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MOBILE OFFSHORE BASEHYDROMECHANICS
Dr. Paul PaloU. S. Naval Facilities Engineering Service Center
Centre for Ships and Ocean StructuresNorwegian University of Science and Technology
29 October 2004
Outline
l Overview: SeaBasing and MOB
l Overview: ONR MOB S&T Program
l MOB Hydromechanics S&Tu Science & Technology (S&T) Evaluation Process
u Hydromechanics S&T and Products
u Supporting S&T Activities
l Summary
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Overview of SeaBasing and MOB
• SeaBase Function: provide completelogistics support for ground personnel
– Transit to site in reasonable time
– Receive supplies from CONUS
– Store, prepare, stage supplies
– Transport supplies to shore
SeaBasing and MOBSample Mission Requirements
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SeaBasing and MOB SeaBase Requirements #1 of 3
Single Module Transit
u Scenariou Full cargo &
u Deballasted on pontoons
u Fastest Transit Timeu Maximum Speed versus
• Sea State
• Incident Wave Direction
SeaBasing and MOB SeaBase Requirements #2 of 3
On-site operations
u Positioned 20-50 miles off coast
u Large Vessel Unloading through Sea State __
u Small Vessel Loading through Sea State __
u Survive Extreme Events through Sea State __
u Accommodate __-number of VTOL aircraft
u Stationkeeping (Dynamic Positioning)
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SeaBasing and MOB SeaBase Requirements #3 of 3
On-site operations: optional?
u CTOL Aircraft Operations
This greatly complicates the engineering:
�Multiple Module Platform• Connect through Sea State __
• Provide acceptable dynamics through Sea State __
• Optional Disconnect through Sea State __
SeaBasing and MOBFamily of SeaBase Concepts
“MOB”Platforms
Navy-favored
“SeaBase” Platform
based on MPF(F)
& V-22 Osprey
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SeaBasing and MOBRange of MOB Platform Configurations
Two ModulesSingle Module
Hinged Platform
Bridged Platform
Dyn. Pos. Platform
Semi-submersible
VLCC
CVN
M O B
300m (DB102)
455m (Seawise Giant)
320m (Nimitz)
Semisubmersible Module300-600m length130-160m beam
35m draft
SeaBasing and MOBMOB Module and Platform Sizes
Platform Configuration300-1500m length
1 to 5 modulesRigid/Hinged/Bridged/DP Connectivity
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MOB S&T Program
ONR MOB S&T ProgramONR Program Overview
♦♦ Program ObjectivesProgram Objectives•• Establish Feasibility and Cost of MOB(s)Establish Feasibility and Cost of MOB(s)
♦♦ FundingFunding•• $36M$36M•• FY96-00FY96-00
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ONR MOB S&T ProgramNFESC Program Overview
♦♦ Program ObjectivesProgram Objectives•• Establish feasibility and costEstablish feasibility and cost
•• Define a consistent design methodologyDefine a consistent design methodology•• Reduce technology gapsReduce technology gaps
•• Advance industry capability to Advance industry capability to DoDDoD req’tsreq’ts•• Make Make DoDDoD a “smart buyer” a “smart buyer”
•• 53-organization Project Team53-organization Project Team•• 26 commercial firms (domestic and int’l)26 commercial firms (domestic and int’l)•• 16 academic institutions16 academic institutions•• 11 government agencies11 government agencies
Identify MissionRequirements
Define EngineeringDesign Requirements
State-of-the-ArtCapabilities
Prioritize Science &Technology Tasks
Prioritize Science &Technology Tasks
• Still undefined as of 2004!
