ABSTRACT
Subsea systems used in combination with host facilities, such as floating, production storage and offloading units (FPSOs), tension leg platforms (TLPs), spars or compliant towers employ supervisory control and data acquisition (SCADA) systems whose architectures differ from SCADA systems found on conventional platforms. In these systems, programmable logic controllers (PLCs) located on the host facility, provide control of subsea valves on manifolds and satellite wells. PLCs also continuously record data from temperature and pressure sensors located on subsea wells. Since it is expensive to repair the electronic systems on the manifolds and satellite wells, redundancy is typically built into the system.
SCADA systems used in deepwater must also provide for control switching to workover vessels. Such vessels must provide control compatible with the existing deepwater multiplex controls and must ensure the safe control of the well.
During 1999, the Minerals Management Service (MMS) in the United States launched a study to assess the current state of SCADA systems used in the territorial waters of the US, with special emphasis on quantifying the reliability of these systems.
This paper describes findings from this study and provides a comparison of trends with other international deepwater SCADA systems.
INTRODUCTION
Deepwater developments have progressed from fixed structures in waters of less than 1400 feet to developments that include an array of structures (FPSO, TLP, Spar) in water depths of 4000 feet or more.
At the same time, subsea well technology has become sufficiently advanced to allow wells to be completed on the seafloor in over 5000 feet of water. Multiplex electro-hydraulic control (MUX) systems interface these subsea wells with the host platform and provide the ability to close valves, sample well pressures and temperatures, test well flow, run shut-in pressure surveys and to control well treatments.
SCADA systems have also evolved from discrete stand-alone systems, to systems integrating a multitude of input and output points through the use of PLCs and PC-based controllers. These modern SCADA systems provide the capability to continuously monitor subsea wells, poll the wells for status of operation, sample and return data on a continuous basis, and provide emergency shut-down controls.
The United States MMS recognized these trends, and the proliferation of SCADA systems being used offshore. In 1999, the MMS initiated a study to determine the state of SCADA systems used offshore and the reliability of these systems. This work identified different SCADA architectures for fixed structures, deepwater developments, pipelines and mobile drilling units. The results reported here focus on deepwater SCADA systems.
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Shari Dunn-Norman1, Kelvin.T. Erickson 2, Egemen K. Cetinkaya3 E. Keith Stanek4 and Ann Miller5
Copyright 2000, Brazilian Petroleum Institute - IBP
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