In order to achieve overall cost effectiveness, all FLNG (Floating Liquid Natural Gas) projects must use large volume of deep sea cold water for their cooling process. This has prompted FLNG projects hanging several risers (about 20 to 30 inch diameter) vertically straight down from the platform to a depth between 500 to 1000 feet. Static and dynamic motion analyses of these risers reveal conflicting design requirements. Low stiffness and light weight help to decrease stress and moment reaction on the FLNG vessel and riser, while associated large motions cause problem with interference between risers. High stiffness and heavier weight help to increase minimum bend radius (MBR) and decrease the riser motions and interference. Weight mass affects the dynamics but provides valuable restoring moment thus reduces static displacement due to current forces. Present work describes details of a concept development study, based on the pertinent design parameters, static and dynamic tuning of riser connection, and the development of an innovative solution. This study has demonstrated that simplest alternative solution is a single caisson type riser employing available large diameter (60 to 70 inch) pipes with different options of connections at the platform. Other potential materials of choice can be Glass Fiber Reinforced Plastic (GFRP), High Density Polyethylene Pipe (HDPE), and high strength steel, or a hybrid combination of steel with titanium or aluminum. Various options of constructions and installations have also been examined
Proceedings of the 19th Offshore Symposium, February 2014, Houston, Texas Texas Section of the Society of Naval Architects and Marine Engineers INNOVATIVE CAISSON TYPE SEAWATER INTAKE RISER DESIGN CONCEPT FOR FLNG GAUTAM CHAUDHURY TECHNICAL AUTHORITY, MCS KENNY VENKAT CHAKKARAPANI STAFF CONSULTANT, MCS KENNY ABSTRACT In order to achieve overall cost effectiveness, all FLNG (Floating Liquid Natural Gas) projects must use large volume of deep sea cold water for their cooling process This has prompted FLNG projects hanging several risers (about 20 to 30 inch diameter) vertically straight down from the platform to a depth between 500 to 1000 feet Static and dynamic motion analyses of these risers reveal conflicting design requirements Low stiffness and light weight help to decrease stress and moment reaction on the FLNG vessel and riser, while associated large motions cause problem with interference between risers High stiffness and heavier weight help to increase minimum bend radius (MBR) and decrease the riser motions and interference Weight mass affects the dynamics but provides valuable restoring moment thus reduces static displacement due to current forces Present work describes details of a concept development study, based on the pertinent design parameters, static and dynamic tuning of riser connection, and the development of an innovative solution This study has demonstrated that simplest alternative solution is a single caisson type riser employing available large diameter (60 to 70 inch) pipes with different options of connections at the platform Other potential materials of choice can be Glass Fiber Reinforced Plastic (GFRP), High Density Polyethylene Pipe (HDPE), and high strength steel, or a hybrid combination of steel with titanium or aluminum Various options of constructions and installations have also been examined Keywords: Seawater Intake Riser, FLNG INTRODUCTION Substantial numbers of discovered gas reservoirs in deep waters around the world are just waiting to be developed as marinization of safe, compact, and cost efficient FLNG process technology matures Present sociopolitical environment around the world, strong demand with favorable pricing, and eagerness to finance are all indicator of a strong growth potential in FLNG projects This is demonstrated in the initiation of several recent FLNG projects such as Prelude, Bonaparte, and Scarborough In addition to large capital costs, FLNG project economics is subject to very high operating cost for cooling The obvious choice is to reap as much benefit as feasible of free unlimited supply of cold deep sea water Cold water is pumped up from a depth of 500 to 1000 feet below the sea surface Depending on the size of LNG production capacity typical volume of water requirement is about 30,000 to 60,000 m^3/hour Most projects so far has opted for pumping sea waters through multiple sea water intake risers (SWIR) each delivering 5,000 to 6,000 m^3/hour Generally, inner diameters of these risers are about 0.75m (30 inch) While these straight down risers appear simple in a vertical configuration, the design is quite challenging when trying to satisfy all design criteria, especially if the design environment is harsh and platform responses are poor A Proceedings of the 19th Offshore Symposium, February 2014, Houston, Texas Texas Section of the Society of Naval Architects and Marine Engineers soft pipe (low bending stiffness) suits better to withstand high drag load from attending wave and current The down side is increased static and dynamic deflections of the risers causing interference between adjacent risers The large lateral separation required to mitigate interference problem affects process equipment layout significantly A pragmatic