OTC-14017-MS.pdf

Copyright 2002, Offshore Technology Conference This paper was prepared for presentation at the 2002 Offshore Technology Conference held in Houston, Texas U.S.A., 69 May 2002. This paper was selected for presentation by the OTC Program Committee following review of information contained in an abstract submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the Offshore Technology Conference and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the Offshore Technology Conference or its officers. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of the Offshore Technology Conference is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of where and by whom the paper was presented.

ABSTRACT: Use of composite materials in the deep sea oil production riser systems may allow a dramatic decrease in weight, as well as improved fatigue resistance to loads induced by environmental conditions. Many concepts have been developed by the industry and could be available in the next years. However, the cost of composite components will always be higher than the one of steel components, and only significant advantages for particular applications will justify their use by the industry. Up to now, cost comparisons have been made essentially for TLP or SPAR riser systems. This paper presents a study that has been carried out to compare steel and composite riser solutions for catenary risers, submerged export lines, and hybrid riser towers in ultra deep water. Specifications were first proposed, following which steel and composite solutions were designed and compared. This was done both from the feasibility point of view as well as from consideration of the operational advantages resulting from the lightweight and the fatigue resistance of the composite. Acceptable costs of composite risers were then deduced. The main conclusions are that in the mild conditions of the Gulf of Guinea or of Brazil, both steel and composite solutions are technically feasible, although steel solutions come close to their limits. Composite riser joints fabricated in moderately long lengths seem to be the most interesting solution for transportation and laying purposes. Large cost advantages may be obtained, particularly during the laying phase, which can justify using the composite solution.

INTRODUCTION: During the nineties, the oil industry proceeded to exploit offshore fields in 1000 metres water depths and beyond. This has been done generally by simple extrapolation of existing architectural floating concepts, such as TLPs and FPSOs, or by using new ones such as SPAR systems. A significant new development has been the introduction of Steel Catenary Risers. Also the exploitation of new lightweight materials has begun with the introduction of the first mooring systems made out of polyester ropes. New problems have also arisen such as the necessity for significant thermal insulation of pipes. With increasing water depth, and particularly in Ultra Deep Water (between 2000 - 3000 m), the need to decrease the weight of the risers and mooring lines will become stronger. The introduction of lightweight materials may become the best economic option, or possibly the only technically available option. Among such materials, high strength composites have been the subject of intensive industrial research and may become commercially available for operational purposes in the present decade. Up to now attention has been generally focused on weight sensitive floating systems, such as TLPs, on which the balance of weight and the advantages induced can be easily deduced. The objectives of the present study were to evaluate the technical and economic interest of other pipe systems, such as catenary risers, export lines, or hybrid towers, where the balance of weight is only part of the interest, and where steel solutions have run into barriers including fatigue behaviour, stiffness, installation loads, thermal insulation, etc. The study was carried out by IFP on behalf of the CLAROM association. IFP dimensioned the composite pipes and calculated their behaviour, while the three offshore contractors calculated the equivalent behaviour of the steel alternatives. They also evaluated the installation methods, and the cost difference by which the composite solutions will have to be justified. CATENARY WATER INJECTION PIPE: Catenary risers would appear to be good candidates for composite applications since composite materials have excellent qualities of lightness and fatigue resistance. A water

OTC 14017

Technical and Economical Evaluation of Composite Riser Systems P.Odru and Y.Poirette, Institut Franais du Ptrole, Y.Stassen, Bouygues Offshore, J.F.Saint-Marcoux, Paragon Litwin, L.Abergel, Technip-Coflexip

