FORTIM Global, Ltd.
Preamble and Predicate Concept Draft for Discussion Purposes Only
Subsea leak detection systems
In April 2010, the same month as the Deepwater Horizon event in the Gulf of
Mexico, Det Norske Veritas promulgated a recommended practice, DNV – RP –
F302, entitled selection and use of subsea leak detection systems.
DNV is an autonomous and independent foundation. With the objective of
safeguarding life, property and the environment, at sea and on shore, DNV
undertakes classification, certification, and other verification and consultancy
services relating to quality of ships, offshore units and installations, and on shore
industries worldwide. DNV carries out research in relation to these functions.
DNV offshore codes consist of a three level hierarchy of documents:
– Offshore service specifications. Provide principles and procedures of the
classification, certification, and verification and consultancy services.
– Offshore standards. Provide technical provisions and acceptance criteria for
general use by the offshore industry as well as the technical basis for the
– Recommended practices... Provide proven technology and sound engineering
practice as well as guidance for the higher-level initial offshore. Initial Service
Specifications and initial Offshore Standards.
The offshore codes are offered within several areas including but not limited to
qualification quality and safety methodology, materials technology structures,
systems, special facilities, pipelines and risers, asset operation Marine
operations cleaner energy and subsea systems.
Of particular relevance to this memorandum are the offshore codes for
pipelines and risers specifically.
Production systems for hydrocarbons are complex installations and it is known in
the industry that smaller and larger unwanted linkages occur and cause
discharge of hydrocarbons to the surroundings. Today, both operators and
authority awareness toward the environmental impact of oil and gas production is
for remote measurement of acute pollution. Regulatory agencies in UK, USA and
In spite of the fact that methods for subsea leak detection have been available in
the market for years, there are still gaps to fill concerning the design engineering
and operation of such methods to make reliable for detection of this change to
the environment. Thus, there is a need for history reference in this technology
field which can serve as a guideline today and is useful for coordinating the
development of the field in the future
This initial Recommended Practice (RP) summarizes current industry
experiences and knowledge with relevance to selection and use of detectors for
a subsea leak detection system. It covers relevant regulations, field experience
and available technologies and also gives guidance on how to design and
operate a system for subsea leak detection and recommendations for
developments needed in this field of technology
This RP is developed within a Joint Industry Project, including: Conoco Phillips,
Shell and Statoil in addition to a broad selection of engineering companies and
subsea leak detection technology suppliers participated in workshops and
The objective of this best practice (RP) is to summarize industry experiences and
knowledge with relevance to selection and use of detectors for subsea leak
detections. The intent is that this document can be used as a technical and
practical guideline and reference for operators, suppliers, regulators and
decision-makers in the field of subsea leak detection. The referencing of this
document will not substitute for the development of a field specific leak detection
strategy, but rather be an element in one.
It is emphasized that the application of subsea league detection system shall not
reduce safety level subsea systems in terms of design, manufacture, quality
It is also emphasized that the performance of a subsea leak detection system is
not determined by the technical specification of the detector technology alone,
but by an overall assessment of technical data, system layout and system
The Norwegian Oil industry Association has through a Joint Industry Project,
taken the initiative to improve practice in the industry regarding detection of
subsea hydrocarbon leaks. Subsea leak detection is becoming more important in
the petroleum industry.
The regulations of PSA (Petroleum Safety Authority) Norway outline requirements
also the European Union also describe requirements for detection of acute
As well as detecting leakages, a leak detection system may be designed to
provide condition monitoring data and may form part of a life extension strategy,
for example, the risks of leakage considered too high to continue production
without continuous monitoring. It is recently been stated by the Norwegian
authorities in relation to future production in the Arctic region that there is a need
for "early warning systems" based upon detecting leaks at the source".
This is the essence of FORTIM ( fiber-optic real-time integrity monitoring) as an
entry strategy into the oil and gas industry that is highly focused upon this area
of challenges, specifically early warning systems based on detecting leaks at the
source with direct continuous sampling of measurands ( as opposed to indirect
methodologies such as vortex induced vibration).
While numerous additional citations could be repeated from the DNV
recommended practice referenced herein, it is noted that additional detail is
provided from Norway the United Kingdom USA and the European Union
Statistical analysis shows that the majority of leaks observed in the North Sea are close
to subsea installations, platforms and flowlines. At the current time, this best practice is
limited to subsea structures with access to the control system.
