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Model Generation Through the ANSYS Mechanical Products or Femgv

Models can be created for analyses in ANSYS ASAS using ANSYS DesignModeler and the ANSYS Mechanical products; the model transfer includes all aspects of the analytical model including the finite element mesh, material and geometric properties, boundary conditions and loading.

The ANSYS ASAS solver has also been integrated with FEMGV to provide powerful finite element analysis facilities including both modeling and post processing.

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Geometry generated in ANSYS DesignModeler

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Model meshed with ANSYS Structural/Mechanical application

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Model transferred to ANSYS ASAS via ANSYS macro

 

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Coupled Wave Structure Interaction with Regular and Random Waves

In addition to being able to generate environmental loading for conventional quasi-static analyses, ANSYS ASAS includes a feature that integrates the wave loading capabilities with the large deflection solver available in the nonlinear product. This facilitates full hydroelastic coupling and makes it suitable for jack-ups, compliant structures, manifold installation and riser analysis.

Features include:

  • Wave, current and wind load
  • Regular waves, airy, Stokes 5th, cnoidal, stream function plus user defined wave grid
  • Random seastates, JONSWAP, Pierson-Moskowitz and user defined plus shell new wave
  • Wave loading within the API code of practice including effects of current on the wave period, current stretching, current blockage factor and wave kinematics factor
  • Tube and beam elements with facilities for modeling appurtenances, anodes, etc
  • Flooded or sealed members
  • Marine growth>
  • Reynolds/KC number effects
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Jacket structure with coupled wave-structure interaction

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Analysis of Offshore Wind Turbines

ANSYS ASAS is increasingly used in the design and certification process of offshore wind turbines (OWTs). For the last four decades, ANSYS ASAS has been used in the Oil & Gas industry for analyzing a large variety of offshore structures, its extensive and well-proven set of FEA capabilities enable comprehensive and reliable design and certification. In particular, ANSYS ASAS is used to simulate the overall OWT system for which a number of effects can be analyzed simultaneously, including wave loads (from regular and irregular sea states and currents), the elastic behavior of the support structure (ranging from monopiles to jackets), and the soil characteristics of the local sea bed, as well as loads from turbulent wind fields and turbine control system when coupled with specialized wind programs such as Flex5, ADCoS and FAST. As required, hundreds of load cases can be easily set up, and from these the ANSYS ASAS software can derive probabilistic based rain flow counting fatigue data such as fatigue life, usage factors, damage per wave and stress histograms.

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Courtesy REpower Systems AG

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Interfaces to ANSYS AQWA

Pressures and motions from a diffraction/radiation analysis in ANSYS AQWA can be transferred to an ANSYS ASAS model. Each combination of wave frequency, heading, height and phase becomes a quasi-static load case. This feature is also available for transferring wave loads for structures made up of both diffracting and tubular elements, such as may occur in semi-submersibles or truss-spars.

The meshes for the ANSYS AQWA and ANSYS ASAS models can be developed independently since structural requirements are quite different from the much simpler hydrodynamic model. The mapping feature automatically interpolates the correct pressures at the structural nodes. For those structural elements that cut the water surface, corrections are made to the resulting pressures to ensure that the total loads applied are correct.

ANSYS AQWA can also be used as a source of response amplitude operator (RAO) information to provide simulation of large floating body response to a wave environment.

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Hull pressure plot and stress resultants in 
ANSYS ASAS application after pressure mapping

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Nonlinear and Extended Solution Capabilities

ANSYS ASAS has a standard set of features that allows you to go beyond simple linear elastic analysis, such as:

  • Large displacement
  • Nonlinear elastic material behavior
  • Plasticity
  • Laminated composite failure behavior
  • Soil and rock-like material behavior
  • Nonlinear boundary conditions, such as gaps and rigid surface contact
  • Creep
  • Nonlinear transient dynamics
  • Stability (buckling loads)
  • Steady state heat
  • General field analysis

In addition to this, there is a special application for investigating pile behavior using non-linear soil properties, and how this interacts with attached structural members (so called pile-structure interaction). This employs an efficient solution based upon P-Y and T-Z soil curves, either explicitly defined or by providing basic soil parameters, such as unit weight, shear strength, etc. Pile group effects are automatically included to account for the interaction between the piles through the soil. Pile code checks to API can be requested.

Whilst conventional analysis of framed structures assumes rigid joint connections between adjoining members, there is much evidence that the effects of including the inherent flexibility at the joints provides for a more rigorous solution. This allows simple beam models to account for chord wall flexibility and the effects of adjacent members, potentially obviating the need for detailed shell models in critical areas. This application is based upon simple parametric formulations that describe the load-deformation characteristics for various joint types.

Code Checking and Fatigue Calculations

A major requirement for many civil engineering (including offshore structures) designs is the need to satisfy regulatory codes of practice for frame and shell like structures. ASAS provides capabilities to undertake checks against the most commonly used codes. Three categories of code checks may be undertaken:

  • Beam member checks for strength and buckling
  • Tubular joint connections
  • Hydrostatic collapse

Present and past codes are available, including API WSD and LRFD, AISC WSD and LRFD, ISO 19902, NORSOK, BS5950 and DS449.

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Member utilizations

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Joint check utilizations

To compliment the code checking there is also a fatigue capability that provides for operational life evaluation in environments where fatigue issues have been identified, for example in the offshore industry. Spectral, deterministic and time history fatigue analysis options are available. Stress concentration factors can either be explicitly defined or a set of empirical formulations may be utilized, including the Efthymiou influence function approach. S-N curves may also be explicitly defined, or use one of the pre-defined curves commonly adopted. Thickness correction effects can be included.

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Direct Access of ASAS Results from MS-EXCEL© and Mathcad©

As part of the ANSYS ASAS installation, functional interfaces are added enabling all ANSYS ASAS results to be accessed and retrieved using MS-EXCEL and Mathcad.

This facility provides a number of major benefits. Firstly it allows the user to perform further post-processing using MS-EXCEL (including Visual Basic) and PTC Mathcad. Secondly, it allows a user to design his/her own report templates. Once designed, templates can be re-used, thus saving considerable effort.

There is also a Windows Dynamic Link Library that provides a toolkit of functions that may be called by user created executable programs that permit access to the ANSYS ASAS results.

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