Skirt Length Research Articles

Numerical investigation of punch-through mitigation in stiff-over-soft clays using skirted spudcan

Punch-through failure, a critical issue in offshore engineering, occurs when a foundation penetrates through strong-over-weak soil layers. This paper presents a study on the effectiveness of skirt addition to spudcans in addressing punch-through failure through numerical simulations using the axisymmetric Particle Finite Element Method (PFEM). The validity of the PFEM is verified by simulating a spudcan penetration centrifuge test. Subsequently, PFEM simulations are conducted to analyse the penetration behaviour of both generic and skirted spudcans in stiff-over-soft clays. Results from the simulations reveal the bearing responses of spudcans as they penetrate into clays, elucidate associated failure mechanisms, and underscore the significant of considering the spudcan-driving process in estimating punch-through failure mitigation. Findings indicate that, the skirt effectively mitigates punch-through failure by increasing plug height in the lower soft layer, thus reducing the rate of bearing capacity reduction. Furthermore, critical factors such as skirt length, upper layer thickness, lower layer strength gradient, and the strength ratio between lower and upper layers are assessed through comprehensive parametric studies to evaluate the efficacy of skirted spudcans in mitigating punch-through failure, with eventual goal to provide essential guidance for practical design considerations in offshore foundation systems.

Seismic response of the composite bucket foundation supported offshore wind turbines under marine environmental loadings

Many offshore wind turbine (OWT) foundations are situated in earthquake-prone areas, where complex environmental loads intensified the seismic responses of the OWTs. In response to this situation, the composite bucket foundations is proposed and extensively applied due to its reliable installation and high seismic resistance. To analyze the seismic response of composite bucket foundations subjected to environmental loads, refined numerical models of the 6.45 MW OWT system are established, and parametric analyses are conducted, considering the influence of seismic frequency contents and intensities, the soil layer distributions and wind velocities. The results show that the composite bucket foundations undergo cumulative rotational deformation around a point biased toward the windward side of the foundation under the seismic and environmental loads. The increasing mean wind velocity significantly enlarges the displacement response envelope, while has limited influence of the acceleration response envelope of OWTs. Increasing the bucket diameter enhances the deformation resistance of the OWT, while the efficiency of this improvement diminishes as the diameter increases. Greater bucket skirt length also enhances its deformation resistance, but may lead to increased lateral acceleration response of the OWT.

Undrained HM bearing capacity of hybrid foundations in spatially variable soils

This paper investigates the probabilistic capacity of hybrid foundations (HF) for offshore structures in spatially variable marine sediments under horizontal (H) and moment (M) combined loading. Non-stationary random fields are used to model the spatially varying undrained shear strength that linearly increases with depth. The results indicate that using hybrid foundations, with peripheral and two internal skirts of identical length (i.e. d/D = 1) is almost similar to simple skirted (SS) foundations for both deterministic and stochastic approaches. Increasing length of internal skirts to twice that of peripheral ones with d/D = 2, results however in significant increase of foundations capacity, mainly under HM combined loading.

Experimental Study on Mechanical Behavior of Skirt–Pile Foundations in Saturated Clay under Horizontal Load

Skirt–pile foundations have gained widespread attention in the field of offshore engineering due to their ease of installation and high bearing capacity. In this study, the ultimate bearing capacity, pile bending moment distribution and development, cumulative deformation characteristics, and cyclic stiffness development of skirt–pile foundations were investigated using physical model tests. The experimental results indicate that the ultimate bearing capacity and deformation resistance of the foundation can effectively be improved by increasing the skirt diameter. The cumulative deformation of the skirt–piles exhibited rapid development during the initial stages of cyclic loading, eventually stabilizing. Under long-term cyclic loading, the existence of the skirt can share the bending moment, which then affects the internal force distribution of the pile foundation along the axis. The pile foundation’s cyclic stiffness reduces as the loading cycles increase and increases as the skirt diameter and length grow. Meanwhile, the horizontal cyclic stiffness decreases as the number of cycles increases, stabilizing after 3000 cycles. This study can not only deepen the understanding of the deformation laws of skirt–pile foundations in clay soil but also offers some references for the design of offshore pile foundations.

