This paper proposes a method for detecting the vibration of subsea spanning pipelines by using a spherical detector (SD) equipped with a triaxial accelerometer. The mathematical analyses and experiments demonstrate that the acceleration modulus square (AMS) and the AMS of the ac components (AMS_AC) of the acceleration signals recorded by the SD have one characteristic component whose frequency is equal to and can be used to determine the pipeline vibration frequency. As the amplitude of this component is immune to the rotation noises of the SD, its signal-to-noise ratio is higher than that of other components whose frequencies are the mixture of the vibration and rotation frequencies. Removing the dc bias along the three sensitive axes can eliminate other characteristic frequencies to make the AMS_AC having the unique frequency to indicate the pipeline vibration. The vibration frequency can be accurately and sensitively identified by using the AMS_AC. The detection thresholds of 5-, 10-, and 15-Hz vibrations are 0.6, 0.4, and 0.2 mm with regard to the amplitude, respectively.

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Subsea pipelines often bend upward due to vertical buckling or downward due to spanning and seabed settling. Typically, harsh environments and the difficulty of accessing underwater pipelines make the inspection and monitoring of subsea pipeline bends a challenging task. This paper demonstrates a lowcost, high-efficiency, and quasi-real-time method for detecting the vertical bends of subsea pipelines by using an in-pipe spherical detector (SD) with rolling features and a low blockage risk. When the SD rolls forward inside a bent pipeline, its rolling speed will change as it moves uphill and downhill, which can be indicated by the centripetal acceleration-the DC component of the recorded rolling acceleration signals. To achieve a high bend detection performance, the mass of the SD should be distributed in a centered disc area to make the SD capable of stably rolling around one of the sensitive axes of the accelerometer, and the accelerometer should be kept as far from the rotation axis as possible. It is experimentally demonstrated that a convex/concave DC component indicates that the pipe is bent downward/upward, and the bend detection resolution can reach 1 cm for a 12 m pipeline.

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