Numerical study of spudcon penetration on two-layered soil with Lagrangian-Eulerian couple

Jackup rigs are one of the most widely used and popular offshore structures in semi-deep waters due to their mobility. The new jackups usually consist of a floating triangular body supported by three independent vertical truss legs that can be raised, lowered and jacked, and each of these legs rests on foundations known as spuds. They ride and reach stability and balance by penetrating deep into the soil. In cases where the seabed soil has a layered structure including a strong layer placed on top of a weak layer, there is a possibility of punch failure during foundation penetration. Knowing about the soil flow mechanisms around the foundation of the spudcon that suffer continuous large penetration and also estimating the probability and degree of intensity of a sudden spudcon penetration are very important issues.

In the present research, a numerical study was carried out to investigate the bearing capacity of the spudken foundation of the offshore jackup structure in soil with a two-layer system of sand on clay. Finite element analysis of large deformations has been used to simulate the continuous penetration of the spudken foundation in a layered soil consisting of a strong sand layer placed on top of a weak clay layer. Numerical simulation is done by "Eulerian-Lagrangian coupling" method in ABAQUS software.In this research, the effect of characteristics of sand and clay layers, including relative density (ID) and thickness of sand layer, shear strength at the boundary of two layers (su0), gradient of shear strength of clay layer on bearing capacity and punch rupture phenomenon are studied. In this research, the Mohr-Columb model was used to model the behavior of sandy and clay soils.

The results of the research show that the parameters related to geotechnical characteristics, including the shear strength of the clay layer, the shear resistance gradient, the relative density of the sand layer, and the thickness of the sand layer have a direct relationship with the bearing strength of the Spadken foundation. So that with the increase of each of these characteristics, the bearing resistance also increases. The obtained results showed that by increasing the thickness (Hs) and relative density (ID) of the sand layer, the load bearing capacity of the pispodken (q) and the volume of the transferred sandy soil mass (the height of the soil mass (hplug) and the width of the soil mass) (bplug) increases, so that with the increase of Hs from 6 m to 8 m, the value of qpeak increases from 263 kPa to 346 kPa, the value of hplug from 6.52 m to 8.26 m and the value of bplug from 6.6 m to 6.9 m and with The increase of ID from 20% to 60% increases the qpeak value from 328 kPa to 367 kPa, the hplug value from 6.85 m to 7.14 m, and the bplug value from 5.7 m to 6 m. Also, the investigation of the effects of shear resistance (su0) and shear resistance gradient (ρ) of the clay layer shows their direct relationship with the bearing capacity and the reverse relationship with the volume of the transferred soil mass, so that with the increase of su0 from 11 kPa to 20 kPa, the value of qpeak from 307 kPa to 390 kPa, the value of hplug from 7.41 m to 7.14 m and the value of bplug from 6.9 m to 6.6 m, and with the increase of ρ from 1 kPa/m to 2 kPa/m, the value of qpeak from 314 kPa At 352 kPa, the value of hplug goes from 7.14 m to 6.9 m and the value of bplug goes from 6 m to 5.7 m.

The results of this research can be summarized as follows:• Parameters related to geotechnical characteristics, including shear resistance of clay layer, gradient of shear resistance, relative density of sand layer, thickness of sand layer have a direct relationship with the load bearing strength of the foundation. So that with the increase of each of these characteristics, the bearing resistance also increases.• The severity and risk of punch breakage increases with the increase in the thickness of the sand layer and the decrease in the shear strength of the clay layer.• The soil rupture pattern under the infiltrating spudken foundation changes with respect to the depth, such that at surface depths, the rupture has two components, shear along the almost vertical shear plane in the sand layer and local shear rupture in the clay layer.