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Subsea leak detection systems


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Subsea leak detection systems

  1. 1. FORTIM Global, Ltd. Preamble and Predicate Concept Draft for Discussion Purposes Only Subsea leak detection systems Background: 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 offshore services. – 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 constantly increasing.
  2. 2. 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 hearing rounds. 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 assurance etc. 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 operation. 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
  3. 3. also the European Union also describe requirements for detection of acute pollution 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 Limitations 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.
  4. 4. 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 Background 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
  5. 5. 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. Field experience 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
  6. 6. 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 detection. 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 potential. 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 actions. 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,
  7. 7. 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. 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
  8. 8. 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. General Requirements 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:
  9. 9. – Mechanical, electrical and communication interfaces – 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.
  10. 10. 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 Communication bandwidth 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
  11. 11. be imposed on the subsea leak detection system depending on the field specifications 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. Power requirements 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 Test methods 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
  12. 12. 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 sensors 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 work Robustness 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. Interfaces 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
  13. 13. 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 developed 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 user interface 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