2 P.ODRU, Y.POIRETTE, Y.STASSEN, J.F.SAINT MARCOUX, L.ABERGEL OTC 14017

injection pipe requiring no insulation in very deep water was chosen for the study. Specifications: The specifications used for both composite and steel pipes were an internal diameter of 11 1/2 and a top end service injection pressure of 1.5 MPa. The water depth considered was 2500 m. In service, the risers are always full of water. To reduce weight the steel pipe is laid empty and has to resist collapse pressure, but the composite pipe can be laid full of water. The characteristics of the spread moored FPSO are identical to the one proposed by Bouygues Offshore for the BONGA field. Two cases were considered: one in the Gulf of Guinea, and the other for Brazil, using corresponding environmental data. Static design: The static design for both steel and composite cases is carried out considering a pipe that withstands the internal and external pressure requirements. The initial profile of the catenary riser is determined by the shape of a riser of a given length installed between the FPSO in its mean position and a fixed point on the seabed representing the extremity of the flowline. Around this mean position, offsets are superimposed, accounting for low frequency motions of the FPSO induced by wind, wave and current. They induce near and far positions (figure 1), for which corresponding stresses are calculated. If they are found unacceptable, a new mean position is chosen, and the calculations are repeated until acceptable results are obtained. Steel pipe: A particular specification for the steel pipe was an additional corrosion thickness of 10 mm, which is probably severe. The pipe was designed to withstand the collapse pressure (without the corrosion thickness), the effects of weight, and the touch down point bending curvature, in near and far positions, corresponding to a drift of 8% of the total water depth. The current was imposed in the direction with the least favorable effect. Two materials were considered: X70 and P110 grades. The global analysis was made using the DEEPLINES@ software. A 13.8 mm wall thickness was found necessary, to which 10 mm of corrosion thickness was added. The X70 steel was found slightly insufficient to withstand the induced stresses with the required safety coefficients. The steel pipe presented the following characteristics: Wall thickness (mm): without additional corrosion thickness: Apparent weight, full of water (N/m): Apparent weight, empty (N/m): Axial stiffness (MN):

23.8 mm 13.8 mm

1500 N / m 900 N / m 4500 MN

Composite pipe: The design of the composite pipe is more complex than the steel one, due to the anisotropy of the basic material. However, the empty pipe being naturally buoyant, and its weight full of water remaining very low, it can be laid full of water. So there is no requirement for the composite pipe to resist collapse pressure. Because of its low apparent weight, the composite pipe is only subject to very small loads, which leads theoretically to a very thin wall thickness requirement. Hence the pipe has to be designed on robustness criteria, in order to be able to withstand shocks and handling loads. To make the pipe robust two versions were studied with overall wall thickness of 10 mm and 20 mm respectively. Four designs were proposed and analyzed (see Table 1), in which carbon fibers are used to resist structural loads, and glass is used to increase wall thickness to make the pipe robust. These designs have been chosen for the following reasons. Two of them (C10 and C20) have the minimum thickness of carbon fibers required to withstand the loads, the remaining thickness being made up of glass fibers in order to give total thickness of 10 and 20 mm respectively. The two others (C10 and C20) are made mainly out of carbon fibers, leading to higher axial stiffness. The four structures were analyzed and the stresses induced in the three cases (near, mean, and far positions) were found fully acceptable. Dynamic and fatigue verifications: Brazil case: The dynamic study was carried out considering a Jonswap spectrum representing hundred year conditions with a significant wave height of 7.8 m and a peak period of 15.35 s. The drift was taken to be 8 % of the total water depth (200 m). The RAOs were obtained using the DIODORE@ software. Two cases were considered: one with the waves perpendicular to the vessel, and one with the waves in line with the vessel. With waves perpendicular to the vessel, important roll movements are induced, which generate high tensile and even compressive loads in the catenary risers. They were found unacceptable for both steel riser and composite case. With waves in line with the vessel and hence perpendicular to the plane of the risers, both composite and steel designs were found acceptable. The level of stresses induced is however far more favorable in the composite version. The case with waves at 45 to the vessel was found to be acceptable for the composite pipe.

OTC 14017 TECHNICAL AND ECONOMICAL EVALUATION OF COMPOSITE RISER SYSTEMS 3

Gulf of Guinea case: Environmental conditions in the Gulf of Guinea are highly unidirectional with narrow frequency-band waves. The Jonswap spectrum with significant wave height of 3.7 m and peak period of 13.9 s was used to represent hundred year conditions, which are less severe than the extreme conditions for Brazil. Analyses of the risers with such waves perpendicular to the vessel or in line with it, showed that both the steel and the composite designs were satisfactory. Although the sea states are mild, they are persistent, and their effects on fatigue must be studied. The fatigue analysis was performed using a wave scatter diagram, using both frequency and time domain methods. The method to assess the fatigue life of steel pipes is well known. A safety coefficient of ten has to be applied to the damage calculated using Miners rule. This method does not apply for composites. Carbon fibers however present excellent fatigue properties and their ability to withstand fatigue loading can be reasonably estimated. The principal result of these studies was that the fatigue life in the environmental conditions of the Gulf of Guinea was acceptable for both steel and composite solutions. Design conclusions: Both steel and composite designs have been found acceptable as 12 OD water injection catenary risers in the deep offshore Gulf of Guinea conditions. However, stress levels in the steel riser are very high requiring the use of high-grade steel such as P110. In contrast the stress levels in the composite are low and such risers only have to be designed for robustness. Their self-weight is low. Difficulties appear when trying to design both pipes for the case of hundred year waves perpendicular to the vessel in the Brazilian environment. Problems result from the roll movements of the vessel, which is not adapted to these environmental conditions, and can be improved by using a FPSO with turret or flexible pipes. This was beyond the scope of the study. Although both materials can be used to achieve satisfactory designs, they lead to major differences in self-weight of the two pipes suspended from the vessel. This results in a 420 T top tension for the steel pipe, and around 28 T for the composite one. Estimate of economic benefits: The estimation of economic benefits resulting from the use of composite materials is a difficult exercise. The methodology used was to estimate the economic influence of the following: the weight of the pipe on the floating support;

