Bundles and towed installation

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Bundles and towed installation

Controlled-depth surface tow can use either fixed buoys or tethered tubular buoys distributed along the length of the bundle as shown above. Early methods made use of fixed buoys, but these accentuate the flexing of the pipeline in the waves. The resulting fatigue can cause a significant loss of strength during the tow-out – generally at the trail end. The newer development, near-surface tow makes use of tubular buoys fixed to lines which trail back at an angle, losing some of their buoyancy. At low speeds, the buoys float at the surface, allowing tow-out to proceed through the peaks and troughs of the waves. The tethers reduce some of the flexing due to wave action and the resulting fatigue found at the trailing end of the bundle on true surface tow. As the tow speeds up, the buoys change their attitude as drag increases. They thus lose some of their buoyancy, allowing the bundle to sink beneath the wave action completely. (This is analogous to the chains losing some of their weight for CDT, allowing the bundle to ‘fly’.) When the speed is reduced, the bundle will float back to the surface again. Tugs need not be as powerful as with CDT. Procedures are required to permanently sink the bundle within the lay corridor in a controlled manner, and to remove the buoys. The method has been used to install the rigid section of hybrid risers. In this instance, the bundle is flooded after the buoyancy is removed, and the tail end was pulled down to the seabed anchorage. At present, the lengths towed by this method are much shorter than other methods. However, the buoys are recovered and reusable – in contrast with the chains with CDT which are left on the seabed.

APACHEBACCHUSBUNDLESVIDEO

APACHE BACCHUS BUNDLES – VIDEO

This video shows the use of a bundle to develop the Apache Energy Bacchus field in the North Sea in 2012. Bacchus is tied back to the Forties field, one of the largest developments in the North Sea. The bundle was fabricated by Subsea 7 at their Wick facility in the north of Scotland. The towhead and trailhead, each weighing over 200 tonnes were fabricated by sub- contractors and after transportation to Wick attached to the ends of the assembled bundle. The 42.5 in carrier pipe for the bundle contains two 6 in production lines and two 4 in heating lines inside a 20 in sleeve pipe.