OTC 17224Daniel Karunakaran, Subsea 7; Trond Stokka Meling, STATOIL; Steinar Kristoffersen, STATOIL and Kjell M. Lund, STATOIL Copyright 2005, Offshore Technology Conference This paper was prepared for presentation at the 2005 Offshore Technology Conference held in Houston, TX, U.S.A., 2–5 May 2005. This paper was selected for presentation by an OTC Program Committee following review of information contained in a proposal 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, its officers, or members. Papers presented at OTC are subject to publication review by Sponsor Society Committees of the Offshore Technology Conference. 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 a proposal of not more than 300 words; illustrations may not be copied. The proposal must contain conspicuous acknowledgment of where and by whom the paper was presented. Write Librarian, OTC, P.O. Box 833836, Richardson, TX 75083-3836, U.S.A., fax 01-972-952-9435. Weight-optimized SCRs for Deepwater Harsh Environments Abstract Steel catenary risers (SCR) have been an attractive choice for recent deep-water field developments. However, design of SCRs for harsh environment is a great challenge, especially from large motion host platforms such as semi-submersible platforms. The key driver for the design of SCR in harsh environment is the fatigue near hang-off and at touch down point. This paper describes the concept of weight optimized SCR design for deep water harsh environment fulfilling both strength and fatigue requirements. The concepts incorporate presently available materials and technology. For the fatigue critical cross-sections, some qualified cost-effective and easily installable solutions are proposed. These proposed concepts are demonstrated through case studies of a few common riser sizes for such deep water field development with large vessel motions. Introduction Steel catenary risers (SCR) have been an attractive choice for recent deep-water field developments. However, design of SCRs for harsh environment has been a great challenge, especially from large motion host platforms such as semisubmersible platforms. The application of SCR from such semi submersible Floating Production Units (FPU) at harsh environment present design challenges due to the large motions of the vessel from waves and large vessel offsets from wind, current and slow-drift wave motions. There are buckling issues at touch down point (TDP) due to large heave and surge motions, and fatigue problems due to vessel motions and soilriser interaction. These design challenges can be successfully addressed by introducing a SCR concept with varying weight along the riser and with lightest possible cross-sections in the Touch Down Zone (TDZ), Karunakaran et al. (2002). This concept is schematically shown in Figure 1. The variation in weight of the riser is achieved by applying different density coatings. In this paper, the availability of such coatings which are qualified and ready to use and their properties are discussed. Also, the choice of material for different applications and their influence on the fatigue performance is discussed. The effects of such weight optimized design of SCRs are demonstrated for North Sea condition for three different applications. These example studies are performed using nonlinear time domain analysis and the fatigue analyses are performed applying a comprehensive irregular wave time domain analysis procedure. The examples clearly indicate that there is a remarkable improvement in the dynamic behavior of SCRs with the application of such weight optimization along the riser length. The von Mises stress at TDP is reduced considerably. Furthermore, the effect of soil-riser interaction on the fatigue response of such lightweight coated SCRs is much reduced as compared to that of a bare SCR. Hence the weight optimized SCRs with lightweight coating at TDZ presents a very attractive solution for deepwater applications in harsh environments with large vessel motions Normal Riser Heavy weight Riser Light weight Riser Figure 1 SCR concept for harsh environment Availability and limitations to coating on SCRs A key issue in the present study is to restrict the weight optimization to use of qualified materials. When moving to deepwater applications, issues like long-term performance with respect to hydrostatic creep and water absorption become key factors. In some cases a single material provides all of these functions. both for factory coating as well as for field joints. Weight coating During the DEMO2000 project Ti-Rise the Vikoweight coating from Trelleborg Viking was qualified for offshore use through an extensive testing program. and the present temperature limit is 140oC. The project readiness of the various materials for deepwater application differ. Seawater absorption After 32 weeks in seawater the rubber weight coating was experience less than 2% volume