the accessories (stress-joints, ball joints, others); the installation procedure. The sum of these influences gives the differences between the steel and the composite concepts, as well as giving indications on the way the composite has to be manufactured. It will then be possible to deduce the acceptable cost for the composite, and in what conditions this can be matched. The influence of the weight of the pipe is not negligible (around 30 T for the composite instead of 420 for the steel) but does not seem to have a significant influence on the direct design of the FPSO structure, which is capable of supporting considerable weight. However, the lightness of the composite pipe, combined with its flexibility, has an important influence on the requirements of the topside connection (stress joint or ball joint) which represents an important part of the global cost. In some cases it governs the feasibility of the catenary riser solution itself. The most important impact seems to be on the installation method. In the steel case a classical procedure has to be followed. The flowline is raised from the seabed to the surface and the riser elements are then vertically welded. The vessel moves towards the FPSO while the elements are added and laid using the J-lay method. The head of the riser is then transferred to the FPSO and finally installed. This requires the use of large laying vessels, such as SAIBOS FDS or the Seaway POLARIS. The laying of the composite version may be completely different. A significant point is that the composite riser joints need to be equipped with steel connectors which are heavy and expensive. Three types of composite solutions have been considered, characterized by the lengths of the riser joints:

Short lengths in the range of 10 to 25 meters can be easily manufactured and handled, but the cost induced by the number of steel connectors does not favor this solution;

Intermediate lengths (100 to 300 m) can probably be manufactured, transported and assembled close to the site at optimal cost;

Long lengths (300 m to 4000 m) may be considered, but they would probably be manufactured in a dedicated site far from the final destination. The required long tow could be very hazardous. Reeled pipe technique could be envisaged, but composites having no plasticity domain, this would induce too large stresses for such types of diameters.

The second option appears as the more interesting, and was chosen as the reference case for the whole study. The composite riser having been assembled and filled with water could then be towed at the mudline (as for the Girassol bundles) by two tugs. Before installation of the FPSO, the riser can be laid directly in the alignment of the flowline and


abandoned. Because of the length of the catenary, the abandonment head would lay on the opposite side of the FPSO. It can then be picked-up when the FPSO is fully moored, raised by a tug that transfers the riser head to a chain operated from the vessel, and installed at its place on the other side of the vessel [8]. If the influence of the difference of weight on the FPSOs structure seems negligible, the economic consequences on the two other items (accessories and installation) have been evaluated in Table 2. The cost for the composite pipe does not include the transportation of the intermediate length joints from the manufacturing plant and their on site assembly, as well as the mobilization cost of the heavy vessel in the steel case. Conclusions: The balance is of around $1.35 million, on which the excess cost of the composite version versus the steel one will have to be justified. It is difficult to estimate the cost of such a product today. It seems however that the C10 version, which includes the least material, could be a candidate for such an application, if manufacturing costs and associated investments are not too high. Estimates have been made for both the Gulf of Guinea and the Brazilian cases. If both steel and composite cases have been found technically feasible, the former requires high-grade steel, where as the composite solution leads to very low stresses. These results suggest that in more severe environments the steel case will reach its limit. On the other hand the composite solution should be more competitive, due to the low stress levels induced by self-weight, and the high compliance and fatigue resistance. APPLICATION TO CATENARY PRODUCTION RISERS: The use of catenary production risers made out of steel or composite for direct production purposes in very deep water will raise the problem of thermal insulation. This was not addressed in this study. The second problem will be that in a production case the riser may be empty, or full of gas. The composite riser will then become buoyant, which is unacceptable. For such an application we propose to equip the composite riser with an internal steel carcass, as in flexible pipes. This will secure the internal liner and allow the weight to be adjusted. The specification for the study comprised an internal diameter of 8, a top service pressure of 10 MPa with a possible maximum at 35 MPa, and a water depth of 2500 m. The characteristics of the FPSO and the environments studied were the same as for the water injection riser. The steel riser was designed following API 2RD, which gave a 9.5 mm thickness on which a 5 mm corrosion thickness

