Wajac Standard

DNV Software Sesam User Course

Wajac – Wave, current and wind loads on fixed frame structures

© Det Norske Veritas AS. All rights reserved.

Sesam Wajac

27 February, 2013 Slide 2

Wajac in Sesam Overview Sesam Manager

INTEGRATED PROGRAM PACKAGES

GeniE

conceptual modeller including Wajac, Sestra, Splice, Framework

DeepC

deep water mooring and riser analysis including Simo, Riflex

HydroD

environmental modeller including Wadam, Wasim, Postresp

SESAM INTERFACE FILE

POST

PRO

CE

SSIN

G

Xtract

presentation & animation

of results

Framework

frame fatigue and earthquake

Stofat

shell/plate fatigue

Profast

probabilistic fatigue and inspection

Cutres

presentation of sectional

results

Platework

plate design

Concode

concrete design ST

RU

CT

UR

AL

E

NV

IRO

NM

EN

TAL

Installjac

launching of jackets

Waveship

wave loads on ships

Wajac

wave loads on frame structures

Wasim

3D wave loads

on vessels

PRE

PRO

CE

SSIN

G

Simo

marine operations

Preframe

frame structures

Patran-Pre

general structures

Submod

sub- modelling

Prefem

general structures

Wadam

wave loads on general structures

Splice

structure- pile-soil

interaction

Usfos

progressive collapse

Mimosa

mooring analysis

Riflex

non-linear riser

Sestra

linear statics and dynamics

Postresp

presentation of statistical

response

Presel

superelement

assembly

Xtract

presentation & animation

of results

Wajac wave loads on frame structures


Sesam Wajac


Purpose and main features of Wajac For fixed frame structures (jackets, jack-ups, etc.) composed of 2 node beams

Computes loads: - Wave - Buoyancy - Wind - Current

Computes mass: - Added mass - Mass of internal water

(flooded members)

wave current

wind

buoyancy

added mass


Sesam Wajac


Fundamentals Z-axis must point upwards,

only 2 node beams considered

Fixed and rigid structure

Hydrodynamic force calculation: - Morison equation for beams - (MacCamy-Fuchs for vertical tubes)

Water particle motion undisturbed (with some modifications)

Buoyancy included, with or without flooding

not floating not flexible

D

Wave length > 5D

D D

Z

flooded air filled


Sesam Wajac


Load calculation Wave and current force (per unit

length) in load calculation points

Two or more segments per beam, increase if wanted

Only submerged part of beams subjected to wave and current

Distributed loads transferred to structural analysis through L*.FEM file

Overturning moment and base shear reported in print file - If the origin is not at mudline

and jacket centre then specify vector to point for overturning moment - On ‘Output > Global results’ tab

of ‘Edit Wave Load Run’ dialog

base shear Z

load calc.

points

segment overturning moment

Z


Sesam Wajac


Load calculation – Morison equation Force per unit length in each

load calculation point is F = FInertia + FDrag

- FInertia is inertia part of force

- FDrag is drag part of force

FInertia = ρ π D2/4 Cm an

FDrag = ρ D/2 Cd vn |vn|

Linear variation of force between load calculation points

Water particle acceleration an and velocity vn are normal to member, vn include current

F

D

vn / an

segment

load calc. points

wave current


Sesam Wajac


Load calculation – MacCamy-Fuchs If wave length λ < 5D then inertia

part of Morison inaccurate

Then MacCamy-Fuchs force calculation is more accurate

MacCamy-Fuchs theory: for single, vertical, cylinder standing on bottom and breaking surface

Use with care in Wajac!

