Subsea pipeline free span would pose a threat to its safety. Most of the existing detecting technologies for free span adopt outer pipeline detecting methods whose detecting results are constantly affected by complex submarine environment. Existing inner pipeline detecting technologies cannot detect free span of subsea pipeline. To avoid the affection of complex submarine environment outside pipeline on the detecting results, an inner pipeline detecting method is proposed in this paper. Two evaluation indicators are used to identify the free span. Experiments are carried out to verify the applicability of the method. Experimental results show that free-spanning segment can be detected by this method.


Subsea pipeline plays an important role in transporting oil and gas. The free span of subsea pipeline would pose a threat to its safety. When free span length exceeds a certain value, subsea pipeline would be more vulnerable in submarine environment. And it maybe results in pipeline leakage and other pipeline failures. So, detection of the subsea pipeline free span is necessary.

Existing pipeline free span detecting technologies mainly include underwater diver detection, sensing and monitoring, side-scan sonar, multi-beam, sub-bottom profiler and underwater vehicle detection. In underwater diver detection technology, diver detects the free span of subsea pipeline with measuring tools. But it can only detect the freespanning segment in shallow water. The sensing and monitoring technology can achieve real-time monitoring of subsea pipeline surrounding condition. Some parameters, such as the temperature of subsea pipeline surrounding (Zhao, Li, Ba and Ou, 2012; Zhao, Li, Song, Zhu and Du, 2013) and the vibration of subsea pipeline (Peng, Hao and Li, 2012; Peng, Hao, Li and Fan, 2013), are monitored and analyzed. However, the laying cost of fiber along subsea pipeline is high and application of this technology is also constrained by complex subsea environment. The side scan sonar technology (Gauer, Mcfadzean and Reid, 1999) detects the free span of subsea pipeline through transmitting sonar to the seabed and analysis of the sonar images received by the towed fish. The multi-beam (Lubniewski and Chybicki, 2008) is a kind of acoustic detecting technology. The acoustic emission transducer array emits a wide coverage acoustic wave to the seabed. The reflected acoustic wave is received by the acoustic receiving transducer array. The seabed topography of pipeline surrounding can be described through analysis of the emission and reflected acoustic waves. The free span can be therefore detected by seabed topography. The sub-bottom profiler (Wang, Tian, Wang, Zhao and Yang, 2007) is also a kind of acoustic detecting technology. The control signal is converted to the acoustic pulses with different frequencies by the acoustic transducer. The acoustic wave would be reflected at different interface of acoustic impedance. The acoustic characteristics of strata can be obtained through analysis of the reflected acoustic waves. The free span of subsea pipeline can be therefore detected. However, side-scan sonar, multi-beam and subbottom profiler are all geophysical investigation technologies. The main disadvantage of these technologies is that the surrounding condition of subsea pipeline can be just qualitatively investigated and it's hard to get the exact free span length. And their detecting costs are high. The underwater robot detection (Narimani, Nazem and Loueipour, 2009) operates remote operated vehicle to detect the free span of subsea pipeline. The condition of subsea pipeline and its surrounding can be videoed and recorded. However, its detection cost and efficiency limit its popularization and application.

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