Flowlines/Umbilicals/Risers

Removal of Protection Mattresses & Spool Pieces

The pipeline protection mattresses can be removed from the import pipeline and umbilical and will be recovered to the DSV to allow access to the spoolpiece and umbilical. The condition of the mattresses may have deteriorated over time or sustained some damage, so recovery will utilise a support cradle or be deposited in a skip placed on the seabed prior to recovery onto the deck. This will assist in the protection of the divers from the dangers resulting from a mattress breaking up whilst transitioning from water to air.

A work-class ROV or divers would then airlift local to the flanged connection between the spoolpiece and the pipeline. Hydratight tools or bolt cutters would then be used to remove the flange bolts.

As the spools are long and would be difficult to recover and store on the DSV deck, sectioning of the spoolpiece would be required. At the cut locations, and at the planned lifting points, the divers or ROV would locally excavate the seabed. This would allow access for installing a diamond wire saw deployed from an ROV, which would then undertake the cutting operation. The sections of the spoolpiece would then be recovered to the deck of the DSV.

Hydratight tools or bolt cutters would then be used to remove the riser flange bolts to allow recovery of the final section of the spool. Any sandbags used to support the spool adjacent to the jacket structure would also be removed to allow verification of seabed clearance. The pipeline would then be plugged in a manner suitable for the planned decommissioning operations.

The umbilical may be disconnected following receipt of the owners permission, either by severing with the diamond wire saw at the base of the j-tube or releasing the umbilical at the topside termination and pulling the umbilical through the j-tube. The umbilical would then be flaked out on the seabed for subsequent abandonment by the owner, who would be notified prior to completion of abandonment.

Removal of the spoolpieces for the 12” and 2” intrafield pipelines would initially follow a similar procedure to that for the 8” Import pipeline. The flanged connection between the two spools used in the tie-in would be utilised as a natural ‘cut’ position in this case. A combined de-watering and pipeline recovery head would be installed at the flanges.

A visual inspection of the area of the operations will then be carried out using the ROV. Pipeline related debris would then be removed prior to de-mobilisation of the DSV.

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