Design and Welding Challenges in the Infield Flowlines of the Encana Deep Panuke Development

Abstract

Designing and constructing subsea flowlines to address the implications of aggressive hydrocarbon well fluids — and selecting suitably corrosion-resistant materials for such applications — typically proves challenging and often leads to the specification of clad, lined, or solid corrosion resistant alloy (CRA) linepipe materials. Design and construction guidance for such flowline systems is presently not comprehensive in offshore pipeline standards, even for cases where the thickness of the CRA layer is ignored in the structural design. Acergy are designing, procuring and installing a series of technically challenging infield flowlines within the Encana Deep Panuke gas prospect located off the coast of Sable Island, Nova Scotia. Presently being developed, first gas from the Deep Panuke field is scheduled for the third quarter of 2010 following the tie-in of the infield flowlines to their respective subsea production wellheads. These flowlines are to be installed using the Acergy Falcon, a vessel which has an installation system based on a variable angle J-lay principle and plastic deformation of the pipe. The four 8in production flowlines are clad linepipe comprising a 12.5 mm WT grade 415 (X60) carbon steel substrate with an internal 2.5mm Incoloy Alloy 825 clad layer that is metallurgically bonded to the mother pipe. The single 3in acid gas flowline is solid Inconel Alloy 625. The nominal level of installation plastic strain for the project ranges up to 1.675% in the case of the 8in line. Both lines will be welded by manual GTAW using Inconel 686 filler material. The pipelines are designed and fabricated in accordance with DNV OS-F101 supplemented by new guidance emerging from a DNV joint industry project on clad and lined materials. Metallurgically clad and mechanically bonded (lined) products present a mixture of common and unique challenges when designing and welding flowlines. The existing production limits for pipe dimensions in clad material have for some time now existed on the very cusp of design requirements, especially when using only the thickness of the steel substrate to resist the design loads. Indeed, recently the design demands of some projects have clashed with the available linepipe geometry and the mechanical properties of the clad layer material have of necessity been taken account of in the structural design. The dominant offshore design code, DNV OS-F101, is presently unable to offer specific guidance for including the clad layer and it is only in 2009 that joint industry research has established a viable design methodology for pressure containment wall thickness design which includes the strength effect of the clad layer. In addition to discussing the Deep Panuke design challenges and the welding philosophy for clad pipe, this paper also draws on approaches to welding and NDT successfully taken for the Statoil Tyrihans project in Norway, which used lined pipe material. The general welding philosophy adopted accommodates the continued inability of AUT systems to reliably inspect CRA weldments without false indications from normal metallurgical weld features. A proven approach is taken using intermediate inspection of the root and hot pass using real-time radiography (RTR); effecting any repairs needed; and then re-inspecting the weld upon fill and completion using RTR again. The importance of — and difficulty in — achieving adequate weld metal yield strength in CRA weldments is also discussed.