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Many offshore geotechnical problems--such as cyclic T-bar penetration and lateral buckling of pipelines--are affected by the tendency of fine-grained soils to strain-soften due to remoulding. Careful calibration of the constitutive model parameters that control strain softening is essential for accurate simulation of these processes. This is usually achieved by matching constitutive model response with standard element test data, which implicitly assumes that homogeneous stress/strain fields exist within the sample. However, popular element test protocols (e.g., triaxial, simple shear) cannot apply sufficient deformation-- at least as measured at the boundaries--to achieve fully remoulded conditions. This work explores the potential to determine strain-softening parameters experimentally, directly from image-based full-field deformation measurements and external loading data. Artificial data generated from finite element simulations of biaxial compression and T-bar penetration tests, using a non-locally regularised strain-softening constitutive model based on Modified Cam Clay, are then used to demonstrate the potential of the proposed technique. The results demonstrate significant potential for the application of the technique to identify constitutive parameters from full-field measurements even when polluted with modest measurement noise. Key words: large deformation modelling, particle image velocimetry, full-field deformation measurement, constitutive parameter calibration, 1. Introduction Geotechnical problems--particularly in the field of offshore geomechanics----often involve significant remoulding of soil due to large movements of structural elements. Selected examples of such problems are cyclic T-bar [...]

The number of incidents worldwide where pile tip damage has occurred during driving has accelerated following the growth of the offshore wind sector, and the expansion into areas with increased risk of embedded boulders or partially weathered soft rocks. The process involving pile–boulder–soil contact can be simulated with advanced three-dimensional (3D) large-deformation dynamic coupled Eulerian–Lagrangian (CEL) finite-element (FE) analysis. However, analyses of this nature are highly time-consuming and computationally expensive. The objective of the paper is to develop a generic framework whereby the potential for denting by a boulder can be assessed a priori. The study encompasses four main components to determine the magnitude of force necessary to create plastic damage of the pile tip and what combination of boulders and soil might generate such reactions during pile installation: (a) a parametric 3D FE structural denting study; (b) a parametric boulder–soil 3D CEL study, where prescribed displacements were applied to the surface of boulders embedded in sand; (c) a range of pile–boulder–soil 3D CEL analyses where a deformable pile and different boulder–soil combinations were simulated; and (d) centrifuge model tests involving tip damage from boulders. Simple relationships and charts have been established from the numerical studies and validated from experimental work. [ABSTRACT FROM AUTHOR]

The risk posed by seabed instability is of increasing significance as offshore activities including oil and gas developments continue to expand in this century. Many studies have considered runout of debris flows resulting from submarine landslides and potential impact on offshore infrastructure. However, initiation of slab failure resulting from shear band propagation (SBP) at the onset of a landslide has received less attention, although it is of key importance for estimating the scale and consequences of landslides. The present paper explores arrest of SBP as well as global slab failure above the shear band. The complete evolution of a submarine landslide from shear band initiation, propagation, slab failure, and arrest of SBP is observed through large-deformation finite element (LDFE) modelling. Governing equations for both dynamic and quasi-static SBP are established and solved, with results showing good agreement with the LDFE results. Lower limits for the final shear band length and for global slab failure are proposed for cases with high strain-rate dependency of shear strength, where shear band propagation is quasi-static; upper limits are proposed for cases without rate effects where the behaviour is dynamic. Key words: failure, landslides, shear strength, slopes, offshore engineering. Les risques poses par l'instabilite des fonds marins sont de plus en plus importants, car les activites d'exploitation de mer, y compris les developpements petroliers et gaziers, continuent de prendre de l'ampleur au cours de ce siecle. De nombreuses etudes ont pris en compte le debordement des coulees de debris resultant des glissements sous-marins et l'impact potentiel sur les infrastructures d'exploitation de mer. Cependant, l'initiation de la rupture de la plaque resultant de la propagation de la bande de cisaillement (PBC) au debut d'un glissement a recu moins d'attention, bien qu'elle soit d'une importance capitale pour estimer l'echelle et les consequences des glissements. Le present article explore l'arret de la PBC ainsi que la rupture de la plaque globale au-dessus de la bande de cisaillement. L'evolution complete d'un glissement sous-marin a partir de l'initiation de la bande de cisaillement, de la propagation, de la rupture de la plaque et de l'arret de la SBP est observee grace a une modelisation par elements finis de grandes deformations (<< LDFE >>). Les equations determinantes pour la PBC dynamiques et quasi statiques sont etablies et resolues, les resultats montrant un bon accord avec les resultats par LDFE. Des limites inferieures pour la longueur finale de la bande de cisaillement et pour la rupture globale de la plaque sont proposees pour les cas ou la vitesse de deformation depend fortement de la resistance au cisaillement, ou la propagation de la bande de cisaillement est quasi statique; des limites superieures sont proposees pour les cas sans effets de taux lorsque le comportement est dynamique. [Traduit par la Redaction] Mots-cles: echec, glissements, resistance au cisaillement, pentes, ingenierie d'exploitation de mer., Introduction Instability of submarine or subaqueous slopes is ubiquitous throughout the world, including much studied examples such as the giant Storegga landslide that occurred 8200 years ago in the North [...]

