WO1997006341A1

WO1997006341A1 - Riser

Info

Publication number: WO1997006341A1
Application number: PCT/NO1996/000181
Authority: WO
Inventors: Arnt Olufsen; Nils Terje Nordsve; Daniel Karunakaran
Original assignee: Den Norske Stats Oljeselskap A/S
Priority date: 1995-08-03
Filing date: 1996-07-15
Publication date: 1997-02-20
Also published as: NO305618B1; AU6710496A; NO953062L; NO953062D0

Abstract

Riser for transferring fluids between an installation (bottom installation) (6) on the seabed (5) and a floating installation (surface installation) (7) at the water surface, comprising a bottom segment (1) adapted to lie with a portion of its length on the seabed (5) from the bottom installation (6), an intermediate segment (2) being provided with buoyancy elements and having a length portion with an upward curvature, and an upper segment (3) the upper end (30) of which is connected to the surface installation (7). The position of the surface installation is adapted to be maintained within a given variation range. The buoyancy elements on the intermediate segment (2) are provided over a length of the same order of magnitude as the water depth at the installation site, and the bottom segment (1), the intermediate segment (2) and the upper segment (3) consist of stiff metal pipe sections.

Description

RISER

The invention relates to risers for transferring fluids between an installation (bottom installation) at the seabed and a floating installation (surface installation) on the sea surface, comprising a bottom segment adapted to lie with a portion of its length on the seabed out from the bottom installation, an intermediate segment provided with buoyancy elements and having a length portion which is upwardly curved, and an upper segment the upper end of which is connected to the surface installation, whereby the position of the surface installation is adapted to be maintained within a given range of variations.

These risers, being extensively employed in offshore oil and gas activities, as a rule have been based substan- tiallly on flexible pipe structures. Typically these are composed of helically wound reinforcement layers of steel with concentric sylindrical layers of rubber and/or plastic materials between the reinforcement layers and outside the outermost reinforcement layer, (and possibly inside the innermost layer) in order to obtain the high bending capa¬ bility considered to be required, i.e. the possibility of assuming configurations having a small radius of curvature at portions of the riser. The rubber and plastic materials are particularly subjected to unfavourable loads at high pressures and temperatures. The designation dynamic riser has been used in view of the fact that these during opera¬ tion will have to adapt themselves to various movements or displacements, specifically of the surface installation, such as a production vessel, to which the upper end of the riser is connected. Even if sophisticated positioning methods are employed, it is unavoidable that surface instal¬ lations or vessels for this purpose will be subjected to position displacements horisontally. In other words the horisontal distance between the anchoring points of the riser at the seabed and the installation or vessel at the surface, will vary between a minimum distance and a maximum distance. In addition to this variation also waves, sea streams and other effects will have influence on the con¬ figuration or curve shape which is assumed by the riser at any time during operation.

Known riser configurations according to what has been stated at the beginning above, have a very pronounced, lying S-shape, wherein a point or a short portion of the inter¬ mediate length range of the riser is elevated by means of consentrated buoyancy elements. Known configurations of this kind were presented at a seminar "Flexible Pipe Technology Seminar" in Oslo 1986 by C. Pettenati-Auziere, also published under the title "Flexible Dynamic Risers - State Of The Art", London Press Center, December 1985. Of particular interest among the riser configurations presented therein, is the one denoted "LAZY WAVE" (LW-riser) . This is the previously known design which it is reasonable to take as a starting point when considering the present invention.

