Offshore Mudmats: Sensitivity of Deflections to Selected Soil Model

Offshore Mudmats: Sensitivity of Deflections to Selected Soil Model

Publish Date: 18th June 2014

Author: Zeynep Ozkul

Conference: NUMGE

In the offshore industry small subsea structures are generally supported by mudmat foundations and these are often loaded with forces and moments acting in three directions. In deep water applications very soft clays are frequently encountered and, depending on the design, finite element analyses may be employed to verify that the capacity of the selected mudmat is sufficient to carry the expected load combinations safely.

Most of these analyses are conducted based on available undrained shear strength data collected in-situ. The use of effective strength soil parameters and evaluation of stresses and deformations that may arise under longterm drained conditions has received less attention. The subsea structures supported by mudmats are generally connected to numerous pipelines and umbilical lines via elements called jumpers.Although there is some inherent flexibility in their design, foundation movements arising from settlement and operational loads can influence the integrity of the connectors at the joint locations. In this paper a mudmat resting on clay is analyzed for deflections and rotations. In particular, the influence of using effective soil strength parameters (instead of the typical undrained shear strength profile) and the influence of including a consolidation stage in the analysis is investigated. Different soil constitutive models, namely the Mohr Coulomb and the Hardening Soil models that are available in Plaxis 3D are used. Undrained and drained loading sequences are applied which reflect expected conditions that mudmats typically encounter over their service life. The results are evaluated with respect to the sensitivity of the calculated deformations to the modeling approach employed. 

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