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Integrated hydrodynamic and structural analysis webinar presentation tcm4 601490

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  • 1. DNV GL © 2014 SAFER, SMARTER, GREENERDNV GL © 2014 29 April 2014 Torgeir Vada SOFTWARE Integrated hydrodynamic and structural analysis 1 DNV GL Tech Talk webinar
  • 2. DNV GL © 2014 About the presenter  Name: Torgeir Vada  Position: Product Manager for floating structures  Background: – PhD in Applied mathematics/Hydrodynamics from University of Oslo, 1985 – Worked in DNV since 1985, with Sesam since 1997 – Worked as developer and in various line management roles – Member of technology leadership committee for hydrodynamics in DNV GL 2
  • 3. DNV GL © 2014 Agenda  Introduction to the tool used in the case study: Sesam for Floaters  Internal dynamics o Quasi-static approach o Full handling of internal fluid dynamics  Case study: Analysis of an FPSO o Loads around the waterline o Checking load transfer quality o Submodelling and fatigue 3
  • 4. DNV GL © 2014 10 Introduction to the tool used in the case study: Sesam for Floaters
  • 5. DNV GL © 2014 11 FE analysis 4. Global stress and deflection & fatigue screening Sesam – a fully integrated analysis system 1. Stability and wave load analysis Wave scatter diagram 2. Pressure loads and accelerations Loadtransfer 3. Structural model loads (internal + external pressure) Local FE analysis 5. Local stress and deflection & fatigue
  • 6. DNV GL © 2014 12 The Sesam floating structure package  Linear structural analysis of unlimited size  Hydrostatic analysis including stability code checking  Hydrodynamic analysis  Buckling code check of plates and beams  Global to sub-model boundary conditions  Fatigue analysis of plates and beams  Coupled analysis, mooring and riser design  Marine operations
  • 7. DNV GL © 2014 The Sesam floating structure package – main tools  Sesam GeniE for modelling and structural analysis  Sesam HydroD for hydrostatics and hydrodynamics  Sesam Manager for managing the analysis workflow  Sesam DeepC for umbilicals, mooring and riser analysis  Sesam Marine for marine operations  Sesam CAESES for parametric modelling and optimization 13
  • 8. DNV GL © 2014 16 What can you do with Sesam HydroD?  Model environment and prepare input data for hydrostatic and hydrodynamic analysis  Perform hydrostatics and stability computations (including free surface)  Calculate still water shear and bending moment distribution  Perform hydrodynamic computations on fixed and floating rigid bodies, with and without forward speed  Calculate wave load statistics and determine design loads  Transfer hydrostatic and hydrodynamic loads to structural analysis HydroDD1.3-04 Date: 31 May2005 15:01:34 0 50 100 150 -2-101234 GZ-Curve HeelAngle[deg] GZ[m]
  • 9. DNV GL © 2014 18 Why Sesam HydroD?  Advanced modeling features – Anchor and TLP elements simulation – Multi-body analysis – hydrodynamic, stiffness and damping coupling are included  Second order motions and forces – Mean drift force – Quadratic transfer function (QTFs) for motions and forces  Non-linear time domain analysis – Hydrostatic and Froude-Krylov pressures to instantaneous free surface – Exact handling of gravity and inertia according to vessel motions – Morison drag force considered in time domain – 5th order Stokes wave for shallow water – Quadratic damping coefficients
  • 10. DNV GL © 2014 19 Internal dynamics Quasi-static approach Full handling of internal fluid dynamics
  • 11. DNV GL © 2014 Quasi-static method  The internal fluid is regarded as rigid body, no internal waves or relative motion wrt. hull structure  Internal free surface is accounted for with additional restoring matrix.  Tank fluid Mass added to the total hull mass, to be balanced with buoyancy force.  Filling fraction is defined in pre-processing.  Reference points for each tanks shall be pre-calculated. – “Acceleration point” (CoG of the tank fluid) – “Zero level point” (geometry center of the internal free surface, or roof center for a full tank) 20
  • 12. DNV GL © 2014 Full dynamic method, introduction  The internal radiation is solved for each tank. _ _ _ _ _  The acceleration point is not needed anymore for calculating local pressure.  More accurate.  Sloshing mode to be captured (Linear). 21 Only known as a global load Computed from a distributed load
