Offshore structures are used in a variety of applications ranging from the traditional oil and gas extraction facilities to emerging renewable energy concepts. These structures must be secured to the seabed in an efficient and cost effective manner. A novel approach is to use shallow inverted buckets as foundations, installed by suction, in place of the more usual piles. These foundations lead to cost savings through reduction in materials and in time required for installation. It is necessary to determine how these foundations perform under typical offshore loading conditions so that design calculations may be developed. This thesis presents experimental data from a comprehensive series of investigations aimed at determining the important mechanisms to consider in the design of these shallow foundations for dense sand. Initially the long term loading behaviour (e.g. wind and current) was investigated by conducting three degree of freedom loading {V:M/2R:H} tests on a foundation embedded in dry sand. The results were interpreted through existing work-hardening plasticity theories. The analysis of the data has suggested a number of improved modelling features. Cyclic and transient tests, representing wave loading, were carried out on a foundation embedded in an oil saturated sand. The novel feature of the cyclic loading was that a 'pseudo-random' load history (based on the 'NewWave' theory) was used to represent realistic loading paths. Of particular interest was the tensile load capacity of the foundation. The results observed suggested that for tensile loading serviceability requirements rather than capacity may govern design. Under combined-load cyclic conditions the results indicated that conventional plasticity theory would not provide a sufficient description of response. A new theory, termed 'continuous hyperplasticity' was used, reproducing the results with impressive accuracy. Surprisingly, under the conditions investigated, loading rate was found to have a negligible effect on response.

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Investigations of suction caissons in dense sand

Suction caissons are a new foundation option for offshore wind turbines. This thesis is focussed on the behaviour of suction caisson foundations in sand and in clay during installation, and under subsequent vertical and combined moment-lateral loadings. The research is based on extensive experimental work carried out using model scaled caissons. The analysis of the results allowed the determination of parameters for hyperplasticity models. Model caissons were vertically loaded in loose and dense sands to study in service states and plastic behaviour. Bearing capacity increased with the length of the caisson skirt. The bearing capacity formulation showed that the angle of friction mobilised was close to the critical state value for loose sands and close to those of peak values due to dilation for dense sands. The vertical load increased, though at a lower rate than during initial penetration, after large plastic displacements occurred. A hardening law formulation including this observed behaviour is suggested. In sand the installation of caissons by suction showed a drastic reduction in the net vertical load required to penetrate the caisson into the ground compared with that required to install caissons by pushing. This occurred due to the hydraulic gradients created by the suction. The theoretical formulations of the yield surface and flow rule were calibrated from the results of moment loading tests under low constant vertical loads. The fact that caissons exhibit moment capacity under tension loads was considered in the yield surface formulation. Results from symmetric and non symmetric cyclic moment loading tests showed that Masing’s rules were obeyed. Fully drained conditions, partially drained and undrained conditions were studied. Caisson rotation velocities scaled in the laboratory to represent those in the field induced undrained response for relevant periods of wave loading, a wide range of seabed permeabilities and prototype caisson dimensions. Under undrained conditions and low constant vertical loads the moment capacity of suction caissons was very small. Under partially drained conditions the moment capacity decreased with the increase of excess pore pressure. In clay, vertical cyclic loading around a mean vertical load of zero showed that in the short term the negative excess pore pressures generated during suction installation reduced vertical displacements. The yield surface and the flow rule were determined from moment swipe and constant vertical load tests. The moment capacity was found to depend on the ratio between the preload Vo and the ultimate bearing capacity Vu. Gapping response was observed during cyclic moment loading tests, but starting at smaller normalised rotations than in the field. The hysteresis loop shape obtained during gapping cannot be reproduced by means of the Masing’s rules.

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Model testing of foundations for offshore wind turbines

Computer simulation of underground blast response of pile in saturated soil

http://sourcedb.imech.cas.cn/zw/lwk1/200908/P020090831402245334251.pdf

The response of bucket foundation under horizontal dynamic loading

Model tests of pile foundations have been widely used to assess tlateral bearing capacities. The difference between the measured and the real lateral bearing capacity of pile foundation may exist due to the boundary effect, and this issue has not been studied thoroughly in the past. This paper, therefore, describes an extensive investigation of the boundary effect of a model container on the lateral bearing capacity of a circular pile foundation in uniform clay through finite element (FE) analysis. Prior to parametric study, the FE model was validated by comparing with the centrifuge test results and good agreement was obtained. Based on the parametric study, including various container sizes and soil properties, it can be concluded that the boundary effect on circular pile lateral bearing capacity can be significantly eliminated when the distance of container boundary to the pile center is larger than 15 times the pile diameter. A formula was proposed, based on FE results, to predict the difference of lateral bearing capacity of a circular pile in uniform clay induced by the boundary effect.

Numerical analysis of the boundary effect in model tests for single pile under lateral load

A test programme was carried out in the small geotechnical centrifuge of the University of Delft to investigate the horizontal bearing capacity of suction piles in sand and clay. Thanks to the small size of the samples the soil density could be accurately reproduced, so that slight differences in design could be made visible. The influence of several parameters was tested, such as height/diameter ratio, the attachment point of the cable and loading angle. In some typical cases the failure mechanism was visualized in a three-dimensional test. The test results were compared with the API standard and with three-dimensional finite element calculations. It appeared that the optimum bearing capacity was achieved if the attachment point of the cable is at 2/5 of the pile height. The API calculations yield rather conservative values for the horizontal loads. The finite element calculations appeared to be in good agreement with the measured tendencies of the test results. It is believed that a combination of numerical calculations and tests in a small centrifuge yield a powerful design tool.

A. A. Kirstein

Papers

Centrifuge And Numerical Modelling of Horizontally Loaded Suction Piles

Design, construction and monitoring of an underwater retaining wall close to an existing immersed tunnel