• Representative missions defined
Candidate PlatformCharacteristics
• Nonquantified missions èèèè unknown length
• Hierarchy of platforms based on length & connection scheme
ONR MOB S&T Program S&T Evaluation Process
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ONR MOB S&T ProgramUnprecedented Design Challenges
l Semisubmersible module size
l Multiple module platformu Connectivityu Length/configuration
l Open ocean dynamicsu Motions (Aircraft operations, vessel motions)u Wavefield (Stresses, Cargo transfer, Air gap)u Failure Modes (1st torquing mode)
�� MOB exceeded state of practiceMOB exceeded state of practice - No consensus methodology to design, down-select, build, and operate a MOB.
�� MOB as anMOB as an ““Innovative StructureInnovative Structure”” - An Innovative Structure -- “is usually the first of its kind;
few, if any, design standards directly apply and there islittle operational experience to relate to the design reviewprocess.” (National Research Council, 1991)
“Technical evaluation of such structures must be basedon fundamental engineering principles, requiring
specialists in the relevant disciplines.”
ONR MOB S&T Program MOB Uniqueness
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ONR MOB S&T ProgramRoadmap = Classification Guide
Requirements
Demands
Capacity
Analysis/DesignAssessment
3. Classification Requirements
2. Requirements & Procedures
1. General
11. Dynamic Positioning
13. Maintenance
14. Environmental Compliance
6. Materials
4. Environment 5. Loads
7. Structural Resistance
8. Engineering Analysis
9. Structural Design
10. Stability
12 . Fabrication
MOB Hydromechanics S&T
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MOB HydromechanicsScience & Technology Categories
1. Operational Dynamics
2. Survival Dynamics
3. Transit Dynamics
4. Validation Tests and Analysis
5. Exercise Models
6. Met/Ocean Specification
MOB HydromechanicsS&T Balance
MOB ClassificationGuideline
Advance & Exercise
Models
ValidationData Metocean
Specification
$4.3M$3.7M
$1.9M
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MOB HydromechanicsScience & Technology Issues
1. Operational Dynamics Models1.1 Platform dynamics1.2 Connection/Disconnection1.3 Fatigue structural loads1.4 Stationkeeping
For Each Issue -
u Identify key requirements
u Assess State-of-the-art
u Highlight advancement and product
MOB Hydromechanics 1.1 Platform Dynamics
Example Calculationsu Motions of one module
u Transitu Operations
u Motions of connected platformu Column and pontoon shape optimizationu Dynamics of berthed vessels
Key Model Requirementsu Inviscid, linear & stationary acceptableu Hydrodynamic coupling; mean drift forceu Highly efficient numerical solveru Hydroelasticity [rigid and/or elastic bodies]
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MOB Hydromechanics 1.1 Platform Dynamics
State-of-the-Art Summary (Frequency Domain)
u HOBEM (high-order, single, rigid body)
u AQWA (rigid, multiple bodies, no coupling)
� MORA (rigid, multiple bodies; adjacent coupling only)
u HYDRAN (hydroelastic, rigid, hydrodynamic coupling; computationally intensive)
� WAMIT (hydroelastic, hydrodynamic coupling; computationally intensive)
� HIPAN (single, rigid body; higher-order; computationally efficient)
MOB Hydromechanics 1.1 Platform Dynamics
MOB Frequency Domain S&T:
1.1.1 Advance WAMIT & HIPAN modelsu Refer to N. Newman presentation
1.1.2 Develop alternative dynamic model
1.1.3 Develop simplified preliminary design tool
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MOB Hydromechanics 1.1 Platform Dynamics
1.1.1 Advance WAMIT & HIPAN models
Objective: provide fully-coupled hydroelastic final design model(s)
Performer: MIT, AeroHydro
Product:u 2 associated deliverables (Fast-WAMIT & HIPAN)u HIPAN: high-order potentials on patches &
panels using b-spline body geometry descriptionè WAMIT: pre-computed FFT solver (up to 3 orders
of magnitude computational increase) makesthis the only model capable of full MOB study
MOB Hydromechanics 1.1 Platform Dynamics
1.1.2 Develop alternative dynamic model
Objective: provide numerically efficient preliminary design modelPerformer: OSA Inc. (Chakrabarti)
