Sevmorneftegaz wanted to install a Gravity Based Structure (GBS) – the “Prirazlomnaya offshore ice-resistant fixed platform” - 50 miles south of Novaya Zemlya. With a height of 100 m, the platform consisted of two sections, the lower section or support (caisson) layer and the top section (top side).
The support (caisson) layer was constructed in the Sevmash factories in Severodvinsk (White Sea). This GBS support layer had to be towed from the factory in Severodvinsk through a channel to a position where the second layer (top side) was to be added.
After tow out the upper and lower structure are assembled. Subsequently, the complete GBS is towed to a position with sufficient water depth where ballast is inserted. After completion of the ballasting the GBS is towed to its final destination.
The Sevmash Project involved the dredging works required for the deepening and widening of the access channel towards the Sevmash factory for both the shallow as well as the deeper areas in order to achieve the required depth and width. This contract was carried out on behalf of Sevmorneftegaz Rosneft & Gazprom.
The scope of work involved deepening and widening the existing channel to the Sevmash factory.
The dimensions of the old channel were:
- Bottom width 100 m
- Channel depth 9.1 m (Baltic Height System)
- Slopes 1:5
- Length approximately 9 kilometres
The dimension of the new channel is:
- Bottom width 190 m
- Channel depth 11.10 m (Baltic Height System)
- Slopes 1:7
- Length approximately 12 kilometres
The dredged material had to be disposed off in a designated dumping location at a distance of 17 kilometres from the outer edge of the proposed channel, in deep water.
Material to be dredged consisted of sand and clayey silt without pebbles and boulders. Prior to the start of the dredging works, extensive bathymetric surveys were carried out.
A vessel chartered from the R.F. Navy, equipped by DI Survey department with a DGPS positioning system and Multi-beam echo sounder was used to monitor daily progress. For the very shallow areas a boat with a minimal draught and equipped with Single-beam echo sounder was used.
The TSHD Uilenspiegel carried out the majority of the dredging works (deeper than 8 meters water depth), while the TSHD Vlaanderen XX dredged the shallower sections (between 6 meters and 8 meters water depth).