and innovative new design concept is to use a single large diameter pipe like a caisson type riser reaching the design depth and pumping cold water from near the surface by several individual suction pipes and pumps The main riser acts like a caisson This arrangement eliminates the interference issue and provides ultimate flexibility in process layout The following sections depict details of the development of such a concept design demonstrating technical feasibility in terms of design, construction, and installation Detail engineering and costing is outside the scope of this present work Candidate project needs to perform due diligence engineering, based on respective operational and functional requirements EXISTING, EVOLUTION, AND PROPOSED DESIGNS At present, designs under consideration are soft bonded rubber hose pipes (ContiTech, Trelleborg, and Dunlop) of about 24 to 30 inch diameter and 12m (40 ft) long They are connected by bolts through built in metallic flanges at their ends The hose pipes have higher bending stiffness and weight mass at the ends compared to the middle section High stiffness at the ends comes from steel reinforcements required to connect the end flanges (complex construction) At the platform interface one special hose with much higher bending stiffness or, in some cases, a bend stiffener is used to smoothly resist the bending moment generated It is envisaged that glass fiber reinforced plastic (GFRP) and high density polyethylene (HDPE) pipes may be used as well (Dong-ho-Jung and Franck Rogez) These pipes require distributed weights for sinking as well as to generate restoring moment Careful investigative study indicates that dynamically optimized weight pipe helps reduce static and dynamic lateral displacements through restoring moments generated due to lateral offset from the hang-off point (Figure 1) However, during angular excursions weight moment is not restoring but enhancing Ideal design will be a weight optimized pipe yet flexible at the hang-off to initiate offset and associate restoring moment to limit the extent of the overall displacement Natural choice is steel pipe with a flex element at the platform connection which has been selected by Shell Prelude through a patented design The design consists of multiple risers and as such subject to interference problem unless risers are laterally separated adequately In this case, the problem is resolved by bundling the risers The system is complex The quest for simplicity in design, and the realization of the fact that weight restoring moment is a major contributor in controlling stress lead the way to a most cost effective and pragmatic sea water intake riser solution, a single caisson riser By eliminating the interference problem, the riser system can be freely tuned to obtain the best compromise and balance between riser weight, flexibility at hang-off, and material combination depending on respective project demand The proposed concept design aims to resolve interference issue with a simple design by selecting one large diameter pipe, a caisson type design The choice of material is preferably steel but GRFP and HDPE pipes may be used with design optimized weights A hybrid combination of materials (Steel, Aluminum, Titanium, GFRP, HDPE, Bonded hose) may also be considered A single large diameter pipe design also benefits from hydraulics (pressure head loss) and heat transfer standpoints Other associated advantages are presented below; Less drag load on the vessel due to net reduction in projected area Less numbers of support structure sections at the vessel Less numbers of installation activities Avoids complexity of multiple riser installations REDUCED PRESSURE HEAD LOSS Pressure head loss may be calculated by use of the simple formula ∆ (1) The friction factor ‘f’ depends on Reynolds number Re and relative roughness Rr, and it can be obtained from Proceedings of the 19th Offshore Symposium, February 2014, Houston, Texas Texas Section of the Society of Naval Architects and Marine Engineers Moody pipe friction chart for any given Re and Rr combination Assuming four small diameter risers replaced by one single large diameter, the diameter ratio (d/D) is 0.5 For any given roughness parameter, the relative roughness of the large diameter is half that of the small diameter and Reynolds number of large diameter is twice that of the small diameter For an example case (Re=2*10^5 and Rr=0.0002 Vs Re=4*10^5 and Rr=0.0001), from Moody’s chart friction factor of large diameter is 1.162 times less Using the reduced friction factor in equation 1, the pressure head loss in large diameter riser is 43% of the head loss in small diameter pipe Figure Force and Weight Moments during Static and Dynamic Responses REDUCED HEAT TRANSFER LOSS From the heat transfer correlation work of Dittus-Boelter (1930) (Rohsenow, W et all), the heat transfer coefficient ‘h” in terms of thermal conductivity ‘Kw’ and caisson diameter ‘D’ is given by the following formula; Where Nusselt number 0.023 (2) By replacing the Nusselt number in equation 2, the heat transfer coefficient ratio of small diameter ‘d’ to large diameter ‘D’ may be obtained as 1.15 from Equation below (3) In terms of net heat gain by the cold water inside there will be another factor of from the