“It takes one incident that is properly handled to pay for the system.”

Mapping the users

With the MPT radios, queueing was a frequent problem during busy periods and users sometimes had to wait as long as 60 seconds to get through. With TETRA, queueing time has been cut to no more than about one second. 

Another big improvement with the TETRA system has been a large increase in the number of talk groups (some 300 now) and categories of priority users. There are 15 priority categories, embracing all the ‘blue light’ emergency services and security staff at the complex, with users such as fire-engine drivers in the top five levels. “After that, the next five are production because, after an emergency, the plant still needs to operate”, John explains. “With MPT, we only had three priorities – Normal, Higher and Emergency priorities. And I used to say, ‘When does an emergency priority become normal priority? If you have only three priorities around the plant, every second person says, ‘I have high priority’. So the object of the exercise is defeated.”

John emphasizes the importance of creating these definitions from the outset in the ‘fleet map’ of the system. “Your fleet map is the most crucial part. It doesn’t matter what system you have – if your fleet map is not healthy, you’ve got nothing. From Day One, it must be good.”

If the fleet map has mistakes in it, it can result in calling all the radios back in to correct them, causing huge disruption. And so, to create the Secunda fleet map, the skills of an outside expert were called in.

“When you design a system and put down your requirements”, advises project manager Christo Kriek, “you need to consider all the different components of the network and how these will be utilized in the future, because you need to get maximum benefit from the system, considering the amount of money invested in it.

“Secondly, don’t paint yourself into a corner, not being able to grow because you have selected the wrong technology. You must think of all the components in the whole system: is a component a possible growth point or a pinch-point for possible growth?”

John Rush adds that it was this kind of proactive planning that guided the choice of the radio towers that they built. “If you’re going to put down a tower, it’s going to be there for the next 50 years. But if you don’t take everything into consideration, you will realize, after you’ve put the dish and the fourth antenna on, that you’ve hit its wind loading spec. So the towers we’ve put down are upgradable.” In this way, Sasol has been able to provide antenna space for the cellular operators.

Christo also recommends preparing proper radio coverage studies and not simply assuming that the old analogue sites will be enough. “We also spent money on additional digital elevation maps with vector information”, he says. “Some buildings and forest, and that, absorb the radio frequency.

“You should also make more than one study with different simulations for repeater stations and different power settings and levels. And you can also move some of the high sites. Radio frequency planning should be an iterative process.”

Technical support

Business units on the site – including production and maintenance personnel and their managers – are supplied with radio units by the Sasol Shared Services unit, and all but a few have now migrated to the TETRA system. The MPT system has been halved in capacity, releasing radio frequencies, but it has not yet been disconnected: it remains a useful resource for use during maintenance shutdowns, to help meet the needs of outside contractors, although at least one of the contractors has already equipped his staff with TETRA radios.

John is aware, however, that the MPT system is becoming prone to problems of old age. By the time it finally fails, he hopes to have acquired enough TETRA radios for it not to matter.

A measure of the TETRA system’s success with users is that the overall number of subscribers connected has grown by almost 30 per cent annually over the past two years. The initial complement of radios was from Selex, which at the time was the only manufacturer able to offer an intrinsically-safe (IS) TETRA handportable, although today the choice is very much wider. IS radios are special models certified for use in flammable or explosive atmospheres, such as in petrochemical plants.

But a persistent problem for the team has been in getting radios repaired. John picks up a radio module which has been knocked out of action by a lightning strike – a frequent hazard in lightning-prone South Africa. Even if a repair is a simple one, he explains, it cannot be carried out locally: a radio’s IS status would be nullified immediately if an untrained person were to open it up. So instead the radio must be returned to Europe for repair – at substantial cost in transport, insurance and customs fees. And he might have to wait many months for it to come back.
(中国集群通信网 | 责任编辑：陈晓亮)