Effect of a Strong Middle Layer on Spudcan Penetration

The rapid penetration of one or more of the foundations of a mobile jack-up rig into the seabed is an ongoing major problem in the offshore industry, with the potential to cause major damage to the structure and endangering any personnel on board. A recent example is the jack-up drilling rig Perro Negro 6 incident happened near the mouth of the Congo river in July 2013 with one of the rig’s crew of 103 reported missing and six others injured. This uncontrollable displacement is due to a form of failure known as punch through failure and commonly occurs on stratified seabed profiles. It has been reported that unexpected punch-through accidents have resulted in both rig damage and lost drilling time at a rate of 1 incident per annum with consequential costs estimated at between US$1 and US$10 million [1]. This paper presents the bearing capacity profiles and associated soil flow mechanisms of a common spudcan foundation penetrating into a three layer soft-stiff-soft clay soil through the use of large deformation finite element (LDFE) analysis. The Remeshing and Interpolation with Small Strain (RITSS) [2, 3] technique was implemented in the software package AFENA [4] to conduct the LDFE analysis. Both soil layer thickness and soil layer strength ratios were varied to study their effect on the spudcan penetration responses. The LDFE results of spudcan penetration into the soft-stiff-soft clay soils were calibrated by existing centrifuge test data. A parametric study was then conducted to study the bearing capacity responses and soil flow mechanisms during spudcan large penetrations by varying the soil layer strength ratio and relative layer thickness to the diameter of spudcan. It was found that there were three types of bearing responses during continuous penetration of spudcan: (a) when the top soft layer is relatively thin, the spudcan bearing response was similar to that of two layer soils with stiff over soft clays; (b) when the top soil layer thickness is medium, a peak resistance is observed when spudcan penetrates into the middle stiff layer followed by reduction; (c) when the soil layer is thick, the peak resistance occurs when spudcan gets into the bottom soft soil layer. The critical thickness of top soil layer is a function of soil strength ratio and middle stiff soil layer thickness. The bearing response types were also corresponding to the soil cavity formations during spudcan initial penetration.

Drift of Piles Subjected to Cyclic Lateral Loading From a Varying Direction: System vs. Soil Element Behavior

Offshore monopiles are usually designed using the p-y method for cyclic loading. While the method works for static loading, it was not developed for high numbers of cycles. Since the turbines are highly sensitive towards tilting, cyclic loading must be considered. The static results should therefore be combined with results from cyclic model tests with a high number of cycles to account for the accumulation of displacement or rotation during the lifetime of these structures. These model tests can underestimate the accumulation, however, as it has recently been shown that a change of loading direction can increase the accumulation considerably. These results have been verified using small scale modeling and centrifuge testing. The results from modeling the full problem of a laterally loaded pile are compared here with results from cyclic simple shear tests with a change of shearing direction during the cyclic loading. For these tests, a newly developed apparatus is used. This allows further insight into the question how a soil can “retain a memory” of its loading history.

Numerical Study of the Combined Loading Envelope of a Skirted Spudcan Foundation

Foundations for offshore structures, such as mobile jack-up units, are subjected to large horizontal (H) and moment (M) loads in addition to changing vertical (V) loads. The use of a combined vertical, horizontal and moment (V-H-M) loading envelope to define foundation capacities has become increasingly applied in recent years. However, there is no study on the skirted spudcan, a new alternative foundation type to the conventional spudcan footing for jack-ups. In this study, the combined V-H-M yield envelope of a skirted spudcan foundation in clay soil is investigated with small strain finite element analyses using 3D modeling. The footing’s uniaxial bearing capacities and failure mechanisms are described. The failure envelope for the combined V-H-M loadings is presented. A comparison of the bearing capacities between the spudcan and skirted spudcan of various dimensions is also presented.

Poro-Elasto-Plastic Model for Wave-Induced Liquefaction

In this study, a two-dimensional poro-elasto-plastic model for the wave-induced liquefaction in a porous seabed was presented. Two mechanisms of the wave-induced pore pressures were considered. Both elastic components (for oscillatory) and the plastic components (for residual) were integrated to predict the wave-induced excess pore pressures in marine sediments. The proposed 2D poro-elasto-plastic model allows for the pore pressure build-up process in a sandy seabed. The proposed model overall agreed well with the previous wave experiments and centrifuge tests. Numerical example shows that the pattern of progressive waves -induced liquefaction gradually changed from 2D to 1D.

Pipeline Upheaval Buckling in Clayey Backfill and Shore Approach

True estimation of soil response during pipeline upheaval buckling is a key parameter in the safe design of subsea buried pipeline. In this paper the effects of sea mean water level over the buried pipeline and the effects of pipe burial depth on the soil response during vertical buckling are investigated. For that purpose a numerical modeling of pipeline upheaval buckling in clayey backfill has been conducted. Different sea mean water levels are considered to simulate the pipeline shore approach. In addition, various pipeline burial depths are considered to predict the soil uplift resistance and the soil failure mechanism. In order to model the large penetration of pipeline into the soft clay, Arbitrary Eulerian Lagrangian (ALE) method is employed. The results reveal that in the shallow water the sea mean water level may have considerable effects on the soil failure mechanism and soil uplift resistance. In addition, as the sea mean water level and pipe burial depth increases, a new transitional failure mechanism can be observed. The mechanism is a combination of vertical sliding block mechanism and the flow-around mechanism.