increased surface Proceedings of the 19th Offshore Symposium, February 2014, Houston, Texas Texas Section of the Society of Naval Architects and Marine Engineers area for the small diameter risers Therefore, for the large diameter riser net gain in terms of not getting heat from water outside is 2.3 times This means water received at the topside process end will be cooler for a large diameter caisson riser ALTERNATIVE DESIGNS Most simple and inexpensive base case main riser pipe is expected to be a steel tube connected rigidly at the platform Detail cost study, including manufacturing and installation, may show otherwise The present base case study is based on a full length steel tube The design variations will be in the way the caisson riser pipe is connected to the platform Depending on the riser size and attending environment, construction, and installation most optimized option may be selected Option 1: The caisson riser pipe is rigidly connected to the platform at any convenient location The pipe will be sized to satisfy water volume need and all design load and fatigue requirements The connection system has to be designed to withstand all the moments and fatigue loads imparted by the riser pipe This option will attract most moment load on the platform/riser interface This is because of rigidity of the pipe Deflection will be less, consequently, less restoring moment from the pipe distributed weight Another variation may be to connect the caisson at two locations (at deck level and keel level) and support moments by corresponding reaction shear forces Option 2: In this option, the riser tube is configured at an angle from the connection point thereby generating some initial restoring moment at the connection By careful design static moment due to current can be reduced to half the maximum moment value of a straight configuration The connection should be made such that it allows the riser to rotate freely about vertical axis, that is, it weathervanes The benefit will be in the maximum design stress due to reduction in mean stress There will be no benefit from the dynamic moments, that is, no fatigue improvement Option 3: In this option, the design and connection system may be more complex but it is suitable for extremely severe environmental condition The design relies on dynamic tuning of the system parameters such as riser pipe weight distribution, flexibility at hull interface, and relative stiffness of caisson and the suction pipes For example, a complex hybrid system, which uses soft small diameter HDPE or hose pipes for pumping water from the caisson The rigid caisson pipe is connected in such a way that it can freely rotate about the two horizontal axes, except for the equivalent rotation stiffness provided by the soft pipes The free rotations may be tuned only up to a small angle (2 to degrees) depending on riser size, respective environments, and design criteria The top ends of the soft pipes are fixed at some point above the caisson riser and the bottom ends are inside the caisson tube at some point below the mean water level thus providing some rotational stiffness depending on their bending stiffness and length If necessary, the system design may be tuned to work with materials (Steel or Aluminum) other than HDPE or hose on a case by case basis For all options, the caisson pipe may be connected at the bottom or at the side of the hull to suit the process lay out and cooling water routing provided there is no interference of the caisson riser with turret or mooring lines CAISSON/HULL/SUCTION PIPE INTERFACE In the caisson riser concept, there are two interfaces which are to be carefully designed, caisson/hull and caisson/suction pipes There are various alternatives to the basic caisson concept depending on the caisson/hull connection which is a function of the environments and attendant platform motions For a simple rigid connection (base case, Figure 2) the primary interface is at the keel level such that minimum moment arm is active If an open top ended caisson is used then the caisson pipe need to continue at a height higher than maximum seawater level at maximum draft of the platform and the bottom end of suction pipes should be at a level lower than the lowest level of seawater at minimum draft of the platform In this case, the moment may be supported as shear forces For a closed and sealed ended caisson design, caisson pipe may end at the rigid connection, and suction pipes should continue below the seawater level at minimum draft This design eliminates extra length of caisson pipe but adds complexity of welding suction pipes to the top end diaphragm plate of the caisson Proceedings of the 19th Offshore Symposium, February 2014, Houston, Texas Texas Section of the Society of Naval Architects and Marine Engineers CAISSON RISER PIPE SIZING The rate of water volume required will depend on the process type, its efficiency, and cooling water temperature On an average, 14,000 m3/hour water is required per M-Ton/year LNG processing Assuming a typical 3.0 M-Ton/year production net water required is 42,000 m^3/hour While in order to avoid impingement of fish and other marine life water intake velocity is limited to 0.5 ft/s for all sea water intake system nearer the offshore platforms, limits on