On the Influence of Mechanical and Geometrical Property Distribution of the Safe Reeling of Rigid Pipelines

The reel-lay method is a fast and cost efficient alternative to the S-lay and J-lay installation methods for steel pipelines up to 20^″ in diameter. The quality of the pipeline construction is high due to onshore welding under controlled conditions. However, reeled pipelines are subject to plastic straining (up to approx. 2.3%) during installation. It is therefore common practice to specify a minimum required wall thickness to avoid on-reel buckling. For a given pipe outside diameter and bending radius, formulae developed for pipes under pure bending are generally used. In addition, to ensure the integrity of pipelines during reeling, a minimum spooling-on tension is specified and tolerances on pipe properties, such as wall thickness and yield strength, are constrained. Tolerance limits are specified to reduce the likelihood of spooling two consecutive pipe joints, which have a significant difference in plastic moment capacity (mismatch). It has been shown previously that high levels of mismatch can trigger an on-reel buckle [1]. The reliability of the reeling process is indeed related to the uniformity of pipe properties. It can therefore be supposed that more uniform pipe properties may allow reeling of thinner-walled pipes, while achieving the same level of reliability. This issue has been investigated as part of a wider evaluation of reeling mechanics and the development of procedures for optimized assessment of the process, including such aspects as the effect of the geometry of pipelay equipment [2]. This paper explores methods that can be used to evaluate the reliability of reeling a given pipe onto a given vessel. Particular focus is given on the selection of appropriate material variation parameter for the assessment. The concept of an averaging factor is introduced as a means to relate variations in individual wall thickness and yield strength measurements to the variation in pipeline cross-section, which determines the likelihood of buckling. It is suggested that, in the future, this factor could be used as a method for optimizing design for reeling when using higher quality pipe.