As exploration moves into ever deeper and more complex fields, the
need for flexible pipe increases. Given that the use of flexible pipe is
relatively recent and the analysis is therefore fairly limited, its integrity
has largely been monitored by visual inspections carried out by ROV operations.
Visual inspection shows only the damage already done. It does not
provide preventative measure to reduce the risk of failure. Emerging new
technologies, such as annulus testing, can improve the integrity of flexible
pipe, reduce costs, and extend their original design life.
During 2002, UKOOA issued guidance entitled “Monitoring methods and
integrity assurance for unbonded flexible pipe.” Since then, as the use of
flexible pipe has increased, a number of lessons have been learned in
management and on-going operation of flexible pipes.
These guidelines should be updated to include the data on reasons
for failure and lessons learned. This would allow the industry to challenge the
inspection strategies currently used as well as to identify emerging
technologies as alternative solutions.
Given the advances in flexible pipe analysis, some of which were
developed under the RealLife JIP, MCS demonstrated that through proper asset
management, flexible pipe technology service life can be extended beyond the
original design criteria. Similarly, flexible pipe previously considered
damaged and in need of early replacement may be justified for extension to
beyond original design life.
Since the mid-1980s, there has been exponential growth in the use
of flexible pipe. Given that most flexible pipe systems are designed with a
20-25 year service life, only a small percentage in operation are close to this
service life.
Should flexible pipe failure rates follow the typical “bath-tub”
analysis curve, it is possible that the type and quantity of flexible pipe
damage/failure mechanisms will increase in the future.
Examining the main causes of damage shows the percentage of
“internal sheath pull-out” incidents is down. This is due to a historical
failure mode associated with early PVDF design structures, identified around
1995, and subsequently designed out.
There also has been a reduction in the number of “aged internal
pressure sheath” incidents. This can be explained by improved industry
knowledge regarding the recommended operating restrictions for polymers.
Another significant variation is related to “external sheath
damage” where there has been a marked increase in the number of occurrences.
This is difficult to explain in terms of changes in flexible pipe operation.
However, one explanation is the increase in vacuum testing and other monitoring
and inspection techniques for the annulus which make it possible to identify
external sheath damage.
A significant proportion of external sheath damage occurs during
installation. There is clear potential for this to be improved with better
installation procedures.
To date, general visual inspection (GVI) of flexible pipe has been
key in maintaining its integrity. However, operators are increasingly
questioning the value of frequent GVI to their integrity management strategy.
The main advantages of GVI are the ability to confirm general
layout and configuration, to identify gross riser/ancillary equipment damage,
and to provide inspection results which can be clearly interpreted.
The main disadvantages are that it is difficult to identify small
outer sheath leaks, annulus flooding from venting systems is not identified, no
inspection of internal layers is possible, there are environmental limits
(especially shallow work), limited inspection contractor expertise for flexible
pipe, and a requirement for ROV support vessel or DSV when vessel costs and
availability are real issues.
External sheath damage would appear to be the most obvious damage
to be spotted by periodic GVI. However, the reality of flexible pipe operation
means that it is only possible to identify reasonably large areas of damage
from the video footage. In conjunction with this, experience has shown that
relatively small areas of damage can cause significant problems.
In addition to this, it is MCS’ experience that areas of potential
damage are routinely identified during GVI but upon closer inspection are only
surface staining. This has a significant cost implication for the operator, who
will invariably have to undertake close visual inspection (CVI) using either
diver or ROV.
The second reason for flexible pipe damage is aging of the
internal pressure sheath. GVI can only identify damage that has already reached
the point where it can be difficult to manage. For example, when fluid release
by the pressure sheath has aged to an extent where is visible externally, it
makes the pipe unserviceable.
There is a need for GVI with flexible pipe systems due to
regulatory requirements and to identify other damage/failure mechanisms such as
those associated with ancillary equipment, e.g. loss of mid-water arch
buoyancy, buoyancy clamp slippage, tether failures, etc.
