Installation of Suction Bucket Foundations in Sand
Authors
Gitsas, Thomas ; Sgourakis, Georgios-Nektarios ; Pandiella Martinez, Hector
Term
4. term
Education
Publication year
2016
Submitted on
2016-06-08
Abstract
Sugebucket-fundamenter bruges i offshore-industrien og er attraktive til vindturbiner, fordi installation med sug er stille og effektiv. Denne afhandling undersøger samspillet mellem et bucket-fundament og mættet, homogen sand under installation (ved sug og ved påført kraft) samt afinstallation (ved induceret vandtryk). Et mellem-skala modelbucket blev testet i laboratoriet, hvor vertikale bevægelser og poretryk blev målt gennem hele forløbet. Forsøgene blev gengivet numerisk i Plaxis 2D for at analysere strømningsveje (seepage), hydrauliske gradienter, poretryksfaktor og kritisk tryk. Med udgangspunkt i CPT-resultater blev de empiriske koefficienter kp (spidsmodstand) og kf (skørtfriktion) beregnet efter en DNV-baseret metode og sammenlignet med tidligere studier; derudover blev β-faktorer udledt for at belyse forskellen i jordpenetrationsmodstand mellem installation med sug og med kraft. Resultaterne viser, at de hydrauliske gradienter er størst ved spidsen af skørtet, hvilket medfører den største reduktion i spidsmodstand under sug; den teoretiske kritiske sugetryk blev i flere tilfælde overskredet under installation uden observeret piping, og de kritiske tryk blev ikke overskredet under afinstallation. De numeriske resultater korrelerer godt med løsninger for homogen sand i litteraturen. De estimerede kp- og kf-værdier følger overordnet kendte tendenser, men enkelte afvigelser forekommer, og de beregnede β-faktorer indikerer en reduktion i penetrationsmodstand ved sug; disse er de første eksperimentelle estimater og kræver yderligere verifikation. Samlet set bidrager arbejdet som et skridt mod en forbedret CPT-baseret model for jordpenetrationsmodstand for bucket-fundamenter i sand.
Suction bucket foundations are widely used offshore and offer a quiet, efficient option for wind turbines because installation by suction is straightforward. This thesis examines how a bucket foundation interacts with saturated, homogeneous sand during installation (by suction and by applied force) and during uninstallation (by induced water pressure). A medium-scale model was tested in the laboratory with continuous measurements of vertical displacement and pore pressures. The tests were reproduced numerically in Plaxis 2D to analyse seepage paths, hydraulic gradients, pore pressure factor, and critical pressures. Using CPT results and a DNV-based approach, empirical coefficients kp (tip resistance) and kf (skirt friction) were calculated and compared with prior studies; β factors were also derived to quantify differences in soil penetration resistance between suction and force installation. Findings show the highest hydraulic gradients at the skirt tip and, consequently, the largest reduction in tip resistance under suction; during installation the applied suction sometimes exceeded the theoretical critical pressure without observed piping, and critical pressures were not exceeded during uninstallation. Numerical results agree well with published solutions for homogeneous sand. Estimated kp and kf generally follow known trends, albeit with some deviations, and the β factors indicate reduced penetration resistance under suction; as first experimental estimates, they require further validation. Overall, the work contributes a step toward improved CPT-based models for soil penetration resistance of bucket foundations in sand.
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