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Jang, Youn-Young (Seoul National Univ. of Science and Technology) | Huh, Nam-Su (Seoul National Univ. of Science and Technology) | Lee, Jae-Bin (Steel Solution Center) | Kim, Ki-Seok (Steel Solution Center) | Cho, Woo-Yeon (Steel Solution Center)
For many structural components in oil and gas transmission pipelines, fracture assessment is one of essential elements for structural integrity since a common failure arises from internal cracks mostly formed during installation or operation in severe environment. In the fracture assessment for pipeline, the fracture mechanics parameters. i.e., J- integral and crack-tip opening displacement (CTOD), have been used, in which these parameters can characterize crack initiation and instability well. In particular, CTOD is widely used for fracture assessment in strain-based design (SBD) concept as well as the transportation pipelines or submarine riser industries subject to large deformation. Many researches have been made to estimate fracture parameters, and several engineering approaches such as General Electric/Electric Power Research Institute (GE/EPRI) and reference stress (RS) method have been suggested. Among them, a GE/EPRI method is one of general estimation approaches based on finite element (FE) analysis, where material behavior is assumed to be characterized by the Ramberg-Osgood (R-O) relation. In GE/EPRI approach, plastic influence functions used to calculate fully plastic term of fracture parameter are calibrated through the detailed FE analyses according to geometries and material properties, respectively. In this context, FE analyses should be systematically performed to estimate CTOD for various cracked pipes based on GE/EPRI method. In the present paper, 3-dimensional (3-D) elastic-plastic FE analyses were carried out to calibrate and propose the fully plastic solutions of CTOD based on GE/EPRI concept for pipes with a semi-elliptical surface crack. The geometric and material variables of a cracked pipe such as pipe thickness, crack length and strain hardening exponent were systematically varied to cover practical ranges of these values. In terms of loading condition, pure bending moment which is most important loading mode in pipeline was considered.
Many pipeline transmission systems have been constructed in all around the world to satisfy the demand for increasing gas and oil as an important energy source and transport safely energy to industry. In the construction industry for transportation pipeline, girth weld procedure has been performed to connect the long-distance pipeline. However, since weld cracking can be created during installation or operation in severe environment (Chiodo and Ruggieri, 2010; Dake et al., 2012), a common failure has been arisen from internal cracks. To secure structural integrity and eliminate uncertainty due to crack initiation, the many crack assessment procedures based on fracture mechanics have been developed. For such a crack assessment, J-integral or CTOD based on elastic-plastic fracture mechanics (EPFM) has been used as fracture parameters to estimate crack driving forces of cracked pipelines. In particular, CTOD has been widely adopted for a crack assessment in SBD concept (Zhang et al., 2014) as well as the transportation pipelines or submarine riser industries subject to large deformation.
Tian, Deqiang (China University of Petroleum-Beijing) | Fan, Honghai (China University of Petroleum-Beijing) | Li, Chaowei (China University of Petroleum-Beijing, CNOOC Research Institute) | Wen, Zixiang (China University of Petroleum-Beijing) | Liu, Yuhan (China University of Petroleum-Beijing) | Jiang, Wenlong (China University of Petroleum-Beijing)
The stability of shallow conductor and subsea wellhead is much important and influenced by kinds of factors; however, few people have considered the effect of sand liquefaction to it. A dynamic response differential equation of lateral deviation of the conductor is established and efficiently resolved by Newton downhill method and the generalized minimum residual method in this paper. Some specific coefficients needed in calculating the lateral subgrade reaction of the conductor are modified when taking account of the sand liquefaction. And the computed results are compared with those which didn't consider sand liquefaction. Some useful conclusions and recommendations are summarized.
There are many uncertainties exist about the soil stiffness, although it is an important parameter in regular pipeline design (Kruisman, 1990, Zhou, 1993). The Soil liquefaction, one of those uncertainties, describes a phenomenon whereby a saturated or partially saturated soil substantially loses strength and stiffness in response to an applied stress, usually earthquake shaking or other sudden change in stress condition, causing it to behave like a liquid. The fine sandy loam and silt soil are widely distributed in the shallow-submarine layer, and which has a certain possibility to be liquefied. When liquefaction occurs, both the vertical support and lateral deviation subgrade counterforce will decrease obviously (Lim.T.K, 2012). The former will lead the subsea wellhead sinking; the later will expand the lateral deviation amplitude of the conductor, and eventually cause uncontrollable wellhead instability. However, few people have considered the sand liquefaction when analyzing the stability of subsea wellhead, presently.