swelling. In the present study the preferred method of weight optimization is by use of external coating due to simplicity during installation. The rig was capable of applying impact energy of 12. Cathodic Disbondment Tests of a titanium plate coated with rubber weight coating produced satisfactory results. Additional weight can be achieved by increasing the steel wall thickness. The bonding properties to the weight rubber were good. Some details about the selected coating systems are given in the following sections. The syntactic PP is based on a polymeric matrix incorporating small hollow glass sphere. KRISTOFFERSEN AND LUND OTC 17224 Polymeric external coatings are generally used on flow lines and risers for: • Corrosion protection • Mechanical protection • Thermal insulation However. where different layers provide the various functions. Syntactic polypropylene 5. see Figure 3. Impact testing A titanium pipe with 45mm rubber weight coating was tested in Statoil’s impact rig. and different low density materials are tested by the coating suppliers. A heavy weight rubber coating was qualified through the DEMO2000 project Ti-Rise. but only the coating system made of syntactic PP can be used with reel lay (with diameter limitations). Low density PP coating There are different methods to achieve low density coating and usually a multi-layer technology is applied to fulfill the various functional requirements in a most optimal way. The fatigue life corresponds at least to an operational lifetime of 20 years. Aging tests Aging tests were carried out at 50o C which lasted for 128 weeks. however Soco-Ril’s five-layer syntactic PP coating has been used in the Roncador project at 2000 m water depth and the Bonga project at 1200 m water depth.g. The most common coating systems used in the oil and gas industry are: • Multilayer polypropylene (PP) or polyethylene (PE) • Polyurethane / syntactic polyurethane (PU) • Rubber coating The various coating systems have different density limits which are qualified for deepwater applications. multilayer systems are used. The following tests were included in the program: Bending tests Extensive testing of surface roughness and primer systems has been carried out with good bonding properties for both titanium and steel. see Figure 2. which provide good thermal performance and high resistance to hydrostatic pressure. The fiver-layer syntactic PP coating consists of the following layers: 1. Figure 2 Impact testing of rubber weight coating Wear in touch down area can be handled with a separate outer layer with a higher abrasive resistance than the standard rubber. and buoyancy can be achieved by attaching buoyancy modules. Solid polypropylene Where the first three layers are for corrosion protection. The key factor in the long-term performance of . e. Polypropylene adhesive 3. Thermal conductivity of a generic material is directly linked to the materials density. Horn et al. Fusion Bond Epoxy 2. whilst in other concepts. (2002). MELING. Both the weight coating material as well as the bonding to titanium demonstrated good properties. foamed rubber. The coating density for Bonga was 680 kg/m3 with an OHTC of 2.2 KARUNAKARAN. The syntactic PP coating is presently qualified for use in water depths greater then 2500 m targeting a service life of 20 – 25 years.2 W/moK. Vikoweight is a rubber based weight coating where the density is equal to 3000 kg/m3. Pipe dimensions installed range from 8” – 12” with coating thickness ranging from 34 mm – 102 mm. Thermal insulation is not always a requirement hence other means of weight optimization exist. syntactic PE and PP.7 kJ. Solid polypropylene 4. There exist qualified low density coating systems of syntactic PP and syntactic PU. coating can also be applied for weight optimization by varying density and thickness of the coating. For topside application fatigue analyses of the fireprotection coating has been carried out with successful results. No cracking or damage to the pipe coating was observed in any of the four impacts. These calculations have been performed by SINTEF based on knowledge from flexible risers. size. Fatigue resistance With risers made out of 12 m pipes. stringent fabrication requirements to reduce hydrogen contamination. Due to diffusion of gas through the plastic liner. The fatigue resistance will to a less degree be dependent on the welding method which will introduce different residual stresses. internal corrosion allowance with possibility of fatigue initiation from bottom of local corrosion attacks will drastically reduce the fatigue resistance. cap and root geometry. solid corrosion resistant alloys (CRA) or clad will be utilized when risk for corrosion attacks exists. This solution may require either S-lay.3” to 2. solutions exist for solving this issue by equalizing the bore and annulus pressure [PBJ pressure balanced joint] now being investigated