A finite element solver for modal analysis of multi-span offshore pipelines

Abstract

Accurate determination of pipeline eigenfrequencies and mode shapes is essential to free span design. For pipelines resting on rough seabeds, multiple free spans are commonly located sufficiently close to be interacting, and finite element analysis (FEA) is then conventionally required to determine the modal response. In the present report, a tailor-made (specific purpose) FEA tool is developed to carry out modal analyses of multi-span offshore pipelines. The specific purpose FEA tool is thoroughly validated by comparisons to analytical results and to results obtained using the general purpose FEA software Abaqus. Several beam and pipeline configurations are studied, ranging from simplified analyses of simply supported beams to sophisticated analyses of challenging multi-span pipeline sections, using actual seabed survey data. The validation study therefore gives valuable insight into the dynamic response of multi-span subsea pipelines. Compared to general purpose FEA modeling, the specifically designed FEA tool offers more flexible adjustment of element resolution in critical areas, more efficient file storage utilization, and also allows the designer to improve aspects of the physical modeling. The latter includes the possibility of using a consistent soil stiffness formulation rather than a traditional lumped soil model with discrete springs, as well as applying different added mass coefficients in axial and transverse directions. The impact on the modal response quantities of adopting a consistent soil stiffness model and directional variation in added mass is investigated. In addition, a methodology for establishing the static configuration and the effective axial force distribution along the pipeline using the general purpose FEA software Abaqus is described.