Integrated Process Control & Shutdown Systems

Abstract

This paper outlines how the separate requirements of process control and plant shutdown can be met using PLCs in redundant configuration to achieve high reliability and availability. System topologies are discussed and an example of an integrated system recently installed on BP's Forties Alpha oil platform is examined.

Introduction

What is an integrated control and plant shutdown system? By "integrated", we mean that the process control system and the shutdown system are supplied by a single company/integrator, based upon the same PLC equipment range with intercommunication between the subsystems and a common operator interface. This approach is being called for increasingly by oil companies around the world.

Prior to the concept of integrated systems, separate contracts for the process control and shutdown elements, involving separate suppliers and technologies could potentially give rise to contractual difficulties and problems with technical interfaces. Also, separate forms of human machine interface (HMIs) could be a contributor to the cause of accidents, due to incorrect interpretation of the state of the plant during process upset conditions.

Another major driver for integrated systems is the cost saving afforded, due to the need to administer only one contract for systems supply, as opposed to two or possibly three associated with earlier approaches. This is in line with the CRINE* initiative of the oil companies operating in the UK Continental Shelf, CRINE being a new culture introduced in the 90's to drive out unnecessary cost. *Superseded by LOGIC

Topology of the Integrated System

Refer to diagram. This illustrates the integration of separate PLC subsystems for process control, Fire & Gas, shutdown and mechanical packages, under the umbrella of a common operator interface, for a typical oil industry project.

PLC System Architectures

A variety of PLC subsystem architectures can be employed dependent upon the reliability and availability requirements:

Single PLC with single I/O

Dual channel PLC system

Redundant processors with single I/O

Redundant processors with dual I/O

Redundant processors with triple I/O

Triple redundant PLC system with triple I/O and 2oo3 voting

Guidelines and Standards

It is necessary to observe the UKOOA guidelines for instrument-based protective systems and the provisions of international standard IEC 61508, e.g. in respect of system integrity levels (SIL), probability of failure on demand (PFD), etc. for all protection and safety functions.

Integrated Control and Shutdown System for BP's Forties Alpha Oil Production Platform

This project has been chosen as a case study, to illustrate the integrated approach.

The flexible characteristics of Rockwell's range of PLCs allows an integrated solution to be achieved to the problems of upgrades on the older North Sea oil platforms.

The installation of these upgrades extends the platform production life by the introduction of:

1. Reduced unplanned shutdowns.

2. Improved maintainability through better information gathering.

It will also reduce operational costs due to the improved production profile and the reduced input from the operations personnel.

The example chosen is that of the BP's Forties Alpha Platform where the NGL plant had become the key area of the operation due to its pivotal role in provision of high pressure gas for the Deep Gas Lift system installed there in the early 1990's. Loss of the NGL means loss of the platform's production.

The Pre-Upgrade Situation

The Forties Alpha NGL control scheme, prior to upgrade, was based on pneumatic controllers and relay based sequencing and production shutdown scheme.

This scheme, due to its age was unreliable and was the cause of numerous unplanned shutdowns. By reduction of unplanned shutdowns the production profile of the plant would be much enhanced producing benefits of greater production returns and reduced manpower demands.

Constraints & Objectives

The following constraints were placed on the design:

1. Shutdown time (oil to oil) was limited to 14 days for completion of installation and commissioning.

2. Physical space was limited

3. Maintenance period to be 5 years

4. Able to be expanded for future upgrades

The Solution

An integrated system was developed based on two independent PLC subsystems and a common PC - based operator interface:

1. Process control PLC subsystem

2. Plant shutdown PLC subsystem

Each PLC had a dual processor hot-backup configuration with BCM & BEM modules for increased fault tolerance. The process control PLC subsystem was configured for 40 AI, 17 AO and 20 DI. All 17 PID control algorithms were configured with full alarm generation for plant parameter deviation monitoring.

The plant shutdown PLC subsystem handled 17 AI, 80 DI and 58 DO. The analogue inputs were configured for trip functions. This gave increased reliability through the use of out-of-range monitoring for fault detection, so removing unseen failures possible with digital inputs, such as pressure switches.

It should be noted that no ESD (Safety Integrity Level 3) functions are involved, these being performed by a hard wired relay system.

The logic allowed for complete sequencing of pump valve arrangements and for the control of the refrigeration and the glycol gas dryer / recovery plant.

The space constraint was answered by only having the client PC in the NGL control room plus one server. A further server was installed in the main central control room.

The PLC's were mounted in an adjacent equipment room.

The Result

The improved control on the plant has given the operator more confidence in the performance of the plant. When shutdowns do occur the information supplied by the SCADA based operator interface allows for speedy recovery. The unplanned shutdowns are running well below 10% of pre-upgrade factors. BP have requested more extensive upgrades to the Forties Delta and enhancements to the Forties Alpha system, which confirms the success of the scheme. The upgrade hook up and commissioning was able to be completed in the shutdown period, the PLC installation and pre-testing having been completed offshore prior to the shutdown.

The Conclusion

The combination of the flexible PLC based systems with a PC based operator interface gives a flexible and efficient solution to the upgrade problem. Production losses which may have been expected in a major upgrade were avoided by careful use of time and technology. This makes this approach more attractive and cost efficient.