OMAE2008-57046 – Actual VIV Fatigue Response of Full Scale Drilling Risers: With and Without Suppression Devices M. Tognarelli (BP), S. Taggart (BP), M. Campbell (2H) 27 th International Conference on Offshore Mechanics and Arctic Engineering June 2008 - Estoril
Outline • Background • Key conclusions • Observed VIV in-field • Instrumentation • VIV Software Calibration • Key findings and software limitations VIV occurrence in the field − Suppression devices − Higher harmonics − Time sharing − • Conclusions and recommendations 2
VIV Monitoring: Background and Objectives • Motivation What is actually − Equipment damage happening to our risers? − Excessive waiting on weather Observed BOP stack VIV − Low confidence in conservative design tools − − Limited performance data on off-the-shelf VIV suppression devices (fairings/fins) • Approach GoM drilling riser VIV monitoring program kicked off − − Drilling risers with and without suppression devices • Objectives of Monitoring Campaign Ensure safe operations and feed back important information to wells teams − − Calibrate design tool (SHEAR7) with field measurements − Identify and understand sources of discrepancies Assess suppression performance via ad-hoc field trials − 3
Key Conclusions • VIV DOES occur in the field • Measured damage is generally less than predicted – Risers are being operated safely • VIV fatigue analysis tool SHEAR7 can be/is being calibrated using field data, however, only in an average sense − Large scatter in predictions requires use of a large FoS • Complex fundamental physical phenomena have been observed that are not included in analysis models • Revised VIV prediction tools/approaches are required to account for the physics involved as well as to reduce scatter • Based on limited data, suppression devices that BP has employed appear to perform well • Sufficient uncertainty in riser VIV response is identified to warrant continued monitoring 4
VIV Does Occur: Riser and BOP Stack Response 5
Gulf of Mexico BP Riser Monitoring • 7 Drilling Risers 4,100 – 6,800 ft WD − 10 – 20 accelerometers each − • Instrumentation – Accelerometers / Angular Rate Sensors pod-M™ 6
Analysis Tools Overpredict VIV Damage of Connected, Unsuppressed Drilling Risers Typical design parameters – AVERAGE factor of 30 overprediction Includes data from earlier monitoring campaigns WoS, Brazil, North Sea (OMAE Paper 2005) 1E+ 02 Overestimated 1E+ 01 Equal 1E+ 00 1E-01 Calculated 1E-02 1E-03 1E-04 1E-05 1E-06 Underestimated 1E-07 1E-07 1E-06 1E-05 1E-04 1E-03 1E-02 1E-01 1E+ 00 1E+ 01 1E+ 02 Measured 7
Adjust Input Parameters to Reduce Conservatism • Overprediction reduced from factor of 30 to 10 • Difficult to reduce further because of scatter 1E+00 Over estimated 1E-01 1E-02 Equal Calculated 1E-03 1E-04 Default 1E-05 1E-06 1E-07 Adjusted 1E-08 Underestimated 1E-09 1E-09 1E-08 1E-07 1E-06 1E- 05 1E- 04 1E-03 1E-02 1E-01 1E+00 M easured A djusted Paramet ers Default Parameters 8
Another Layer of Conservatism: VIV Does Not Occur as Often as Predicted • Higher VIV occurrence in the field is observed for higher fatigue damage estimates • Analysis tools almost ALWAYS predict VIV 100 1 90 0.9 80 0.8 VIV % Occurrence 70 0.7 60 0.6 PoO VI V 50 0.5 40 0.4 30 0.3 20 0.2 10 0.1 0 -7 -6 -5 -4 -3 -2 -1 0 1 2 log 10 (mean(D SHEAR7 )) (1/ yr) Increasing Average Predicted Damage Rate 9
Suppression Performance: Full-wrap fairings 4669 ft Drill Floor Fairings Data: Diverter & Upper FJ 4647.8 ft –––––– Inner Barrel & Pup Joint 4620 ft –––––– Tensioner Ring • 7 buoyant joints (525’) equipped with fairings MSL 4595 ft Outer Barrel 4542.8 ft –––––– Intermediate FJ • Shell Global Solutions, Inc. (SGSI) - full wrap Termination Joint & Pup design 4412.8 ft –––––– 7 joints 3000ft rating Buoyancy with F airings 3887.8 ft –––––– • Chord/Diameter (C/D) Ratio: 1.5 Staggered section 4 jts 3000ft rating Buoyancy 5 jts Slick • Length: 6 ft 3212.8 ft –––––– 21 joints 3000ft rating Buoyancy Monitoring Data: 1637.8 ft –––––– • 9 months of monitoring 6 joints 5000ft rating Buoyancy 1187.8 ft –––––– • 1.6% VIV occurrence 4 Slick Joints WT = 0.750" 887.8 ft –––––– • Max current below 0.5 knots except for period 11 Slick Joints WT = 0.875" of 21 st July to 27 th July 07 when a max current Lower FJ 62.8 ft –––––– LMRP speed of 2.1 knots observed 39.0 ft –––––– BOP 13.0 ft –––––– Well head Seabed 36” & 28” Conductor (157.5 ft) 10