(probably pessimistic for most of usage) had to be added. The linear weight in water was then of 675 N/m. The composite riser was designed to withstand the tensile and pressure specifications. Its apparent weight full of gas was adjusted to be zero through the use of an internal steel carcass. Carbon fibers orientation: Carbon composite wall thickness: Glass fibers orientation: Glass composite wall thickness: Pipe axial Modulus: Apparent weight with end fittings: Apparent weight with internal carcass:

60/20 6.5 mm

55 13.5 mm 11 GPa

160 N/m 440 N/m

Both steel and composite risers were found able to work in static conditions for a given mean position and a drift of 8%. Problems were found when the empty pipes were subjected to a current perpendicular to the axis of the FPSO. The steel pipe showed a corresponding displacement close to 70 m, which is probably acceptable in 2500 m water depth. However the composite riser in the same conditions showed a displacement of 600 m (figure 2), which is certainly unacceptable, due to the risks of contact with the other risers. Further analysis indicated that this was due to the very low weight of the empty composite pipe (40 N/m). This displacement was found dependant only on the weight and not on the stiffness. It can be concluded from this approach that weight may be necessary to withstand these extreme conditions. EXPORT LINE: The second composite application studied was the case of an export line (Figure 3). The export line of the Girassol field is made out of two 16 steel pipes, shaped in a W form by the use of syntactic foams [ref 9], and called for this reason lazy W. Export lines with larger diameters such as 22 cannot meet the required fatigue safety coefficient of ten due to the motions of the Surface Buoy. The interest of a composite version was its good fatigue resistance to surface buoy excitation. Specifications: Specifications for the line were following: Length (steel Lazy W): Diameter: Service pressure:

2100 m 22

2 MPa The line is always filled with water or treated oil (density 0.8); no corrosion thickness was required.


Environmental specifications: The FPSO is considered as fixed. The buoy is located at a nominal distance of 1850 m of the FPSO, making an angle of 64 with the axis of the vessel. The movements of the buoy are described by RAOs. The environmental data (centenary wave and sea state scatter diagram) correspond to the Gulf of Guinea. Steel pipe design and analysis: The steel pipe design was following: Nominal diameter: Steel grade: Yield limit: Wall thickness: Apparent weight full of water: Apparent weight full of oil:

22, 559 mm API 5L X 65

450 MPa 25 mm

2850 N / m 2450 N / m

The extreme conditions analysis was carried out with the ORCAFLEX@ software, considering a hundred-year wave in the direction of the vessel with a significant wave height of 3.62 m and a peak period of 15.9 s. A wind wave in the direction of the line with significant height of 1.9 m and a peak period of 6.3 s was added. No current was considered. The fatigue simulation was carried out considering a representative scatter diagram and using a spectral method. A statistical variation of the stress in the pipe can be deduced from each wave period. S-N curves are then used to evaluate the damage associated with the number of cycles and the global fatigue life is estimated with a safety coefficient of ten. Only first order movements are taken in count; damages due to installation, second order movements and VIV were ignored. Global calculation showed that the maximum damage zones were located close to the buoy, and that the pipe could not fulfill the safety coefficient of ten. Two solutions can be then proposed: use of three smaller diameter lines, like 16 lines

for instance; use a subsurface buoy, in order to decrease the

movements induced by the environment; this solution was evaluated during the study, and found acceptable, maximum damage zone being then in the middle of the pipe.