for West Africa applications. The H2S partial pressure might also influence the material selection.g. except practical issues regarding coating production line and installation. and reduced load and strain utilization factors. However. For reeled installation. see Figure 4. Full scale reeling tests have also been successfully carried out for 12” pipes with 3.5” coating thickness. has been extensively used for flow lines. The fiver-layer syntactic PP coating system can be applied to pipes installed with the standard methods. This field joint coating has good flexibility and resistance to hydrostatic pressure and can therefore be used with most common laying methods for water depths over 2000 m. typically less than 30 butt-welds for each riser will be highly fatigue loaded and fatigue resistance beyond what is normally recommended by design standards required. Also the type. By applying the different layers simultaneously a strong adhesion between the layers is secured which is crucial for the coating’ s capacity to withstand the installation loads and also for avoiding disbonding. As fatigue will be the most challenging design criteria. and carbon manganese (C-Mn) steels can be utilized. and additional onshore sorting to select pipes without ovality and by match pipe ends. However for reeling installation the limit will be dependant on the thickness of the various coating layers and the pipe dimensions. while on Roncador the installation method was reeling of the 8” risers with coating thickness ranging from 1. cracking and disbanding. Dissimilar steel qualities can be joined by welding and rather expensive steel quality optimal for fatigue utilized in certain sections. 10” and 12” pipes with 4” and 3. Material selection The flow line risers will internally be exposed to the well fluids. Steel. location and orientation of weld defects are important. where hydrogen from cathodic protection (CP) or welding consumables are one of many factors leading to the failures. The choice of CRA material is dependent on the specific well-stream environment and mechanical properties required. Bell and Tough (2001). Some brittle failures have been discovered in 13% Cr. a maximum coating thickness of 4” can be applied. Hence. With S-lay and Jlay installation methods the limiting factor will be the capacity of the installation vessel and not the coating crushing capacity. Testing so far indicates no cracking nor disbonding and an acceptable level of creep and water absorption. The Bonga risers. the potential for an implosion during sudden pressure release can limit the use of this concept to water injection lines where corrosion resistance is required and where gas will not be present. Figure 3 Hollow glass sphere and syntactic PP coating Figure 4 Water absorption of solid propylene A multi-pass system is developed for application of the coating where multiple layers of different types of polypropylene are applied by a side extrusion process.OTC 17224 Weight-optimized SCRs for Deepwater Harsh Environments 3 the coating is hydrostatic creep. J-lay or towing due to possibly differences in stress-strain behavior which can lead to unacceptable plastic strain-concentrations during reeling. In theory there are no limitations in number of layers that can be applied using this method. Kvaale et al (2004). e. Technip has qualified a solution with carbon steel with plastic liner which requires installation by reeling or towing. So thicknesses of 12” are quoted as practical. For coating of field joints. high quality coatings to reduce the risk of exposing welds to seawater and CP. For dynamic risers only a relatively small part near the hang off and touch down will be significantly fatigued during service. water absorption. Internal weld geometry can be much improved by specifying tight production tolerances and calibration of pipe ends. For export of processed gas and oil corrosion will not be an issue. which will be corrosive mainly due to CO2. The major oil companies have however confidence in these materials provided appropriate measures. were installed with the J-lay method. steel.6” . Also. where Statoil in total is operating more than 400 km. Duplex (22%Cr) and Super Duplex (25%Cr). The coating can be applied to a pipe range from 2” to 48” . On the Norwegian Continental Shelf (NCS) supermartensitic stainless steel (SMSS) which is 13% Cr. Fatigue resistance of welds in highly fatigued areas will be strongly dependent on weld geometry inclusive eccentricity.5” coating thickness respectively. The water absorption of solid propylene at 140o C has been measured to 2-3%. by applying needle hammering or ultrasonic peening residual . a solid elastomer polyurethane insulation material is qualified. g. ULS and FLS limit state criteria In the strength analysis the von Mises stresses are checked along with local buckling checks based on DnV-OS-F101 and DnV-OS-F201. Design