GB2394017A

GB2394017A - Pipe-in-pipe flowline joint

Info

Publication number: GB2394017A
Application number: GB0223197A
Authority: GB
Inventors: Mark Alexander John Dixon; Daniel Edward Jackson
Original assignee: Deepsea Engineering & Man Ltd
Current assignee: Deepsea Engineering & Man Ltd
Priority date: 2002-10-07
Filing date: 2002-10-07
Publication date: 2004-04-14
Also published as: GB0223197D0

Abstract

The joint comprises an inner pipe end 12 joined to a corresponding pipe end 14, a respective outer pipe end 18, 20 surrounding each inner pipe end and from which the inner pipe ends project axially, and a fibre reinforced plastics collar 28, 30 mechanically connecting the inner and outer pipe ends. Thermal insulation 34, 36 surrounds the inner pipe ends. An outer shell 32 is applied between the collars 28, 30. Plastics rings 24, 26 are used to space the inner and outer pipe ends, to seal the interpipe annuli and as formers for the collars 28, 30.

Description

PIPE-IN-PIPE FLOWLINE JOINT

Background of the Invention

This invention relates to joints formed between pipe sections used for transporting 5 hydrocarbons and other fluids in offshore developments. Deepwater field developments

often require Bowlines and risers with high thermal efficiency both under normal flowing conditions (steady state) and during shut-down/shut-in (transient). The temperature of the production fluid at the wellhead can be in excess of 100 C, but must not be allowed to fall below a critical value, to avoid blockage of the Bowlines and other production equipment by 0 precipitated solid hydrates and paraffinic deposits. The ambient temperature at the seabed is often close to 0 C so heat loss can be a problem in developments where the production fluid has to be transported long distances through subsea Bowlines or risers. Insulated Bowlines allow extended cooldown of passive systems and/or reduced energy consumption of systems using active (additional) heating.

Pipe-in-pipe systems are a proven technology for Bowlines requiring a low OHTC (overall heat transfer coefficient) and installation of hundreds of kilometres are planned over the next few years. Pipe-in-pipe systems comprise an inner pipe containing the fluid to be transported, enclosed within an outer pipe designed to withstand hydrostatic pressure and 20 other external loads. The annulus between the pipes can be filled with high efficiency thermal insulation materials, as well as accommodating other components, such as active electrical heating systems or thermal energy stores (e.g. phase change materials). The insulation therefore does not have to be rigid and the annulus can also be evacuated and/or filled with insulating gas to further decrease the OHTC.

The joints between adjacent sections of a pipe-in-pipe Bowline must be able to transmit the forces experienced during installation and service. Known joints in widespread use have inner pipe sections extending beyond the outer pipe ends and butt-welded together. The outer pipe ends are swayed and circumferentially welded onto the inner pipe. (See, for 30 example, W00033990). To stiffen these joints in bending and sometimes to allow transmission of tensile stress between adjacent sections of outer pipe, an outer sleeve is slid over the outer pipe and in some cases this sleeve is mechanically connected between the

outer pipe ends. The annulus between the sleeve and inner pipe is filled with insulation. The swayed outer pipe ends can serve to transmit stresses between the inner and outer pipes.

These stresses can be very large, particularly as the Bowline is bent and as the necessary tension is applied during the laying operation. The welded and swayed outer pipe ends s provide a thermal bridge between the inner and outer pipes and the circumferential weld of the swage onto the Bowline adversely affects the inner pipe's main Bowline weld carrying the product by changing the inner pipe's mechanical properties locally requiring extensive Bowline weld qualification testing. The heat loss at such joints can be reduced by providing a large overlap between the sleeve and outer pipe ends, with insulation extending into the lo annul) between the outer sleeve and each pipe end. However this greatly increases the bulk of the joint, making it more expensive and time consuming to install. In a further attempt to reduce heat loss at the joints, pairs of elongate metal sleeves, each circumferentially welded at one end to the inner pipe and at the other end to the outer pipe, have been used to provide the mechanical connection between the inner pipe and outer pipe ends. (See, for example, 5 US 5447339). However, in all of these known joints, the heat loss is still significant.

