Hydrotest on offshore pipeline

Leak detection equipment adapted for pipeline hydrotests, rehab projects

Pipeline leaks involve transition of the fluid from the internal pressure to the lower external pressure. This generates an acoustic signal, due to the turbulence and sudden expansion of the fluid mass.

Co.L.Mar's Acoustic Leak Detector (ALD) technology is designed to acquire and process the acoustic data and to extract the leakage from the ambient noise. The system's main components are an underwater acoustic sensor for acquiring the data along the pipeline; a transmission line to relay data to the surface vessel; a reception unit; and PC-based, proprietary software that evaluates in real time the signal acquired and its development along the pipeline track, analyzing the data from statistical, energetic, and spectral viewpoints.

ALD sensors in towed version.

The signal generated by a leak is detectable mainly as ultrasound, which the ALD receiver converts to an audible lower frequency. Different sensors are used depending on the operational mode:

• Towed fish, in which the sensor is towed along the pipeline track by a vessel at speeds of up to 6 knots, typically in water depths of up to 100 m (328 ft). This technique is suited to line inspections, with optional use of a USBL system for greater positioning accuracy
• Diver-manipulated (ALD-DIVER), in which the diver drives the system around the flange to be inspected, with the data sent via soft cable to the surface receiver
• Vertical deployment, suitable for line and flange inspections. The sensor is lowered from the vessel's side and kept vertical by means of a clump weight
• ROV-deployed, in which the sensor is installed on the vehicle, with the latter tracking the pipeline at a speed of around 0.5-1 knots.

Early this year, BJ Services contracted Co.L.Mar to support a pre-commissioning program for two newly installed, 48-in [1.2-m] gas pipelines in the Baltic Sea. ALD equipment and personnel were on standby during hydrotesting of the first of the 1,200-km (745-mi) lines, the equipment being supplied in both ROV and towed fish versions.

"There is an increasing need for pipeline contractors to have contingency services in case of leakage occurring during hydrotests," said Co.L.Mar Managing Director Luigi Barbagelata. "The client will not accept a pipeline if there are any leaks."

Pipeline inspection with ALD installed on an ROV.

For this project, Barbagelata noted, "the client asked us to adapt our towed fish system to work in water depths of up to 250 m [820 ft], which involved design and manufacture of a new ‘fish sensor' and towing system. Data acquired by the sensor are modified and transmitted by means of a single armored coaxial cable, also used for power injection."

During April and May, Co.L.Mar undertook a similar job in the Kazakh sector of the Caspian Sea. ALD equipment was sent offshore while personnel were on standby at the company's headquarters in La Spezia, northern Italy, after securing visas to avoid delays in any necessary response.

Acoustic leak detection is also used for pipeline rehabilitation projects.

"With this kind of inspection," Barbagelata explained, "it is common to encounter large leaks and low pressures, and in these cases chemical or optical sensors detecting particles in seawater can give positive results. For that reason, the ROV and diver-deployed ALD versions now feature a channel that can host an additional sensor – either a dye detector (fluorescin) or a hydrocarbon detector. Data from the extra sensor are multiplexed with acoustic information and sent to the surface via the same transmission line. In this way, it is possible to run a chemical and optical inspection of the pipeline simultaneously."

Recently, Co.L.Mar took delivery of an underwater leak simulator which simulates liquid and gas leaks across a range of pressures (2-200 bar, or 29-2,901 psi) and leak sizes.

"The on-site conditions can be replicated and the acoustic signal generated can be measured and used as a precise target reference," said Barbagelata. "During trials with the simulator, a signal is acquired by calibrated hydrophones and the data are processed using a proprietary software, allowing definition of each leak's source level and spectral distribution. This information allows us to better set up the equipment prior to an inspection with knowledge of the pressure and flow conditions."

For over a year, Co.L.Mar has also been working on a new monitoring system for leak detection on subsea structures. This would involve permanently installed sensors on critical equipment such as christmas trees, manifolds, subsea valves, and flanges.

"We are working on two types of sensors – one short range and omni-directional, which basically could detect a leak within a few meters of the subsea structure," Barbagelata noted. "The other would be bigger, and with a much larger operating range – perhaps 50 m [164 ft] – which would in addition give information on the direction of a leak. The main challenge is ensuring long-term reliability, because maintenance costs for subsea equipment are prohibitive."

Reference: http://www.offshore-mag.com/articles/print/volume-71/issue-11/italy-supplement/leak-detection-equipment-adapted-for-pipeline-hydrotests-rehab-projects.html