Data The riser design is to comply with the DnV-OS-F201. MELING. but the technology exists and it is a question of time and money for optimization and qualifications. A realistic fatigue class for girth welds in air made from two sides is C1 and D for the internal surface for girth welds made from one side. The test environment is extreme and will be very conservative for the NCS. However. The design lifetime shall be obtained using a factor of 0. KRISTOFFERSEN AND LUND OTC 17224 stresses close to the surface from welding can be removed and replaced by compressive residual stresses.4 KARUNAKARAN. (2003). Also. High fatigue resistance requires a reliable detection of very small defects. penetrating the riser into the soil and thereby increasing the soil resistance. However. the weld quality must be controlled and proven free from deteriorating imperfections. regulations and API RP2D/APIRP1111. Environmental conditions The riser is designed for the 100-year wave condition in combination with 10-year current profile. Residual stress methods can only be used on non-plastically deformed risers and has the potential to give a significant improvement to fatigue resistance. Automatic ultrasonic testing (AUT) on solid or clad CRA is not straight forward. based on DnV-RP-C303 are applied: • Production riser C1 or C2 • Gas export riser D The production riser is expected to be fabricated on-shore and installed by a reeling lay vessel. Presently. The water depth at this location is 1000 m. ref Buitrago et al. With the necessary incentive and time virtually defect free welds can be made and verified both for large diameter offshore welded and small diameter reeled risers. The common practice is to model the soil riser interaction by linear soil stiffness and friction. E. 100-year design wave: • Significant wave height • Corresponding wave peak period 15. both the fatigue classes and the influence from H2S will require a qualification program where the production procedure is full scale fatigue tested with sufficient number of valid fatigue tests results. 45º and 70º). Steel • Gas export riser X65 Carbon steel For the fatigue analysis the following S-N curves. a significant effort is put into the problem worldwide with positive results. The riser dimensions are: • Production riser 254 mm ID (10” ) • Production riser 304. the fatigue limit state is the most critical one. Pipeline welding is normally performed by internal clamping. Eddy Current and Time of Flight Diffraction (TOFD). Material data For these riser configurations.0 sec The riser is designed for 10-year current velocity along with the above mentioned wave condition. This together with further development of the systems will find and determine much smaller defects than typically specified for risers today with a high probability of detection (PoD). However. fatigue tests in sour environment based on NACE TM0177 solution B found H2S decreasing the fatigue life significantly for C-Mn and Super Duplex Welds. grinding. the following materials are selected for different applications: • Production riser 13% Cr. Soil-Riser interaction When the riser is subjected to oscillatory motion. By grinding the cap and root noise in the AUT scans will be reduced. The minimum fatigue life required for the risers is 20 years Riser data Three different riser types and dimensions are used. needle or ultrasonic peening and non-destructive testing (NDT). and traditionally the internal clamp is equipped with copper-backing and back purging when welding CRA materials. Pulse Echo (normal probe.1 on the calculated fatigue lives. very limited fatigue data is available on clad material. AUT on clad material is not straight forward. which means that: • • • The developed configurations are to fulfill PLS. Based on the discussions in the materials selection section. Statoil has used a rather complex pig for inspection of a 12” internal pipe with features such as internal grinder supervised by camera. The internal clamp can however also include additional features for internal welding. camera for inspection with high magnification. there is a complex interaction between riser movement and the sea-bed at touch down point (TDP). So the selection of steel quality is dictated by the fatigue performance of the steel quality.6 mm ID (24” ) The production risers have an internal pressure of 345 bars (5000 psi) and the gas export riser has an internal pressure of 200 bars. where offshore welded or towed risers additionally can be improved by introducing beneficially residual stresses. The following parameters are found to be representative for this work: .5 m 16. To use clad in dynamic risers a qualification program is necessary. Both seamless and longitudinal welded pipes can be used where the adjoining butt-welds in any circumstance will limit or be challenging for the fatigue utilization.8 mm ID (12” ) • Gas export riser 609. but