1. BEST PRACTICES
PAM BOSCHEE l OIL & GAS FACILITIES EDITOR
2. perators... BASED ON THE PROJECT
DATABASE, KEY FLEXIBLE PIPE
STATISTICS ARE AS FOLLOWS:
are increasingly recognizing the need for a systematic
assessment and management of flexible pipe integrity,
but the identification of critical criteria and the means to
best achieve valid and efficient inspection and monitoring
(I&M) continues to evolve. Emerging technologies in
I&M and operators’ expanding implementation of risk-
based management are leading the way toward achieving 58 % of installed flexible
pipes are risers.
a comprehensive integrity management approach for
flexible pipeline and riser systems worldwide.
Flexible pipes are being installed and operated of all flexible pipes
in more marginal and challenging offshore conditions,
adding to the complexity of acquiring complete and valid pressures of less
data for the determination of their integrity. Especially than 5,000 psi
important is the accurate assessment of the remaining (345 bar).
life of a flexible riser so operators can avoid costly
premature change outs.
of all flexible pipes
To further develop the definition of best practices
in flexible pipe integrity assurance, the SureFlex Joint 90 % are less than 10-in.
Industry Project (JIP) presented key findings from its
extensive 20-month-long survey work, including flexible
pipe use worldwide, statistics on design limits, damage,
and failure incidences. Conducted under the auspices of
the Oil and Gas UK, a trade association for the United < 50,000 Pressure by internal
Kingdom upstream oil and gas industry, the “State of psi-in. of the majority of
the Art Report on Flexible Pipe Integrity and Guidance flexible pipes.
Note on Monitoring Methods and Integrity Assurance for
Unbonded Flexible Pipes (2010)” revisited the state of
of flexible pipe has
flexible pipe since the first survey in 2001 to 2002. The
been designed for a
scope of work was international in its content and had the
support of international companies outside of the UK. 70 % temperature of
less than 176°F
O’Brien et al. (2011) reported the outcomes of
the JIP’s data gathering from flexible pipe operators
worldwide, manufacturers, and specialists in the field.
From the time of the first survey, MCS Kenny compiled of operating flexible
risers are in a water
an in-house database of flexible pipe use, damage, and
failure incidents. A comprehensive literature review was 70 % depth of less
than 3,281 ft
also performed. The resultant database covered 1,900
flexible risers and 1,400 static flexible flowlines; 130 (1000 m).
production facilities worldwide; and 315 individual
damage and failure incidents from around the world.
16 Oil and Gas Facilities • February 2012
Water Depth, m
0 2 4 6 8 10 12 14 16 18
Internal Diameter, in.
Fig. 1—Water depth vs. internal diameter for flexible pipe in operation worldwide.
0 2 4 6 8 10 12 14 16 18 20
Internal Diameter, in.
Fig. 2— Design pressure vs. internal diameter for operating flexible pipe.
The deepest water depth in which a flexible riser is annulus between the internal barrier sheath and the external
installed is about 6,234 ft (1900 m) with a flexible pipe sheath of the flexible pipe.
internal diameter (ID) of about 7.5 in. as shown in Fig. 1. Patrick O’Brien, group director of strategic business and
Although flexible risers with ID of more than 16 in. have marketing at Wood Group Kenny, said, “We might attribute
been installed offshore, these are in water depths not this increase to people not being fully aware of the real extent
exceeding 1,312 ft (400 m). Fig. 2 shows the largest p×ID of external sheath damage in 2001, as they were likely not
value in operation, 80,000 psi-in. for a 12-in. flexible pipe. testing for it. However, when the issue was raised in the first
The majority of flexible pipe in use has a p×ID value of less report, more operators by good practice would regularly test
than 50,000 psi-in. the annuli of their flexible risers, and we began to see a
higher incidence of the damage.”
In 2001, external sheath damage was identified when
Flexible Pipe Failure/Damage Mechanisms measurement of annulus pressure through a vent valve at
The most recent survey found that external sheath damage the riser vessel connection revealed that the annulus did not
remains the most common failure, showing an increase since hold its pressure, indicating that the sheath may have been
2001 (Fig. 3). The external sheath is an external polymer breached, most likely by a small crack or a pinhole in the
barrier applied to the flexible pipe to resist mechanical external sheath farther down the riser. In these cases, gross
damage and seawater corrosion of the tensile and pressure structural damage of the armor wires within the annulus was
armor wires of the flexible pipe. Its presence creates an not found to have occurred.
