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Problems pile up for UK contractor

Monopiling has been used to deliver a mammoth jetty against tough deadlines and in a harsh environment

Tides in the Gulf of Kachchh, north west India, rise and fall by 6m. The Gulf is also internationally recognised for its delicate maritime environment. Taken singly these factors would pose a challenge to marine contractors.

Jointly, construction of a jutting, 3.5km jetty and tanker berth facility has called for sophisticated piling techniques.

The £125M facility serves the vast Jamnagar refinery complex in Gujarat state. In addition to the fantastically long main jetty, it consists of an 850m jetty adjoining the main structure with four tanker berths, 23.2km of sub-sea pipeline, and offshore single point mooring dolphins. It was delivered by main contractor Saipem UK under a design and build contract.

Client Reliance Petroleum needed deep water berthing for ships of up to 350,000t. Shallow waters in the Gulf meant major dredging or construction of a very long jetty were unavoidable.

Cost and speed of delivery tilted the decision towards a jetty.

To meet the tight deadline and contend with the area's notoriously difficult weather conditions, Saipem opted for a modular structure above a piled foundation. Large sections of the superstructure were pre-assembled and barged to site for installation. To minimise seabed damage Saipem opted for monopiled foundations.

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

Marine subcontractor Seacore won the specialist drilling contract for pile construction and brought two reverse circulation hydraulic drilling rigs to site to penetrate the fractured 200MPa basalt peppering the sea bed. Seacore created deep and partially cased rock sockets for jetty, tanker berth and mooring dolphins, into which Saipem placed tubular steel monopiles.

At the end of 1997, five months after work started, the drilling programme was thrown badly off track. Saipem diverted one of the jack-up barges supporting Seacore's work on to critical jetty pile installation, forcing Seacore to shut down one of its drilling rigs.

A month later, a vicious cyclone hit the exposed site, washing all Saipem's floating plant away and beaching much of it inland in areas of mangrove swamp.

'In all, 26 barges were grounded, ' says Seacore contracts manager Julian Cockett.

Four and a half months were lost while Saipem recovered its plant. Seacore was forced to race to make up time, drilling from two separate jackups, completing the smallest diameter piles at the end of February 1999. Large diameter drilling was completed in May after further juggling of plant.

Seacore carried out work from a combination of two jack-up platforms on the jetty works and two floating barges on the offshore single point mooring location. Operating in water up to 30m deep, the company drilled 214 rock sockets ranging from 1.2m to 3m diameter, to depths between 8m and 18.5m into the seabed. In total 2,160m was drilled.

The operation itself involved pitching a long tubular steel pile hole casing on to the seabed from a jack-up barge.

This was driven through an initial layer of silts and shells, just into the underlying and weakened basalt. Once the casing was in position a 'bottom hole assembly' - drill bit, stabiliser and drill string - was installed and the drill rig clamped on top of it.

Once drilling was complete the casing was cut off leaving an approximate 1m tall stub sticking out of the seabed.

Saipem brought in a separate jack-up for placing the steel monopile, which was 200mm smaller in diameter than the drilled socket. The socket was air flushed to remove any debris. Grout was 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 about 24 hours while grout, tremied in to fill the annulus between the pile, hole and casing, set.

The inside of the tube was finally filled with grout to seabed level and steel caps welded to complete the monopile.

Saipem brought in pre-fabricated substructure and superstructure elements by barge from a fabrication yard in Dubai. These comprised composite steel frame sub-structures, complete with oil transfer pipes and sections of concrete roadway deck. Using a large floating crane they were lifted into place atop the pile caps at high tide. Once in position the deck joints were filled in and oil transfer pipes connected and welded together with expansion joints.

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