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深水中的模型试验


MODEL TESTING FOR DEEPWATER CONCEPTS
C.T. Stansberg Norwegian Marine Technology Research Institute A.S (MARINTEK), Trondheim, Norway

OGP Workshop on Technology Requirements for Floating Systems, London, UK, 23-24 April, 2001

Contents of presentation
- Deepwater metocean conditions - physical modelling
- Deepwater floating systems - physical modelling - Particular areas of experimental investigation - Laboratory limitations - and solutions - Areas of uncertainty and further development

Deepwater oil and gas fields (d ~ 500m - 3000m): Critical metocean design conditions
Waves
Norwegian Sea Atlantic Margin High High

Current
High

Wind
High

Others

Moder./High High

Gulf of Mexico
Offshore Brazil West of Africa (Newfoundland)

Steep/High
Moderate Low High

High
High High High

High
Moder. Moder. High Ice

Physical modelling of deepwater metocean conditions in a laboratory basin

MARINTEK’s 50m x 80m x 10m Ocean Basin
The Ocean Basin Laboratory
Double-flap wave maker Multi-flap wave maker Double-flap wave maker Multi-flap wave maker 50 m

Cross-section of Ocean Basin

Length: 80 m - Width: 50 m - Depth: 0 -10 m

S:\ ve rh a \ o sto \mta .p t o e dh u n vd p

80 m

OCEAN BASIN

TOWING TANK

20 01-0 4-17

Modeling of waves items of particular interest
- Nonlinear effects (crests; wave heights; kinematics) - Extreme waves (probability; mechanism; freak waves?) - Multi-directionality - Multiple-peak spectra (in frequency & in direction) - Non-stationary hurricanes (in frequency & in direction) - Repeatability - Minimum scale of reproduction - 1:150 ?

Second-order deep-water random wave model (numerical)

Modeling of waves Items of particular interest
- Nonlinear effects (crests; wave heights; kinematics) - Extreme waves (probability; mechanism; freak waves?) - Multi-directionality - Multiple-peak spectra (in frequency & in direction) - Non-stationarity (in frequency & in direction) - Repeatability - Minimum scale of reproduction?

Measured vs. second-order wave model

Modeling of waves Items of particular interest
- Nonlinear effects (crests; wave heights; kinematics) - Extreme waves (probability; mechanism; freak waves?) - Multi-directionality - Multiple-peak spectra (in frequency & in direction) - Non-stationarity (in frequency & in direction) - Repeatability - Minimum scale of reproduction?

Modeling of deep-water currents - challenges:
- Vertical profile (magnitude & direction) - Homogenous & constant current velocity / turbulence? - Full-depth limitations in available laboratory basins - Combine with equivalent force / numerical models
u

Example: 3000m depth trunc. at 1000m

Modeling combined metocean:
Wind waves + swell + current + wind
- Collinear & non-collinear - Optimal model scale - Modeling of rapid change in hurricane system?

Deepwater floating systems

Deepwater floating systems - physical modelling
Traditional hydrodynamic verification:
- Modeling of “complete” system hull+mooring+risers (+DP) - Scales ~1:50 - 1:100 - Dynamic (& static) coupling between floater & lines/risers - Individual line models - dynamic line tension - Line drag induced slow-drift damping - Complex behaviour of total system / “new” effects? - Extreme nonlinear responses in storm conditions / need for calibration of numerical models - Operations - Measurements: Vessel motions - Line forces - Relative wave Green sea - Slamming - Video observations

FPSO in extreme wave event

Semisubmersible in extreme wave event

Particular areas of experimental investigation
Motions - slow-drift forces in extreme waves with current - viscous damping - motion coupling effects - Dynamic line tension in extreme wave groups - Dynamic coupling to vessel motions

Mooring

Risers

- Steady drag forces - VIV (model testing of separate components)

Relative wave / Green Sea - Probability of green sea / negative air gap - Impact loads & structural responses Extreme responses Numerical analysis - non-Gaussian processes - combined / integrated with experiments

Numerical visualisation (from coupled analysis study)

Dynamic line tension: 1:55 model tests vs. coupled analysis

Laboratory limitations - and solutions
Challenge:
Depths ~ 1000m - 3000m: Too deep for testing at “conventional” scales (1:50 - 1:100) in available basins How to keep the benefits from “complete” system - couplings etc.?

