Royal J. Richoux
Amoco Production Co.
Lafayette, La.
Increased two-way radio congestion among Gulf of Mexico offshore operations prompted Amoco Production Co. to design and build a 900-mhz trunked radio system for its gulf operations.
Trunked radio, which resembles trunked lines used in telephone networks, is a method by which several radio users share relatively few channels. It offers clear, uninterrupted communications with highly efficient channel usage.
The multi-net trunking system selected by Amoco provides wide area coverage across the gulf, with a variety of sophisticated features. The system gives Amoco ample room to expand both voice and data communications.
The trunking system currently covers Amoco's gulf operations through eight offshore and four coastal repeater sites, and hundreds of mobile, portable, and base-station radios.
COMMUNICATIONS
Throughout the 1980s, the major petroleum production companies operating in the Gulf of Mexico began selling many of their offshore platforms to numerous smaller, independent producers.
While divestment makes good business sense, it also adds to the already congested radio spectrum in the gulf region. Because the independent producers need radio communications to manage their operations, more transmitters are now on the air.
The petroleum industry shares radio frequency spectrum with manufacturing and forestry in all bands: low band, high band, and 450 mhz (or UHF) band. The industry depends on its frequency coordinator to minimize interference between radio users.
When frequencies are scarce, the coordinator works to reduce the number of co-channel users and tries to keep adjacent channel users separated as far as possible (both in frequency and horizontal separation).
However, the fundamental problem remains: There are simply more transmitters in the gulf, causing more interference and congestion.
This was the communications situation that Amoco Production Co.'s offshore business unit faced in the late 1980s. Unlike Exxon, Shell, and other producers, Amoco had not over the years applied for 450-mhz channels for each field as the channels became available.
As a result, Amoco had 100-200 open-squelch transmitters on the same 153-mhz frequencies, with transmitters in different fields interfering with each other.
In addition, the radio equipment was nearing the end of its useful life and needed to be replaced. Amoco decided that there was no advantage to buying new tone-or-digital-squelch conventional radios.
Because the FCC was unlikely to assign Amoco additional frequencies, this older 1950s technology would do little to relieve the channel congestions problems.
Amoco's best option was to build a completely new system in another band using a new (to Amoco) trunked-radio technology.
TRUNKING OPTION
Even 15 years after trunked two-way radio had been introduced, it was still virtually unknown in Gulf of Mexico operations. This is unfortunate because trunking was created to relieve crowded radio traffic typically found in large cities, the same kind of congestion that plagues radio users in the gulf.
Trunked radio is based on the idea that individual radio users access the system only for a small percentage of the time and that several users will not all jump on the system at the same time.
With a trunking system, radio users share a number of repeater-based channels, controlled by an automatic switching system. When you push-to-talk on a trunked radio, the system automatically finds an available channel and switches your radio to it, usually in a fraction of a second.
The channel is for your use alone and the only people listening are the ones you want to talk with at that moment.
Because of the low probability that all channels are busy at the same instant, especially in larger systems, the chance of being blocked is far less than when using the one-channel conventional radio.
TESTING CONCEPT
Before Amoco committed to building a trunked radio system, it studied the signal propagation of 900-mhz radio communications in the gulf.
In October 1990, Amoco placed a conventional 900-mhz repeater on Amoco's Eugene Island 215 platform. Then, a crew boat with test equipment left Amoco's support base at Fourchon, La., sailing west toward Eugene Island 215.
Test engineers aboard the crew boat periodically made contact with the repeater, measuring received signal strength indicator (RSSI) voltage and decibel (db) level.
The engineers also noted reception quality and weather conditions. All data were logged with the time of day and a Loran-C position fix.
The signal from the repeater was judged "loud and clear" within 74 miles of EI 215, with RSSI between 0.800 and 0.930 v and a level of - 98 db. Naturally, the signal improved as the crew boat approached EI 215.
Slightly less than a mile from the platform, the RSSI was 2.052 v at a level of 0 db. The boat continued west, and at the final test location, 55 miles from EI 215, the signal had 1.320-1.352 v RSSI and - 89 db.
At this final location, the engineers also noted that the 900-mhz signal was "loud, clear and sharp" while a 153-mhz transmission "had static and was hazy."
MULTI-NET TRUNKING
The field survey convinced Amoco that 900 mhz was viable for its Gulf of Mexico operations.
Amoco began looking at different trunked radio systems and selected the multi-net trunked radio system built by the E.F. Johnson Co.
The multi-net system is a highly flexible, reliable, and expandable trunking system. Each system supports up to 30 channels (systems can be linked together for more channels), with up to 75 radios/channel. Each channel requires a repeater.
