In-Place Free Span Assessment Using Finite Element Analysis

The DNV RP 105 standard provides guidelines for evaluating the fatigue damage of pipelines over free spans, where guidance is provided for calculating the approximate Vortex Induced Vibration (VIV) response, considering the pipe properties, the span geometry, the pipe-soil interaction and the effective axial force. The approximate response models are limited, however, to single spans with leveled shoulders, short length, i.e. span lengths less than 140 times the pipe diameter, and bar buckling not influencing the pipeline dynamic response. To overcome these limitations, specifically for “long” spans and multi-spanning pipelines, RP F105 recommends that eigen-value analysis be performed using Finite Element (FE) method to calculate the natural frequencies, mode shapes and corresponding stresses associated with the mode shapes considered for VIV fatigue assessment. In this sense, a methodology and suite of Finite Element (FE) based tools for multi-mode/multi-span VIV fatigue assessment have been developed. The FE methodology accounts for the initial static equilibrium configuration of the pipeline in the as-laid condition followed by application of the subsequent load steps, such as flooding, hydrotesting, dewatering, start-up, etc. It also considers the non-linearity of the seabed stiffness and the effect of geometric non-linearity/large deflections on the dynamic response of the pipeline. In addition, the FE approach allows the determination of the dynamic response at every location of the free span for the different mode shapes and hence offers the ability to calculate the distributed fatigue damage along the spanning section of the pipeline instead of assuming all damage occurring at a single location. The methodology was applied in recent projects to allow for a better estimate of the requirements for free span correction with significant cost savings anticipated. The proposed methodology also has the potential for post-lay assessment of the pipelines and for through-life in-service assessment of existing pipelines, where new free spans are often observed during inspection due to soil movements along the lifetime of the pipeline. This paper addresses the in-place FE methodology, the validation process and the tools that were developed to speed up the fatigue assessment procedure, which is a key factor especially when analyzing post-lay survey in real-time for determination of requirements for free span correction in the field.