Pipeline Inspection

Subsea systems should be monitored or inspected regularly, internally and externally. The inspection can provide such information as: geometry variation (dent, wrinkle, buckle, etc.), wall thickness variation (metal loss), corrosion, crack, leak, etc. The advantages and disadvantages of internal and external inspections are as follows:
  • Internal inspection
Applicable for inaccessible (buried or concrete/insulation coated) pipes. May have to shut-down the system to send pigs. Pigs may be stopped or lost due to pipe buckle or pressure loss due to large hole on the pipe.
A variety of intelligent pigs have been employed for pipeline inspection purposes, including detection of not only dents and buckles but also corrosion pitting, cracks, spanning and burial, and measurement of wall thickness. The information obtained from the pigging operations is used for assessment of pipeline safety and integrity. Magnetic-flux leakage pigs have been used for detection of dents and buckles, and measurement of pipe ovality and wall thickness over the entire pipe surface. The principle of magnetic-flux leakage detection relies on measurement of metal loss, and hence the size of defect. Usually a series of survey runs over years are required to establish trends. Magnetic-flux leakage pigging can be utilized in liquid and gas pipelines.

Ultrasonic intelligent pigs are used to make direct measurement of wall thickness of the entire pipe surface. They are better suited to liquid pipelines and cannot be used in gas pipelines without a liquid couplant. Pipeline spans have traditionally been found by external inspection using side-scan sonar or ROVs. In recent years, neutron-scatter pigs have been employed to detect spanning and burial in subsea pipelines with lower cost and better accuracy.
    MFL Pigging Magnetic Flux Leakage
    Source: http://dacon-inspection.com/wp-content/uploads/2013/03/MFL-Pigging01.jpg
  • External inspection
Applicable for un-piggable line. No need to shut-down the system. Good for partial suspicious area inspection, such as manifold, jumper connection, riser, etc.
Self-crawling intelligent pigs have been developed to perform the In-line inspection (ILI) without interrupting the production. The external inspections or integrity monitoring systems are performed by ROV or tools mounted on the pipeline.

Magnetic and ultrasonic tools are commonly used to detect corrosion, crack, geometry and wall thickness variations. Detecting a leak as early as possible will reduce the environmental damage. The current leak detection systems available for subsea pipelines are:

-  Ultrasonic - transmit ultrasonic waves and receive/record reflected waves
Ultrasound is a non-destructive testing technology which has been applied for a variety of inspection tasks for many years now. A major advantage of ultrasound is the ability to provide quantitative measurements. This means that the actual wall thickness of a pipe section can be determined with high accuracy and reliability. The reporting accuracy regarding depth measurement for the latest generation of tools is around ± 0.4 to 0.5 mm. The highest possible resolution that can be achieved today is 0.06 mm. Usually thresholds for depth measurement of metal loss or cracks are set at 1 mm, however lower thresholds are possible.

There are different ways, using different types of transducers, how the ultrasound principles are applied technically, for instance piezo-electric transducers or transducers based on electro-magnetic acoustic transmission. The most widely used tools available from several vendors make use of piezo-electric transducers.

Ultrasound Principle Wall Thickness Measurement
Source: http://ppsa-online.com/papers/2007-9-Beller.pdf
  -  Acoustics - monitor/detect noise or pressure change being created by a rupture or sudden leak
Acoustic leak detection (ALD) systems use hydrophones (underwater microphones) that ‘listen’ for ultrasound generated by leaking fluids under pressure. The acoustic signals generated by a leak tend to be at frequencies well above the audible range, i.e. above 20kHz, thus requiring sophisticated sensors and software to reliably determine the difference between leak generated and ambient ‘noise’. The major problems with this method are the sounds caused by the attendant (ROV) and other vessels in the vicinity. Thrusters and manipulators are constantly moving during subsea operations causing highly variable acoustic signals to be generated over a wide spectrum.

A diver inspecting a flange with ALD
Source: http://www.offshore-mag.com/content/dam/etc/medialib/new-lib/offshore/print-articles/2010/nov/10725.res/_jcr_content/renditions/pennwell.web.400.353.jpg
These signals are additional to any leak-generated sound. It has been difficult, therefore, to differentiate an acoustic leak signal from these other sources and, for this reason, it has not been frequently used as a mobile sensing method. Instead acoustic sensing is commonly used for fixed leak detection sensing, e.g. by mounting hydrophone arrays on subsea equipment. However, modern data handling and spectral analysis techniques have improved the method sufficiently such that in the right conditions the method can be very successful.

