The Capacity Increase Program.
By: J. Makkinje
When the pipeline was still in the design stage, provisions were incorporated to boost the throughput capacity by installing additional pumps at points approximately midway between the main pump stations. The pipeline has a tapered wall thickness, i.e. the thicker pipe is installed on the discharge side of the pump stations while the thinner pipe is installed in the regions of normally low pressure. The same practice was followed at these projected locations. For example at the projected location of Shubah the incoming line is 31 X 1/4 which is then tied into a 30 X 3/8 section.
In 1956 the choice was made to install 5,000 horsepower G.E . Frame 3 Dual Shaft gas turbines at each of the intermediate stations as well as replacing the 5 Worthington diesel units at Turaif with two gas turbines. One Worthington diesel removed from Turaif was installed at each of the three other main stations. In addition one 180,000 bbls. floating roof crude storage tank was added at Rafha, Badanah and Turaif. These additions completed in late 1957, increased the throughput by approx. 140,000 bpd. The article Kilometer 1213: Tapline Today has some pictures of the turbine van units as installed at Wariah ( In the Aramco section) Shubah, Uwaigilah and Jalamid.
The turbine control system was a Bailey pneumatic system that controlled the discharge pressure by changing the pump speed and it provided crude oil temperature, discharge and suction pressure signals via a VHF link to the upstream base station.
Initially the turbines were supposed to be controlled through a complex relay multiplexing system, in practice it never worked, there were just too many mechanical contacts in a very dusty environment to be able to work reliably. This control system was therefore never used.
In addition the hydraulic problems that these APU's created were not solved by the increases wall thickness at the Auxiliary Pumping Units (APU's) alone. In fact they created new operating conditions with new problems at different locations.
The APU's were unattended and therefore they could trip off without warning. This increase in suction pressure would initiate a high pressure wave travelling back towards the preceeding pump station and thus cause higher than normal pipe stresses in thin wall sections of the line. The fact that the discharge pressures of the main stations was also increased already subjected some thin wall sections to higher stresses than before and to compound this with the surge pressure wave could be disastrous.
The obvious solution would be to limit the normal discharge pressures and thus to allow for surges. But again this would limit the increase in throughput to approx. 100,000 bpd.
Tapline's Engineering Department under A. E. Olson made an extensive study of transient pressures in pipelines. A paper on this subject written by R.R. Burnett , Assistant Chief Engineer was presented to the Annual Pipeline Conference In Tulsa , Oklahoma on April 28 -29 1960.
It became apparent that under the circumstances, i.e. the fixed wall thickness transitions the only solution would be to reduce the upstream station discharge pressure after a certain time delay and thus initiating a rarefaction wave to counter the surge wave.
The amount of delay was dependent on the exact locations of the critical line sections. The Turaif - Qaryatain section was totally different and needed a different arrangement with a pressure relief system and overflow tank.
To accomplish this pressure reduction at the upstream station, a continuous tone was sent back from the APU. This tone would allow the base station to go up to a certain maximum discharge pressure. This maximum pressure then depended on whether the APU was on line. If it was on line the pressure at the base station could be increased to maximum and maximum throughput could be achieved. However if the APU tripped off line, the tone would shift in frequency and this would trip 3 of the 6 diesel engines to idle, thus providing the required rarefaction wave. A loss of tone would be interpreted as an APU tripping off line, this was the inherent safety feature, as communications with the APU had to be maintained to know what was going on.
This system worked reasonably but it was far from ideal. There were many instances whereby the APU tripped off line for unexplained reason or the VHF link that carried the Surge protection tone faded which then tripped three units to idle.
The search for increased throughput never ended. Further increases were developed by improvements in prime mover capability, surge analysis and control, improved APU control and a continuous program of increased operating pressures.
Again the Engineering Department made extensive studies and performed more extensive tests to search for increased safe operating pressures. Operating experience and testing indicate that the test program should be a function of the system environment and operating stresses. Normal practice is to test to a specific minimum ratio of circumferential stress, excluding longitudinal tension or combined stress and the effects of pipe temperature to the ductility transition temperature. The latter is very significant in Tapline's working environment where pipe temperatures may vary between 35 and 100 F.
