Overturning Analysis of Maleo Jack-Up Mat Foundation on Soft Clay

2008 ◽  
Author(s):  
James Murff ◽  
Alan G. Young
Keyword(s):  
Soft Clay ◽  
Mat Foundation ◽  
Jack Up

Effect of a Strong Middle Layer on Spudcan Penetration

2014 ◽  
Author(s):  
Wen Gao ◽  
Tom Harrup ◽  
Yuxia Hu ◽  
David White
Keyword(s):  
Bearing Capacity ◽  
Layer Thickness ◽  
Soft Clay ◽  
Soil Layer ◽  
Strength Ratio ◽  
Top Soil ◽  
Flow Mechanisms ◽  
Soil Flow ◽  
Jack Up ◽  
Peak Resistance

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.

scholarly journals Simplified analysis of chord and brace effects on jack-up leg penetration for preloading in soft clay

2018 ◽  
Vol 55 (12) ◽  
pp. 1900-1907 ◽  
Author(s):  
R. Aagesen ◽  
E.T.R. Dean ◽  
F.H. Lee ◽  
Y.P. Li
Keyword(s):  
Soft Clay ◽  
Centrifuge Model ◽  
Beneficial Effects ◽  
Codes Of Practice ◽  
Disturbed Soil ◽  
Form Part ◽  
Simplified Analysis ◽  
Jack Up ◽  
Special Case ◽  
Centrifuge Model Tests

The main codes of practice governing jack-up preloading presently do not consider the possibility of beneficial effects of soil resistance generated by leg chord and leg brace members as they move downwards through disturbed soil that has been squeezed past the spudcan to form part of the backfill. This paper argues that these effects can be significant for the special case of deep penetrations in soft clay. An approximate method of estimating the effects is proposed and discussed. Results are found to be broadly consistent with recent centrifuge model tests and numerical analyses. Further work is recommended to explore these potentially important and certainly complex effects.

Predicting the overall horizontal bearing capacity of jack-up rigs using deck–foundation–soil-coupled model

2020 ◽  
pp. 147509022092590
Author(s):  
Qilin Yin ◽  
Jinjin Zhai ◽  
Sheng Dong
Keyword(s):  
Bearing Capacity ◽  
Failure Mode ◽  
Double Layer ◽  
Soft Clay ◽  
Coupled Model ◽  
Single Layer ◽  
Horizontal Load ◽  
Foundation Soil ◽  
Failure Envelopes ◽  
Jack Up

The overall bearing capacity of a jack-up rig under horizontal load is conducted using finite element models that consider the deck–foundation–soil interaction. In these models, the simplified horizontal load acts on the deck and increases until the platform loses its stability. The effects of the self-weight of the platform W and load direction α on the ultimate horizontal bearing capacity Hult are investigated, and W- Hult failure envelopes under different α conditions are obtained. Two typical seabed types, including the double-layer seabed of sand overlying soft clay and the single-layer seabed of sand, are considered. The results show that a critical self-weight Wcritical exists in the double-layer seabed. Based on Wcritical, the failure of the platform presents two different modes. When W <  Wcritical, the windward leg is pulled up, and Hult increases with the increase in W. When W >  Wcritical, the failure mode is the leeward leg or legs puncturing the bearing sand layer, and Hult decreases with the increase in W. In the single-layer seabed, the failure mode is the windward leg being pulled up, and Hult increases with the increase in W throughout the whole range. The W- Hult envelopes in these two types of seabeds are basically the same when W <  Wcritical.

scholarly journals Jack-up push-over analyses featuring a new force resultant model for spudcans in soft clay

