The Application of Open Control Systems to Offshore Automation for Reduced Costs and Increased Flexibility

Abstract

Abstract Automation of offshore platforms and other offshore facilities have always presented unique challenges. Traditionally only the largest platforms could accommodate a DCS but the continuing evolution of Open Control Systems (OCS's) has opened the door for modem automation on nearly all offshore facilities. This paper is based upon several offshore automation projects which demonstrate the unique and valuable benefits of OCS's applied offshore. Actual project details will be presented in as much detail as permitted by the end users. Offshore automation options will be summarized along with the features and benefits of various options. The use of conventional analog controls, PLC's, DCS's, and OCS's will be considered. Design guidelines for automation of new offshore facilities as well as retrofits of existing facilities will be included. The material to be presented is highly significant in that it covers the application of leading edge automation technologies for economic benefit in offshore applications. Introduction Prior to the introduction of microprocessor based control systems, plant automation was, for the most part, limited to Single Input - Single Output (SISO) continuous control and electromechanical relay based discrete control. Operators worked with such control systems at a panel board filled with dials, gages, strip charts, lights, and switches. Using such systems, the operators job was to keep the plant operating safely. The introduction, in the late 1970's, of microprocessor based, or "Distributed Control Systems (DCS)," opened the door for the operator to not only focus on safe operation but, really for the first time, to also accept some additional responsibility for the profitability of the process. The first DCS's emulated their panelboard predecessors in look and feel but did so utilizing CRT displays. These early systems were limited to continuous control functions. They did significantly enhance the practical ability to implement more sophisticated control strategies whose purpose was to improve the process economically. Other features of early DCS's which were of benefit included their ability to interface with a supervisory computer and to automatically produce reports. The application of early DCS's was essentially limited to large, continuous processes such as those found in the petroleum refining industry. With these early systems, it was rarely economically feasible to implement DCS control on any process, including offshore facilities, where continuous I10 quantities were less than approximately 300 inputs and 100 outputs. Offshore facilities presented additional beamers to DCS control due to the often harsh environments and impracticality of "computer rooms" offshore. During the 1 980's' numerous microprocessor based systems came onto the market; each one an attempt to overcome one or more of the limitations presented by other systems. Programmable Logic Controllers (PLC's) came into common use in discrete control applications. Relative to electromechanical relays, PLC's had many advantages in terms of lower engineering and maintenance costs. Personal Computer (PC) based systems were introduced to address the smaller 110 count market. PC reliability was significantly less than that of DCS's which was a significant problem in many applications. Hybrid systems which combined DCS's, PLC's, and I or PC's were also offered in an attempt to offer the advantages of each type of these control systems.