Application of Engineering Simulation

Solutions in Offshore and Marine

Dr. Steven M. Varnam

European Technology Group
ANSYS
1 2011 ANSYS, Inc. October 30, 2014
 Agenda
Introduction
ANSYS Workbench
Analysis Capabilities of ANSYS Mechanical Products
Analysis of Ocean Environments
Examples
Roadmap

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 Introduction

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 Offshore Structures

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 Key Market Problems

Reliable and Safe Product and Processes:

Deep-waters, high pressures
Temperature variation
Hurricane, waves
Dense areas, combustible and hazardous
products
Drilling through complex geology, long
distances
Many production and processing
equipment : Topside, subsea
Hundreds or thousands of load cases
required, particularly for fatigue.

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 Key Market Problems

Cost of Failure
Human life
Environmental concerns
Delays and fines
Loss of capital, time and equipment

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 Fracture, creep,
fatigue

Buckling

Failure is (very)
expensive

Erosion, shocks

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 ANSYS Workbench

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 ANSYS Workbench
ANSYS Workbench acts as a common environment
for all of our mainstream products structures,
fluids, thermal, electronics ....
It enables us to interface software modules directly
without going through intermediate files and
manual processes
For Structural Mechanics users it means Global &
Local analyses in the same system
For detailed component analysis, ANSYS Structural
Mechanics has a wealth of features for linear and
nonlinear simulation.

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 ANSYS Workbench

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 Analysis Capabilities of
ANSYS Mechanical

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 Analysis Capabilities of ANSYS Mechanical

Static analysis Dynamic

Modal Analysis
LINEAR

Vibrations - eigenmodes
Spectral
Cour
tesy:
NAS
A
Vibrations - Probabilistic

Stresses Linear model Harmonic

Vibrations - Harmonic

Implicit
NONLINEAR

Buckled shape
under hydrostatic Time histories
load
Rigid Dynamics
& Rigid/Flex

Explicit
Drop test, shocks, Blast Mechanisms
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 Analysis of Ocean
Environments

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 Modeling of Ocean Waves
Wave loading effects on offshore structures is a critical design
feature for both strength and fatigue performance
Design requirements vary depending upon the type of structure
being designed
Slender Bodies ANSYS
DNV Recommended Practice Mechanical
discusses four loading conditions Global Assessment of
1. Slender Members Large Bodies ANSYS Aqwa
2. Large Volume Structures
3. Air Gap/Slamming Detailed Assessment
4. Vortex Induced Motions of Response to ANSYS CFD
Complex Wave
Loading

Increasing
Complexity

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 Modeling of Waves

The solution of the complete wave equation is not possible without approximations.
Different wave theories exist with different mathematical approximations.

In deep water, the phase speed depends on wave length: Longer waves travel faster.
In shallow water, the phase speed is independent of the wave; it depends only on
the depth of the water.

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 Slender Body Loading

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 Slender Body Wave Loading
When the characteristic dimension of a body is less than 1/5 of the shortest
wavelength, it is common to refer to this as a slender body
Use Morisons equation to solve generated load

w CaAU
F 0.5CdD Ur Ur CmAU s Fw MaU
s

Wavelength
Cd is drag coefficient Height
Cm is inertia coefficient Depth

Ca is added mass coefficient; defined as Ca = Cm 1

Ur is relative velocity of fluid to structure
Us is structural acceleration
Uw is wave kinematic acceleration

Current
profile

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 Slender Body Wave Loading

ANSYS Mechanical enables Morison loading

on
PIPE288/289
BEAM188/189 New at v15
LINK180 New at v15

Wave theories
AIRY - Small amplitude Airy wave without modifications (default).
WHEELER - Small amplitude wave with Wheeler empirical modification of
depth decay function.
STOKES - Stokes fifth order wave.
STREAMFUNCTION - Stream function wave.
RANDOM - Random (but repeatable) combination of linear Airy wave
components.
SHELLNEWWAVE - Shell new wave.
CONSTRAINED - Constrained new wave.
DIFFRACTED - Diffracted wave (using imported hydrodynamic
data)

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 Large Body Loading

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 Large Body Wave Loading
When a body is large enough to interfere with an approaching
wave then it is necessary to include wave interference effects. If
the body is also free to move this can generate additional waves.
A common solution for global analysis of such systems is to use
three dimensional radiation diffraction theory which computes
the linearized hydrodynamic fluid wave loading. This is based
upon fluid potential flow.