deep sea water intake velocity is around to m/s Assuming m/s intake velocity leads to a riser pipe inner diameter of 1.93m ndustry survey confirmed that 2.5m diameter pipes (LSAW, SSAW, and DSAW) are available Due to low heat transfer a single large diameter riser may require less volume of water or riser length may be reduced Figure Basic Conceptual Layout (Outside shown, similar arrangement is possible at bottom of keel through a moonpool) CONCEPT DESIGN VERIFICATION Design verification of the simplest concept (single rigid connection – option 1) has been performed employing a steel riser pipe (1.83m (6ft) diameter and 50mm (2-inch) thick) and 450 MPa yield strength) A typical NorthWestern Australian environment combined with a generic highly conservative FLNG platform RAOs have been used to compute static and dynamic response and stress calculations All first pass base case analyses were performed using a 152m (500ft) long steel caisson riser in a vertically straight configuration connected rigidly at the platform keel level The industry standard program Flexcom was used in all analyses and some further validation check was conducted in ABAQUS The present scope is limited to concept verification and not full detail engineering Static Strength Highest static stress is at the base of the riser connection due to maximum current which is well within the Proceedings of the 19th Offshore Symposium, February 2014, Houston, Texas Texas Section of the Society of Naval Architects and Marine Engineers allowable limit The maximum stress for a 2.7m/s (9 ft/s) current is 140 MPa (20.4 ksi) This means, the concept is expected to satisfy static strength criteria in all environments worldwide Extreme Dynamics Strength First pass dynamic analysis was conducted, based on stress RAO, using conservative regular beam sea waves in time domain taking account of all non-linearities For further conservativeness, unit wave amplitude was used in the stress RAO generation For a 10m amplitude wave, maximum stress (Static + Dynamic) at the base is approximately 440 MPa The high stress value lead to generation of realistic stress RAOs using steepness based wave amplitudes that better accounts the velocity squared damping Using steepness based wave RAOs, the maximum stress is approximately 310 MPa It is likely that the design would satisfy dynamic design storm criteria for all environments worldwide At locations where environment is very harsh turret moored FLNGs are used which will not result in such high stress as obtained here with beam sea wave In addition, irregular wave run will improve dynamic stress a lot Wave Fatigue Resistance Fatigue damage calculations have been conducted by using simple conservative spectral procedure Stress RAOs (Figure 3) are determined from regular wave runs within the frequency range of interest where wave energy is present The RAOs were synthesized with yearly fatigue sea state spectra to generate corresponding response stress spectra and associated stress standard deviations Damage for each sea state was calculated by employing the well documented formula (Equation 4), based on S-N approach and narrow band approximation of the stress spectrum Damages from Individual sea states were then added assuming Palmgren-Miner (PM) hypothesis S-N parameters were selected from the DnV “C” type fatigue curve, based on the reason that welds can be performed from both inside and outside and toes grinded at least for the critical joints Figure Stress RAOs (4) Proceedings of the 19th Offshore Symposium, February 2014, Houston, Texas Texas Section of the Society of Naval Architects and Marine Engineers All fatigue seas were considered as omnidirectional and long crested For the base case rigid connection hangoff, unit beam sea wave run RAOs are too severe to satisfy extremely severe NW Australian fatigue seas (worse than UK North Sea) For example, the wave scatter diagram used here has 73% of waves between 1.5m Hs to 3m Hs and 74% of the waves are between 10Sec to 16Sec periods as opposed to typical North sea wave scatter as 95% below 2m Hs, 50% below 1m Hs, and 95% are below 7.5s period (Hogben N et al) A new set of wave steepness based (wave amplitude = 0.05.T^2) RAOs were generated which showed some improvement but not as much to satisfy 20 years of factored fatigue life So far the RAOs were generated by passing beam sea waves As FLNG vessels in harsh environments will be turret moored a further set of RAOs were generated, employing quartering sea waves This is still very conservative for a turret moored platform, especially when fatigue stress is governed by angular excursions The improvement was enough to just satisfy fatigue requirement (unfactored life = 250 years) for a rigidly connected hang-off when subject to extremely harsh NW Australian fatigue sea states On a case by case basis, fatigue life may be further improved from the benefit of irregular wave analysis, rainflow cycle count technique, and taking account of the wave spreading (short crested seas) The benefit of directionality will be automatically included when full account of the platform weather vanning is considered Further investigation revealed that during angular rotations weight induced moment actually enhance riser moment at the base rather than