The best practice focuses on continuous monitoring principles for leak detection
For pipeline systems, the reader may refer to DNV–RP–F116 Guideline on
integrity management of submarine pipeline systems
Leakages are in this context limited to those originating from production of
hydrocarbons in the form of natural gas, oil or multiphase, as these are
considered to have the greatest environmental impact .Leak detection may also be
of interest for CO2 injection systems and hydraulic control systems. Technologies for
leak detection are, however, still immature, and this document will focus on
leakages from oil and natural gas production, as this considered to have the
greatest environmental impact. Some detection principles prescribed in this guideline
will in theory detect any liquid leaking at a certain pressure; others are depending on the
chemical compound for detection.
The focus of FORTIM technology is direct measurement of liquid leaking at a
certain pressure as opposed to the chemical compound variation.
The maturity of the technologies described is varying from concepts to
commercially delivered products, However, the operational experience with
subsea leak detection only goes back to the 1990s and is limited to a few of the
described technologies. New developments and further developments of existing
technologies are expected and the descriptions and recommendations given in
this document should be developed accordingly.
This document has been developed based on input from suppliers, integrators
and end-users of leak protection technologies as well as PSA Norway
Experience to date
Subsea production systems have been developing since the first subsea completion as
early as 1943. Currently, most operating companies use a combination of technical and
organizational measures to detect unwanted discharges of oil and gas, including visual
observation. Experience has shown that the current methods and equipment available
are not as effective as desired. The documentation reference herein is based on a JIP
of OLF with the oil and gas industry during the period of 2005 2007.
Phase 1 was a database review of available information on reported subsea leaks up to
the year 2005 mainly in the North Sea. Main conclusions were that most leak
incidents occur at or close to subsea installations. Most leaks are small and from
small diameter pipes.
Phase 2 was a review of available technologies for subsea leak detection. The main
conclusion was that there are several different available technologies potentially
suitable for continuous monitoring of subsea installations
Phase 3 consisted of comparable experimental tests of different types of subsea leak
section principles. Main conclusions were that all the technologies tested in these
experiments work well under laboratory conditions; both crew oil and gas leakages
were detected. However, the technologies perform differently under varying conditions,
due to their different strengths and limitations. Generally, gas leakages are easier to
detect than crude oil leakages.
In past decades, detection of gas pipeline ruptures near platforms has been in focus, to
protect personnel and structures from explosion risk. Pipeline emergency shutdown
systems based on pressure monitoring (blowout preventers) have been installed
by most operators. These systems generally work satisfactorily under normal
production operations and when automatic warnings are included in the system.
However, problems may arise during transients, for example, at start-up, after
shutdown periods and when systems rely on the operator reacting to the data
instead of automatic warnings.
A good example of the weakness of relying on operators reacting to data instead
of ordering automatic warnings was the mud observed on surface during the
Deepwater Horizon pre-events (writer note-.this needs to be verified with post
event investigation documentation).
The forward trend in Norway seems to be adding permanently installed leak
detection systems. The international trend points towards leak detection systems
mounted on mobile units (ROVs) and deep water applications. Inspection
surveys are typically done once a year. However this may not prove sufficient for
an increasing number of complex and large subsea structures.
The existing experience in this field of technology is generally young, limited primarily to
the Norwegian sector and includes problems with false alarms and consequently
disabled sensors as well as fields where one has accomplished solutions that are
described as promising.
Subsea leak detection should be regarded as a monitoring system and not and is
not at present mature enough to fulfill the requirements normally set for a safety
system (herein lies the need, challenge and opportunity for FORTIM).
Field developments cited the DNV RP F302 shall not be repeated herein but are
referenced accordingly. It is noted that the predominance of location and
reported leaks on installations in the Norwegian sector are very small leaks
equally distributed between mid line (> 500 m from installation) and subsea
well/template/manifold (500 m radius).
The leak sizes are not listed in the PTIL/NPD file. However, they fall into four
categories: large, medium, small and very small.
The large leak size type is used where a large crack, split rupture or substantial volume
of fluids have leaked out. The medium leak size is used for incidents where the leak
seems to have been significant but not large.