  • 13. DNV GL © 2014 Comparison with Molin’s experiment  Two rectangular tanks next to each other with the same geometry.  The fluid level are set as 19cm for both tank in case1  The fluid level are set as 19cm for one tank and 39cm for the other in case2  Roll motion to be investigated. 22 Experiment layout Panel model in HydroD (filling height 19cm & 39cm)
  • 14. DNV GL © 2014 Comparison with Molin’s experiment, continue  The 1st peak corresponds to the eigen period of the hull in water  The 2nd & 3rd peaks relate to the sloshing modes of the tanks  Smaller filling fraction, smaller sloshing frequency  Sloshing modes captured very well. Linear effects only. 23 Case 1 19cm in both tanks Case 2 19cm & 39cm
  • 15. DNV GL © 2014 LNG carrier study  Dynamic pressure in compartments Compartments included in Panel model to calculate internal hydrodynamics
  • 16. DNV GL © 2014 LNG carrier study  Dynamic pressure in compartments 25 compartments with 4 for liquid cargo tanks balancing Automatic balancing
  • 17. DNV GL © 2014 LNG carrier study  Dynamic pressure in compartments – Surge, heave and pitch not so affected – Sway, yaw and roll affected both for full and half tank 26
  • 18. DNV GL © 2014 Case study: Analysis of an FPSO
  • 19. DNV GL © 2014 FPSO ULS and FLS analysis modelled in Sesam Manager 28 Wave load computation + Structural analysis
  • 20. DNV GL © 2014 The load transfer workflow  This is the core workflow in both ULS and FLS analysis 29 Compute hydrodynamic loads Transfer loads to FEM model Load transfer verification Structural analysis HydroD Sestra Cutres (global model only)
  • 21. DNV GL © 2014 Loads transferred from HydroD to Sestra  Hydrodynamic pressure on the outer hull  Hydrodynamic pressure from internal fluid  Inertia and gravity loads  Line loads on beams (Morison’s equation)  Nodal loads – Anchor and TLP elements – Pressure area elements => axial loads on beams  Ma = F => sum of all transferred loads should (ideally) be zero 30
  • 22. DNV GL © 2014 The FPSO used in this study Length 165.7 m Beam 43 m Full load condition: All cargo compartments full All ballast compartments empty Mass 111,180 tonne COG (78.6m, 0, 12.35m) Radii of gyration (19.6m, 95m, 95m) Draft 15.5 m Half load condition: All compartments half filled Mass 77,047 tonne COG (78.6m, 0, 7.5m) Radii of gyration (19.7m, 95m, 95m) Draft 10.8 m 31 FEM model Compartment model
  • 23. DNV GL © 2014 Half filled compartments – rigid body motions 32 Blue: Dynamic Red: Quasi-static Surge Sway Heave Roll Pitch Yaw Wave heading: 135°
  • 24. DNV GL © 2014 Full compartments – rigid body motions 33 Blue: Dynamic Red: Quasi-static Surge Sway Heave Roll Pitch Yaw Wave heading: 135°
  • 25. DNV GL © 2014 Half-filled compartments – pressure distribution on foremost bulkhead 34 Wave period = 10s Wave heading: 135° • Significantly lower pressures in dynamic solution • Zero pressure at waterline in dynamic solution
  • 26. DNV GL © 2014 Full compartments – pressure distribution on foremost bulkhead 35 Wave period = 10s Wave heading: 135° • Up to 10% lower pressures in dynamic solution • In general quite similar
  • 27. DNV GL © 2014 Half-filled – MVonMises stress distribution on foremost bulkhead 36 Wave period = 10s Wave heading: 135° • 20% lower maximum stresses in dynamic solver • Lower stress level in most of the bulkhead in dynamic solver
  • 28. DNV GL © 2014 Half-filled – stresses on all bulkheads 37 Wave period = 10s Wave heading: 135° • 30% lower maximum stresses in dynamic solver • Lower stress level in most of the bulkheads in dynamic solver
  • 29. DNV GL © 2014 Full load – MVonMises stress distribution on foremost bulkhead 38 Wave period = 10s Wave heading: 135° • 10% lower maximum stresses in dynamic solver • Difference within a few per cent on most of the bulkhead • Much smaller difference on stresses than on pressures
  • 30. DNV GL © 2014 Full load – stresses on all bulkheads 39 Wave period = 10s Wave heading: 135° • In general very small differences
  • 31. DNV GL © 2014 Loads around the waterline
  • 32. DNV GL © 2014 Pressure scaling for waterline elements  Retain correct pressure, but get incorrect force – Constant pressure centroid Or  Scale pressure at waterline elements to get correct force – Area adjusted  padjust = A1/A2 x poriginal – where A1 is the wet area of the element and A2 is the total area of the element – This is applied whether or not the centroid is below the free surface A2 A1
