24Feb10 - Rio OSAP HydroLoads

7/29/2019 24Feb10 - Rio OSAP HydroLoads

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Technical Basis ofHydrodynamic Load Analysis

Rio de Janeiro: 24 Feb. 2010

Overview of OSAP 2.0

Gwo-Ang ChangPrincipal Engineer

ABS Corporate Technology


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Outline

Load Generation

Design Wave Calculation

Load Mapping and Balancing

Summary


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3D HydrodynamicAnalysis Model

Geometry and MeshGenerator

HydrodynamicAnalysis Program

FEA Solver

Global Structural FE Model(Coarse Mesh for Yielding and

Panel Buckling Check)

Motion RAOsPressure RAOs

Fluid Velocity (optional)

Global Structural FE Model(Locally Refined Mesh for

Fatigue Check)

Structural Responses(Stress, Strain and Displacement)

OSAP 2.0

User Selected Third-party Applications

Typical Design Workflow Using OSAP

Need LocalFEA

Local FE Modelwith Refined Mesh

Y

N

End

Yielding, Buckling &Fatigue Code Check

Yielding, Buckling &Fatigue Code Check

Global FE Model, Loads,

and Constraints Input

Load Mapping& Balancing

Load Mapping

& BalancingLoad Cases for Strength

& Fatigue Assessment

Design WaveCalculation

Design WaveCalculation

LoadGeneration

LoadGeneration

Critical Responses

RAOs


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OSAP 2.0 Technical Basis

Load Generation


Load Generation


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OSAP Load Generation Module

Calculate inertia and drag forces for Morison elements

Dynamic forces calculation for mooring or tendons

Sectional forces and moments calculation

Interface with AQWA and WAMIT


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Elements Used for Hydrodynamic Modeling

For WAMIT Model

Panel element

For AQWA Model

Panel element

TUBE element

STUB element

DISC element

PBOY element


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Panel Elements

Hydrostatic pressure

Hydrodynamic pressure

Hydrostatically variant pressure

)( 0ZzgPStatic +=

tPDynamic

=

)( 213 xygP cVariationHydrostati +=

Global Coordinates

Body Coordinates


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Panel Elements

Force and moments in body coordinate systemare calculated by integrating the total pressures

over the body surface

where

: position vector (x,y,z) with respect to thereference point for the moment calculation

: outward normal vector of body surfaceexpressed in the body coordinate system

S : wetted body surface

= S dSnPF

dSnrPMS = )(

r

n


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TUBE and STUB Elements

Circular (TUBE) and non-circular (STUB)Morison elements

Hydrostatic loads Buoyancy forces

Hydrodynamic loads

Drag forces

Inertia forces

Froude-Krylov forces

Diffraction forces

Added-mass forces

Hydrostatic variation


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DISC Elements

A DISC element has no thickness and nomass, but has drag coefficient and added-

mass coefficient in its normal direction No hydrostatic force or Froude-Krylov force

Hydrodynamic loads

Drag forces

Diffraction forces

Added-mass forces


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PBOY Elements

PBOY represents a constant point buoyancy

Hydrostatic forces

Point buoyancy force in either upwards ordownwards direction normal to the mean stillwater line

Hydrodynamic loads

Hydrostatic variation


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Mooring or Tendon Model

For Static Case

Mooring/Tendon loads can be directly definedin OSAP *.STAT files.

Alternatively, mooring/tendon loads can beimported from AQWA-LIBRIUM analysis

results associated with any of the followingloading methods

LINE, NLIN, WNCH, FORC, LNDW (Deck 14)

THRS (Deck 10)


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Mooring or Tendon Model

For Dynamic Case

Mooring/tendons are modeled as masslesslinear springs, with one end attached at thefloater and the other end attached to theseabed.

