Bundles and towed installation

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Bundles and towed installation

WHAT ARE BUNDLES?

WHATDOWEMEANBYBUNDLESANDTOWEDINSTALLATIONS?

WHAT DO WE MEAN BY BUNDLES AND TOWED INSTALLATIONS? ƒ Flowlines and carrier strings welded ashore ƒ Held together with bolted-on spacers ƒ Filled with pressurised nitrogen ƒ Support for carrier against later hydrostatic loading ƒ Chains added to balance buoyancy

ƒ ‘Trimmed’ in sheltered bay nearshore ƒ Towed out to field – up to 4 days

ƒ DP tugs allow to settle within lay corridor ƒ Treated seawater displaces N2 in annulus ƒ Provides permanent stability on seabed ƒ Towed installation – flowlines with no carrier

Bundles are commonly used in the North Sea and West Africa (Girassol). They have been used elsewhere such as the Gulf of Mexico, Middle East as well as in the Artic and Australia in order to avoid the need for a laybarge. The assembly of flowlines is welded onshore and fitted within a thin-walled carrier pipe. This is not designed to resist the final water pressure at the seabed. Instead, the annulus between the flowlines and the carrier are filled onshore with pressurised nitrogen. The whole assembly including tow and trail heads is launched into an adjacent sheltered shallow bay where the weight of the bundle and chains is ‘trimmed’ or balanced to ensure the correct amount of buoyancy. Some short lengths of chain are attached to the bottom of some of the main chains using wire. Divers can cut through this and release some weight. Large tugs then tow the bundle to the field – usually at mid-depth so that it is beneath the effects of waves and yet not at risk of touching the seabed. This may take up to four days. If the tow stops, the bundle gently sinks to the seabed. At the field, the tugs manoeuvre the bundle within the lay corridor using their thrusters and DP. A survey vessel monitors the position. The annulus (and possibly the flowlines) are flooded with treated seawater, displacing the nitrogen. The additional weight – together with the chains – provides permanent stability to the bundle against environmental forces (current and wave action).