The Application of Reliability Based Methods in the Optimisation of Reeled Rigid Pipeline Wall Thickness Requirements

Abstract

Reel-lay is a fast and cost-effective means for the installation of subsea flowlines and Pipe-In-Pipe systems with outer diameters up to 18”. Pipelines installed by the reel lay method are plastically deformed during installation. The most critical step in the installation of a reeled pipeline occurs at the spool base, when the assembled pipeline is spooled on to the hub of the reel. The nominal level of deformation is dictated by the vessel equipment geometry, applied back tension, and pipe dimensions. Localised increases in deformation are caused by mismatches in bending stiffness between adjacent pipes. The mismatch potential is dictated by the natural variation of yield strength and by dimensional variation that is inherent to linepipe manufacturing processes. Reliability based assessments are commonly applied in the assessment of minimum acceptable wall thickness for reeling. These assessments enable the minimum acceptable wall thickness to be determined with a defined target reliability level, assessing mismatches based upon distributions of wall thickness and yield strength. The mismatch parameter calculation method and the definition of appropriate acceptance criteria are the two most important factors in reeling assessments. Neither of these two factors has been specified in a pipeline design code or a recommended practice available in the public domain. However, there is an increasing level of familiarity in industry; mismatch calculation methods and strain or ovality based acceptance criteria, defined by installation contractors are gaining widespread acceptance. This paper presents a review of the application of reliability based methods currently under use, focusing on mismatch calculation methods, acceptance criteria, and probability of failure calculation methods. Minimisation of costs is of particular importance in the current oil and gas industry climate. Because of this, the ability to specify an optimum wall thickness enables installation contractors to provide more cost effective reeled rigid pipeline solutions. After reviewing the subject matter and existing body of work this paper looks in detail at the deformation responses and failure modes for a range of sizes of reeled pipelines with mismatches. The assessment of deformation responses demonstrates a significant level of conservatism in recently proposed acceptance criteria that is based upon averaged axial strain levels. This conservatism is quantified by probability of failure calculations and provides a strong justification for further optimisation of the minimum wall thickness for reeling. Finally, the beneficial effect of increased reeling tension is quantified in terms of its effect upon probability of failure.