Current site investigation practice for offshore pipeline design relies on soil parameters gathered from boreholes or in situ test soundings to depths of 1-2 m below the mudline. At these depths, the fine-grained seabed is very soft and possesses low undrained strength, which can be difficult to measure. This paper describes a centrifuge test programme undertaken to evaluate the feasibility and performance of a novel penetrometer designed to assess the shallow strength of soft seabed over continuous horizontal profiles. This device is termed the vertically oriented penetrometer (VOP). Tests were performed on a normally consolidated kaolin sample, with the VOP translated horizontally at velocities ranging from 1 to 30 mm/s, after embedding the VOP at 30 and 45 mm depths. All tests involved many cycles of VOP forward and backward movement to assess its potential to derive the ratio of intact to fully remoulded strength. Strength determination is achieved by dragging the VOP at a specified embedment depth along the soil surface, and deriving the soil strength from the measured resistance as if the VOP were a laterally loaded pile. The VOP is shown to yield comparable strength measurements to that of a T-bar penetrometer. The VOP is a potentially valuable addition to the range of tools used to characterize soil strength, both in small-scale centrifuge models and, following practical development, potentially also in the field. Key words: clays, penetrometer, pipeline, shear strength, site investigation. Pour concevoir des pipelines en mer, on se base habituellement sur les parametres du sol mesures dans les trous de forage ou lors de sondages d'essai sur le terrain realises a des profondeurs comprises entre 1 et 2 m sous le fond marin. A ces profondeurs, le sol finement granuleux est tres mou et presente une faible resistance non drainee, qui peut etre difficile a mesurer. Le present article decrit un programme d'essais en centrifugeuse visant a evaluer la faisabilite et les performances d'un nouveau type de penetrometre permettant d'estimer la resistance superficielle d'un fond marin mou sur des profils horizontaux continus. Cet appareil est appele penetrometre oriente verticalement (POV). Les essais ont ete effectues sur un echantillon de kaolin normalement consolide, le POV etant soumis a une translation a des vitesses variant entre 1 et 30 mm/s, apres enfouissement du POV a des profondeurs de 30 et 45 mm. Tous les essais ont consiste en nombreux cycles de mouvements du POV en avant et en arriere, afin que l'on puisse evaluer sa capacite a mesurer le quotient de la resistance au cisaillement a l'etat intact par la resistance au cisaillement a l'etat totalement remanie. On a pu determiner la resistance en tramant le POV a une profondeur d'enfouissement specifique le long de la surface du sol et en deduisant la resistance du sol a partir de la resistance mesuree, comme si le POV etait un pieu charge lateralement. On constate que le POV fournit des mesures de resistance comparables a celles obtenues a l'aide d'un penetrometre a barre en T. Le POV est donc tres utile et pourrait s'ajouter a l'eventail des outils servant a caracteriser la resistance du sol dans le cadre d'essais en centrifugeuse sur des modeles reduits et possiblement sur le terrain apres une phase de developpement pratique. [Traduit par la Redaction] Mots-cles: argiles, penetrometre, pipeline, resistance au cisaillement, etude sur le terrain., Introduction This paper describes the development of a novel device for measuring the near-surface soil undrained strength, named the vertically oriented penetrometer (VOP). This device is a short rigid cylinder, [...]

