Chapter 10 A COMPREHENSIVE STUDY ON THE EFFECT OF SOIL MOVEMENT DUE TO MANMADE OPERATIONS ON SUBSEA PIPELINES

Geohazards for pipelines are typically generated by natural events such as mudslides or earthquakes. Additionally, deformation or energy released due to manmade geohazards near pipelines can inflict significant damage. These types of events may affect a limited pipeline length but can cause substantial local deformation potentially resulting in leakage. Spudding near fixed jackets is a typical example of such risks. Jack-up rigs are often utilized nearby shallow water fixed platforms and pipelines for well intervention operations. The effects of spudcan penetration/extraction operations on nearby platform piles have been studied extensively and generic guidelines are adopted by many regulatory bodies throughout the world. The interaction between such operations or other potential earthwork related activities and nearby subsea pipelines, and potential long-term integrity effects, are lacking in the literature. This study aims to fill the gap by providing findings from numerical assessment of interaction between displaced soil and pipelines.

This paper presents results and observations from recently carried out case studies. Spudcan-soil-pipeline interaction was studied using a commercially available, large scale, non-linear finite element analysis tool using Coupled Eulerian-Lagrangian (CEL) approach. The investigation also covers potential effects of spudcan extraction and subsequent soil collapse potential. In addition, sensitivity studies were carried out by varying soil profile parameters and pipeline end conditions. Buckling of pipeline and associated potential integrity implications due to soil movement are discussed in detail. Based on the findings, a set of generalized best practices were developed.

It was observed that spudcan penetration increases stresses in the pipeline within one spudcan diameter due to seabed soil movement. Soil movement causes localized deformation to the nearby pipeline and can increase the risk of subsequent inelastic buckling under operating conditions. The most notable aspect of these simulations was the effect of spudcan extraction. It was found that spudcan extraction conditions are more critical for soil displacements and degradation.

Results of the case studies and generalized recommendations provided in this study are expected to provide guidelines for screening level assessments and further precautionary requirements. In addition, observations are reported in this study on the effect of accidental drop of heavy equipment or structural components on pipelines during installation/removal procedures. This study would help industry in assessing and mitigating risks to subsea pipelines in a more effective and comprehensive manner. Methodologies discussed in this study can also be utilized for assessment of pipelines subjected to natural geohazards.