Severe Sour Pipeline Lateral Buckling Mitigation Design Optimisation

Abstract

Abstract This paper presents the optimisation journey of lateral buckling mitigation design for a severe sour service pipeline. The journey, which was in a front-end stage of a project, involved preliminary in-environment fracture toughness testing program, preliminary engineering critical assessments (ECA), and pipeline design activities to achieve a feasible pipeline design. A thermal buckling study concluded that the pipeline is susceptible to lateral buckling with unacceptable consequences. Lateral buckling mitigation options were investigated and confirmed that the Zero Radius Bend (ZRB) buckle trigger remains an effective solution. The pipeline design thus focused on the optimization of the material, number of the trigger points, and its locations. In-environment fracture toughness testing program was conducted using representative girth welds made by carbon steel consumables and representative carbon steel pipes to cater for the required design scenarios. Concurrently, an ECA was performed to establish allowable pipeline longitudinal stresses as input to pipeline design for optimization of lateral buckling mitigation measures. This assessment was iterative to define a compromise between the welding acceptance criteria, the material capacity (fracture toughness in sour) and the number of buckle triggers to be installed. The ECA outcome provided the allowable longitudinal stress for use in the pipeline design. Normally, ECA is performed in the execution stage of a project to define the welding acceptance criteria for known material properties and final pipeline design stresses. It is also common that pipeline design is driven by factors that do not consider the material capacity limitations on large stress (strain) utilisation in sour service and rather aim to comply with a given design code. It is less commonly known that these design codes do not account for significantly reduced carbon steel material properties in severe sour therefore there is a gap in the pipeline design codes. The main learnings from this journey are that allowing for pipeline utilisation up to the SMYS value of the material (defined at 0.5% strain in DNV-ST-F101 and API 5L) is not safe for sour pipeline design in severe sour conditions (NACE TM 0175 Region 3). Instead, severe sour pipeline design should: (1) Not exceed the Sulphide Stress Cracking (SSC) material qualification stress limits; and (2) Satisfy ECA defined limits for the longitudinal stress in the girth weld. If these are not considered, the pipeline may (i) initiate cracking in the crown of the lateral buckle in service due to SSC degradation, (ii) it may become impractical to construct if the welding acceptance criteria become too stringent in the execute project phase, or (iii) it may be unsafe to operate for the intended design envelope.