Composite pipe design and analysis: The mechanical and environmental specifications of the lines being mild, the use of carbon fibres does not seem to be necessary. A composite material made out of glass fibres in a thermoset matrix was proposed, for cost purpose. Two cases were considered, differing by the orientation of the fibres and

the corresponding axial and bending stiffness. The design criterion was in fact the external pressure. The wall thickness found seems to be able to insure sufficient robustness of the pipe (see Table 3). The lazy W form, as the analysis showed, is required to avoid the apparition of negative effective tension or real compression loads in the pipe. It is obtained through the use of syntactic foams blocks adequately disposed. With the composite design, the difference of apparent weight between the case of the pipe full of oil or of water becomes very significant although weight is very low, and this induces very different and incompatible behaviours (the Lazy W form cannot be kept). It was so proposed to use the composite pipe as a double catenary line, which will have for interest not to use the expensive syntactic foams blocks. Results of the dynamic analysis showed that the stresses induced in the composite for both designs were fully acceptable, even if the radius of curvature in extreme conditions could be quite small. However, important values of negative effective tension or even compressive loads are found in the proximity of the buoy and of the vessel. They do not seem to be a problem for the pipe, for they apply only for very small time (2 to 3 s), reasonably avoiding buckling risks. But some real compressive loads may be transmitted to the flex-joints, which are not designed for. If the flex-joints are suppressed, the stresses in the direction of the fibres become too important, needing the use of an adequate stress joint. Economical evaluation: The economic interest of the composite solution will be evaluated through the comparison with the cost of the steel version, including transport and laying. The economic advantages induced then by the composite solution are quantified, and the acceptable cost of the composite pipe can be deduced. The steel version is made of two flex-joints with their supports and of steel lines assembled by welds with their coatings, anodes and buoyancy modules. The scenario for the installation of the steel pipes would be the following. The pipes are manufactured by 40 lengths and transported to the site. They are then welded two by two and transported on the laying barge. The floaters are installed during this operation. When the line is laid, hydrostatic tests are performed and the connection to the floating supports is achieved (FPSO and buoy). The scenario for the composite line has some differences. As previously stated for the composite catenary riser, middle long lengths (100 m or more) are manufactured in a distant manufacturing plant, and are then transported to the yard, where the pipes are assembled and the hydrostatic tests performed. The two lines are then towed to the site by a tug at


each extremitiy. On site, the lines are installed and connected to the floating supports. Admissible cost for the composite solution: Considering the simplification of the offshore operations, the admissible cost for the following furniture: composite material pipes with their connection systems, necessary flex-joints and connecting systems, buoyancy if needed, transport from the manufacturing plant to the yard, would be of around $ 6 millions. The advantage may be possibly more, for the need for additional steel lines or for a subsurface buoy was not quoted. But the composite solution will probably have other specific problems, as the necessity to include adequate stress joints at each long length connecting system, which was not studied in details. Here too the authors of the paper are not composite manufacturer and cannot answer definitely, but such a cost seems possible to reach, if conditions are favourable. HYBRID TOWER: Hybrid tower is an innovative concept developed for the giant Girassol field in the Gulf of Guinea, and installed during summer 2001. It consists (in this particular case) in three vertical towers made of a core pipe sustained by a subsurface buoy, along which the risers and facility systems are assembled (see figure 4). Syntactic foams disposed around the pipes insure both buoyancy and thermal insulation (a joint Stolt Offshore Doris Engineering patent). The connection to the 300 000 T FPSO is made through a subsurface flexible pipes system. The objectives of this study were not to evaluate directly the economic advantages able to be brought by a composite version, for the design of such a system is quite complex. But to evaluate the advantages in terms of weight saving and possible thermal insulation, as well as the problems that could arise in a composite version. Specifications: The water depth considered is of 2500 m. Production risers: Internal diameter: Maximum service pressure, sea floor : Normal fluid density: Service temperature:

103/4, 273 mm 35 MPa

0.8 T/m3 40C

Core pipe: The core pipe has to be designed for a 1000 T tension. Composite riser design and analysis: One of the problems of the design of such a system is that the syntactic foams are used for both buoyancy and thermal insulation. But when the pipes expand under thermal or end pressure effects, gaps appear between the blocks of syntactic foams, inducing thermal convection and a loss of insulation efficiency. Composite pipes however can be tailored to reach specifications impossible with isotropic materials, such as a Poissons ratio close to .5 (which would considerably reduce elongation under end pressure effect), and a thermal expansion close to zero. In this case however, the initial axial expansion under the pure effect of the weight will stay important, and needs the use of carbon fibre composite to be reduced. The second problem arising with the composite solution will be that, when full of gas or empty, the pipe will be buoyant, creating an axial compressive force able to induce buckle. This problem is well known for instance for the peripheral lines of drilling risers, and the problem can be solved by increasing the axial stiffness of the pipe and displaying lateral guides along it, calculated in order that the critical buckle length cannot be reached. An other solution could be to increase the weight by displaying an internal or external steel carcass able to protect the internal and / or external liner, and adjust it so that the weight is never negative. The design of the carbon composite pipe could be the following, without additive weight: Fibres orientation: Wall thickness: Poissons ratio: Apparent weight, full of oil: Apparent weight, empty: Tensile stiffness: Bending stiffness: Collapse pressure: Thermal elongation: Critical buckling length:

