Subsea leak detection

Neptune Oceanographics Ltd is a global leader in the field of detection of leaks in subsea pipelines, risers and control systems. The company places strong emphasis on R&D and is constantly striving to develop systems that are more efficient in terms of cost and detection success rate and simultaneously reduce the adverse effect on the environment.

Several methods are available for subsea leak detection but generally they use fluorescence or acoustics. In some instances temperature differential and direct hydrocarbon measurements are used. 
Traditionally, leaks have been found using fluorescent dyes detected by ‘black light’ (unfiltered ultraviolet [UV] light) but the major problem with this is that the dye concentration has to be high (see Figure 1) and visibility must be good. Another problem with un-tuned ‘black light’ is that some marine organisms fluoresce thus causing the observer to see a ‘fog’ that masks the location of the leak. 

Fluorescence

Deploying submersible ‘tuned’ fluorometers that send data up to the attendant vessel providing a real time visual display has largely solved these problems. These submersible fluorometers are very sensitive and will detect dye at concentrations so low as to be invisible to the naked eye or underwater camera. 
Neptune Oceanographics has recently and successfully introduced their remote detecting ‘Long Ranger’ family of fluorometers. These ‘Long Ranger’ sensors have a forward facing ‘tuned’ light source (like a torch) producing a beam of excitation light spreading to about 40cm diameter at 3m. Fluoresced light signals generated in the leaked fluid are detected by the sensor and converted for transmission to the surface via the ROV umbilical for display and recording.

By mounting the sensor on the ROV manip and because the sensors have high spatial coverage, quick and easy scanning for leaks is achieved without needing to consider tidal flow direction to ‘capture’ dye as is necessary with most fluorometers.
Other major advantages of the ‘Long Ranger’ include the detection of leaks from risers while working at safe distances and detecting leaks within confined structures where ROV access is not possible or too hazardous.

Fig 1 – Example of control fluid leak with a very high concentration of Fluorescein and excellent visibility

Where subsea control systems or hydrostatic pressure testing are concerned, fluorescent dye is normally added as a component solely for the purpose of leak detection, however, Fluorescein, the most commonly used dye, is being phased out as it no longer complies with the latest legislation for discharges at sea. Other leak tracers such as UV clear dye, Roemex’s 9022 red dye, Rhodamine, etc., have been tested to demonstrate their compliance with the latest legislation and are now widely used.

Fig 2 – Long Ranger Leak Detector

During 2008, Neptune Oceanographics will phase in their latest version of the ‘Long Ranger’ family of leak detection sensors that have the potential to detect fluorescent dyes, crude oil, hydrocarbon based control fluids, etc. at distances of up to 20m from the leak.
Fig 3 – Long Ranger in operation

Long Range Sensor. 

Two blue excitation light beams are visible.

Fluoresced light produced by the dye shows as a green cloud at the leak location.
Tracer dye is Fluorescein

Leak detection system output is graphically displayed on the onboard PC.

Fig 4 – Screen display

The on-board PC displays data a colour time series plot in real time allowing the operator to easily see changes in signal that indicate the presence of a leak. The software also allows the user to set alarm levels.

Hydrophones

These are effectively 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 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. 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 Group to develop an acoustic leak detection sensor to complement the existing optical detection system. The sensor, which may be diver-held or ROV mounted 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. The detected acoustic signals are 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.

Direct Hydrocarbon Leak Detection

Hydrocarbons in oil form such as crude oil can be detected using Neptune’s Long Ranger leak detector, however, a different method is required for gas. A direct reading hydrocarbon sensor has been added to the range of Neptune Oceanographics leak detection systems. This sensor is essentially a methane detector but it is equally a general hydrocarbon detector that will respond to most hydrocarbons, however, the potential for oil contamination on the sensors membrane make it less suitable for oil detection but good for gas detection. The very high sensitivity of the sensor also makes it ideal for the detection of gas seepage from the seabed. By careful investigation around an area of leakage from the seabed, or any other source (pipelines, etc.), an estimate of the quantity of leaking fluid can be made.

Fig 5 – Refraction patterns caused by leaking fluid

Differential temperature sensors

Under some circumstances such as with water injection flow lines and high pressure control systems, leaks can be identified using the temperature difference between the surrounding seawater and the water leaking from the flowline. The flowline water temperature is normally above ambient seawater temperature due to the high pressure pumping required. Neptune has developed a differential temperature technique that can detect very small temperature changes. The sensors comprise fast, high precision thermistors connected through the Neptune leak detection system to the ROV umbilical for onboard display and recording.
The temperature probes are designed for use with the 2-channel leak detection system and can be used simultaneously with a fluorometer or acoustic sensor connected to the second channel. Data are displayed graphically and recorded on the onboard PC.

Other news

The company has teamed with Aberdeen based Dynamic Positioning Services (DPS) who provide full workshop facilities, technical expertise and experienced offshore engineers to support Neptune’s global leak detection operations.

Contact information

Chris Teal
Neptune Oceanographics Ltd
Sapharey House, Sturt Road
Charlbury, Oxon OX7 3SX
United Kingdom
Tel:  +44 (0)8453 707177
Direct tel: +44 (0)1608 819177
Fax: +44 (0)8704 581979
Email: info@neptune.gb.com
http://www.neptune.gb.com

Scott Johnstone
DPS
Unit 2  
Denmore Place, Bridge of Don
Aberdeen, AB23 8JS, United Kingdom
Tel: +44 (0)1224 226850
Fax: +44 (0)1224 226851
Email: sjohnstone@dynamic-positioning.co.uk
http://www.dynamic-positioning.co.uk