AQWA Suite Solution

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 Large Body Wave Loading
ANSYS Aqwa is a multi-body hydrodynamic program that utilizes
the three dimensional radiation/diffraction theory for global
loading and motions simulations

Hull Pressures Coupled Motions

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 Hydrodynamics and Mooring Systems
ANSYS Aqwa for hydrodynamic analysis and vessel motions
Applications include (but not limited to):
Moored multi-body systems
Sea keeping (eg ship performance)
Dynamic positioning systems
Ports and harbors (eg breakwaters)

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 ANSYS Aqwa

Wave Surface
Contours

Hull Pressures
Articulations

Moorings

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 ANSYS Aqwa

ANSYS Aqwa can be utilized for

Connect /offloading /disconnect scenarios
Floatover installations
Launching installations
Lifting operations
Transportation
Failure conditions
Air gap
User definable functionality

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 Large Body Wave Loading

Diffracted Wave Patterns Motions Simulation

Global Load Mapping to Structural Model

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 AQWA Case Study -
Soft Yoke Mooring & Offloading

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 SBM SYMO Design
Soft Yoke Mooring & Offloading (SYMO)

Mooring function Offloading function

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 Specific Design Aspects

Connection operation
Limiting sea states
Winch speed
SYMO dynamics
Loads in connecting rope

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 Specific Design Aspects

Disconnection operation
Limiting sea states
SYMO dynamics
Position of counterweight
Clearance with LNG carrier bow
area
Clearance with superstructure

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 Aqwa SYMO Model

Pin articulations:
Roll & pitch

Pitch only

Roll & pitch pin

Yaw bearing

Pitch pin
Roll & yaw bearing

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 Aqwa Analyses - Connection

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 Solutions For Complex
Ocean Environments

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 Need for More Rigorous Solutions
Limitations of Potential Flow Solvers
Not capable for viscous dominated problems
Green water behavior,
FPSO roll motions,
VIV,
Hull drag calculation etc
Non linear waves
Turbulence
Importance
Key factor in design of offshore structures
Green water behaviors on the deck of ship (FPSO)
Wave run up on the structures
General Requirements
Wave-in-deck loads ( LRFD design criteria)
Hydrodynamic force on the structure
Maximum and Minimum pressures
Duration of pressure peaks
Wave induced motion ( violent free surface)
Green water behavior
Wave run-up
Challenges
Complex environmental loads
Turbulence,
Non linear waves,
Wave interaction with the structure

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 Computational Fluid Dynamics ( CFD)
Examples using Linear/Stokes Wave Theories
CFD is the simulation of fluid flow and heat and mass transfer
by solving conservation equations for mass, momentum, and
energy
Wave interaction with hull

Wave slamming on submarine

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Wave-body interaction
 Current loading on Fixed Structures
CFD can be used to look at viscous and form
drag on a subsea drill fixed to seabed
Upstream velocity 1/10th power law profile
Calculation of force components on structure
in three directions
Drag force ~ hundreds of kN
Lift force ~ tens of kN Bauer Renewables Ltd.

