Gas and Petroleum

Gas Transmission System (GTS) in a simplified form consists of a linear part (pipelines) and gas compressor stations (pumps). By the linear part are meant the main gas pipelines and gas pipeline branches, as well as some other technological objects that will be described below. Gas Compressor Stations (pumps) are extremely complex technological objects. It should be noted that Gas Compressor Station (GCS) which has a corresponding dispatch point is the control center of both the compressor station, and the linear part (in a certain area of responsibility). The mentioned area of responsibility can occupy an area of 300 km x 300 km. The most common technological objects of the GTS linear part include crane platforms (CP), gas distribution stations (GDS), cathode protection stations (CPS). CPs serve to redistribute gas flows between gas pipelines, to localize some areas of gas pipelines in emergency situations or repair work, etc. CPSs are set to protect pipes of the main gas pipeline from corrosion. GDS is intended for gas preparation for the end user: decrease and adjustment of pressure, odorization, etc. All listed objects, excepting GDS, operate without participation of a person, offline, and are automated to a varying degree.

Object management is carried out from dispatch center of the compressor station. Considering the fact that an object can be situated 150200 km far from the GCS, the task of organizing data transmission channel is not easy. In addition, between the dispatcher, that is at the GCS, and the operator, that is at the GDS, it is necessary to organize additionally voice communications. Copper four-wire cables have been mainly used to ensure dispatching voice communications up to the last few years. When it was necessary to transmit data to technological objects, special adapters were installed in order to transmit data at a speed of up to 2.4 kbps.

The speed is relatively small, but it’s sufficient to solve main tasks. The problem is actually how to divide one cable between voice communication and data transmission, which is performed in on-line mode, that is, not occasionally, but constantly? There are several options to solve this problem:

to install a sealing equipment on the cable;
to install one more cable;
to use radio channel additionally.

Recently, radio channel gets increasingly distributed for both voice communications and data transmission. A typical option to arrange management of geographically distributed objects is to install a master controller (a computer with SCADA system) on GCS and emulation ("extension") of RS-422/485 tires using "long-range" radio modems or cable modems. At the same time, the corresponding modems and controllers are installed at the technological objects. Retransmitters or regenerators can be applied to increase communication range.

Exchange with remote controllers is carried out according to the "Request-Response" scheme. Request frequency determines the access time of the remote controller to the transmission medium and receiving delay of data from a specific controller. Advantages of such technical solution - simplicity and relative low cost, but it doesn’t solve all problems and it works only under certain conditions. What does it mean? Each remote controller together with a master controller have to support a single exchange protocol. This condition is difficult to fulfill even for construction of a new gas pipeline with a new communication system, when it should seem that a single exchange protocol can be provided for a CP-, CPS-controllers and for GDS automatic controller. But even under these conditions the commercial accounting computer of gas, installed on GDS, will have another "classified", that is, a known to nobody protocol. Actually the controllers for these technological objects are different types, are installed at different times and support different exchange protocols. What does this lead to? For example, the CP controller supports the ModBus protocol, the automation of GDS - DNP3, and the CPS controller - some kind of "non-standard" protocol. There are also situations where one part of the CP controllers uses one protocol, and the other part – the other one. The problem occurs: the SCADA system on the master controller doesn’t allow "to associate" several protocols with one СОМ-port, which the radio modem is connected to. It’s extremely difficult to solve such kind of problem. But if it’s even possible to solve it, another problem arises: one physical medium (cable and radio channel) transmits packages of different protocols. It’s difficult to predict how remote controllers will respond to the packages of "not their" protocol. Of course there are checksums, CRC, etc. But without a doubt the probability of checksum coincidence increases significantly. In practice this task is accomplished by sealing a channel (cable or radio channel), which ultimately leads to a significant increase in the cost of the project. When considering data issues it was not taken into account, that unlike CP and CPS gas distribution stations are technological objects that are served by people (operators). This is due to not only their technological complexity, but also as a rule to low level of their automation. Basically all GDS management is implemented in manual mode by operator by the commands of the dispatcher, that is located at the GCS. In this regard there must be provided the additional voice communication between a GDS operator and a dispatcher. GDS operator can be on duty not only in the GDS building, but also in his house, which is usually located nearby. This means that the voice connection with the dispatcher is needed not only for GDS, but also for the operator’s accommodation. In addition, it is necessary to provide transmission of various emergency, fire and security alarming data. Voice communication is essential for service and emergency brigades at any point of the gas pipeline. Bandwidth requirements for the voice communication system are minimal, but it’s needed. Considering all the listed tasks, here are general requirements for some communication system and data transmission, which effectively accomplish the task.

A large coverage area (300 x 300 km).
Data transmission between master and remote controllers according to the "point - multipoint" scheme in on-line mode.
Capability to organize several virtual tires (channels) described in the previous paragraph (each - for its exchange protocol).
Voice communication Dispatcher - GDS (dispatcher - GDS operator’s house).
Voice communication with mobile subscribers.