In summer 2005, the taps were turned on in the BTC oil pipeline running from theCaspian Sea at Baku, Azerbaijan, through Tbilisi, Georgia, to the MediterraneanSea at Ceyhan, Turkey.
Meanwhile, the SCP gas pipeline linking Baku to the Georgian coast of theBlack Sea is progressing well. This $4 billion BTC-SCP pipeline project, capableof transporting one million barrels of oil and 22 million m3 of gas per day, isbeing built by a consortium of international oil companies led by BP, theoperating company for the project. In an interview with BBC News on May 25,2005, David Woodward, the head of BP’s operations in Azerbaijan, said: “Thispipeline represented Azerbaijan’s rebirth as an important country for the oilindustry, just as it was more than a century ago.”
One of the final links in the pipelines was the crossing of the Kura EastRiver in Georgia near the City of Rustavi, about one hour drive from the capitalCity of Tbilisi.
The final success of this challenging enterprise was made possible thanks tothe excellent collaboration between the various teams from: the owner BP; maincontractor Amec Spie-Petrofac International JV; the engineer, CB&I JohnBrown and its consultants Geoengineers; HDD contractor, Horizontal DrillingInternational, a French HDD company with both the experience and the drillingrigs capable of installing the largest of crossings, and the guidance systemcontractor Prime Horizontal Ltd., a U.K. company experienced in the use of theParaTrack-II subsurface steering system.
Kura East River Crossing Project
Theproject called for the installation of three parallel crossings comprising the1,168-mm diameter BTC oil pipeline, the 1,066-mm diameter SCP gas pipeline and a150-mm diameter product pipe designed to carry a fiber-optic bundle. Thecrossing of the river was not possible with open-cut methods because the riveris quite deep with strong currents. In addition to using HDD, the maincontractor also investigated the use of microtunneling. HDD technology wasfinally selected as the preferred crossing technique to save money, and, moreimportantly, time. The microtunneling method was kept in reserve should the HDDmethod fail.
It is believed that the successful completion of the crossing would make itone of the largest diameter pipes ever installed in solid rock using HDDtechnology, if not the largest.
Project specifications called for a minimum of 10 m of separation between theBTC oil line and the SCP gas line. At the crossing point, the Kura River’s spanis 360 m, giving an actual drilling distance from the entry point on one side ofthe river to the exit on the other of 780 m, to keep above the minimum allowablebending radius of the product pipes and to clear a rail track running parallelto the riverbank. While the design called for a minimum radius of curvature of1,400 m for the BTC and 1,200 m for the SCP over an arc length of three joints(about 29 m), the actual in-field design was 1,600 m, giving a wider margin ofsafety. This requirement on both entry and exit ends resulted in a 10-degreeentry angle and a 6-degree exit angle.
The downhole motor used, therefore, had to be set to build a maximum bend ata rate of 0.34 degrees/9.5 m (the nominal length of one drill pipe) around anentry curve from 80 degrees to 90 degrees and an exit curve from 90 degrees to96 degrees. The maximum allowable was 0.4 degrees, or 1.2 degrees over 29m.
Fifteen investigation boreholeswere drilled prior to the start of the HDD projects. Bore holes were verticallydrilled to the planned depth of the crossings on both sides of the river.Results provided unconfined compressive strength and abrasivity values for thevarious rock formations encountered and a supplementary geophysical surveydisclosed a 20-m wide relic river channel in the river bed, filled with packedcobbles and boulders, a feature that was drilled beneath during the HDDoperations.
Ground conditions comprised vertical rock layers, alternating hard sandstone,softer claystone and siltstone, overlain by a 6-m thick surface layer of rivergravel and cobbles on the exit side. Furthermore, the alignment of the verticalrock layers was almost parallel with the crossings axis, with an average offsetof only 12 degrees. Even using a downhole motor with a rock bit, this geologyproved extremely difficult to drill, since a prerequisite for the success ofthese two parallel crossings was to drill straight holes and achieve large radiiof curvature due to the extreme stiffness of the large diameter and high steelgrade of the two pipelines.
The continually changing formation dip and strike relative to the bore holesmade it difficult to establish a single trend of tool face commands. Forexample, hydrated sandstone would quickly drop angle, whereas hard sandstonerequired hours of work to shave down the angle. So drilling proceeded slowly inorder to remain in specification on the designed track. Also the lag timebetween the position of the bit and the location of the probe, housed in thenon-magnetic drill collar behind the bit, proved difficult to compensate forwhen the rate of build would occasionally go above the acceptable 0.4 degreesper pipe joint.