February 2012 • Oil and Gas Facilities 17
2002 UKCS and Norway only Others:
Smooth bore collapses
% of Failure/Damage Incidents
30% 2010 Worldwide
25% Excess Torsion
Armour wire failure
it t i
Fig. 3— Flexible pipe failure/damage mechanisms.
their focus on how they connect and install their flexible risers
to the floating production facility so as to avoid this failure
mode in the future, he added.
Two failure/damage mechanisms showing significant
decreases in incidence since 2001 were related to the
internal sheath—aged internal sheaths and polyvinylidene
fluoride (PVDF) internal sheath pullout failures (Fig. 3).
The oil and gas industry has studied the causes of aged
internal sheaths, largely affected by high water content
and elevated bore temperatures on polyamide 11 (PA11),
a high-performance polymer material that allows for higher
operating pressures than does high-density polyethylene
(HDPE). As a result, operators gained a better understanding
of the properties of PA11 and how to monitor, control, and
Fig. 4— General armor wire corrosion. predict its life cycle.
PVDF internal sheath pullout failures have dropped with
One finding related to external sheath damage has the introduction of new end fitting designs developed by the
only emerged since the original study, O’Brien said. “There flexible pipe manufacturers.
is a subset of external sheath damage where general and
extensive armor wire corrosion (Fig. 4) occurs due to gross
external sheath damage along a region of the riser within Early Planning for Integrity Management
the splash zone and where the riser may also be shielded O’Brien noted that the survey results point to the need for
by vessel structures. The shielding effect of the vessel operators to consider their flexible pipe integrity management
structures can prevent proper operation of the riser’s strategies at the earliest stages possible. “From the minute
corrosion protection system, while the splash zone causes you think about designing a flexible riser or flexible pipeline,
intermittent wetting effects that provide a supply of oxygen, even at concept, front-end engineering design, and into
which encourages a highly corrosive environment. Hidden operations, the guidance note recommendations propose
locations at the riser top section close to end fittings, for what should be done at those stages to consider integrity,”
example at J-tubes, are susceptible to this type of external he said.
sheath breach and subsequent armor corrosion. Operators One of the challenges in developing an early I&M
were generally not aware of its incidence or prevalence in the strategy is the lack of coordinated efforts between project
first study, but there have been significant instances of it in teams charged with the design and the operations personnel.
the last number of years,” he said. Operators have increased “How do you get a change in behavior at the project stage?
18 Oil and Gas Facilities • February 2012
5. exposing armor wires to the same conditions. However, there
is significant evidence for that not being the case.”
PA-11 External Sheath
5 Operators have made decisions to change out risers
Carbon Steel Tensile Armors
4 because their integrity prediction methods indicated that the
Carbon Steel Pressure Armor
3 riser was approaching its predicted fatigue life. Regarding
PA-11 Pressure Sheath
2 the selection of appropriate I&M methods, O’Brien said,
1 Interlocked “Part of the problem is that there is no single magic piece
Carcass of technology out there that is able to properly inspect
flexible pipe. It is a bit like detective work, selecting a range
of alternative inspection and monitoring techniques, which
when utilized together help you to establish the current
integrity of your flexible pipe.”
In a recent case, following the decommissioning
Fig. 5 — Flexible pipe layers of dissected riser. Source: OTC 22398. of a 10-year-old flexible riser with a flooded annulus,
the dissection, inspection, and laboratory analysis of
components showed that the condition of the armor wire
Sample 1 Sample 2 remained comparable to the as-manufactured conditions
(Charlesworth et al. 2011).
End Fitting Bend Stiffener
I-Tube Flexible Pipe
Fatigue Performance of a Flooded Annulus
Charlesworth et al. described the dissection process and
Fig. 6 — Locations of flexible pipe dissection. Source: OTC 22398. condition of a flexible, high-pressure gas riser following its
decommissioning from the West of Shetland (WoS) region.
