There is no doubt that the only permanent repair of a subsea pipeline is hyperbaric welding. Mechanical connectors or clamps exposed to the product can only be considered temporary repairs due to concerns over the corrosion with the seal mechanism. Given that there is a general lack of hyperbaric welding capability generally available in various regions of the world
The options for a damage repair are:
- To monitor the damage, or leak, or contain using sleeves or clamps where possible until a planned shutdown period, then repair using hyperbaric welding, if the damage is sufficiently light;
- To execute a temporary repair in the form of mechanical connector installation, and follow up at the first convenient planned shutdown with a permanent hyperbaric repair within two years;
The decision will be made in the particular circumstances during engineering assessment, considering:
Planning & Control
All the repair options have to be carried out in a planned and organised fashion, an immediate threat means an immediate shutdown/depressurisation of the pipeline and /or associated subsea systems with remedial actions implemented as soon as possible. A planned repair correction, meaning production can continue as normal or the pipeline depressurised within acceptable limits, until a planned repair.
All the inspections and repair types will have to be organised with the correct procedures in place, the list below represents some of the procedures/manuals that should be already be in place or implemented to assist with the planning, preventative and offshore construction activities:
- Offshore Marine manual (Regularly updated)
- Diving Operations Manual (Saturation & Air diving regularly updated)
- Field charts (Regularly updated)
- Emergency contact list (Regularly updated)
- Inspection & Maintenance manual (Regularly updated)
- Bridging document proforma (updated for each contract)
- Generic risk assessment record
The repair scenarios listed below can be applied to repair categories:
- Pipeline Sleeve/clamp repair, grouted or sealed.
- Diver Habitat Hyperbaric welding repair.
- Mechanical connector subsea repair with spool
- Above water tie-in, for dry pipeline welding or flanged with subsea spool tie-in (shallow water)
Note: All the repair scenarios will require detailed risk assessments, planning, procedures and SOW working with the diving/marine/construction/fabrication contractors.
Repair Sleeves
There are a number sleeve repair clamps for different damage applications can be used for pipeline repair.
- Grouted /epoxy sleeve repair (Primarily used for dents and gouges)
- Pressure sealed sleeve repair (Primarily used for small leaks)
- None standard sleeve (Primarily used for dents and gouges on bent pipelines)
The sealed pressure clamps and the grouted sleeve clamp. Both are similar for preparation and installation but have fundamental differences in reasons for remedial rectification of the pipeline damage.
None standard sleeve repair clamp would be fabricated to fit a certain criteria of damage on a pipeline that is outside the tolerances for a standard sleeve repair.
The grouted repair sleeve that uses concrete/grout to fill the analysis or void from the outer parent pipeline metal to the sleeve inner clamp shell. This repair is normally associated with pipeline dents and gouges that are within the pipeline tolerance, but represent a long-term threat. This type of repair system is the most commonly used for immediate repair to a pipeline throughout the world, this is due to the common occurrence of drop objects, dragging anchors, and fishing trawl boards. The pipeline does not normally have to be fully depressurised, but depended on the risk analysis assessment the pressure will have to be reduced and isolations in place for the repair period.
Mechanical Connectors
A pipeline repair using a mechanical method to seal the connectors to the pipeline ends can be achieved on the seabed, rather than pulling both ends of the pipeline to surface to install welded flange connections. The actual connection or tie-in to a spool is the same for mechanical or welded connectors.
Hyperbaric welding habitats
Hyperbaric chambers in which divers enter to carry out welding in a dry atmosphere, for over 25 years the marine, offshore and oil and gas industries have utilised this technique to perform various construction work on pipelines. Its application and methods have been the preferred method for tie-ins both on spool installation and pipeline repair.
Hyperbaric welding is the term giving to welding when it is performed under pressure. There are two forms of this in the diving industry, “wet welding” carried out in, but unprotected from, the surrounding water, normally a non-critical weld. The other form is "habitat welding" this process is carried out Sub Sea, but in the dry, and is often the clients preferred choice when specifications required are stringent.
The costs associated can be high in comparison to a flanged tie-in, which as left hyperbaric welding to a secondary method and a flange connection a cost effective solution.
The most common type of welding performed within a habitat are pipeline welds, carried out on either new laid pipelines that are tied into the platforms, damaged pipelines were sections are cut out and replaced with a section of new pipe.
With specifications and criteria being high it is normally only those divers with a welding background that get involved. The welder divers are all experienced & highly qualified; they execute this trade with the utmost professionalism and develop the much sought after skills.
The hyperbaric diver welders can enter the Habitat from underneath, strip out of their diving equipment and prepare the area for welding. All the services are deployed from the DSV support Vessel including gases for breathing, hot water for heating, communication cables and an electrical supply for power tools and the welding plant.
Above Water tie-in
Mobilisation of a pipelay/davit barge capable of lifting the ends of the pipeline to the surface, to allow either surface tie in (only in shallow water or dewatered pipeline), or welding of flanges to permit subsequent subsea connection by installation of a flanged spool.
Both repair methods are large scale repair operations that whilst simple in concept requires considerable engineering planning to achieve a successful outcome, especially in deeper water.
The probable damage inflicted on a pipeline that requires a repair by above water tie-in, would in most cases mean that the pipeline is already flooded due to the ingress of water through the damage area, or that the damage was substantial enough for the section to be cut out and replaced.
|
