Electronic Emergency Shut Down System For Well Testing
Authors: Godfrey Craig , Holloman Rick , Beharie Colin , Sims Gary 
Published: October 2008

Awareness of safety requirements during well testing from semi-submersibles has increased considerably in recent years. As a result, electronic equipment and controllers are being employed to provide immediate detection of unsafe conditions so that performance of emergency shut down (ESD) systems can be improved. These systems utilize modern data acquisition techniques, which use the data for both safety purposes and permanent record keeping.

This paper will discuss a portable ESD system that uses a programmable logic controller (PLC) to monitor and control the safety systems by acquiring data and maintaining safety requirements based on that data. The PLC communicates with a personal computer running supervisory control and data acquisition (SCADA) software. The SCADA system discussed is designed to make operator input easy and intuitive during well testing. The simplicity of the user inputsallows anyone that understands the process to monitor and control the operation without being a computer programmer. The SCADA system is graphically based, logging data so that reports can begenerated, alarms can be logged, and data can be plotted.

The programmable logic controller is responsible for the primary control functions. It is pre-programmed and controls the basic process while maintaining safety. The SCADA system can have input into the process, but will not override safety logic. The SCADA system is used to monitor selected sensors and log events related to those sensors. [t interfaces with the primary controllers by sending set points or calculated values when appropriate. The SCADA software is the interface between the operator and the operation, and as such, is supervisory in nature sinceit is not solely responsible for the primary control functions.

The ESD System has the capability to monitor and control virtually any end device. Typically, there are analog inputs, digital inputs, pulse inputs, digital outputs, analog outputs, and smart devices used in well testing. All of these devices are constantly monitored, and the information can be used to determine operational safety. If unsafe conditions are detected, the controller can automatically initiate either a warning to the operator or a shut-down sequence, depending on the severity of the alarm condition.

If any critical component in the system fails, causingan invalid input to the system, the system is designed to shutdown in an orderly sequence. If power is lostto the control system, devices shutdown to a "known" condition.

INTR0DUCTI0N

The primary function of a process safety system is 1) to detect undesirable situations that can represent asafety hazard and 2) to take the corrective action(s) necessary to regain a safe operational state. Automatic safety control must be used when dangerous conditions cannot be counteracted in a timely manner by manual intervention; e.g., if the process is distant from personnel on duty. Theseautomatic safety systems are designed to keep process variables within the limits specified for the machinery (the process) during its normal working conditions and to monitor physical changes in the process that result from wear, fouling of pipes, filters and heat exchangers, characteristics of control valves.