  • 33. DNV GL © 2014 Pressure reduction zone  Modify pressure in the area +/- A around the free surface level to account for the presence of water above the mean waterline and absence of water below. Default: A=0.0 (i.e. no change)  DNV class note 30.7: A zAz r wl p 2  Reduction factor when |z-zwl| < A
  • 34. Example Pressure reduction zone
  • 35. DNV GL © 2014 User defined pressure reduction region User defined wall-sided part  Apply a user defined pressure reduction region on a selected part of the vessel – The method is only recommended on the part of the vessel which is wall-sided and should thus be controlled by the user
  • 36. DNV GL © 2014 Checking load transfer quality
  • 37. DNV GL © 2014 Check report from hydrodynamic analysis 46 This number should ideally be 0
  • 38. DNV GL © 2014 Reaction forces – global load balance  These are quite small in all four analyses indicating good global balance in the load transfer – Example below is for half filled condition with dynamic solver 47
  • 39. DNV GL © 2014 Half-filled – sectional load comparison – load distribution consistency – internal dynamics 48 Fx Fy Fz Mx My Mz Blue: HydroD - load integration Red: Cutres – stress integration
  • 40. DNV GL © 2014 Recommendations for load transfer to FEM model  Avoid loads with unknown distribution – E.g. additional damping or restoring matrices  Use FEM model as mass model for the hydrodynamic analysis – To obtain identical mass matrices  Avoid putting the fixed nodes close to “interesting” parts of the structure – There will always be some imbalance which will create artifical reaction forces at these nodes  Convergence of local loads may require a finer mesh than convergence of global responses – Use fine mesh in areas with large curvature 49
  • 41. DNV GL © 2014 Submodelling and fatigue
  • 42. Fatigue analysis workflow and Submod properties 51
  • 43. DNV GL © 2014 Global model and sub-model 52
  • 44. DNV GL © 2014 Sub-modelling procedure  Do first the global analysis in Sesam Sestra  Then create the sub-model in e.g. Sesam GeniE – With prescribed displacement boundary conditions where geometry is cut  Submod: – Reads the sub-model – Reads the global analysis results file – Compares the two models and fetches displacements from global analysis – Imposes these as prescribed displacements on the sub-model boundaries with prescribed b.c.  Perform the structural analysis for the sub-model, this is a standard Sesam Sestra analysis  It is important to perform load transfer from Sesam HydroD to the local model since the loads must be the same as on the global model. Slide 53 analyseanalyse analyseanalyse SubmodSubmod global model sub-model prescribed b.c.
  • 45. DNV GL © 2014 Sesam HydroD to compute local wave loads  Rerunning Sesam HydroD for the sub- model is easy: – Panel and mass models are typically the same as for the global model – Wave periods and directions must be the same as for the global model – The basic hydrodynamic results from the global analysis can be reused so the local analysis is much faster – Structural model in Sesam HydroD: Simply replace global model with sub- model – Pressure loads for panels outside the sub-model are discarded – Also needed if there are no wet surfaces since the inertia and gravity loads will still apply Slide 54 Global model Sub-model Panel model
  • 46. DNV GL © 2014 Typical workflow – Local analysis 55 Local structural analysis Stress extrapolation Stress distribution for each load case RAO’s •Local stress/deflections Local stress/deflections Input •Hot spot location Result •RAO •Principal hot spot stress Principal hotspot stress Principal stress 0.E+00 1.E+07 2.E+07 3.E+07 4.E+07 5.E+07 0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 Wave period [ s] 0 45 90 135 180 Local stress transfer functions Fatigue calculations Input •Wave scatter diagram •Wave spectrum •SN-curve •Stress RAO •=> Fatigue damage Stress Hot spot Geometric stress Geometric stress at hot spot (Hot spot stress) Notch stress Nominal stress Scatter diagram SN data
  • 47. DNV GL © 2014 SAFER, SMARTER, GREENER www.dnvgl.com Thank you! software@dnvgl.com 56

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