A precise prediction of seabed stability involving the fluid-pipe-soil interaction can lead to significant cost reductions by optimising design. Unlike previous investigations, a three-dimensional numerical model for the wave-induced soil response around an offshore pipeline is proposed in this paper. The numerical model was first validated with 2-D experimental data available in the literature. Then, a parametric study will be carried out to examine the effects of wave, seabed characteristics and confirmation of pipeline. Numerical examples demonstrate significant influence of wave obliquity on the wave-induced pore pressures and the resultant seabed liquefaction around the pipeline, which cannot be observed in 2-D numerical simulation.
Nowadays, many offshore structures have been commonly constructed over the last few decades due to the growing engineering resource in the ocean. Submarine pipelines, as one of the popular offshore infrastructures, have been extensively used for transportation of natural gas and oil from offshore platform, and disposal of industrial as well as municipal waste. To ensure the safety of usage of such submarine pipelines, the coastal engineers have to consider the unexpected loads including the wave, current, and anchor dropping/dredging, which might cause the its stability and decrease its life span. Thus, it is customary to bury the pipeline by trenching and refilling soil whose cost is relatively high and time-consuming (FredsØ e, 2016).
As reported in the literature, two well-known main mechanisms of dynamic wave-induced seabed liquefactions are the momentary liquefaction and residual liquefaction, based on in the field measurements and laboratory experiments (Zen and Yamazaki, 1991). The fist mechanism, momentary liquefaction, can occur beneath wave troughs when the great seepage flow is upward directly. Since this kind of liquefaction may be happen within a short duration as the passage of wave trough, it is also called instantaneous liquefaction. The other mechanism, residual liquefaction, takes place as a result from a compacted and cyclic shearing process that the build-up of excess pore pressure in the seabed (Seed and Rahman, 1978). As mention previously, the waves also can induce shear stress in the soil when the waves propagate, which has been analytically investigated by Yamamoto et al (1978). Whereas the wave-induced shear stress has less impact on seabed instability compared to that caused by the previous two mechanism above. This study only focuses on the wave-induced seabed liquefaction incorporating both instantaneous mechanism.
In this paper, an integrated numerical model is developed to investigate wave-seabed interactions around multiple submerged permeable breakwaters in an elasto-plastic seabed foundation with Bragg effects. In this model, the wave motion is governed by VARANS equation and the Biot's u-p approximation is used to govern soil-pore fluid interactions in porous medium. The advanced elasto-plastic constitutive model (PZIII) is used to reproduce the plastic soil behaviour in seabed foundation under long-term cyclic wave loading. Numerical results show that the wave motion can be largely changed due to Bragg effects. Under the strongest Bragg effect, the soil permeability and degree of saturation can significantly affect the liquefaction potential in seabed foundation.
Breakwaters are the most commonly used offshore structures for protecting coast lines by reflecting incident waves back to offshore area. There are many forms of breakwaters, among them, the submerged permeable breakwaters have advantages like eco-friendly and wave energy dissipation efficiently because the breakwaters are under water surface that have less impact on ocean environment and porous medium allows it to reduce wave impact more efficiently. Bragg effect is defined as amplification of reflected waves by multiple breakwaters when the distance between two adjacent breakwaters are about half of incident waves length (Mei et al, 2005). Bragg effect is beneficial for breakwaters to protect the coast line as more waves will be reflected, on the other hand, it will imply more chance for seabed foundation to be liquefied as the wave height become larger in front of breakwaters. As reported, numerous damages of submerged breakwaters are caused by wave-induced seabed instability rather than from the construction deficiencies (Sumer, 2014). Therefore, it is essential to have a better understanding of wave induced seabed response around multiple breakwaters with Bragg effects.
In the past few decades, considerable studies have been conducted to investigate the wave induced soil response and wave-structures-seabed interactions (Jeng, 2003). In the early stage, analytical solutions are the most widely used method to study the wave induced soil response around submerged permeable breakwater (Jeng, 1996), in which the breakwaters are simplified as lines without width and weight. Later, numerous numerical models are developed to study the wave-breakwater-seabed interactions. These models normally consist of two sub-models: fluid model for fluid-structure interactions and soil model for dynamic soil response. The fluid-structure interactions have been intensively studied previously (Lin and Liu, 1998; Liu et al., 1999; Hsu et al., 2002). By integrating fluid and soil models, the integrated numerical models can simulate the more realistic engineering problems as the e_ect of structure shape, porous properties of structures and the gravity of structures can be included (Ye, 2012).
