Full scale VIV response measurements of a drill pipe in Gulf of Mexico loop currents (OMAE2008-57610) Beynet, Shilling, Campbell, Tellier, Howells Estoril Portugal - June 2008
The Test • September 2004 Vessel mounted Vessel mounted • Test of opportunity motion logger motion logger Drill floor 6133 ft Drill floor 6133 ft • Waiting on weather to run conductor (~2 knot surface currents) 6040 ft 6040 ft • VIV monitoring system already on the Keel 6000 ft Keel 6000 ft rig • 6-5/8 inch drill pipe was instrumented and deployed to 1,000ft water depth 1080 ft 1080 ft • On-board acoustic doppler current profiler (ADCP) measured current 5 motion loggers 5 motion loggers
Did it VIV?
Agenda • Background Test set up − Monitoring system − Test timeline − • Observed response VIV occurrence and effect of changing current − Single mode, multi-mode or time sharing? − Higher harmonics − Standing or travelling wave − • Conclusions
Test set-up Vessel mounted Vessel mounted • 1080 ft length motion logger motion logger • 6-5/8 inch OD Drill floor 6133 ft Drill floor 6133 ft • 0.492 inch wall thickness 6040 ft 6040 ft • 32.2 lb/ft (47.9 kg/m) in air Keel 6000 ft Keel 6000 ft • 80ksi strength, carbon steel • Drill pipe was free flooding – water filled up to mean water line 1080 ft 1080 ft 5 motion loggers 5 motion loggers
VIV monitoring system • 6 standalone motion sensors, 5 on drill pipe, 1 on vessel • INTEGRIpod: Field proven, high reliability 1 • Acceleration in 3 DOFs and 2 DOF angular rates 0.9 • Continuous logging at 10 Hz sampling frequency 0.8 • Test specific limitations: 0.7 − Low sampling frequency for measured response 0.6 − Number of instruments x/L 0.5 • Drill pipe base is location of max displacement for all modes 0.4 0.3 Logger Depth Below x/L ID MWL (ft) 0.2 17 Vessel Vessel 0.1 22 52.0 0.866 0 21 443.2 0.503 -1 -0.5 0 0.5 1 13 713.2 0.253 Normalized Displacement 29 848.3 0.128 1 2 4 6 8 10 14 983.3 0.003
Current measurement • 38 KHz Acoustic Doppler Current Profiler (ADCP) Current Direction (degrees) Current Speed (knots) 0.0 100.0 200.0 300.0 • 0.0 0.5 1.0 1.5 2.0 Designed for measurement of ocean 0 KEEL currents 100 • Provides 10 minute average speed 200 and direction 300 • Measures 95ft to 3,600ft below Depth Below MWL (ft) surface every 100ft 400 • Max measured current = 1.8 knot 500 • Strouhal (0.20) frequency = 1.1 Hz 600 • 50ft missing between drill ship keel 700 and first data point 800 • Vessel mounted system measures 900 effective current on drill pipe whilst drifting (which we want) 1000 # 2 # 5 # 13 # 19
Test timeline • 2 hour test with 16:03 (4:03pm) start time Vessel drift = surface current Vessel drift = surface current • Vessel drift – vertical pipe Vessel drift – vertical pipe Vessel drift relative to the current was varied Vessel stationary Vessel stationary • Objective: Determine effect of maintaining vertical pipe Time Vessel Drift Information Current ~1.8 knots Current Current Current ~1.8 knots Current ~1.8 knots Current ~1.8 knots 16:00 Varying vessel drift, speed unknown Vessel drift at 1.8 knots in current 17:08 direction 17:17 Reduce drift speed to 0 knot 17:28 Vessel at 0 knot Increase vessel drift to 1 knot in current 17:38 direction
Drill pipe response at base – 0 to 30 minutes Varying vessel drift with current – Water fall Plots X and Y (Lateral) Accelerations Objective: maintain vertical pipe • X and Y waterfall plots side by side Current Speed (knots) -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 0 KEEL 100 200 300 Depth Below MWL (ft) 400 500 600 700 800 Approx max cross flow frequency (St=0.20) 900 1000