Possible solutions:
- Ultra small scales (1:100 - 1:200) - scale effects?* - Integrated tests & computer analysis (“hybrid techniques”)* - Outdoor testing* - Numerical analysis only - New ultra-deep basin?

Not recommended: Truncation without subseq. computer-extrapolation*
* Studies carried out at MARINTEK: VERIDEEP; NDP; Deepstar

Ultra-small scale model testing: Comparing 3 scales

Verification tests on the P-26 project, a polyester taut mooring system

Testing in scales 1:100 - 1:150 (200) is feasible,
depending on floater, condition etc. Practical limitations today (environmental modelling) Scale effects on line tension can be accounted for; smaller effects on slow-drift Particular attention and care in preparation and execution Special limitations: Thruster modelling (> 1:100) Truss structure details Spar models with moonpool

Hybrid methods: An “off-line” procedure
Hybrid verification:
Computer program Calibration through numerical reconstruction Reduced depth Model tests

Numerical extrapolation

Full depth simulation

Design of truncated system:
(hybrid verification)

- Horizontal restoring force characteristics - Vertical coupling mooring / floater - Quasi-static single-line characteristics - “Representative” damping levels

Numerical reconstruction & extrapolation
(hybrid verification)

- Calibration / check of numerical code - coupled analysis

- System identification; in particular: slow-drift excitation & damping
- Sea state dependent parameters - Final full-depth simulation with calibrated code (coupled analysis)

Coupled analysis (RIFLEX-C) model of an FPSO system
ZG ZV

master node Vessel node XV beam element slav e node XG 15.28 m

bar elements

Empirical surge drift coefficients (semi in irreg. waves)

Example (NDP study):
Semi-submersible system in 3000m steel catenary mooring(semi-taut) Scale 1:150, truncated at 1100m (7.3m mod sc) Norwegian sea 100yr: Hs=16m Tp=18s Cu=1.3m/s Wi=48m/s

Initial check of method: An 1100m system truncated at 550m Extrapolated results compared to full-depth 1100m measurements

Final results: Results from truncated set-up (1100m) numerically extrapolated to 3000m

Promising experiences with the “off-line” hybrid procedure
Some notes for future applications: - Scale of truncated set-up should be > 1:100 - 1:125 - The method works technically fine, while improvements for efficient use are underway (efficient link between experiments and numerical analysis etc.) - Procedures for design of truncated set-up should be established - Uncertainties of 2-step method should be assessed - Guidelines for hybrid verification have been suggested, but should be further discussed and completed

Other hybrid methods:
- On-line (active) integration between truncated test set-up and computer simulations
Potentially a very interesting method. Sophisticated, power-consuming computer tools required: The need for “intelligent” algorithms should be evaluated (How intelligent should it be to represent a real verification?) Need for very large actuators in 6 DOF? - Verification of parts of the system (e.g. the floater only), or of the computer program itself?

Areas of uncertainties and further development:
- When do we need to use scales > 1:100? - More standard procedures to be established for hybrid techniques - Uncertainties in hybrid techniques - What is required for software used in deepwater verification - qualification? - On-line hybrid technique: intelligent software & actuator / control

- More standard procedures for extreme value estimation from model tests
- Viscous drift forces in high waves on currents - Higher-order drift forces on ship in high and steep waves - Metocean input: Multi-directionality / multiple-peaked spectra Deepwater currents: profile / turbulence Rapidly changing weather conditions?

- Particular problem arising in ultra deep waters: Operations in connection with intervention etc. / multi-body dynamics / floating pipelines

2000 m


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