The previously known very flexible risers being refer¬ red to above, among other things have the serious drawback that under the varying loads occuring in practical opera- tion, they may be subjected to damage with time and for example suffer from leakage, which involves complicated and expensive repair or replacement works. Another problem has to do with the large level differences that will occur between the elevated or upwardly curved portion of these risers, and an accompanying bight or curvature hanging down¬ wards, which together has an effect similar to a water seal or trap. An important factor in this connection is that the depending bight must not under any operational condition be allowed to get into contact with the actual seabed. In general there are a number of parameters having a more or less decisive influence on the choice of riser configurations and the behaviour of the riser under varying operational conditions. Among such parameters the following are mentioned here: The total length of the riser, the horisontal distance between its ends and besides the water depth at the installation site. The distribution of the buoyancy elements and the magnitude of buoyance per meter riser also are very significant, in particular when con- sidered in relation to the actual weight of the riser per meter and the weight or load represented by the products being conducted throught the riser. The operational pressure in the riser is a further parameter which must apparently be taken into account.

As numerical examples for illustration of the varying conditions to which these risers should be able to be adapted, the following are mentioned here:

Water depth from 300 to 1300 meters Internal riser diameter from 6 to 30 inches

Density of the riser contents - product from 180 to

1025 kilo/cubic meters

Internal pressure in the riser from 30 to 350 bar

Drift of the surface installation or vessel: 10% of the water depth, possibly 15% of the water depth during abnormal operation or a damaged anchoring system.

On the above background there is according to the invention provided an improved form of riser based upon the introductory statements above, where the novel and specific features in the first place consist in that the buoyancy elements on the intermediate segment are arranged over a length being of the same order of magnitude as the water depth at the installation site, and that the bottom segment, the intermediate segment and the upper segment consist of stiff metal pipe sections.

The combination of stiff metal pipe sections in con¬ trast to the previously employed flexible risers, and the dimension or length relationships given for the arrangement of buoyancy elements, have proven to lead to a very favourable riser configuration having a number of advan¬ tages. When the buoyancy elements shall cover a length of the same order of magnitude as the water depth at the installation site, this means that the buoyancy length shall be substantially comparable to the water depth, and in practice may vary within a range of 0,7 to 1,5 times the water depth. Preferably according to the invention however, the buoyancy elements of the intermediate segment are pro- vided over a length being within a range of 0,8 to 1,3 times the water depth. The relatively larger lengths of the inter¬ mediate segment with buoyancy elements will usually apply to shallow water depths, whereas deeper water will indicate a relatively shorter segment of buoyancy elements.

Compared to previously known riser designs the solution given here involves a substantially novel concept both with respect to the length or dimension relationships of the buoyancy or intermediate segment in relation to the water depth and the riser as a whole, and as far as its function in practical operation is concerned.

In addition to the novel and specific features just mentioned, there are stated in the claims further particular features that contribute to the advantages obtained with the invention. One such particular feature consists in the pre- bending of at least some of the stiff metal pipe sections, for use in the portions of the riser with the smallest radius of curvature in the installed riser.

The solutions given here for a novel riser configu- ration involve advantages in that the dynamic excitation will be taken up by movements of the intermediate segment of the riser without subjecting the riser to high bending stresses. Moreover only small movements will occur where the riser contacts the seabed, which among other things will reduce possible problems associated with interaction between this riser portion and a soft clay bottom, which can have an unfavourable suction effect. Another advantage consists therein that the present novel riser configuration is favou¬ rable with respect to the flow conditions compared to the previously known designs discussed above, among them the LW riser, as a result of the substantially reduced vertical distance between the highest and the lowest point or portion of the wave form ("water seal") , which contributes to re¬ ducing the risk of formation of liquid plugs at low flow rates. This new riser configuration also has the advantage of reducing the risk of collision in the case of several risers from the same underwater installation, because such risers can be deployed with a radial distribution, i.e. with a mutual angular distance. Finally is mentioned that the pre-bending of pipe sections as referred to above, will involve reduced stress levels on movements away from the static configuration in the middle position or normal condi- tion of the riser.

In the following description the invention will be explained more closely with reference to the drawings, where:

Fig. 1 in schematic elevation illustrates an embodiment of the riser according to the invention, extended between a bottom installation on the seabed and a surface vessel at which the upper end of the riser is suspended. Fig. 2 in longitudinal section shows a short portion of the riser provided with buoyancy elements.

