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Research Papers: Offshore Technology

Experimental Investigation of the Effect of Cyclic Loading on Spudcan Extraction

[+] Author and Article Information
Omid Kohan

Centre for Offshore Foundation Systems,
ARC Centre of Excellence for Geotechnical Science and Engineering,
University of Western Australia,
Perth, WA 6009, Australia

Mark J. Cassidy, Christophe Gaudin

Centre for Offshore Foundation Systems,
ARC Centre of Excellence for Geotechnical
Science and Engineering,
UWA Oceans Institute,
University of Western Australia,
Perth, WA 6009, Australia

Britta Bienen

Centre for Offshore Foundation Systems,
ARC Centre of Excellence for Geotechnical
Science and Engineering,
University of Western Australia,
Perth, WA 6009, Australia

1Corresponding author.

Contributed by the Ocean, Offshore, and Arctic Engineering Division of ASME for publication in the JOURNAL OF OFFSHORE MECHANICS AND ARCTIC ENGINEERING. Manuscript received June 16, 2015; final manuscript received November 7, 2015; published online February 12, 2016. Assoc. Editor: Charles E. Smith.

J. Offshore Mech. Arct. Eng 138(2), 021301 (Feb 12, 2016) (10 pages) Paper No: OMAE-15-1047; doi: 10.1115/1.4032157 History: Received June 16, 2015; Revised November 07, 2015

Self-elevating mobile jack-up units have been employed in offshore exploration and development in shallow waters at depths of up to approximately 150 m. Jack-ups are designed to move to a new site after operations are completed. The spudcan footings, which can be embedded up to three diameters deep in soft soil, must therefore be extracted by jacking down the hull into the water and then floating it beyond the neutral draft. This provides the maximum pull-out force to overcome the soil resistance to the jack-ups, but this force may not be sufficient. Problematic cases of this offshore are reported to take up to 10 weeks to extract, a costly exercise for the industry. A method sometimes used offshore is to cycle the spudcans vertically in an attempt to free them. This can be achieved by pushing and pulling the leg by leaving the hull afloat in the water and allowing the impact of small amplitude waves on the hull to generate cyclic loads on the spudcan. This paper reports a series of centrifuge tests investigating the ability to extract a spudcan under regular and irregular cyclic loading. Spudcan extraction tests were performed from a depth of three spudcan diameters in normally consolidated clay in a geotechnical beam centrifuge. The results demonstrate that successful extraction is dependent on the combination of mean pull-out load and the amplitude of the cycling. It is also shown that insufficient tensile static loads and prolonged small cyclic loads result in the dissipation of the negative excess pore pressure at the spudcan invert caused by the buoyancy of the hull in excess of neutral draft. It results in consolidation of soil and changes in the shear strength of the soil and consequently either extraction of the spudcan after a long period of time or unsuccessful leg extraction.

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Figures

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Fig. 1

Model spudcan and location of the pore pressure transducers (dimensions in millimeter)

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Fig. 2

Regular versus irregular

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Fig. 3

Schematic of testing program

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Fig. 4

Penetration and extraction resistances for Test01 and Test02

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Fig. 5

Development of excess pore pressure at the spudcan invert for Test01 and Test02

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Fig. 10

Penetration and extraction resistances for Test01, Test04, and Test06

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Fig. 11

Development of excess pore pressure at the spudcan invert for Test01, Test04, and Test06

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Fig. 12

Development of excess pore pressure at the spudcan top for Test01, Test04, and Test06

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Fig. 6

Excess pore pressure responses to the first 500 cycles for Test02

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Fig. 16

RAOs for heave for freely floating jack-up barge

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Fig. 17

RAOs for roll of freely floating jack-up barge

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Fig. 18

RAOs for pitch for freely floating jack-up barge

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Fig. 19

Vertical spudcan reaction at leg1

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Fig. 7

Penetration and extraction resistances for Test01 and Test03

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Fig. 8

Development of excess pore pressure at the spudcan invert for Test01 and Test03

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Fig. 9

Development of excess pore pressure at the spudcan top for Test01 and Test03

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Fig. 13

Preliminary contours of hull buoyancy load level as a function of cyclic amplitude ratio to reach to failure

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Fig. 14

Proposed contours for the number of cycles to failure (contours represent preliminary estimation)

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Fig. 15

Geometry of the jack-up hull used in the hydrodynamic analysis

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