Li, Yanghui (Dalian University of Technology) | Song, Yongchen (Dalian University of Technology) | Liu, Weiguo (Dalian University of Technology) | Yu, Feng (Dalian University of Technology) | Wang, Rui (Dalian University of Technology) | Nie, Xiongfei (Dalian University of Technology)
To acquire more knowledge about the mechanical properties of gas hydrate-bearing sediments and assess the longterm stability of the strata, samples of clayey sediments containing methane hydrate were prepared and tested for their mechanical properties under various conditions by triaxial compression. The effects of temperature and confining pressure on the mechanical properties of methane hydrate-bearing sediments were analyzed. And a strength criterion considering the influence of temperature was developed for methane hydrate-bearing sediments. It agrees well with the experimental data, it and can be taken to predict the shear failure strength of methane hydrate-bearing sediments under subzero conditions.
Gas hydrate has attracted global attention due to its widespread occurrences and large potential as an energy resource (Brown et al., 2006; Glasby, 2003). It generally exists in the regions of Arctic permafrost and submarine continental margins where there are acceptable P/T conditions (MacDonald et al., 1994). Additionally, the naturally occurring hydrates are always associated with large quantities of gas trapped underneath, with the hydratebearing strata acting as seals for trapped free gases. Gas may release abruptly during mining and well drilling, which can lead to blowouts and well-control problems (Cameron et al., 1990; Vanoudheusden et al., 2004). Characterizing the mechanical properties of gas hydrate-bearing sediments will provide a scientific basis for the evaluation of the safety of the structure and the stability of the reservoir. Up to now, the mechanical properties of gas hydrate-bearing sediments have not been fully understood. Durham et al. (2003) and Stern et al. (1996) indicated that the mechanical properties of methane hydrate are very different from those of water ice. Winters et al. (2004, 2007) studied the strength and acoustic properties of laboratory-formed methane gas hydrate. However, the analyses of the mechanical properties of gas hydrate-bearing sediments under various conditions are qualitative.
This paper concerns the response of a single-layered stranded cable of helical wires with wires-to-core contact under constant curvature constrained bending. The stranded cable under static-loading conditions experiences any combination of tension, torsion and bending. A linear elastic model for helical wire stranded cable running over a drum has been derived by using the thin rod theory. The stiffness matrix has been developed and the relations are presented for axial, torsional and flexural rigidities and for coupling parameters. The present work emphasises the contribution of the effect of the drum on the cable together with the coupling of tension, torsion and bending of the stranded cable.
Helically wound strands have numerous engineering applications. Each strand of a wire rope consists of a central core as a straight wire surrounded by a number of wires wound helically in a single layer or a multilayer. In practical applications, stranded cables often impose some transverse loading or curvature, such as ropes running over a sheave and drum, wind-induced transverse vibration of conductors, etc. They are classified as free bending, as in vibrating cables and constrained bending, as in the case of cable running over the drum, or in the case of cable bent on a pulley. Although cables have been used for many centuries, theoretical models for bending of helically wrapped cables are few and relatively recent. Costello and Butson (1982) proposed a model for cable free bending (including twisting) based on Love’s (1944) equations of equilibrium of a thin rod model. Lanteigne (1985) added wire bending with the wire axial force for determination of the cable’s bending moment. Knapp (1988) also derived the wire curvatures and twist for a kinematic model of a cable bent in a planer circular arc.
Minagawa, Hideki (National Institute of Advanced Industrial Science and Technology) | Egawa, Kosuke (National Institute of Advanced Industrial Science and Technology) | Sakamoto, Yasuhide (National Institute of Advanced Industrial Science and Technology) | Komai, Takeshi (National Institute of Advanced Industrial Science and Technology) | Tenma, Norio (National Institute of Advanced Industrial Science and Technology) | Narita, Hideo (National Institute of Advanced Industrial Science and Technology)
A proton nuclear magnetic resonance (NMR) system combined with a permeability measurement system has been used to clarify the relation between permeability and a methane-hydrate saturation in methane-hydrate-bearing sediment with regard to effective pore-size distribution. Pore-size distributions of sediments have been calculated using the relaxation time distribution of NMR-T2. Two different laboratory methods for growing gas hydrate in sediment cores have been used to determine the relationship between hydrate saturation and permeability: a conventional approach called the connate water method, and a dissolved-gas method. The 2 methods produced different permeability and pore-size distribution of sediment.
