Piggyback configurations of pipelines, such as a Direct Electrical Heating (DEH) cable mounted on production flowlines, are becoming a common occurrence in the offshore oil and gas industry and they have been observed to excite into a type of flow-induced vibration called galloping in the presence of strong currents at free span locations.

This work was aimed at studying potential flow induced galloping vibrations of piggyback type of pipelines commonly used on offshore installations in the oil and gas industry. Tests were carried out in a 12m long, 0.7m wide and 1.2m deep current flume tank located at the NTNU/SINTEF Hydrodynamic Laboratory in Trondheim, Norway. The tank has a test rig with cylinders suspended horizontally on a set of springs mounted on it. Reduced velocities were ranging from 4 to 15, depending on the equivalent diameter of the piggyback pair.

In this experimental work, the effects of different diameter ratios and the angle of attack of the flow on the cylinders arranged normal to the flow were investigated. Three different diameter ratio cases were investigated: D+0.5D, D+0.32D and D+0.25D. Attack angles 0°, 30°, 60°, 90°, 120°, 150° and 180° were tested for these 3 diameter ratios giving a total of 21 test cases.

The results obtained show that, for all cases of diameters ratios, high response amplitude ratios, as high as 1.7, can occur at reduced velocities less than 10 when the angle of attack is at 90°. It was also observed that vibrations that are characteristic of galloping instabilities occurred at an attack angle of 180° for the D+0.5D and the D+0.32D configurations. For the D+0.25D case, the response amplitudes were similar to a VIV situation. Comparing the response of the different diameter ratios show that the largest pipeline to piggyback ratio gives the largest responses for all attack angles.

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