ANSYS AQWA – An Integrated System

ANSYS AQWA is an integrated system for undertaking hydrodynamic and mooring analyses. Functionally it may be split into six operations:

  1. Diffraction/radiation
  2. Static and dynamic initial stability including the effects of mooring systems and other physical connections
  3. Frequency domain dynamic analysis
  4. Time domain with irregular waves including slow drift
  5. Non-linear time domain with large amplitude regular or irregular waves
  6. Hydrodynamic load transfer to a structural finite element analysis

A graphical interface provides the common tool to facilitate visualization and results processing for the AQWA system.

In addition an optional facility for coupled cable dynamics is available with the static and dynamic solvers (in both the frequency and time domain) to provide more rigorous modeling of the loading and response of mooring systems, especially in deep waters.

Modeling Flexibility

For large floating bodies ANSYS AQWA requires a description of the hull form. A standard panel discretization of the hull is utilized in the radiation/diffraction stage of the simulation (and may also be used in subsequent solvers, such as the static and dynamic time domain solutions). This can be achieved in one of several ways.
For ship shaped vessels there is an automated mesh generator that works from the ship’s lines plan (or offsets).

Modeling Flexibility

For more general vessel forms the ANSYS DesignModeler may be utilized to either generate the geometry directly, or to import geometry from a wide range of CAD systems. This geometry can then be imported into the new ANSYS AQWA hydrodynamic diffraction analysis system that includes its own meshing capabilities. Alternatively, suitably defined external CAD geometry may be directly imported into ANSYS AQWA hydrodynamic diffraction.

Total Capability for all Hydrodynamic Applications

ANSYS AQWA is not just for moorings or diffraction/radiation but is a general purpose hydrodynamics analysis tool providing enormous flexibility to address most types of problem. Examples of its use include:

  • Design and analysis of mooring systems including intermediate buoys and clump weights
  • Motions analysis of FPSOs
  • Determination of air gaps
  • Calculation of the shielding effects of ships and barriers
  • Multiple body interactions during LNG transfer
  • Coupled cable dynamics
  • Sectional force calculations
  • TLP tether analysis
  • Dropped object trajectory calculations
  • Concept design and analysis of wave and wind energy systems
  • Simulation of lifting operations between floating vessels
  • Discharging landing craft from mother ships
  • Transportation of large offshore structures using barges/ships
  • Float over analyses
  • Motion analysis of spar vessels

Hydrodynamic Interaction Between Bodies

ANSYS AQWA can take account of hydrodynamic interaction between adjacent vessels and structures. Thus the motions of one structure can affect the motions of another. The structures can be articulated, connected by cables or independent. A typical application would be shielding effects. Forward speed effects can be included in the interaction.

Hydrodynamic interaction between bodies

This image shows the shielding effects of a pier adjacent to a ship, an important aspect 
in the design of breakwaters and how they affect mooring systems

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Multiple Body Articulations

ANSYS AQWA has the ability to handle articulated structures, up to a maximum of 50 structures and 50 articulations. This permits the modeling of combined hydrodynamic and mechanical dynamic configurations. Articulations can have stiffness, friction and damping included. If the mechanical connections are represented as tubes these are allowed to become wholly or partially submerged and Morison types forces are calculated.

Multiple body articulations

Powerful Modeling and Results Interrogation

The ANSYS AQWA-Graphical Supervisor is the key module within the suite. It can perform a range of tasks including:

  • Data review
  • Automatic mesh generation
  • Scaling of existing models
  • Results presentation
  • Powerful graphing facilities
  • Export to spreadsheets
  • Function processing e.g. Nodal RAOs
  • Transformation analysis e.g. time domain to frequency domain
  • Bending moment/shear force calculation including forward speed
  • Sectional force calculation
  • Online tutorial/demos
  • Control and monitoring of analyses
Powerful modeling and results interrogation

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Coupled Cable Dynamics

As an optional extra, it is possible for ANSYS AQWA to include the effects of line dynamics by modeling the mooring lines as a series of rod elements. This has the added feature of including drag loads and added mass effects of the line, something that can be significant for the long lengths of line encountered in deep water applications. The feature is available for both static (drag effects) and dynamic analyses. For the dynamic response the capability is available within both the frequency domain and time domain. Frequency domain solutions are very fast and can help determine whether cable dynamics need to be considered as part of the analysis. Time domain solutions can provide much greater accuracy but take longer to run.

