Design and Modelling of Riser Turret Mooring Systems Dr.Fabrizio Pistani, Floating Systems Group, Perth AOG2015 Perth WA March 11-13 Presentation outline The concept Design philosophy and motivations The software tools Calibration of the numerical models RTM column modelling Some results Tandem offloading Disconnection Conclusions Further Developments 2 - www.woodgroup.com – AOG2015 Perth 11-13 March RTM concept Buoyant column about 100m Universal Joint connection Chain table for mooring Risers connected at mid length 3 - www.woodgroup.com – AOG2015 Perth 11-13 March RTM concept De-coupling of motions Reduction of loads on the FPSO arm 4 - www.woodgroup.com – AOG2015 Perth 11-13 March RTM concept Evasion from tropical cyclones RTM sinking under water surface Will re-connect when cyclone passes All RTM columns are in Australian waters 5 - www.woodgroup.com – AOG2015 Perth 11-13 March Project Scope Mooring design analysis of an FPSO moored through a disconnectable, bow mounted, Riser Turret Mooring system RTM Analysis scope aimed to verify that the mooring system meets all operational, survival, and installation requirements Tasks: Connected strength assessment (including tandem offloading) Disconnected strength assessment Service life (fatigue) assessment Disconnection / reconnection analysis Mooring / risers interference assessment 6 - www.woodgroup.com – AOG2015 Perth 11-13 March Design Traditionally => Extensive model test campaigns Presently => Numerical tools 7 - www.woodgroup.com – AOG2015 Perth 11-13 March Scope Design the mooring system for a vessel that is already existing… …but will be refurbished and installed: • With a different mooring layout • On a different site • With a similar RTM column Using: model tests carried out for the original configuration for calibration of the new system layout; in order to avoid repeating model tests 8 - www.woodgroup.com – AOG2015 Perth 11-13 March Software Packages Screening Analysis Ariane: Fast Limited Detailed Analysis Orcaflex: Accurate Time consuming 9 - www.woodgroup.com – AOG2015 Perth 11-13 March Calibration Model test results used to calibrate the numerical models Statically Pull models for mooring stiffness Dynamically surge extinction and model test runs 10 - www.woodgroup.com – AOG2015 Perth 11-13 March Calibration – Orcaflex model Mooring stiffness curve and characterisation 11 - www.woodgroup.com – AOG2015 Perth 11-13 March Calibration Dynamic calibration: surge extinction Adjustment of the damping values Discrepancies due to non linearities 12 - www.woodgroup.com – AOG2015 Perth 11-13 March Calibration Dynamic calibration of Orcaflex model with connected tests • Low frequency motion of the vessel (standard deviation of X and Y offset) showed good correspondence between the tests and the Orcaflex numerical model • Mean offsets were also generally close, but tended to be underestimated for collinear or near collinear cases • Mean yaw heading and yaw oscillation generally matched well. In fatigue environments, the yaw oscillation was somewhat higher in the Orcaflex runs • Good correspondence was found between the UJ angles and loads and maximum line tensions • Reasonable match when no additional damping was considered • Actual measured wave spectrum had to be used • Transverse motions not replicated 13 - www.woodgroup.com – AOG2015 Perth 11-13 March Calibration RTM Column Modelling in Orcaflex Disconnected model: Calibration of hydrodynamic parameters DNV-RP-C205 recommended coefficients for drag with reduction factors for finite length cylinders 14 - www.woodgroup.com – AOG2015 Perth 11-13 March Modelling the link in Ariane Overcoming limitations Orcaflex Ariane 15 - www.woodgroup.com – AOG2015 Perth 11-13 March Calibration – Ariane model Ariane Calibration Static : pull models for mooring stiffness Dynamic: surge extinction 16 - www.woodgroup.com – AOG2015 Perth 11-13 March Calibration Ariane Dynamic calibration with connected results • Ariane used to model low frequency motions for screening purposes: only vessel offsets were compared • The low frequency motion of the vessel showed reasonable correspondence • Mean offsets for collinear cases tended to be underestimated • The mean yaw heading and yaw oscillation found in Ariane matched reasonably well • The Ariane results were comparable to the Orcaflex simulation results • Separate calibration of low frequency damping levels for Ariane is required; best match obtained by considering % of values suggested in hydrodynamic study 17 - www.woodgroup.com – AOG2015 Perth 11-13 March Results Environments: The 100 year return period 1yr and 100yrs non-cyclonic 2yrs cyclonic conditions some environments with sea and swell steady current dominated non-cyclonic case is governing 18 - www.woodgroup.com – AOG2015 Perth 11-13 March Tandem offloading 19 - www.woodgroup.com – AOG2015 Perth 11-13 March Tandem offloading: results Maximum loads in the offloading hawser for different environments and tankers 20 - www.woodgroup.com – AOG2015 Perth 11-13 March Disconnection analysis 21 - www.woodgroup.com – AOG2015 Perth 11-13 March Disconnection analysis Selection of the right moment and tug assistance force for disconnection 22 - www.woodgroup.com – AOG2015 Perth 11-13 March Conclusions • The verification of the mooring design has been successfully carried out without the need for an additional campaign of model tests • The use of two different software packages provided means for screening a large number of cases and then refine the investigation with additional details 23 - www.woodgroup.com – AOG2015 Perth 11-13 March Conclusions • The availability of the refined model allowed to investigate details of the procedures for disconnection and reconnection providing important insight • Overall WoodGroup Kenny Perth has delivered a mooring design of a complex system supporting the client also in the installation and procurement phases 24 - www.woodgroup.com – AOG2015 Perth 11-13 March Further Development • Coupled analysis • Usage of load RAOs for the vessel • Account for different periods and dynamics of the floating boadies • VIV/VIM in disconnected conditions • Still not well established • Time consuming • Lack of in field measurements • CFD as alternative? 25 - www.woodgroup.com – AOG2015 Perth 11-13 March Thanks to: The MCS office in Perth (now WoodGroup Kenny) Azad Hessamodini Johann Melillo 26 - www.woodgroup.com – AOG2015 Perth 11-13 March
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