In this paper, a modified PZIII model by considering the impact of principal stress rotation for the wave(current)-induced soil response in a sandy seabed is cited. Unlike the previous works, the proposed model considers the effect of PSR by treating it generating the plastic strain rate independently. Then, the proposed model was incorporated into the finite element analysis procedure DIANA-SWANDYNE II. Both wave and current loadings are considered in the present model. The proposed model was first validated through comparisons with the previous experimental data for the soil response under wave and current loading. Adapting the proposed model, effects of the PSR on the fluid-seabed interactions will be investigated.
Recently, the human exploration and development of ocean are more frequent due to the abundant marine resources and colossal development. However, in the complex marine environment, the constructions of offshore engineering projects have huge challenges in the present due to various uncertain factors. The cyclic dynamic load which is produced by wave and current propagation on the seabed surface will cause the fluctuation and accumulation of pore water pressure. When the pore pressure extreme growth, the effective stress will decrease, which will lead to instability of the soil as the consequence of the horizontal or vertical movement of the soil particles, resulting in instability of the soil (Sumer, 2014). Therefore, determining pore-water pressures within a porous seabed is particularly crucial for coastal geotechnical engineers involved in the design of the offshore infrastructures foundations. Numerous studies have been carried out to calculate the stability of the seabed in the past, including poro-elastic model and poro-elastoplastic model, but most of them did not consider the effects of PSR (Jeng, 2013).
The continuous rotation of the principal stresses in a seabed is an essential feature of the dynamic response of soil under cyclic wave loading. Unfortunately, due to the assumption of pure PSR, this process cannot be trapped by conventional elasto-plastic theory without changing the cyclic deviatoric stress amplitude of the plastic strain. However, several experimental results have confirmed that the plastic strains are generated merely through altering the principal stress orientation in both monotonic and cyclic rotational share tests (Ishihara and Towhata, 1983; Towhata and Ishihara. 1985; Sassa and Sekiguchi, 1999; Jafarian et al. 2012; Konstadinou and Georgiannou, 2013). However, the inhomogeneity materials determines the effect of PSR. If the seabed soil is always homogeneous, even if the wave loading is rotating, the principal stress axis may not produce a significant effect.
Soil permeability is one of key factors in the prediction of the wave- induced seabed response, which will directly affect the design of foundation around marine installations. Most previous studies in the field treated the soil permeability as a constant, although it depends on numerous soil parameters. In this study, the soil permeability is considered as a function of pore water pressure as reported in the literature, with this new feature, the governing equation will become non-linear differential equations. Numerical examples demonstrate the significant influence of dynamic soil permeability on the wave-induced pore pressure and effective stresses.
In the past a few decades, considerable efforts have been devoted to the phenomenon of the wave-soil interactions. One of the major reason is that the evaluation of the wave-induced soil response and its resultant seabed instability is particularly important for coastal geotechnical engineers involved in the design of foundation of the offshore installations (Rahman, 1991; Sumer, 2014). Another reason is that the poro-elastic theories for wave-soil interaction have been applied to field measurements such as determination of the shear modulus of seabed (Yamamoto et al., 1991) and the direction spectra of ocean waves (Nye and Yamamoto, 1994), as well as acoustic waves propagating through porous media (Yamamoto and Turgut, 1988).
Based on Biot's poro-elastic theory (Biot, 1941), several classic investigations for the wave-induced soil response have been carried out since the 1970s. Among these, Yamamoto et al (1978) proposed an exact closed-form analytical solutions for the wave-induced oscillatory soil response in an isotropic, poro-elastic and infinite seabed. This model was further extended to three-dimensional cases by Hsu and Jeng (1994) for various soil conditions. Another different approximation, so-called boundary layer approximation, was proposed by Mei and Foda (1981), which provided a simple formulations of the wave-induced soil response. This approximation can provide precise prediction of soil response in fine sand rather than coarse sand, as reported in Hsu and Jeng (1994). Okusa (1985) investigated the effects of degree of saturation on the wave-induced soil response based on plane stress conditions. A detailed review of previous relevant research can be found in Jeng (2003).
The use of fiber reinforced polymer (FRP) materials in civil engineering structures is relatively new. FRP reinforcing bars are starting to be used in lieu of conventional steel reinforcing bars where improved corrosion resistance is required in soil nailed reinforced earth retaining structures. Presently, there is only a relatively small body of knowledge on numerical modeling techniques and the response of FRP reinforcing nails under load. This paper provides an overview of research completed to date on both steel and FRP reinforced soil nails, and an outline of planned future research.