Methane hydrates (MH) in sediment are expected to be developed as a resource for natural gas and have been studied as a possible future energy resource. In-situ dissociation of the naturalgas hydrate is necessary for commercial recovery of natural gas from natural-gas-hydrate-bearing sediment (i.e., mainly MHbearing sediment) (Makogon, 1981, 1988). Various methods of producing methane gas from MH have been proposed for exploiting MH (e.g., depressurization (Makogon, 1981, 1986, 2005; Sakamoto, 2007a, 2007b), thermal stimulation (Makogon, 1981, 1986, 2005; Sakamoto et al., 2007a, b), and inhibitor injection (Makogon, 1981, 1988; Makogon and Holditch, 2005; Kawamura et al., 2006). With any method, the gas permeability and water permeability of MH-bearing sediments are important factors for estimating the efficiency of methane-gas production. Sediment permeability is generally determined by measurement using gas or liquid flow. For example, the permeability of an MH-bearing layer is measured by using gas or liquid flow through the MH-bearing sediment, which can be explored using a pressure-temperature core sampler (PTCS). The permeability of MH-bearing sediment is considerably affected by several properties of the sediment (e.g., the pore-size distribution, porosity, cementing, MH production characteristics and MH saturation). This method can be used at high pressure but is limited to samples with water-saturated pores.
Lee, Yun-Hee (Korea Research Institute of Standards and Science) | Kim, Yongil (Korea Research Institute of Standards and Science) | Park, Jong Seo (Korea Research Institute of Standards and Science) | Nahm, Seung Hoon (Korea Research Institute of Standards and Science) | Yoon, Ki-Bong (Chungang University)
In order to estimate the hardness and yield strength of an indented material, advanced methods have been developed for extracting closed boundaries of the contact area and the plastically deformed zone from 3D nanocontact morphologies. However, this image processing technique cannot be applied to shallow indentations as it results in weak surface pile-ups. Based on the modified volumetric approach, the new hardness and yield strength of Au film and fused quartz are compared with those from the indentation curve analysis and differential contact analysis.
Nanoindentation measuring applied load and indenter penetration depth during a contact deformation is one of the most powerful techniques for evaluating the mechanical properties of small volume materials (Oliver and Pharr, 1992). Typical nanoindentation researches have been constrained within the determination of elastic modulus and hardness. However, the research scope is now being expanded to the analysis of plastic flow curve, yield strength, residual stress, fracture toughness, interfacial adhesion and various tribological properties (Ahn and Kwon, 2001; Lee et al., 2006; Lee and Kwon, 2002; Lee and Kwon, 1999). However, since the deformation morphology under indentation loads less than mN cannot be easily observed, 2 models have been developed (Oliver and Pharr, 1992; Doerner and Nix, 1986) for characterizing Ac at the peak indentation load from the nanoindentation curve. The method commonly used for analyzing the nanoindentation load-depth curve is that proposed by Oliver and Pharr (1992), expanding on an earlier work by Doerner and Nix (1986). Below, the analyzed data based on the Oliver and Pharr method will be denoted as O&P. However, the O&P method (Oliver and Pharr, 1992) can strongly underestimate the contact area if a material pile-up is involved, as reported in the finite element simulation work of Bolshakov and Pharr (1998).
The starting and steady flows past a circular cylinder placed near a planar boundary are investigated both experimentally and numerically. The flow is accelerated until it reaches an ultimate speed and then remains steady. The flow field is visualized using a hydrogen bubble technique. In numerical simulations, the Navier-Stokes equations for unsteady incompressible viscous flow combined with a k- turbulence model are solved via a finite volume method with SIMPLEC algorithm. The investigation reveals that vortex shedding takes place at the starting flow stage but is suppressed when the gap ratio G/D becomes smaller than 0.3.