MacCamy-Fuchs only for inertia part of force

For frequency domain only (linear harmonic Airy wave)

On ‘Special options’ tab of ‘Edit Wave Load Run’ dialog

D

λ

MacCamy-Fuchs according to theory

Use of MacCamy-Fuchs

in Wajac


Sesam Wajac


Load calculation – buoyancy forces Buoyancy forces

- Optionally contributing to wave load in deterministic load analysis - Irrelevant for frequency analysis

- Or as separate load case

Contributing to buoyancy: - Parts of members above below surface - Steel - Pipe sections non-flooded by default

Difference is net

pressure downwards


Sesam Wajac


Model – diameters and marine growth Diameter for force calculation

Tubes: - Structural diameter, D, by default - Alternatively, manual specification

(‘Hydrodynamic diameter’ in ‘Hydro Properties’) - Adding marine growth to diameter for drag forces - Optionally adding marine growth to diameter also

for inertia forces

Non-tubes: - Equivalent diameter by default - Alternatively, manual specification as for tubes

D

D D

D + marine growth

marine growth


Sesam Wajac


Model – hydrodynamic coefficients Hydrodynamic coefficients: inertia Cm and drag Cd

Constant assigned to whole or part of structure - Different constant values to different parts of structure

Function of diameter

Function of roughness and Reynolds number - Rn = vn D / ʋ, ʋ is kinematic viscosity

Function of roughness and Keulegan-Carpenter number - KC = 2 ρ vn / ω D

By API rule

Coefficients are normally on T*.FEM file created by GeniE

Coefficients on T*.FEM are overruled if coefficients are given in Wajac input file - A few additional ways of specification


Sesam Wajac


Wave theories an = an(h,H,T,z,t) and vn = vn(h,H,T,z,t)

- h = water depth - H = wave height - T = wave period - z = height above mud-line - t = time

Wajac offers five wave theories: - Linear harmonic theory, Airy - Stoke’s 5th order – steep waves, deep waters - Dean’s Stream Function – numeric approximation of given wave - Cnoidal – shallow waters - NEWWAVE – theory introduced by Shell

- Cannot be selected in GeniE, specify in Wajac input file

H

h z

T

vn / an


Sesam Wajac


Linear harmonic wave theory, Airy Harmonic waves (sinus)

Linear theory, i.e. velocities and accelerations are linear with respect to wave height

While other theories describe wave crest and trough, Airy describe up to still water level

Alternative assumptions: - Constant above still water level - Extrapolation of exponential curve

- Cannot be selected in GeniE - Wheeler stretching

constant

extrapolation

Wheeler

extrap.

Wheeler


Sesam Wajac


Current Application of current:

- Contributes to drag force for deterministic wave

- Contributes to drag force for time domain simulation of short term sea state

- Used in equivalent linearisation of drag force for spectral wave

Direction of current: - X-, Y- and Z-component - Horizontal and given direction - Horizontal and parallel with wave


Sesam Wajac


Wajac input & output Input to Wajac:

- Model of structure on Input Interface Files (T*.FEM) created by: - GeniE - (Patran-Pre or other preprocessor)

- Model of hydrodynamic environment on input file (Wajac.inp) created by: - GeniE - Manual editing – see example inputs in the following

Output from Wajac: - Print file (Wajac.lis) – for control of analysis

- Interpretation of input and possibly important messages - Optional amount of results

- Loads Interface Files (L*.FEM) – input to Sestra - Superelement analysis: Loads must be combined in Presel

- S-file (S*.FEM) in certain cases – input to Sestra - Correspondence between load cases and wave directions/frequencies – important for fatigue analysis

- Hydrodynamic Results Interface File (G1.SIF) if requested – input to Postresp - Display transfer function for base shear and overturning moment in Postresp – useful in fatigue analysis


Sesam Wajac


Overview of analysis capabilities 1. Design (100 year – ULS) wave analysis, deterministic analysis *

- Static analysis in Sestra - Code checking in GeniE

2. Deterministic fatigue wave analysis * - Static analysis in Sestra - Deterministic fatigue analysis in Framework

3. Spectral fatigue wave analysis, calculation of load transfer functions - Frequency domain quasi-static or dynamic analysis in Sestra - Spectral (stochastic) fatigue analysis in Framework

4. Time domain simulation of wave loads in short term sea state - For detailed study (time domain dynamic analysis in Sestra)

