Lubricated friction on pipeline pigs

Abstract

Pigs are devices that are transported from time to time through a pipeline, mainly to either clean the pipeline or to inspect the integrity of the pipeline wall. Conventional models for the friction between the pig and the pipe wall, as used in the oil and gas industry, are often criticized for their inaccuracies. To obtain a more accurate model, this thesis investigates the detailed physics behind the frictional forces. The effect of the sliding conditions on the wall-normal force and on the friction force was investigated by conducting 1D lubricated sliding experiments for a rubber lip moving along a metal surface and by performing model simulations, including a parameter sensitivity study. The experiments were conducted for dry water, dodecane and castor oil under varying sliding velocities and loads. The loads were measured with a dual axis transducer and the lubrication film was observed during the experiments with a microscope set-up. The experimental results consist of friction forces for various configurations, which show that the frictional behaviour depends strongly on the lubrication regime in which the rubber lip is operating. The friction coefficients for the boundary lubrication regime and the mixed lubrication regime were measured and compared with results from the model. Among the tested fluids, castor oil showed a non-proportional decrease in friction force when the sliding velocity was increased. This decrease in friction force was interpreted as a transition from the boundary lubrication regime to the mixed lubrication regime. The other tested fluids remained in the boundary lubrication regime in the experimental runs. A validation study was performed by fitting the friction model to the experimental results. Only the dry friction coefficients are in good agreement with the model. Both the water and dodecane experiments show a slight increase for the friction coefficient with increasing lubrication factor and are in mediocre agreement with the friction model. The castor oil experiments are in the mixed lubrication regime of the Stribeck curve and are in reasonable agreement with the model. The results can be linked to the oil and gas industry as pigs can operate in different regimes of the Stribeck curve. Stalled pigs can be the result of transitions from the mixed lubrication regime to the boundary lubrication regime. This can happen when a pig is momentarily slowed down in the pipeline. Recommendations for future experimental work are: performing experiments at higher sliding velocities, using an emulsion as lubricant and finding suitable methods to calibrate models that have a range of input parameters. This work was carried out by the author at the Shell Technology Centre in Amsterdam. The supervision by the Shell staff and the support obtained in carrying out the experiments in the Shell lab are greatly acknowledged.