Riser DOES NOT vibrate in one instance of high current in faired zone BP Explorer Drilling Riser Monitoring Back Analysis Full-wrap Fairing Performance Deployment I : May 2006 - July 2006 CURRENT SPEED WI TH RESPECT TO EVENT NUMBER, DATE, AND TI ME VS DEPTH Current Speed (knots) 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 0 100 FAI RI NGS 200 300 STAGGERED JOI NTS 400 500 Depth (m) 600 700 BUOYANT JOI NTS 800 900 1000 1100 1200 SLI CK JOI NTS 1300 1400 Event 909 - 7/ 24/ 06 16:00 Measured Current • From 21 st July to 27 th July 07 max current speed of 2.1 knots observed • VIV did not occur with highly sheared 2 knot current loading on the fairings 11
Riser DOES vibrate when moderate currents are below faired region BP Explorer Drilling Riser Monitoring Back Analysis Full-wrap Fairing Performance Deployment I V: Nov 2006 - Feb 2007 CURRENT SPEED WI TH RESPECT TO EVENT NUMBER, DATE, AND TI ME VS DEPTH Current Speed (knots) 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 0 100 FAI RI NGS 200 300 STAGGERED JOI NTS 400 500 Depth (m) 600 700 BUOYANT JOI NTS 800 900 1000 1100 1200 SLI CK JOI NTS 1300 1400 Event 767 - 1/ 31/ 07 20:00 Strouhal 0.17 Strouhal 0.22 Measured Current • 4 days of measured currents with ~0.8 knot mid-depth current • VIV events identified during this period • Buoyant joint VIV excitation: Mode = 2-3, Frequency = 0.0475 – 0.0627 Hz 12
Assessment - Fairings • Field measurements indicate that fairings suppress VIV • The measured response of the risers with fairings showed no unstable behavior • Operational personnel confirmed that the fairings improved operability • Data are VERY limited and more are needed to confirm the effectiveness of fairings 13
Suppression Performance: Riser Fins Slick (Finned Joint) Lankhorst Fins Staggered Finned/Buoyant Joints Continuous Buoyancy Staggered Finned/Buoyant Joints Buoyant Joint 14
Percentage Occurrence of VIV: Riser with Fins • VIV (above threshold) occurred only 2.02% of the time in 17 months • For a similar period, VIV occurred 13.8% on a drilling riser w/o suppression • 31.3% of finned riser VIV occurred when riser disconnected (fins retrieved) Riser Hang Off 15
Example: Fins Reduce Damage Finned Riser Response: Connected Finned Riser Response: Hung Off Expected power in region; Expected power in region; Critical velocity = 0.3 kts Critical velocity = 1.7 kts Show scary test result Damage Rate = 2.2E-6 /yr Damage Rate = 0.2 /yr 16
VIV response frequency comparison on finned riser data SHEAR7 frequencies match measured VIV frequencies assuming finned joints have 100% suppression BP - DDII Drilling Riser VIV Monitoring MEASURED vs SHEAR7 ZERO CROSSING FREQUENCY Drilling Phase - B Class - SCF=3.30 0.18 Overestimated 0.16 0.14 0.12 Shear7 (Hz) 0.10 0.08 Event 353 Event 486 0.06 0.04 0.02 Underestimated 0.00 0.02 0.03 0.04 0.05 0.06 0.07 0.08 Measured (Hz) Equality Calibrated Reduced Lift Curve 100% Suppression in Finned Joints 17
Higher Harmonics Prolific in Model Tests Example higher harmonic response • Higher harmonics observed in recent model tests − NDP high mode Deepstar Miami - Gulf Stream − • In the tests higher harmonic fatigue damage can exceed cross flow VIV by > Time factor of 10 • Currently not considered by any of the existing industry VIV tools • BP field measurements used to confirm the risk of higher harmonic VIV in full scale risers Frequency (3X) Inline (2X) Crossflow (1X) 18
Higher Harmonics Contribute Little in the Field 1.000 0.900 ias 0.800 e B 0.700 ag 0.600 am 0.500 e D 0.400 u atig 0.300 F 0.200 0.100 0.000 1 2 3 4 5 Schiehallion Svinoy WOS Assynt WOS Reki Brazil DEN GoM WOS 1181ft 3510ft 3143ft 3173ft 6000ft Riser # Cross-flow I n-line Higher harmonic • Higher harmonic fatigue does occur in full-scale drilling risers • Resulting fatigue damage is minimal compared to cross flow VIV with exception of one occurrence during rig move • Differences between tests and full scale are not yet fully understood • Additional data would assist in understanding when higher harmonics are a risk 19
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