03 Wadam Standard
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Transcript of 03 Wadam Standard
DNV SoftwareSesam User CourseGeneral wave load analysis
Revised: August 22, 2012
© Det Norske Veritas AS. All rights reserved.
Wave Analysis by Diffraction And Morison theory
Computation of
wave loads and
global response
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Diffraction & radiation theory
� Viscous effects neglected
� Distortion of waves due to presence of structure included
� Waves created by the motion of the structure included
� Linear theory
� Structural part with dimensions comparable to wave length (large volume part)
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The problem to be solved
Moving bodyIncoming wave
Radiated and diffracted wave
Equations:
Conservation of mass => field equations inside fluid
Conservation of momentum => Equations of motion for body
Boundary conditions
Basic assumption: Inviscid fluid
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Continuity equation (conservation of mass)
General equation: vdt
D r•−∇=ρρ1
00 =•∇⇒=⇒ vdt
D rρAssumption 1: Incompressible fluid
Assumption 2: Irrotational motion ϕ∇=⇒=×∇⇒ vvrr
0
Assumptions 1 and 2 give the following field equation in the fluid domain:
( ) 0,,,2 =∇ tzyxϕ
(Velocity potential)
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Boundary conditions
� At sea bed: 0=∂∂=
nvn
ϕ
Iϕϕ =
(zero normal velocity)
� At infinity: (no disturbance of incoming wave)
� At free surface:
� Kinematic boundary condition: A water particle in the free surface will remain in the free surface
� Dynamic boundary condition: Atmospheric pressure
� Can be set to zero since a constant pressure give no force on a body
� On vessel: 0==∂∂
nVn
ϕ(normal velocity in fluid equals normal velocity of body)
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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
Amplitude: 3m, direction 135°
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Linear theory - view from below
Amplitude: 3m, direction 135°
Slide 9
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Non-linear theory - view from below
Amplitude: 3m, direction 135°
Slide 10
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Morison theory
� Viscous effects included
� Empirical formula
uuDCdt
duC
DF dm ρπρ
21
4
2
+=
Structural parts with dimensions much smaller than wave length (small-volume part)
Slide 11
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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)
)(21
21
Bwreldd uuuDCuuDC −≈ ρρ
Excitation
Damping
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Xtract in SESAM Overview
INTEGRATEDPROGRAMPACKAGES
SESAM INTERFACE FILE
PO
STP
RO
CE
SSIN
G
Xtract
presentation& animation
of results
Framework
framedesign
Stofat
shell/platefatigue
Profast
probabilisticfatigue andinspection
Cutres
presentationof sectional
results
Platework
platedesign
Concode
concrete designST
RU
CT
UR
AL
Manager
EN
VIR
ON
ME
NT
AL
Installjac
launching of jackets
Waveship
wave loadson ships
Wajac
wave loads on framestructures
Wasim
3D wave loadson vessels
PR
EP
RO
CE
SSIN
GA
SSO
CIA
TE
D
Proban
probabilisticrisk and
sensitivity
Workflow
Simo
marineoperations
Preframe
framestructures
Patran-Pre
general structures
Presel
super-elementassembly
Submod
sub-modelling
Prefem
general structures
Wadam
wave loadson generalstructures
Splice
structure-pile-soil
interaction
Usfos
progressivecollapse
Mimosa
mooring analysis
Riflex
non-linearriser
Sestra
linearstatics anddynamics
Postresp
presentationof statistical
response
Genie
conceptual modeller including:Wajac, Sestra, Splice, Framework
DeepC
deep water mooring analysisincluding: Simo, Riflex
FPSO
programs needed for FPSO designintegrated in Workflow
Xtractpresentation& animation
of results
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Procedure for hydrodynamic analysis
HydroD
Mass Model(Patran-Pre, GeniE)
Panel Model(Patran-Pre, GeniE)
(Presel)Model of panel & mass model
Analysis control parameters
Seastate Transfer function Response
Postresp - short term
Scatter diagramLong term response
Postresp - long term
Output from Postresp:• Long term statistics• Display response variables• Combine response variables• Display response spectra
Wadam
Load transfer to structural analysis:• Inertia load• Wave pressure
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Wadam input
Preprocessor(GeniE, Patran-Pre)
Panel ModelMorison Model Structural Model Mass Model
Presel
Hydro Model
Wadam
Environment
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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
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 Wadamoutput
� 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.