The system also offers more than 8,100 unique radio IDs, five priority levels, telephone-interconnect operations, and can accommodate other types of radios and signaling protocols, such as digital or tone-controlled squelch UHF radios,
A wide variety of mobile, portable (hand held), and fixed-base stations (control stations) are used in the system. These radios can be programmed for a large number of system and group combinations and priority levels.
These can switch to a conventional "talk around" mode for direct radio-to-radio communications without going through a repeater.
With optional digital tone modulating frequency (DTMF; or "touch tone") keys-pads, the radios can also place telephone calls if the multi-net systems have interconnect capability.
Multi-net mobiles and portables are available as intrinsically safe models, a critical feature in petroleum operations. All of Amoco's multi-net radios are intrinsically safe.
Each multi-net site is an independent trunking system with repeaters that process calls between radio-frequency (RF) units, maintain fleet partitioning, and provide priority access control.
To build a complete multi-net trunking network, the sites are interlinked through a backbone that is usually microwave, fiber optic, or four-wire voice-trade telephone circuits.
The hub of the backbone is the radio network terminal (RNT). It serves as the central switch for communications between multi-net sites, and auto-registration of radios as they roam from site to site. It provides the interface between the system and dispatch consoles, telephone interconnects, and connections to other radio systems.
The RNT also collects data for system accounting and supports the system management module (SMM), an IBM PC-compatible computer that controls the data base for system configuration and network call processing. The terminals for use by the accounting system are shown in Fig. 1.
PLANNING THE SYSTEM
Amoco's operations in the gulf cover thousands of square miles from the Matagorda field off the coast of Texas to the West Delta field southeast of New Orleans.
Unlike land-based radio systems, the biggest terrain obstacle offshore is the curvature of the earth. There are no natural barriers such as mountains to isolate one part of the system from another.
Radio waves have good propagation over water. Therefore, if a system is not carefully designed, an offshore worker can find himself inadvertently talking to someone on a platform 100 miles away.
With the help of Johnson engineers, Amoco designed a 10-channel system for wide area coverage. Eight offshore repeater sites and four sites along the coast can cover Amoco's operations, with three channels (repeaters) at each site.
Two RNTs, east and west, were installed at the offshore business unit's operations center in Lafayette, La.
Dozens of control stations were installed throughout the gulf, both at offshore platforms and support bases such as Galveston and Port O'Conner, Tex., and Fourchon and Intracoastal City, La. (Fig. 2).
The entire system is linked via microwave and telephone circuits owned either by Amoco or leased from other oil companies and communications services (Fig. 3).
Amoco spent much time studying its offshore operations to determine the right mix of radio equipment, system, and talk groups. With the huge expense of maintaining offshore operations, efficient and reliable communications are essential.
Helicopters must talk to crew quarters and support bases. Support bases must talk to supply boats. Supply boats must talk to crane operators on the offshore platforms. The operations office must talk to just about everyone in the field.
Currently, 14 systems have been programmed within the area, with 11 talk groups assigned within each system. Two of the talk groups are allocated for unique applications such a telephone interconnect or unit-to-unit directed calls. One group is reserved for emergencies.
All portable radios in the system have large, red emergency buttons on top. If a worker is injured, a push of the button sends a top-priority message throughout the system.
Amoco also plans to install multi-net radios in all offshore survival capsules.
FUTURE APPLICATIONS
Substantial extra capacity was designed into the trunking system. Now three channels/site are used, but if traffic level increases, five or more channels can be accessed by addition of the necessary repeaters.
Amoco is also considering the addition of automation data communications to the system. Throughout the gulf, Amoco operates satellite platforms which are monitored and supervised by remote terminal units (RTUs). These RTUs report back to master computers located on the central tank battery (CTB) platforms.
By putting the automation communications on the trunking system, satellite platform RTUs could also report production data directly to the operations office in Lafayette.
Because the trunking system is easily interconnected through a PBX (private branch exchange) and public switched telephone networks (PSTNs), it can always be used for offshore telephone service, either on an emergency basis or on platforms without dedicated telephone access.
The experience of planning and installing a 900-mhz trunked radio system in the gulf has demonstrated to Amoco that trunking is the future of communications in the gulf. As more operations require more channels, the existing conventional radio frequencies will become even more crowded.
Amoco anticipates that more offshore producers will take a hard look at trunking at 900 mhz and possibly in the future at 450 mhz.
Copyright 1993 Oil & Gas Journal. All Rights Reserved.
