SOFTWARE. Sesam user course. 20 February 2017 Wadam - General wave load analysis. Ungraded SAFER, SMARTER, GREENER DNV GL 2014

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1 SOFTWARE Sesam user course DNV GL 1 SAFER, SMARTER, GREENER

2 Wave Analysis by Diffraction And Morison theory Computation of wave loads and global response 2

3 Diffraction & radiation theory Structural part with dimensions comparable to wave length (large volume part) Viscous effects neglected Distortion of waves due to presence of structure included Waves created by the motion of the structure included Linear theory 3

4 Linear theory Assuming that the wave amplitude is small Expanding all conditions on free surface around mean sea level and keep only terms proportional to the wave amplitude Motion of structure is of the same order as the wave amplitude Expanding all conditions on structure around mean position and keep only terms proportional to the vessel motion Computational grid (panel model) will be the same at all times 4

5 Linear theory - top view 5

6 Linear theory - view from below 6

7 Non-linear theory - view from below 7

8 Morison theory Structural parts with dimensions much smaller than wave length (small-volume part) Viscous effects included Empirical formula

9 Drag linearization methods Linearizing the non-linear drag force: Excitation 1 DC 2 d u u 1 DC 2 d u rel ( u w u B ) Damping Regular wave linearization (iteration process) Find u rel as the local relative velocity in each harmonic wave Wave amplitude must be given Stochastic linearization (iteration process) Find local u rel from a wave spectrum (Short crested or) long crested Give u rel as a global constant (no iteration) 9

10 Morison section HydroD will list the cross-sectional data for the predefined Morison model From the T-file The user must define additional data like drag and added mass coefficients Diameter (and mass) may be changed Diameter must be specified for non-pipes Note the information symbols 10

11 Panel model For the large-volume part of the structure Created by GeniE, Patran-Pre, Presel Shell or solid elements Single superelement or hierarchy of superelements External wet surface identified by the Wet Surface property in GeniE or Hydro load in Patran-Pre. This must be assigned to load case number 1. No, one or two symmetry-planes can be used Arbitrary position of origin Maximum panels 11

12 Adjustment of panel model to actual wet surface This adjustment is done automatically in Wadam by adjustment of those panels that intersect the free surface Warning: For load transfer the structural mesh should not have elements intersecting the free surface 12

13 Morison model Used for the small-volume part of the structure Created by GeniE or Patran-Pre 2-node beam elements One single first level superelement No symmetry planes Defined by assigning hydrodynamic properties in HydroD 13

14 Reference frame for Wadam output Motions and forces are by default referred to Wadam s internal frame of reference. The motion reference point can be user specified (from Wadam version 8.2) Motion directions are in the global system Heave is motion vertical to the free surface In this system the mean free surface is identical to the xy-plane. 14

15 Hydro models in Wadam Hydro model Panel model Morison model Composite model 15

16 Mass model Global mass data Given in HydroD Centre of gravity, radii of gyration, products of inertia, total mass or Mass matrix Sufficient for computation of rigid body motion and pressure distribution Given by a superelement model This can be the panel model, the structural model, the Morison model or a separate model Needed for computation of sectional loads Alternatively the mass may be given by a point mass file 16

17 Definition of waves Incoming wave defined as:,, Y Wave amplitude Wave number Angular frequency Wave direction ( going to direction) X Input to Wadam: Wave direction + Wave length or Wave period or Angular frequency 17

18 Wadam output Listing file: Contents determined by PRINT-SWITCH Datacheck + normal output Can be VERY large with high print switch Global response & Load transfer Loads Interface Files (L*.FEM) Load transfer Loads transferred to structural analysis in Sestra Load cases produced must be accounted for in Presel load combination (need not be done prior to Wadam run) S-file (S*.FEM) - part of Sestra input file Correspondence between load cases and wave directions/frequencies Essential for spectral fatigue analysis in Stofat / Framework (Optional for a non-fatigue analysis) Created when the first load case no. is 1 18

19 Wadam output listing file The list of contents is useful and is also showing what is printed for different settings of the print switch 19

20 Wadam output Results Interface File - the G#.SIF (or G#.SIN) file (for Mimosa, DeepC, Postresp, Xtract) Rigid body motion RAO Mass, added mass, damping and restoring matrices Excitation forces Mean drift force Wave elevation at specified points Wave kinematics at specified points Pressure RAO on selected panels Global loads RAO (sectional loads) Optional 20