This paper reports advanced three-dimensional large-deformation finite element analyses investigating the effect of the padeye offset on the behaviour of plate anchors. The analyses varied the normalized padeye offset from 0 to 0.5 and identified two counterbalancing effects on the anchor bearing capacity induced by the padeye offset. These are the change in anchor inclination with respect to the direction of loading (detrimental due to the reduction of bearing area) and the reduction of loss of embedment (beneficial, as the anchor mobilizes stronger soil). The results provide insights to optimize the design of plate anchors. The underlying concepts also have applications for other types of anchors. Key words: plate anchor, large-deformation finite element analysis, clays, offshore engineering, loss of embedment, bearing capacity. Cet article presente des analyses par elements finis avancees en trois dimensions et en grande deformation servant a etudier l'effet de l'espacement des mains de fer sur le comportement des ancrages fi plaque. Les analyses ont fait varier l'espacement normalise des mains de fer de 0 a 0,5 et a identifie deux effets contre-balanciers sur la capacite portante des ancrages causes par l'espacement des mains de fer. Il s'agit de la variation de l'inclinaison de l'ancrage selon la direction de la charge (effet negatif du a la reduction de la surface portante) et de la reduction des pertes d'enfouissement (benefique puisque l'ancrage mobilise un sol plus fort). Ces resultats offre des informations pour optimiser la conception des ancrages a plaque. Les concepts de base ont aussi des applications pour d'autres types d'ancrage. [Traduit par la Redaction] Mots-cles: ancrage fi plaque, analyse par elements finis en grande deformation, argiles, ingenierie en mer, perte d'enfouissement, capacite portante., Introduction Plate anchors are an economic and efficient deep-water mooring solution, which have been used increasingly in recent years (Dove et al. 1998; Murff et al. 2005; Yang et al. [...]

Shallow skirted foundations can mobilize uplift resistance from end bearing in the short to medium term. However, uncertainty exists over the magnitude of reverse end bearing resistance compared with resistance in compression, and how this might be affected by a gap between the external face of the foundation skirt and the adjacent soil. The study presented in this paper explores this problem through centrifuge model tests, investigating the effect of skirt embedment ratio on (i) the magnitude of reverse end bearing capacity compared with compression capacity, (ii) the uplift displacement associated with spontaneous loss of suction during uplift, and (iii) the existence of a vertical gap along the external skirt-soil interface. The results show that (i) peak uplift capacity equivalent to compression capacity can be mobilized for a fully sealed foundation with an intact skirt-soil interface, (ii) suction required for reverse end bearing can be maintained through considerable foundation displacement, even for a low skirt embedment ratio, and (iii) the presence of a vertical gap along the external skirt-soil interface causes abrupt loss of suction beneath the top plate after minimal foundation displacement, with subsequent uplift capacity being markedly reduced. Key words: bearing capacity, centrifuge modelling, clays, footings-foundations, offshore engineering, suction. Les fondations chemisees peu profondes peuvent mobiliser de la resistance au soulevement a une extremite sur une periode de temps courte et moyenne. Cependant, il y a des incertitudes en lien avec la magnitude de la resistance a l'extremite inverse comparativement a la resistance en compression, ainsi que la facon dont cette magnitude est affectee par l'espace entre la face externe de la chemise de la fondation et le sol adjacent. L'etude presentee dans cet article explore cette problematique a l'aide d'essais de modelisation par centrifugeuse servant a evaluer l'effet du ratio d'ancrage de la chemise sur (i) la magnitude de la capacite portante de l'extremite inverse comparativement a la capacite en compression, (ii) le deplacement en soulevement associe a la perte de succion spontanee durant le soulevement et (iii) l'existence d'un espace vertical sur l'interface externe entre la chemise et le sol. Les resultats demontrent que (i) une capacite de soulevement maximale equivalente a la capacite en compression peut etre mobilisee dans le cas d'une fondation completement scellee avec un interface chemise-sol intact, (ii)la succion necessaire pour la capacite portante de l'extremite inverse peut etre maintenue malgre des deplacements considerables de la fondation, meme pour un faible ratio d'ancrage de la chemise et (iii) la presence d'un espace vertical le long de l'interface externe chemise-sol entrame une perte de succion abrupte sous la plaque superieure suite a un deplacement minimal de la fondation, et la capacite de soulevement est reduite significativement par la suite. [Traduit par la Redaction] Mots-cles: capacite portante, modelisation par centrifugeuse, argiles, semelles/fondations, ingenierie en mer, succion., Introduction Skirted foundations Skirted foundations comprise a top plate with a peripheral 'skirt'. The skirt penetrates the seabed until the top plate comes into contact with the seabed, confining a [...]