also concluded that there should be no change in slope which is found in international standards at typically 10M cycles. The gas export riser may be installed by a J-lay or S-lay vessel. ExxonMobil has in an extensive study of 56 full scale fatigue tests of C-Mn single sided welds with tight tolerances on eccentricity justified the D-curve. Internal welding of small diameters should be possible down to around 8” . the fatigue issue near the flex-joint is normally addressed by a 5 to 10 m long taper section. Vessel data The static vessel offset in connection with the extreme response analysis is 10% of water depth for intact mooring and 12% for one mooring line failure condition. most severe weather comes from North. Drag and inertia forces in the riser axial direction are not considered.8. Upper-end termination In this study the top end of the riser is assumed to be equipped with a flex joint attached to the riser termination point. However. • Sum-up the fatigue damage over all the blocks and obtain the fatigue damage for that direction • Perform the same procedure for all 8 directions and sum-up the total fatigue damage is estimated by applying directional probabilities. However. • Nonlinear dynamic response analysis is used applying irregular waves. Northwest. Also in the dynamic response analysis the vessel motions are generated based on the vessel RAOs . each at one sea state and the fatigue life was estimated. In Northern North Sea. In the fatigue analysis vessel offsets proportional to significant wave height is applied. West and Southwest. • Perform non-linear time domain response analysis and calculate stress time series • Estimate the fatigue damage with in each simulation using rain-flow-counting procedure and weight that with the probability of each block. In this work. The fatigue damage is estimated at 16 points along the circumference of the pipe. the flex-joint stiffness will not influence the response. Hence. Riser configurations Riser hang-off locations The risers are hung from the Northern part of the pontoon as shown in Figure 5 and Figure 6. In this analysis a total time series length of 45 min is used for every sea state. Also. The variability of predicted fatigue life associated with the simulation length was studied in other studies by performing three independent simulations of 45 min. The step-by-step procedure used in the time domain fatigue analysis is described below: • Divide the wave scatter diagram into various blocks. The dynamic response is obtained by performing non-linear dynamic response analysis using random irregular waves applying Airy wave model. No marine growth was explicitly included. • Calculate local force by Morison equation with relative velocities. rotational stiffness of the flex-joint will influence the fatigue response of the riser cross-section closer to the flex-joint. • The static riser configuration is established. Hence. This representative sea state has the highest occurrence rate with in that block.0. . in the fatigue analysis. with constant average acceleration algorithm.method. The riser configuration and tension are calculated at each time step by an iterative procedure and the dynamic response of the riser system is estimated using the Newmark. • Perform non-linear time domain analysis for one representative sea state for each of these blocks. it is concluded that this simulation length is sufficiently long to estimate fatigue response. so is not considered. SINTEF (1995). The variation in fatigue life due to statistical spread was much less than 1 % between the simulations. The dynamic analysis was performed for both 0o and 180o wave directions (in plane with the riser configuration). The large diameter gas export riser is placed within 3 m from the centre of the platform. in the global analysis model the top end is modeled as pinned. The inertia coefficient CM used in this analysis is 2.7-0. Analysis procedure Ultimate limit state analysis The riser configuration is developed by satisfying the ULS design conditions. a comprehensive non-linear time domain fatigue analysis is performed. Hence when the risers are placed in North Western part of the platforms they attract severe fatigue loading both from in-line and from transverse direction. Current is not used in fatigue analysis. The basic configurations are obtained by performing nonlinear dynamic response analysis using the dynamic analysis program RIFLEX. For the extreme loading. The risers have top angle between 15o for the production risers and 17o for gas export riser in vessel mean position. The lateral load case is not critical for riser dimensioning. Lateral load cases may be critical for interference between risers. Fatigue analysis In this work. The 12” production riser is placed 10 m from the centre.9 is used to include a possible increase in external diameter from marine growth.5 10 kN/m2 600 kN/m2 Hydrodynamic