Flow around a circular cylinder near a planar boundary may be an ideal simplification of many practical problems, such as the hydrodynamics of marine pipeline and cables. With regard to steady incoming flow, Bearman and Zdravkovich (1978), Grass et al. (1984), Taniguchi and Miyakoshi (1990), Buresti and Lanciotti (1992), Lei et al. (1999) and others have experimentally investigated the flow past a circular cylinder above a planar boundary. The features of flow field and the effects of the boundary to the hydrodynamic loads on the cylinder are the major concerns. Advanced flow visualization techniques, such as the particle image velocimetry (PIV) technique, have been used during research on this subject (Price et al., 2002; Wang and Tan, 2008; Lin et al., 2009). Plenty of related works also involve local scour and shear stress distribution around submarine pipeline, both experimental and numerical, such as Mao (1986), Fredsøe et al. (1992), Sumer et al. (2001), Chen and Cheng (2004), Zhao et al. (2006), Zhao and Cheng (2010), and Chen et al. (2010). These works investigated the flow around the pipeline from the view of local scour as the vortices’ motion plays an important role on scour.
This paper reviews recent developments in the prediction of the likely future corrosion losses and of the maximum pit depth for steels exposed to marine environments. A robust mathematical model based on corrosion science principles and calibrated for immersion conditions to an extensive range of literature and new data is described. The model has provided explanations for the effects of steel composition, water velocity, depth of immersion and seawater salinity and also has facilitated new interpretations of data for long-term pitting corrosion. This paper briefly overviews these developments and refers to some typical applications, including marine corrosion of ship ballast tanks, corrosion of sheet piling in harbours and corrosion of offshore platform mooring chains.
Physical infrastructure plays a major role in the most modern societies. So-called whole-of-life assessments increasingly are being used for decision processes. Such algorithms require models of sufficient rigor and robustness to represent (a) the demands or loadings expected to be placed on the system; (b) the ways in which the system may respond; and (c) prediction of likely future response, including deterioration and effectiveness of repairs. Consistent with modern decision theory, the models required for (a) and (b) are probabilistic (Melchers, 1998). Until recently, models for (c) were largely ignored. Most infrastructure has expected lives of several decades. As argued previously (Melchers, 2005), the only way such predictions can be made is to invoke a combination of scientific understanding of deterioration processes and sound mathematical modeling. The present paper is concerned with the development of corrosion models, particularly for longer-term exposures. Despite good maintenance regimes, and the availability of protective coatings and of various forms of cathodic protection, field evidence shows that existing infrastructure often shows signs of corrosion, particularly in severe environments, such as for offshore facilities, along marine coastlines and in harbors.
The coastal development on the Persian Gulf has increased recently with the development of Abu Dhabi, the United Arab Emirates. To ensure the stability and serviceability of the coastal structures, the resistance against the horizontal and uplift forces should be considered in the design process of the foundations supporting those structures. Unlike the resistance against compression and lateral forces, however, the pullout resistance of piles has not yet been fully investigated; also, belled piles are known to be very effective against pullout forces, but research on their pullout behavior has been limited. Hence, in this study, pullout load tests of belled tension piles were performed at 4 sites in Abu Dahbi, and subsequently the bearing capacity, characteristics of load-displacement of piles and load distribution considering skin friction were investigated. Based on the numerical simulation analyses, proved to capture the ultimate uplift capacities from the load tests, the shape and size of the bell has influence on the load-displacement behaviors of belled piles rather than the ultimate uplift capacity of the belled piles in the weathered sandstone ground conditions. In addition, the limit pullout bearing capacity calculated by 3D finite element analysis and theoretical methods were compared. The theoretical methods overestimate the ultimate pull out capacity regardless of the bell-shape considerations.
Drilled shafts (piers or caissons) are the most common type of foundation for tall structures in coastal areas. To ensure the stability and serviceability of the coastal structures, the resistance against horizontal and uplift forces should be considered in the design process of the foundations supporting those structures. It is necessary to characterize the pullout behavior of pile foundations due to the increasing demand on the offshore and onshore construction and the structures resisting the wind and earthquake loadings.
A 3D coupled fluid-sediment interaction model applicable to suspended sediment analysis is developed in this study. For validation, the developed model is applied to hydraulic experiments on suspended sediment in a steady uniform flow and a swash zone. For both flow conditions, the predictive capability of the model is demonstrated against experimental data in terms of time-averaged suspended sediment concentration. Numerical results for the swash zone show that complex suspended sediment transport relates to 3D vortex motion due to plunging breaking waves, and the results demonstrate the usefulness of the developed model in investigating suspended sediment transport under breaking waves.