Total forces/moments due to one spring

+=

ationStaticVarim

ationStaticVarim

m

m

m

M

F

M

F

K


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Linearization of Drag Force

The drag term of the Morison equation can be writtenin a generic linear form:

where is the linearized drag coefficient

Assuming a zero current velocity, the stochastic

linearization of drag force due to a random oscillationin irregular waves is

where is the standard deviation of relative velocity;is the fluid velocity; is the body velocity

OSAP requires the input of a representative sea statefor calculating the linearized drag force.

uCDLF dLIN 2

1

=

))(8

(2

1sfudLIN uuCDLF

=

ufu su

dC


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Sectional Force/Moment Calculation

External forces applied on Part 1and theinternal forces applied on the cutting plane on

Part 1 side follows Newtons Second Law

M=

+

+

ExternalInternal

ExternalInternal

MM

FF

Part 2Part 1n


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Sectional Force/Moment Calculation

Internal Forces Sectional forces andmoments to be solved

External Forces

Include all the forces applied on the Panel,TUBE, STUB, DISC and PBOY elements as

well as mooring/tendon loads Include the variation of gravity forces (g-effects)

M 6 X 6 mass matrix for Part 1

6 DOF acceleration vector at thereference point&


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Validation Case Study

Longtudinal Shear Force - FLDraft = 23.5m

0

200

400

600

800

1000

1200

1400

1600

1800

0 5 10 15 20 25 30 35 40

period(s)

RAO(KN/m

)

0deg - AGS

30deg - AGS

45deg - AGS

60deg - AGS

90deg - AGS

0deg - ABS

30deg - ABS

45 deg - ABS

60deg - ABS

90deg - ABS

Bending Moment - My

0

20000

40000

60000

80000

100000

120000

0 5 10 15 20 25 30 35 40

period(s)

RAO(KN

-m/m)

0deg - AGS

30deg - AGS

45deg - AGS

60deg - AGS

90deg - AGS0deg - ABS

30deg - ABS

45deg - ABS

60deg - ABS

90deg - ABS

Panel + TUBE Elements


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Validation Case Study

Longtudinal Shear Force - FL

0

200

400

600

800

1000

1200

1400

1600

1800

0 5 10 15 20 25 30 35 40

period(s)

RAO

(KN/m)

0deg - AGS

30deg - AGS

45deg - AGS

60deg - AGS90deg - AGS

0deg - ABS

30deg - ABS

45 deg - ABS

60deg - ABS

90deg - ABS

Bending Moment - My

0

20000

40000

60000

80000

100000

120000

140000

0 5 10 15 20 25 30 35 40

period(s)

RAO

(KN

-m/m)

0deg - AGS

30deg - AGS

45deg - AGS

60deg - AGS

90deg - AGS0deg - ABS

30deg - ABS

45deg - ABS

60deg - ABS

90deg - ABS

DISC + TUBE Elements


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Design Waves

What is a design wave?

A means to transfer the responses from a

hydrodynamic model to a global structural FEAmodel

A regular wave that generates the same

magnitude of critical response in the structure How to calculate a design wave?

Deterministic approach

Stochastic approach


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Design Wave Analysis Deterministic Approach

Calculate RAOs of critical responses (loads andaccelerations)

Define regular wave steepness S and maximum wave height Calculate the maximum response, Rmax, and the period,

Tmax, and heading where Rmax occurs

Design wave period = Tmax, the period where Rmaxoccurs

Design wave heading = wave heading where Rmaxoccurs

Design wave height = H(Tmax)

NjTRAOTH

MAXRj

j,1,)(

2

)(

max

=

= S

gTTH

j

j 2)(

2

=


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Design Wave Analysis Stochastic Approach

Short term approach

Use waves from a specified return period contour

(for example, 100-year return wave contour) Use multiple 100-year return waves

The maximum response is the design criticalresponse.

Long term approach

Predict based on the statistics of the wave scatterdiagram at the site

The long term response is predicted to a probabilityof exceedence based on the specified return period(for example 10-8.7 for 100-year return period).


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Using Short Term Approach (Available in OSAP)

Calculate RAOs of critical responses (loads and accelerations)

Derive wave energy spectrum (Sw)

Calculate response spectrum

Predict most probable extreme response (Rmax) for each seastate

Determine the maximum Rmax among all sea states

Calculate the design wave height curve (LF load factor )

Select design wave height

=== 02

00max_ )(2)

10800ln(2 dSmm

mT

TmR RnnZ

Z

j

)()]([)(2 WR SRAOS =

NjRMAXR j ,1),( max_max ==

LFTRAO

RTHD

)(

2)( max=

))(min( THH DD =


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Load Factor

CSDUs had been designed based on the deterministic

method of approach for many years before thestochastic method of approach was used. Thus, mostof the experience gained by the industry are from thedeterministic design

