aqwa-intro 14.5 l02 basics and theory

© 2012 ANSYS, Inc. March 21, 2013 1 Release 14.5

14.5 Release

Lecture 02

ANSYS Aqwa Basics & Theory


A. Description of program capabilities • Modelling in Aqwa

– Diffracting or non-diffracting panels – Morison elements

• TUBE • STUB • DISC

– Point masses – Mixed models – Moorings

• Linear springs • Elastic catenaries • Intermediate buoys • Tethers • Pulleys

– Fenders – Articulations

• New for v14.0

ANSYS Aqwa Workbench Integration – Time history animation

Frequency Domain Drag Linearization – Extension of drag linearization in Aqwa-Line to include drag on DISC

and STUB elements as well as TUBEs – Linearized drag in Aqwa-Fer

Multi-directional Wave Spectra – Extension to Aqwa-Fer


... Description of program capabilities

Modelling in Aqwa – Environment

• Waves

• Regular / irregular

• Time history

• Wave Spectra

• Pierson-Moskowitz

• JONSWAP

• User-defined

• Cross-swell

• Current

• Uniform

• Profile

• Wind

• Uniform

• Spectra

• Time history



Radiation/Diffraction

• Multiple structures including hydrodynamic interaction

• Hydrodynamic coefficients (added mass and damping)

• RAOs

• Drift coefficients (Near / far field, full QTF matrix)

• Shear Force / Bending Moment

• Splitting forces

• Pressure distribution for transfer to structural model



Load Transfer

• Calculation and transfer of hydrodynamic loads to ANSYS Mechanical

Aqwa

Hydrodynamic

Model

ANSYS

Mechanical

Model

Motions & Pressures

AQWA

WAVE

Aqwa Pressure plot ANSYS Pressure plot


... Description of program capabilities Equilibrium and Stability • Computes static equilibrium position for multiple environmental and mooring

configurations

• Preliminary mooring design

• Calculates static / dynamic stability

Frequency Domain • Significant motions at low frequency/wave frequency in frequency domain

• Permits rapid analysis using linearized parameters of mooring systems

• Graphs for response spectra / RAOs & other parameters

• Wave spreading and 2D spectra

Time Domain • Time-history analysis of multiple structures with irregular waves

• Can use full QTF matrix for shallow water conditions

• Import of wave height time history

• Input of forces via user-defined .dll

• Output of motions and forces

• Graphical and animation results



Large amplitude motions

• Non-linear time-history analysis with large (survival) waves

• Regular or irregular waves

• Integration of pressure over wetted surface

• Standard plus short crested wave option

Cable dynamics

• Additional cost item for more rigorous simulation of mooring line behaviour

• Provides full coupled vessel/mooring line analyses

Excel Interface

• Aqwa specific functions add-in for Excel for data and results retrieval, processing and report generation


...Description of program capabilities

Currently in Workbench (Release 14.0)

• Import of geometry from ANSYS DesignModeler

• Interactive data modification and editing

• Native meshing

• Diffraction/radiation analysis

• Definition of moorings, articulations and fenders

• Definition of environment

• Time domain analysis

• Graph plotting

• Wave surface and pressure contour plots


B. Aqwa Basics

Aqwa Global Coordinate System

• Aqwa Global Coordinate System is referred to as the Fixed Reference Axes (FRA):

• The origin lies in the still water plane

• The positive z axis is vertically upwards

• A right handed system

• It is not related to the directions

North, South, East and West

• Rigid body motions:

• Surge, Sway, Heave – translational

• Roll, Pitch, Yaw - rotational

• The direction of motion are relative to the geometry definition of vessel/structure (x is not surge by default!!)

0

z

y

x W.L.

z

Portside Bow Stern

Starboard x

y


... Aqwa Basics

Hydrostatics

• Archimedes’s principle

• Buoyancy of an immersed body = weight of the fluid displaced

• Hydrostatic pressure

G: centre of gravity

B: centre of buoyancy

Buoyancy is the resultant of all hydrostatic force over wetted surface

0gZp G

Z0 B


... Aqwa Basics

Environmental direction in Aqwa

• The wave, wind and current directions are defined in Aqwa as the directions which they are travelling towards.

• The direction is defined as the angle between the wave (or current, wind) and the positive x axis measured anti-clockwise (For a ship facing forward to + x axis, this means 0 is astern seas and 180 is head seas).

• Directions in AQWA are input and output in degrees.

