of the compartment. This means that the outer pipe traverses the respective hole in the wall of the compartment. This may facilitate arriving at a reliable seal between said outer pipe and wall In one embodiment the outer pipe in a sealed and mov able manner is connected to said wall part of the compartment by a flexible joint. Such a flexible joint, for example, may be of a bellow type or of a sliding type.

Preferably the inner pipe is intended for conveying a cryogenic liquid.

In a second aspect the invention relates to an assem bly of stationary offshore structure resting on the seabed and subsea pipe-in-pipe (PIP) system according to the present in vention .

Hereinafter the invention will be elucidated while referring to the drawings, in which:

Figure 1 in a schematical cross section illustrates a first embodiment of the PIP system;

Figure 2 in a schematical cross section illustrates a second embodiment of the PIP system.

Firstly referring to figure 1 a lower part of a sta tionary offshore structure 1 (for example a loading crane for transferring a cryogenic liquid to a vessel) resting on the seabed 2 is illustrated. The offshore structure 1 comprises an internal piping 3 which in the illustrated embodiment is con nected to the structure 1 using an anchor plate 18. A subsea pipe-in-pipe (PIP) system (or pipeline) 4 for the thermally controlled transport of a cryogenic liquid towards the off shore structure 1 is provided which generally, as is known per se but is not visible here, for at least its major part rests on or (at least partly) is buried in the seabed 2. The PIP system 4 comprises an inner pipe 5 for conveying the cryogenic liquid and an outer pipe 6 surrounding the inner pipe. Between the inner pipe 5 and outer pipe 6 an insulating space 7 is de fined (which may be evacuated, but which also may house an in sulating material, for example material 17 surrounding inner pipe 5 as illustrated in figure 1 only) . At (regular) distanc es the inner pipe 5 and outer pipe 6 are connected by connect ors 8 (as is known per se too) .

The offshore structure 1 has a wall 9 in which a hole 10 is formed through which the PIP system 4 extends. Internal ly, the offshore structure defines a closed compartment 11 which defines a dry space. A length section (or part) of the length of the PIP system 4 within said closed compartment 11 does not comprise the outer pipe 6 but only the inner pipe 5.

It is noted that seawater may be totally absent in the closed compartment 11, but it is also conceivable that seawater has been removed from the closed compartment only to such an extent that the PIP system 4 within the closed com partment 11 does not come into contact with the seawater.

The section of the inner pipe 5' within the closed compartment 11 extends in a pattern for defining at least one expansion arrangement or loop 12 (of which the shape and ori entation may differ from what has been illustrated in figure 1) . The expansion loop 12 allows to reduce thermally induced stresses in the PIP system 4.

In the illustrated embodiment this section 5' of the inner pipe 5 defines an end section which connects to the pip ing 3 of the offshore structure 1 (for example at a joint 13 of any appropriate type) .

The outer pipe 6 has an outer pipe end 6' which ex tends through the hole 10 in the wall 9 and, in the illustrat ed embodiment, ends just inside of said wall 9. This outer pipe end 6' is connected to the inner pipe 5 in a sealing man ner, for example through a weld or special pipe piece 14. A flexible joint 15 (for example of a bellow type or a sliding type able to cope with the local water pressure) assures that the outer pipe 6 connects to a wall part surrounding the hole 10 in a sealing and movable manner. Thus the outer pipe 6 may move relative to the offshore structure 1 for coping with thermally induced deformations (expansions/contractions) .

In the embodiment illustrated in figure 2, the closed compartment 11 is defined in a separate structure 16 resting on the seabed 2. This structure 16 comprises two holes 10 in its wall 9 and the outer pipe has a first outer pipe section 6" which in a sealed and movable manner is connected to a wall part surrounding a first one of said holes 10 in the wall 9 of the compartment 11 and a second outer pipe section 6 ' " which, likewise, in a sealed and movable manner is connected to a wall part surrounding a second one of said holes 10 in the

where only the inner pipe 5 is provided defining the expansion loop 12 (thus inner pipe section 5 ' ) is located between the two holes 10 inside of the closed compartment 11. Both outer pipe sections 6" and 6' " are connected to the wall 9 through flexible joints 15. An anchor plate 19 may be provided con necting the inner pipe section 5 ' to the structure 16.