Probabilistic failure envelopes of tripod bucket-supported offshore wind turbines in spatially variable clay based on a novel continuous method

Marine geologic conditions play key roles in offshore geotechnical preliminary design and construction decisions. Tripod bucket foundations are considered promising geotechnical foundations for offshore engineering. Their bearing characteristics are often determined based on tripod foundation system failure modes in uniform clay without considering spatially variable soil properties. Therefore, this study investigates the combined bearing capacity of tripod bucket foundations in spatially variable clay based on comprehensive finite element analysis with Monte Carlo simulation (MCS) using a novel simplified modified swipe (SMS) method considering the effects of bucket spacing and foundation skirt length. The SMS method can directly represent the ultimate combined bearing capacity of the whole space efficiently and accurately. The results show that a reduced bucket spacing or increased skirt length changes the failure mode of the foundation from independent caisson failure to global failure. Moreover, the spatial variation in soil leads to an asymmetric failure mode, which is more remarkable for foundations with small bucket group effects. An independent safety factor design approach is adopted to represent failure envelopes with different recurrence probabilities, which should be cost-effective, especially for foundations sensitive to soil variability. Finally, an application based on a convex polynomial optimization strategy for representing failure envelopes is demonstrated.

Numerical simulations of wave-soil-structure interactions for a subsea cover

Glass-reinforced plastic (GRP) subsea protection covers are widely used to prevent damage to offshore pipelines placed on the seabed from dropped objects, hydrodynamic wave induced loads, and trawling. The light GRP subsea covers could be stabilized by using skirts which penetrate the seabed soil. The dynamic wave pressure acting on the cover will transfer to the cover bottom and the skirt and further influence the pore pressure and seepage flow inside the soil beneath the cover. The present study performs a numerical analysis for the wave-soil-structure interaction (WSSI) of a subsea cover. Two-dimensional (2D) numerical simulations are carried out using an open-source numerical toolbox for modeling the porous seabed interaction with waves and structures under the framework of the finite-volume-method (FVM) based OpenFOAM. The nonlinear waves are solved to obtain the dynamic wave loadings on the cover and the pressure on the seabed. A soil consolidation model is used to provide the initial effective stress in the soil. Then, a one-way coupling algorithm is applied for the WSSI analysis to obtain the soil response in the vicinity of the cover. The distributions of the wave-induced pore pressure, the soil shear stress, and the seepage flow within the seabed are studied and the influences of the wave heights and the skirt lengths are discussed.

Development of a test method for the dynamic drapability of fabrics using reciprocating motion

This study aimed to develop a method for measuring the dynamic drapability of fabrics using a reciprocating motion device that mimics the movement created by shaking a piece of fabric. Five types of fabrics were selected based on their drape coefficients and weights, and the lengths of fabrics were determined based on skirt length (50 cm) and dress length (100 cm). The width and speed of the reciprocating motion were considered as the experimental variables. Results of reciprocating motion-based fabric movement analysis revealed that the number of nodes, the position of the first node, amplitude, total length, and area are significant factors for measuring dynamic drapability. It was found that the longer fabrics had more nodes, enabling measurement of various types of fabrics. Furthermore, it was found that a wider reciprocation width under the same speed and a faster reciprocation speed with the same width favorably demonstrated dynamic drapability, resulting in proportionality between the reciprocating motion and the force transmitted to the fabric. Movement analysis of seven selected conditions and additional samples showed that the optimal condition was a 100-mm reciprocation width and a 150-rpm speed and that the first node’s position is the most significant factor for a dynamic drapability. To verify the proposed measurement method, measuring four pairs of fabrics with similar drape coefficients but different shapes showed that as fabric weight increased, the node moved downward, despite similar drape coefficient values. This confirms that the proposed method can overcome the limitations of the existing drape coefficient.

Bearing capacity and settlement of inclined skirted foundation resting on sand

Shallow foundations behavior is improved by using skirts to confine the soil under the foundation beneath. The bearing capacity and settlement of the skirted foundation resting on sand are affected by the angle of the skirt's inclination. Numerical analysis using Plaxis-3D software was conducted on an axially loaded square-skirted footing in this investigation. In addition, experimental modelling was carried out to validate the numerical analysis findings. The experimental findings revealed an acceptable agreement with the numerical analysis conclusion. The results of the investigation showed that the bearing capacity of the skirted foundation enhances as the skirt's inclination and embedded skirt length increase. Settlement reduction exceeds the value of 80% in the case of inclined skirts with the same skirt length/footing width of 1.0. The deformation shape showed the collapse of the top surface surrounding the soil of the skited foundation. Charts are provided to evaluate the bearing capacity and settlement of the inclined skirted foundation.