35 2011 ANSYS, Inc. October 30, 2014 Courtesy of Mojo Maritime

 Wave Impact Loading on an Offshore Oil
Rig (Animation)

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 Load History During the Wave Impact

Load history during the wave impact

[Fx-horizontal load, Fz-vertical load]
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 Wave-in-deck Analysis of Offshore Oil Rig:
Model Details
Fifth Order Solitary Wave Theory
Wave Details:
Wave Height : 20m
Water Depth: 41m
Time Period:22s
Domain Size:
400 x 100 x 100 m
Deck Height ( Oil rig height from the lower part of the deck to liquid surface) : 13.375m
Mesh Count: 7M
Time Step Size: 0.05s
Computational Time: ~26 hours in 24 CPU

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 Mechanical Examples

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 ANSYS Mechanical Applications
Offshore Structures
Pipelines and Risers
Tubulars, connectors
BOPs
Pressure vessels
Seals
Hulls
Etc.

Courtesy of ACA Engineering Consultants Courtesy of Delta Marine Engineering Company

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 Examples Linear Static

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 Jacket Structural Analysis
Project Page showing various
Mechanical Systems

Detailed jacket model in

DesignModeler

Jacket model in Mechanical

Bending moment plot

SF/BM plots
along member

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 Offshore Structures - Design Solutions
Joint Check

Transportation
Installation
Wave loading
Pile/soil modelling
Beam joint fatigue assessment
Member and joint code checking
Decommissioning

Member Check

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 Code Checks
Joint Code & Member Code checks including:
AISC 10th edition working stress and 2nd edition LRFD
API RP2a-WSD 21st edition working stress
RP2A-LRFD 1st edition
BS5950 part 1 1992
NORSOK 2000
NORSOK NS3472 1984
NPD 1992
DS449 1984 (with 1994 amendments)
DS412 1984 (with 1994 amendments)
ISO 19902

Easy-to-use code check facilities including:

Code checks on time histories
Code checks on combined load cases
Visualization of code checks
Ability to use them in combination with ANSYS calculations
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 Fatigue Assessments
FATJACK module offers both deterministic and spectral fatigue capabilities
for tubular frame structures subjected to waves and current or wind including wind
gusts
can be used in frequency and time domain
sea states: JONSWAP, Pierson-Moskovitz, Occhi-Hubble, Scot-Weigel and Shell New
Wave, or user-defined wave spectra

FATJACK includes explicit SCF definitions

SCFJ if crown & saddle SCF is known e.g. from empirical formulae
SCFA if SCF is known at specific locations e.g. from FE
SCFB if SCF is constant across a section
SCFP if SCF values vary with location

Automatic (empirically derived) SCF definitions based on

Efthymiou, Wordsworth, Kuang or DS449
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 Fatigue Assessments

Rainflow counting fatigue

Reduces spectrum of varying stress into simple stress reversals
Allows the application of Miners rule to assess fatigue life of structure subject
to complex loading
Based on ASTM E1049-85 (2005) Standard Practices for Cycle Counting in
Fatigue Analysis
It is possible to use results from up to 1000 different transient dynamic analyses and
loading (i.e., multi-directional wave spectra)
Uses Rainflow counting method to produce stress range histogram

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 Combined Global and Detailed
Simulations
Detailed joint analysis is possible:

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 Submodelling

Alexander Kielland platform accident during

1980

Submodelling
allows to study
only a portion
of the model
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 Global to Local Analysis
Reproduced from T-Rex ANSYS Advantage Article 2012

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 Global to Local Analysis

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 Monopile with multiple load cases
Wind turbine example showing spectrum loading at different
directions in time domain, fatigue summed in Fatjack.

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 Bolted Connections

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 Examples Linear Dynamic

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 Modal Analysis
Jacket Structure 1st Mode 0.321Hz i.e. a Period of ~3 seconds.

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 Frequency Domain Fatigue
Wave loads applied to the previous modal model in the frequency domain
(rather than over time) to drastically increase the solution time.
Topside lateral deformation increased x3 due to excitation of the structure at
resonance.

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 Fatigue with Mechanical
Using the results from a frequency domain (Harmonic) analysis,
a fatigue analysis can be carried out using Fatjack.
See below for other wave fatigue options Fatigue can be
carried out in the time domain and the frequency domain.