2014 ◽  
Vol 81 ◽  
pp. 139-149 ◽  
Author(s):  
Youhu Zhang ◽  
Britta Bienen ◽  
Mark J. Cassidy
Keyword(s):  
Soft Clay ◽  
Jack Up

scholarly journals A plasticity model for spudcan foundations in soft clay

2014 ◽  
Vol 51 (6) ◽  
pp. 629-646 ◽  
Author(s):  
Youhu Zhang ◽  
Mark J. Cassidy ◽  
Britta Bienen
Keyword(s):  
Finite Element ◽  
Load Capacity ◽  
Soft Clay ◽  
Plasticity Model ◽  
Vertical Load ◽  
Geotechnical Centrifuge ◽  
Centrifuge Tests ◽  
Model Components ◽  
Jack Up ◽  
Selection Of

A plasticity model for predicting the load displacement behaviour of a typical spudcan foundation for offshore jack-up platforms under combined vertical, horizontal, and moment loading in soft clay is presented. Results from geotechnical centrifuge experiments of a spudcan embedded vertically to 0.7, 1.0, and 1.45 footing diameters are described. Augmented with finite element results, these centrifuge experiments are used to evaluate the plasticity model components. As a result of soil backflow on top of the spudcan, enhanced combined bearing capacity was measured and this is reflected in increased yield surface size. A tensile vertical load capacity is also incorporated. The excellent predictive capabilities of the model are demonstrated by retrospectively simulating a selection of centrifuge tests.

Study of Soil Responses Adjacent to Jack-Up Spudcan Foundation

2010 ◽  
Author(s):  
Yi Xie ◽  
C. F. Leung ◽  
Y. K. Chow
Keyword(s):  
Soft Clay ◽  
Digital Camera ◽  
Extraction Process ◽  
Processing Technique ◽  
Image Processing Technique ◽  
Image Velocimetry ◽  
Adjacent Field ◽  
Jack Up ◽  
Soil Responses ◽  
Centrifuge Model Tests

Centrifuge model tests have been conducted to investigate the soil responses during penetration, jack-up rig operation and extraction of a jack-up spudcan foundation in soft clay. The penetration and extraction process of a half-cut spudcan were captured by a digital camera at 100g. Image processing technique involving Particle Image Velocimetry was then applied to evaluate the induced soil movements in the adjacent field of the moving spudcan. The development of pore pressures adjacent to the spudcan was also monitored.

Using a thin sand layer to ease spudcan extraction in clay

2015 ◽  
Vol 52 (8) ◽  
pp. 1023-1034 ◽  
Author(s):  
Muhammad Shazzad Hossain ◽  
Stefanus Safinus ◽  
Mark J. Cassidy
Keyword(s):  
Penetration Depth ◽  
Soft Clay ◽  
Extraction Process ◽  
Sand Layer ◽  
Pipe System ◽  
Bottom Face ◽  
Jack Up ◽  
Underlying Soil ◽  
Insight Into ◽  
Centrifuge Model Tests

Difficulties are encountered, in soft clay and silt sediments, during extraction of the spudcan foundations of “mobile” jack-up rigs. This paper reports results from centrifuge model tests undertaken to provide a simple, effective, and practical means for easing spudcan extraction in soft normally consolidated and lightly overconsolidated clay. The tests were carried out on model spudcans of two different geometries typically used in the field. The spudcans were extracted from penetrations of 2∼3 diameters and after an operation period of ∼19 months. The measured pore pressure at the bottom face of the spudcan during extraction process provided insight into the development of suction at the interface between the spudcan base and the underlying soil. The tests were carried out with and without a sand layer placed locally in between the spudcan and the mudline at the onset of penetration. In the field, a gravel layer can be placed by means of a flexible fall pipe system. The presence of a sand layer (with a thickness less than the spudcan tip length) beneath the penetrated spudcan assisted in easing leg extraction difficulties allowing the base suction to be reduced by 73%∼83% and hence the breakout force by 31%∼32%. For a given penetration depth, the installation resistance was also increased by 24%∼26%. This yielded a 15%∼17% reduction of spudcan penetration depth under a given preload and hence the breakout force as much as 50%. Guidelines and cautions for using the proposed measure are provided. Additional advantages of the new recommended method in enhancing the efficiency of the conventional measures of lessening spudcan extraction difficulties are discussed.

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