In two horizontal directional drillings (HDDs) over 1,350 and 1,450 metres in length, VBMS (as part Visser & Smit Hanab) installed a 48" gas transport pipeline under the river Schelde for the Dutch gas board Nederlandse Gasunie, along the North–South route. One of the project’s more spectacular features was a temporary working island, built in the Western Schelde as a platform for the 450-tonne drilling rig.
The Western Schelde crossing between Ossendrecht (NL) and Zelzate (B) is part of the so-called Wijngaarden–Zelzate section. For this unique crossing, Gasunie called in the expertise of Visser & Smit Hanab. VBMS (part of Visser & Smit Hanab until 2012) assisted the drilling department on the project. In fact, the project involved two crossings: an HDD across the Schelde-Rhine canal, along a length of 600 metres, and the crossing of the river Schelde, which consisted of two HDDs, 1,450-metres and 1,350-metres long, installed from a temporary working island (approx. 80 x 30 metres) in the middle of the river. This method was designed to ensure minimum disruption to the area.
The work consisted of the following:
With regard to the logistics surrounding the working island platform, this included co-ordinating the transport of the 450-tonne drilling rig to and from the island in the river. A shear leg hoisted the rig from the quay onto a transport pontoon, then positioned it at the proposed location upon reaching the island.
After the two pipelines were successfully pulled in, excavation on the inner cofferdam started. This provided a construction pit with a depth of no less than 12 metres, 10 metres of which lay below water level. On the newly formed riverbed, the two pipelines were welded together with spool pieces that had been brought in, which were then cut to size on-site. One limiting factor was the tidal range: at low tide, the working island dried 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. When all was said and done, Visser & Smit Hanab was able to safely drill over 2,700 metres of pipeline under the Western Schelde.
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