Diseño de embarcaciones de alto desempeño usando Simulaciones

Abstract

Simulation-based design increasingly replaces traditional experience-based design. This article gives an overview of techniques now used in advanced industry practice, with particular focus on navy applications. The article covers the basics of the techniques, illustrating approaches and state of the art with applications taken from the experience of Germanischer Lloyd.

Full text

9
Design of High-Performance Ships
using Simulations
Simulation-based design increasingly replaces traditional experience-based design. is article gives
an overview of techniques now used in advanced industry practice, with particular focus on navy
applications. e article covers the basics of the techniques, illustrating approaches and state of the art
with applications taken from the experience of Germanischer Lloyd.
El diseño basado en simulaciones crecientemente está remplazando al diseño basado en la experiencia.
Este trabajo presenta una visión general de las técnicas empleadas actualmente en prácticas industriales
avanzadas, con particular énfasis en el diseño de buques militares. El trabajo cubre los aspectos básicos e
ilustra el estado del arte con aplicaciones tomadas de la experiencia del Germanischer Lloyd.
Key words: CFD, ship design, simulation, structural analysis.
Palabras claves: CFD, diseño de embarcaciones, simulación, análisis estructural.
Fritz Grannemann Auorth1
Volker Bertram2
Abstract
Resumen
1 Germanischer Lloyd, Mexico, City. e-mail: fritz.grannemann@gl-group.com
2 Germanischer Lloyd, Hamburg/Germany. e-mail: volker.bertram@gl-group.com
Diseño de embarcaciones de alto desempeño usando Simulaciones
Date received: March 4th, 2010 - Fecha de recepción: 4 de marzo de 2010
Date Accepted: October 6th, 2010 - Fecha de aceptación: 6 de octubre de 2010
Ship Science & Technology - Vol. 3 - n.° 6 - (9-17) January 2010 - Cartagena (Colombia)

