Ovality Prediction for Reel-Laid Pipeline Using Empirical Formulas

Abstract

In 2014 Asia Pacific, McDermott International, Inc (NYSE: MDR) completed installation of its first pipein-pipe (PIP) flowline system in water depths in the range of 1200m to 1500m by reel-lay using its reel Lay Vessel North Ocean LV105. During spooling onto the installation vessel and offshore installation, the pipeline is reeled, unreeled, straightened and deployed to the seabed. These operations subject the pipe to cyclic plastic deformation and induce significant residual ovality in the pipe after reeling and straightening. An accurate prediction of this residual ovality is a key parameter for deep water pipe collapse checking and is a dimensional requirement to determine expected welding hi-lo during offshore tie-in welding. An empirical design formula for ovality prediction has been provided in DNV-OS-F101 [1] (which makes reference to Murphey and Langner (1985) [2]. However the formula does not take into account the effect of pipeline contact pressure with the reel and therefore its predicted ovality is not in line with that of experienced during actual pipeline reeling. Alternative methods for ovality prediction are to conduct a full scale bending test (FSBT), or to perform a finite element analysis (FEA). However, a conventional bending rig test generally results in higher ovality because its boundary conditions such as back tension and shear force are different from those of actual reeling. On the other hand, FEA can predict the initial reeling-on ovality accurately, however it over-estimates the residual ovality. The consequences of over-conservative ovality estimation are that it increases the pipeline design wall thickness requirement and limits the number of re-reeling cycles that can be allowed during pipelay operations (reversing the pipe back onto the reel or back over the aligner). This paper tackles the prediction of ovality for reeling and straightening processes using the finite element simulation software ABAQUS [3] by addressing the over-estimation issue caused by the standard isotropy plasticity model with a simplified but effective and practical post-processing method. A new empirical formula for predicting ovality under pure bending has been formulated based on curve fitting results from FEA simulation of pipeline pure bending and a novel approach has been developed to estimate the on-reel ovality for actual reeling. It started with estimating the pure bending ovality using the newly developed formula as mentioned above, followed by another newly developed formula that accounts for the effect of contact load on reel. This contact load ovality empirical formula is developed considering the pipe stiffness definition used in buried pipe design for plastic pipe [4] and steel pipe [5]. Combining both the ovalities due to pure bending and contact load respectively ultimately gives the on-reel ovality of interest. Finally, a new simplified approach for estimating as-laid ovality has been developed. This new approach is based on the above mentioned newly created on-reel empirical formula together with reduction factors based on in-house data.