X axis

Wave direction (or current, wind)

positive angle


... Aqwa Basics

Phase angle

• In Aqwa, the phase angle (Φ in degrees) of a parameter defines the time difference (dt) from the time when the wave crest is at the COG of the structure to the time when the parameter reaches its peak value.

dt= Φ*T/360, where T is the wave period

• A positive phase angle indicates that the parameter lags behind the wave.

CoG

dt


... Aqwa Basics

Regular waves:

• Airy Waves (linear wave)

a = A cos (-ωt + kx)

(ω: frequency in radians/sec; k: wave number)

• Stokes 2nd Order Waves

a = A cos (-ωt + kx) + 0.5 k A² cos2(-ωt + kx)


... Aqwa Basics Irregular waves:

Wave spectrum types accepted in Aqwa are:

• Pierson-Moskowitz (PM) spectrum

• JONSWAP spectrum

• Gaussian

• User defined spectrum

• Cross Swell (using any of the above)

Irregular waves can be in the form of:

• Long crested waves; OR

• Short crested waves, i.e. a directional spread sea (available in some features so far)


... Aqwa Basics

Wind:

• Uniform wind

• Ochi and Shin wind spectrum

• API wind spectrum

• NPD wind spectrum

• User-defined wind spectrum

Current :

• Uniform velocity current

• Velocity profiled current


C. Theory in Aqwa Hydrodynamic Forces on Structures (on diffraction elements)

Fluid force

HydrodynamicHydrostatic

Wave exciting force

Ambient pressure

(incident wave or

Froude-Krylov force)

Effect of structure

on waves

(Diffraction)

Radiation force due

to structure motion

In-phase

(Added Mass)

Out-of-phase

(Radiation

damping)

F() C.xMa().x B().x


... Theory in Aqwa

Wave Forces for Diffracting Structures (modelled with plate elements)

• Incident wave force (Froude-Krylov force): from the pressure in the undisturbed waves.

• Diffraction force: due to stationary structure disturbing the incident waves.

• Radiation force: due to structure’s oscillation which generates waves.

• Drift force (net force due to high order effect)


... Theory in Aqwa

Hydrodynamic forces for non-diffracting structures (modelled with Morison elements e.g. TUBE, DISC)

• For slender cylindrical elements (D/λ<0.2), e.g. tube elements, the hydrodynamic force (including drag) on unit length of the element can be calculated using Morison equation:

• Ca and Cd are the added mass and drag coefficients of the element;

• is the volume of the element (per unit length)

• D is the element diameter,

• V is the relative velocity,

• λ is the wave length.

VDVCXCaCaF dawaw 2

1

Froude-Krylov force Wave inertia force

Radiation force Drag force


... Theory in Aqwa 3D Potential Theory solution • Viscous forces are not taken into account

• Ideal fluid, irrotational, incompressible

• Small wave elevation

Boundary condition problem is solved by satisfying • Body Boundary condition (Timman-Newman relations)

• Linearized free surface condition

• Sea bed boundary condition

• Radiation condition

• Solution for diffracted and radiated wave potentials uses a pulsating source distribution (zero speed solution with forward speed corrections)

Theory applies to finite depth and diffraction problem is solved in frequency domain

• Shallow water solution is available

Both 1st Order and 2nd Order wave forces are calculated • 2nd Order forces can be calculated from either near field or far field solutions

For more details on the exact formulations please refer to Aqwa Theory Manual


... Theory in Aqwa Full Quadratic Transfer Function (QTF)

• Components at both difference and sum frequencies

• Each with in-phase and out-of-phase parts

• The second order wave potential does not contribute to the diagonal terms of the QTF matrix, so that it has no effect on the mean wave drift force. However, the second order wave potential contributes to the off-diagonal terms of the QTF. It has been found that in shallow water the QTF’s (drift force coefficients) can be increased significantly by the second order potential. Therefore, the inclusion of the second order incident and diffracted potential is necessary for the accurate evaluation of the second order wave exciting forces in shallow water. In Aqwa this is done using the Pinkster approximation (Pinkster, 1980).

N

i

N

jjijiijjijiij

N

i

N

jjijiijjijiij

tQtQ

tPtPtF

1 1

1 1

)2(

sinsin

coscos)(


... Theory in Aqwa

Equations of Motions

• The response X of a structure in waves is calculated by solving the equation of motion in the frequency domain for unit wave amplitude:

• where Ms is structure mass

• Ma is added mass (frequency dependent)

• B is damping (frequency dependent)

• C is hydrostatic stiffness

• F is wave force (incident and diffracting forces).

