StructuralAnalysesOfJack-upsInElevatedCondition

8/13/2019 StructuralAnalysesOfJack-upsInElevatedCondition

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Properties of Jack-ups in elevated condition

Stiffness Hull is stiff

Legs relative soft

Masses

hull mass is high

leg mass relative small

Percent critical damping e

Soil 0% 2%

Hydrodynamic 2% - 3%

Material/Friction 0%

2%

Total: often 7% assumed

Natural Periods:

between 4s und 10s


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Modal analysisTranseverse and Longitudinal Modes

Thormodal_wd-50_he9_r0-quer.avi

Transverse Longitudinal

Thormodal_w d-50_he9_r0-laengs.av i


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Loads in a global analysis

constant, quasi-constant Loads

Weights

Crane

Wind

CurrentTime varying loads

Waves

(irregular sea with varying wave

periods and wave heights)

Wave periods between 4s-18s

RESONANCE possible


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Method for dynamic analyses of jack-ups (1/2)

Determination of DAF (DynamicAmplification Factor) due to wave excitation

Comparison with a single degree of freedom

system (SDOF) or

Statistial assessment of the reponses of the

jack-up due to time history wave loads (1-3hours simulation time of qualified seastate)

Defintion:

DAF=Fdynamic/Fstatic

=(Fstatic+Finertia)/Fstatic,,i.e.

Finertia=(DAF-1)* Fstatic

22

2

21

1

T

Te

T

T

DAF

nn


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Method for dynamic analyses of jack-ups (2/2)

Base Shear BS is the total

horizontal load at reaction point

BSmax=1500 kN

BSmin=-800 kN

BSamp=(BSmax-BSmin)/2= 1150 KN

BSmean=(BSmax+BSmin)/2

= 350 KN

Inertia Load:

Finertia=BSamp*(DAF-1) acting at

CoG or distributed over the

legs

Wave according WAVEL

-1000

-500

0

500

1000

1500

2000

0 45 90 135 180 225 270 315 360 405 450 495 540

Phase [Deg]

Force [kN]

-2.000

-1.500

-1.000

-0.500

0.000

0.500

1.000

1.500

2.000

Elevation [m]

Wave Load Contour [kN] Wave Elevat ion [m]


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Morison equation (empirical formula for hydrodynamic loads on cylindrical member)

Fd= Drag force

Fm= Inertia forcer= Density of water

D = Reference diameter of cylindrical member

A = Total cylindrical area of member

Cd= Drag coefficient

Cm= Mass coefficientv = relative particle velocity normal to member axis ( v~H/2*w*sin[wt-a] + vcur)

u = relative particle acceleration normal to member axis ( u~H/2*w**2*cos[wt-a] )

Wave loads on transparent Structures

uCAvvCDF

FFF

md

md

2

1


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Total Loads and non-linear Effects

Total Load on a Wind Turbine Installation Vessel

F = FGravity+ FCrane+ FWind+ FWave/Current+ FInertia

All Loads are depend on load directions except gravity

Non-linear effects at spud can reaction point due to non-linear soil condition

wave loads according to drag term of Morison equation

P-Deffect on legs (equilibrium at deformed shape)


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References:

Society of Naval Architects & Ocean Engineers (SNAME):

Technical & Research Bulletin 5-5A

Guidelines for Site Specific Assessment of Mobile Jack-up

Units, Rev.3 2008

and

ISO 19905-1 and ISO 19905-2

Site Specific Assessment of Mobile offshore units

Thank you

Thomas Jahnke, Tel: +49 40 36149 106

Jochen Knzel, Tel: +49 40 36149 7424

Documents

StructuralAnalysesOfJack-upsInElevatedCondition