Product:u diffraction theory model (fully-coupled, rigid-
body)u two-stage analysis: (1) potentials for isolated
individual modules; (2) superpositionè avoids large panel problem entirely; allows
for efficient parametric configuration studies
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MOB Hydromechanics 1.1 Platform Dynamics
1.1.3 Develop simplified preliminary design tool
Objective: provide numerically efficient preliminary design model
Performer: Off Coast Inc. (Ertekin & Riggs)
Product:u based on a simplified (uncoupled, rigid body)
version of the HYDRAN diffraction theorymodel
u menu- & library-based interactive preprocessoru Rayleigh damping for [equiv] viscous dragè deliberately simplified to allow for efficient
parametric configuration studies
Key Model Requirements:u Inviscid, linear & stationary acceptableu Multiple modules with varying mean positions and
arbitrary approach pathu Impact and/or elastic loads desirable
State-of-the-Art Summary (Time Domain)u LAMP (described next)u CFD viscous models: not sufficiently mature
MOB Time Domain S&T:1.2.1 Advance LAMP model1.2.2 Develop simplified connection model (MORA)
MOB Hydromechanics 1.2 Connection/Disconnection
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MOB Hydromechanics 1.2 Connection /Disconnection
1.2.1 Advance LAMP model
Objective: advance accuracy of this nonlinear model
Performer: SAIC (Annapolis).
Product:u “fully-nonlinear” time domain model (rigid-
body; large waves and responses; multiplebodies with nonstationary mean positions)
u computationally-intensive modelè advanced free surface condition from incident
to instantaneousè Encountered fundamental air gap deficiency
(see S&T topic 2.1)
1.2.2 Develop simplified connection model
Objective: develop “piecewise stationary” prelimi- nary analysis model for wave motions
Performer: C. J. Garrison and Assoc
Product:u efficient preliminary analysis tool (via MORA)u three stage solution: (1) determine frequency
domain behavior for two bodies at finite numberof fixed mean positions along approach path; (2)convert to time domain “retardation functions”;(3) interpolate for continuous dynamics
èavoids intensive time domain solution
MOB Hydromechanics 1.2 Connection /Disconnection
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MOB Hydromechanics 1.3 Fatigue structural loads
Key Model Requirementsu Inviscid, linear & stationary acceptableu Interface pressure distribution from arbitrary hydro panel
into arbitrary structural element surface meshes
State-of-the-Art Summary (Time Domain)u None (SAS; restricted to identical meshes)
MOB Time Domain S&T1.3.1 Universal loads generator
MOB Hydromechanics 1.3 Fatigue structural loads
1.3.1 Universal loads generator
Objective: Develop a universal pressure loadsgenerator post-processor
Performers: MIT, Aerohydro Inc, McDermott
Product:u b-spline representation for body geometry
and potentialsu convert Fast-HIPAN pressures into local
pressures for locally nonlinear responsesè allows for optimum hydro and structural
discretizations (maximum accuracy)
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StructuralABAQUSStructural
ABAQUSHydrodynamic
MIT / HIPANAeroHydro/HIP2FEA
DefineGeometry
MOB-HyLoads
Wind & Current Loads
Time history wave loads
Analysis Procedures
Mean Drift Forces
Sp
ectru
m
frequencyω1 ω2 ω3 ω4 ω5 ω6 ω7 ω8 ω9 ω10 ω11 ω12
Spre
adin
g Fu
nctio
n (G
)
Heading Angleθ1 θ2 θ3 θ4 θ5 θ6 θ7 θ8 θ9 θ10 θ11 θ12
DefineSea State
MOB Hydromechanics 1.3 Fatigue: universal loads generator
MOB Hydromechanics 1.4 Stationkeeping
Key Model Requirementsu Current and wind loads on semisubmersibles, including
viscous wakes, wave, and free surface effectsu Desirable: multi-body capability for connect simulations
and shielding effects
State-of-the-Art Summary (Time Domain)u CFD (not sufficiently mature)
MOB S&T None. (1) Hydro CFD not pursued because there are no
apparent short-term opportunities to advance modeling offree surface and high Reynolds Number flows. (2) WindCFD for aircraft landing/takeoff environment not pursuedas not critical to feasibility objective.
