WIND LOADS / MOORING & FISH TAILING. Arjen Koop, Senior Project Manager Offshore Rogier Eggers, Project Manager Ships

Transcription

1 WIND LOADS / MOORING & FISH TAILING Arjen Koop, Senior Project Manager Offshore Rogier Eggers, Project Manager Ships

2 OVERVIEW Wind Loads Wind shielding Fish tailing? 2

3 WIND LOADS FOR OFFSHORE MARS TLP after Katrina, 2003 Wind forces are important for DP/mooring system Extreme wind loads important for structural integrity Fluctuating wind forces due to gusts Wind gusts can excite resonant behavior -> e.g. fish-tailing 3

4 costs METHODS TO DETERMINE WIND LOADS Tunnel Tests CFD Building block approach Rules of classification societies Cheap and accurate does not come fast Fast and cheap does not come accurate Fast and accurate does not come cheap 4 accuracy

5 WIND TUNNEL TESTS Accurate geometric modeling is possible Shielding, focusing, lift effects, spatial and time variations included Blockage effects should be checked Relatively expensive Once model is made it is fast to run multiple wind headings and different configurations Additional costs and time: Wind velocities around object Pressures and forces on substructures Smoke visualization 5

6 COMPUTATIONAL FLUID DYNAMICS (CFD) CFD has become engineering tool in last few years with good accuracy Proper Verification and Validation needed! Important physics included: wind profiles, shielding, lift effects, etc. Scale and blockage effects can be investigated Good flow field visualization Faster than wind tunnel testing (and the same costs) Limitation on geometry complexity 6

7 WIND LOADS LNG CARRIERS 7

8 WIND LOADS SHUTTLE TANKER 8

9 WIND SHIELDING SHIELDED ZONE Shielding: reduction of forces on a downstream object Focusing: increase of forces on a downstream object Varying wind loads important for fish tailing Shielding by environment? (mountains, harbors, channels, buildings, etc?) 9

10 WIND FIELD PREDICTION Good agreement in velocity field between CFD and windtunnel 10

11 WIND SHIELDING MODELING 11

12 WIND SHIELDING MODELING 1. Determine unshielded wind coefficients for each segment using CFD or wind tunnel 2. Obtain velocity field using either CFD or wind tunnel 3. Determine local velocity direction and magnitude in each volumetric segment 4. Using local velocity, determine unshielded coefficients for each segment 5. Correct unshielded coefficients for atmospheric boundary layer profile 6. Calculate shielded coefficients using local velocity and corrected unshielded coefficients for each segment. House segment Wind Direction 12

13 EXAMPLE WIND SHIELDING yaw moment 300m shielded, Cxysh wind direction at tanker [deg] Measured in wind tunnel ( ) Estimated with model using CFD ( ) Estimated with model using wind tunnel ( ) 13

14 OFFSHORE OFFLOADING OPERATIONS Tandem spread moored Single point moored Tandem with turret Side-by-side Side-by-side 14

15 FISH-TAILING Instability Discomfort for passengers and operations Sensitive to: (Unsteadiness of) Wind field Wind Shielding Line length and tension Wind loads and deck configuration Under water shape (damping) Maneuvering stability of vessel On-off phenomena 15

16 TANDEM OFFLOADING AND FISH TAILING Identical wind-only condition (basin measurement). Spread moored Single Point Mooring

17 SENSITIVITY FISH-TAILING ON/OFF PHENOMANON 27 m/s wind 28 m/s wind

18 SQUALL AND FISH-TAILING 18

19 TYPICAL QUESTIONS OFFLOADING -> YACHTS? DP strategy? Following Weather vaning? Shielding effects in wind, waves and current? Crossed conditions? Fish-tailing instabilities? Line length and tension? Relative headings? Wind, wave and current conditions? Motions of vessel and comfort? Similar questions for yachts? 19

20 WIND LOADS FOR YACHTS? Often not only emphasis on forces and wind fields, but also Propagation of exhaust gasses Helicopter operations These topics require further research if to be done with CFD However, if only forces and wind fields are important, then CFD provides a cost effective option Input for DP capability investigations Input for fish-tailing phenomenon? 20

21 FISH TAILING AT ANCHOR FOR YACHTS Not a typical research area, however Many yachts also fish tail when at anchor (just look at the AIS data off Monaco) Large freeboard and superstructure combined with a small displacement appears to be particularly problematic Difficult to solve for existing yachts: small changes in geometry are typically not sufficient to reduce problems Operational solutions, e.g. double anchor arrangement, are typically effective, but not liked Active devices (e.g. a quiet low power stern thruster) can be very effective 21

22 FISH TAILING AT ANCHOR FOR YACHTS (CONT.) MARIN can assist to quantify the reduction of fish-tailing using free-anchored model tests or using simulations Set-up can be kept more simple than what is done for offshore industry oriented model tests Challenge? Research needed? 22

23 Offshore Yachts Arjen Koop Rogier Eggers