Mobilization wascompleted over two months during 2004. Site preparation involved building accessroads on both sides of the river, excavating the river gravel at pipe side andexcavating the side of a hill at the entry side to prepare the drillingplatform. Upon project completion, demobilization again took two months.
Afirst attempt to install the BTC crossing in 2004 was unsuccessful, after thehole partially collapsed in areas of fractured rock and cobble near theBaku-Tbilisi railway lines bordering the river. The railway tracks were quicklysecured with grouting works before subsidence was observed.
The HDD project resumed at the beginning of February 2005 and ended in August2005. Almost seven months were necessary to complete the three crossings andtie-ins although originally planned for completion in six months. Timing for theindividual operation stages were: fiber pipe pilot hole, 10 days; fiber pipepullback, one day; BTC pilot hole, 18 days; BTC reaming to 1,625 mm diameter, 30days; BTC pipe pullback, one day; SCP pilot hole, 15 days; SCP reaming to 1,372mm diameter, 25 days; SCP pipe pullback, one day. All reaming, swabbing andpulling works were double shifted, with crew changes onsite.
The HDD rig and backupequipment used for all of the crossing bores and reams included: a newHerrenknecht AG HK 250 drill rig, with 250-tonne nominal pulling force (itsmaximum being 330 t) and 100 kNm nominal torque; three high-pressure pumpingskids (1 x Ellis Williams 446 and 2 x Shäffer & Urbach) with capacity of2,000 l/min. each; two mud recycling units at the rig side including an SS150and an SS240 manufactured by PSD, which were able to process 2,500 and 4,000l/min., respectively and supported by a primary shaker at the pipe side.
The guidance system was a ParaTrack-II surface system provided by PrimeHorizontal, using Vector probes owned by HDI.
The 172-mm diameter mud motors were supplied by INROCK, while the split bithole-openers used, from 965 to 1,625 mm in diameter, were manufactured by PrimeHorizontal and Sharewell. Rock reamers from 508 to 762 mm in diameter weremanufactured by HTI. All hole-openers were fitted with TCI cutters. Barrelreamers utilized during reaming, from 864 to 1,422 mm in diameter weremanufactured by SMFI. The drill pipes utilized were 127- and 168-mm diametergrade S135.
Due to the remoteness, all the critical equipment components were redundant(one spare power pack for the rig, 3 HP pumps so that always two would work atall times, two mud cleaning units, two steering kits, etc.)
As the HDD project wascrossing the Georgian national railway, the product pipes were installed withinsolid bedrock at a minimum depth of 17 m below the railway tracks. The railwaytraffic speed was reduced during completion of the HDD crossings in order toreduce the vibrations above the reamed holes. No settlement was observed duringthe final installation works.
Considering the large diameter of both main product pipes, the pilot holesfor the BTC oil line and the SCP gas line had to be carefully completed to theplanned large radius of curvature and with 10 m of separation distance. In orderto remain on track with the necessary precision while drilling, three differentconfigurations of AC guidewire reference sources were utilized for the magneticsteering tool in the three pilot holes so that continuous guidance data werealways available when drilling under the river.
Each pilot hole utilized a different guidewire configuration to provideoptimum guidance information. Steering for the first pilot hole to contain the150-mm duct for the fiber-optic bundle utilized an entry coil extending to thenear river’s edge and an exit coil from the far river’s edge to the exit point.Steering for the second pilot hole for the BTC oil line, utilized a guidewirepulled through the fiber-optic duct, as well as entry side and exit side ACcoils. An entry side coil and exit side coil, as well as a metal guidance cableattached to two metal poles and stretched across and above the river was used tosteer the third and final pilot hole for the SCP gas line while under theriver.
The most difficult pilot hole to navigate was the SCP pilot hole. In spite ofthe variable signal strength caused by flooding from snow melt, the real timeposition fixes, as validated with an independent survey performed by HDI afterdrilling was completed, verified that the SCP pilot hole separation from the BTCline was greater than the minimum 10 m required.
Reaming & Pipe Installation
Bothproduct pipes were API 5L x 70 steel pipes coated with three layers of PE. Thefiber pipe was a basic steel product pipe without coating.
Due to the large size of hole-openers used, high torques were expected on thedrill string when cutting the rock. The weight of the larger hole-openers was inexcess of 4.5 tonnes, thus generating large longitudinal torques along thedownhole assembly.