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MOB HydromechanicsScience & Technology Issues
2. Survival Dynamics
2.1 Extreme Motionsu Air gapu Wave impactu Run-up on columns
2.2 Extreme structural loads
MOB Hydromechanics 2.1 Extreme Motions
Key Model Requirementsu Goal = required column height for semi designu Accurate modeling of instantaneous free surface and
[nonstationary] wetted surface of bodyu Single (disconnected) rigid body; uncertain if inviscid
models are necssary
State-of-the-Art Summary (Time Domain)u LAMP model (simplified to incident free surface)
MOB S&T2.1.1 Apply LAMP model (see previous Task 1.2.1 & next
visual)2.1.2 Advance AEGIR model
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MOB Hydromechanics 2.1 Extreme Motions
2.1.2 Advance AEGIR model
Objective: advance development of this nonlinear time domain model
Performer: MIT
Product:u “fully-nonlinear” time domain model (rigid-body;
large waves and responses; inviscid;nonstationary mean positions)
u highly flexible formulation; includes run-upu Incomplete development at end of MOB program
MOB Hydromechanics 2.2 Extreme Structural Loads
Key Model Requirementsu Accurate pressures induced on [nonstationary] wetted
surface of bodyu Multiple bodies, elasticity (if connected in extreme events)u Viscosity preferrable
State-of-the-Art Summary (Time Domain)u Morison Equation, based on long wavelengths in typical
extreme seas and MOB characteristic dimensions ofcolumns and pontoons. Inviscid models (e.g., LAMP) donot include viscous damping and have been shown toyield misleading results. Uncertainties with: coefficients,shielding, and wave crest kinematics.
MOB S&T None.
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MOB HydromechanicsScience & Technology Issues
3. Transit Dynamics Models
3.1 Platform dynamics (stability, accelerations)
3.2 Nonlinear Stability
3.3 Dynamics while in damaged condition
MOB Hydromechanics 3.1 Transit Dynamics Models
Key Model Requirementsu Accurate modeling of instantaneous wave pressures and
buoyancy on [nonstationary] wetted surface of body aspontoons intermittently submerge and waves overtop
u Single, rigid body models acceptable; importance ofviscosity unknown
State-of-the-Art Summary (Time Domain)u No industry experience similar to MOB transit. LAMP
allows for changing wetted surface, but does not modeldynamics of waves above the pontoons.
MOB S&T3.1 Apply LAMP model (see previous Task 1.2.1)
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MOB Hydromechanics 3.2 Nonlinear Stability Model
2.1.2 Evaluate Stability during transit
Objective: investigate single, deballasted module dynamics (with pontoon immersion)
Performer: Univ of New Orleans
Product:u Estimation of orbits, attractors, etc to nonlinear
buoyancy and “representative” viscous dampingu Not a solved topic!u Continued development of the “Reverse MI/SO”
system identification technique @ UNO (see Task4.4)
MOB Hydromechanics 3.2 Damage Dynamics Models
Key Model Requirementsu Assess stability and motions for large heel and trim static
conditions [due to explosive detonations]
u Accurate modeling of instantaneous free surface and[nonstationary] wetted surface of body. Internal voids?
u Single, rigid body models acceptable; importance ofviscosity unknown
State-of-the-Art Summary (Time Domain)u No specific criteria yet developed. LAMP allows for
changing wetted surface.
