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Model testing of offshore structures
Experimental Methods in Marine HydrodynamicsLecture in week 41
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Offshore structures
All other applications except ships in transitExamples: Floating platforms and ships applied for production and/or storage
of oil and gas Fixed structures Risers Mooring systems Floating and submerged bridges Fish farming structures
Commonality: Hydrodynamic problems are important. In most cases are also surface waves involved
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Typical Test Objectives
1. Concept Verification studies Apply design loads to a completely modeled structure to verify
that it satisfies requirements Typically an oil installation (drilling rig, production or storage
ship)2. Operational limits studies
Typically the limiting sea state of a demanding marine operation3. Parts testing studies
Experiments with parts of a complex system Determination of coefficients (drag , added mass, damping, RAOs
) for input to numerical simulations4. Validation and/or verification of software
For actual type of structure and loading condition
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Typical Test Requirements- For concept verifications and operational limits studies
Zero (or very low) speed High-accuracy modeling of complete environment
Multi-directional and short-crested waves Time-varying wind (correctly modeled gusts) Depth- (and time-) varying current
Correctly (Froude) scaled water depth is often important
Correctly (Froude) scaled risers and mooring lines Low speed and requirement for high accuracy waves
implies a short but wide tank (=basin)
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The Ocean Basin Laboratory
Length: 80 m - Width: 50 m - Depth: 0-10 m
TOW
ING
TA
NK
OCEAN BASIN
50 mMulti-flapwave maker
80 m
Cross-section of Ocean Basin
Double-flapwave maker
Multi-flapwave maker Double-flap
wave maker
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Typical test set-up
Measurement of: 6 DoF motions by use of
optical position meas. system Mooring line forces Wave elevation close to
structure Riser forces
Observation (by video): Green water Motions of mooring lines and
risers (by underwater video)
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Scaling considerations
Floater (ship or platform) built to geometrical scale No particular scaling problems of motions and global loads
Risers: Correct drag coefficient of sections
Scale effects modify diameter to obtain correct forces Froude-scaled bending stiffness Correctly scaled weight in water
Mooring lines Axial stiffness might need to be modeled, but bending stiffness might be
neglected Solution:
Non-homogeneous models of risers and mooring lines
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Non-homogeneous models of risersExample:
Steel core gives correct bending stiffness
Material of outer pipe to give correctbuoyancy force
Diameter to give correct drag force
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Risers
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Non-homogeneous mooring lines
Might be a combination of: Thin rope (fishing line) Wire Chain Springs Discrete lead weights Floats
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Small riser models
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Mooring line
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Deep-water challenges Time-variant current forces increasingly important
Large offsets Positioning difficulties Risk for line entanglement Manoeuvring from the surface is difficult
Vertical resonance, resulting in motion amplification and reduced limiting sea states
Wire weight Lifting gear capacity New liftline materials
Increased operation time (more uncertain weather forecast)
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Deep water testing
Deeper basin! Ultra small scale model testing (=1:>>100) Passive equivalent mooring system or truncated hybrid
system Solutions with active control systems Outdoor testing
Exististing basins
Deep water
Mooring lines
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Too expensive!
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Verification of Deepwater Systems by Physical Model Testing
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Ultra small scale model testing Model scale =1:>>100 Challenges:
Weight and accuracy of models difficult to make Viscous effects and surface tension might influence floater Risers and mooring lines become extremely small
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Ultra-small scale model testing: Comparing 3 scales
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RAO & PhaseTension vs. fairlead motion
RAOWF motion vs. wave
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Damping from decay tests Empirical drift coefficients3 scales compared 2 scales compared
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Passive equivalent mooring system Mooring stiffness can be correctly modeled Mooring and riser dynamics usually not correctly
represented
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Hybrid verification procedure
Stansberg et.al. 2002
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Trade-off between model size and use of hybrid techniques
Stansberg et.al. 2002
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Solutions with active control system
Exististing basins
Deep water
Mooring lines
Basin
Full water depth
Mooring lines
Mooring line servo units
Mooring line servo unit
Power
Controlsignals
Output motionsof mooring line endFloater motions
Mooring line tension
Control systemD/A
A/D
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Outdoors testing
Verification tests with complete system cannot be done outdoors, due to lack of control of environment
Outdoors testing have been done for investigation of Vortex-Induced Vibrations (VIV) of risers Hanytangen Skarnsundet
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Slowdrift (2nd order) forces
Very important for design of mooring and dynamic positioning systems
Stiffness of mooring system is of vital importance(provides the restoring force in the dynamic system)
Natural periods of deep water moored units
Unit Natural periods (s)
Surge Sway Heave Roll Pitch Yaw
FPSO >100 >100 5-12 5-30 5-12 >100
Semi >100 >100 20-50 30-60 30-60 >100
Spar >100 >100 20-50 50-100 50-100 >100
TLP >100 >100
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Testing of Marine Operations
Determination of environmental limits for specific operations Wind Waves Current
Trying out of different procedures Typical operations
Heavy lift Installation of bottom equipment Pipe laying Towing operations
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Operation planning
Selectvessel
Wavestatistics HAZOP
ObjectSimple analysis DnV Rules
Criteria
Verification:Detailed analysisModel tests
Selectcases
Feasibility,limits
Riskelements
Defineoperation Costs
Criticalissues
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Installation of slender structures
Dynamic forces during lowering Wave forces in the splash zone,
incl. slamming Force contribution from crane
motion Forces from waves and current
towards the sea bed
Identification of critical steps Recommendations to limiting
seastatePhoto: Halliburton
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512384
TML Lift of Frigg jacket, model tests
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Installation of a subsea production structure1 - 4
Steel structure with GRP protection coversDimensions: 18 x 18 x 7 mMass: 180 tonnes
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Installation of a subsea production structure5 - 8
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Software verification and validation studies Examples:
Green water on deck Run-up and air-gap/deck slamming
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Extreme events:SEMI
Wave runups and slamming
Extreme wavesand deck slamming
Bow slamming
FPSO
Green waterslamming on deck
Green waterslamming on deck
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Testing in extreme wave events - in order to study rare responses
- Response examples: Ringing; slamming and other strongly nonlinear phenomena.
- Irregular waves, simulating e.g. 3 hours storms:Some times they produce only 2 3 criticalresponse events. Many realisations may be needed to give reliable statistics.
- Alternatively: Test in selected, transient wave groups. Problem: How do we select the wave groups?Specific, designed waves? Which characteristics?
One option: Pre-calibrate full irreg. records.Then pick out selected time windowsand put them together.(Selection criteria?)
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A 3-hour random realization of a 10-000 yr North Sea storm
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Extreme groups put together, picked out from many realisations
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Green Sea and Wave Impact on FPSO
MARINTEK study: Model tests with turret moored FPSO in 100-year storms
Measured: Motions - relative waves - water on deck water impact on deckhouse and bow flare line loads
- Investigate physical effects leading to critical green sea and impact load events in irregular wavesWhat are the dominating (linear & nonlinear)
mechanisms?
- Benchmark data for numerical tool development
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Video observations - examples
From the side bow Forward from deckhouse From above - bow
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Two successive green sea events in irregular sea:
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VOF model of water on deck flow from high wave (time-varying incident flow at bulwark) (Ref. Ernst Hansen)
Time-varying boundary conditions taken from elevation probes from the measurements
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Testing of part problems
Dynamics of risers are most important subject! Vortex Induced Vibrations (VIV) Structural testing
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Vortex Induced Vibrations
Current
Vortex shedding
Th
