The future for protected pipelines
Statoil has examined PU field joints. At the Asgard site cracks were observed, so the system was tested for 2 years in salt water at 140C. There was no steel corrosion, so the cathodic protection was working and there were no external signs of hydrolysis. At Gullfaks a similar situation was observed. The coatings were tested for accelerated ageing under a variety of conditions to ascertain service life, which should be at least 20 years.
The ideal heat shrink sleeve for 3-layer field joint coating, according to Canusa CPS, combines epoxy, adhesive and a crosslinked polyolefin (either PE or PP). The force cure system was tested in the Waupisoo pipeline carrying bitumen from Fort McMurray to Edmonton.
KWH Pipe has been working in partnership with Borealis on a new field joint coating system, which has been used on the Gasum Oy Mäntsälä-Siuntio natural gas pipeline project in Finland by the subcontractor Stroytransgaz. The operating temperature of the pipe ranges from -30 to +30C and much of the installation took place in subzero conditions. The patented technique involves the equipment rotating around the pipe applying hot melt film of PE or PP, which requires a 400mm space. The process involves blast cleaning of the joint, the machine is then lifted onto the pipe and induction heating and FBE application take place, followed by polymer in a second induction step.
There are three main types of deep sea field joint coating: injection moulded polypropylene or polyurethane and elastomeric coatings. Exova is involved in independent testing and quality control. Pipes are subject to extremes of temperature during installation such as heating during welding, re-heating for field joint coating, quenching in salt water and cooling during lowering to the ocean bed. They are also subject to physical stresses from winding and laying. All of these factors should be taken into allowance when designing a new pipeline and coating system.
A new polymer system for deep sea field joints is under examination by IFREMER (the French Ocean Research Institute). The current materials have limitations – polyurethane by temperature and polypropylene by process. The new plastic is polydicyclopentadiene (pDCPD), trade name Telene. It has been subjected to simulated offshore accelerated ageing tests. Water absorption is low (less than 1%) and thermal conductivity testing at Exova in Canada shows that the material is okay after 6 months of ageing at 160C. The overall results indicate that this material is a candidate for field joint coating.