BP managed riser integrity using a risk-based approach
Project teams are focused on design, aiming for controlled on its WoS flexible risers installed in the late 1990s. As
costs. Then suddenly, it is handed over to operations,” part of this process, fatigue of armor wires in those risers
O’Brien said. with flooded annuli was identified as a failure mode. As
Despite recognized issues related to flexible pipe fatigue occurs in many flexible risers worldwide, external sheath
and armor wire corrosion, and the availability of monitoring breaches occurred during installation of the WoS risers,
techniques such as fiber optics embedded in armor wires causing annulus flooding. BP sealed the locations of the
to allow measurement of real-life stress, operators will risers’ breaches, displaced the seawater in the annuli with
sometimes decide at the design stage to forego their a corrosion inhibitor, and implemented a program of regular
inclusion to cut costs. O’Brien said, “They’re not thinking fatigue life reassessments for the affected risers.
about the benefit that might bring to operations. In 2008, one of BP’s WoS gas risers with a flooded annulus
“Some operators are now making sure that they are was identified by calculations used in the integrity management
bringing their operations people into the project team so process to be approaching its predicted fatigue life, indicating
no decision is made that might be favorable for capital that replacement was the appropriate course of action.
expenditures, but would be a poor one for subsequent Following decommissioning of the riser, it was dissected
operating expenditures.” at Technip’s Le Trait Manufacturing Unit and its various
metallic and polymer layers were inspected. Armor wire
samples were taken from the fatigue critical region of the
Guidance for I&M riser and tested at Marintek to establish S-N curves in an
High-strength armor wire used in flexible pipe systems for environment representative of the affected riser’s annulus.
offshore applications is subject to harsh service conditions, (An S-N curve shows the results of a fatigue test as a plot of
including service stress loadings and corrosive environments. stress [S] against the number of cycles to failure [N]. A log
In efforts to accurately measure the condition and remaining scale is used for N.)
life of the armor wires, one of the major issues is the The major structural layers of the riser are shown in
determination of the real conditions within the flexible pipe Fig. 5. Dissection locations are shown in Fig. 6, and findings
annulus. O’Brien said debate continues about whether or of the dissection in Table 1.
not armor wire in an annulus is fully exposed to presumed The S-N testing of the armor wire samples from
environmental conditions in the pipe bore, or if the actual the decommissioned pipe was carried out following the
environment is much more benign than what is being guidelines of a test protocol recently developed as part of a
predicted. “Today, we assume the worst case scenario, Marintek-run JIP on flexible pipe corrosion fatigue, as was
such as if there is hydrogen sulfide and carbon dioxide in the testing of the as-manufactured armor wire samples.
the bore of the pipe, it will permeate out into the annulus, Fatigue testing of the tensile armor wires taken from the
February 2012 • Oil and Gas Facilities 19
6. TABLE 1— FLEXIBLE PIPE DISSECTION FINDINGS
Layer Sample 1 Sample 2
Carcass • No unlocking • No unlocking
• Isolated pitting • Isolated pitting
• No visible cracks or breaks • No visible cracks or breaks
Pressure sheath • Regular creeping into pressure armor • Regular creeping into pressure armor
• Carcass rigidly held • Carcass rigidly held
Pressure armor • No unlocking • No unlocking
• Limited corrosion/pitting • Limited corrosion/pitting
• No visible cracks or breaks • No visible cracks or breaks
Tensile armor • No abnormal gaps or overlap • No abnormal gaps or overlap
• Limited corrosion • Limited corrosion
• No visible cracks or breaks • No visible cracks or breaks
External sheath • Longitudinal surface scratches • Longitudinal scratches
• Discoloration • Marine growth stains and discoloration
• Local minor damage on internal skin
Anti-wear tapes • Regular distribution • Regular distribution
• Local overlap under the bend stiffener
• Creeping under the bend stiffener
High-strength tape • Good general condition • Major damage under the bend stiffener
• No disorganization
• Local slight tape stretching
Source: OTC 22398.
decommissioned pipe showed fatigue lives comparable to by MAPS Technology as an inspection tool that can be
the as-manufactured wire. connected to a flexible pipe to monitor for armor wire
Charlesworth et al. said the findings demonstrated the damage and breakage.
conservative nature of current fatigue prediction models. The MAPS technology capitalizes on the magnetic
As flexible pipe installations increase in more challenging properties of iron, steel, and other ferromagnetic
environments, overly conservative fatigue prediction materials and their sensitivity to stress. The internal
methodologies may limit the applicability of technology, magnetic structure of these materials is divided into
they added. domains, which are strained along the direction of their
One factor that may contribute to overly conservative magnetization, a property known as magnetostriction.