The small leak size category is used for incidents where the leak seems small but
corrective actions are initiated. The very small leak size category was used for
incidents where no corrective actions were initiated. Typically the risk reduction
measure was to observe the leak for further development.
It should be noted that the observation of the small leaks for further development
can be very costly, highly dependent on ROVs and optical or human inspection
Very small leakages are interesting for monitoring but not repair purposes and
detection methods reported are mainly by ROV and the inspection human
observations with a few incidents reported as detected by pressure testing and
automatic shutdown/pressure loss of well.
It should be noted that statistics show the largest hydrocarbon leaks occur when
the operation is unsteady (shut –in, start up, maintenance etc.). These are phases
when mass balance cannot be used for leak monitoring due to unstable flow and
pressure readings, therefore creating a high level of vulnerability and threats
Structure components reported being subject to leaks are connections, connectors,
flanges, seals, valves and welds. These can be considered critical points for
monitoring of possible leaks.
The remainder of this document is focused on flanges. Leaks caused by materials
failure, due to corrosion, cracking, and external impact such as vortex induced
vibration, etc. can in theory occur in all structures and critical points for these failures
may be harder to identify. However it is possible also for these failure modes to identify
where a failure is most likely to occur based on design and environmental parameters
and define these locations as critical points for monitoring particularly when used in
extended life simulations to predict the mean expected failure point.
Leak detection technologies and their characteristics
Principles and methods for leak detection are in the literature associated with
many concepts including surface detection inspection and permanent subsea
monitoring. An important target for detection of leakages subsea is to achieve
early warning of small to medium-sized leaks for monitoring and corrective
The remaining focus of this text is permanently installed subsea sensors that
require access to the subsea control system for continuous monitoring subsea.
Some of the principles are independent of the subsea control system.
Some of the principles described below provide area coverage which means they can
place a leakage relative to the position of the sensor. The accuracy of the placement,
the range covered and positioning parameters vary from principle to principle. Other
technologies are point sensors that detect a leakage in their vicinity but cannot
determine the precise location of the leak. Point sensors may be an option for
monitoring high risk leak points.
The concept of point sensors using fiber-optic methods is an additional drill
down and narrowing in of this document.
The available technologies include active acoustic, biosensor, capacitance, and fiber-
optic, fluorescent, methane sniffer, and optical camera, passive acoustic and mass
balance. The methods studied previously hereto include limited intelligible results which
are shown in Table 3-1 of the DNV RP.
All of these technologies are efficacious in the detection of gas with mixed
results for the detection of crude oil. All are equipped with a high level of
sensitivity and broad area coverage.
With the exception of capacitance, there is no response time for detection noted
in any of these studies. Similarly, with the exception of optical camera, passive
acoustic and active acoustic, there is no leak positioning.
Fiber-optic methods are used for locating and measuring mechanical
disturbances at acoustic frequencies along the continuous optical fiber note the
word continuous (which implies a series circuit as opposed to a parallel circuit).
Disturbances can because by e.g., fibrillation, seismic waves or acoustic
signals from, for example, leaking gas or liquids. Simultaneous disturbances
may be detected in position to approximate 1 m accuracy along the continuous
(i.e., series circuit) fiber.
There is a trade-off between spatial resolution along the sensing fiber and
detection sensitivity. For example if it is not necessary to isolate the detection of
vibration between every adjacent meter of fiber 10 m would be acceptable. Then
the detection sensitivity can be increased by roughly a factor of 10. A benefit
with fiber-optic methods is that no power or electronics is required along the
length of the cable and it is immune to electrical interference (and also the
creation of voltage).
For monitoring the subsea structures as technology has not yet been taken beyond
the conceptual stage. The concept has been tested for pipelines on shore. This
technology is not covered further in this version of this document.
Design of subsea leak detection systems
The design of the leak detection system should be integrated in the overall subsea
system design. According to industry input, the structures where leak detectors have
been installed to date are mainly Christmas trees, templates and manifolds. The
feedback from subsea system integrators is that integrating the sensors to subsea
structures mechanically, and to the control system, in most cases is solvable, but that it
is important that this requirement is identified early in the design process.
The leak detection system should therefore be included as a primary design
requirement and not considered as an add-on late in the design process.