21 Response Amplitude Operators (RAOs) Response per unit wave amplitude as function of wave period and heading Input: cos Output:, cos, Wadam Seastate Transfer function Response Postresp 21

22 Complex variables The RAO (Transfer function) is most conveniently treated as a complex variable Input: Output: cos,,,, Positive phase angle,, means that the response peak occurs before the wave crest reaches the origin The phase angle is model dependent, only relative phase angles have a physical meaning 22

23 Example: Heave RAO A typical RAO for a semi-submersible 23

24 Example: Heave RAO A typical RAO for a ship 24

25 Fluid dynamics in compartments New method in Wadam , described in this paper: OMAE : ADAPTING A LINEAR POTENTIAL THEORY SOLVER FOR THE OUTER HULL TO ACCOUNT FOR FLUID DYNAMICS IN TANKS Fluid dynamics in the compartments need to be accounted for In assessment of offshore structures, like FPSOs and Semisubmersibles, there is a growing requirement that the fluid dynamics in the compartments is accounted for in a way consistent with the fluid dynamics outside the hull. More correct motions and consistent load transfer on the structure Better representation of the contribution from tank dynamics to the crosssectional loads and the local pressure loads on the structure. The added mass and restoring stiffness of each tank enter into the global motion equation More correct contributions to the global moments of inertia. Non-wave type of dynamics is occurring for full tank 25

26 Fluid dynamics in compartments, cont. More features in the new method The radiation potentials with the resulting motions determine the actual pressure loads in the tanks with the given internal free surface level. Tank definitions shall be included also in the panel model, following the same practice as for the tanks in the structure model. The geometry, wet surface, and the load case numbering shall be correspondent to those in the structure model. The zero pressure point shall be given to each tank (automatic) The tank fluid shall be excluded from the mass model. Panel model Structure/mass model 26

27 Comparison with Molin s experiment Experiment layout Panel model in HydroD (filling height 19cm & 39cm) 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. 27

28 Comparison with Molin s experiment, continue Case 1 19cm in both tanks Case 2 19cm & 39cm The 1 st peak corresponds to the eigen period of the hull in water The 2 nd & 3 rd peaks relate to the sloshing modes of the tanks Smaller filling fraction, smaller sloshing frequency Sloshing modes captured very well. Linear effects only. 28

29 Waves in shallow water Validity of result is limited by the validity of the wave theory (Airy) which is used in Wadam and Wasim. The limit depends on the wave lengths studied and the size of the waves. Tentative minimum water depth (the smaller value requires small amplitudes) 40-70m for T=15s (wave length 340m infinite depth = 170m) 20-40m for T=10s (wave length 150m infinite depth = 75m) 5-10m for T=5s (wave length 38m infinite depth = 20m) The requirement on water depth increases linearly with wave length for constant wave steepness (steepness is wave height/wave length). The requirement on water depth increases linearly with wave steepness for constant wave length. Wave length increases with wave period squared. 29

30 Gap resonance and irregular frequencies 30

31 Gap resonance and irregular frequencies 10.0 Gap resonance 8.0 Elev IRR0 Drift ,000 3,500 3,000 2,500 2,000 1,500 1, Frequency Irregular frequencies Frequency IRR0 31

32 Irregular frequencies 2 ; ;, Same equation system for internal and external solution Equations are singular for internal sloshing modes => equations are singular for these frequencies also for the external problem Non-physical resonance Solution to problem: Put a lid on the internal free surface to suppress the sloshing modes 32

33 Gap/moonpool resonance Linear theory will overpredict response at resonance Physical resonance => should be modelled by physical arguments Origin of damping is primarily from viscous effects How to model this within potential flow theory? more or less empirical methods, often calibrated against model tests Our solution: Add damping term in kinematic free surface condition => Damping Correction of dispersion relation 33

34 Validation 34

35 Moonpool resonance 35

36 Wave elevation and torsional moment 36

37 Moonpool resonance 37

38 Running Wadam Start from the Activity Monitor May include both Stability analyses and Wasim analyses Use of HydroD is described in a separate presentation in the training course Possible to execute multiple activities (Threads) in parallel 38

39 Wadam additional features Time domain (deterministic) output Extensions (additional licences): Multibody computations Second-order response and excitation forces Wave Drift Damping Wave/current interaction (forward speed) 39

40 The world-leading provider of software for a safer, smarter and greener future DNV GL Software software.support@dnvgl.com SAFER, SMARTER, GREENER 40