5. Calculation of static (stationary) wind loads * - Combine with deterministic wave analysis or wind fatigue, static analysis in Sestra

6. Calculation of added mass * - For subsequent dynamic (e.g. eigenvalue) analysis in Sestra

* Analysis may be controlled from GeniE


Sesam Wajac


1. Design (100 year – ULS) wave analysis Stepping true waves through structure

- Design waves from different directions - Any wave theory - Calculation of drag force - Current included - Optionally according to API 2A-WSD

Distributed wave loads transferred to structural analysis (L*.FEM files) - For steps giving maximum overturning

moment and base shear

Overturning moment and base shear reported in print file (Wajac.lis) - Sum of distributed wave loads - Phase giving maximum overturning

moment and base shear

step


Sesam Wajac


2. Deterministic fatigue wave analysis Stepping true waves through structure

- Several waves, different: - wave heights - wave frequencies - wave directions

- Any wave theory - Calculation of drag force - Current included

Distributed wave loads transferred to structural analysis (L*.FEM files) - For all steps of waves

Prepares for deterministic fatigue analysis (in Framework)


Sesam Wajac


3. Spectral fatigue wave analysis Unit waves, different frequencies and directions

Linearity between wave height and wave force - Linear harmonic Airy wave - Linearise drag term (FDrag = ρ (D/2) Cd vn |vn|)

- Equivalent linearisation - Linearisation with respect to characteristic wave height –

maximum submergence through cycle - No contribution from current to loads

Distributed wave loads (load transfer functions) transferred to structural analysis (L*.FEM files)

Frequency domain → complex loads

Prepares for quasi-static or dynamic structural analysis (Sestra) and spectral (stochastic) fatigue analysis (Framework)

Edit Wajac input to give wave data

1

1


Sesam Wajac


3. Further processing of spectral loads Wajac

ω

load

hydrodynamic transfer function

Framework spectral fatigue

ω

energy Sw(ω) wave

spectrum

Sestra

ω

stress

H(ω) transfer function

for structural response

ω

load Sresp(ω)

response spectrum

= ×

2 Sresp(ω) = |H(ω)|2 Sw(ω)

H(ω) is complex transfer function

Real stress is:

σ(ωt) = a Real [H(ω) eiωt]

= a [ HR(ω) cos(ωt) –

HI(ω) sin(ωt)]

a is wave amplitude


Sesam Wajac


4. Time domain simulation Random short term sea-state

generated based on wave energy spectrum - JONSWAP - Pierson-Moskowitz

Sea state stepped through structure

Distributed wave loads transferred to structural analysis (L*.FEM files)

Static or time domain dynamic analysis (Sestra)

Time series results for stresses/forces

Statistical postprocessing (Postresp)

Edit Wajac input to wave spectrum and simulation length

ω

energy Sw(ω) wave

spectrum


Sesam Wajac


5. Calculation of static wind loads Static wind = stationary wind

Relevant for: - Deterministic wave load analysis - Wind fatigue analysis

Drag force per unit length in each load calc. point is: FDrag-w = ρa (D/2) Cdw vnw |vnw| - ρa is air density - Cdw is air drag coefficient - vnw is wind velocity normal to member

Linear variation of force between load calc. points

Wind loads transferred to structural analysis (L*.FEM files)

FDrag-w

D

vnw

segment

load calc. points

wind

SWL


Sesam Wajac


6. Calculation of added mass Combine with other load calculation in

Wajac

Added mass based on D including marine growth: ρ π D2/4 (Cm – 1)

Including weight of marine growth (ρg is density of marine growth): ρg π (D tg – tg2)

Optionally including internal water

Added mass transferred to structural analysis (L*.FEM files) as nodal masses - Free vibration - Dynamic forced response

Added mass summed over all elements and reported in print file (Wajac.lis)

Added mass

D

Marine growth

Internal water

tg

Dsteel

Acceleration in X

Transverse component of X-acceleration giving rise to added mass

Transverse added mass decomposed to X-direction

Nodal added masses

Wajac Standard

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