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 Fatigue with Mechanical
Using the results from a frequency domain (Harmonic) analysis,
a fatigue analysis can be carried out using Fatjack.
See below for other wave fatigue options Fatigue can be
carried out in the time domain and the frequency domain.

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 Fatigue with nCode DesignLife

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 Examples Nonlinear Static

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 DNV-RP-C208
DNV recently introduced RP-C208, detailing nonlinear
analysis methods.
RP-C208 gives guidelines to be used when carrying out
Nonlinear analyses.
ANSYS is an a member of the JIP (Joint Industry Project) for
RP-C208.

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 Nonlinear - Buckling

Nonlinear Buckling analysis on the Laggan-

Tormore project, the cost savings on steel was
aprox. NOK 2.000.000-, (40 tonnes).

The procedure is approved by DNV

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 Pushover of Oil and Gas Platform
The ocean loads applied to an
offshore structure are calculated
using Morisons equation, which
calculates the loads on line elements
due to wave and current. This load is
calculated for the worst position of
the 100 year storm wave in relation
to the structure by running one
complete wave through the
structure and extracting the wave
with the largest base shear. This
worst case load is then incremented
linearly on the structure until the
analysis can no longer converge,
indicating failure of the Structure.
The dead load of the structure is
kept constant.

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 Complex Contact

Non-linear analyses of umbilicals

- Monitor applied force on clamps vs plastic deformation during
loading
- Investigating distortions in umbilical parts after unloading

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 Complex Contact

Results: Steel parts Mises Stress

After loading:

After unloading:

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 RAO application
Example showing RAO
application for linear waves
and transient spectra.

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 Examples Nonlinear Dynamic

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 Ship impact

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 Explicit - Impact

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 Explicit - Impact

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 Explicit - Explosion

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 ANSYS Explicit application examples

Deck penetration

Pipe cutter
Model Courtesy of Prospect

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 Floating Wind Turbine base
Example showing Random wave loading on a slender member
floating structure. Currently producing verification of this compared
to AQWA.

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 Floating Wind Turbine base
Example showing Random wave loading on a slender member
floating structure. (Currently producing verification of this model
comparing Mechanical to AQWA).

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 Aeroelastic coupling for
Wind turbine modelling

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 Coupling Mechanical with 3rd Party
Aeroelastic Tools for Offshore Wind
Turbine Modeling

Aeroelastic coupling (for wind turbine support

structures)
Sequential
Allowing structural (ANSYS) and aeroelastic (3rd
party) analyses to be run independently
Just use a provided MAPDL macro to write out input
data for the aeroelastic analysis
Fully coupled
Co-simulation of structural and aeroelastic tools
Custom build of MAPDL required, with a macro to
manage the data availability from and to MAPDL
Images Courtesy of REpower Systems AG
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 Sequential Solution Example

Mass matrix
Damping matrix
MAPDL Stiffness matrix
External force time series
Substructure analysis to generate
matrices and load history for aeroelastic code

Top node force Aeroelastic software

or displacement Wave-Wind Analysis
time series

MAPDL
Analysis of foundation structure

Beamcheck FATJACK
Strength calculations Fatigue calculations

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 Fully Coupled Solution
Aeroelastic interface to MAPDL using the USER300 element. This
element allows user defined stiffness, damping and mass data.
This utilizes a shared memory dynamic link library, so requires
modification to the aeroelastic code to facilitate the interface.

