Mar 13 2005
As the BTC management explained in one of its recent news bulletins, for the Kura River West crossing in Azerbaijan, the oil pipeline is being taken under the river in a small shaft or micro-tunnel. The 46 in dia. pipeline encounters the Kura again in Georgia where on the western side of the river it will be placed in an open-cut trench known similarly as the Kura West crossing. In the most recent BTC bulletin, this work is reported to be some three-quarters complete. Both these West crossings have their counterpart East crossings. It is understood that the horizontal directional drilling technique (HDD) was intended for use at both East crossings but so far BTC has made no mention of progress at either of them. BTC’s spokesman in London confirmed to CIOB International News a report that the first attempt to form the Kura East crossing in Georgia had failed but said that a fresh attempt was being made by the contractors.
The project management is confident, he said, that the delay will not affect the overall program for the pipeline’s completion. Construction of the Baku-Tbilisi-Ceyhan oil pipeline and the South Caucasus gas pipeline which runs parallel to it in Azerbaijan and Georgia is proving a test-bed for the principal techniques employed in the oil and gas industries to take pipelines under natural obstacles such as rivers or through high ground.
There are of course other kinds of obstacle, such as roads and railways, where pipe-jacking or directional drilling have been equally successful in many locations around the world. The decision on which method to use depends of course on the location and its geology. The Georgian landscape is at all not favourable to the construction of pipelines. A quotation from the paper given by the pipeline geological team at the Geopipe conference last year gives an indication of the difficulties. “Geologically, the region is part of an active plate collision zone and the combination of uplifted, faulted weak sedimentary rocks and steep, dissected slopes presents major challenges for pipeline routing. “Amongst the most significant geohazards encountered along the AGT route in Georgia are landslides, seismicity, river channel erosion and scour, and soil erosion. In south-central and western Georgia, the pipeline route traverses mountainous terrain at altitudes of more than 2200 m.”
Constraints of this nature, together with the difficulty of trench excavation in such conditions, were recognised at an early stage of project planning. The river crossings have also been described as a challenge by the BTC management who said in the latest news bulletin that 46 major crossings are required in constructing the oil pipeline. Five of them are associated with the same Kura River that flows through the three host countries. As this indicates, the Kura is a long river, some 1400 km in all, not far short of the length of the oil pipeline that has to cross it so many times on its way to the Mediterranean marine terminal. But the Kura flows in the other direction, rising in the mountains of the Caucasas. After traversing Georgia and Azerbaijan it discharges into the Caspian Sea south of Baku. Not far west of Tbilisi, the river, which at one time was navigable from the Caspian as far as the Georgian capital, becomes braided into a series of shallow interconnected channels separated by alluvial deposits. Horizontal directional drilling consists basically of three operations, pilot hole drilling, reaming (opening up the hole) and lastly pulling the pipeline through the hole.
The description usually applies to large scale crossings in which the pilot bore filled with bentonite is enlarged by a washover pipe and back reamer. At Kura East in Georgia, it is understood that it was at the reaming stage that the first operation had to be halted due to a ground collapse above the bore. The team from Australia’s AJ Lucas Group, who in a paper at the Geopipe conference expounded the advantages of HDD in terms of both distance and pipeline diameter, described it as a ‘black art’. With today’s knowledge of pipeline steels, pipe coatings, cathodic protection and high level testing techniques, they said, there is no reason for a buried pipeline to fail. But they described cobbles and gravels as the nemesis of HDD, because the method relies on an open hole being available to pull back the product pipe. They gave the example of encountering uncemented cobbles and gravels, where gravity tends to fill the drill path such that there is no opening through which to pull either the hole openers or the pipe. Something of the kind appears to have happened at the Kura East crossing in Georgia. HDD preferred option at Kura The question of which construction methods should be employed in crossing the many obstacles presented in this difficult terrain was as one would expect subject to careful study prior to the BTC project being launched.
Many of the major rivers which in the opinion of the hydraulic engineering consultancy HR Wallingford would have benefited from HDD were deemed unsuitable from a geotechnical perspective. This assessment concluded that HDD would be the preferred option for the Kura River crossings. As things have turned out, the prospects of a quick and economical procedure on the east side of the river in Georgia have been frustrated by what looks like a piece of obtuse geology. According to the HR Wallingford contribution to the Geopipe conference, they foresaw the type of problems that might arise with HDD crossings in certain circumstances. Mentioning specifically the Kura River, they pointed out its braided nature and said that such systems are subject to large and rapid lateral movement, behaviour that demands specification of appropriate set back distances. Set back is explained as the width of the active river and the floodplain if the location of the main channel is deemed likely to move during the lifetime of the pipeline. That indicates a fairly long crossing at Kura River East, taking into account the proximity of the railway running between Baku on the Caspian and Supsa on the Black Sea coast. The authors of that paper closed their review of the river crossings in Azerbaijan and Georgia with a comment that brings home the risks involved in this type of work. “Whilst pipeline exposure in a river crossing does not necessarily lead to failure, inspection and remediation should take place as soon as possible to prevent additional spans being exposed and permit the maximum throughput of pipeline to be maintained. If a pipeline crossing needs to be replaced during the operations phase this is a very costly and prohibitive operation.”
Fortunately the setback at the Kura River is one that will be rectified before the oil starts to flow. The microtunnel technique being employed at the Kura River West site in Azerbaijan is a method of remote control pipejacking, employing hydraulic or other forms of jacking from a drive shaft to form a continuous string in the ground. This operation as reported in the February progress bulletin from BTC is virtually complete.
Execution of the Georgian section of the BTC and SCP pipelines is in the hands of the AMEC Spie-Petrofac International Joint Venture.