reducing as does lateral displacements This poses fatigue problems because vessel motions RAOs have unusually high roll responses even at short wave periods The conclusion is that the concept design is governed by fatigue The hang-off connection and riser weight may be dynamically tuned, based on specific environment and vessel motion RAOs, to achieve the best compromised design VIV Fatigue Increased diameter will help reduce the number of possible frequencies of excitations and increase the period of oscillations thereby reducing the VIV induced fatigue damage Table shows limits of frequencies for a given current velocity It is prepared by using the formula for reduced velocity presented in Equation (5) Table presents the range of natural frequencies for a 2m diameter pipe between a range of current velocities from 0.5m/s to 2m/s assuming that self-excitation of VIV may synchronize within a reduced velocity range between to From the Table range of frequencies of possible excitation are between 0.03125 Hz to 0.25 Hz, that is, a period range of 4s to 32s Fundamental frequency of a free hanging vertical riser is a pendulum mode type Therefore, in most cases, only one or may be two modes may get excited thereby limiting possible VIV fatigue damage First three natural periods were 19.6, 3.6, and 1.3 seconds This means, only the first mode is likely to excite In the extreme cases, any VIV fatigue issue can be easily resolved by using suitable strake Table 1: Natural Frequency Limits of VIV Excitation, Based on Reduced Velocity Band of to Natural Frequency Limits of VIV Excitation, Based on Reduced Velocity Band of to Current velocity (m/s) NF Lower limit in Hz (Ur = 8) NF Upper limit in Hz (Ur = 4) 0.5 0.03125 0.0625 1.0 0.0625 0.125 2.0 0.125 0.25 VIM and Second Order Wave Fatigue A ship shape hull is unlikely to generate well defined and correlated shedding of alternate vortices necessary to gradually synchronize and lock-in with its natural frequency A comprehensive literature survey did not reveal any reported case of VIM of ship shape platform Therefore, VIM induced fatigue may be ignored Second order slowly varying lateral motions of a platform is the result of resonance of the platform at its natural frequency with the wave energy at difference frequencies Natural periods of lateral motions for large FLNG platforms are in the range of 200 seconds Therefore, resulting stress computation may be treated in a quasi-static manner From API RP 2SK maximum RMS response during design storm is estimated about 15ft (Amplitude is 22ft) Average long term response amplitude for fatigue may be considered about 10ft Maximum velocity Proceedings of the 19th Offshore Symposium, February 2014, Houston, Texas Texas Section of the Society of Naval Architects and Marine Engineers amplitude is 0.314 ft/s Cyclic stress range due to drag resistance force on the riser is negligibly small to cause any appreciable fatigue damage DESIGN CONSTRAINTS AND OPTIMIZATION Response behavior of a vertical end free riser is quite different to those with end fixed vertical top tensioned and catenary configured risers Static current force causes large displacement at the free end providing restoring moment at the base there by reducing overall design moment Similar benefit is also observed during lateral motion of the platform Drag resistance force generates moment at the base which gets reduced by the weight restoring moment Unfortunately, during angular excursion the opposite happens Moment due to weight in this case is additive to the base moment Therefore, design optimization requires careful dynamic tuning of the primary parameters such as riser weight distribution, bending stiffness, compromise between strength and fatigue, and implementation of fit for purpose engineered solution NEW TECHNOLOGY RISK ASSURANCE A caisson riser with a simple rigid connection system has no new technology Introducing complex system to attain flexibility in the proposed design concept may require some new technology or new applications Otherwise, while the concept is novel and it may appear to be application of new technology, the constituent elements are essentially tried, tested, and proven Large diameter LSAW pipe manufacturing is well matured, including girth weld joining Cold form LSAW pipes are known to loose collapse strength due to cold work strength hardening Seawater intake risers are nearly pressure balanced, except for small suction pressure Therefore, collapse is not an issue even if the D/t ratio is high Weld quality is expected to be better because of access from inside and outside Tight control should be required for misalignment, perhaps some pipe matching may be used for few critical joints at the top end LSAW pipes have better thickness tolerance In relation to construction issues, it is in a better position than rubber hose with complex steel reinforcements at ends Quality assurance of rubber/steel hose manufacturing is extremely difficult Even though the consequence of failure of seawater intake riser is minimal, focus should be given to the support design such that high reaction loads are robustly designed against Therefore, there is no added risk associated with the large diameter caisson riser concept In operational consideration, overall