DNV has developed a protocol to ensure diver safety and weld integrity for the most challenging subsea welding jobs where traditional pressure testing is not possible.
The welding of new connections on to operational subsea pipelines as satellite wells are added to increase production in an offshore oil or gas field minimises down time but these hot tap welding operations pose a risk to the integrity of the pipeline and the divers undertaking the work.
Typically pressure testing is used to ensure the strength and integrity of the welded joints but this is not always possible depending on the geometry of the connection and the dimensions of the pipes involved. The root of the main weld can be particularly difficult to access and inspect, and in some cases, the requirements of a standard strength test could lead to distortion or collapse of the pipes.
Such welds are called golden welds and, speaking at Offshore Europe 2011, DNV’s Dr Alan Smith presented a methodology for managing these new pipeline connections in order to qualify the welding work done.
“ It is important to realise that strength tests are recommended practice if possible but this work undertaken by DNV and Intecsea demonstrates that a comprehensive, evidence-based justification can be given that golden welds have the integrity required to operate safely, ” says Dr Smith .
“ Design verification requires evaluation of the entire hot tapping process before work commences to ensure the mother pipe will not yield during the preheat stage and that burn through cannot take place. Modelling these failure mechanisms requires care, skill and a good understanding of the engineering and material issues involved.”
Based on a combination of fracture mechanics and non-destructive testing, the welding work can be verified once completed. This weld qualification includes evaluation of potential flaws in the weld, determining stress distributions at maximum operating pressure and worst case environmental loading, measuring fracture toughness and ensuring that critical flaw dimensions are not exceeded. Several non-destructive inspection methods can be employed including magnetic particle inspection (MPI), compression wave ultrasonic testing (UT) and creeping wave UT.
The presented paper “Ensuring the Integrity of Subsea Hot Tap Welded Joints in Lieu of Hydro Testing” was co-authored by Robbie Williamson of DNV and Andrew Low of Intecsea.
By Per Wiggo Richardsen
[mappress] Source: DNV, September 07, 2011;