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e word simulation is derived from the Latin
word “simulare” which can be translated as
“to reproduce”. e VDI (Society of German
Engineers) denes the technical term “simulation”
as follows: “Simulation is the reproduction of a
system with its dynamic processes in a running
model to achieve cognition which can be referred
to reality”. According to the Oxford dictionary
“to simulate” means “to imitate conditions of
a situation or process”, specically “to produce
a computer model of a process”. In this sense
virtually all computer models used in the design
and construction of ships would qualify as
simulations. Indeed, we see an ever increasing
scope and importance of simulations in our work.
e trend in modern classication society work is
also towards simulation-based decisions, both for
design and operation of ships.
Ship design is increasingly supported by
sophisticated analyses. Traditionally, ship design is
based on experience. is is still true to some extent,
but increasingly we rely on “virtual experience”
from dedicated and well chosen simulations.
Scope and depth of these simulations guiding our
decisions in design and operation of ships have
developed very dynamically over the past decade.
We describe here the state of the art as reected
in our work, building on previous work, Fach and
Bertram (2006), Bertram and Couser (2007), but
now with particular focus on applications for navy
ships.
FEA for global strength within the elastic material
domain have been standard for a long time, Fig.1.
ese simulations were the starting point for
more sophisticated analyses, e.g. fatigue strength
assessment, ultimate strength assessment, etc.
Until 1998, the SOLAS regulations on subdivision
and damage stability specied damage stability
Fig.1. Global strength analysis; grid and
stresses for frigate
Introduction
Structural Analyses
Finite-element analysis (FEA)
requirements only for cargo ships longer than 100
meters. Since 1998, this limit has been lowered to
80 m for new cargo ships. Additional transverse
bulkheads to full damage stability requirements
are costly and restrict operations. However, the
new SOLAS regulations permit for some ships
alternative arrangements, provided that at least
the “same degree of safety” is achieved. is
notation allows some exibility of structural
designs supported by advanced simulations. E.g.
a structural design having increased collision
resistance thus reducing the probability of
penetration of the inner hull could eliminate the
need for additional bulkheads. Based on extensive
FEA simulations for ship collisions, Germanischer
Lloyd developed an approval procedure which
provides the rst such standard for evaluation
and approval of alternative solutions for design
and construction of these ships, Fig.2, Zhang et
al. (2004). e basic philosophy of the approval
procedure is to compare the critical deformation
energy in case of side collision of a strengthened
structural design to that of a reference design
complying with the damage stability requirement
described in the SOLAS regulation.
Finite-element analyses (FEA) require load
specications which for ships involve frequently
external hydrostatic and hydrodynamic loads.
Grannemann Auorth, Bertram
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GL.ShipLoad, Cabos et al. (2006), supports
ecient load generation for global FEA of ship
structures. Hydrostatic and hydrodynamic
computations are integrated into the program.
GL.ShipLoad supports the generation of loads
from rst principles (realistic inertia and wave
loads for user supplied wave parameters), but the
program also aids in the selection of relevant wave
situations for the global strength assessment based
on bending moments and shear forces according to
Germanischer Lloyd’s rules. e result is a small
number of balanced load cases that are sucient
for the dimensioning of the hull structure.
Advances in computer methods have made 3-d
FEA today the standard choice for ship vibration
analyses, Asmussen and Mumm (2001). e
computations require longitudinal mass and
stiness distribution as input. e mass distribution
considers the ship, the cargo and the hydrodynamic
'added' mass, Fig.3. e added mass reects the
eect of the surrounding water and depends on the
frequency. One can either use estimates based on
experience or employ sophisticated hydrodynamic
simulations. For local vibrations analyses, Fig.4,
added mass needs to be considered if the structures
border on tanks or the outer hull plating. Because
of the high natural frequencies of local structures,
FEA models must be detailed including also the
bending stiness of structural elements.
Fig.2. FEA for collision of two ships Fig.3. Global FE A of vibrations
Fig.4. Local FEA of deck vibrations
Vibration analyses
Acoustics
0,63 Hz
0,91 Hz
For very high frequencies (structure-borne noise),
the standard FEA approach to vibration analyses
is impossible due to excessive computational
requirements. For a typical passenger vessel
for a frequency of 1000 Hz, a FEA vibration
model would lead to several million degrees of
freedom. However, the very fact that information
is required only averaged over a frequency band
allows an alternative, far more ecient approach
based on statistical energy analysis (SEA). e
Noise Finite Element Method (GL NoiseFEM)
of Germanischer Lloyd, Cabos and Jokat (1998),
Cabos et al. (2001), is based on a related approach.
GL NoiseFEM predicts the propagation of noise
by analyzing the exchange of energy between
weakly coupled subsystems. Validation with full-
scale measurements shows that the accuracy of GL
NoiseFEM is sucient for typical structure-borne
sound predictions for the frequency range between
80 Hz and 4000 Hz, Wilken et al. (2004). While
further development is still needed, structure-
borne noise analyses have been validated with good
Design of High-Performance Ships using Simulations
Ship Science & Technology - Vol. 3 - n.° 6 - (9-17) January 2010 - Cartagena (Colombia)

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agreement on the wetted shell. Reliable prediction
of the structure-borne noise is an important step
towards predicting radiated noise of vessels. In the
meantime, GL NoiseFEM structure-borne noise
analyses are already applied to support the design
of navy ships, cruiseships and customer-made
yachts, Fig.5.
Fig.5. Structure-borne noise computation for Blohm&Voss cruiseship (up) and mine hunter (down)
8th Deck
7th Deck
6th Deck
5th Deck
4th Deck
3rd Deck
2nd Deck
1st Deck
Tween deck
Tank Top
B.L.
- 10 dB -
4500
4500 Ft. 2
Ft. 12
Ft. 22
Ft. 32
Ft. 42
3000
3000
Cl
H-Deck
Boden
Basis
Ship Science & Technology - Vol. 3 - n.° 6 - (9-17) January 2010 - Cartagena (Colombia)
Grannemann Auorth, Bertram