coefficients Common practice for this type of marine structures is to apply a CD in the range of 0.OTC 17224 Weight-optimized SCRs for Deepwater Harsh Environments 5 • • • Lateral friction coefficient Lateral soil stiffness Vertical soil stiffness 0. The response analysis is characterized by: • The riser is modeled by FEM principles using discrete beam elements. in this work a CD of 0. while the 10” production riser is placed 30 m off centre of the platform. the flex-joint stiffness is not modeled as the critical cross-section for fatigue is at TDZ which is not affected by flex-joint stiffness. the maximum coating thickness that can be applied without damaging the outer surface of coating is 100 mm. forged or machined ends with locally increased wall thickness can also be used. The gas export riser may be installed by J-lay or S-lay method. However. If forged sections are used. This coating is also qualified for use. This reduces the dynamics of the straight section. So the bare riser is coated with 3 mm PP coating.Coating @ 2800 kg/m3 • Riser configuration As discussed earlier and shown schematically in Figure 1. For deep water SCRs the key design issues are local buckling and severe fatigue damage at TDZ. The variation in weight along the riser is achieved by applying different density coatings along the riser length. in harsh environment.6 KARUNAKARAN. This varying density coating will also act as insulation. for the small area where the fatigue life enhancement is needed. The riser configuration for the Gas export riser is presented in Figure 9. the thicker sections will be offshore welded with class D S-N curve. This way the fatigue performance in the riser in critical areas can also be improved. Hence for production risers. The riser configuration for the 12” and 10” production risers are shown in Figure 7 and Figure 8. the maximum coating thickness is restricted to 100 mm. The features of the riser design are: 0 -200 -400 -600 -800 -1000 0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 X-coordinate [m] Figure 7 10” Production riser configuration Z-coordinate [m] . the wave and vessel motion induced dynamics of the straight section of the SCR will also influence the stresses at TDZ. This is a qualified product and used in many projects. This helps to reduce the propagation of compression waves from vessel heave motion. the riser configurations are developed with varying weight of cross-sections along the length. where there is a need to increase or lighten the weight of cross section. MELING. The riser cross-sections at TDZ are made light by providing insulation coating with a density of 670 kg/m3. As discussed earlier. Heavy stresses at TDZ are due to the “compression waves” due to vessel motions and also due to the up and down motion of long riser sections at TDZ. Also it reduces the dynamic stresses at TDZ arising from the up and down movement of long riser section at TDZ • Heavy riser cross-section on the straight part of the SCR. Hence.coating @ 670 kg/m 3 Heavy riser . this increases the hang-off loads and also increases the dynamic axial stress closer to the hang-off. KRISTOFFERSEN AND LUND OTC 17224 SCR hang off location at pontoon SCR at vessel mean position SCR TDZ Figure 5 Riser hang-off from pontoon 12” Production 10” Production 24” Gas Export Fwd NORTH X Y Figure 6 Riser orientation Lightest possible cross-section at TDZ. Further. The production riser may be installed by reeled lay vessel. a feasible riser configuration is developed by varying the weight of the riser along the riser length. while thin sections which are welded onshore will have class C1 or C2 weld. a coating with varying density is applied. as it is inefficient to weld many joints of 13% Cr. The cross-section at the straight section of the catenary is made heavy by partly increasing the wall thickness and also by providing a coating with density 2800 kg/m3. thereby reducing the dynamic stresses at TDZ. a feasible configuration can only be obtained by successfully controlling the dynamic stresses at TDZ. For reeled risers. For the gas export riser there is no need for insulation coating. for deep water applications. However. Norm al riser Light riser . Steel riser offshore. for the large diameter export riser. Further. 