A series of precautions was taken to reduce the risk of twist-offs and otherproblems downhole including: Premium larger diameter drill pipe was used for allhole opening passes of 762 mm in diameter and above; front centralizers made ofheavy-duty barrel reamers were used for all phases of 965 mm in diameter andabove; all threaded connections were specially engineered to obtain homogeneousbending strength ratio (BSR) averaging 2.25 along the downhole assemblies forall reaming phases; and torque was intentionally limited to 60 kNm, even thoughthe HDI rig delivers a nominal torque of 100 kNm. Systematic swabbing passeswith barrel reamers were also performed between the reaming phases in order toreduce the torque on the string. Finally, the tail string was screwed to thebottom hole assembly (BHA) without a swivel joint so that it could be used toretrieve the BHA from the pipe side if necessary with rotation being appliedfrom the pipe side.
Upon the initial completion of the pilot hole for the BTC oil pipe, the rigbegan opening up the bore size with staged reamers in readiness to pull in thefirst large diameter pipe. Unfortunately at this stage, the hole could not beheld open long enough to install the BTC product pipe due to the unconsolidatedgravel and cobble beneath the railroad. So the rig was moved 18 m to the rightof its original location and a new profile was designed for a drill path thatwent below this problem zone. The drilling pattern was aggressive to counter aright and then left drift caused by the dipping ground formations. Finally, thishole was stage reamed to 1,625 mm and the BTC pipe was successfully pulled intoposition.
The mud formulation usedwas based on Premium High Gelling Bentonite from Egypt Bentonite &Derivatives Co., which was enhanced with Xantham Gum as a fluid lossreducer/stabilizer, and with a cellulosic polymer as a rheological additive,provided by Süd Chemie of Germany.
The mud was monitored by a mud engineer and two qualified mud technicians, tomake sure that it was recycled adequately and that the viscosity was maintainedbetween 60 and 75 seconds/qt, thus avoiding unnecessary torque generation on thestring and tools.
Despite these precautions, a few twist-offs occurred during the last reamingphases. Because the hole was clear and clean, tooling was quickly recoveredwithout any further complication. Three roller cones were lost during the finalBTC opening, and two additional swabbing passes were necessary prior to the pipepull to ensure the hole was clear of any object likely to damage the pipecoating.
For both large diameter product pipes, a buoyancy control system was utilizedcomprising three 450-mm diameter HDPE pipes previously installed inside the BTCpipe and then transferred to the SCP product pipe. The HDPE pipes werepressurized during the pullback operations and the annulus was progressivelyfilled with water while the pulls progressed. This system made both pipes almostweightless once immersed in the drilling fluid. This, combined with carefullycontrolled drilling radii, enabled two smooth pull-in operations, with lowpulling forces and very minimal damage to the PE coatings despite the abrasivesandstone side walls.
This project was bothimportant to the world’s energy economy and to the consortium of companies thatown and operate the pipelines. Due to its importance, the design specificationsand in-field quality control were stringent, especially in light of the verticalhard rock formations encountered.
The project demonstrated that the use of HDD technology is highly applicableto river crossings in hard rock for large diameter product pipes. Geologicalsurveys prior to field operations are necessary in order to design both thedrilling plan and the downhole drilling string. The ParaTrack-II magneticguidance system is an optimum system for downhole steering. For this project, itwas felt that the ParaTrack-II system was the only subsurface guidance systemthat would work. A magnetic guidance system based on DC electric current wouldnot work because of the magnetic interference from nearby pipes and northseeking laser gyros are not currently available.
On completion of the works during his address giving special thanks to allthose involved, Chris Walker, CB&I, who had overall responsibility for rivercrossings in both Azerbaijan and Georgia, said, “The crossing of the Kura EastRiver in Georgia was one of the greatest technical challenges faced during theconstruction of the BTC and SCP pipelines. The successful outcome can beattributed to the dedication and hard work of those involved. Integration ofboth construction and engineering teams played a key part in achieving the finalgoal and forms a necessary part of constructing HDD crossings in adverse terrainand geological conditions.”
In addition, special acknowledgments are given to Chris Walker and ReesBrislin from CB&I John Brown, Paul Bearden and Allan Snider, GeoengineersHDD consultants, Sebastien Duval, SPJV supervisor for the Kura East crossings,Alexis Filliette and Guus De Rechter, project managers for HDI, Philippe Mathy,Jean-Yves Aera and Andrew Horn, HDI drilling superintendents, Serge Thomas,Olivier Arnould and Valery Caucigh, HDI drillers, and David Court, PrimeHorizontal guidance engineer.
Ian Clarke is a freelance writer with No-Dig Media Services, based in theUnited Kingdom. He wrote the article from information provided by DenisPellerin, Horizontal Drilling International S.A.S., along with Thomas L. Teer,Ph.D., and David Court, Prime Horizontal Ltd.