Summary of the Invention

We have sought to provide a pipe-in-pipe joint for a Bowline or riser system that is less bulky, uses less materials and is quicker to install in the field than prior joints, whilst still

20 giving adequate or even improved mechanical properties and thermal insulation.

Accordingly, the present invention provides such a joint comprising an inner pipe end joined to a corresponding pipe end, an outer pipe end surrounding the inner pipe end and from which the inner pipe end projects axially, a collar mechanically connecting the inner and outer pipe ends and thermal insulation surrounding the inner pipe end, in which the collar is 25 formed from fibre reinforced plastics (FRP) bonded to the inner and outer pipe ends. The FRP collar has a much lower thermal conductivity than the previously used welded swages or metallic collars. At the same time, surprisingly, it has been found that the FRP collar and its bonds to the inner and outer pipe ends will perform adequately in bending of the pipe joint and in transmitting axial forces between the inner and outer pipes.

Because the FRP is itself a reasonably good thermal insulator, the thermal bridges found in prior art joints between the inner and outer pipe ends are substantially eliminated. Thus there

may not necessarily be the need to extend the insulation applied to the exterior of the joint so as to overlap the outer pipe ends. Therefore, in one preferred form, the FRP collar of the present invention has a first end bonded to the outside of the outer pipe end, the joint comprising a surrounding outer shell having one end fitted to the collar first end and another s end fitted to the first end of a corresponding collar of an adjacent outer pipe end, an annular space defined between the outer shell, inner pipe ends and outer shell providing or containing thermal insulation.

To keep the inner and outer pipe ends properly (e.g. concentrically) spaced as the joint is lo formed, thermally insulating spacing rings may be inserted between them. These may be formed from a suitable plastics material, e.g. LDPE, PU, Ureol bi-resins, either solid, as rigid foam or fibre reinforced. Because the FRP collar may be slightly permeable to seawater, the spacing ring can also be used to seal the annulus between the inner and outer pipes.

Preferably therefore, it is made from an impermeable material and forms a continuous Is annulus sealingly bonded to the inner and outer pipe ends. The outer surface of the spacing rings may be shaped to provide a smooth transition between outer surfaces of the inner and outer pipe ends, thereby serving as a former for laying up the FRP collar.

The FRP collar can be fabricated by numerous known techniques, such as filament winding, 20 or by using pre-preg "socks" slipped over the assembled pipes and laid up in position in the joint to form a multilayered construction. Vacuum infusion may also be used as a manufacturing technique.

The reinforcing fibre may be glass fibre and/or carbon fibre. The innermost layers of the 2s FRP collar are preferably reinforced with glass fibre, preventing any corrosive effects between the carbon and the steel pipe. The FRP matrix material is likewise chosen for compatibility with the reinforcing fibre, fabrication method and service temperature. Resins and fibres suitable for use in the present invention are available commercially (for example T700 Carbon Fibre from Toray Carbon Fibers (America) Inc., 2030 Highway 20, Decatur, 30 AL 35602, USA; 20 epoxy resin from Bakelite AG, Varziner Strasse 49, D-47138 Duisberg Meiderich, Germany and TCS Composites UP 3325. The fibre may be impregnated with resin shortly before being used in preparing the collars. Alternatively the fibres may be

employed in the form of strips of pre-impregnated fibres (so called 'prepegs').The collar-to-

pipe bonds can be achieved using high-performance pre-formed adhesive tapes, applied to the pipe ends before the FRP collar. The adhesive is then cured simultaneously with the collar. Additional additives may be used to provide additional adhesive qualities to the fibre resin matrix when using a wet fibre resin system before the system is cured. Alternatively for pre-

preg fibre resin systems suitable adhesive films may be employed between the pre-preg and the metal surface before curing. Other systems would include the use of a suitable adhesive 0 film or paste to be applied to either the pipe surface or the underside of the fibre resin system prior to curing.

Sections of pipe-in-pipe can be fabricated on land, with interpipe insulation installed and an FRP collar already formed and bonded in place at each end. Then the only finishing operations required in the field are to join the inner pipe ends (usually by butt-welding) and

apply the outer shells and joint insulation. However, care should be taken and possibly cooling measures and/or long projecting inner pipe ends used to ensure the FRP collars are not overheated by the welding operation. If filament winding is used, the winding apparatus may be of the type that orbits the pipe ends, or the pipes can be rotated to wind the collars.

