The Span Mitigation Analysis With Use of Advanced FEA Modeling Techniques

Abstract

Spans occur when a pipeline is laid on a rough undulating seabed or when upheaval buckling occurs due to constrained thermal expansion. This not only results in static and dynamic loads on the flowline at the span section, but also generates vortex induce vibration (VIV) which can lead a fatigue issue. The phenomenon, if not predicted and control properly, will result in significant damage to the pipeline integrity, leading to expensive remediation and intervention works. There are various span mitigation methods in use for both over stressing and fatigue concerns. The mitigation methods, if not analyzed properly, may result in much unnecessary work or generate more problems or concerns in the future. The mitigation analysis can become very challenging due to many restrictions in the field such as the minimum and maximum heights or lift of mechanical supports or grout bags, and bearing capacity vs. cost of supports. The cost of different mitigation methods and their interactions are the other considerations along with the installation tolerances, challenges associated with the water depth and uncertainties in seabed properties. This paper describes the latest developments in use of finite element analysis to investigate associate mitigation solutions given the governing practical limitations and cost factors. The ULS and fatigue lift criteria are used as the guidelines. The methods presented within this paper are applicable for various span conditions. Conclusions are then drawn to the impact of these various scenarios so that the pipeline integrity can be assured with confidence.

Review of pipeline span analysis

“…Wang et al (2011) described the latest developments in the use of FEA to examine the associated mitigation solutions, given the dictating practical limitations and cost factors. This was carried out to optimise mitigation methods such that unnecessary works or concerns in future are avoided.…”

“…The theoretical background on fatigue phenomenon can be found in Det Norske Veritas (2000, 2007); Rippon et al (1986) and Ruby and Hartvig (2008). Relevant references on span-creation mechanisms include Det Norske Veritas (2007); Shittu (2012); Shittu and Kara (2018), Wang et al (2011) and Wei et al (2010). Span-creation mechanisms due to environment include scouring, sandwaves, liquefaction, underwater landslides, etc.…”