89 0. It is clearly seen that the fatigue critical location is short. it is important to note that the strength analysis is performed with the same vessel offsets and wave and current data for both near and far positions. This can be improved by using larger wall thickness. all three risers have sufficient strength capacity.80 Accidental 0. the entire length of riser has an insulation coating thickness of 100 mm. The summary of the strength analysis are presented in Table 1.91 0.curve C2 – curve D . Where the riser is made light or heavy 75 mm coating is provided with 670 kg/m3 and 2800 kg/m3 coating densities respectively. The fatigue life along the riser length for 12” production riser is shown in Figure 10.curve At top C1 .91 0. This shows that it is easily possible to achieve a robust riser design. This is due to the dynamic axial stress due to vessel motions. For the gas export riser the entire length of riser has a 3 mm corrosion protection coating. At TDZ C1 – curve C2 – curve D .83 0 -200 -400 -600 -800 -1000 0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 X-coordinate [m] Static top 15o angle in mean Top tension Static 2441 kN Maximum 4033 kN Normal Maximum 4385 kN Accidental Buckling utilization Normal 0.74 7 Norm al riser Light riser .93 0. Also.Coating @ 670 kg/m 3 Heavy riser .curve . Production risers The fatigue performance of the production riser is presented in Table 2.78 Accidental 0. The von Mises stress utilization presented in Table 1 is by normalizing the calculated maximum von Mises stress by the allowable stress. The most critical section for buckling and von Mises stress is at TDZ. The strength analyses were performed for both near and far position vessel offsets with 100-year wave and 10-year current. it is shown in Table 2 when the riser is placed on the Southern side of the platform. Where the riser needs to be light.64 0. Table 2 Fatigue response (years) – Production riser 10” production riser 12” production riser Riser in Riser in Riser in Riser in North side South side North side South side 1205 721 454 585 354 227 1778 1086 698 2102 1234 764 750 447 306 479 290 202 2033 1173 779 2331 1359 910 Figure 9 24” Gas Export riser configuration For the production risers.OTC 17224 Weight-optimized SCRs for Deepwater Harsh Environments Table 1 Strength analysis results 10” production 12” production riser riser 15o 3297 kN 5599 kN 6018 kN 0. Riser response Strength performance All the risers are analyzed for operating extreme condition and accidental extremes.8 times yield stress for normal extremes and 1.75 m m coating @ 670kg/m 3 Heavy riser with 75 m m coating @ 2800 kg/m 3 Z-coordinate [m] 0 Z-coordinate [m] -200 -400 -600 -800 -1000 0 500 1000 1500 2000 2500 X-coordinate [m] As seen from the results shown. Further. All three riser configurations have sufficient strength capacity both for normal and accidental extreme cases.58 Figure 8 12” Production riser configuration Bare riser L ight riser .92 0. all configurations have sufficient margin for strength. where the weld quality and pipe matching tolerance can be improved thereby C1 or C2 weld can easily be obtained to get fatigue life in excess of 1000 years. which is 0. Fatigue performance The wave induced fatigue analysis of all three riser configurations are performed using a comprehensive time domain fatigue analysis procedure explained earlier.Coating @ 2800 kg/m 3 24” gas export riser 17o 5505 kN 7777 kN 7978 kN 0. It is seen that for both riser dimensions.0 times yield for accidental extremes. Further. the fatigue performance of the risers are improved considerably. As seen the riser configurations have sufficient fatigue life even with D-curve. which is conservative. The top 500 m section has fatigue life less than 500 years. The critical locations for fatigue are at TDZ and close to the top end. It is noted that about 150 m at TDZ has fatigue life below 500 years. the coating density is reduced to 670 kg/m3 and to make heavy the density is increased to 2800 kg/m3. the fatigue life is in excess of 750 years even with D-curve.67 Von Mises stress utilization Normal 0. This is due to the dynamic axial stress due to vessel motions.00E+03 TDZ Fatigue limit .0E+02 1000 1500 2000 2500 3000 Riser length [m ] Figure 13 Fatigue life along the riser due VIV – 24” Gas Export riser – Riser in North side of the FPU . The fatigue damage from the different current profiles was weighted and total accumulated damage during 20 years service life predicted. It is seen that the fatigue critical location is short.200 years 1. if the riser is placed on the Southern side.0E+03 Fatigue damage due to vortex-induced vibrations (VIV) An assessment of the fatigue contribution from VIV was carried using VIVANA (2004). MELING. 1.0E+04 1. Further. As seen the minimum fatigue life is at least 1000 years with a uni-directional approach. Here the current is in-line with the riser and D-curve is used. The critical locations for fatigue are at TDZ and close to the top end.200 years 100 1000 1500 1.D-curve 1. The fatigue life along the riser length for the gas export riser is shown in Figure 11.3 – 1. As seen the riser configurations have sufficient fatigue life even with D-curve. the configuration has much better fatigue life as seen from the same table. Ten 2D current profiles ranging from 70% exceedance probability to the 1-year profile were used – surface velocity ranging from 0.0E+02 1000 1500 2000 2500 3000 Riser length [m] Figure 12 1. By simply adding the VIV damage to the WF damage a target service life of 20 years is still reached with a safety factor of 10.24" Gas Export riser . This can be improved by using larger wall thickness.0E+04 1. 