Bundles and towed installation

Bundles and towed installation

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The completed bundle system was launched into the North Sea during a suitable weather window and then towed 190 km (120 miles) to its subsea location in a 36 hour period. During the tow, the bundle crossed over existing pipelines in controlled depth tow configuration with the positioning data being transmitted to the towmaster on the control vessel. Once the bundle reached its final location the carrier pipe was flooded to stabilise the bundle on the seabed. The tie in spool pieces were then installed prior to final commissioning of the system

TOWINGMETHODSSUMMARY

TOWING METHODS – SUMMARY

Tow method

Advantages

Disadvantages

■ Minimum bundle weight ■ No additional chains or buoys needed ■ Can be installed in gentle curve ■ No issues with crossings or surveys ■ Bundle towed below wave-affected zone ■ DP tugs can install within curved corridor

■ Accurate seabed surveys required ■ Safety issues at crossings ■ High bollard pull (twin tugs) ■ Accurate seabed surveys ■ Permission and protection for crossings ■ Requires accurate control of tension ■ Close tolerance to ensure buoyant weight ■ Large tugs for depth control ■ Requires sheltered trimming bay adjacent to a long construction site ■ High risk of fatigue at trailing end ■ Requires calm conditions or short tows ■ Removal of any buoys (not Flowlay) ■ Requires separate sinking operation ■ Removal of buoys

Bottom tow

Off- or near- bottom tow

Controlled-depth tow

■ Simplest towing configuration

Surface tow

Controlled-depth surface tow

■ Reduces wave fatigue risk of pure surface tow

Any questions?

The advantages and disadvantages of the five methods of towing bundles are shown in the above slide.