In this paper, a dynamic response differential equation of the conductor lateral deviation is established and numerical discretized by the Finite Element Method (FEM). Then, the integral dynamic equation is resolved based on Newmark's-Beta method in time domain by Newton downhill method and the generalized minimum residual method. Besides, the internal friction angle, initial modulus and soil stress coefficient of liquefied sand soil are all modified by the residual pore pressure ratio and residual pore pressure reduction coefficient of weakening soil while considering the sand liquefaction effect. Finally, the lateral subgrade reaction force of the conductor, caused by the liquefied sand, is calculated by the p-y curve method considering the weak-hyperbolic form; and the influence of sand soil liquefaction impacted on the dynamic responses of shallow pipe string is systematically analyzed.
In this paper, the water entry problem of free-falling twin wedges is studied based on an in-house code, named a Constrained Interpolation Profile (ClP)-based model in Zhejiang University (CIP-ZJU). The model is built on a Cartesian grid system and governed by the incompressible Navier-Stokes equations. The CIP method is used for flow solver, the free surface capture scheme, tangent of hyperbola for interface capturing/slope weighting (THINC/SW) scheme combined with immersed boundary method is employed to solve the interaction between fluid flow with free surface and moving bodies. In comparison with previous numerical and experimental results of single wedge entering water surface, the accuracy of present model for solving water entry problems of wedges is validated. Subsequently, considerable attention is paid to the water entry problem of twin wedges. Various results are provided for velocity, acceleration and force components. The effects of initial impact velocity and the horizontal distance between two wedges on those results are investigated.
Due to excellent seakeeping quality, high speed catamarans with v-shaped section lines are widely used for pleasure, polar scientific research and transportation. When a high speed catamaran sails in large waves, the large-amplitude hull motion can generate bow-flare water impact. To study the hydrodynamic of this kind of water impact, the hull is simplified to twin wedges in present work. The water-entry problem of wedge has drawn attention from many researchers. Von Karman (1929) first proposed an analytical method to predict impact pressure of water-entry. Then Wagner (1932) improved Von Karman's method to predict pressure distribution on a two-dimensional wedge. Many analytical studies have followed Wagener's method. Based on Vinje and Brevig's (1981) nonlinear method, Greenhow (1987) used Cauchy's formula to study water entry of wedges of various deadrise angles. In the frame work of potential flow theory, the Boundary Element Method (BEM) has been adopted by many researchers to solve water entry problem. Zhao and Faltinsen (1992) studied the water entry of wedges with deadrise angles varying from 4 to 10, however, the liquid in the thin jet zone was not taken into account in the computation. Wu et al. (2004) used an analytical solution for the jet based on the shallow water approximation to solve water entry of a wedge in free fall motion. Wu (2006) extended Wu et al.'s (2004) method to study the problem of twin wedges impacting water surface vertically at constant velocity. Based on Green's Theorem, Yousefnezhad and Zeraatgar (2014) applied linear free surface boundary condition to analyze water-entry problem of twin wedges at constant vertical speed.
When an air cushion supported vessel navigates in the waves, the air pressure in the cushion pulsates due to the pumping effect of waves. The pulsating pressure could induce significant waves to have an impact on the aerodynamics of air cushion. To evaluate the waves, a linear 3D potential method has been proposed by Doctors (1974) and Kim and Tsakonas (1981). However, the algorithm of the 3D method is too complicated and so far the 3D method has not been widely utilized in the air cushion hydrodynamics. In this paper, a 2.5D method for calculating the waves due to the pulsating pressure was firstly presented. The 2.5D method is much simpler and more efficient than the 3D method. The numerical results suggest the sufficient conditions for the 2.5D method to perform as well as the 3D method.
The problem of pulsating pressure induced waves, which is also known as the free surface Dirichlet problem, is common in air cushion hydrodynamics. When an ACV (air cushion vehicle) or a SES (surface effect ship) navigates in waves, the air cushion pressure pulsates due to the wave pumping effect. The pulsating air cushion pressure inversely reacts on the free surface in the cushion and makes waves that could influence the dynamics of the air cushion by changing the cushion volume or air leakage area. Therefore, it is important to evaluate the air cushion hydrodynamics.
