Ocean Loading in Ansys 14.5

© 2012 ANSYS, Inc. February 13, 2013 1 Release 14.5

14.5 Release

Lecture 6

Ocean Loading


Overview of Existing Pipe Elements

Various pipe elements have been present in ANSYS Mechanical for many years:

• PIPE16: elastic straight pipe

• PIPE17: elastic pipe tee

• PIPE18: elastic curved pipe (elbow)

• PIPE20: plastic straight pipe

• PIPE59: immersed pipe (wave loading)

• PIPE60: plastic curved pipe (elbow)

The capabilities, namely elastic vs. plastic behavior as well as ocean loading, were dependent upon the element selection

• The user needed to select the pipe element type based on whether plasticity or hydrodynamic loads were present


Overview of Existing Pipe Elements (cont.)

At ANSYS 12.0, three new pipe/elbow elements were introduced:

• PIPE288: 2-node finite strain pipe

• PIPE289: 3-node finite strain pipe

• ELBOW290: 3-node finite strain elbow

With the new pipe elements, the user only needs to decide the topology during element selection:

• New pipe elements are based on Timoshenko beam theory

– Ability to model both thin and moderately thick pipes

• Both linear and nonlinear material models are supported

– Plasticity, hyperelasticity, viscoelasticity, viscoplasticity can be defined

Thick Pipe Thin Pipe

Stress state Full 3D plane stress

Thickness change independent variable, directly solved

recovered from membrane strains


Overview of Existing Pipe Elements (cont.)

• Hydrodynamic loading is supported for PIPE288/289

– Wave loading is defined with OCTYPE, OCTABLE, OCDATA and SOCEAN commands

• In MAPDL use SECTYPE,SECID,PIPE to define cross-section information

– No need to use real constants anymore

– Section integration points exist to capture nonlinear behavior

• Additional section-related items can be defined:

– SFLEX allows for definition of flexibility factors

– SSIF allows for input of stress intensity factors

• Internal, hydrostatic and hydrodynamic loads can be considered


PIPE288/289

Element Connectivity

PIPE288 PIPE289

Kinematic Large rotations, finite strains

Shear deformation 1st order approximation (transverse shear strain is constant through the cross section)

Material Elasticity, plasticity, viscoelasticity, viscoplasticity, hyperelasticity, creep, more …

3D Pipe Element Topology


Pipe 288/289 Elements

1. Stiffness matrices are the same for BEAM188 element, specialised to a round cross section. A twist-tension option is included

2. Mass matrix is also the same as for a BEAM188 element. It is of the consistent form and it includes the effects of:

- Pipe wall mass

- Internal structural components and fluids

- Added mass

3. The load vector includes effects of:

- Self weight

- Thermal expansion/contraction

- Hydrostatic effects

- Hydrodynamic effects


PIPE288 Input Summary

The pipe element is a 1-dimension element in space capable of capturing ovalization

Nodes

• I, J, K (optional orientation node)

Degrees of Freedom (per node)

• UX, UY, UZ, ROTX, ROTY, ROTZ

Cross Section Information

• Accessed via SECTYPE,SECID,PIPE and SECDATA commands in MAPDL

Material Properties

• EX, EY, ALPX, ALPY, PRXY, DENS,...

Pipe 288/289 Elements (cont.)


– KEYOPT(3)

• Shape functions along the length:

• 0 -- Linear (default)

• 2 -- Quadratic

• 3 -- Cubic

– KEYOPT(4)

• Hoop strain treatment

• 1 -- Thin shell theory (Kirchoff)

• 2 -- Thick shell theory

– KEYOPT(6)

• End cap loads

• 0 -- Internal and external pressures cause loads on end caps

• 1 -- Internal and external pressures do not cause loads on end caps

– KEYOPT(8)

• Shear stress output

• 0 -- Output a combined state of the following two types (default)

• 1 – Output only torsion-related shear stress

• 2 -- Output only flexure-related transverse-shear stress

– KEYOPT(12) (n/a with HROCEAN)

• Hydrodynamic output

• 0 -- None (default)

• 1 -- Additional centroidal hydrodynamic output

Pipe 288/289 KeyOptions


Typical Pipe Cross Section


Pipe 288/289 Element Section When SECTYPE,,PIPE --> SECDATA, Do, Tw, Nc, Ss, Nt

Do = Outside diameter of pipe

Tw = Wall thickness

Nc = Number of cells along the circumference. Must be 8 or greater (Default value is 8)

Ss = Section number of the shell representing the pipe wall

(valid with ELBOW290 only)

Nt = Number of cells through the pipe wall. Valid values are 1, 3, 5, 7, and 9. The default value is 1. Cells are graded such that they are thinner on the inner and outer surfaces.


Pipe 288/289 Element Section (cont.) When SECTYPE,,PIPE --> SECDATA, ..., Mint, Mins, Tins

Mint = Material number of fluid inside of the pipe. The default value is 0 (no fluid)

Mins = Material number of material external to the pipe (such as insulation, biofouling, or armoring). The default value is 0 (no external material).

