The pipeline pigging operation for emptying purposes is a common practice in the petroleum and gas transport industry. The emptying operation is employed for removal of the pipeline liquid products and substitution for an inert gas like nitrogen. This operation is necessary before pipeline maintenance or hydrostatic test procedures. The emptying operation applied for oil pipelines usually demands large volumes of nitrogen because of the pressure difference that is necessary to maintain the pig in a velocity that guarantee an efficient and safe operation. The nitrogen that is originally stored inside cryogenic vessels in liquid phase is pumped and vaporized to be injected into the pig launcher, after heating the gas. The gas injection and expansion inside the pipeline propel the pig, delivering the product that was in the pipeline at the receiver station. It is common to cut the nitrogen injection in a certain instant of the operation, before the pig reaches its destination. From then on, the expansion of the gas is able to finish the operation alone.
A dynamic simulator called DESLOCAN2 was developed by SIMDUT/PUC-Rio to simulate the pig motion during emptying operations with nitrogen in TRANSPETRO crude oil and refined products pipelines. In the operation planning phase the simulator is used to evaluate the gas mass flow rate, the inlet gas pressure and the nitrogen cut instant that can propel the pig to its destination with a minimum volume of nitrogen. The outlet liquid pressure is calculated using two simultaneous controllers: outlet flow rate based on maximum pig velocity and minimum outlet pressure that avoids slack line condition. The dynamic simulator also can be used as a forecast monitor of the pipeline pigging operation given the current inlet gas and outlet liquid conditions, allowing the visualization of actual and forecasted pipeline pressure profile, the pig velocity and position, the accumulative gas inside the pipeline and the volume of product removed.
The main objectives of this paper are: Present the mathematical modeling and considerations built in the simulator; Validate the model’s main hypothesis; Present the gained experience on building the model and planning an emptying operation of an existing pipeline: OSBAT 24.