MOB S&T3.2 Apply LAMP model (see previous Task 1.2.1)
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MOB HydromechanicsScience & Technology Issues
4. Validation Tests and Analysis
4.1 Hydroelastic Tests
4.2 Limited Hydroelastic Validations
4.3 Transit Dynamics Tests and Analysis
4.4 Air gap Tests
MOB Hydromechanics 4.1 Hydroelastic Tests
Key Data Requirementsu Accurate structural knowledge of platform responseu Accurate knowledge of spatial wavefield
State-of-the-Art Summary (Time Domain)u Limited; typically very small scale and for mats
MOB S&T4.1 Conduct hydroelastic tests of generic, connected MOB semisubmersibles
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MOB Hydromechanics 4.1 Hydroelastic Tests
4.1 Conduct Tests at NSWC-CD
Objective: provide a hierarchy of data for 1, 2, and 5module elastic MOB platforms with connectors
Performer: Naval Surface Warfare Center, Carderock Detachment (MASK facility)Product:
è guided by 2-day workshop of government,academia, and industry, and real-time QA
u Four 6m fully-elastic modulesu Multi-axis spring connectors
� Limited. Exhausted funding before dataanalysis. See next topic.
MOB Hydromechanics 4.1 Hydroelastic Tests
6m Elastic Module
Connector
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MOB Hydromechanics 4.1 Hydroelastic Tests
27.5m Elastic Platform Model
MOB Hydromechanics 4.2 Limited Hydroelastic Validations
Objective: conduct preliminary validations usingsubsets of NSWC-CD data
Test Performer: McDermott Technologies; OSA;University of Hawaii; University of Maine
Products:u limited evaluations for 1 and 2 module
configurationsu Opportunity??
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MOB Hydromechanics 4.3 Transit Dynamics Tests and Analysis
Key Data Requirementsu Rigid body motions of a semisubmersible at transit draft
(minimal freeboard up on the pontoons)u Nonlinear buoyancy (pontoons versus column
waterplane) results in complicated dynamics best treatedwith nonlinear phase plane techniques
State-of-the-Art Summary (Time Domain)u None; unique problem to MOB
MOB S&T4.4 Conduct transit dynamics tests
MOB Hydromechanics 4.4 Transit Dynamics Tests and Analysis
Objective: examine transit dynamics
Test Performer: U. S. Naval Academy (Annapolis)
Data Analysis: University of New Orleans
Products:u 4m model length; same as NSWC moduleu direct use of data for validation of LAMP, etc.u indirect use of data to qualify and quantify
equations of motion using “Reverse MI/SO”nonlinear systems identification technique (perNFESC).
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MOB Hydromechanics 4.4 Transit Dynamics Tests
4m Rigid Module
MOB Hydromechanics 4.5 Air Gap Tests
Key Data Requirementsu Rigid body motions and wavefield for a semisubmersible
at survival draft
State-of-the-Art Summary (Time Domain)u Proprietary (e.g., LAMP)u Industry divided as to the accuracy of state-of-the-art
modeling
MOB S&T4.5 Conduct air gap tests
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MOB Hydromechanics 4.5 Air Gap Tests
Objective: provide data for subject conditions
Test Performer: U. S. Naval Academy (Annapolis)
Pre-test Simulations: SAIC (Annapolis)
Products:u same model as for transit dynamics testsu 3 response channels; 13 wave channels
(sensors on model and fixed)u ultimately intended for validation of LAMP and
other model predictions.
