STUDY OF POTENTIAL GALLOPING MOTIONS OF PIGGYBACK PIPELINE GEOMETRIES IN STRONG CURRENTS

Abstract

This thesis entails an experimental work that tries to investigate the effects of different diameter ratios and the angle of attack of steady flow on the piggyback cylinders arranged normal to the flow. Three different diameter ratio cases were investigated: D+0.5D, D+0.32D and D+0.25D, where the main cylinder diameter, D = 5cm. Attack angles of 0°, 30°, 60°, 90°, 120°, 150° and 180° were tested for these 3 diameter ratios giving a total of 21 cases.

The work was aimed at studying a flow induced vibration called galloping on piggyback type of pipelines commonly used on offshore installations in the oil and gas industry. The 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 the cylinders suspended horizontally on a set of springs mounted on it. Reduced velocities range from 4 to 15 depending on the equivalent diameter of the piggyback pair.

The results showed at the end of this work 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°.

The results also showed vibrations that are characteristic of galloping instabilities at 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. However, this results were limited by the reduced velocities (max=15) and the damping of the system.

Comparing the response of the different diameter ratios showed that the largest ratio always gave the largest responses for all attack angles.