By two horizontal directional drillings of over 1,350 and 1,450 meter in length, Visser & Smit Hanab installed a 48" gas transport pipeline under the river Westerschelde for the national gasboard ‘Nederlandse Gasunie', North-South route. A spectacular feature of this project is a temporary working island in the Westerschelde where the 450 ton drilling rig has been set up.
The Westerschelde crossing between Ossendrecht (NL) and Zelzate (B) forms part of the Wijngaarden - Zelzate section. For this unique crossing, Gasunie called in the
expertise from Visser & Smit Hanab.
Temporary working island
In fact, the project comprises two crossings: the crossing of the Schelde-Rhine canal with a HDD over a length of 600 m and the crossing of the Westerschelde. These consist of 2 HDD's of 1,450 m and 1,350 m in length, from a temporary working island (apporx. 80 x 30 m) in the middle of the Westerschelde. This method ensures a minimum of disruption in the area.
Drilling rig: a unique position
The coordination of the logistics surrounding the working island included the transport of the 450-tonne drilling rig to and from the working island in the Westerschelde. A shear leg hoisted the drilling rig from the quay onto a transport pontoon. At the working island, the shear leg positioned the drilling rig at the proposed location.
After the successful pulling in of the two pipelines, the excavation of the inner cofferdam started. This provided a construction pit with a depth of no less than 12 metres, 10
metres of which lies below the water level. On the new seabed, the two pipelines were welded together by means of spool pieces brought in, which were then cut to size on
One limiting factor was the tidal range; at low tide, the working island dries out. Because of the dynamic area surrounding the working island, weekly surveys were taken
to monitor the ground coverage of the nearby pipelines and backfill it if necessary.
As a result of this project, Visser & Smit Hanab was able to safely drill over 2,700 metres of pipeline under the Westerschelde.
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