Tins = Thickness of material external to the pipe, such as insulation. The default value is 0 (no external material)

External material (Mins) adds mass and increases hydraulic diameter, but does not add to stiffness


Pipe288/289 Ocean Loads

Hydrostatic Effects

1. Internal pressure due to uniform internal pressure

2. Internal pressure due to hydrostatic effects of internal fluid

3. External pressure due to hydrostatic effects of external fluid

4. Buoyancy

where: {F/L}b is the vector of loads per unit length due to buoyancy

Cb = coefficient of buoyancy

{g} = acceleration vector

}{4

}/{ 2 gDCLF ewbb


The hydrodynamic effects are computed from a generalized Morison’s Eq.

where:

{F/L}d = vector of loads per unit length due to hydrodynamic effects

CD = coefficient of normal drag

ρw = water density (mass/length3)

De = effective diameter of the pipe with insulation (length)

= normal relative particle velocity vector (length/time)

CM = coefficient of inertia

= normal particle acceleration vector (length/time2)

CT = coefficient of tangential drag

= tangential relative particle velocity vector (length/time)

Pipe288/289 Ocean Loads (cont.)

}{ nu

}{ nv

}{ tu

tte

wT

newMnne

wD

d

uuD

C

vDCuuD

CL

F

||2

4||

2

2


Pipe288/289 Ocean Loads (cont.)

1. The relative particle velocities include the effects of water motion due to current and waves, as well as motion of the pipe itself

2. Two integration points along the element length are used to generate the load vector

3. Ocean loading is nonlinear based on the square of the relative velocity between the structure and the water


Marine Growth Definition

Marine growth has a number of effects on loading:

a. increase in structural diameter and displace volume

b. increase in force coefficients

c. increase in structural weight

d. Increase in mass and hydrodynamic added mass* --> decrease natural freq.

e. increase flow stability

CD and CM will be affected in Morison’s Eq as a result of the above

Marine growth is specified as a thickness (average) addition to selected line bodies characterized by its density. Named selections are implemented in order to define marine growth

*added mass acts only normal to axis of element


Ocean Commands Overview

Syntax:

OCTYPE,OCID,BASIC,OCNAME,IDCUR,IDWAV (compulsory)

OCDATA,DEPTH,MATOC

OCTABLE,RE,CDy,CDz,CT,CMy,CMz

where

RE = Reynolds number for coefficients for this command. Input these values in ascending order from one command to the next.

CDy = Drag coefficient in the element y direction (normal).

CDz = Drag coefficient in the element z direction (normal). Defaults to CDy.

CT = Drag coefficient in the element x direction (tangential).

CMy = Coefficient of inertia in the element y direction.

CMz = Coefficent of inertia in the element z direction. Defaults to CMy.


Ocean Commands Overview (cont.)

Syntax:

OCTYPE,,CURRENT (optional)

OCTABLE,DEPTH,CURVEL,CURDIR,T

OCTABLE,-DEPTH,CURVEL,CURDIR,T

where

DEPTH = Coordinate location of the drift current. Input these values in descending order from one command to the next. (The first Z value must be zero and the last one must be -Depth.)* If the current is constant, only one OCTABLE value is required

CURVEL = Velocity of the drift current

CURDIR = Angle of the drift current

T = Temperature at this location

*up to eight different vertical stations can be defined



Syntax:

OCTYPE,,WAVE (optional)

OCDATA,KWAVE,WAVEDIR,...

OCTABLE,HGT,PERIOD,Ps,L (dependent on KWAVE)

where

KWAVE = wave theory (Airy=0, Wheeler=1, Stokes=2,...)

WAVEDIR = Wave direction θ

HGT = wave height

PERIOD= wave period

Ps = phase shift*

L = wavelength (optional)

* the phase shift is measured in degrees. It determines the wave position, i.e. the point at which the load is applied.



Syntax:


OCDATA,KWAVE,WAVEDIR,WAVELOC,KCRC,KMF

where

WAVELOC = wave location type (valid when KWAVE = 0~3)

KCRC = wave-current interaction (see next slide)

KMF= MacCamy-Fuchs adjustment key (valid when KWAVE = 0~3)


Wave-current interaction option Syntax:


OCDATA,WAVETHEORY,WAVEDIR,WAVELOC,KCRC,KMF

This adjustment is usually applicable when the wave amplitude is large relative to the depth. The options are the following:

KCRC = 0 – no stretching, current as defined in OCTABLE,DEPTH,CURVELO

KCRC = 1 – linear stretching from seabed to the top of the wave

KCRC = 2 – same as KCRC=1 but also adjusts the current profile horizontally

KCRC = 3 – nonlinear stretching, valid only when KWAVE = 5~7 (see API RP 2A Codes of Practice)


Definition of Wave Loading

How do I define the wave load?