Characterisation of Residual Shear Strength at the Sheringham Shoal Offshore Wind Farm

For offshore foundations, the residual shear strength is an important soil parameter for the evaluation of installation resistance and axial pile capacity (for jacket foundation). Estimation of residual shear strength can be conducted in a shear box test in the conventional way, or with the introduction of an interface to evaluate the change in residual shear strength under influence of friction between soil and the interface. In addition, the residual effective friction angle can be measured in the ring shear test using the Bromhead apparatus. In this study, the three above-mentioned methods are employed to estimate the values of residual shear strength of two soil units: the Swarte Bank Formation and the Chalk Unit sampled from the Sheringham Shoal offshore wind farms. The Swarte Bank Formation is dominated by heavily over-consolidated stiff clay, while the Chalk Unit is characterized by putty white chalk which behaves in a similar manner to stiff clay if weathered, or to soft rock if unweathered. These soil units are located at the bottom of the soil profile at the Sheringham Shoal wind farm and hence are important in providing axial capacity to the foundation. Samples from the two soil units are tested and compared at different rates of shearing to evaluate the change in axial capacity and installation resistance of the offshore wind turbine foundations under various possible loading and drainage conditions. Comparison is also made between residual shear strength with and without a reconsolidation period to assess the potential for soil set-up and its influence on the soil capacity. The results show that, for both the clay and the chalk, the estimated residual shear strengths are quite similar between the conventional and interface shear tests and tend to increase with increasing shearing rate. This can be attributed to the increasing dominance of the turbulent shearing mode. Relative to the peak shear strength, the values of residual shear strength are approximately 5 to 35% lower in most cases. Reconsolidation for a period of 24 hours appears to have, if any, marginal positive effect on residual shear strength of the two soils in both shear box and interface shear box tests. The residual friction angles derived from the shear box and ring shear tests are comparable and fall in the immediate range of shear strength. The various test results imply that the pile foundations at the Sheringham Shoal would have considerably large axial capacity, assuming that the horizontal stress is similar to the normal stress used in testing. The test data however should be used with caution and combined with piling experience in comparable soils where possible. The study aims to provide a source of reference for design of pile foundations for sites with similar soil conditions.

Experimental Study for Wave-Induced Oscillatory Pore Pressures in a Sandy Seabed

In this paper, an experimental study for wave-induced pore pressures in marine sediments was reported. In the experiment, a one-dimensional facility was set up with a vertical cylinder and a 1.8 m thick sandy deposit and 0.2 m thick water above the deposit. Unlike the previous experiments [1], additional static water pressures were added on the harmonic dynamic wave pressure and more pore pressure gauges were buried in the deposit, which allowed us to simulate the case with larger water depth and better describe the distribution of pore pressure trend. A series of experiments with 3000 cycles in each test were conducted under numerous different wave and soil conditions, which allowed us to examine the influence of wave and soil parameters on the wave-induced pore pressures as well as liquefaction. The experimental results show the significant influence of liquefaction on sandy seabed in shallow water. Furthermore, some new experimental phenomenon was observed. The depth of sandy deposit was usually considered to be unchanged in theoretical calculation, while the depth of which was indeed changed periodic with wave loading, which was observed and recorded in the experiments.

Dynamic Model Test of Monopile for Offshore Wind Turbines

In current practice, the most common foundation type for Offshore Wind Turbine generators (OWT’s) at moderate water depths is the monopile. A model pile in scale 1:20 of a typical monopile foundation for offshore wind turbine has undergone lateral vibration testing in dry laboratory sand. Eigen-frequencies are determined based on acceleration measurements. The aim of the tests is to provide benchmark results for validation of different calculation methods for offshore wind monopile foundations. The stiffness contribution from the sand is evaluated on behalf of measuring the first natural frequency of the pile-soil interaction system. Preliminary results from back-calculations of the model tests using both 3D -FEM and a simple beam on elastic foundation model indicate that strain-dependent soil stiffness plays an important role for determining the system stiffness. In this paper, the model tests and their results are presented, along with the preliminary results from the back-calculation.

Large Deformation Finite Element Analysis of Gravity Installed Anchors in Clay

The OMNI-Max anchor is a multi-directional, self-inserting, gravity-installed anchor and used as foundation for mooring deep water offshore facilities, including risers and floating structures. The OMNI-Max anchor offers a cost effective anchoring solution with improved reliability in the mooring system. Pullout capacity and keying behavior are two important issues in the design of the OMNI-Max anchor. In this paper, the pullout capacity and the keying process of a vertically installed OMNI-Max anchor embedded in normally consolidated clay were simulated using three dimensional large deformation finite element analysis. In these numerical analyses, 10-node tetrahedral elements were used to predict the collapse loads of undrained geotechnical problems involving material incompressibility. Nodal joint elements were used to simulate the interaction between the anchor and soil. The effect of the loading angle on the keying behavior of the OMNI-Max anchor was considered. The analyses clearly show the two important processes (1) “keying”: the anchor rotates rapidly until reaching the best bearing capacity position; (2) “diving”: the anchor mainly translates with tiny rotation. It agrees well with the keying and diving phenomenon in published model test results.