Bundles and towed installation

Bundles and towed installation

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Bundles can be positioned in curve by placing concrete blocks. Because the load is lower, a single tug can be used.

CONTROLLED-DEPTHTOW

CONTROLLED-DEPTH TOW

ƒ Traditional method used in the North Sea

Powerful lead tug

Trailing tug

Chain ballast

Large towhead

Controlled-depth tow: This is as for the off-bottom tow, but the tension in the bundle is increased by using a tow and trail tug. As the speed of the tow increases, the hydroplane effect of the chains causes the bundle to rise in the water column and ‘fly’; This is the traditional method used in the North Sea; In deeper water, the bundle adopts a snaking shape, reflecting previous lifting and lowering of the towhead as the tug speeds up or slows down. Towheads are typically 20 m to 30 m (60 ft to 100 ft) below the surface and lowest point of the bundle 20 m to 50 m (60 ft to 160 ft) above the seabed; Most third party pipelines operators request 15 to 20 m (50 ft to 60 ft) of vertical clearance when the bundle is being towed over their line; The average speed of the tow is about 4 knots (2 m/s); The bundle starts the tow in off-bottom mode and is returned to this configuration, by reducing speed and tension, for the final positioning in the field.

TOWCONFIGURATIONFORCDT