The pulsating pressure induced waves or the free surface Dirichlet problems were concerned long time ago. Stoker (1957) and Wehausen et al. (1960) deduced the mathmatic models for solving the free surface Dirichlet problems in 2D case and 3D case, respectively. The 2D model is not practical since the air cushion is always three-dimensional. Meanwhile, it is difficult to directly perform the calculation under the 3D model given by Wehausen. Later Doctors (1974), Kim and Tsakonas (1981) made great efforts to develop a more solvable 3D linear potential method for evaluating the free surface elevation due to a rectangular uniformly-distributed pulsating pressure patch. In order to calculate the waves due to a non-rectangular pulsating pressure patch under an air-lifted vessel, Xie et al. (2008) discretized the irregular pressure patch to a set of rectangular ones, using the method developed by Kim and Tsakonas (1981) to obtain the free surface elevation due to each rectangular patch, and then linearly superimposed them as the final results. In another case, Guo et al. (2016) discretized the nonuniformly-distributed pulsating pressure patch to a series of approximately uniformly-distributed pulsating pressure patches, and then adopted similar procedures to get the free surface elevation. However, it is still not trivial to solve the free surface Dirichlet problem using the upper mentioned 3D linear potential method, since there exist some singular and oscillating integrals.
CFD is becoming an integral part of the vessel's innovation and design process. Our work attempts to predict hull hydrodynamics of a semi- planing wave-piercing craft both in calm water and waves by numerical simulations based on CFD. Numerical methods solving unsteady RANS equations were adopted. Resistance and hull motions of a slim semi-planing wave-piercing craft in calm water and waves were calculated. Numerical results are compared to experimental results. Comparisons show that although the special hull shape and operation mode cause trouble, numerical simulations based on CFD are still suitable tools for hydrodynamics prediction of a semi-planing wavepiercing craft, and can offer vivid results of sufficient accuracy and reliability. In addition, a simulation in the steep wave was conducted to figure out how a semi-planing wave-piercing craft behaves in an adverse sea condition at high speed.
Wave-piercing boats, especially high-speed wave-piercing boats, are popular in recent years. When a wave is encountered, a wave-piercing boat pierces through the water rather than ride over the top. A bow of a wave-piercing boat is slender, and the freeboard is usually tumblehome or vertical rather than flared. This kind of bow shape can reduce the wave disturbance and make a hull move softly in waves. A semi- planing hull is a good alternative for a high speed craft. A semi-planing wave-piercing craft combines a semi-planing hull and a wave-piercing bow and tries to take advantages of both of them.
Published studies on mono wave-piercing boats focus on a wavepiercing tumblehome vessel published by the US Office of Naval Research (ONR). Studies on the ONR tumblehome hull were conducted mostly for safety consideration such as parametric roll (Mccue et al., 2007; Olivieri et al., 2008; Sadat-Hosseini et al., 2010), stability (Bassler et al., 2007a; Hashimoto, 2009), and some other topics (Carrica et al., 2008; Umedao et al., 2008). In addition, Bassler (2007b) experimentally measured the hydrodynamic coefficients on the ONR hull.
A non-reflection internal wave-maker using a momentum source function is coupled with a CIP-based model. The momentum source term derived from the Boussinesq water wave theory equations is employed and added to the Navier-Stokes equations for wave generations. The influence of reflected waves, which is a big problem for a boundary wave maker when containing the interaction of waves and structures, can be avoided by using the internal wave-maker method, because the waves generated by the source function do not interact with reflected waves. A constrained interpolation profile (ClP)-based model is employed to solve the incompressible Navier-Stokes equations with the free surface boundary condition to deal with the water-air-structure interactions. In addition, the VOF-type tangent of hyperbola for interface capturing/slope weighting (THINC/SW) method is used to capture the free surface. The structure is treated by an immersed boundary method. A series of numerical simulations using the momentum source wave-maker are compared with the analytical solution to verify the applicability in a two dimensional numerical wave flume. Furthermore, a more challenging wave decomposition process over a submerged trapezoid breakwater is presented. The numerical results of wave surface profile show a good agreement with the experimental results. It indicates that the CIP-based model with the non-reflection internal wave-maker can provide a more robust modeling of wave generation and wave-structure interaction without being affected by the reflected wave.