Lateral ultimate bearing capacity of modified suction caisson in uniform and linearly increasing strength undrained clays

AbstractThe modified suction caissons (MSCs) have a cylindrical external skirt surrounding the top of the traditional suction caisson to increase the horizontal bearing capacity. MSCs will be subjected to inclined lateral loadings for anchoring offshore floating facilities, and the maximum lateral bearing capacity will be reached when the loading direction passes through the rotation center of the MSCs to generate horizontal translation in the seabed. This paper proposed an analytical model consisting of a meniscus failure wedge near the mudline and a purely lateral flow‐around mechanism in deeper soil for MSCs moving laterally in clays. The upper‐bound analysis is conducted to obtain the ultimate bearing capacity of MSCs by optimizing only one parameter to approach the critical failure mechanism that offers the minimum energy dissipation. The influences of the external skirt length (L) and radius (R) and adhesion factor (λ) at the caisson‐soil interface on the bearing capacity of MSCs are investigated. The finite element limit analysis (FELA) results are used to verify the rationality of the presented failure mechanisms and upper‐bound predictions. It is proved that the presented upper‐bound analyses can well predict the lateral ultimate bearing capacity of the MSCs with a horizontal translation failure.

Effects of the skirt length and embedment depth on the capacity of a skirted spudcan foundation under combined loading

Skirted spudcan foundations can provide a considerable bearing capacity and moment fixity for resisting the complex combined vertical, horizontal, and moment loadings arising from the harsh environment in the offshore oil and gas industry. These potential benefits are conducive to the extension of mobile jack-up drilling rigs into deep water. In this study, a series of probe tests were carried out using finite element analysis to investigate the effects of the skirt length and embedment depth on the bearing capacity of skirted spudcan foundations in soft clay under combined loading. The results indicate that both the skirt length and the embedment depth can significantly affect the behavior of a skirted spudcan and apparently improve the various bearing capacities under combined loading. As the skirt length and embedment depth increase, the failure envelope's size gradually expands and the failure envelope's shape changes accordingly. A spudcan with a skirt length of 0.25D embedded in about 2D of sediments is suggested to obtain an optimal application in practice. A closed-form algebraic expression was derived to interpret and predict the behavior of skirted spudcan foundations, which properly considers the effects of the skirt length and embedment depth.

Mitigation of punch-through in stiff-over-soft clay by changing spudcan shape

This study focuses on easing or eliminating potential punch-through in two-layer stiff-over-soft clay deposits by tweaking spudcan shapes. The effectiveness of skirted spudcans with and without holes, relative to a generic spudcan used conventionally, is tested. Three-dimensional large deformation finite element (LDFE) analyses are carried out systematically; encompassing a parametric study on the length of the skirt on the bottom periphery, number of holes through the spudcan base, and the slope of the spudcan base. Numerical results lead to selection of two skirted spudcans with and without holes, and both with a skirt length of ∼0.23 spudcan diameter. The performance of those two spudcans is then tested through a series of centrifuge model tests; varying the strength ratio between the two layers, and the thickness of the stiff layer relative to the spudcan diameter. Overall, the results of eighteen penetration tests suggest that the skirted spudcan without holes is more effective in easing or even eliminating the punch-through that is associated with the penetration of the generic spudcan. For using the skirted spudcan without holes in the field, an improved analytical model is proposed for assessing penetration resistance profile or embedment depth under a given preload.

Combined bearing capacity of bucket foundations in soft-over-stiff clay

In this study, a series of three-dimensional small-strain analyses are carried out to explore the combined capacity of bucket foundations in soft-over-stiff clay under general loading. It aims to provide a supplement to existing design codes. Numerical models are generated and validated with existing data in open literature, where good agreements are obtained. The effects of the layered deposit are explored, including the thickness of the upper soft clay, the length of skirt, the magnitude of the vertical load, and soil properties. Due to the presence of soft clay, the uniaxial bearing capacities reduce significantly, reaching as low as 25% reduction compared to the caisson in uniform stiff clay. In the V-H, V-M and V-H-M spaces, the failure envelopes are normalized in unique shapes according to the geometry of the bucket. Based on the numerical results, an empirical design procedure is proposed to evaluate the failure envelopes of bucket foundations subjected to complex loading conditions in soft-over-stiff clay.