From the foregoing, it can be seen that the FRP collar plays an important part in the present invention. Therefore, in another aspect, the present invention provides a fibre reinforced plastics collar for forming a thermally insulating mechanical connection in a pipe-in-pipe joint for a Bowline or riser system.

The invention and its preferred features and advantages can be further understood from the following description of an illustrative embodiment, made with reference to the single figure

of the drawings, in which Figure 1 shows a cross section of an assembled pipe-in-pipe joint.

30 Description of the Preferred Embodiment

As the FRP collar and pipe-in-pipe joint of the present invention can be used with any insulation material it provides competitive advantages by permitting a single flowline weld

and a rapid offshore field joint, thereby significantly reducing offshore installation time when

compared to any current field jointing system on the market.

The illustrated joint is made up of a composite structure connecting the inner and outer pipes 5 and improves various aspects of known pipe-inpipe systems: (1) increases thermal performance at the field joint through elimination of the metal

Bowline to carrier connection, in particular providing improved (longer) cooldown. FRP composites have excellent thermal properties when compared to steel. A vastly reduced field

lo joint size also results from the high thermal performance of the FRP collar.

(2) Reduces installation activities to completion of one offshore weld and injection of a small amount of insulation material.

5 (3) Provides reliable and robust mechanical connection between the bowline (inner) and carrier (outer) pipes to ensure both axial and lateral structural compliance. There is no weld on the Bowline from a metal swage or bulkhead.

(4) A further advantage of this joint is that it is compatible with all forms of pipe-in-pipe 20 system and is designed for use in the field. Hence it is a 'universal' field joint.

The universal field joint 10 shown in Figure 1 comprises a pair of inner pipe ends 12, 14 butt

welded together at 16. Also shown is a pair of outer pipe ends 18, 20 coaxal with the inner pipe ends 12, 14. The annul) thus defined contain a high performance insulation system 25 schematically indicated at 22, which may be of known material, such as a microporous silica, ceramic or Polyurethane foam. Polymer rings 24, 26 are interposed between the inner and outer pipe ends and FRP collars 28, 30 are formed in place, bonded to the outer pipe ends 18, overlying polymer rings 24, 26 and tapering so as to be bonded to the inner pipe ends 12, 14.

An outer shell 32 is applied to the joint 10 between the FRP collars 28, 30 where these 30 overlie the outer pipe ends 18, 20. The annular void thus formed between the collars 28, 30, inner pipe ends 12, 14 and outer shell 32 is filled with field insulation 34, 36. Several of

these components and design considerations for the preferred field joint are further discussed

below. Polymer Ring 24, 26 5 The polymer ring is a profiled conical-type section cast or moulded from an appropriate polymeric material or ceramic or manufactured from fibre reinforced plastics material. It is machined to close tolerances prior to fitting and is bonded to both the inner pipe ends (bowline) and outer pipe ends (carrier pipe) with an appropriate adhesive such as Araldite (RTM) 2014 or a methacrylate adhesive such as Plexus MA550. It therefore serves to seal lo the interpipe annul). The exposed face 38 of each ring is chamfered and a radiussed shoulder 40 is provided in front of the outer pipe ends 18, 20 to provide a smooth surface that is ready for lay up of the FRP collars 28, 30 and free of stress concentrations. Material selection for this component is based on the required thermal and mechanical properties with suitable materials having the following characteristics: (i) Low thermal conductivity and high specific heat capacity (ii) Mid-range strength and stiffness values for polymers (iii) Low cost (iv) Good bonding affinity The polymer ring is profiled to provide a 'mandrel' onto which the FRP collar is either 20 wound or moulded or vacuum formed during fabrication, its shape dependent on the magnitude of the load to be transferred and the system geometry.

Expandable foams may also be employed between the outer surface of the polymer ring and the inside/outside of the outer/inner pipe respectively to completely seal the gap and reduce 2s or eliminate the tolerance issues of the polymer ring and pipe.