24 / 76 19 mm

0.55 180 N/m

- 290 N/m 1 030 MN

8.3 MN.m2 41 MPa

-0.05.10-6 10.7 m

Comparison with steel pipe:

Composite pipe

Steel pipe

Wall thickness: Apparent weight, full of oil: Apparent weight, empty: Total weight, full of oil:

19 mm 180 N/m

- 290 N/m 45 T

15 mm 758 N/m 290 N/m

190 T

The suspended weight of the pipe full of oil is reduced from 190 T for the steel pipe to 45 T for the composite pipe.


Thermal aspect: The oil temperature of the Girassol field is quite low (60C) and an important insulation is necessary to keep the fluid from reaching wax or hydrates formation points during transportation. The specification was a thermal exchange coefficient of 1.5 W/m2.K. Table 4 presents the thermal properties of the different materials. Calculations of required insulation material for steel and composite solution give 210 mm in the first case and 200 mm in the second one. The difference is quite negligible. The second aspect is the relaxation of temperature when the production is stopped. The calculations show that in the steel solution the required time for the temperature to fall from 40C to 20C will be of 20 hours, although it is only of 15 hours for the composite version, due to the lower thermal capacity of the material. As a conclusion, using composite in these conditions will bring a marginal advantage from the insulating point of view, and a disadvantage from the thermal capacity point of view. Core pipe: The specification of the core pipe is a tensile capacity of 1000 tonnes. It can be designed as an empty steel pipe, and so be neutrally buoyant. But this becomes impossible in very deep water, for the collapse design will necessitate thick walled and heavy pipes. The solution seems to be here the use of pultruded carbon fibre rods. A tendon of 1000 T capacity could be made of 300 rods of 6 mm diameter, for a linear apparent weight of 42 N/m without connections, which leads to a total apparent weight of 13 tonnes in 3000 m water depth. Such a product is to day very close to be industrially available. Conclusion hybrid tower: From a technical point of view, use of composite could allow a very important mass reduction of hybrid towers, as well for the riser system as for the central tension member in Ultra Deep Water. Problems should arise however as the pipes full of gas are buoyant, needing to take care of possible buckling. The use of composite material does not seem to bring thermal properties advantages, although the composite is more insulating than the steel. CONCLUSIONS: The general conclusions relative to each of the pipe systems that have been studied can be summarised as follows:

Catenary water injection risers have been found to be technically feasible in 2500 m water depth for the environments of Brazil and Gulf of Guinea for both composite and steel versions. The steel solution however requires the use of a high grade material, whereas the composite one can be dimensioned without difficulty. The economic evaluation shows then that the very large saving in riser weight has little effect on the FPSO itself. The major economic benefit comes from the simplification of the laying method, which allows small vessels to be used for the composite solution. The excess allowable cost of the composite version versus the steel one was estimated to be $1.3 million. It seems possible to the composite solution to reach this objective. The fabrication of intermediate lengths (100 m) would appear feasible and reasonably well suited for transportation and on site assembling, but the cost involved is difficult to establish.

The technical feasibility of a composite production

catenary riser has also been evaluated. The weight of the composite has to be adjusted in order to avoid it being buoyant when full of gas. The resulting lightness will lead to very large lateral deflections under the effect of currents.

The feasibility of the steel version of the Lazy W export

line system is limited to diameter of 16. The composite version on the other hand has been found to work as a catenary with the required diameter of 22. It will require stress joints at extremity instead of flexjoints because of the low tension. The economic advantages induced have been quoted in the paper at around $ 6 millions and a composite version may be able to reach the objective, particularly because low cost glass fibres could be used. Here too, this will depend on the availability of long lengths of composite pipes and their costs.

