The need for subsea leak detection

Globally, the environmental aspects of subsea pipeline systems are of major concern and are increasingly so as regulatory authorities around the world become less tolerant of the potential release of polluting material into the marine environment.

These concerns are of particular relevance to subsea pipeline installations and, in this respect, the ability to detect and locate any leakage of gas or oil to the surrounding waters is of paramount importance. Historically, two main methods of subsea leak detection have been used where obvious visual signs of leaks such as bubbles, large clouds, etc. are either not present or have not been visually located. In general, the main methods used are in situ fluorometric measurement and acoustic listening. Neptune Oceanographics is a leader in the field of detection of leaks in subsea pipelines, risers and control systems and, in collaboration with Aquatec Group, has made significant technical advances in the techniques mentioned above. In addition, the company is constantly striving to develop systems that are more efficient in terms of cost, detection success rate and the effect on the environment. Two such new developments are described later in this article.

Fluorometry
To date, 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 (see Figure 1) to allow visual observation. Also, general visibility must be good.

Deploying submersible fluorometers that send data up to the attendant vessel providing a real time visual display has, to a large extent, solved these 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. The commonly used dyes, such as fluorescein, are becoming less favoured as a means of detection, as they cannot meet the zero discharge requirements for environmentally harmful pollutants now being sought.

Increasingly, regulatory authorities around the world are asking the oil and gas industry to find ways of detecting leaks that do not require the introduction of these additional potential pollutants. Where subsea control systems are concerned, the control fluids normally have fluorescein dye or similar, as a component solely for the purpose of leak detection. As with all control systems, leakages through seals, flanges, etc., do occur and, therefore, means of detection and location are essential. Whatever means of observation is used to detect fluorescence, dye is still required and it will, therefore, always be a potentially polluting method.

In some circumstances, fluorescence can be used directly on the pipeline load as some of these hydrocarbons are known to fluoresce in the UV part of the spectra and thus detection can be made using suitably tuned fluorometers. However, other fluids and hydrocarbons do not fluoresce or the level of fluorescence is too low for practical use. Additionally, fluorescent techniques have several drawbacks; the seriousness of which will depend on site conditions:

• Until now, spatial coverage has normally been very small because the sensor has to be in the leak plume, thus making detection rather hit and miss if great care is not taken to observe position in relation to pipeline and tidal flow, etc. However, Neptune Oceanographics have recently developed a ‘long range’ fluorometer that can scan remotely at distances of several metres from the leak using a ‘cone’ of light. This greatly increases spatial coverage as the sensor no longer has to be in the leak fluid path thus speeding up the process of searching for a leak and considerably increasing leak detection rate.
• Being optical, the sensitivity of a fluorometer is subject to reduction and even total loss due to turbidity, i.e. suspended material in the seawater such as sediment reduces the transmission of light, therefore, in high turbidity situations, detection ability can be seriously affected.
• Dye is required to be added to pipeline systems thus incurring an additional environmental downside plus consequential operational and financial costs.

Hydrophones
The other method used in the past, and to some extent now, is the use of hydrophones. Effectively, these are 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. These signals are additional to any leak generated sound. It is has been difficult, therefore, to differentiate an acoustic leak signal from these other sources and, for this reason, it has not been frequently used. However, modern data handling and spectral analysis techniques have improved the method sufficiently such that in the right conditions the method can be highly successful.

Neptune Oceanographics have collaborated with the Aquatec to develop an acoustic leak detection module to complement the existing optical detection system. The module, which may be diver-held or mounted in a manipulator, incorporates a directional hydrophone. The system includes noise filters to remove the effects of ambient noise, including mechanical noise from an ROV or support vessel. It has been tuned to cover the range of frequencies known, through experimental verification, to be emitted by high pressure leaks through small apertures.

The detected acoustic signals in the frequency band of interest are amplified, digitised, and sent to the leak detection system data logger, where acoustic intensity may be displayed either alongside fluorometer readings in dual capacity mode or as a single channel. In common with the fluorometer system, the sensitivity is controlled by the data logger to obtain the maximum dynamic range.

Requirements for leak detection
The requirement to conduct subsea leak detection falls mainly into the following categories:

•Commissioning of new pipelines and subsea installations;
•Subsea structures;
•Risers;
•Subsea control systems;
•Water injection systems;
•Maintenance and repair of existing systems.

New methods in use and under development
In an effort to find new leak detection methods that improve detection efficiency and also eliminate the need to introduce additional potential pollutants to the pipeline system such as dye, Neptune Oceanographics in parnership with Aquatec has developed, or are in the process of developing, several new systems.

One such system is SNIFFIT, a generic name for an in situ modular leak detection system. The system has been developed to allow different sensors, or a mix of sensors, to be plugged in depending on the leak detection requirement. These sensors can be acoustic, fluorometric, electro-chemical (hydrocarbons) or differential temperature. The hydrocarbon sensor (METS) presently used was developed and built by Capsum and does not rely on optics and is thus unaffected by turbidity.

A second system, SONIC, is under development and uses active acoustic backscatter to detect the presence of a leak. This system will detect any fluid, whether it is water, oil, gas or a control fluid, leaking into the surrounding sea. The method uses high-frequency acoustic backscatter by sending a signal from a scanning fan beam sonar module in the direction of the pipeline or structure and receiving backscattered signals returned from scatterers within the water. Acoustic discontinuities and Doppler shift caused by turbulence, changes in the water density, and scatterer properties, as the acoustic beam passes through leaked fluid will be detected and displayed on an on-board PC. The acoustic imaging provides a three dimensional visualisation of the leak position. The advantages over existing methods are:

•The proposed system will detect all leaks with or without the presence of dye.
•Sensing can be made several metres from the pipe.
•High spatial coverage from scanning fan beams allowing detection in ‘one pass’.
•The method has nil environmental downside as no dyes are released into the environment.
•Cost of dye and introduction to pipeline system is eliminated.
•The sensors are non-optical, therefore, they are unaffected by turbidity. All optical systems, including lasers, are affected by turbidity.

It is considered that no one system or method will be appropriate for all leak detection situations and, therefore, it is essential that whatever method is selected for inspection of leaks, it must be chosen for the right technical and environmental reasons.