The nature of aggressive hydrocarbon reservoir fluids places demands upon material selection for linepipe that can only be met by the use of corrosion resistant alloys (CRAs): either in solid form; or as an internal liner or clad layer combined with a carbon steel substrate. Design and construction guidance for such flowline systems is presently not comprehensive in offshore pipeline standards, even for cases where the CRA layer could be ignored in terms of structural design. Offshore pipelines designed and fabricated in accordance with DNV OS-F101 benefit from the standard allowing flaw acceptance levels for girth welds to be determined based on an engineering critical assessment (ECA).The linepipe materials presently available fall into two main categories: clad, where the CRA layer is metallurgically bonded to the carbon steel substrate; and lined, where the CRA liner is mechanically bonded in place within the carrier pipe. These products present a mixture of common and unique challenges when designing and welding flowlines. In particular, the welds in these materials are typically more complex than in rigid C-Mn flowlines and this fact is reflected in the difficulty in conducting ECAs using the available conventional guidance. Due to production limitations on linepipe dimensions, it may also be necessary to explicitly take account of the strength of the clad layer in the overall design, including assessing integrity and fracture control across the full (composite) wall thickness.This paper discusses conducting ECAs in such complex weldments whilst addressing the implications of these challenges. Reference is made to experience gained from two projects; where, in the most recent of these (the Deep Panuke project), new guidance on conducting such ECAs has been implemented for the first time. The Deep Panuke flowlines comprise: four 8in production flow-lines in clad pipe with a 12.5mm WT grade 415 (X60) carbon steel substrate and aninternal 2.5mm Incoloy Alloy 825 clad layer; and a single 3in acid gas flowline in solid Inconel Alloy 625. Both lines will be welded by manual GTAW using 686 filler material. The nominal level of installation plastic strain for the project ranges up to 1.7% in the case of the 8in line so the additional complexities of cyclic plastic deformation during installation must also be addressed by the ECA using constraint-matched SENT fracture mechanics specimens and a tearing instability fracture assessment.The challenge of achieving adequate strength in the weld is ever-present but the intrinsic yield strength limitations of CRA materials makes the probability of an undermatching condition high enough (despite a strong focus during weld procedure development) that the ECA philosophy has to be able to accommodate a potential weld undermatching condition. Broadly speaking, the strategy adopted is to use finite element analysis (FEA) to model the crack driving force (in terms of J or CTOD) of a flaw in an undermatched weld used in order to support and, where necessary, calibrate BS7910 type fracture mechanics assessments. The assessment will thus be fine-tuned to account for the actual level of undermatch present. The methodology is a new one and is a first for the Deep Panuke project. A case study from an earlier project on lined pipe is also presented for comparison.
展开▼