Supposing a flow of the cryogenic liquid from left to right in figure 2, the section of the PIP system 4 comprising the inner pipe 5 surrounded by the second outer pipe section 6' " of the outer pipe is intended to be connected to a piping of, for example, a stationary offshore structure resting on the seabed (not illustrated in figure 2) . Because the expan sion loop 12 within the closed compartment 11 of the separate structure 16 can cope with thermally induced deformations, the configuration of said section of the PIP system 4 may be con ventional .

For long PIP systems the axial displacement can be typically several centimetres. Their bending flexibility (par ticularly of relevance next to the stationary offshore struc ture) will be based on the soil characteristics of the seabed. For weak soils the PIP system may displace overtime to a deep er location, resulting into a local bending of the PIP system at the stationary offshore structure.

Preferably the PIP system will be installed at the seabed using a pull-through method, in which the PIP system will be outfitted with an end cap and a pulling head. In the pull-through method the PIP system will be assembled above the water (typically onshore, for example at a beach area), and next will be pulled towards the offshore structure using a pull-in wire that is attached to the pulling head (for example using a pull-in arrangement that can be located in-line of the arriving PIP system but at a distance from the offshore struc ture; typically such pull-in arrangement consist of a wire winch located on a floating vessel or barge; large angles in the pull-in wire are avoided by sufficient distance between the pull-in winch and the hawser pipe end or by using clump weights resting on the seabed that guide the pull-in wire) . In the dry space inside the closed compartment of, for example, the offshore structure a split type hawser pipe will be pro vided that allows the pulling through of the PIP system through the structure. Typically the hawser pipe will be in line with and at approximately the same elevation as the PIP system to ensure that only axial pull forces are applied to the PIP system.

The PIP system will be pulled-in the split type haws er pipe until the outer pipe can be attached to the wall of the closed compartment using the flexible joint. After estab lishing this flexible joint the pull wire can be disconnected and after removal of this pull wire the hawser pipe can be blinded-off (for example using watertight closures at both its ends) to ensure that the hawser pipe becomes watertight. Fi nally this split type hawser pipe can now be opened in the dry space of the closed compartment to connect the inner pipe of the PIP system to the piping of the offshore structure.

As an alternative, the step of establishing a pull- through method for installation of the PIP system, the off shore structure can be delivered at the onshore PIP system ar ea to connect the PIP system to the dry space inside the closed compartment of the offshore structure. The dry space will provide buoyancy to the offshore structure when it is pulled to its offshore location. This buoyancy will limit or even completely reduce the friction between the offshore structure and the soil (seabed) during the pull. During the pull of the offshore structure the PIP system's pulling head will be fixed to the offshore structure to avoid excessive loads on the flexible connection between the outer pipe of the PIP system and the closed compartment of the offshore struc ture .

As yet an alternative, the step of establishing a pull-through method for installation of the PIP system, the PIP system can be pulled through the closed compartment in the offshore structure, while this closed compartment is flooded. After provision of the flexible joint (s) between the PIP sys tem and the offshore structure as well as after installation of the blind on the hole in the structure that allows the pull through of the PIP system pulling wire, the water in the closed compartment can be removed. Inside the closed compart ment the outer pipe of the PIP system may be connected to the inner pipe. The inner pipe will be routed inside the now dry space of the closed compartment in a way that allows expansion or contraction of the PIP system to avoid too high stresses in the PIP piping (thus defining at least one expansion loop) .

The invention is not limited to the embodiments de scribed before which may be varied widely within the scope of the invention as defined by the appending claims. As such it is noted that the definition of a "PIP system" as used in the present context may encompass a single pipe-in-pipe system but also multiple pipe-in-pipe systems, such as for example triple pipe-in-pipe system where two insulations are applied between the void spaces of three pipe-in-pipes.