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Learning the drill

Construction of a major tanker berth facility was made more complex by a combination of tight deadlines, a remote location and tropical cyclones. Rodney Byles reports

A major cyclone which swept away most of the plant and equipment, high temperatures and a telecommunications network which could be down for two weeks at a time; these were just some of the difficulties experienced by the team constructing a jetty and tanker berth facility in the Gulf of Kachchh in north west India.

The facility serves the vast Jamnagar refinery complex in Gujarat State. The project consisted of a 3.5km access jetty, an adjoining 850m jetty with four tanker berths and 23.2km of sub-sea pipeline, with offshore single point mooring dolphins. Client Reliance Petroleum awarded the $200M design and build contract to Saipem UK which brought in specialist marine contractor Seacore as drilling subcontractor.

Speed of construction was essential so Saipem opted for a modular structure carried on a piled foundation. Large sections were fabricated in a yard in Dubai and shipped to site for installation by a floating crane. This minimised the amount of work over water, but the foundations presented a problem.

'The ground conditions dictated we couldn't drive clusters of piles for the jetty. Working in a 6m tide and in an environmentally sensitive area, we had to go for monopiles installed into sockets drilled in the seabed, ' said Saipem UK deputy project manager Piergiorgio Mariani.

Seacore had been working from marine plant supplied by Saipem UK - a combination of two jack-up platforms on the jetty works and two floating barges on the offshore single point mooring location.

Mounted on these were two of its own reverse circulation hydraulic drilling rigs, T10 and T40.

Working in water up to 30m deep, in temperatures of up to 45degreesC, Seacore had been drilling through fractured 200MPa basalt and peppering the bed of the Arabian Sea to form deep and partially cased rock sockets. All the clearance holes accepted tubular steel foundation monopiles being placed by Saipem UK.

But five months into the job, at the end of 1997, Saipem diverted one of the jack-ups Seacore was using on to the critical jetty pile installation, forcing it to shut down one of its drilling rigs. A month later a vicious cyclone hit the exposed site, washing away all Saipem's floating plant and beaching much of it inland.

Seacore lost four and a half months while Saipem recovered the plant. One of its drills was found beached in a mangrove swamp.

Seacore restarted drilling from two separate jack-ups and at the end of February 1999 had finished the smallest diameter piles and shipped the T10 drill back to the UK.

A further three months of drilling was lost shortly afterwards when Saipem again diverted the jack-up platform carrying the T40 drill. The company restarted in late May and completed the remaining rock sockets without further interruptions In total, 214 rock sockets were drilled, of five diameters from 1.2m to 3m, to depths between 8m and 18.5m into the seabed. Originally Seacore expected to drill just under 1,700m of socket, but Saipem found additionally weathered material in some locations and instructed the contractor to go deeper, resulting in an extra 460m.

The same basic drilling technique was used throughout. Saipem first positioned the jack-up and pitched a long tubular steel pile hole casing onto the seabed. The internal diameter of the casing was 200mm larger than the diameter of the drill bit. Using a D55 Delmag diesel hammer Saipem drove the vertical casing through a layer of silts and shells into the underlying and weakened basalt, ensuring the toe was not damaged or deformed.

Once it was plumb, Seacore placed the drill rig on top of the flooded casing and clamped it with integral pneumatic gripper jacks.

The rig mast was pivoted back to allow the jack-up crane to lower the bottom hole assembly, comprising drill bit, stabiliser and a section of drill string, through the drill deck doors and into the casing. The assembly was temporarily supported on fingers in the drill deck to allow a further section of drill pipe to be added. This procedure was repeated until the drill bit was just resting on the seabed.

Both Seacore's drills used a modular bit with a central core capable of accepting interchangeable cutter wings, which in the case of the larger T40 can cope with holes up to 4m diameter.

The 330kW reverse circulation T40 hydraulic drill, developing 40tm of torque, was originally designed and built at Seacore's quayside headquarters at Gweek, Cornwall in the UK to drill a pair of 4.1m diameter, 75m deep blind holes for offshore sewerage riser shafts in Hong Kong. As the bit, complete with an array of button cutters, was slowly rotated within the casing, a 23m 3/min compressor provided the reverse circulation air lift. This forced air down tubes within the 550mm diameter drill string to the bit and pushed cuttings and water up the 300mm diameter muck pipe. Spoil was forced through the rig's rotary swivel and discharged overboard. Samples of cuttings were taken regularly to determine final depth of each hole.

Seacore's crews worked two, 12 hour shifts, seven days a week and achieved an average penetration rate of between 300mm/h to 500mm/h.

Once drilling was complete the bit and drill string were removed.

Saipem then lowered a special frame, complete with radial cutting disks, inside the casing to just above seabed level. The casing was cut off leaving a 1m tall stub.

Saipem brought in a separate jack-up to place the steel monopile, which was 200mm smaller in diameter than the drilled socket. The socket was air flushed to remove debris and grout tremied into the bottom of the hole to form a plug and bring the pile toe to its correct level. After curing, the pile was lifted in and held plumb for 24 hours while grout, tremied in to the annulus between the pile, hole and casing, set. The inside of the tube was also filled with grout to seabed level.

With the piles installed, Saipem followed on with the jetty and tanker berth structures, together with seabed pipeline manifolds and offshore single point floating mooring dolphins. The dolphins accommodate tankers up to 350,000t.

By July 1999 the first of the four jetty berths was ready for its first 65,000t vessel unloading a cargo of crude oil. And Saipem was able to make up much of the lost time to complete the project by September.

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