Reverse circulation proves to be forward-thinking

The job was never going to be easy, comprising 295 piles of approximately 600mm diameter, most 14° off vertical to an average depth of around 17.5m, in testing ground conditions and the majority 5m from an existing building.

This work is piling for the new crane base on the west side of Dry Dock 1 within the Rosyth Royal Naval Dockyard and it represents the first time the large-diameter down-the hole (DTH) reverse circulation (RC) system has been used in the UK.

Last year the Ministry of Defence signed contracts worth circa £3 billion to build two new aircraft carri ers for the Royal Navy. The carriers, to be named HMS Queen Elizabeth and HMS Prince of Wales, will be the biggest and most powerful surface warships ever constructed in the UK. They will be built in sections at Govan on the Clyde, Portsmouth and Barrow-in-Furness with final assembly and completion by Babcock Marine taking place at Rosyth.

The dry dock is going through a major upgrade carried out by BAM Nuttall for client Babcock Marine to accommodate assembly of the ships.

The crane base on the east side had been piled conventionally but the ground conditions on the west meant a different approach was needed. Piling and ground engineering specialist BAM Ritchies was engaged and had to come up with a drill system that would handle the difficult ground conditions and stringent environmental site constraints.

BAM Ritchies and H&F Drilling Supplies put their heads together, and an RC simultaneous casing system was decided upon. RC had never been used with large DTH before in the UK, so the challenge was of great interest to H&F. Its experience of putting cutting edge drilling systems together over the last 24 years came to the fore and the system was delivered and drilling completed within the time scale required.

The plant department of BAM Ritchies also came into its own. H&F supplied Rab Forsyth, BAM Ritchies' workshop manager, with drill string weights, drilling parameters and procedures. He set about building two drilling rigs to the specifications required and also designed and built two bespoke rod and casing handlers. The handlers were designed in such a way that a 2m length of 610mm casing and a 2m RC drill pipe, 775kg in total, could be offered to the rig at any angle.

At the sharp end of the system are Numa RC210 DTH RC hammers equipped with RC T560 Super Jaws under-reaming bits. Chris Beare, European sales manager for Numa, ensured that the hammer specification was well suited for the 632mm rock socket. The hammer requires 2500cfm of air at 12bar. This air is being provided by three Atlas Copco 860cfm compressors into one of H&F's specially-designed air receivers, then into the drill string by a side inlet swivel located under the drill head. For the hammer lubricant, Matex environmentally-friendly DTH hammer oil was chosen; it is being regulated and applied by an H&F oil-injection pump unit. Water is also be ing injected to help cope with the difficult conditions and dust control.

For the drill string design and manufacture, H&F teamed up with Driconeq from Sweden and design engineer Christer Axelsson worked with H&F to overcome technical design challenges with the hammer assembly and dual wall drill string. Much time was spent making sure the system was balanced and the air input and velocity in the inner return tube were correct. Andreas Pikowski of Eurodrill Gmbh also provided some design changes to ensure the smooth connection of the drill string to the Eurodrill RH6002 rotation heads supplied by Casagrande UK.

H&F's in-house design and manufacturing facility ensured that the hammer assembly, drill string and casing drive assembly integrated and matched up to allow the system to drill and drive casing efficiently. The design allowed the inner string to advance through the lead casing to drill the rock socket without having to trip out. Care was also taken to allow the hammer assembly back up into the casings smoothly, on pulling out of the hole at an angle.

The system drives 610mm casing and finding tube with the correct tolerances was not easy. In total, 2,500m of material was eventually located in Houston, Texas, and, after samples were flown halfway around the world, followed by a rigorous quality assurance inspection, the pipe was shipped to the UK. H&F set up a threading operation near the job site and has supplied 850 lengths of casing to keep up with the site production.

The rock depth and ground conditions varied from one end of the site to the other and the system has had to cope with infill, clay, timber,'obstructions' boulder clay, mudstones, quartzitic sandstone and igneous intrusions.

With an average of 5m3 of drilled material coming out of every hole, one of the big benefits of RC is control of the cuttings. The cuttings are evacuated from the bottom of the hole through the hammer and via the inner return pipe in the dual wall drill rods straight through the drill head and through a top swivel gooseneck into the 'bluey' return pipe that is fed into a skip. These arisings are then taken away to an approved nearby disposal area. This makes for a clean and safe site.

The cuttings are not in contact with the sides of the hole, which decreases the degradation of the hole itself during drilling, especially in softer formations. Every pile socket is inspected with borehole CCTV and the system has proved to leave a clean rock socket. Once reinforcement is correctly installed to depth in the hole, concrete is placed through a 6in internal steel tremie pipe to complete the pile.

Article courtesy of GeoDrilling International - April 2009