FRP Collar 28, 30 The FRP collar is the component providing the primary mechanical connection between the Bowline and the carrier pipe. It can be manufactured by the filament winding method or 30 other methods such as vacuum infusion and may be for example either GRP (glass reinforced plastic) or CFRP (carbon fibre reinforced plastic) or a combination of both. It is bonded to the Bowline outer surface at one end and the carrier outer surface at the other through use of

adhesive tapes, such as Redux 610, which cures at the same time as the FRP collar. The polymer ring provides the continuous, smooth surface between the two pipes during winding of the FRP collar and these two components are also bonded together. The FRP collar is designed to withstand the loading seen during installation and operation for the life of the 5 system in the subsea operating environment.

A number of manufacturing methods can be used. If filament winding is chosen the field

equipment preferably uses fibre bobbins rotating around the pipe ends, rather than rotating the pipes. Pre-pregs can also be used, for ease of fabrication. Should vacuum infusion be lo chosen as the manufacturing method, the field equipment is also simplified. The equipment

required for the field application of the filament winding method is available off the shelf and

is used for such applications as attaching together consecutive lengths of glass fibre drainage pipes. The vacuum infusion method can utilise layers of pre-preg. materials pre-

manufactured in 'sock' type arrangements that can be rolled over the conical polymer ring.

IS Outer Shell 32 The outer shell is a composite tube made of either GRP or CFRP and is the external covering of the field joint] 0. It provides a contained annulus around the field joint region into which

suitable insulation material 36 may be injected. The field joint itself may contain half shells

20 of an alternative insulation material 34 to reduce the amount of injected material thereby reducing field jointing cycle time. The outer shell may be designed purely to form a housing

for the injection of field joint insulation in which case its structural requirements are minimal

and weight should be minimised. In that case, it may be assembled from a number of part-

cylindrical sectors, or from a rolled flexible sheet as currently as used for most mastic filled 25 field joints. Alternatively the outer shell can be designed to provide a pipe-in pipe system

with continuous, constant bending stiffness, i.e. the bending stiffness of the field joint is

equal to that of the main body of the pipe-in-pipe system. For projects with lateral buckling concerns this continuity in the bending stiffness eliminates the possibility of a lateral buckle initiating at a field joint due to discontinuous stiffness. The outer shell, if required to carry

30 substantial loads between the outer pipe ends, can be fabricated by similar techniques to the FRP collars. If necessary, a suitable layup support or former (not shown) is first applied to the joint between the parts of the collars 28, 30 overlapping the outer pipe ends 18, 20.

Wet Insulation 34, 36 As mentioned above the field joint insulation is located within the outer shell 32. It is

required to be suitable for wet operation, i.e. exposed to seawater, and obviously must be s thermally efficient. There are many proprietary and non-proprietary products available to fulfil this role and material selection for this component is project-specific.

Joint Design Two main areas of the joint design have been modelled by the applicants: the mechanical lo design for installation and the thermal design for steady state and cooldown. The basic inputs for the Figure I embodiment are shown in Table 1 and are suitable for a large deepwater development.

Flowline OD (inch) 12.75 (324mm) Flowline wall thickness (inch) 0.625 (15. 9mm) Carrier OD (inch) 16 (406mm) Carrier wall thickness (inchl 0.625 (15. 9mm) Design water depth (m) 1200 Steady state U-value (W/m2K) 1.0 Cooldown 30 C drop in 12 hours . Operating temperature ( C) 50 15 Table 1 - Design inputs Mechanical Design The focus of the mechanical design modelling has been the installation phase. Two loading scenarios have been considered: ( 1) Maximum installation tension of 400 tonnes force + 40 tonnes force dynamic component.

It is assumed that the tensioners or friction clamps act on the carrier pipe and this load must be fully transferred to the inner pipe through one field joint. As there are two collars per pipe

section (one at either end) the actual load would be half this value thereby giving a safety 25 factor of 2 for in-line tension rating.

(2) Sag bend minimum radius of 160m + 40 tonnes force residual tension + 12 MPa external pressure. This load combination represents the main loads experienced in the sag bend during installation. Apart from the external pressure these values require determination by installation analysis for the configuration in question.