Numerical Analysis for the Response of Skirt Circular Shallow Footing Resting on Sandy Soil under vertical loads

In this paper, the Static response of a circular skirt foundation resting on a sandy soil subjected to pure vertical load was analyzed numerically using finite element software, PLAXIS 3D 2020. Linear elastic model was adopted to describe the behavior of the shallow concrete circular foundation with (1m) diameter and (0.2 m) thick, as well as the soil underneath, with and without skirt . Different skirt's length to diameter ratios (L/D) (0.25, 0.5, 0.75, and 1.0), in addition to different skirt thicknesses (0.1 m, 0.15 m, 0.2m) were anticipated. The results shown that, in dry sand, existence of skirts improves the foundation performance effectively by increasing its load carrying capacity particularly when the skirt (L/D) ratio become larger. However, the increase in the thickness of the skirt has marginal influence and it can be neglected for design purposes.

신한복 온라인 플랫폼 구축을 위한 추천 치마디자인 개발

This study establishes an online customization platform for New-hanbok based on analyzing the hanbok brands and the preference for new-hanbok skirts. This study aims to develop a design that can recommend a modern hanbok skirt so that customers can purchase it online. Design elements according to body parts of each age group were extracted in two-dimensional and three-dimensional techniques, and short, medium, and long skirt designs were selected. The pattern for the selected design was made through YUKA Apparel CAD, and the design was confirmed by simulating by using a CLO 3D program. Pattern production dimensions were produced by substituting the dimensions of women in their early 20s from the 7th body dimension survey report of Size Korea. The applied dimensions are 161.0 ㎝ in height, 71.0 ㎝ in waist circumference, and 93.0 ㎝ in hip circumference. The exact dimension was applied to the virtual model for 3D simulation. As a result of examining design elements for 3-Dimension, young people in their 20s and 30s preferred long skirts of around 80㎝, and older people preferred medium-length (60㎝). Except for those in their 20s, short length (45㎝) was the least preferred, so skirt length was found to be related to age. Regardless of age group, the waist width was about 5㎝, which was the most preferred. Next, it seems that the simple shape of the skirt was designed with a change in the waist of the skirt, preferring a wide width. The most preferred length was an average length of about 60㎝, followed by about 100 ㎝. Young people preferred monochromatic colors for skirts, and older people preferred chromatic colors such as red and blue. Patterns were preferred more in the older age group than in the younger age group, and flower patterns were preferred the most. Among the developed skirt designs, the top three types with high preference seem to be related to individual tastes and age. Customers can purchase a recommended skirt simultaneously or re-recommendation for a new skirt by revising design components.

The remedial performance of suction caisson foundations for offshore wind turbines under seismically induced liquefaction in the seabed: Shake table testing

The assessment of liquefaction hazards to offshore wind turbines (OWTs) requires an improved understanding of the interplay among the foundation geometry, ground shaking, and liquefaction-induced deformations. In this note, shaking table model tests were used to evaluate the behavior of suction caissons due to seabed liquefaction. Spectral accelerations and settlement of OWT foundations of varying skirt lengths on liquefiable sand were analyzed and compared.Insights into the performance of caisson skirts as a liquefaction countermeasure relative to the benchmark models are provided. The experimental results identified the two controversial mechanisms controlling shear-induced caisson settlement; the liquefaction-induced bearing capacity failure as the skirt length increases, whereas the larger net excess pore-water pressures within skirts contribute more significantly to reducing seismic demand.Overall, the skirted foundation with the longest skirt length experienced the least settlement and acceleration, which may be ascribed to the interplay of various mechanisms, i.e., the confining effect of the skirt and a slower pore-water pressure dissipation that limits the possibility of drainage.

Elastic stiffness coefficients of a skirted spudcan foundation in clay

Skirted spudcan foundations can potentially provide considerable bearing capacity and moment fixity that can withstand the complex combined vertical, horizontal, and moment loadings arising from the harsh environments encountered in the offshore industry. The analysis of a skirted spudcan foundation's stiffness should be given special consideration during the preliminary stage of the design process. This technical note carries out a series of probe tests using finite element analysis to investigate the elastic stiffness coefficients of a skirted spudcan foundation in soft clay. The elastic stiffness coefficients of skirted spudcan footings with different soil shear strengths, embedment depths, and skirt lengths are presented. The results indicate that skirted spudcans can provide higher elastic stiffness coefficients, and the elastic stiffness coefficients are significantly influenced by the soil shear strength, embedment depth, soil backflow, and skirt length. An approximation expression with three special variables is developed for predicting the elastic stiffness coefficients of a foundation, except for the footing on the surface. This may potentially complement the elastic stiffness coefficients in the existing jack-up assessment guidelines, which do not account for skirted spudcan shaped footings embedded at different depths.

