OMAE2008-57046 Actual VIV Fatigue Response of Full Scale Drilling - - PowerPoint PPT Presentation

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) 27th International Conference on Offshore Mechanics and Arctic Engineering June 2008 - Estoril

2

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

3

VIV Monitoring: Background and Objectives

• Motivation

− Equipment damage − 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

What is actually happening to our risers?

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Key Conclusions

• VIV DOES
• ccur 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

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VIV Does Occur: Riser and BOP Stack Response

6

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™

7

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-07 1E-06 1E-05 1E-04 1E-03 1E-02 1E-01 1E+ 00 1E+ 01 1E+ 02 1E-07 1E-06 1E-05 1E-04 1E-03 1E-02 1E-01 1E+ 00 1E+ 01 1E+ 02

Measured Calculated

Overestimated Underestimated

Equal

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Adjust Input Parameters to Reduce Conservatism

• Overprediction

reduced from factor of 30 to 10

• Difficult to reduce further because of scatter

1E-09 1E-08 1E-07 1E-06 1E-05 1E-04 1E-03 1E-02 1E-01 1E+00 1E-09 1E-08 1E-07 1E-06 1E- 05 1E- 04 1E-03 1E-02 1E-01 1E+00

M easured Calculated

A djusted Paramet ers Default Parameters Over estimated Underestimated Equal Adjusted Default

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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
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• 6
• 5
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• 2
• 1

1 2 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

log10(mean(DSHEAR7)) (1/ yr) PoOVI V

10 20 30

VIV % Occurrence

90 100 80 40 50 60 70

Increasing Average Predicted Damage Rate

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Suppression Performance: Full-wrap fairings

Fairings Data:

• 7 buoyant joints (525’) equipped with fairings
• Shell Global Solutions, Inc. (SGSI) -

full wrap design

• Chord/Diameter (C/D) Ratio: 1.5
• Length: 6 ft

Monitoring Data:

• 9 months of monitoring
• 1.6% VIV occurrence
• Max current below 0.5 knots except for period
• f 21st

July to 27th July 07 when a max current speed of 2.1 knots observed

11 Slick Joints WT = 0.875" BOP LMRP Lower FJ 21 joints 3000ft rating Buoyancy 13.0 ft –––––– Seabed 39.0 ft –––––– 62.8 ft –––––– 1187.8 ft –––––– MSL 4595 ft Well head 4 Slick Joints WT = 0.750" 887.8 ft –––––– 1637.8 ft –––––– Drill Floor Intermediate FJ Termination Joint & Pup Tensioner Ring Outer Barrel Inner Barrel & Pup Joint Diverter & Upper FJ 4542.8 ft –––––– 4669 ft 4620 ft –––––– 4647.8 ft –––––– 4412.8 ft –––––– 6 joints 5000ft rating Buoyancy 3887.8 ft –––––– 7 joints 3000ft rating Buoyancy with F airings Staggered section 4 jts 3000ft rating Buoyancy 5 jts Slick 3212.8 ft –––––– 36” & 28” Conductor (157.5 ft)

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Riser DOES NOT vibrate in one instance of high current in faired zone

• From 21st

July to 27th July 07 max current speed of 2.1 knots observed

• VIV did not
• ccur with highly sheared 2 knot current loading on the fairings

BP Explorer Drilling Riser Monitoring Back Analysis Deployment I : May 2006 - July 2006 CURRENT SPEED WI TH RESPECT TO EVENT NUMBER, DATE, AND TI ME VS DEPTH

100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4

Current Speed (knots) Depth (m)

Event 909 - 7/ 24/ 06 16:00 Measured Current

STAGGERED JOI NTS BUOYANT JOI NTS FAI RI NGS SLI CK JOI NTS

Full-wrap Fairing Performance

12 BP Explorer Drilling Riser Monitoring Back Analysis Deployment I V: Nov 2006 - Feb 2007 CURRENT SPEED WI TH RESPECT TO EVENT NUMBER, DATE, AND TI ME VS DEPTH

100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 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

Current Speed (knots) Depth (m)

Event 767 - 1/ 31/ 07 20:00 Strouhal 0.17 Strouhal 0.22 Measured Current

STAGGERED JOI NTS BUOYANT JOI NTS FAI RI NGS SLI CK JOI NTS

Riser DOES vibrate when moderate currents are below faired region

• 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

Full-wrap Fairing Performance

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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

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Slick (Finned Joint) Lankhorst Fins Buoyant Joint

Continuous Buoyancy Staggered Finned/Buoyant Joints Staggered Finned/Buoyant Joints

Suppression Performance: Riser Fins

15

Percentage Occurrence of VIV: Riser with Fins

Riser Hang Off

• 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)

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Example: Fins Reduce Damage

Show scary test result

Expected power in region; Critical velocity = 0.3 kts Expected power in region; Critical velocity = 1.7 kts