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Combining intelligently linear frequency-domain
methods with nonlinear time-domain simulations
allows exploiting the respective strengths of each
approach, El Moctar (2005). e approach starts
with a linear analysis to identify the most critical
parameter combination for a ship response. en a
non-linear CFD (Computational Fluid Dynamics)
analyses determines motions, loads and free surface
(green water on deck). We employ the commercial
RANSE solver Comet for our purposes, e.g. Fig.6.
Fluid-structure interaction is a topic of increasing
importance in our experience. In a weak coupling,
the computed pressures from the seakeeping
analyses are used to compute the structural
response to these forces. In a strong coupling,
the hydrodynamic and the structural problem
are solved simultaneously. e hydrodynamic
model then considers the deformation of the
hull, the structural model the loads from the
hydrodynamics, Oberhagemann et al. (2008).
CFD is the most appropriate tool to support
practical rudder design, Fig.7 (see page 12). e
propeller is typically modelled in a simplied
way using external forces distributed over the
cells which cover the location where the propeller
would be in reality. e sum of all axial body
forces is the thrust. e body forces are assumed
to vary in radial direction of the propeller only.
is procedure is much faster than geometrical
modelling of the propeller (by two orders of
magnitude) at a negligible penalty in accuracy
(about 1%). e procedure has been extensively
validated for rudder ows both with and with-
out propeller modelling. e same approach for
propeller and rudder interaction can be applied for
podded drives, Junglewitz and El Moctar (2004).
Comet allows also the treatment of cavitating
ows, Fig.8 (see page 12). e extensive experience
gathered in the last 5 years has resulted in a GL
guideline for rudder design procedures, GL (2005),
El Moctar (2007).
Design of High-Performance Ships using Simulations
Fig.6. CFD simulation of ships in extreme waves; up: fast
trimaran; down: frigate
Computational Fluid Dynamics
(CFD)
For many seakeeping issues, linear analyses
(assuming small wave height or small wave
steepness) are appropriate and frequently applied
due to their eciency. e advantage of this
approach is that it is very fast and allows thus
the investigation of many parameters (frequency,
wave direction, ship speed, metacentric height,
etc.). Non-linear computations employing time-
domain approaches are usually necessary for the
treatment of extreme motions. ese simulations
require massive computer resources and allow only
the simulation of relative short periods (seconds to
minutes).
Seakeeping
Rudder ows
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Fig.7. CFD model for hull-propeller-rudder interaction Fig.8. Cavitation predicted at propeller
Fig.9. CFD aerodynamic simulation
Aerodynamic ows around ship superstructures can
be computed by CFD, Fig.9, although wind tunnel
tests still are popular and widely used. CFD oers
the advantage of overcoming scale eects which
can be signicant if thermodynamic processes are
involved, El Moctar and Bertram (2002). HVAC
(heat, ventilation, air condition) simulations involve
the simultaneous solution of uid mechanics
equations and thermodynamic balances, often
involving concentrations of dierent gases. Navy
applications include for example the smoke and
heat (buoyancy and turbulence) conditions on
helicopter decks aecting safe helicopter operation.
HVAC and re simulations
Hub vortex cavitation
Tip vortex cavitation
Velocity
16,7173
15,0456
13,3738
11,7021
10,0304
8,35864
6,68691
5,01519
3,34346
1,67173
0
Ship Science & Technology - Vol. 3 - n.° 6 - (9-17) January 2010 - Cartagena (Colombia)
Grannemann Auorth, Bertram