Wave generation plays an significant role in numerical wave flumes. During the development of wave simulation, three types of numerica methods for wave generation have been developed based on continuity and Naiver-Stokes equations: internal generation of waves, static boundary wave generation, and moving boundary wave generation. A key challenge for numerical models is to maintain the incident wave and minimize unwanted reflected waves from structures. The static boundary wave generation and moving boundary wave generation usually need an special treatment in generation boundary in order to avoid reflected waves. The internal wave-maker, which generates wave from a region inside the numerical flume and absorbs wave at two ends of the flume, provides a good non-reflecting property. The reflected wave propagates over the wave maker zone without affecting the wave generation and is finally absorbed by the sponge layers.
Ohta, Yutaka (Japan Agency for Marine-Earth Science and Technology (JAMSTEC)) | Ishibashi, Shojiro (Japan Agency for Marine-Earth Science and Technology (JAMSTEC)) | Watanabe, Yoshitaka (Japan Agency for Marine-Earth Science and Technology (JAMSTEC)) | Sugesawa, Makoto (Japan Agency for Marine-Earth Science and Technology (JAMSTEC)) | Tanaka, Kiyotaka (Japan Agency for Marine-Earth Science and Technology (JAMSTEC)) | Iseki, Toshio (Tokyo Univ. of Marine Sci.) | Yamanaka, Shintaro (Tokyo Univ. of Marine Sci.) | Suzuki, Hiroyoshi (Osaka Univ.)
In the field of deep-sea exploration, autonomous underwater vehicles (AUV) have been developed, and various research results are given by operating them. However, existing operation method of an AUV, it's a problem to take the much operating cost about an AUV. In order to reduce the cost, it's researched about underwater docking technology for operate an AUV based on an underwater station. This paper describes a concept design of observation which is using an AUV and an underwater recharging station (URS), and improves a design of the AUV “OTOHIME” for substantiate underwater docking.
A variety of study about oceanography has been conducted around the world. For example, ocean physics, biology, topography and so on. In the field of deep-sea exploration, it has been researched and developed underwater vehicles, and various research results are given by operating them. Research and development on underwater vehicle especially autonomous underwater vehicle (AUV) has proceeded in recent years. As a result, AUV enables us to collect more efficient survey data in the field of deep-sea.
Moreover, Japan Agency for Marine-earth Science and Technology (JAMSTEC) has been conducted these research and development. There are many research fields about oceanography, especially in Japan, there has glowing interested in natural sea-bed resources. Japan has the world 6 th largest exclusive economic zone (EEZ) and it's reported that there are natural sea-bed resources for example methane hydrates, rare metals and so on. If it's possible to utilize the natural resources, it results in significant effects for economy. Sea-bed resources are expected as one of new economic potential. In order to confirm the amount and distribution, and to utilize the sea-bed resources, we researched and developed AUV and its operation system.
It's necessary for AUV construction to research and develop elemental technology such as vehicle design, propulsion, control system, electronics and so on. Among them, especially vehicle design is important for efficiency of AUV performance. AUV can be classified roughly into two types, one is cruising-type and another is hoveringtype. Former one is mainly use to bottom topography with using sonar, it's able to observe wider area than later one with its high propulsive performance. And later one is mainly use for details observation with camera system, it's able to cruise with high mobility in near the sea bottom at narrow area. In addition, AUV is required the performance which is equipped with both high propulsive and high mobility to conduct using the both of sonar and camera for more efficient survey. Therefore, it needs to decide the vehicle design according to observation purpose or mission. We have been researched about vehicle design to achieve more efficient AUV system.
Wu, Nai-Long (Shanghai Jiao Tong University) | Wu, Chao (Shanghai Jiao Tong University) | Ge, Tong (Shanghai Jiao Tong University) | Wang, Xu-Yang (Shanghai Jiao Tong University) | Yang, Rui (Ocean University of China)
Models for underwater vehicles explaining its relationship between movement and the force exerting on the robot permit a wide range of development to be used in control and navigation. On the premise of previous evidence, system identification is usually utilized to explore the empirical data by determining coefficients in the equation. However, no general method arrives a better model with structure and parameters for vehicles automatically when laws underlying vehicle models and laws reflecting the underwater environment are unknown. In this work, symbolic regression based on genetic programming is adopted to discover the representation and parameters of vehicle model via mining dataset. The solution for the six degree of freedom of underwater vehicle model can be obtained via evolution after function set and terminals set are chosen. Results show that it achieves an more general, accurate model and less optimization time than parameter identification methods like Levenberg-Marquardt algorithm.
