On-Bottom Stability Analysis of Subsea Pipelines using a Finite Element Model
Abstract
The dynamic on-bottom stability analysis represents a fundamental task in the design process of the subsea pipelines. Such analysis ensures the stability of the as-laid pipeline on the seabed against the lateral displacements which are induced by the surrounding hydrodynamic forces. In this paper, the dynamic on-bottom stability analysis is performed using a finite element-based advanced offshore engineering simulation software called “Flexcom” to predict the pipeline response in a time-domain simulation based on a given environmental condition (i.e., the sea state, the soil frictional resistance, and the non-linear behavior of the pipeline). A case study is conducted on a 22-inch pipeline, which is placed on sandy soil in shallow water, under different loading combinations from two-dimensional irregular waves and a steady current. The resultant maximum lateral displacements and the associated stresses are examined via different thicknesses of the concrete weight coating. Pipeline response from the numerical simulation is investigated and reasons behind the dynamic response are defined, seven simulation runs using random seed numbers are performed to generate different wave patterns for the selected concrete weight coating thickness to confirm the on-bottom stability against different random sea states, comparison with other conventional stability methods is conducted to study the effect of ignoring passive soil resistance and the importance dynamic stability analysis to optimize the on-bottom stability design. It is observed from the results the importance of several factors on pipeline stability such as pipeline submerged weight using the simple friction model, hydrodynamic loads induced by random sea states on the stresses and lateral displacements, and soil friction model being used incorporating the passive soil resistance term. The comparison has emphasized the reduction in concrete weight coating using dynamic stability analysis if the actual case is modeled accurately.