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Fig.10. CFD re simulation
Fig.11. Steps to AENE AS model from CA D model to cells
with assigned information
At present, zone models and CFD tools are
considered for re simulations in ships. Zone
models are suitable for examining more complex,
time-dependent scenarios involving multiple
compartments and levels, but numerical stability
can be a problem for scenarios involving multi-
level ship domains, HVAC systems and for post-
ashover conditions. CFD models can yield detailed
information about temperatures, heat uxes, and
species concentrations, Fig.10. However, the time
penalty of this approach currently makes CFD
unfeasible for long periods of real time or for large
computational domains. Nevertheless, applications
have graduated from preliminary validation studies
to more complex applications for typical ship rooms
(accommodation, atrium, engine room), Bertram et
al. (2004).
Evacuation assessment became a major topic
at the International Maritime Organization
(IMO) after the loss of the ‘Estonia’, resulting in
new requirements for evacuation analyses in an
early stage of the design process, IMO (2002).
Germanischer Lloyd and TraGo have developed
the software AENEAS for this purpose. Evacuation
analyses focus on safety, but the tool can be used
Evacuation simulation
also for the optimization of boarding and de-
boarding processes, Petersen et al. (2003), or space
requirements for promenades on cruise ships and
large RoPax ferries. ese simulations are very
fast, allowing typically 500 simulations within
one hour, to gain a broad basis for statistical
evaluation. e ship is represented by a simplied
grid of dierent cell types (accessible oor, doors,
stairs, obstacles/walls), Fig.11. Passengers and crew
are represented by intelligent agents. e same
approach can be used to simulate crew movement
on board of navy ships, e.g. time to man battle
stations.
Germanischer Lloyd has developed an integrated
methodology called NESTOR, Petersen and
Voelker (2003), combining re simulations with
the Multi Room Fire Code, evacuation simulation
with AENEAS and an Event Tree Analysis for
risk assessment. Meyer-König et al. (2005) coupled
seakeeping simulations and evacuation simulations
in a semi-empirical approach to nd the inuence
of ship motions on evacuation times. Since trim
and pitch angles are usually relatively small, their
eect is mostly negligible. Roll motions were found
to be less critical than static heel for evacuation
time.
Design of High-Performance Ships using Simulations
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e technological progress is rapid, both for
hardware and software. Simulations for numerous
applications now often aid decisions, sometimes
‘just’ for qualitative ranking of solutions, sometimes
for quantitative ‘optimization’ of advanced
engineering solutions. Continued validation
feedback serves to improve simulation tools as well
as it serves to build condence.
However, advanced simulation software alone is
not enough. Engineering is more than ever the art
of modelling, nding the right balance between
level of detail and resources (time, man-power).
is modelling often requires intelligence and
considerable (collective) experience. e true value
oered by advanced engineering service providers
lies thus not in software licenses or hardware, but
in the symbiosis of highly skilled sta and these
resources.
Many colleagues at Germanischer Lloyd have
supported this paper with their special expertise,
supplying text and/or gures, namely (in
alphabetical order) Christian Cabos, Bettar El
Moctar, Jürgen Jokat, Axel Köhlmoos, Holger
Mumm, Stefan Nusser, Ulf Petersen, Helge Rathje,
Pierre Sames, Tobias Zorn.
ASMUSSEN, I.; MUMM, H. Ship vibration,
GL technology, Germanischer Lloyd,
Hamburg, http://www.gl-group.com/
brochurepdf/0E094.pdf. 2001.
BERTRAM, V.; COUSER, P. CFD possibilities
and practice, e Naval Architect, September
2007, pp.137-147 2007.
BERTRAM, V.; EL MOCTAR, O.M.; JUNALIK,
B.; NUSSER, S. Fire and ventilation
simulations for ship compartments, 4th Int. Conf.
High-Performance Marine Vehicles (HIPER),
Rome, pp.5-17. 2004.
CABOS, C.; EISEN, H.; KRÖMER, M.
GL.ShipLoad: An Integrated Load Generation
Tool for FE Analysis, 5th Int. Conf. Computer
and IT Applications to the Maritime Industries,
Leiden, www.compit.info. 2006.
CABOS, C.; JOKAT, J. Computation of structure-
borne noise propagation in ship structures using
noise-FEM, 7th Int. Symp. Practical Design of
Ships and Mobile Units (PRADS), e Hague,
pp.927-934. 1998.
CABOS, C.; WORMS, C.; JOKAT, J. Application
of an energy nite element method to the
prediction of structure borne sound propagation in
ships, Int. Congr. Noise Control Engineering,
e Hague. 2001.
EL MOCTAR, O.M. Computation of slamming and
global loads for structural design using RANSE,
8th Num. Towing Tank Symp. (NuTTS),
Varna. 2005.
EL MOCTAR, O.M. How to avoid or minimize
rudder cavitation, 10th Num. Towing Tank
Symp. (NuTTS), Hamburg. 2007.
EL MOCTAR, O.M.; BERTRAM, V. Computation
of viscous ow around fast ship superstructures,
24th Symp. Naval Hydrodyn., Fukuoka. 2002.
FACH, K.; BERTRAM, V. High-performance
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2006.
GL. Recommendations for preventive measures
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Germanischer Lloyd, Hamburg. 2005.
IMO. Interim guidelines for evacuation analyses for
new and existing passenger craft, MSC/Circ.1033,
International Maritime Organization 2002.
Final remark
Acknowledgements
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Ship Science & Technology - Vol. 3 - n.° 6 - (9-17) January 2010 - Cartagena (Colombia)
Grannemann Auorth, Bertram

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