MOB HydromechanicsScience & Technology Issues
5. Exercise Models
5.1 Benchmark Comparison of Existing Diffraction Models
5.2 Benchmark LAMP Exercise
5.3 Berthed Vessel Simulations
5.4 Industry Experience with Seakeeping Models
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MOB Hydromechanics 5.1 Benchmark Comparison of Diffraction
Models
Objective: benchmark existing models for MOB applications
Test Performer: Bechtel
Products:u parametric assessment of small amplitude,
linear diffraction theory modelsu HOBEM, AQWA, MORA, WAMIT, HIPAN and
HYDRAN exercised for single semisubmersiblesand connected platforms
èquantitatively established general guidance onhow to properly apply diffraction theory modelsfor VLFS semi seakeeping studies
MOB Hydromechanics 5.2 Benchmark LAMP Exercise
Objective: investigate numerical use of LAMP
Test Performer: Naval Facilities Engineering Service Center (Port Hueneme CA)
Products:u investigate discretization of: body, free surface,
and matching surface for transit conditionsu preliminary guidance on LAMP relative
accuracy versus number of panels
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MOB Hydromechanics 5.3 Berthed Vessel Simulations
Objective: examine dynamics of vessels in the confused wavefield associated with large MOB semisubmersibles
Test Performer: McDermott Engineering (Houston)
Products:u preliminary assessment of dynamic motions
during cargo handling operationsu quantify errors if incident waves only are
[incorrectly] used as excitation
MOB Hydromechanics 5.4 Industry Experience with Seakeeping
Models
Objective: indirect assessment of industry capability to correctly apply seakeeping models
Test Performers: McDermott Engineering (Houston) Kvaerner Bechtel AkerProducts:
u valuable insight regarding transition of newmodels and extrapolation of old models to MOB
èconsensus opinion: their original expectationsthat MOB was a direct extension of state-of-the-art was too simplistic
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MOB HydromechanicsScience & Technology Issues
6. Metocean Specification at O(km) Scales
6.1 Metocean Specification6.2 Wave Spatial Coherence: Data Analysis6.3 Wave Spatial Coherence: Modeling
Critical for accurate motion and stress estimatesu Low order modal responses will be excited only if
waves are long-crested and narrowbanded.
MOB Hydromechanics 6.1 Metocean Specification
Key Requirementsu Knowledge of wind/wave/current phenomena at MOB O(2
km) scale; internal waves and solitons includedu Necessary for elastic responses and cell/environmental
contour design methods in MOB Classification Guide
State-of-the-Art Summary of Wave Fieldsu None.
MOB S&T6.1 Develop general engineering-oriented specification6.2 Numerically investigate wave properties from existing data sets6.3 Advance physics-based models into “3+1” dimensions
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MOB Hydromechanics 6.1 Met/Ocean Specification
Objective: compile engineering-oriented guidance forwind/wave/current excitation of MOB
Performers: Bechtel (numerous subcontractors)
Products:u Extensive MOB Environmental Specificationu 2 Extensive MOB Climatological Databases
� 23 sites, 20 years @ 6 hrs� Pacific storms, 1 km mesh, 1 hr intervals
uExcellent general guidance for marine structures6 Missing critical wave coherence information
MOB Hydromechanics 6.2 Wave Spatial Coherence: Data Analysis
Objective: provide “quick-look” guidance regarding open ocean wave crest lengths
Performers: JHU/APL; ERIM; WHOI/UMiami; UWyoming/NASAProducts:
u Guided by ONR Workshop, Aug 1997u numerically investigate wave properties from
existing SAR, SRA, wave gauge data setsu Unresolved question: “what is a ‘wave’?”èFirst-ever measurements of the wavefield in a
Hurricane
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North Panel Northeast Panel East Panel
Allpanels:
~6x1 km
WAVES IN HURRICANE BONNIEWAVES IN HURRICANE BONNIEScanning Radar Altimeter (NASA)Scanning Radar Altimeter (NASA)
MET/OCEAN DESCRIPTORSMET/OCEAN DESCRIPTORS
MOB Hydromechanics 6.3 Wave Spatial Coherence: Modeling
Objective: advance numerical modeling of evolving 3D wavefields
Performers: MIT; UHawaii; UTorinoProducts:
u Guided by ONR Workshop, Aug 1997u MIT and Hawaii models balance physics and
computational burdenu Torino model addresses fundamental physics of
rogue waves
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