When the APU's came on stream in late 1957, some entire sections of 30 X 3/8 and 30 X 5/16 pipe downstream of the APU's were operated above the original hydrostatic test pressure, this was also the case with the sections downstream of the main stations but only on very short sections.
Pressure tests were performed on many sections and the maximum allowable discharge pressures were increased accordingly. In 1957 an increase in maximum allowable pressures from 65 to 70 percent of the specified minimum yield strength to 69 and 75 percent for various critical points of the system.
Further tests in 1959 and 1960 resulted in a decision to raise several critical points to an allowable operating level of 80 percent of the specified minimum yield strength.
At the same time many modifications were made to the Byron Jackson main line pumps to increase their efficiency at higher flow rates. The combustion systems of the gas turbines were extensively tested to determine which fuel nozzle and airflow pattern would provide the longest service life of the caps and liners, this to minimise downtime for combustion system change-outs and to allow the firing temperatures to be increased
This was in line with efforts to increase the maximum output of the turbines by 2,000 horsepower. The buckets on both wheels were replaced with long stem buckets and major changes were made to the cooling air distribution system and the combustion liners.
There remained one problem, being the inability to determine what was happening at the APU especially when the turbine tripped off line. Often when maintenance personnel went to the APU, they could not determine the fault that caused the trip. At other times the VHF link failed because of periodic fading and it caused the main station to trip to idle, thus dropping the suction pressure at the APU below its trip setting and it would drop off line.
The Communications , Electrical and Instrumentation (C.E.& I) Department employees worked long hours and covered many miles, until 1964 on dirt roads, to find out what tripped the APU and to put the APU's back on line.
Over a period of time, the Bailey control system was refined so that the turbine would stay on line unless it developed a serious mechanical problem or the upstream station dropped off line or a main line gate valve in the line section broke its stem and thus dropped the gate thereby completely shutting off the flow.
To do this the turbine now was controlled by not only it own safety devices but also the following pipeline variables:
The minimum suction control ensured that a drop in suction pressure for whatever reason would lower the turbine speed and it would stay on suction control. If the main station dropped off line however, the minimum suction at the APU could not be maintained and the turbine would drop off line as well.
The two other variables controlled the turbine in a similar fashion, by lowering the speed of the turbine to stay within the limits set in the Bailey system.
Normally the APU would run at its Exhaust Temperature Control with suction pressures in the 60 psi. region and approx. 25 - 50 psi below maximum discharge pressure depending on the ambient temperature at the time and the state of the combustion system and fouling of the compressor stage.
All these changes helped to boost the maximum throughput. In 1964 for the first time an average throughput of 480 MBD. was realised.
In 1968 -1969 problems started to appear on the horizon in the form of the slowly rising sodium content in the crude oil. The turbines used crude oil from the main line as fuel. As the life of the transition pieces of the combustion system is affected by this sodium, the turbines had to be taken out of service for more frequent and sometimes lengthy overhauls.
There was a relationship between the sodium content and the area from where Aramco shipped the crude to Tapline. Initially it was agreed that only crude from low sodium wells would be shipped, this arrangement worked reasonably well for a number of years. As time went by, it was no longer possible for Aramco to be that selective. It got so bad that combustion systems had to be changed at 1700 hours.
The solution was to install small fuel treatment units at each APU and Turaif.
These units were manufactured in the United States and shipped in 1974. They were offloaded in Beirut port and following the disturbances there, they were almost completely destroyed in the fighting and a subsequent fire.
It did not really matter, because the pipeline was virtually shut down by that time, only crude oil deliveries to Jordan were maintained using only a few of the diesel units at the main stations to fill the tanks at Turaif. Turaif then used an electric booster pump to pump crude in small batches to Jordan.
That was in effect the end of Tapline. The turbines stood idle, never to be started again.
In 1983 Aramco removed the turbine from the foundation at Wariah, capped the APU loop and removed everything that was there, including the large concrete foundations for the turbine and control vans.In Tapline as long as people were manning the stations, there was always hope that things would be back to normal in the not too distant future.
In 1986 Tapline started to cap the APU pipeline loop at Jalamid, the others followed later.
The turbines were never removed because nobody wanted to collect the old Frame 3 locomotive turbines at considerable cost. They sit there to this day as silent witnesses to a great undertaking.
- February 21, 1999