As a further level of investigation, individual lines may be processed through the graphical supervisor, providing detailed information about tensions and motions along the line.

This screen shows the difference which can occur between including and excluding the effects 
of coupled cable dynamics. Also shown is the distribution of energy during the analysis.

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External Force Dynamic Link Libraries

ANSYS AQWA has the ability to include a user defined external force algorithm via a dynamic link library (DLL). This feature permits the user to create his or her own DLL to calculate a force based on the time, the position and/or the velocity of a structure. Example interfaces are provided for C or FORTRAN, but any programming language that can produce a DLL can be employed. An added mass matrix can also be computed at each time-step to simulate inertia forces. The calculation can be controlled by a set of up to 100 integer parameters and 100 real parameters that may be input and then passed to the external force routine.

This facility could be used to model:

  • A dynamic positioning system
  • A steering system
  • The towing force provided by a tug
  • A damping system with unusual characteristics
  • The suction force between two ships close together, or between a ship and the sea-bed
  • Fluid transfer between vessels
  • To calculate the energy extracted from a wave-power device

Functional Interface to Microsoft Excel

When ANSYS AQWA is installed a Microsoft Excel add-in is made available to permit direct interrogation of the ANSYS AQWA database via the insert function facility. This allows for custom report generation, plotting, and additional post-processing that can be defined by the user.

An example is shown below of the extraction of response amplitude operator information from a diffraction analysis.

Functional interface to Microsoft Excel

Available functions include:

FUNCTIONS

DESCRIPTION

Aqlcogcoord

Returns x, y or z coordinate of the centre of gravity of a structure

Aqlcoord

Returns x, y or z coordinate of a node

Aqlelcent

Returns x, y, or z coordinate of the centroid of a diffracting element

Aqlelpres

Returns pressure at element centroid

Aqlfrequency

Returns wave frequencies analyzed in ANSYS AQWA-LINE

Aqlglobals

Returns global parameters

Aqlmessage

Returns an error message for a given aql error flag number

Aqlndirns

Returns the number of wave directions for a structure

Aqlnfreqs

Returns the number of wave frequencies for a structure

Aqlnlines

Returns the number of mooring lines in a model

Aqlnstructs

Returns the number of structures in a model

Aqlposition

Returns the position of a structure for a time history analysis

Aqlrao

Returns RAO information from the hydrodynamic database

Aqlrao2

Returns interpolated RAO information from the hydrodynamic database

Aqlstatmooring

Returns mooring line information for a static (ANSYS AQWA-LIBRIUM) analysis

Aqlstatposcog

Returns position of the CoG for a static (ANSYS AQWA-LIBRIUM) analysis

Aqlstatposnod

Returns position of a specified node for a static (ANSYS AQWA-LIBRIUM) analysis

Aqlthacccog

Returns the acceleration of the cog for a time history analysis

Aqltharticulation

Returns articulation reactions for a time history analysis

Aqlthfender

Returns fender information for a time history analysis

Aqlthmooring

Returns mooring line information for a time history analysis

Aqlthnumsteps

Returns number of time steps for a time history analysis

Aqlthposcog

Returns the position of the CoG for a time history analysis

Aqlthtime

Returns the time associated with a time step in a time history analysis

Aqlthvelcog

Returns the velocity of the cog for a time history analysis

Aqlwavedirn

Returns wave directions analyzed in ANSYS AQWA-LINE

Aqlzcge

Returns the ZCGE parameter for a structure in an ANSYS AQWA-LINE analysis

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Transfer of Results for Stress Analysis

ANSYS AQWA-WAVE is a linking program between the ANSYS AQWA diffraction calculation and a finite element model. It takes the results in the form of pressures and motions for given wave directions and frequencies and automatically applies them as pressures and accelerations to an ANSYS or ASAS finite element model. Differences in mesh density between the two models are automatically accounted for.

Motions and pressure