Traditionally, soil nail earth retaining structures use steel reinforcing bars. Where steel is used, some standards and guidelines require up to three forms of corrosion protection. This typically involves hot dipped galvanizing the steel bar, an enveloping plastic sheath and a grouted annulus enveloping the bar. Advances in the development of fiber composites has resulted in the use of fiber reinforced polymer (FRP) reinforcing bars in lieu of steel bars for civil engineering structures.
FRP composites are a composition of fiber bound within a resin. FRP is an anisotropic material that is characterized by a high tensile strength in the direction of the reinforcing fibers. Fibers used in composites can be categorized based on their molecular structure using three groups: (1) polymeric; (2) carbon; and (3) inorganic. These fibers are typically set in an epoxy, polyester or vinyl-ester resin.
A key advantage of FRP materials is their high stiffness-to-weight ratio when compared to steel, being approximately 10 to 15 times higher. However, this advantage is offset by several shortcomings in comparison to steel. Firstly, the modulus of elasticity of FRP reinforcing bar is approximately four times less, resulting in larger displacements under pullout loads (Zhu, Yin et al., 2010). FRP also has a much lower shear modulus that can result in a lower ultimate shear resistance at a slip surface in a reinforced soil slope. Furthermore, when subjected to high tensile loads, FRP generally exhibits significant time-dependent elongation. Finally, FRP has a brittle failure mode.
Zhou, Jialin (Griffith University) | Oh, Erwin (Griffith University) | Zhang, Xin (Shandong Jianzhou University) | Jiang, Hongsheng (Shandong Jianzhou University) | Bolton, Mark (Griffith University) | Wang, Peisen (Tianjin University)
This paper provides an investigation of shaft and base grouted concrete pile behavior by conducting vertical compressive and uplift static load tests (SLTs) in Jinan, China. Three concrete piles were tested by compressive SLTs. Two of these piles were applied with shaft and base grouting, and base grouting technology respectively, and the third was applied without any grouting. Two uplift SLTs were carried out on one shaft and base grouted pile, and one pile without grouting. Traditional methods which include Load-settlement curvatures (L-s), Settlement-log time curvatures (s-lgt) and Time-log Load curvatures (t-lgQ) analysis were provided to check if the bored piles reached the design requirement. In addition, an interpretation of the test results from Double-tangent, DeBeer's, and Chin's methods (for compressive SLTs) and Mazurkiewicz's method (for uplift SLTs) were provided for determining the ultimate pile capacity where piles experienced non-plunging failure. Results from the five SLTs program indicates that the Double-tangent and DeBeer's test results are close to each other whereas Chin's method overestimates the pile capacity; the base and shaft grouting pile and base grouting pile (compressive load) increases 9.82% and 2.89% of its capacity, respectively. Compared to the ultimate uplift SLTs, there is a 15.7% increment of pile capacity after using base and shaft grouting technology.
Piles are the long and slender structural components which transfer loading from upper structures (Tomlinson and Boorman, 2001). It is common for these piles to experience vertical compressive loading and sometimes these piles may also experience uplift loading, as in for example, the piles under a wind turbine. Pile foundation is popular because some situations occur where shallow foundations are not appropriate for resisting the loading transferred from structural elements (Samtani and Nowatzki, 2006). Such situations are: loads applied to foundation are very large; properties of some soil layers are not good enough; requirement needed as in the case where displacement must be kept small etc. (Knappett and Craig, 2012).
External pressure loadings in sub-sea pipelines can generate catastrophic structural instabilities such as propagation buckling. This failure mode is typified by a pipe collapse (snap-through phenomenon) that occurs at an initiation pressure PI and a subsequent propagation of the collapse to pipe ends that occurs at a propagation pressure PP. Recent studies have shown that pipelines with a textured geometry, corresponding to the post-buckled shape of a thin-walled cylindrical pipe under axial compression, are able to substantially increase PI, PP, and thus resistance to propagation buckling, compared to conventional smooth pipelines. This study investigates the performance of alternative post-buckled shapes observed in thin-walled pipelines under hydrostatic loading. These shapes correspond closely to a geometric family known as curved-crease origami and so a geometric definition is developed to map geometric parameters from origami to pipelines. A numerical analysis is then conducted on two curved-crease forms and comparative smooth and textured forms. Textured and smooth numerical models show good correspondence with previously reported post-buckling behaviour. One curved crease form is shown to have an increase in PP that is 10.8% greater than the textured pipeline and 131.8% greater than smooth.
