Anti-Slug Advanced Control for Offshore Production Platforms

Abstract

The application of advanced control techniques in offshore oil production plants is a challenge. There are many changes in operational points in time, for example, the process is affected by the natural oil well's behavior dynamics. Besides that, limited instrumentation available has to be considered when thinking in oil optimization and control. In order to improve the scenario, taylor-made advanced control modules have been developed for those units. The present article will present development, implementation and results of anti-slug control for four platforms located at Campos and Santos basins. Severe slugging flow regimes are one of the major disturbances for the operation of offshore production platforms. Slugs can occur, for example, in pipeline-riser systems that transport oil and gas mixture from the seabed to the surface and they are characterized by severe flow and pressure oscillations, with moments of high instantaneous liquid or gas flow, which is then followed by moments of almost no flow. This cyclical phenomenon can cause many problems like reduction in process efficiency, and production losses due to many unplanned shutdowns. One of the ways to minimize issues associated with slugs is to use advanced control techniques in wells. These techniques utilize the available measurements, as downhole pressure (PDG) and top side pressures, to define a choke valve position that minimize or even eliminate slug instability. The objective is to have a stable flow together with the maximum possible production rate. One great challenge for this anti-slug control is how to deal with plant nonlinearity, because the process gain of the system changes drastically at different operating conditions. So, we will discuss in this paper different control algorithms to deal with this problem. This paper will present an anti-slug advanced control, which has three subsystems: one responsible to diagnose and identify slugs, considering some pressures patterns; other to control or minimize slugs; and finally one to protect the process plant in case of severe disturbances. Finally, this paper will show and discuss some economic and safety results of the implementations of these anti-slugs advanced control systems in offshore platforms, emphasizing the benefits obtained, which are related with stability, energy efficiency, and profitability due to a important decreasing in unscheduled compressor shutdown events, as well as increasing in operational efficiency. It will be shown that operators have well accepted this system and are maintaining this controller in automatic mode in more than 80% of the operating time.