In this study we investigate the development of a speed controlled pig in a low pressure pipeline. This is known to be a challenge due to the compressibility of the gas which can induce velocity surges of the pig. In order to reduce these velocity surges, a speed controlled pig with active by-pass control has been designed, fabricated and subsequently tested on a laboratory scale. The experimental setup consists of a 52 mm diameter pipe with a length of 62 meter. The working fluid is air. The speed controlled pig has an adjustable by-pass valve which can provide the right amount of by-pass to regulate the by-passing fluid force. First test runs show that the velocity surges can indeed be reduced, which demonstrate the feasibility of realizing fully autonomous control for bypass pigs in low pressure pipelines.
Pipeline maintenance in the oil and gas industry is usually done with a so-called ‘pig’, which is a cylindrical or sometimes spherical device travelling through the pipeline while being propelled by the production fluids [8, 10]. Such pipelines can transport oil/water, dry gas, or multiphase flow such as gas/condensate/water.
When operating under low pressure conditions, while the flow is gas-dominated, the compressibility of the gas may lead to an unsteady, oscillatory motion of the pig through the pipeline. This is because compressed gas pockets may build up behind the pig when it is moving slower due to for example irregularities in the pipe diameter. When the pressure build-up in such a pocket is sufficiently high, it is able to catapult the pig, resulting in high pig velocity excursions. This can lead to an unsafe and inefficient pigging operation. It can even result in a so-called ‘stick-slip motion’, where the pig slows down completely after a period of high velocity. Apart from possible damage that can occur to the pipe or the pig, velocity excursions may have an adverse effect to the quality of the inspection data that are collected during an intelligent pigging run [9, 11]. It is therefore desired that the velocity of the pig is controlled in low pressure gas-dominated pipelines.