A subsea pipeline is a slender structure used for long-distance transport in oil and gas industries. It can experience a number of instabilities such as lateral (snaking) buckling, upheaval buckling, and propagation buckling (Karampour et al., 2013). Among these, lateral buckling is a global buckling that is due to restrained axial expansion caused by combination of seabed friction and high temperature pipeline contents. Upheaval buckling is akin to this but occurs in subsea pipelines that are trenched (Karampour et al., 2015). The most critical instability is propagation buckling, a snap-through phenomenon that can rapidly damage a large length of the pipeline, particularly in remote deep subsea regions (2-3km depth).
In this paper, new formulae are presented to estimate the wave overtopping rate of mass-armoured berm breakwaters. In addition to the effects of dimensionless crest freeboard and crest width, the formulae consider the influences of water depth at the toe of the structure and structure slope on the overtopping rate through simple dimensionless parameters. The performances of the new formulae were then compared with those of the existing empirical prediction formulae. Statistical indicators such as Root Mean Square Error (RMSE) showed that the new formulae are better predictors than the existing ones.
Breakwaters are built to protect coastal region from wave action and consequently flooding and erosion (Lara et al., 2008). Safety in a harbour can depend on the wave overtopping rate at breakwaters and therefore their appropriate design is critical (Van der Meer and Bruce, 2014). The mean wave overtopping must be less than the allowable rate under the design and operating conditions to ensure the safety of people, land and property behind the breakwater (Goda, 2009). Additionally, Zviely et al. (2015) reported that excessive wave overtopping can lead to extensive damage in port infrastructure and moored vessels. Thus a reliable estimation of wave overtopping rate is essential to reduce the risk associated with the failure of the breakwaters.
The propagation buckling response of Pipe-in-pipe (PIP) system subjected to external pressure is investigated in this paper. Experimental study has been performed on buckling of aluminium PIPs with outer diameter to wall-thickness ratios (Di t) of26.7 and 30, inside a hyperbaric chamber. The experimental protocol is comprised of endsealing concentric PIP with a length of 1.6 m, inserting the PIP inside the 25MPa hyperbaric chamber and increasing the pressure until collapse of the system due to external pressure has occurred. This paper proposes a simple Ring Squash Test (RST) for estimating the propagation buckling pressure of PIPs which is compared against hyperbaric chamber results. The proposed ring squash test is a much expedient test to implement in comparison to hyperbaric chamber test and estimates the propagation pressure of PIPs with reasonable accuracy. Experimental results from the ring squash tests, confined ring squash tests (CRST) and hyperbaric chamber tests are presented, and are compared with empirical equations. The RST gives a lower bound of propagation pressure of PIPs. Previous empirical results agree well with current hyperbaric chamber results.
Pipe-in Pipe (PIP) systems are extensively being used in the design of high pressure and high temperature (HP/HT) flowlines due to their outstanding thermal insulation. A typical PIP system consists of concentric inner and outer pipes, bulk heads and centralizers. The inner pipe (flowline) conveys the production fluids and the outer pipe (carrier pipe) protects the system from external pressure and mechanical damage. The two pipes are isolated by centralizers at joints and connected through bulkheads at the ends of the pipeline. The annulus (space between the tubes) is either empty or filled with non-structural insulation material such as foam or water. A sub-sea pipeline can experience a number of structural instabilities, such as lateral (snaking) buckling, upheaval buckling, span formation and propagation buckling. Among these, propagation buckling is the most critical one, particularly in deep water, and can quickly damage many kilometers of pipeline. Local collapse, irrespective of how it is induced, usually will initiate a propagating buckle which in a short time can catastrophically flatten significant sections of the structure (Palmer and Martin, 1975; Kamalarasa and Calladine, 1988; Xue and fatt, 2001; Albermani et al., 2011).
This paper presents the research work to investigate the impact of different parameters on the results of a numerical modeling for a nailed wall constructed in residual soil. Stability analysis of a soil nailed wall was carried out with Strength Reduction Method (SRM) and in the modeling stage, different conditions were considered. Element type, mesh density and excavation stages were three criteria whose effect was investigated in the current work. Finally, the result of analyses for the above-mentioned cases are to be presented and discussed in this paper in terms of Factor of Safety, wall deformation and structural forces.