Fig. 3 shows dynamic movements of a riser according to

Fig. 1, at a close position of the surface vessel, and Fig. 4 in a corresponding way shows dynamic movements of the riser at a far position of the surface vessel.

The schematic illustration of a riser 1, 2, 3 in Fig. 1 is related to a diagram showing the water depth in meters and horisontal distance also in meters. A surface instal¬ lation in the form of a vessel 7, for example a production ship, is anchored or positioned by means of some kind of known anchoring or positioning system (not shown) so as to be maintained in location within a limited range of varia¬ tions in relation to a bottom installation 6 at the seabed 5. As will be seen from the diagram in Fig. 1, the water depth in this example is approximately 300 meters.

The complete riser in Fig. 1 comprises three segments, namely a bottom segment 1, an intermediate segment 2 and an upper segment 3 the upper end 30 of which is connected to a suitable coupling device which may well be provided at the bottom of the vessel 7. There is indicated an angel 30A between the upper riser portion and the vertical in the connection point 30.

In a manner per se the bottom segment 1 is adapted to lie with at least a part of its length on the seabed out from the bottom installation 6, whereby the lower end 10 of the riser under all circumstances shall be supported by the seabed and to the greatest possible extent never be elevated from the seabed, in order thereby to avoid tensional stresses at the point of entering into the bottom installa¬ tion 6.

The intermediate segment 2, which has substantial significance in the present context, extends from a point where the riser, i.e. the bottom segment 1, in general is always elevated from the seabed 5, with a transitional portion 12 between the bottom segment and the intermediate segment. Substantially the whole length of the intermediate segment 2 is provided with buoyancy elements, which can be of a previously known design. See Fig. 2 which is to be described below. Another transitional portion 23 is indicated between the upper segment 3 and the intermediate segment 2.

The length of the intermediate segment 2 with buoyancy elements in the examplary embodiment of Fig. 1, is approxi¬ mately equal to 1,2 times the water depth. The upward cur¬ vature of the intermediate segment 2 preferably is clearly smaller than the downward curvature at and near the transitional portions 12 and 23, in particular the latter transitional portion. These geometrical relationships give this riser configuration a characteristic curve which in¬ volves the location of the buoyancy elements further down on the complete riser and over a substantially longer length than in previously known riser configurations, at the same time as the dimensions of the buoyancy elements, i.e. the thickness or outer diameter, is smaller than what has been usual previously.

A further characteristic feature being associated with the geometrical relationships mentioned above, consists therein that the vertical distance Dv indicated between the highest point of intermediate segment 2 and the lowest point of transitional portion 23, is substantially smaller than the length of the intermediate segment 2 with buoyancy elements. The buoyancy elements are shown purely schema¬ tically at 2A in Fig. 1, but are shown somewhat more in detail in Fig. 2.

In Fig. 2 there is shown a length of a stiff metal pipe section 2S on which there is mounted a number of buoyancy elements 2A. These are accordingly separate elements being attached with a spacing along the length of pipe section 2S. In a practical example with a metal pipe 2S having an inter¬ nal diameter of 12 inches and a wall thickness of 20 milli- meters, the radial thickness of buoyancy elements 2A can be for example 90 millimeters, i.e. with an outer diameter at the buoyancy elements 2A of a little more than 40 centime¬ ters. It will be realized that such separate buoyancy ele¬ ments 2A could easily be attached also to pre-bent metal pipe sections when such sections shall be employed.

The diagrams in Figs. 3 and 4 relate to the embodiment of riser described with reference to Fig. 1, and the curve shapes in Figs. 3 and 4 are calculated on the basis of a mathematical model, since experiments at full scale is difficult and expensive with structures of the kind discus¬ sed here. The dynamic movement of the riser in the two alternatives in Figs. 3 and 4, is illustrated in the form of the outer contours of maximum excursions or dynamic move¬ ments in the close position (Fig. 3) and the far position (Fig. 4) , respectively. It can be added that the riser is here considered to be made of titanium, which is of interest in view of, inter alia, the lower modulus of elasticity of titanium compared to steel. This choice of material in certain cases can be of much significance for a successful solution based upon the invention.

