چكيده به لاتين
Abstract:
Employment of oil and gas pipes is the most common and safest method for their delivery. However, after a while, the interiors of the pipes suffer from debris, corrosion, abrasion and fractures, which in turn reduce efficiency of the pipe. Thus, pigging is performed to remove debris, localize fractures and corrosions, and inspect pipe interiors. It also cleans pipe interiors and hence increases its efficiency. Furthermore, localization of fractures and corrosions results in suppression of their progress. Nevertheless, in order for pig to perform efficiently, it needs to move in a specific speed range. If moved in a higher or lower speed than the predetermined range, pig does not perform properly and pigging operation will not be correct. Since pigs commonly move due to an upstream pressure, their speed can be affected by factors such as friction, sediments, pipe’s slope and intersections. Therefore, possible increments or decrements in pig’s speed result in incomplete (or no) pigging in some parts of the pipe. Hence, a system is needed to monitor pig’s speed to avoid aforementioned problem. Such a system should be able to –when necessary– increase or decrease pig’s speed without damaging it, which in turn yields in raising pigging efficiency. Meanwhile, due to the low viscosity of gas in gas pipes, and thus pig speed control is more important. Otherwise, the rapid changes in pig speed and acceleration not only cause several damages on the pig, but also results in problematic pigging operation.
In this thesis, first, pig’s movement in oil and gas pipes is dynamically investigated. Partial differential equations including mass, linear momentum and energy conservation equations and state equations are converted into normal differential equations using characteristic method. Then, a MATLAB code is employed solve equations for pig’s movement equations as well as equations concerning flow, in order to calculate boundary conditions in pig’s nose and tail in each time slot. Results are in good agreement with those obtained by Tan Tin Nguyen et al. on modelling and investigation of gas pipelines [9], which implies the validity of the proposed model for pig movements.
After studying control systems, pig’s speed is controlled by a by-pass aperture as a valve placed in the middle of the pig. By opening (closing) the aperture, fluid flow is allowed (prevented), so the pressure difference between pig nose and tail is changed and hence its speed is controlled. A MATLAB code then models pig movement as a mid-pass flow, the results of which are in good agreement with those by Tan Tin Nguyen on pig control is natural gas pipelines with bypass flows [12].
Finally, an experimental setup is designed and fabricated to investigate the effects of the geometry of the by-pass valve on pressure changes in pig’s nose and tail. By changing flow levels and geometry of the valve, the patterns of the pressure changes in nose and tail, and hence the operation of pig speed controller is examined.
Keywords: pig, pigging, speed control, pressure difference, bypass valve