In a marine environment, fluid flow can induce sediment transport and resulting seabed profile change. In the literature, a 2-way coupling scheme has been used as one of the computational techniques dealing with fluid-sediment interaction (e.g., Roulund et al., 2005; Lee et al., 2008). In the coupling scheme, seabed profile change is computed from fluid motion, and the updated seabed profile is taken into account in computing the fluid flow at the next time step. Nakamura and Yim (2011) developed a 3D coupled fluid-sediment interaction model (referred to below as FSM), in which the coupling scheme was employed to ensure fluid-sediment interaction. The predictive capability of the FSM was verified against experimental data on cross-shore beach profile changes due to solitary waves (Nakamura and Yim, 2011) and jet-induced local scouring in front of quay walls (Mizutani et al., 2009). In the FSM, seabed profile evolution due to bed-load sediment transport is computed using a module based on the model of Roulund et al. (2005). However, consistent with the rationale provided by Roulund et al. (2005), suspended sediment transport is assumed to be neglected. Accordingly, the FSM cannot be applied to a fluid-sediment interaction phenomenon dominated by suspended sediment.
Microorganisms play a vital role in many ecological, environmental and engineering phenomena: Examples include plankton blooms in the oceans and bioreactors for algae fuels. In the last decade, the mathematical models and numerical methods used in this field have improved significantly. In this paper, we review recent advances in the simulation of the individual and collective behaviors of swimming microorganisms, as well as discrete modeling of individual microorganisms for simulating large-scale flow structures. Because we have recently reviewed the biomechanical aspects of suspensions of swimming microorganisms (Ishikawa, 2009), we mainly focus on methodological aspects here.
Microorganisms play a vital role in many ecological, environmental and engineering phenomena. Plankton blooms in the oceans, for instance, are at the bottom of the food chain and affect the oceanic ecosystem. They sometimes form harmful red tides in coastal regions of the ocean that cause serious damage to fish farms. Algae in the oceans absorb much CO2, which affects the global climate. Microorganisms are also used in bioreactors for medicine and food, such as bread, cheese and beer. Bioreactors for algae fuels are currently a hot topic because of the worldwide energy crisis, and they have the potential to generate an energy revolution (Service, 2011). Because microorganisms have a considerable influence on the global environment, industry and human life, mathematical models that predict their behavior are an important subject of scientific research. The flow field around a microorganism can be considered a Stokes flow, because the size and swimming velocity of a cell are usually small enough to neglect inertia. In terms of fluid mechanics, then, swimming microorganisms may be modeled as singularities, i.e., multipoles (Kim and Karrila, 1992). When a cell is denser than the surrounding fluid, external force is generated on the cell body.
Reel-laying is a fast and cost-effective method to install offshore pipelines. During reel-laying, repeated plastic strain is introduced into the pipeline which may, in combination with ageing, affect strength and ductility of the pipe material. The effect of reel-laying on the pipe material is achieved by small- or full-scale reeling simulations followed by mechanical testing according to corresponding standards. In this report an appropriate test setup for full-scale reeling simulation is presented. The fitness-for-use of the test rig is demonstrated by finite element calculations as well as by full-scale reeling simulations on different pipes of various grades. Plus, small-scale reeling simulations with subsequent ageing and mechanical testing are performed on the same pipe material. A comparison of results from mechanical tests after small- and full-scale reeling simulations is given. Additionally results from collapse tests on pipes after full-scale reeling simulations are presented, and the influence of repeated bending of the pipe on its collapse behavior is discussed.
Two main concepts are normally used for laying offshore subsea pipelines. In the S- and J-lay method a pipeline is fabricated on the deck of a lay barge by welding individual lengths of pipe as the pipe is paid out from the barge. The pay-out operation must be interrupted periodically to permit new lengths of pipe to be welded to the string. The S- and J-lay method requires skilled welders and their relatively bulky equipment to accompany the pipe-laying barge crew during the entire laying operation; welding must be carried out on board and often under adverse weather conditions. Further, the S- and J-lay method is relatively slow, with even experienced crews laying only few miles of pipe a day. This can subject the entire operation to weather which can cause substantial delays and make working conditions quite harsh.