redundancy is less because of one single caisson Therefore, the design and construction must be robust such that probability of failure is extremely small CONSTRUCTION AND INSTALLATION A pipe manufacturer survey results show that LSAW, SSAW, and DSAW pipes within the size range are available It is envisaged that LSAW pipe will be better for fatigue, in that, it has substantially less weld length than spiral weld pipe Primary joining method of the pipes should be by means of girth welding, preferably from both inside and outside For multiple piece transportation and installation few welded flange or threaded connectors may be used A single piece installation is probably the simplest Actual method of installation will depend on the riser length and other onshore facilities nearby Installation from platform In this procedure, suitable lengths of pipe with welded on end connector will be made at onshore facility Actual joint length should be carefully determined, based on attending transportation and handling limits Pipe joint lengths will be stalked like the way vertical top tension risers are made up and deployed Installation from Outboard In this procedure, final installation on to the vessel will be as a single piece, while the transportation could be in two or three pieces depending on the total riser length, transportation method, and local onshore facilities Single or Proceedings of the 19th Offshore Symposium, February 2014, Houston, Texas Texas Section of the Society of Naval Architects and Marine Engineers multiple pieces may be brought in by barge or towed In case of multiple piece construction and transportation, they should be joined to a single piece at site and floated out At this point both ends of the riser are sealed, attached with pull in cables, and cables passed on board the platform pull in system The seal at top end of the riser should be fitted with a control valve that can gradually release air from inside as required Once all systems are ready, the bottom end seal cover should be removed initiating a gradual submergence of the bottom end Control valve will release air slowly such that the complete riser sinks gradually and attains vertical configuration, while transferring the load on to the platform pull in system (Figure 4) Air release rate should be carefully calculated such that a safe velocity of descend is maintained throughout the operation If necessary, a tension wire may be connected at the bottom to control submergence without overstressing Finally, the risers will be pulled in and placed in its proper location Figure Installation Sequence CONCLUSIONS AND FUTURE WORK The work has demonstrated that proposed large diameter deep sea water intake riser system concept is technically feasible, simple, eliminates challenging interference problems, and provides ultimate flexibility for process train layout Riser self-weight induced moment (enhancing during angular excursion and restoring otherwise) is a very important parameter for highly tuned and optimized design A single large diameter caisson type riser concept can be constructed from various types of materials, including hybrid combinations However, structural integrity needs to be satisfied In a single rigid connection design, there is no added risk in comparison to multiple rubber hose design There are a lot of associated benefits to be gained from a single large diameter riser as opposed to several small diameter risers Design and construction must be very robust Further investigation is required to perform detail engineering design of different alternative options and materials, including respective installation considerations and cost comparisons REFERENCES Design and Analysis of Station keeping Systems for Floating Structures, API Recommended Practice 2SK, 3rd Edition, October 2005 Dong-ho-Jung, Hyeon-Ju Kim, and Deok-soo Moon., “Dynamics of Large Diameter Riser”, Proceedings, Twentieth International Offshore and Polar Engineering Conference, 2010 Proceedings of the 19th Offshore Symposium, February 2014, Houston, Texas Texas Section of the Society of Naval Architects and Marine Engineers Franck Rogez, “Deep large seawater intakes: a common solution for Floating LNG in Oil & Gas industry and OTEC in Marine renewable energy”, 4Th International Conference in Ocean Energy, Dublin, 17th October, 2012 “The Moody chart for pipe friction with smooth and rough walls”, Glasgow College of Nautical Studies, Faculty of Engineering W Rohsenow, J Hartnet, Y Cho., “Handbook of Heat Transfer (3rd Edition), McGraw-Hill Hogben N, Dacunha N.M.C and Oliver G.F., “Global Wave Statistics” British Maritime Technology, London, 1986 10 ... the caisson riser with turret or mooring lines CAISSON/ HULL/SUCTION PIPE INTERFACE In the caisson riser concept, there are two interfaces which are to be carefully designed, caisson/ hull and caisson/ suction... sea water intake riser solution, a single caisson riser By eliminating the interference problem, the riser system can be freely tuned to obtain the best compromise and balance between riser weight,... respective project demand The proposed concept design aims to resolve interference issue with a simple design by selecting one large diameter pipe, a caisson type design The choice of material is preferably