An offshore oil platform requires seawater to be pumped into its reservoir well. Photo by Weir Westgarth
Click here to enlarge imageDesalination specialists Weir Westgarth supplied a packaged plant complete with a Siemens Process Control System (PCS 7) on an offshore platform in the Danish sector of the North Sea oilfields to solve reservoir scaling problems for Amerada Hess.
The oil production process requires seawater to be pumped into the reservoir well to raise the pressure and improve the flow of crude oil. The seawater used for this process contains high sulphate levels, so antiscalants are usually injected to prevent reservoir scaling and subsequent loss of production.
Amerada Hess contracted Weir Westgarth to supply a Sulphate Reduction Package (SRP). Sulphate removal operations are controlled by the PCS 7 system, which is in turn linked to the platform's main control system, located centrally. The project was completed in two phases. Phase 1 consisted of one stream at 40,000 barrels per day (BPD) of seawater. Phase 2 followed with two streams of the same capacity, with additional pre-filtration equipment.
One of the major factors influencing the decision to select the Siemens PCS 7 was the time available for development, installation and commissioning. The fast tracks project, Timescales, meant that the control blocks used by the PCS 7 from previous Weir Westgarth membrane projects could be reused.
Siemens ET200 I/O units interface the control system with motor control centres, while a Siemens Modbus interface links the SRP equipment to platform central controls, which communicates across a Modbus network. Elsewhere, Profibus fieldbus communicates with the intrinsically safe I/O units.
Although time was limited, systems integrators Booth Welsh incorporated the latest version of the PCS 7 for the first time in this type of installation. Martin Welsh, the company's technical director, explains that although some reprogramming was needed, the modular approach used in the PCS 7 architecture made the project relatively straightforward. "We knew that time would be at a premium, but the ability to use both a database and library containing common instruction code reduced software development significantly." Booth Welsh is based in Irvine, Scotland.
Once the control system had been configured, Booth Welsh carried out acceptance tests in its Scottish facility before the entire system was shipped to the North Sea. Remote diagnostics have been incorporated into the control system so that fault can be traced by telephone, avoiding the need to visit the platform. "This is an important factor in the system's reliability," explained Martin Welsh. "Getting to a platform is not only expensive and time consuming, health and safety regulations mean that all personnel have to undergo survival training. Anything we can do to reduce site visits is a bonus for all concerned parties."
The Siemens Process Control System controls all chemical dosing, water pumping and membrane filtration, including membrane cleaning, motor control and pump management for the SRP unit. Distributed control using a fieldbus was essential since the nearest I/O device is located some 150 metres from the Simatic Processor. This significantly reduced installation times during phase two because additional nodes were linked directly into the network, rather than having to rewire each component individually. This minimised the downtime for phase 2 because it only required a minimal period for tie-in.
The PCS 7 system is playing a vital role in oil production because no oil production is possible without seawater of adequate quality being pumped into the reservoir well.
Author's note
Neil Lancett works for Siemens Process Automation, which is located in Worcester, England.