As a minimum comply with the requirements found in ISO-13628 –6. Specific
contents of the qualification activities must be tailored for each technology .DNV
– RP – 8203 “qualification procedures for new technology” may be used as a
guideline for the qualification process
System performance requirements and technology selection
For designing an appropriate detection system to be applied for specific field,
performance requirements for the system should be developed. The requirement
should be based on authority requirements, environmental and safety risk
analysis for the specific field, corporate requirements, field specific conditions
that will affect the performance, interface to the control system and integration
into the overall operational management philosophy of the subsea equipment.
A leak detection system comprises selected detection technologies as well as tools for
data management, operational procedures and layout/placement of sensors.
The total system layouts should fulfill the developed field specific performance
requirements. This is also discussed in the OLF guideline “recommended
procedure for evaluating remote measuring initiatives"
The selection of sensors must be optimized for each application. A risk assessment
may be valuable to identify the locations on subsea production system where it is most
likely that leakages will occur and hence where to place the detectors. It is also
necessary to take into account process medium, process pressure and external
conditions at the site.
To enable operators to select the right sensor types for their system, all
commercial available leak detection technology should have a specification
sheet specifying parameters like:
– Mechanical, electrical and communication
– Bandwidth requirements
– Power requirements
– Requirements to location in environment
– Sensitivity to noise and other effects from other components
– design pressure
– design temperature or
– Test conditions, e.g., water depth, pressure and temperature, for qualification
testing and FAT testing
– Sensitivity and drift
– Ability to determine location of leak
The subsea Leak detection system is tailored based on specific conditions based on
available technologies, the field specific conditions, connectivity to the control system
and operational management philosophy of the subsea equipment.
There may be gaps between the desired and achievable performance of leak
detection system. Gaps may be addressed by adjustment of the mass balance
system on one hand, or the inspection methods/intervals on the other.
Leaks over a certain size C will be detectable by mass balance .The smallest leaks
below a threshold A may not be detectable by any methods.
Conditions specific to field (environment, production process, feel layout, feel
composition etc.) will together with the performance specifications of the technologies
determine the threshold be between what will be detectable by the leak detection
system and what must be detected during inspection and approval must be determined
how long a leak of size B can be allowed when considering the HSE risk.
Combining technologies combined to a more types of sensors may provide more
confidence in the overall leakage detection system. Complementary sensor
technologies should be selected to compensate for the respective weaknesses
and enable indication of any event from one sensor types to be confirmed by
positive indications from the other sensor type.
The SIIS (Subsea Instrumentation Interface Standard) JIP is an initiative from the
industry. The goal is to standardize the interface between subsea sensors in the
One of the selected technologies should be able to perform detection over an area (not
limited to point sensing). Point sensors may be installed at critical leak points.
A general approach to how to operate a measuring system is found in ISO10012
“measurement management systems – requirements for measurement
processes and measuring equipment”
Warning display and action
Most permanent subsea leak detection principles rely on the subsea control system to
convey the detection of hydrocarbons to a topside or onshore for the signal root trigger
a form warning
It is assumed that a warning signal will either be transformed to an audio or visual
warning E. G. Signal lamp, and subsequently this signal will be observed by
human operator who will decide on the actions to follow
The maturity of subsea leak detection technologies is today too low to have
independent safety of independent safety function. Setting firm limits for what detector
response represents a positive finding of a leak is challenging further investigation of
leakage warning is needed for confirmation and quantification of the week further
investigation may include –
– check apparel systems (E. G. Mass balance)
– downloading of raw data from the detector giving the warning for further
analysis – Inspection on surface visually, by satellite, radar, plane etc.
– Inspection subsea E. G. By ROV
The operator response upon a warning from the leak detection system must
be captured and operational procedures for the leak detection system
Writer’s commentary is in the addresses sensitivity and response time including
requirements from operators in finding from laboratory tests a general observation is
that in the absence of an absolute inherent direct measurement of a leak many false
positives and/or false negatives could occur this is the heart of the mission of FORTIM
Bandwidth capacity for subsea leak detectors is in general a lesser challenge for
new fields than for retrofit to existing fields. However bandwidth limitations can
be imposed on the subsea leak detection system depending on the field
Communication interface subsea control system.
