MODULARITY ADDS FLEXIBILITY TO PROCESS CONTROL ON VESLEFRIKK

Modular distribution controls have added flexibility to Norway's offshore Veslefrikk platform operations.

This control system allows creating, configuring, and testing new control logic. Modifying or expanding the system can be done as needed to improve operations.

VESLEFRIKK PLATFORMS

Den norske stat oljeselskap A.S. (Statoil) developed the Veslefrikk field, located 145 km (about 90 miles) northwest of Bergen, Norway. Water depth is 175 m (575 ft).

Veslefrikk A, the drilling and wellhead platform, is a fixed steel-jacket platform. Veslefrikk B, a floating production and accommodations platform, is a converted drilling vessel (Fig. 1).

Veslefrikk provided technological innovations in several areas, and is the only one of its kind in the North Sea, Statoil says. Several flexible pipes in catenary suspension between the fixed wellhead platform and the floating producer transfer the well streams.

A separate redundant catenary bundle contains power cables, fiber-optic cables, and hard-wired emergency shutdown (ESD) signals. Members of the 90-person crew use a gangway to cross between the two units.

The 44,000 ton floater is moored to the seabed with 12 anchor chains, each over 1 km long. Normally anchored 40 m from the fixed platform, it can be withdrawn in adverse weather up to 70 m with no impact on production. The A platform is connected to the shore via undersea gas and oil pipelines.

CONTINUOUS MONITORING

Twenty-four-hour monitoring and control of the two Veslefrikk platforms is provided by a Network 90(R) distributed, digital system from Bailey Controls Co. The system distributes processing power to the module level, eliminating the need for a central process computer.

Reliability is further enhanced through a redundant configuration for the distributed processors. The system allows full distribution and segregation of control functions, while maintaining integrated information acquisition and centralized operation. Redundant fiber-optic cables transfer signals between the two platforms.

The system has modular architecture. This facilitates expansion by using standardized, plug-in modules for control and communications.

Network 90 was selected for all process control, data acquisition, emergency shutdown, fire and gas, and drilling systems, plus telecommunications interface to shore.

The central control room (Fig. 2) on Veslefrikk B is connected to process and wellhead control systems both directly and remotely. Smart Transmitters (Fig. 3) provide input/output in safe and hazardous areas.

The control room includes five management command system operator stations and ten process control unit cabinets with a total of 7,320 input/outputs. Serial interfaces to eight external units accommodate 1,200 additional 1/0 points.

Veslefrikk A, the drilling and wellhead platform, is designed to be operated by remote control from the central control room on the floating 6 platform. If the communications cables are broken, operation of the safety systems (ESD and fire and gas) will continue on the A platform.

The safety systems for fire and gas and ESD are implemented with redundant multifunction controllers and 1/0 modules in completely independent paths. Operator interface is via matrix pushbutton mimic panels and management command systems consoles. These systems have over 3,200 1/0 points.

Petrovest A.S. of Bergen, Norway, a Bailey affiliate, provided the detailed engineering, procurement, construction management, and testing of instrumentation and safety systems on both Veslefrikk platforms. It also completed the telecommunications system on both platforms, which includes ship radar, wave radar, closed-circuit television surveillance, and real-time receivers for communications between the platforms and shore operations.

APPLICATION ENGINEERING

Petrovest and Statoil application engineers configure all control strategies off-line using PC-based Bailey engineering work stations and predefined control algorithms called "function codes."

The engineering work stations are used onshore as training stations and on the platform to make modifications. Work is done in the Norwegian language. Each I/0 can be independently selected.

The system is easy to configure so that modifications can be tested on the work station and then downloaded to platform systems. This gives the flexibility of working with the main control system and the fire and gas system from the same work station.

CUSTOM GRAPHICS

Wellhead logic and process shutdown are part of the primary process control and data acquisition system. Both use the same plant communication loop, but the process shutdown system has independent control logic and 1/O circuitry.

From the central control room, operators can control the entire process that includes:

• The main power

• Drilling

• Crude handling and treatment

• Gas treatment

• Flare

• Vent

• Blowdown systems to seawater, freshwater, and heat ventilation, and air conditioning (HVAC) systems.

Custom graphics on the management command system CRT screens give operators rapid access to information on the platform's operation. These include displays of process variables and equipment status, process control functions, alarm handling (annunciation, logging, and summaries), overall platform status, system management, trend recording, and analog values.

The process control and data acquisition system interfaces via serial links from the multifunction controllers with dedicated systems such as gas chromatographies, oil and gas custody metering, and the main generator.

FIRE AND GAS SYSTEM

Each platform uses individual fire logic divided into ten subsystems called "super fire zones" (Fig. 4). Failure in a subsystem will affect only that limited area. Maintenance or minor modifications to hardware or software can be made without interrupting the protective functions of the subsystem.

The primary operator interface for handling fire or hazardous situations is the control room mimic and matrix panel. The fire pump control panel gives operators a visual overview of the pump system for both platforms, as well as the ability to start the pumps manually. The system also includes a matrix panel in the toolpusher's office and an alarm "repeat panel" in the driller's cabin.

The fire and gas system operates completely independent of the process control system.

However, operators can monitor the subsystem with the management command system, which provides logging and archiving of alarms and faults via the plant communications loop.

Copyright 1991 Oil & Gas Journal. All Rights Reserved.