DNVGL - DNV-RP-F113
PIPELINE SUBSEA REPAIR
|Publication Date:||1 October 2007|
This Recommended Practice (RP) applies to fittings used for repair and tie in of submarine pipelines. These fittings include: Couplings, Clamps, T-branch connections and Isolation Plugs. Mechanical means connect these fittings to the pipeline, however, sleeves/couplings and T-branches may also be welded.
The section on the strength of the mechanical attachments is also applicable to pipeline recovery tools.
Refer to Figure 1-1 for typical fitting applications.
Couplings connect pipes by direct attachment to the pipe walls via mechanical or welded joints. Flange connectors differ from mechanical couplings as flanges join pipes via thick, machined pieces of additional material that is welded or forged to the pipe ends prior to installation.
Clamps are fitted externally to the pipeline to prevent leaks or add strength.
Hot-tap T-branch connections are fitted externally to the pipeline assembly even during operation. A pressurized pipeline would be machined open to allow fluid flow through the branch.
Pipeline isolation plugs are pumped with the pipeline fluid to the repair site and then activated in order to form an isolating barrier that can resist differential pressure.
The pipe itself represents the key member of the repair assembly with consequential limitations such as, but not limited to, pipe wall strength, surface irregularities, and deviations in shape. Fittings for subsea repair must be installed with caution to reduce the likelihood of damage, (e.g. seal damage).
Coupling strength shall be sufficient in resisting stresses from all relevant loads, within a factor of safety as defined in Sec.4.6. Pipeline damage
Pipeline damage after installation may be caused by internal and external corrosion, hydrogen induced stress cracking (HISC), unstable seabed conditions, anchors, and dropped objects from the surface. The risk of damage depends on the intensity of surface activities such as ship transport and offshore operations, depth, seabed conditions and the design of the pipeline itself. The extent of possible damage will vary from insignificant to a fully buckled or parted pipeline. Consequently, the repair and repair preparedness strategy depends on this. Ref. 10 & 11 (CODAM & PARLOC) gives an overview of pipeline damage statistics.
Historically shallow water repair have mostly been performed by divers. The water pressure, however, limits human hyperbaric intervention to a few hundred meters water depth due to the human physiology. National authorities further regulate this diving to more shallow depth limits as a mean to safeguard the divers. 180 m water depth presently represents such a limit in Norwegian waters. This depth limit is only a small step to the 2500 m viable for present deep water pipelines.
Consequently, pipeline repair in deeper waters has to be carried out based on remotely controlled techniques.
A pipeline repair in general requires a range of planning and investigations prior to the actual repair:
- Investigation of the damage, the pipe condition and consequences for the pipeline operation, i.e. will any repair be required? Should pollution counter measures be started? Should water ingress in the pipe be limited?
- Planning of uncovering and seabed preparation for the repair including calculations of the pipeline response from this action.
- Planning the repair operation based on the state of emergency preparedness and the results of the investigations. (Planning, ordering of equipment and support)
- Seabed preparations, pipeline pressure adjustments, repair - Test to confirm the repair quality, protection of the repaired section, clean up and finish.