Recent years have seen the long discussed offshore decommissioning of platforms becoming a reality with size reduction being performed both offshore and inshore. Diamond Wire Cutting Systems (DWCS), which produce self verifying cuts by the diamond wire physically passing through the target, ensuring that no inclusions are left to prevent lifting operations, are frequently utilized in platform decommissioning operations.
The removal of large offshore structures, with its inherent dependence on high cost heavy-lift vessels, has led to an in-depth appraisal of the facilities being included in sub-sea cutting spreads to meet today's requirement of reducing the time that lift-vessels and work-class remotely operated vehicles (WRovs) are on station while simultaneously maintaining the self verification requirement of the cut line.
To meet the challenges of Frigg, North West Hutton and Ekofisk removals the CUT group has developed a new range of features on its patented Diamond Wire Cutting System.
The large range of section sizes that must be cut in order to remove a platform, eg braces, risers, jacket legs, piles, pile guides, launch runners, etc, often leads to a requirement for a large range of different size cutters to be mobilised.
To reduce this requirement a new range of modular cutters capable of handling all sizes from 60 (1524mm) to 120-in (3048mm) diameter has been developed.
Apart from reducing the number of cutters required this also leads to a large reduction in deck space requirements and operational planning.
The principal of the system is to utilise a common, and standard, in-feed system but to also allow the width of the cutter to vary by the insertion of buoyant spacers. This is especially useful for cuts across launch runners where the target is an irregular shape.
Baskets
In operations it is commonly required to move from horizontal to vertical cuts with various degrees of batter and angled cross bracing cuts in between. This would normally require separate deploymen/parking baskets for each size cutter and orientation.
To simplify deployment a new range of baskets has been designed to allow vertical, horizontal and any degree of pitch or roll stowage of the cutter. All inside the one outer frame.
The basket is fitted with positive WRov locks and also an internal hydraulic lock to prevent movement of the cutter when passing through the splash zone.
Additionally the option exists to be able to hang the basket on the structure if mid water cuts are required, for example if the jacket is being removed in two or more parts.
This saves considerable time compared to the basket being laid on the seabed with the WRov having to transit to and from the cut site.
Cutting inside the jacket
To obtain initial separation of the major sections it is common for internal bracing to be removed at the split line. This previously meant flying the cutter inside the structure, with its surface umbilical attached.
The new generation of cutters can be flown into place and locked to the structure, without the power umbilical being attached, this and the emergency release are connected later.
This system gives easier WRov positioning without umbilical drag being present and also increased protection against entanglement and abrasion when flying to the site.
Previously it was required to rely on the WRov vision system when approaching the target, and especially during the cut, which could be difficult and time consuming for the WRov, especially when working around the large diameters found at skirt piles, etc.
Electro/hyraulic umbilical
The new modular systems are now complete with their own front and rear view systems operated through the electro/hydraulic umbilical making it independent of the WRov in cutting operations and consequently allowing the WRov to move off station for other operations.
Having cameras and electro/hydraulic umbilicals as above, it has now been possible to extend the instrumentation of the new cutters to give real time recording of the cut progress and also of the tension present in the diamond wire, independent of the WRov telemetry, so further allowing the WRov additional freedom to conduct other operations.
Structural integrity
As the main jacket/pile sections are cut the platform obviously looses integrity and to counter this, the new range of cutters are complete with the ability to give a step (or castellated) cut, as illustrated, which constrains the section against lateral movements. This is achieved by including an extra degree of freedom in the cutter to track not only across, but also along the structure axis.
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Cpt Giuseppe Ummarino BSc, General Manager, and Peter J. Russell, MSc CEng Sales and Marketing Manager, are with Cutting Underwater Technologies AS, Ålgård, Norway. www.cut-group.com