Research on the transverse stability of an air cushion vehicle hovering over the rigid ground

ABSTRACT The transverse stability of air cushion vehicles (ACVs) hovering over the rigid ground has always been the challenging problem involving cushion aerodynamics and flexible skirt structure dynamics. The transverse stability is investigated numerically and experimentally based on a particular ACV model. The physical process and mechanism are analysed and a numerical method for evaluating the transverse stability is presented. A cushion–skirt communication platform is built for interchanging the force, hull displacement, and skirt configuration between cushion aerodynamics and flexible skirt structure dynamics. The transverse stability is calculated numerically and the stability model experiments are conducted based on a scale craft model, demonstrating that the numerical result is valid well against the model experimental result. The influences of the bag-to-cushion pressure ratio, the length and effusion height of the longitudinal stability skirt on the transverse stability are discussed systematically by the numerical approaches, giving insight to the design of ACVs.

Capacity of caissons in stiff-over-soft clay under combined V–H-M loadings

This paper carries out a numerical study to investigate the bearing capacities of caissons in stiff-over-soft clay under combined V–H-M loadings. A finite element model is generated and validated with available data in open literature, where a good agreement is obtained. A parametric study is then performed to explore the effects of the upper clay thickness, skirt length, the mobilization of vertical loading and different soil properties on the failure mechanism and corresponding bearing capacity of caissons in stiff-over-soft clay under combined V–H-M loads. The results show that the failure mechanisms of caissons in stiff-over-soft clay are significantly different from those in uniform clay; hence a dramatical distinction between the failure envelopes in stiff-over-soft clay and uniform clay can be found. Last but not the least, based on the numerical results, an empirical design approach is proposed to predict the failure envelopes of caissons under various loading situations.

Identification of the extinction mechanism of lean limit hydrogen flames based on Lewis number effect

The conventional flame stretch analysis showed some technical limitations in describing the observed extinction mechanism without considering the heat transfer characteristics related to flame flow patterns. To improve the flame stretch analysis, this study investigated the extinction mechanism of lean limit hydrogen flames with the stabilized flame method. Multidimensional lean limit hydrogen flames stabilized in a tube of inner diameter of 25 mm were simulated numerically. Unlike the methane flames, the positive effect of a stretched hydrogen flame (Le≪1) on the flame tip dominated the negative stretch effect by increasing the preferential diffusion. Hence the primary extinction of hydrogen flames occurred at the trailing edge by heat loss mechanism. The simulation confirmed that the radiation heat loss rate did not attain 5% of the conduction heat loss rate at the trailing edge. When the flame skirt length was shortened enough by the primary extinction, an enhanced recirculation flow was generated. The combined effect of conduction and recirculation flow associated with the Lewis number effect was the key to understand the extinction mechanism of lean limit hydrogen flames. Finally, generation of recirculating flow causing the stretch extinction plays a dominant role in final flame extinction.

The Skirt and Collar of The Valsalva Graft: One Dress Fits All

In the original pre-formed Valsalva graft, the skirt is the defining feature. It is a self-expanding portion, obtained by 90° rotation of the Dacron fabric corrugation with respect to the rest of the graft. Due to this manufacturing feature, the skirt length is equal to the graft diameter and, once pressurized, it expands by 25-30% (up to 10 mm) from the nominal size. Proximal to this bulged portion, a small collar completes the prosthesis. By virtue of this anatomical design, the Valsalva graft is recommended for most aortic root surgeries including valve-sparing and Bentall procedures. The skirt, by recreating the pseudo-sinuses, represents the key feature of the graft when used for aortic valve-sparing procedures (remodeling and reimplantation). However, in the Bentall procedure, the graft collar is an important feature because it is useful to attach the Valsalva graft to the valve prosthesis sewing ring when making a valved conduit. Moreover, once the graft is pressurized, the whole volume of the skirt better accommodates the struts and the leaflets of biological and mechanical prostheses. Furthermore, the expanded skirt reduces the distance and the tension on the coronary button anastomosis, increasing safety and possibly reducing complication rates. This review demonstrates the versatility of the Valsalva graft in various scenarios of aortic root surgery, with special emphasis on different uses of the skirt and the collar.