The stochastic design predicts lower values than thedeterministic design. To accept the stochasticallypredicted values, the industry suggests load factors forcalibrating the stochastic value in line with thedeterministic value

Site specific factors


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Using Long Term Approach

Repeat the short term prediction for each sea state inthe wave scatter diagram

Assume the short term response to follow the Rayleighdistribution

Take each sea states joint probability of occurrence

into account to calculate the total number ofoccurrences for each response amplitude

Assume the distribution of all the response amplitudesto follow the Weibull distribution

Predict the long term response to a probability ofexceedence based on the specified return period


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Wave Return Period

For MODUs

Owner/designer to specify the design wave

conditions.

50-year or 100-year return storms arecommonly selected by the Owners and

designers. For Floating Production Installations

Part 5B of ABS Guide for Building and Classing

Floating Production Installations(2009) 100 year return storm is required.


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Maximum Wave Height limit

For MODUs

For severe storm conditions, owner defined 1/10 wave

slope and the maximum wave height limit is typicallyused for worldwide operation except North Sea

For normal drilling conditions, owner defined 1/14 regularwave slope and the maximum wave height limit isnormally used

Although, the MODU rules no longer mentions the 1/10wave slope, the designers still choose the 1/10 waveslope

For Floating Production Installations

Wave height limit is not required to be defined by owner.

Site specific scatter diagram is required.


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Critical Responses for Twin-Hull Semis

Prying force

Summation of all transverse forces on a vertical plan

cut through the centerline.

The maximum occurs at beam seas when wave lengthis approximately equal to 2 times the distance betweenthe outer edge of the lower hulls.


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Torsional moments

Summation of all torsional moments on a vertical plan

cut through the centerline.

The maximum occurs at diagonal seas when wavelength is approximately equal to the diagonal distancebetween the lower hull ends.


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Longitudinal shear force

Summation of all longitudinal forces on a vertical plan

cut through the centerline. The maximum value occurs at diagonal seas when

wave length is approximately equal to 1.5 times thediagonal distance of the lower hull ends.


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Transverse racking forces

Summation of all transverse inertia forces due to the

mass of the deck and upper columns. The maximum occurs at beam sea in shallow draft.


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Longitudinal racking force

Summation of all longitudinal inertia forces due to the

mass of the deck and upper columns. The maximum value occurs at head seas in shallow

draft.


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Vertical accelerations

Summation of all vertical inertia forces due to the mass

of the deck and upper columns. The maximum value occurs at head seas in shallow

draft.


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Critical Response for Ring Pontoon Semis and TLPs

Prying/Squeezing loads between columns

Critical value diagonal seas, with a wave

length slightly more than twice the diagonalcolumn centerline spacing.

A second important case beam/head seas


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Critical Response for Ring Pontoon Semis and TLPs

Torsional moments (about transverse andlongitudinal axis)

Longitudinal shear forces between parallelpontoons

Transverse, longitudinal and vertical

accelerations of deck masses


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OSAP Output of Design Wave Curves

Zoom-in View


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Load Mapping


Load Mapping


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OSAP Load Mapping Module

Capability of mapping pressures, forces, andinternal tank loads from a hydro model to a FEAmodel

Functions of assisting load balancing

Interface with ANSYS and NASTRAN


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Load Mapping Procedure

OSAP

Load Generation

Translate FEModel

Model MeshMapping

Load Mapping

FEA LoadInterface

FEA Input toNASTRAN or

ANSYS

FE ModelTank Models

FE MassConstraints

HydrodynamicAnalysis Model

Motion RAOsForce RAOSPressure RAOsDesign WavesStatic Loads


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Supported Hydro Model Mesh Type

Five Types of Hydrodynamic Element

Panel element

TUBE element

STUB element

DISC element

PBOY element


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Supported FE Model Mesh Type

For Stiffeners Rod or beam

Truss element (rod element in NASTRAN or

link in ANSYS) with axial stiffness and aconstant cross-sectional area.

Beam element with axial, torsional and bi-directional shear and bending stiffness.