Damage Rate = 2.2E-6 /yr Damage Rate = 0.2 /yr

Finned Riser Response: Connected Finned Riser Response: Hung Off

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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

0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.02 0.03 0.04 0.05 0.06 0.07 0.08 Measured (Hz) Shear7 (Hz)

Equality Calibrated Reduced Lift Curve 100% Suppression in Finned Joints Overestimated Underestimated Event 486 Event 353

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Higher Harmonics Prolific in Model Tests

• 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 > 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

Example higher harmonic response

Frequency Time Crossflow (1X) Inline (2X) (3X)

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Higher Harmonics Contribute Little in the Field

0.000 0.100 0.200 0.300 0.400 0.500 0.600 0.700 0.800 0.900 1.000 Schiehallion WOS 1181ft Svinoy WOS 3510ft Assynt WOS 3143ft Reki Brazil 3173ft DEN GoM 6000ft

Riser # F atig u e D am ag e B ias 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
• ne 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

1 2 3 4 5

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Time Sharing of Modes Prolific in Model Tests

• Definition: Multiple VIV frequencies
• ccurring non-concurrently under the

same flow conditions

• SHEAR7 Version 4.5 has been

developed based on the principle of time sharing

• Full scale drilling riser measurements

used to confirm, or otherwise

Time sharing example from Deepstar Gulf Stream test

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Time Sharing of Modes is Rare in Field Observations

• Single mode, multi mode and time sharing VIV observed
• Single mode VIV occurs majority of time
• Occurrence of time sharing is low
• Time sharing typically observed during rig moves –

varying effective current speed

Single mode VIV Multi mode VIV Time sharing

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Conclusions and Recommendations

Conclusions

• Drilling riser monitoring has confirmed safe operations
• VIV design tools are conservative, on average

− Lot of scatter >> high factor of safety

• Phenomena identified that are not currently considered in VIV design

− Similarities and differences from model-scale − Suppression devices

• At this time uncertainties (and limited data to resolve them) justify continued monitoring

Recommendations

• Continue monitoring as the most reliable means of assessing VIV fatigue and extracting

fundamental physics at full scale

• Pursue smarter monitoring that provides operational assistance

− 1 rig day saved >> annual cost of instrumentation

• Champion development of improved software tools that reflect observed physics

− DATASETS AVAILABLE ON BENCHMARKING WEBSITE − INITIATIVES IN DEEPSTAR AND RPSEA JIPs

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Back up slides

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BOP stack VIV

• BOP

stack natural frequency excitation has been observed during relatively high-speed loop currents

• This can result in high accumulated

fatigue damage at the fatigue critical conductor connector below the mudline

• Motion

data collected from an accelerometer placed on the LMRP of the Ocean Confidence drilling riser is presented here

BOP stack excitation

Seabed BOP LMRP Wellhead

36” / 20" pipe 38” / 36"pipe

Conductor_____ Connector Drilling Riser

BOP Stack Excitation

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Just how bad is it?

Summary Acceleration Spectra for 10 min Window

0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1 Frequency (Hz) Acceleration Amplitude (m/s2)

BP Ocean Confidence Event 112 - Magnitude of Peak Response vs. Frequency

Z Axis Spectra Maxima Spectra

1st Harmonic 2nd Harmonic 3rd Harmonic

• BOP stack vibration at 0.16Hz (6.25 sec stack natural period), and

2nd and 3rd harmonic

• Total fatigue consumption at the first connector is 40% in 12 days

BP Ocean Confidence Riser VI V Monitoring CUMULATI VE FATI GUE DAMAGE AT CONDUCTOR CONNECTOR DnV B-Class Curve, SCF = 8.0

5 10 15 20 25 30 35 40 45 50 8/3/05 8/8/05 8/13/05 8/18/05 8/23/05 Date Cumulative Fatigue Damage (% ) 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 Current Speed (knots) Cumulative Fatigue Damage, SCF= 8.0 Max Current Speed

Event 133 8th Aug,05 00:00 Event 97 5th Aug,05 00:00 Event 112 6th Aug,05 06:00

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Conclusions

• Loop currents can cause BOP stack excitation which poses serious

threat to conductor integrity

• Resulting fatigue damage consumption can be high (40% in 12

days in the example here)

• This may result in insufficient fatigue margins for future well

intervention or work-over activities

• Conditions that lead to large BOP motions are poorly understood
• Many unknown factors about what's going on below mudline

(soil properties, quality of cement job)

• It is recommended that:

− contributing factors to BOP VIV receive further study − fatigue critical connectors be placed well below the mudline to improve fatigue capacity

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Gulf of Mexico BP riser monitoring

