Simulation and optimisation of the separation process in offshore oil and gas platforms.

Abstract

Hydrocarbon separation in offshore oil and gas platforms is the process that transforms extracted crude oil into transportable oil and gas. Temperatures and pressures of the separation system can be adjusted to modify the separation of the hydrocarbons. Discovery of the optimal settings of the separation system requires the use of simulation and optimisation techniques which are able to identify high quality solutions from a large and complex decision space. The main focus of this research is to find such approach for simulation and optimisation in offshore oil platforms and suggest directions for future research. In the first part of this thesis, we provide a novel approach for simulation of separation systems based on the flash algorithm. The flash algorithm is an integral part of many reservoir and chemical simulations. The development of a simple, accurate and efficient such algorithm is highly desirable in the oil and gas industry. However, solution of such calculation using the Peng-Robinson equation of state is a complex problem due to the iterative nature of the algorithm and various uncertainties. Our analysis shows that the sources of these uncertainties are poor estimation of initial K-factors, incorrect and/or slow solution of the cubic equation of state and the use of invalid compressibility factors in the approach of the solution. This work presents an improved flash algorithm for real multicomponent hydrocarbon mixtures that can include pure and pseudo components. All computational steps required in order to avoid the various uncertainties are described. Algorithm accuracy was tested and validated by comparing the vapour-feed ratios of different real world hydrocarbon mixtures calculated using both the proposed algorithm and the AspenTech HYSYS tool. In the second part of the thesis, we formulate an optimisation problem which aims at maximising the profitability of the system by tuning temperatures and pressures in the separation system. The profit is affected by the quality of the separation (market value of hydrocarbons recovered in oil and gas) and the operating costs of the separation process. Our formulation takes into account all the physical constraints of such a system in order ot make it as realistic as possible. Finally, we apply differential evolution to three real-world problem instances. We see that optimising the settings of the separation process in an offshore oil and gas platform can save between 10,000 and 100,000 USD per day for operators.