For Plates plate or shell

Membrane plate element (i.e., plane-stresselement) with bi-axial and in-plane shearstiffness and constant thickness.

Bending plate element with in-plane stiffnessas the membrane element plus out-of-planebending stiffness and constant thickness.

L d M i M i


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Load Mapping Matrix

Mooring/TendonForce

PBOY

DISC

STUB

TUBE

Panel

NodeBeamRod/LinkPlate/Shell

FE Model Mesh TypeHydro Model Meshor Load Type

L d M i F ti


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Automatic Mapping function for external wet surface

OSAP automatically searches the matching FE structuremodel with the elements in hydro model within a given

tolerance. The hydro model needs closely match the FE structure

model to generate accurate load mapping.

Mapped FE model is loaded with hydro loads for all

supported hydro elements Option load mapping function for internal tank

User needs to define tank boundary in FE structuremodel

OSAP will find the FE tank boundary, calculate tankpressure on the boundary, map the pressure load andtransfer the load back to the FE tank boundary foranalysis.

Load Mapping - Functions

L d M i P


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Load Mapping - Pressure

Hydro pressures on diffraction panels are mappedto the matching plate elements in FE model.

To smooth the pressures on FE plate elements,the following procedure is implemented

Calculate the nodal pressure in FE model using thepressures on the matching hydro panel.

Calculate the FE plate element pressures byaveraging nodal pressures in each element

Calculate pressure induced nodal forces using the

element area weighted average method for eachFE node.

Wave profile is not taken into account.

L d M i F


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Load Mapping - Force

Morison forces on TUBE and STUB in a hydro modelare mapped to the matching plate elements or lineelements in structure FEM using linear interpolation.

Loads on a DISC element in hydro model is mapped toa group of matching plate elements in structure FEMusing the similar averaging process for pressuremapping.

Loads on a PBOY in hydro model is mapped to thenode of structure FE closest to the PBOY location.

Loads on a mooring/tendon in hydro model is mapped

to the node of structure FEM closest to themooring/tendon attachment location.

Load Mapping Inertia Loads


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Load Mapping - Inertia Loads

Nodal Acceleration

where= acceleration at a node

= translation acceleration at a node

= node coordinate with respect to the reference point

= rotational acceleration about the reference point= gravity acceleration

= vertical direction unit vector

Nodal Inertial Force

m is the nodal mass

)()( 00 kgrraa

++=

a

0a

0rr

g

k

amF

=

Load Mapping Internal Tank Pressures


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Load Mapping - Internal Tank Pressures

Dynamic internal tank pressure is the motion-relatedload components due to rigid body motion and inertialcomponents.

Total Pressure at tank boundary points

2/1222

0))()()(( zzyyxxt agagaghPP +++=

where

P = total internal tank pressure at a tank boundary point

P0 = value of the pressure relief valve setting

= density of fluid cargo or ballast

ht = total pressure head in the direction of total

instantaneous acceleration vector

ax, ay, az = longitudinal, vertical and lateral instantaneousacceleration

gx, gy, gz = longitudinal, vertical and lateral instantaneous gravity

Load Mapping Internal Tank Pressures


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Load Mapping - Internal Tank Pressures

Static Internal Tank Pressure in Filled Tanks

ghPP t+= 0

Load Mapping Load Balancing


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Load Mapping Load Balancing

The unbalanced forces for each load caseneed to be determined and resolved.

OSAP provides extensive output informationfor load balance conditions of each load case.

Functions of applying displacement

constraints and mass points are available inOSAP.

Automatic load balancing is not implementedin OSAP.


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Summary of

Hydrodynamic Load Analysis


Summary of

Hydrodynamic Load Analysis

Summary


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Summary

In OSAP, Morison loads can be calculated and mapped tothe structural FE model using a rational approach.

In OSAP, dynamic forces exerted by mooring or tendons can

be calculated and transferred to structure FEM

In OSAP, sectional forces and moments due tohydrodynamic loads can be accurately calculated at anydefined section, which is very important on determining the

maximum global response at specified sections A robust algorithm is implemented in OSAP to map the

hydrodynamic and hydrostatic loads from the hydro model tostructural FE model.

The loaded FE model is ready to be solved by eitherNASTRAN or ANSYS.


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24Feb10 - Rio OSAP HydroLoads