)()(])())(([ 2 FXCBiMM as


... Theory in Aqwa

Long Story Short: Fluid force

HydrodynamicHydrostatic

Wave exciting force

Ambient pressure

(incident wave or

Froude-Krylov force)

Effect of structure

on waves

(Diffraction)

Radiation force due

to structure motion

In-phase

(Added Mass)

Out-of-phase

(Radiation

damping)

F() C.xMa().x B().x= + + + MS x’’

Structural Mass


D. Workbench HD Basics

WB HD shares common conventions with other Workbench products where appropriate

The basic interface consists of a number of areas

• Toolbars

• Analysis tree

• Details panel

• Graphical model representation and results presentation or textual results


... Workbench HD Basics Standard Workbench Toolbars

View manipulation

Selection control

File shortcuts

Aqwa Specific Toolbars

Show/Hide element boundaries

Turn on/off object highlighting

Show/Hide Seabed and Water Surface

Solve shortcuts


... Workbench HD Basics WB HD Tree view

• As with other Workbench applications the tree and details panel are used to define the organization of the simulation requirements and associated data.

• As a tree object is selected the data related to that object will be presented in the details panel


Bodies make up a Part. These are defined in

DesignModeler

Some Aqwa specific geometry based objects may

be added directly within AqwaWB

Objects that define the additional data required for

undertaking the hydrodynamic analysis

Results objects that may be added as required.

When selected they change the view in the main

visualization window

... Workbench HD Basics WB HD Tree view

Object excluded

Object included and up to date

Object changed since last solve, or no solve has been undertaken

Object is up to date for the Hydrostatic solve

Each vessel/structure is associated with a Part, in

this case Ship


... Workbench HD Basics Model and results presentation

• The main graphical area responds to what is selected in the object tree

– If geometry or mesh based information is selected then this will show a visualization of the information requested on the Geometry tab.

– If textual results are requested (such as Hydrostatic information) this will be shown on the Properties tab.


E. Workbench HTR Basics WB HTR extends the functionality of the WB HD system by providing definition of connections, such as moorings, and environmental data, such as waves, wind and current

WB HTR allows time history dynamic analyses to be undertaken with various wave definitions

• Regular wave

• Irregular wave (large amplitude)

• Irregular wave with slow drift

• Slow drift effects only


... Workbench HTR Basics When applying current and/or wind loading (which are viscous effects) coefficients have to be provided that relate applied force to the relative current/wind velocity for a given vessel under Geometry

The rotational terms about X and Y allow the inclusion of the moment arising from the distance between the effective centre of loaded area to the COG (where Aqwa applies the actual load)


... Workbench HTR Basics

Mooring lines require connection points on both the vessel geometry (fairleads) and at anchor locations (for moorings connected to ground)

Vessel connections are defined underneath the associated structure geometry entry. Connections can either be defined explicitly (as X, Y, Z coordinates in the global system), or as offsets from existing vertices in the model.


... Workbench HTR Basics Similarly anchor locations are defined at the Geometry level. Coordinates are always in the global system.

Mooring lines can then be added from Connections. These lines can be between two structures (vessels), or from a structure to a fixed point (anchor location). The Connection Stiffness shown here is typically used to model the effects of mooring lines on the HD system, where moorings cannot be explicitly utilized.


... Workbench HTR Basics Joints, or physical connections also require connection points to be defined. Joint types are Ball & Socket, Universal, Hinged or Rigid.

Joints can then be added from Connections. These joints can be between two structures, or from a structure to a fixed point.

Joint local axes can be defined to correctly orientate the desired connection.

Stiffness, damping and friction may be associated with the joint freedoms.


Workshop 2.1 – ANSYS Aqwa HD

Workshop 2.1 – ANSYS Aqwa Hydrodynamic Diffraction

Goal:

• Create a hydrodynamic radiation/diffraction model

AQWA-Intro_13.0_WS02.1_AQWAWB_HD.pptx








Workshop 2.2 – ANSYS Aqwa HTR Workshop 2.2 – ANSYS Aqwa HydrodynamicTime Response

Goal:

• Create a mooring system and run a time history analysis

AQWA-Intro_13.0_WS02.2_AQWAWB_HTR.pptx










Workshop 2.3 – ANSYS Aqwa Articulations Workshop 2.3 – ANSYS Aqwa Articulations

Goal:

• Create an articulated system consisting of a turret moored FPSO

AQWA-Intro_14.0_WS02.3_AQWAWB_Articulations.pptx







aqwa-intro 14.5 l02 basics and theory

Documents

ansys aqwa basics theory

aqwa diffracting

workbench release

release lecture

parameters wave

transfer of hydrodynamic

forces pressure distribution

response spectra raos