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Wadam - General wave load analysis20 February 2017 SAFER, SMARTER, GREENERDNV GL © 2014

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SOFTWARE

20 February 2017DNV GL

Wadam - General wave load analysis

Sesam user course

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Wadam - General wave load analysis20 February 2017

Wave Analysis by Diffraction And Morison theory

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Computation of wave loads and global response

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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

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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

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Linear theory - top view

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Linear theory - view from below

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Non-linear theory - view from below

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Morison theory

Structural parts with dimensions much smaller than wave length (small-volume part)

Viscous effects included Empirical formula

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Drag linearization methods

Linearizing the non-linear drag force:

Regular wave linearization (iteration process)– Find urel as the local relative velocity in each harmonic wave– Wave amplitude must be given

Stochastic linearization (iteration process)– Find local urel from a wave spectrum– (Short crested or) long crested

Give urel as a global constant (no iteration)

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)(21

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Bwreldd uuuDCuuDC

Excitation

Damping

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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

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Note the information symbols

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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 15000 panels

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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

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Warning: For load transfer the structural mesh should not haveelements intersecting the free surface

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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

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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.

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Hydro models in Wadam

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Hydro model

Panel model Composite modelMorison model

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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

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Definition of waves

Incoming wave defined as:

, ,

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Wave amplitude

Wave number

Angular frequency

Wave direction (“going to” direction)

X

Y

Input to Wadam:Wave direction +

Wave length or Wave period or Angular frequency

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Wadam output

Listing file:– Contents determined by PRINT-SWITCH– Datacheck + normal output– Can be VERY large with high print switch

Loads Interface Files (L*.FEM)– 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

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Global response& Load transfer

Load transfer

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Wadam output – listing file

The list of contents is useful and is also showing what is printed for different settings of the print switch

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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)

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Optional

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Response Amplitude Operators (RAOs)

Response per unit wave amplitude as function of wave period and heading

Input: cos

Output: , cos ,

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Wadam

Transfer functionSeastate Response

Postresp

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Complex variables

The RAO (Transfer function) is most conveniently treated as a complex variable

Input: cos

Output: ,

, , ,

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Positive phase angle, , means that the response peakoccurs before the wave crest reaches the origin

The phase angle is model dependent, only relative phase angles have a physical meaning

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Example: Heave RAO

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A typical RAO fora semi-submersible

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Example: Heave RAO

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A typical RAO fora ship

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Fluid dynamics in compartments

New method in Wadam 9.0-04, described in this paper:

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 cross-

sectional 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

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OMAE2013-10284: ADAPTING A LINEAR POTENTIAL THEORY SOLVER FOR THE OUTER HULL TO ACCOUNT FOR FLUID DYNAMICS IN TANKS

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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.

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Structure/mass modelPanel model

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Comparison with Molin’s experiment

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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.

Experiment layout

Panel model in HydroD(filling height 19cm & 39cm)

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Comparison with Molin’s experiment, continue

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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.

Case 1 19cm in both tanks Case 2 19cm & 39cm

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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.

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Gap resonance and irregular frequencies

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0

500

1,000

1,500

2,000

2,500

3,000

3,500

4,000

5 6 7 8 9 10

Dri

ft1

Frequency

IRR0

0.0

2.0

4.0

6.0

8.0

10.0

5 6 7 8 9 10

Elev

Frequency

IRR0

Gap resonance

Irregular frequencies

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Irregular frequencies

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

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2;

; , ∈

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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

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Damping Correction of dispersion relation

=>

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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

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Possible to execute multiple activities (Threads) in parallel

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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)

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