Field/MODU Water depth Monitoring system Period of monitoring 1 6000 ft 13 INTEGRIpod-SM data loggers May 04 – Present 2 6800 ft 1 INTEGRIpod-SM data logger Nov 04 – Apr 05 3 6800 ft 20 INTEGRIpod-SM data loggers Nov 05 – Present 4 4100 ft 10 INTEGRIpod-SM data loggers Apr 06 – Present 5 4600 ft 10 INTEGRIpod-SM data loggers May 06 – Feb 07 6 6000 ft 14 INTEGRIpod-SM data loggers 3 real-time strain sensors Jan 07 – Present 7 4500 ft 12 INTEGRIpod-SM data loggers Jan 07 – Present

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• Motion
• Strain

Instrumentation for VIV stick™ pod-M™

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VIV software calibration

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DD2 riser response whilst connected

• Measured frequency = 0.025 Hz
• This indicates likely excitation in continuous buoyancy section
• Staggered finned and buoyant arrangement are not powered-in

in spite of being subjected to high currents

Damage Rate = 2.2E-6 /yr

Expected power in region; Critical velocity = 0.3 kts

31

Hang Off Riser VIV

• Measured frequency = 0.13 Hz; Excitation in continuous buoyancy
• High currents in continuous buoyancy yields orders of magnitude

higher fatigue damage than high currents in staggered buoyant and finned joints

Damage Rate = 0.2 /yr

Expected power in region; Critical velocity = 1.7 kts

32

Key findings and software limitations

33

VIV occurrence in the field

• VIV occurs 17% of the time for our monitored drilling risers
• VIV design tool identifies VIV majority of the time

5 10 15 20 25 30 Assynt DEN (Depls. 2-4) Reki Schiehallion Svinoy

Asset Field VI V Event Occurence (% )

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Field data for determining higher harmonics

6040 ft Keel 6000 ft Drill floor 6133 ft 1080 ft Vessel mounted motion logger 5 motion loggers 6040 ft Keel 6000 ft Drill floor 6133 ft 1080 ft Vessel mounted motion logger 5 motion loggers

Drill pipe test

Field Location Rig Water Depth (ft) Schiehallion West Of Shetland Paul B. Llloyd 1,181 Svinoy Faroes West Navion 3,510 Assynt Faroes West Navion 3,166 Reki Amazon Basin C.R. Luigs 3,172 Thunder Horse Gulf of Mexico DEN 6,200

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Conclusions

• DD2

riser typically demonstrates mild VIV response compared to Enterprise which is in a similar water depth and current conditions and does not have any suppression.

• The difference between DD2 and Enterprise

is the presence of finned slick joints and staggered finned and buoyant joints

• Maximum VIV damage on DD2

is accumulated when it is partially deployed and the continuous buoyancy section is exposed to high surface currents.

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Tail-fin drilling riser fairings

Fairings Data:

• 14 buoyant joints (1050’) equipped with

fairings

• SGSI -

two strap “Tailfin” fairings

• Chord/Diameter (C/D) Ratio: 1.45
• Length: 6 ft

Monitoring Data:

• 2 months of monitoring
• 4.1% VIV occurrence
• Max current speed: 0.95 knots

35 ft Termination Joint BOP LMRP Lower FJ 10 ft –––––– Seabed 63 ft –––––– 473 ft –––––– MSL 4065 ft 5 Slick Joints 2273 ft –––––– Drill Floor Intermediate FJ Termination Joint & Pup Joints Tensioner Ring Outer Barrel Inner Barrel Diverter & Upper FJ 4033 ft –––––– 4153 ft 4112 ft –––––– 4135 ft –––––– 3998 ft –––––– 24 joints 5000ft rating Buoyancy 3173 ft –––––– Well head 98 ft –––––– 12 joints 3000ft rating Buoyancy (3 Top joints with Fairings) 11 joints 2000ft rating Buoyancy (with Fairings) 1125 ft of Fairings

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Case study – Does time sharing occur for a riser undergoing VIV?

Sustained currents > 2 knots and VIV observed 42% of time

1 2 3 4 13-Apr-07 28-Apr-07 13-May-07 28-May-07 12-Jun-07 27-Jun-07

Date Current Speed (knots)

Latch BOP Intermittent Drilling

Unlatch BOP and Rig Move

Latch BOP Stop Operations FJ Angle above Operating limits

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Case study – Does time sharing occur?

Fatigue damage rates are 10 to 100 times higher during hang-off

BP Enterprise Drilling Riser Monitoring FATI GUE DAMAGE RATE AT CRI TI CAL POI NTS Riser Fatigue Critical Point and Conductor Connectors : Dep XV

1.E-17 1.E-15 1.E-13 1.E-11 1.E-09 1.E-07 1.E-05 1.E-03 1.E-01 1.E+ 01 1.E+ 03

Date Fatigue Damage Rate (1/ yr)

0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

Current (knots)

Conductor Connector MC 776# 4 Conductor Connector MC 775# 4 Maximum Riser Damage (3.6 m above LFJ) Current (at 36 m depth)

Latch BOP Intermittent Drilling Unlatch BOP and Rig Move Latch BOP Stop Operations FJ Angle above Operating limits