It is obvious that the invention makes possible a number of modifications and variants in relation to what has been described above, in particular with reference to the Figs, of drawings. As far as choice of materials as just mentioned, is concerned, it is obvious that certain length portions of the complete riser can be made of steel pipe secctions, whereas other portions can be based on titanium. This among other things, may lead to a riser having a weight per running meter with a varying magnitude along the riser.

Correspondingly it applies to the buoyancy elements that these can have varying dimensions or numbers along the intermediate segment, so that the buoyancy will be relatively larger along certain portions of the intermediate segment, for example at the end portions of the segment. All in all it is important that the buoyancy is adjusted accor¬ ding to the conditions of interest for the riser concerned, including the fluids or products it shall convey. In this connection it is clear that the buoyancy per meter riser is substantially smaller in embodiments based on the invention, than in previously known riser designs, where the buoyancy to a high degree is concentrated at a point or a short portion of the riser, i.e. the portion which it is desired to elevate in order to form the pronounced S-shape. The relationships can also be described by saying that the intermediate segment 2 with buoyancy according to the invention, only is required to support relatively very short, adjacent riser segments, corresponding most to the transitional portions 13 and 23 being indicated in Fig. 1. Thus the buoyancy of intermediate segment 2 is to a high degree adapted to only support this segment itself and its contents of fluids.

As far as the stiffness or the flexibility of such a riser composed of individual or stiff pipe sections is concerned, it is clear that a certain bending will always be possible and that the invention is based on such geometric relationships that acceptable curvatures and loads will occur. In this connection also the product or fluid pressure in the riser plays a role, whereby an increased pressure to some degree will lead to an increased stiffness. However, in practical applications being of interest, this effect on the stiffness of the riser will not represent any problem of significance.

Claims

C l a i m s

1. Riser for transferring fluids between an installation (bottom installation) (6) on the seabed (5) and a floating installation (surface installation) (7) at the water sur¬ face, comprising a bottom segment (1) adapted to lie with a portion of its length on the seabed (5) from the bottom installation (6) , an intermediate segment (2) being provided with buoyancy elements (2A) and having a length portion which is curved upwardly, and an upper segment (3) the upper end (30) of which is connected to the surface installation (7) , whereby the position of the surface installation is adapted to be maintained within a given variation range, c h a r a c t e r i z e d i n that the buoyancy elements (2A) on the intermediate segment (2) are provided over a length of the same order of magnitude as the water depth at the installation site, and that the bottom segment (1) , the intermediate segment (2) and the upper segment (3) consist of stiff metal pipe sections.

2. Riser according to claim 1, c h a r a c t e r i z e d i n that the buoyancy elements (2A) on the intermediate segment (2) are provided over a length comprising a range of 0,7 to 1,5 times the water depth, preferably within the range 0,8 to 1,3 times the water depth.

3. Riser according to claim 1 or 2, c h a r a c t e r i z e d i n that the upwardly curved length portion of the intermediate segment (2) is less curved than a downwardly curved transitional portion (23) between the intermediate segment (2) and the upper segment (3).

4. Riser according to any one of claims 1-3, c h a r a c t e r i z e d i n that the vertical distance (D_v) between the highest point of the intermediate segment (2) and the lowest point of the downwardly curved transi- tional portion (23) between the intermediate segment (2) and the upper segment (3) , is substantially smaller than the length of the intermediate segment (2) being provided with buoyancy elements (2A) .

5. Riser according to any one of claims 1-4, c h a r a c t e r i z e d i n that some of the stiff metal pipe sections are pre-bent at least for the portions being or becoming most curved.