1.0E+06 Fatigue life [years] 1.0E+05 Fatigue life along the riser due to VIV – 12” Production riser in North side of FPU At TDZ Normal riser 10% thickness increase 20% thickness increase At top Normal riser 5% thickness increase 498 711 338 407 1413 2015 1274 1557 Fatigue life [years] 341 964 1.8 KARUNAKARAN. Using few thick end joints will not increase the total cost of the riser significantly The top 500 m section has fatigue life less than 500 years.0E+03 1.00E+02 1000 1500 2000 Rise r le ngth [m] 2000 2500 3000 2500 3000 Riser length [m] Figure 10 Fatigue life along the riser – 12” Production riser in North side of FPU Figure 11 Fatigue life along the riser – 24” Gas Export riser – Riser in North side of the FPU Gas Export riser The fatigue performance of the gas export riser is presented in Table 3. This shows that the developed configuration if placed in some favorable parts of the platform will have very good fatigue performance. Table 3 Fatigue response (years) – Gas Export riser Riser in North side Riser in South side The un-factored fatigue life for the gas production riser is shown in Figure 12 and for the gas export riser in Figure 13. The fatigue life at these cross-sections can be improved by using thicker end forged or machined pipes. It is noted that about 200 m at TDZ has fatigue life below 500 years.12" Production riser D-curve 10000 Fatigue life along riser length .00E+04 Fatigue lfe [years] Fatigue life [years] 1000 TDZ Fatigue limit .0E+05 1.2 m/s. KRISTOFFERSEN AND LUND OTC 17224 Fatigue life along riser length . A.02 References .STF70 F95218. A. These design challenges can be successfully addressed by varying the weight of the riser along the length. • The application of this lightweight coating at TDZ reduced the effects of soil-riser interaction on the fatigue response at TDP. “Design of Risers for Floating Production Systems (FPSs) and Tension-Leg Platforms (TLPs).Flexible Riser System Analysis Program User Manual". MARINTEK and SINTEF Division of structures and concrete report .. Marintek Report 516419. P. M.3. Wan C.OTC 17224 Weight-optimized SCRs for Deepwater Harsh Environments 9 Conclusions In this paper the design challenges for the SCRs from large motion FPU in harsh environment are discussed. T and Lund. 2000.M. Clausen. “Fatigue design and performance verification of deep water risers” . 2001. P. The weight optimization is achieved by qualified and readily available coatings within their tested and qualified density and thickness limits. The variation in riser weight is obtained by applying heavy and light weight coating along the riser length. and Fossesholm. thereby reducing the effect of uncertainty in the soil stiffness parameters Acknowledgement The authors would like to acknowledge Subsea 7 and STATOIL for permission to publish the results presented in the paper. “Design.” 1st ed. Oslo. 2004: “VIVANA – User’ s Manual. 2001 Horn. D. It is emphasized that the conclusions put forth reflects the views of the authors alone.W. Michael S. OMAE 2004-51536 SINTEF. Version 3. The availability of such coatings and their properties are discussed.6” .E. OMAE2003-37492 DNV-RP-C203. Bjørnbakk. 1995. 2002: “Steel Catenary Riser Configurations for Large Motion Semi Submersibles with Lightweight Coating” . K.M. API RP 1111..” 3rd ed. Rørvik G. 2000 DNV-OS-F201. and not necessarily those of Subsea 7 or STATOIL. Bell. 1995: "RIFLEX . Construction.. OMAE 2001 Buitrago J. Dynamic Risers. T.K. June 1998. The following conclusions can be drawn from this paper: • Feasible robust design of SCRs for harsh environment from large motion FPUs are possible by applying weight optimized risers. Røstadsand.. Submarine Pipeline Systems. B. Håbrekke T. New Orleans Kvaale P. Hauge. Karunakaran. Combining both VIV and wave induced fatigue gives fatigue life in excess of 200 years even with D-curve. DOT-2002.02. 2001 “Fatigue strength analysis of offshore steel structures” DNV-OS-F101. By varying the weights along the riser feasible SCRs are designed for 10” and 12” production risers and a 24” gas export riser from a semi-submersible platform. Dutta. VIVANA. API RP 2RD.. This riser concept is demonstrated for three riser applications. July 1999. 2002:” Cost Effective Fabrication of Large Diameter High Strength Titanium Catenary Riser” . • Sufficient fatigue life is achieved even for large diameter risers from semi-submersible in harsh environment • All the riser configurations have sufficient fatigue capacity due to VIV also. Dahlberg.. Operation and Maintenance of Offshore Hydrocarbon Pipelines (Limit State Design). 2004:” Experience with qualification and use of stainless steels in subsea pipelines” . OMAE2002. H and Tough G 2001: “Plastic Lined Steel Pipelines for Hydrocarbon Transport’ .