| dc.description.abstract | Foundations are made to withstand and transfer loads to the soil layer beneath. Soil bearing capacity is defined as the soil's ability to support all acting loads. It is preferable to have a foundation with pure vertical loading, because the bearing capacity will be at its maximum, but this is the slightly less likely loading scenario for offshore structures. In subsea conditions, the loading is mostly coupled with vertical, horizontal, and different direction moments, which may be the effects of thermal expansion of pipelines, trawl loads, and environmental loads, thus the ultimate bearing capacity of the foundation may significantly decrease as a result of such combined loadings.
The investigated subsea structure is a towhead, which is used to connect pipeline bundles, and the foundation is rectangular with sand assumed to be beneath. Trawl load acting on the top upper corner of the towhead causes torsional loading which reduces effective bearing area, consequently lowering the ultimate capacity.
This study analyzed the ultimate bearing capacity of a towhead foundation with effects of torsional loading by using finite element limit analysis and the conventional finite element method. Hand calculations based on DNV [1] are also completed and presented in the appendix section. The finite element limit analysis approach was tested on Optum G3 software and compared to Plaxis 3D which is a strain element-based software.
Plaxis 3D software requires skilled expertise especially for the drained condition where the user must know how to assign interfaces with the accurate properties as well as make a decision on dilation angle. Furthermore, in addition to strength parameters, the deformation parameters should be assigned before initiating the analysis. However, Optum G3 runs the model with a single calculation and with only the strength parameters. The associated flow rule is taken into account in the limit analysis, so the user is not required to decide on a dilatancy angle. Collapse load is between upper and lower bound solutions, providing users with an accuracy mechanism to check the result. Furthermore, the Optum G3 analysis duration was significantly shorter than that of Plaxis 3D with the long run times of each case.
Although the minimum deadweight calculated with the 3D finite element approach with Optum G3 and Plaxis 3D are similar, the most challenging aspect of working with Plaxis 3D was the lack of an accuracy mechanism to check on the final result, the only solution for it may be to compute several stress-strain curves with more elements and finer meshes, which is very time - consuming process. Additionally, VH diagrams constructed for comparison of Optum G3 and hand calculation results define a perfect fit of FELA. | |