Regular wave definition:

Regular wave theories available are: • Linear Wave Theory (Airy)

• Solitary Wave Theory (Cnoidal)

• Stokes 5th Order Theory

• Stream Function Theory (3 to 9)

• User defined wave data

Wave height & period are user-defined.

User defined waves require: • Grid of velocities & accelerations

• A free surface profile


Definition of Wave Loading (cont.)

From experiments Re can be approached as a function of (deep) wave height

For example a 4m dia. cylinder experiences: reflection, diffraction, inertia, viscous-dominant drag forces as the wave height increases.


Pipe288/289 Assumptions & Restrictions

• Geometry in Mechanical should have Model type set to “Pipe” so that the ocean load can be applied

• Command objects are needed to define the ocean environment (OCxxxx commands, SOCEAN, marine growth, added mass etc)

• Pipes cannot have zero length

• KEYOPT(12) is not available in analyses that include HROCEAN

• The global origin must be at the mean sea level, with the global Z-axis pointing away from the seabed


Ocean Loading

• Jacket structures can be solved in the following ways:

• Static, with a single loadstep.

• Static, with multiple load steps to track a wave through a structure.

• Transient, to include mass and inertia effects.

• Frequency Domain Static Harmonic (to calculate the effect of a wave passing through the structure via real and imaginary components).

• Frequency Domain Dynamic Harmonic (to calculate the effect of a wave passing through the structure via real and imaginary components including mass and inertia effects).

• For the Harmonic solution methods, the command HROCEAN is used. Discussed in Lecture 11 (Fatigue Checking)


Ocean Loading with multiple Cd and Cm values

• Most certainly a structure has elements with different Cd and Cm values. To define different Cd and Cm carry out the following:

1. Define a named selection (out of edges) with the members that have the different Cd and Cm. Rename it to say, “BracesCD1”

2. Create a command object and define the ocean environment:

octype,OCENV2,BASIC,ocean2,...

ocdata,depth,matdem,...

octable,ReyNum,arg1,arg1,,arg2,arg2

where arg1 and arg2 are the Cd and Cm values


Ocean Loading with multiple Cd and Cm values (cont.)

3. Create a command object for the braces included in BracesCD1. Paste the following in:

FINI /PREP7 *GET,AR22,ETYPE,,NUM,MAX! max elem type # AR24=1200 ! marine growth density (kg/m^3) AR25=0.1 ! marine growth thickness (m) *DO,AR21,1,AR22,1 CMSEL,S,BracesCd1 ESLN,,1,ACTIVE ESEL,R,TYPE,,AR21 ! reselect from current set, elem type # *GET,AR23,ELEM,,COUNT *IF,AR23,GT,0,THEN *GET,AR31,PIPE,AR21,DATA,1 ! get 1st location of SECDATA for SECID, Do *GET,AR32,PIPE,AR21,DATA,2 ! get 2nd location of SECDATA for SECID, Thk MP,DENS,AR21+AR22,AR24 ! marine growth density SECTYPE,AR21+AR22,PIPE SOCEAN,OCENV2 ! associate ocenv2 w/ element selection SECDATA,AR31,AR32,,,,,AR21+AR22,AR25! 7th location of SECDATA, ! mat dens external to pipe EMODIF,ALL,SECNUM,AR21+AR22 *ENDIF *ENDDO ALLSEL FINI /SOLU

Optional, if not needed comment out by putting ! at the beginning of the line


Ocean Loading

HROCEAN HROCEAN, Type, NPHASE

Program calculates forces on each load component of each element at NPHASE solutions. Min and Max forces are then returned

Type Specifies how to include ocean wave information in a harmonic analysis:

HARMONIC — Performs a harmonic analysis using both real and imaginary load vectors (default). This option works by performing a harmonic analysis running at a frequency determined by the wave period (specified via OCDATA & OCTABLE).

STATIC — Performs a static analysis using both real and imaginary load vectors (calculated via HOWP). This option works by performing a harmonic analysis running at a frequency of 0.0Hz.

OFF — Deactivates a previously activated harmonic ocean wave procedure (HOWP) and performs a standard harmonic analysis.

NPHASE Positive number specifying the number of phases to calculate forces. This value must be at least 8 (Defaults to 20).

Note: Only one ocean loading is supported at a time. Irregular wave types are not supported


• Workshop 4 – Ocean Loading

• Goal:

– Use command objects & become acquainted with MAPDL

– Use local parameters in command objects

– Define marine growth

– Promote design variables

– Post process results and study the effects of the parameters

Workshop 4 – Ocean Loading

../Workshops/WB-Mech_120_WS_03.1.ppt






McCormick, M.E. (1973) Ocean Engineering Wave Mechanics. Wiley & Sons

Hallam M.G., Heaf, N.J., Wooton, L.R. (1977) Dynamics of Marine Structures: Methods of calculating the dynamics response of fixed structures subject to wave and current action. CIRIA Underwater Engineering Group Report UR8

Further Reading

Ocean Loading in Ansys 14.5

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