The FRP collar was sized for determination of the lay angles and required number of plies and was modelled with finite element methods to permit examination of the stresses in the individual lamina that comprise the collar. The outer shell 32 and the wet insulation have not been included at this point as they do not significantly influence the structural response.

To comfortably meet the structural requirements, in the illustrated embodiment the FRP collars 28, 30 are 22.5 mm thick. They consist of 80 plies of filament wound carbon fibre (foray T700 with Bakelite L20 epoxy resin) at a lay angle of +/-45 degrees. The curing method and duration is very important in achieving the design strength and is undertaken is fully in accordance with the materials suppliers' specifications. The length of the bond or

overlap between the FRP collars 28, 30 and the respective inner 12, 14 and outer 18, 20 pipe ends, is approximately 100-200 mm.

Results: Scenario 1 20 At a load of 440 tonnes force first ply failure is observed on the FRP collar according to the Tsai-Hill failure criterion. This gives a factor of safety of two on the design of the FRP collar. The maximum stress in the fibre direction in the innermost ply is around 560 MPa (at the concave bend in the collar inner surface). The maximum stress in the fibre direction in the outermost ply is around 480 MPa (at the concave bend in the collar outer surface). Such 25 values are approximately 25% of the ultimate tensile stress for the fibre concerned. Table 2 summarises the maximum and minimum values for the various stresses in the FRP collar at 440 Te load. The values presented are in MPa.

In-plane Transverse Shear Innermost Layer +560 / +199 +47 / +17 +60 / +3 Outermost Layer +498 / -232 +41 / -19 +1 / -26 Table 2 - Max. Stress Summary: Loading Scenario 1

The maximum and minimum values of the shear stress in the Bowline (inner pipe) over the region where the FRP collar is attached are +5.1 MPa and 30.6 MPa respectively. The shear strength of the adhesive must therefore be greater than the largest of these values. Table 3 s shows shear strengths for high performance adhesive bands available from various suppliers.

(Hexcel: \\\.hexcelcomposilcs.con,; Vantico: rww.a1hesives.vantico.com). The Hexcel products meet the design requirements. Vantico EPlS9S only just falls below the required shear strength, but may be used if a safety factor of just less than two is acceptable, or if the length of the overlap region is increased slightly. As a further example of adhesives lo availability and properties, Table 3 also shows the overlap lengths needed with each adhesive on an 8" (203mm) Bowline under 200 tonnes force tensile load (safety factor = 1.0).

Shear Strength Length required Supplier Type Product PSI MPa Inches Hexcel Epoxy Redux 610 4800 33.10 0 09 3 66 Hexcel Epoxy Redux 641 6000 41.37 0.07 2.92 Vantico Epoxy 1595 4351 30.00 0.10 4.03 Table 3 - Adhesives IS Finally, modelling of the distribution of von Mises stress within the polymer ring in loading scenario 1 shows a maximum value of 66 MPa which is approximately 60% of the yield stress of the Ureol Bi-resin material. (This occurs in the angle just inside the outer pipe end).

20 Results: Scenario 2 Under bending at a radius of 1 60m and an axial load of 40 tonnes force, the stress levels in the FRP collar 28, 30 and the polymer ring 24, 26 are drastically reduced from the levels experienced under scenario 1 loading. Table 4 summarises the maximum and minimum values for the various stresses in the FRP collar under scenario 2 loading. The values 25 presented are in MPa.

In plane Transverse Shear Innermost Layer +206 / -198 +18 / -15 +28 / -23 Outermost Layer +194 / -179 +16 / -15 +13 / 17 Table 4 - Max. Stress Summary: Loading Scenario 2

s Therefore the governing design condition for the illustrated embodiment will be the maximum installation tension. The FRP collar is the critical component under scenario I loading and requires a thickness of 18 mm carbon fibre to achieve a safety factor of 2 on the overall loading as the 440 tonnes force loading is shared between the two FPR collars at each end of the outer pipe section.

lo Thermal Design Transient thermal analysis of the field joint region has been performed with representative

materials and a typical sizing. From the results it is evident that the illustrated FRP field joint

offers significantly improved thermal performance over a system with a steel connection 5 between Bowline and carrier. At steady state flowing condition and a 50 C temperature difference between the fluid conveyed and ambient sea water temperature, heat flux through the FRP collar is 697 W/m2 as compared with 22620 W/m2 for the steel equivalent.