The case of the hybrid tower has only been studied from a

technical point of view. Composite production risers lead to large weight savings. However their negative apparent weight, when full of gas, will induce compressive loads, needing stiffness or additional weight to keep from longitudinal buckling. The thermal properties of the composite materials have not been found to be a significant advantage, because of their low thermal capacity. An interesting application of composites could be to replace the central core pipe by carbon tendons in very deep water.

As an overall conclusion composites may find applications of great interest for these types of offshore systems, but their use will depend strongly on the availability of pipes fabricated in long lengths at low cost. These advantages could be widely increased in more severe environmental conditions.


Acknowledgements: The authors are grateful to all those who participated in the study and in particular C.P.Sparks from IFP, E.Coche from Bouygues Offshore, and T.de.Kerdanet from Paragon Litwin. REFERENCES: 1. C.P. Sparks, P.Odru, H.Bono, G.Metivaud, Mechanical

Testing of High Performance Composite Tubes for TLP Production Risers, OTC 5797, Houston 1988.

2. P.J.C. Tamarelle, C.P. Sparks, High Performance Composite Tubes for Offshore Applications OTC 5384, Houston 1987.

3. K.H Lo, J.G Williams, M. Karayaka, M.M Salama, Progress, Challenges and Opportunities in the Application of Composite Offshore, CMOO3, Houston 2000.

4. T.M.Hsu, J.Skogsberg, M.Karayaka, Composites Utilization on a SPAR Platform Potential Economic Impact and Technical Gaps, CMOO3, Houston 2000.

5. Composite Riser Workshop, Statoil Research Centre, Trondheim, 14-15 June 1999.

6. D.B.Johnson, Rigid Composite Risers for Deepwater Oil Production, Intertech Second International Conference on the Global Outlook for Carbon Fibre, San Diego, California, November 1999.

7. B. Melve, T.Meland, P.A.Bergh, K.Klovfjell, Continuous Composite Lines for Methanol Injection on the Asgard Field Experiences from Production and Installation, OMAE2000/MAT-2422.

8. X. Wang, X. Zhou and A.J. Ginnard, J.T. Sprot, 'Installation method evaluation of export SCRs', OTC 12969, Houston 2001.

9. Legras J.L, Traube D, New Concept of Export Line for Deepwater Fields, ISOPE 99.

10. Hanna S.Y, Salama M.M, Hannus H. New Tendon ans Riser Technologies Improve TLP Competitiveness in Ultra Deep Water, Paper 12963, SPE, 2001.

TABLES

DESIGN:

C10

C10 C20 C20

Carbon fibers (, th): Glass fibers (, th): Axial stiffness (MN): Apparent weight (N/m):

90/10, 1.9 mm

55, 8.1 mm 175 95

90/10, 8 mm

10, 2 mm 1020

63

60/20, 2.6 mm 55, 17.4 mm

175 187

90/10, 16 mm

10, 4 mm 2040 118

Table 1: Catenary water injection composite versions design.

STEEL SOLUTION COMPOSITE SOLUTION

Connections: Cost (*) :

Flowline connector Flexjoint

20%

Flowline connector Flexjoint

10%

Offshore operations: Cost (*):

Raising of flowline Riser to flowline connection

Riser laying Riser transfer to the FPSO

80%

Towing Riser to flowline connection Riser transfer to the FPSO

20%

(*): Cost given as a percentage of the steel solution

Table 2: catenary water injection steel and composite version laying operation costs.


Design 1

Design 2

Fibres orientation: Glass composite wall thickness: Apparent weight full of water: Apparent weight full of oil: Axial modulus:

+/- 55 34 mm

580 N/m 160 N/m

58 MN(*)

+/- 10, +/- 90 30 mm

510 N/m 80 N/m

1450 MN(*)

(*): considering a degraded matrix.

Table 3: composite design of 22 export line.

Steel Composite Foam

Thermal capacity (Cp): Thermal conductivity : Density:

470 J / kg. K 45 W / m. K 7850 kg / m3

900 J / kg. K 0.5 W / m. K 1500 kg / m3

1400 J / kg. K 0.17 W / m. K

850 kg / m3

Table 4: thermal properties of steel, composite, and foams.

FIGURES

Figure 1 : Catenary water injection riser in far, mean and near

positions.

Figure 2 : Shape of composite catenary production riser,

subjected to current, weight having been adjusted to be just positive when full of gas.


Figure 3 : Lazy W export line.

Figure 4 : Hybrid Tower concept.

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