The main areas of concern with ancillary components are the bend
stiffener, buoyancy module, mid-water arches, clamps, and tethers. Even here,
data quality can be increased and costs can be reduced with innovative
approaches. For example, the FPSO turret area is difficult for an ROV to
inspect due to surface waves, currents, and congestion. An alternative might be
to deploy a fixed controllable camera below the turret. This allows inspection
as frequently as desired and without ROV support.
Operators spend £30,000 and £200,000 ($61,411 and $409,410) per
flexible per annum on GVI. This variation is due to a number of reasons,
including:
- Small number of flexible pipes in
the field which can increase the costs of vessel mobilization per riser
- Level of integrity management
strategy optimization. An optimized integrity management strategy allows
the operator to target inspection in the high risk locations most
frequently
- Uptake of alternative integrity
management tools instead of GVI
- Age of the asset
- Operator / regulator attitude to
risk.
Depending on the operator strategy in use, it is possible for
operators to reduce their annual inspection costs for flexible pipe. This can
be achieved by:
- Developing and maintaining an
integrated integrity management strategy, which incorporates the use of
focused GVI and alternative assessment / inspection techniques
- Using GVI in high value reward
situations, where the data recovered can be used to identify specific
types of damage, e.g. ancillary equipment
- Considering integrity management
at the asset procurement and design stage to incorporate available online
monitoring techniques which are difficult and expensive to retrofit
- Using previous operational experience
at the design stage
- Development and use of detailed
inspection manuals for the inspection contractor, providing examples of
as-built state, as last-observed state, typical anomalies, and definition
of records required for each type of new anomaly.
One significant way manage to the integrity of flexible pipe can
be by assessing the annulus environment. There are a number of techniques to
accomplish this, including vacuum testing, pressure build-up testing, gas
sampling, and online monitoring. Each provides benefits and levels of
information. Increasingly, combinations of these strategies are being adopted.
The key to all these techniques is that they measure the flexile pipe annulus
properties to identify changes such as the presence of seawater/water, H2S and CO2 composition, gas permeation rates, and
“unflooded” volume in the annulus. Through the correct analysis of this data,
it is possible to determine the presence of external sheath damage, topside
flooding, and pressure sheath aging which then can direct further
investigation.
There are limitations to annulus testing such as the potential to
pull water in to the annulus and the perceived inability to assess riser
systems with hog and sag bends. But these are minimal compared to the value of
knowing whether or not there is a breech in the external sheath.
It still is important to confirm the layout of flexible pipe
subsea. However, this does not necessarily require the level of GVI which is
currently being employed. To demonstrate that the layout has not changed
significantly, a survey from an increased distance covering a number of
flexibles may be sufficient. Equally, it is common practice during GVI on
flexible pipe to survey both sides of the structure. If external sheath damage
is being assessed by alternative techniques, it could be argued that this is
not necessary. This alone would reduce GVI time for flexibles by 50%.
Operators look to extend the life of flexible pipe assets. To
determine whether life extension is feasible, analysis of the flexible pipe
history is required. This generally includes assessment of fatigue (in which
annulus condition is of paramount importance), polymer aging (based upon coupon
samples, operating pressures and temperatures and chemical injection data), and
anomalies identified from GVI.
Given a proper asset integrity management strategy, that is, one
in which data is collected diligently and anomalies are identified and
understood, there is potential to use the methodologies defined as part of the
RealLife JIP to derive an accurate assessment of flexible pipe fatigue damage
rate. This generally results in an extension to previously calculated fatigue
lives. The single caveat to this is that the methodologies that were used to
calculate fatigue life at the design stage must have incorporated the effects
of resonant frequency response on the flexible pipe system. Using RealLife
methods with a proper asset integrity management results in an accurate and
balanced value for the flexible pipe fatigue life.
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