Raising the safety standard

The BP gas reception and processing terminal at Bacton on the Norfolk coast was one of the first facilities to begin handling North Sea gas when this valuable energy resource was first exploited in the late 1960s. While the processes involved in purifying the gas, ready for forwarding to the national pipeline operator Transco, have changed little, environmental and safety standards have moved on.

A major refit of the site has focused on improving its performance in these two areas, and a significant part of the upgrade was the installation of a new plant control system. BP Bacton is a 24/7 operation, with only a short maintenance shutdown during the off-peak summer months, and the control system had to be installed and commissioned in a carefully planned and implemented programme.

The terminal receives gas by pipeline from several production platforms in the North Sea, processing it in up to five separate streams before delivering it literally across the road to the Transco distribution centre. Bacton draws gas from the platforms at the rate required to maintain a pressure of 1000psi at Transco's inlet manifold.

The gas comes ashore mixed with liquid hydrocarbons and a solution of water and MEG, monoethylene glycol, added by the production platforms to absorb water and prevent it freezing.

The process control system has to manage the plant to produce the right amount of gas to meet Transco's demand safely and cost effectively, says Ian Pinkney, BP's control systems technical authority.

While BP and Transco have an annual contract for target gas production, demand can fluctuate in the short term depending on consumption. The control system has to be able to respond quickly and effectively, and alert the operators to any alarm conditions so action can be taken before the plant has to be shut down. If Bacton shuts down for more than a few hours, the platforms have to stop producing gas, and Transco will turn to another supplier to meet demand.

An Allen-Bradley ProcessLogix plant control system was chosen to meet all these criteria and the system was developed jointly by Rockwell Automation's engineering team at Milton Keynes and Servelec, a Sheffield systems integrator.

The new system provides the shift manager in the Master Control Room (MCR) with far more information on what is happening on site, as well as remote control of key plant, machinery and valves, so many alarm conditions can be corrected without manual intervention.

'Before, most of the controls were in the local plant rooms, and the control room just received alarms telling us that the plant had shut down but not why,' says shift manager John Clay. 'Now we get a lot more information, and can put right a lot of problems from the control room, which is a lot safer than sending a man down into the unknown.'

In addition, reliability has been improved by integrating the plant management at one central point. 'Centralised control means we can respond to problems much faster than when the operators had to go around the plant controlling it manually, so reliability is better,' explains Ian Pinkney. 'For example, we can now quickly inject MEG back into the system if a stream begins to freeze, a procedure that never worked effectively before.'

When the gas lands at Bacton it first goes through a 'slugcatcher' to remove large slugs of liquid before being chilled to -13 degrees C in two refrigeration plants to condense out the remaining liquids. The liquid hydrocarbons and the water/MEG solution are then separated. The hydrocarbons are stabilised at atmospheric pressure and stored in a small tank farm before being transported off-site by pipeline, while the MEG is reclaimed before being returned to the platforms.

Before the installation of the new control and monitoring system, this process was largely under local, manual control, with operators having to investigate and correct alarms received in the control room.

Rockwell Automation provided technical back-up to Servelec's engineers in the development of the application software and the custom-built mimic diagrams used to display plant status on the workstations. The control panels were designed and built by Servelec and installed by AMEC Process and Energy, BP's main contractor.

The client/server system is based on two identical servers located in the MCR, operating in dual redundant 'hot standby' mode so that the spare server can instantly take over if the duty machine fails.

The servers are connected to the client PC workstations via fibre-optic Ethernet links and to two identical sets of ProcessLogix controllers by ControlNet. The ProcessLogix racks are housed in two new Local Equipment Rooms (LERs), which are located close to the process plant and form the interface between the control system and the intrinsically safe I/O. Both LERs have uninterruptible power supplies and dual redundant banks of controllers to ensure control of the plant is not lost in the event of a power loss or hardware failure.

Rockwell and Servelec are now working on phase 2 of the plant upgrade, part of which is the development of an e-process approach that will see the integration of the plant control system with BP's management level IT systems. BP managers anywhere in the world will then be able to interrogate the system via standard web browsers.