Drill pipe response at base – 30 to 60 minutes Varying vessel drift with current – Objective: maintain vertical pipe • X and Y waterfall plots side by side Increase vessel drift to ~equal surface current
Drill pipe response at base – 60 to 90 minutes • X and Y waterfall plots side by side Current Speed (knots) -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 0 KEEL 100 200 300 Reduce drift to zero knots, return to loop current profile Depth Below MWL (ft) 400 500 600 700 800 Multi-mode cross flow VIV 900 6 th Higher harmonic 1000
FEA vs. Actual Modal Response Waterfall Δ f = 0.0195 Hz 40 Mode Number 30 20 10 0 0 1 2 3 4 5 Frequency (Hz)
Drill pipe response at base – 90 to 120 minutes • X and Y waterfall plots side by side Current Speed (knots) -0.5 0.0 0.5 1.0 1.5 2.0 0 KEEL 100 200 300 Depth Below MWL (ft) 400 500 600 700 800 Return to varying vessel drift with current 900 Objective: maintain vertical pipe 1000 Single mode cross flow VIV Strong 6 th harmonic
Severity of higher harmonics – 90 to 95 minutes 1.2 Cross flow 1 Normalized to Maximum 0.8 0.6 0.4 3 rd 6 th In-line Harmonic Harmonic 0.2 0 0.55 1.1 1.65 3.3 Frequency (Hz) Displacement Fatigue Damage • Higher harmonic fatigue damage is negligible compared to cross flow • Conflicts with test findings – fatigue from higher harmonics > factor of 10 • Fatigue damage calculation assumes standing wave
Standing or travelling wave? • Standing wave typically assumed in design • High fatigue damage along the entire length if travelling wave • If 100% standing wave there will be locations of zero measured motion and fatigue along length • If 100% travelling wave measured motion envelopes and fatigue along length will be similar Standing Wave Travelling Wave
Spectral response along pipe - 90 to 95 minutes 3 rd Crossflow 6 th Harmonic VIV Harmonic?
Theoretical standing wave vs. measurements 1.6 1.4 1.2 Acc (m/ s^ 2) 1 0.8 0.6 0.4 0.2 0 0 0.2 0.4 0.6 0.8 1 x/ L Measured Theoretical at Measurement Locations Theoretical • Measured Accelerations at 0.552Hz with Mode 10 Superposed • Measured response fits standing wave
Are the higher harmonics standing wave? Varying amplitude: standing wave? 3.3 Hz response is theoretically mode 32
Conclusions • Strong BP on-shore and offshore teamwork allowed test of opportunity • Valuable data set that complements and extends existing tests • Observed single mode, multi-mode and time sharing VIV • Time sharing typically coincides with changes in vessel drift speed • Higher harmonics up to 6 times cross flow VIV observed • Cross flow VIV fatigue damage dominates • Contribution of dynamic positioning prop wash excitation is uncertain • VIV response is standing wave, up to mode 14, possibly mode 32 • Greatest VIV risk: short term temporary operations in high currents • Drifting to maintain verticality recommended to minimize VIV
Questions?
Spatial aliasing example S pacial A liasing Spacial A liasing M ode M ode 1.5 N o.of 1.5 N o.of S hape Shape Loggers Loggers 1 1 0.5 0.5 Amplitude (units) Amplitude (units) 0 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 -0.5 -0.5 -1 -1 Logger Logger S tart Locn Start Locn -1.5 -1.5 Length (units) Length (units) N um ber of loggers 5 N um ber of nodes/antinodes 7 N um ber of loggers 5 N um ber of nodes/antinodes 3
FEA vs. Actual Modal Response Waterfall Δ f = 0.0195 Hz 40 Mode Number 30 20 10 0 0 1 2 3 4 5 Frequency (Hz)
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