The Shah Deniz gas field project in the Caspian Sea, near Baku Azerbaijan, involved a pre-drill construction phase consisting of the placement of a 15 slot, 42 tonne, seabed guide template and installation of 15 nr 42” (1067mm) diameter pre-conductor well casings, through the template, 27m into the seabed, all in 103m water depth. Anticipated ground conditions were 15m of coarse sand and the remainder stiff clay. The pre-conductor well casings were installed to provide primary ground support through the upper seabed deposits to prevent any ground disturbance during main conductor drilling, which could compromise spud can foundation and stability of the main platform.
BP (Shah Deniz) commissioned jetting trials for the pre-conductors in the summer of 2001. The system utilised a conventional forward flush, but the sand/clay interface proved problematical in getting the casings to full depth, furthermore the jetting installation technique greatly compromised the integrity of the seabed deposits, producing a large pit at seabed. The seabed erosion and washing out of the sand layers could have seriously compromised the template pitch and roll installation tolerances as well as the overall platform stability, especially during installation of the main spud cans in which “rat-holing” effects could have been experienced seriously impacting the suction foundation of the spud cans. BP therefore started to search for an alternative solution which would produce minimum ground disturbance during installation of the pre-conductors. BP’s in-house discussions suggested that one potential solution was to employ a reverse circulation drilling (RCD) method.
BP approached Seacore Ltd in March 2002 with a view to developing a system of installing the 42” casings to full depth with minimal ground disturbance, on the semi-submersible “Istiglal”. Seacore initially proposed installing their Teredo 40 power swivel onto the link arms suspended beneath the main drawworks. This method was quickly dismissed as the “Istiglal” derrick structure was not rated to withstand the high torque loading of the Seacore power swivel (337,000 lbs/ft). The solution Seacore developed was to provide a compact, completely self-contained heave compensated drill system and mount it directly on the “Istiglal” drill floor. Seacore’s standard Teredo 10/40 containerised drill unit was modified for this purpose.
The compensator system was designed to bolt directly to the rotary table supporting beams under the drill floor. Installation was a relatively quick operation, taking approximately 10 hours to remove the existing rotary table and replace it with the compensator. The compensator was designed to have a working stroke of 300mm (maximum 500mm) over a period 5 seconds.
The key to the actual pre-conductor installations was the development by Seacore of a novel, complete, down-hole conductor installation system. The Seacore PDS was developed for this project. This consisted of Seacore’s Packer Drive System (PDS); RCD technology, under reamer deployment and proven skills and experience in the installation of large diameter casings (up to 4500mm in the past). In the past Seacore had utilised retracting locking beams and pins to drill piles and sacrificial shoes into overburden and rock. However, the remit from BP was to leave the conductor walls smooth and unencumbered for future drilling operations, therefore the PDS was developed which required no external fixtures to aid installation. The PDS used a series of concentric rubber bladders which, when inflated, allowed the driller to grip the 42” conductor and impart both axial and torsional forces, up to the maximum torque and pull back of the T10/40, if required, to aid installation. The PDS also contained a mud injection facility that allowed discreet injection of a mud slip coat through specific mud ducts near the conductor toe into the small annulus between the side wall of the pile and the surrounding strata, to further aid conductor installation, if required.
The complete system consisting of two packer drives, under reamer, RCD and mud injection facility was assembled into a single Bottom Hole Assembly (BHA). The driller can make an independent or a combination selection of the down hole installation services, remotely from the surface, to achieve optimum conductor installation as ground conditions may dictate.
The equipment spread was designed and built in-house by Seacore at their facility in Cornwall, UK over a period of 5 months. All equipment was subjected to a rigorous commissioning and testing programme prior to shipment to Baku, Azerbaijan and transfer to the “Istiglal” during March 2003. Vessel mobilisation, assembly and testing was trouble free and conducted to programme over a period of about 3 days.
Field operations commenced with placement of the seabed template. This was connected to the Seacore NW300 HT drill string by means of a three pin running tool which in turn located into a triple J-slot built into the central slot. The template was placed on the seabed well within the tolerance of 2 degrees and 1 degree for heading and pitch/roll respectively. Seacore’s RC drill pipe is stronger than conventional pipe typically used for template placement thus the template could be lowered in increased sea conditions. Further, using the Seacore system allowed the template to be connected, via the drill pipe, directly to the Teredo 40 high torque power swivel enabling the template to be accurately rotated in a controlled and safe manner for precise positioning. The RC bolted, flanged heavy-duty drill pipe was built into 15m stands and laid out on the pipe rack; the 24m long BHA was racked back in the finger boards.
The 42” conductor casings, each supplied in two halves, were connected with a proprietary “Puma” threaded connector, manufactured by Oil States International, who also supplied the conductors. Conductor installations commenced with assembly of the two sections. The complete conductor was then suspended below the drill floor and the Seacore BHA inserted, which picked up the conductor using a special latch system built into the BHA. The conductor was then tripped to mudline and stabbed, under ROV guidance, into the template. The BHA latch was then disengaged and conductor drilling operations could commence. RCD was initiated and the casings advanced as material was excavated. All excavated material was recovered to surface and allowed to disperse naturally through a side discharge in the drill pipe just above sea level. Verticality checks were carried out on the casing with a simple bulls-eye visually inspected by a ROV. The conductor was advanced to total depth by the driller using a discretionary combination of the PDS, mud injection and under reamer with minimum ground disturbance. Seacore’s RCD and drill bit design ensures material is excavated and removed with minimum disturbance to the surrounding annulus material.
It is believed to be the first time this operation has been carried out from a floating vessel in this depth of water and the first installation of large diameter conductor sleeves through drilling template slots. The first installation took 56 hours deck to deck to complete but this decreased to an average 22 hours per casing for the last 9 casings. Actual conductor drilling to 27m took as little as 6 hours. It took a total of 36 hours to demobilise all the drill spread and re-install the rotary table into the drill floor.
The contract was completed on time and to budget much to BP’s satisfaction who described Seacore’s performance as an “Excellent job”. Since completion the project reached the final 11 in the BP Helios world wide awards for innovation and technology out of some 121 entries.