Slide 19
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Hydro models in Wadam
Hydro model
Panel model Composite modelMorison model Dual model
A dual model is only needed for load transfer to a beam
structural model
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Mass model
� Global mass data- Given in HydroD (Prewad)
- 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:
))sincos(cos(),,( ββωη kykxtAtyx +−=
Akω
Wave amplitudeWave number = 2π/Wave lengthAngular frequency = 2π/Wave period
β Wave direction (“going to” direction)
Input to Wadam:
Wave direction +
Wave length or Wave period or Angular frequency
β
X
Y
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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
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
� 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)
Results Interface File - the G-SIF or G-SIN file(for Mimosa, DeepC, Postresp, Xtract)
Optional
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Response Amplitude Operators (RAOs)
� Response per unit wave amplitude as function of wave period and heading
Input: )cos( tω Output: )),(cos(),( βωδωβω +tT
Wadam
Transfer functionSeastate Response
Postresp
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Complex variables
� The RAO (Transfer function) is most conveniently treated as a complex variable
Input:
Output:
CRi iTTeTH +== ),(),(),( βωδβωβω
)Re()cos( tiet ωω =
Positive phase angle, δ, means that the response peakoccurs before the wave crest reaches the origin
)),(Re( tieH ωβω
The phase angle is model dependent, only relative phase angles have a physical meaning
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Example: Heave RAO
A typical RAO fora semi-submersible
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Example: Heave RAO
A typical RAO fora ship
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Roll damping methods in Wadam
1. Use an external damping matrix
2. Use the roll damping model in Wadam (ships only)- Requires an iteration since maximum roll angle is a parameter
If maximum roll angle is from short term statistics automatic iteration can be performed- Involves definition of a 2D strip model- One symmetry plane (XZ plane) must be used
3. Use the quadratic roll-damping coefficient- Requires stochastic iteration
4. Use a composite model
Only option 3 allows for load transfer of the roll-damping force
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Typical approach
1. Use the roll damping model in the global analysis
2. Use a composite model in the load transfer. Damping coefficients are tuned so that the resulting damping matrix is the same as in the global analysis
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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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Wave theories and shallow waterExample 1:
T=15s, d= 22m / 45m / 220m
Linear: H < 0.9m / 1.25m / 2.25m
Stokes: H < 6m / 22.5m / 68m
Breaking: H > 13.5m / 34m / 68m
Example 3:
T=5s, d=2.5m / 5m / 25m
Linear: H < 0.1m / 0.15m / 0.25m
Stokes: H < 0.65m / 4m / 7.5m
Breaking: H> 1.5m / 4m / 7.5m
Example 2:
T=10s, d= 10m / 20m / 100m
Linear: H < 0.35m / 0.55m / 1.0m
Stokes: H < 2.5m / 10m / 30m
Breaking: H > 6m / 15m / 30m
In practice the linear limits can be pushed quite a lot
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Airy wave vs. Stokes 5th order wave
100 105 110 115 120 125 130 135 140 145 150
-5-4
-3-2
-10
12
34
56
7
Time
Incoming wave - WasimActivity_h10_lin_U0 Incoming wave - WasimActivity_h10_stokes_U0
100 105 110 115 120 125 130 135 140 145 150
-1-0
.8-0
.6-0
.4-0
.20
0.2
0.4
0.6
0.8
100 105 110 115 120 125 130 135 140 145 150
-2e+
009
02e
+00
94e
+00
9
Wave height: 10mWave period: 17.27sWater depth: 30m
Wave elevation
Heave
Midship Vertical Bending
Containership
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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
Possible to execute multiple activities (Threads) in parallel
Slide 35
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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
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