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Numerical analysis of pipeline-seabed interaction using a constitutive model that considers clay destructuration
Pipelines and risers form an essential part of the infrastructure associated with offshore oil and gas facilities. In deep water environments, the seabed is typically soft clay. Many geotechnical processes regarding offshore foundations and pipelines on clay soils involve cyclic movements that remould and soften the surrounding soil. This disturbance leads to significant changes in the operative shear strength and the basic constitutive properties of the soil such as critical state, which must be assessed in design. Prediction of soil strength and the resulting pipeline-soil interaction forces is essential to provide an appropriate idealization of the soil-structure interaction. However, due to the severity of the cyclic action, it is important to account for the changing constitutive properties of the soil (due to the combined effects of remoulding and reconsolidation) during episodes of disturbance.
The aim of this research is to tackle these problems of pipe-soil interaction, involving remoulding and reconsolidation, using numerical modelling. To do so, it is necessary to adopt a constitutive model that captures changes in clay soil behaviour after destructuration caused by the cyclic movements mentioned above A destructuration theory recently proposed in the literature, which is applied to the SANICLAY Constitutive Model, will be used in this research. SANICLAY is a constitutive model for clays within the framework of critical state soil mechanics, employing the new concept of the bounding surface in plasticity theory. In this research, we have written the algorithm for single element test of SANICLAY constitutive model and we are going to add the destruction theory in it and implement it in ABAQUS.
The suction embedded plate anchor is being used increasingly to moor large floating structures for development of hydrocarbon fields in deep and ultra-deep water. The plate anchor is inserted vertically into the soft clay, and then is pulled to rotate to become normal to the cable load. The loss in anchor embedment during keying reduces the uplift capacity of anchors in normally consolidated clay. For industrial applications, reduction of embedment loss is argued the most efficient way to lower the installation and operation costs of plate anchors. In this research several new methodologies are explored to constrain the embedment loss. Model tests in UWA centrifuge will be conducted to valid the new designs, followed by parametric studies using the LDFE approach to capture the key factors affecting the embedment loss. For plate anchors manufactured through the new methodology recommended, an analytical model based on yield function will also be developed to describe the trajectory and monotonic uplift capacity.