Acoustic signature of a leak
Source: http://www.offshore-mag.com/content/dam/etc/medialib/new-lib/offshore/print-articles/2010/nov/90957.res/_jcr_content/renditions/pennwell.web.400.190.jpg

-  Dye detectors - detect optical fluorescent leak visually by a laser beam
Until fairly recently, the most successful method of detecting leaks has been the use of fluorescent dyes detected by ‘black light’ (unfiltered ultraviolet light) with visual observation either directly by diver or by underwater camera.

The major problem with this method is that the dye concentration has to be high to allow visual observation and general visibility must be good. Deploying submersible ‘tuned’ fluorometers that include an excitation and detection unit and that send data up to the attendant vessel providing a real time visual display has solved many of the problems. These submersible fluorometers are very sensitive and will detect dye at concentration so low as to be invisible to the naked eye or underwater camera.
C-Dye rhodamine based fluorescent leak tracing dye
Source: http://static1.squarespace.com/static/5346c808e4b07f8376a22bd5/t/559a8d04e4b014bac5eb1cbb/1436192009846/C-Dye-Fluorescent-Tracer-Dye.jpg
Because hydrocarbons and some hydraulic fluids have specific fluorescence signatures, they can be targeted by a fluorescence detector. However where subsea control systems or hydrostatic testing are concerned, fluorescent dye is normally added as a component solely for the purpose of leak detection. Until recently, there was a choice between wide area detection of a leak or accurate location. For example a narrow beam (e.g. from a laser) is long-range, sensitive and accurate, but would require accurate aiming to ensure all possible locations of a leak plume were scanned. It would be possible to completely miss a leak depending on tidal flow etc. If the light beam and sensor field of view is wide e.g. a broadly focused LED lamp, then the sensor would have a wider spatial coverage, but would be shorter range and difficult to pinpoint a leak location.

-  Fiber optics - detect leaks by hydrophones, accelerometers, temperature monitoring sensors installed on a distributed fiber optic cable along the pipeline.

Loss of transported medium due to pipeline leaks typically results into one or more of the following detectable effects:
  1. Local cooling due to Joule-Thomson effect (high pressure gas pipelines) 
  2. Soil temperature change due to temperature difference between soil and emanated fluids and due to evaporation effects.
  3. Especially in high pressure applications the emanating medium generates detectable sounds.
Based on Raman or Brillouin scattering effects the temperature changes can be detected, if the medium temperature is different from the soil temperature. Hence distributed temperature sensing has been reported to be applied for natural gas, brine, phenol, sulfur, LNG, crude oil and other mediums and allows detecting even very small leaks. Compared to the conventional intrinsic Pipeline Monitoring methods this approach has the additional advantage to be completely independent of any process conditions.

Even the periodical opening and closing of small leaks in gas pipelines due to freezing effects can be identified with modern signal analysis methods. For offshore pipelines the application of leak sound detection is reported based on a Brillouin strain measurement system.

-  Flow balance - detect leak by monitoring volumetric flow rate, pressure, and temperature
These methods base on the principle of conservation of mass. In the steady state, the mass flow \dot{M}_I entering a leak-free pipeline will balance the mass flow \dot{M}_O leaving it; any drop in mass leaving the pipeline (mass imbalance \dot{M}_I - \dot{M}_O) indicates a leak. Balancing methods measure \dot{M}_I and \dot{M}_O using flowmeters and finally compute the imbalance which is an estimate of the unknown, true leak flow. Comparing this imbalance (typically monitored over a number of periods) against a leak alarm threshold \gamma generates an alarm if this monitored imbalance. Enhanced balancing methods additionally take into account the change rate of the mass inventory of the pipeline. Names that are used for enhanced line balancing techniques are volume balance, modified volume balance, and compensated mass balance.

The effective integrity monitoring and management planning allows the operator to reduce uncertainties and risks associated with riser fatigue, corrosion build-up, hydrate plug or wax formation conditions, etc.

The subsea integrity monitoring service providers include:
  • Genesis SIG (Subsea Integrity Group)
  • Come Monday, Inc.
  • IICORR (Integrity Inspection Corrosion)
  • Fugro Structural Monitoring (FSM)
  • 2H Offshore
  • MCS
  • DeepSea Monitoring Solutions (DMS), etc. 

Introduction to Offshore Pipelines & Risers - Jaeyoung Lee


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