A cooldown simulation (50 C initial product temperature and 0 C ambient sea water 20 tcmpcraturc) produced rllirlirnlurnl temperatures in 'the field joint region after 12 hours of

15.5 C for the illustrated field joint compared to 6.5 C in the equivalent with a steel swage.

The product conveyed has been taken as gas of density 18 kg/m3, thermal conductivity of 0.13 W/m K and a specific heat capacity of 2400 W/kg K. 25 These preliminary analyses demonstrate the magnitude of the thermal efficiency the joint of the present invention offers over an alternative with a steel swage between Bowline and carrier. Optimised design of this field joint will therefore eliminate cooldown concerns

whilst reducing the procured cost of the field joint and the installation time.

The applicants have undertaken some preliminary cost analysis and believe the joint of the present invention offers significant savings over alternative solutions on the market. Current jointed pipe-in-pipe systems on the market have the following major cost items: Onshore pipe swaying and additional handling 5 Welding to the Bowline Larger field joints requiring more material

Large steel formers However, by far the largest cost item is the additional installation and laybarge time.

0 In the applicants' joint the amount of FRP material is low, the insulation material volume is reduced, the former is considerably smaller, and most importantly the offshore installation time is reduced. Therefore it is expected that both the procured costs and as-installed costs are potentially an order of magnitude lower than for previously known pipe-in-pipe field joint

options.

Claims

l. A joint comprising an inner pipe end joined to a corresponding pipe end, an outer pipe end surrounding the inner pipe end and from which the inner pipe end projects axially, a 5 collar mechanically connecting the inner and outer pipe ends and thermal insulation surrounding the inner pipe end, in which the collar is formed from fibre reinforced plastics bonded to the inner and outer pipe ends.
2. A joint as defined in claim l, in which the fibre reinforced plastics collar has a first lo end bonded to the outside of the outer pipe end, the joint comprising a surrounding outer shell having one end fitted to the collar first end and another end fitted to the first end of a corresponding collar of an adjacent outer pipe end, an annular space defined between the outer shell, inner pipe ends and outer shell providing or containing thermal insulation.

5
3. A joint as defined in claim l or 2 comprising a thermally insulating spacing ring Inserted between the inner and outer pipe ends.
4. A joint as defined in claim 3 wherein the spacing ring is used to seal the annulus between the inner and outer pipes.
5. A joint as defined in claim 4 wherein the spacing ring is made from an impermeable material and forms a continuous annulus sealingly bonded to the inner and outer pipe ends.
6. A joint as defined in any of claims 3 - 5 wherein the spacing ring serves as a former 25 for laying up the FRP collar.
7. A joint as defined in any preceding claim wherein the FRP collar is fabricated by filament winding.

30
8. A joint as defined in any of claims 1 - 6 wherein the FRP collar is fabricated using pre-preg socks slipped over the assembled pipes.
9. A joint as defined in any of claims 1 - 6 wherein the FRP collar is fabricated using hand lay-up and vacuum infusion.
10. A joint as defined in any preceding claim wherein the reinforcing fibre comprises 5 glass fibre.
11. A joint as defined in any preceding claim wherein the reinforcing fibre comprises carbon fibre.

lo
12. A joint as defined in claim l l wherein the innermost layer(s) of the FRP collar are reinforced with glass fibre.
13. A joint as defined in any preceding claim wherein the collar-to-pipe bonds are formed using high-performance pre-formed adhesive tapes or liquid adhesives.
14. A joint as defined in claim 13 wherein the adhesive is cured simultaneously with the collar.
15. A fibre reinforced plastics collar for forming a thermally insulating mechanical 20 connection in a pipe-in- pipe joint for a Bowline or riser system.
16. A field joint for a pipe-in-pipe Towline or riser substantially as described with

reference to or as shown in the drawing.