An overview of the issues with offshore drilling activities in high current environments. As a solution, structural monitoring systems are suggested as a way of providing actual riser response data to the operator

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VIV Monitoring:
enhancing the safety of
drilling operations in high
current environments
August 2014

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Agenda
• Environmental loading
• Vortex induced vibration
• GOM loop currents
• Riser monitoring
• Case study: VIV monitoring

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Vortex induced vibration
• Caused by strong, steady currents when
vortices shed by current flow around the riser
matches a natural frequency of the system,
resulting in amplified lateral motions (resonance)
of the riser;
• Generally the governing environmental load in
water depths exceeding 250m;
• Can cause excessive motion of the riser.
Overview

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Industry issues
Vortex Induced Vibration
BOP stack excitation
Seabed
BOP
LMRP
Wellhead
36” / 20" pipe
38” / 36"pipe
Conductor_____
Connector
Drilling Riser BOP Stack Excitation
• High amplitude movements in
riser system can lead to:
• Accelerated fatigue;
• System degeneration;
• Riser failure.
 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.

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VIV in the Gulf of Mexico
• The loop current in the deep water Gulf of Mexico is
the primary driver of high speed ocean currents;
• Excessive loop currents have been observed a
number of times over the past decade, causing
drilling operations to be halted and risers to be
retrieved;
• Some of these loop currents have led to current
speeds of up to 4 knots. Historically, current speeds
of over 1.5 knots have been seen as problematic;
• In the strongest currents it may not even be
possible to unlatch the BOP and retrieve the riser
due to issues with excessive riser motion during
hang-off.
Effects of the loop current

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Vortex Induced Vibration
• Data loggers using a combination of accelerometers and
angular rate sensors located at predefined strategic
locations along the length of the riser, on the BOP stack and
on the vessel;
• Data loggers along the riser measure riser motion to identify
mode shape;
• Strain sensors can also used for localized strain
measurements at fatigue critical locations:
• Bonded strain gauges;
• Subsea strain sensors;
• Dynamic curvature sensors.
Riser Monitoring

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Standalone Acoustic Hardwired Eexd
Vortex Induced Vibration
Data logger technology
 There are a number of data logger options available, the choice of which will depend on monitoring parameters,
logging location and communication preference;
 Hardwired and acoustic communication provides real time operational data to the rig, allowing for improved
decision making based on accurate information. .
 Off-the-shelf rental systems are available at short lead time

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Logger 56
x/L= 0.000
Logger 55
x/L= 0.112
Logger 54
x/L= 0.160
Logger 53
x/L= 0.208
Logger 52
x/L= 0.257
Logger 51
x/L= 0.305
Logger 50
x/L= 1.000
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5
0
0.1
0.2
0.3
0.4
0.5
Frequency (Hz)
Acceleration
Amplitude(m/s2
)
Schiehallion Event 91 - Magnitude of Peak Response vs. Frequency
Accelera
tion
Amplitud
e (m/s2)Logger 56
x/L= 0.000
Logger 55
x/L= 0.112
Logger 54
x/L= 0.160
Logger 53
x/L= 0.208
Logger 52
x/L= 0.257
Logger 51
x/L= 0.305
Logger 50
x/L= 1.000
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5
0
0.1
0.2
0.3
0.4
0.5
Frequency (Hz)
Acceleration
Amplitude(m/s2
)
Schiehallion Event 91 - Magnitude of Peak Response vs. Frequency
Accelera
tion
Amplitud
e (m/s2)Logger 56
x/L= 0.000
Logger 55
x/L= 0.112
Logger 54
x/L= 0.160
Logger 53
x/L= 0.208
Logger 52
x/L= 0.257
Logger 51
x/L= 0.305
Logger 50
x/L= 1.000
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5
0
0.1
0.2
0.3
0.4
0.5
Frequency (Hz)
Acceleration
Amplitude(m/s2
)
Schiehallion Event 91 - Magnitude of Peak Response vs. Frequency
Accelera
tion
Amplitud
e (m/s2)
INTEGRIpods
Riser Monitoring
Vortex Induced Vibration
 Data loggers installed along the riser allow
visualisation of riser mode shape;
 Using a placement strategy based on preliminary
analysis, these motion loggers do not have to be
located along the entire length of the riser. Instead,
a strategic clustering of loggers at one or both
ends of the riser can still allow prediction of global
riser response with the required accuracy

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Vortex Induced Vibration
• Using an online monitoring system provides
real time data on the rig allowing operational
decisions to be based on observed riser
behavior;
• Post processing is conducted on the measured
data in time and frequency domain, allowing
for the source of motions to be identified;
• Amplitude and frequency of stress cycles is
used to obtain fatigue calculation;
• Data also allows for calibration of analysis
model.
Data analysis
Real time data displayed on the rig

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Case Study: VIV
monitoring in the
Gulf of Mexico

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Deep Water VIV monitoring
• Drilling campaign in Gulf of Mexico
• 2000m water depth using 6th
Generation semisub
• Analysis showed conductor fatigue life
of just 8 days
• Conductor top weld shown as fatigue
critical location
• Standalone monitoring requested to
record actual motion responses of the
system
Project Background

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Monitoring System
Deep Water VIV monitoring
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• Eexd-rated INTEGRIpod on vessel
to measure vessel motion
• 10 standalone INTEGRIpods along
riser to measure riser motion
• 2 standalone INTEGRIpods on BOP
to measure angular rate of BOP/
LMRP
• Subsea data loggers installed using
magnetic interfaces

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ROV INTEGRIpod installation
Deep Water VIV monitoring
13

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Deep Water VIV monitoring
• Monitoring data used to determine actual conductor fatigue incurred during operation;
• Data showed acceleration threshold for conductor top weld not exceeded;
• None of VIV events resulted in above-threshold riser fatigue damage;
• Client did not have to undertake detailed fatigue analysis of drilling campaign.
Results & Lessons Learned
14

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Summary
VIV issues and solutions:
• Environmental loading can cause fatigue
damage to subsea risers, wellheads and
conductors;
• In the deep water Gulf of Mexico, the loop
current can lead to vortex induced vibration
and thus excessive loading in the riser
system;
• Structural monitoring systems can be
installed to provide a detailed
understanding of asset behavior. Real-time
data can provide instant warning of
excessive motion.