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 Case Study
Storage Vessel Design

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 Hydro-structural design

Geometry
AQWA CFX/Fluent Mechanical
modelling

HYDRO-STRUCTURAL DESIGN

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 Storage Vessel Design
Effects of FPSO Movement
Liquid-gas interface unstable
Need to reduce sloshing to maintain separation efficiency
What stresses are seen by components?
Can we still use standard baffle configurations?
Bolts and welds
Fatigue loading
Not something thats easy to do experimentally!
Potentially dangerous
Significant cost of rig & instrumentation

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 Objectives
Design study for a 12m long storage tank on board an FPSO
Design considerations:
Internal baffle arrangement to reduce sloshing
Operational load characteristics
Sloshing loading
Fatigue
Welds and bolts

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 Vessel Motion y

Non-accelerating motion
Ship moving at fixed speed
No waves or swell
No acceleration force x
No sloshing!
z

Free motion
Bow of the ship in a storm
x
3 Rotations y
Roll, Pitch & Yaw
3 Linear Accelerations
Surge, Sway & Heave
Sloshing expected!
z

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 Methodology

Hydrodynamic analysis with a given sea state provides motion

profile for CFD and FEA
Velocity motion profiles applied using Six Degree Of Freedom
model in CFD solve
Volume of Fluid model used to model gas-liquid interface in
CFD solver
Transient one-way FSI, surface pressures mapped from CFD
analysis to FEA model
Displacement profiles from Hydrodynamic solver applied to
FEA model to account for inertia of solid structure

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 Simulation Process
Step Solver Design Consideration Output Data

Motion Profiles:
Assess ship hydrodynamic
1 Hydrodynamic Velocities for CFD
response to different sea states
Displacements for FEA
Computational Analyse baffle design to assess
2 Surface Pressure Profiles
Fluid Dynamics sloshing
Analyse stresses to look at
Structural Finite
3 welds and bolt arrangements
Element
and fatigue loading

Velocity Pressures &

Hydrodynamics Profiles CFD FEA
Displacements
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 ANSYS Workbench Project

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 Hydrodynamic Analysis

Motion Profile Output for all six motions, used for CFD and
Structural FE models
Displacements, Velocities and Accelerations
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 Surface Pressure
Profile used for
Structural model

Computational Fluid Dynamics Analysis

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 Structural FEA

Pressure Loading ONLY

Fluid inertia considered
Fixed constraints to feet
Inertia of solid structure
ignored

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 Structural FEA with CFD Free Surface

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 Offshore Roadmap

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 Offshore Roadmap
- Outline

ANSYS Offshore Strategy

Roadmap and Future Directions

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 ANSYS Offshore Strategy
Aqwa Mechanical

+ Asas

CFD (in terms of Global and Local (detailed) FE

analysis of offshore structures from a
linkages to the others) single integrated product (ANSYS
Mechanical)
Supplemented by Aqwa (& CFD) for
hydrodynamics and other fluid flow
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and FSI
 Roadmap
- Asas to Mechanical
Aim to retire Asas in favour of Mechanical
Selected capabilities of Asas introduced in Mechanical
Key capabilities introduced in Mechanical
Wave theories and loadings
Improved beam modeling Images Courtesy of
REpower Systems AG

Soil-Pile-Structure interaction (Splinter)

Coupling to 3rd party aeroelastic tools for OWTs
New Design Assessment system in Workbench Mechanical for
customized post-processing including management of code checking
2 New Code Checking Products
Beamcheck
Fatjack

FATJACK
Fatigue
Result

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 Roadmap
- Aqwa

Integration of Aqwa in Workbench with other ANSYS

products
Migration ongoing over past 3 releases
Providing links to other relevant modeling and analysis
tools
Providing parameterization
Active core developments to Retain Aqwa as a
leading software for general purpose hydrodynamics
e.g.
Multiple spectra
Linearized drag
Improved performance (64bit, increased problem size)

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 Future Directions

Mechanical
Replace/repackage current Beamcheck & Fatjack tools
Ongoing offshore specific developments
Improved beam & shell modeling
Replace Splinter in Mechanical
Integration in Next Generation Workbench-based UI
Aqwa
End-to-end solution in Workbench
Improved performance (HPC)
Enhanced capabilities for risers/moorings & renewable energy
devices
Improved links with other products (including FE and CFD)
Integration in Next Generation Workbench-based UI

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