criteria used in prediction models is the lack of transparency When stress occurs, the degree of domain alignment is
about armor wire testing methods and results by flexible pipe altered, causing a measurable change in the material’s
manufacturers. O’Brien said that although manufacturers magnetic properties.
perform comprehensive testing and qualification of their In experiments with armor wire, the wire’s
armor wires, the results are not made publicly available. He magnetostriction was altered with a change in stress applied
said, “Without wanting to impact in any way or take from to it. The applied stress and measured stress correlated
the commercial business of these manufacturers, as an well, enabling operators to monitor the stress in armor
industry, we need to get more transparency in terms of how wires, during and after a significant wave event. Field trials
fatigue calculations are performed, as this would facilitate of MAPS were being conducted on a Petrobras platform
the ability to radically improve our assessment of the at the time of writing of the JIP report to determine the
remaining life of flexible riser systems.” effectiveness of the method in a real environment.
Tritech recently launched its riser and anchor chain
monitoring system (RAMS), a multibeam sonar technology
Emerging and New I&M Technologies for floating production, storage, and offloading (FPSO) units.
The magnetic anisotropy and permeability system (MAPS), RAMS is deployed beneath the FPSO’s turret and extends
an emerging technology listed in Table 2, was developed down below the level of the bend stiffeners. The transceiver
20 Oil and Gas Facilities • February 2012
7. TABLE 2 — Inspection AND MONITORING techniques
Technique/Method Take-up (Rated 1 to 5) Industry JIP feedback (Rated 1 to 5)
1 – Limited, specific applications 1 – Under development, unproven
5 – Common practice 5 – Highly reliable
Proof pressure testing 2 3 (short-term assurance only/ad hoc method)
Visual inspection 5 4 (anomaly tracking/gross defects)
Ultrasonic 2 (some examples to detect 3 (unproven at detecting wire defects; access to critical
annulus flooding) region may be problematic)
MAPS* 1 (offshore trials ongoing) 2
Polymer coupons 3 (commonly used for 4 (limited implementation, but can provide assurance
high-temperature applications) for PA11)
Annulus monitoring-various 4 (significant increase in recent years) 3–4 (can detect flooding, though dependent on access)
Eddy current 1 2 (new tool under development)
Radiographics 2 (historically limited to topsides) 3 (digital method under development)
Intelligent pigging 1 1
Torsion monitoring 1 (focused on deepwater applications) 1 (alternative systems under development)
Curvature monitoring 1 (focused on fatigue) 2 (no operational feedback)
Acoustic emission 1 2 (not field-proven)
Sonar monitoring (RAMS)† 1 (limited use to date) 4 (has detected bend stiffener loss)
FPU excursion/environment 4 3 (several systems have failed to properly record and
monitoring log excursion data)
Fiber optic monitoring 2 (embedded in tensile armor 2 (no operational feedback)
wires to detect strain cycling)
* MAPS — magnetic anisotropy and permeability system
† RAMS — riser and anchor chain monitoring system (Tritech)
For Further Reading
emits a horizontal sonar beam through 360° to generate “State of the Art Report on Flexible Pipe Integrity and
and record sonar images of the spatial positions of the Guidance Note on Monitoring Methods and Integrity
risers and mooring chains. RAMS has the option to provide Assurance for Unbonded Flexible Pipes (2010)”
the FPSO operator with an alarm-based system that detects at www.oilandgasuk.co.uk, publications, code OP010
out-of-specification movement of risers and mooring lines,
OTC 22398 “Operational Experience of the Fatigue
and allows the measurement and export of trend data for Performance of a Flexible Riser with a Flooded Annulus”
use in material fatigue analysis. by David Charlesworth, BP, et al.
The system has been installed since 2009 on the
Teekay Petrojarl Foinaven FPSO, operating on BP’s OTC 21844 “Developments in Managing Flexible Risers
deepwater oil field off the West Coast of Shetland. BP and Pipelines, a Supplier’s Perspective” by C.S. Dahl,
NKT Flexibles I/S, et al.
reported the system to have been 100% effective.
O’Brien said that because flexible pipe I&M techniques OTC 21524 “Outcomes from the SureFlex Joint Industry
continue to improve and develop at a reasonably fast pace, Project—An International Initiative on Flexible Pipe
the guidance note will be reviewed annually to keep pace Integrity Assurance” by Patrick O’Brien, Wood Group
with developments and to keep operators up to date. OGF Kenny, et al.
February 2012 • Oil and Gas Facilities 21