The concept of models has been used to represent an observable state of a system in many scientific domains, where the models are usually seen as the generative process(Menezes and Roth, 2014). The generative processes consist of an approximated model that appears to describe the relationships between states, forces, moments and environments when systems are not understood clearly. Although current technological advances have been making it much easier to collect datasets for underwater vehicles system, it is difficult to extract models from this data without knowing the specific mathematical structure of model. This difficulty can be attributed both to the quality of massive data and to the non-linear dynamics of many of these complex systems, which cannot be defined as the object function. Another difficulty is brought by the mapping between the models and data since there is a generative process in environments vehicle operates.
To address this the novel symbolic regression machine learning method are introduced in this article. An obscure relationship are defined for variables in the datasets when the expression of model is unknown. The approach can be straightforwardly applied to datasets. At the underwater vehicle system discovery for model parameters and model structure, there is a set of possible functions that could be created. The model of underwater vehicle becomes fully defined if it provides a way to prefer some better equations over the others. In addition to the precise model obtained by identifying key parameters in equation of underwater vehicle, a very simple one only laying some variables of the datasets can also be created without knowing the mapping between. Besides, the simple equation can be treated as simplification of complicated system model for control scheme.
Metallic strips flexible pipe has been favored in the offshore pipelines engineering for its good corrosion resistance, high strength, easy installation etc. This new composite pipe can be regarded as promising alternative for submarine pipelines. In this paper, the cross-sectional design process for this specific kind of pipe is illustrated. Three formulas for calculating the individual strength capacities of the pipe when subjected to internal pressure, external pressure and pure bending are presented for initial screening assessment. And then a case study for an 8 inch metallic strip flexible pipe based on a shallow water application is carried out. Several FE models are established by using commercial software ABAQUS to verify the designed cross section. The two methods presented could be used to assess the structural performance of the pipe in the early design phase, which might be interesting to the manufacture engineers.
Composite pipes are extensively used in the offshore oil/gas industries for decades. Typical composite pipe, take reinforced thermoplastic pipe (RTP) as example, is favored in engineering for its good corrosion resistance, high strength, high flow rates and etc. Recently, metallic strips flexible pipe is emerging in the offshore application. It has the same advantages as RTP. Furthermore, it exhibits better on-bottom stability due to its relatively greater weight and the production costs are quite low. Fig.1 shows the typical configuration of metallic strips flexible pipe. Generally, it can be divided into three components: (1) an inner extruded thermoplastic tube that seals the transported products. (2) metallic strips reinforced layers with winding structure that provide the strength against internal pressure and tension. (3) an outer PE sheath that isolates the underlying layers of the pipe from external environments. The winding angle and the geometry of the metallic strips can be selected based on the design pressure and the corresponding functional requirements. During the metallic strips flexible pipe's installation and service periods, it will inevitably carry operational and environmental loads such as external pressure, internal pressure, bending, tension, torsion and etc. The requirements of these capacities of metallic strips flexible pipe have a significant impact on the cross-sectional design. The derivations of the individual capacities for the metallic strips flexible pipe are illustrated step by step in this paper, which may be of interest to manufacturer engineers.
The importance of hydroelastic analysis of large and flexible container ships of today has been pointed out for structure design. As the size of container ships increases, whipping phenomena has been one of the important design considerations due to its significant effect on wave loads and fatigue analysis. In this paper, a 3D nonlinear hydroelasticity theory was introduced which is fit for time history simulation in irregular wave conditions. The numerical analysis method utilizes a 3D Green function method and a 2-D momentum model, which are coupled in the time domain. In the present study, theoretical calculation combined with model experiment is treated as the most effective approach. Therefore reliable model test data is needed to verify and validate the accuracy and applicability of the computational programs, thus it's possible to make use of the programs at the preliminary stage of ship design in order to decrease the cost. A model system for measurement of nonlinear wave loads which is composed of a backbone and segmented container ship model in which natural frequency of vertical bending is matched using a beam with rectangular-shaped variable cross-section, has been already built up and applied to a series of large ship models especially. A series of model tests have been carried out for selected cases of whipping in regular and irregular waves. The computational results were compared with those of a model test of a 10000-TEU containership. Verifications and validations between model tests results and theoretical calculation have been conducted. Discussions are focused on the asymmetry of wave elevation, vertical ship motion and vertical sectional loads responses, which are observed and calculated in irregular wave conditions, and they show good agreement between experimental and simulated results, basing on this the design vertical bending moment at midship can be obtained by using the nonlinear short-term and long-term analysis and the comparison between direct calculation and classification society rules is carried out, which shows good agreement with each other.