SIIS has developed levels for defining subsea and leak detection interfaces
referring to said definition. Advanced detectors should typically have an SIIS
level III interface (Ethernet TCP/IP), while more proven detectors could use an
SIIS (CA and OPG in space fault-tolerant, ISO 11898 –3) interface in the future
sensors complying with the SIIS standards are what oil companies most likely
will request and what will be the easiest to interface to their systems.
ISO 13628– 6 may serve as a reference for the power requirements for subsea
leak detectors’ Subsea control systems are optimized for power. Keeping the power
requirements for the subsea leak at a minimum will always be a benefit
FAT is described in ISO 13628– 6
System-level tests are described in ISO 13628-1 and 13628–6
Descriptions of sensor specific test methods should be provided by the
vendor for all commercially available leak detection systems
Technology specific requirements
As minimal work has been done in the area of fiber optics of a comprehensive
nature, this is an area of work yet to be done
Calibration inspection and intervention practical procedure for calibration
should be available.
Industrialized sensors and systems
The accuracy and the stability of the sensor reading should be quantified for
detectors against experience. Some kind of saturation, recalibration from
topside subsea should be described.
Lifetime for subsea equipment should be long enough to keep the intervention
frequency and cost at an acceptable level. Typical lifetime for non-retrievable
equipment is 25 years. The lifetime and requirements regarding maintenance
and whether the units are retrievable must be described in procedures. There
must also be available supply information on calibration inspection intervention
Writer’s comment for fiber optics
There is currently no calibration specified. For typical supplier specifications,
however, a two-year interval is specified for inspection and intervention
Further research and development
The message from the industry today is if there is a need for improvement of the
performance of leak detection technologies. A leak detector should ideally be
able to perform identification, localization, quantification and classification
(medium, harmful/harmless) of a leakage. Today no technology alone can
perform all these operations and it may even be hard to accomplish by combining
One aspect that is mentioned in particular for sensor performance is unwanted
warnings. As mentioned previously false positives are often generated due to
natural seepage of hydrocarbons from the seabed. Handling of natural seepage
without giving unwanted warnings is a challenge for further development of
subsea leak detection
To improve performance of detection leakages may be beneficial to develop a
better understanding of leakages and their characteristics through e.g. modeling
Any technology that is deployed subsea should work reliably over a defined
design life in the robust.
Vortex Induced Vibration and other impacts and loads that are present subsea
environment suggest the need for maintenance. Calibration should be well
defined so that this can be taken care of through planned operations. Potential
failure modes for the technology should be thoroughly identified so that
mitigations can be designed into the technology or plan such mitigation as built
in self-diagnosis and redundant functionality of a detector for known technical
failure modes and automatic compression for these failure modes.
Basing the detector design on standardized interfaces mechanically and for
communication and power is enabling for easy integration into the subsea production
system technologies that can offer this fulfill one important criterion for being preferred
in the design phase
For leak detection technologies with relatively high power consumption
integration into the subsea control system power supply can be a challenge.
Solutions for lower power consumption for technologies may be an issue and will
be an important test for the future thus the attractiveness of fiber-optic sensors
Qualification and testing
Proper qualification is important for “for technologies being deployed subsea”. Qualifying
a product should be done through defining its functions and limits, assessing what
functions are not proven, identifying failure modes in carrying out activities those through
objective elements. Minimize failure modes improved functionality
Requirements are found in ISO 13628.
However the specific contents of the qualification activities must be tailored for
each technology as an example deriving the qualification process may be done
according to DNV– P– A203 “qualification procedures for new technology”.
It is important for the operator to be able to verify the function of leak detectors with one
method for testing the detectors after installation subsea (less than 200 m) should be
Pilot projects can be a means for providing interfaces required for gathering real data of
this would potentially allow for improvements for the integration to power and
communication, detector out the rhythms and to find better solutions to the mechanical
integration leak detection technologies in subsea systems pilot projects can also give a
possibility for trying combinations of different technologies into systems and give input
to procedures for management operation of such systems requirements for a surface
Requirements for leak detection will be based on field specific conditions and
will vary from field to field. The operating companies should for each field slot
define clear and quantified requirements for each subsea leak detection system.
For each field the magnitude of the minimum detectable leak should be defined.
Further leaking medium to be detected should also be defined
More work remains to be done
Edward Cook April 22, 2014