A numerical model is developed for 3-D floating body motions in fully nonlinear waves using the BEM. This BEM solves simultaneously the boundary integral equations for the velocity and acceleration fields and the motion equations of a floating body with six-degrees of freedom. The movement of the nodes on the free water surface is evaluated in accordance with the 3-D motion of fluid particle by the Mixed Eularian Lagrangian method (MEL) and it satisfies strictly satisfy the dynamic boundary condition on the water surface. After that, by the coordinate transformation of the six-degrees of freedom motions, the movement of arbitrary nodes on the body surface is evaluated and updated for the next time step. Validity of this numerical model is verified through comparisons with theoretical solutions of nonlinear waves such as the 5th order Stokes and the 3rd order Cnoidal waves, computed results by a fully nonlinear 2D-BEM, and 3-D experimental results of a moored floating plant barge.
It is necessary to develop a numerical model capable of reproducing fully nonlinear 3-D interactions between a moored floating structure and nonlinear waves. However, such numerical model is still being confined to fully nonlinear 2-D interactions using BEM and FEM. Especially, for the BEM, using boundary surfaces alone, surrounding the fluid domain, the problem of boundary values can be solved. So, the efficiency of computation is much better than the FEM and the BEM is more suitable for 3-D computation. For correct evaluation of floating body motions, it is necessary to correctly calculate the time variation of the fluid pressure acting on a floating body. However, the fluid pressure are affected by the motion acceleration of a floating body. So, the motion equation of the fluid must be strictly coupled up with the motion equation of the floating body.
A new project management model which handles the risk factors and uncertainties encountered in the design and construction of coastal projects is developed and named as the Optimum _Risk Management (ORM) model. The ORM model, contemplates to maintain the project objectives within the predetermined time and budget constrains by optimising the construction operations with the incorporation of risk factors and uncertainties inherent in the execution of coastal projects. In Turkey, the overruns in completion times of activities are unanticipated anomalies in project management arising from special problems. The project objectives can not be maintained within the time and budget constrains due to these problems encountered during construction, the most consequential and recurrently observed one being the allocation of inadequate yearly funds to projects. The project management model developed can be used for risk identification and response intentions under these circumstances in Turkey. The reliability-based risk assessment model developed, can also be utilized for the prediction of project parameters, when coupled with a macro-scale political and economical prediction model.
The ORM model aims to perform the following items:
I) Coordinate the logical precedence relationships of project activities,
II) Determine sources of uncertainties in design parameters,
III) Evaluate the damage risk during the construction phase,
IV) Import the risk factors and uncertainties into network computations,
V) Compute the optimum time cost trade-off strategies,
VI) Allocate the resources in the most economical way.
In order to achieve the objectives listed, the Optimum Risk Management (ORM) model, includes two interrelated but independent sub-models, namely the Optimum Port Construction _Planning Model (OPCPM) and the Reliability-BAsed Design (REBAD) models. In the ORM model, the inherent variability which exists in the modelling of the construction activities of coastal structures due to the presence of risk factors has been quantified via the employment of the reliability-based sub-model.
To discuss the shear behavior of clay under the real element test condition, the authors carried out a conventional drained test and two kinds of drained test simulations for a remolded clay by using the strain path controlled mini-triaxial testing apparatus. In the simulation A, the excess pore pressure was kept zero during compression and in the simulation B, the strain path was controlled along that recorded in the conventional drained test. The drained test simulation A could sustain the uniformity of water content distribution within specimen and the volumetric strain during compression was smaller than that in the conventional drained test, because of the uniform water content distribution. In the simulation B, the negative excess pore pressure appeared during compression because of the over drainage. As a conclusion, the shear behavior of clay under the real element test condition can be investigated by the drained test simulation A.
The uniformityof stress and strain distributions in a triaxial specimen is essential to satisfy the requirements of an element test. However, the requirement is not satisfied in the conventional drained test (Uchida & Vaid, 1994, Matsui et al., 1997). A strain path controlled mini-triaxial test can sustain the uniformity of stress and strain distributions within clay specimen during a compression stage comparing to the conventional drained test, because the volume change is directly controlled along an arbitrary strain path. This suggests that shear behavior of clay under the real element test condition may be investigated by using the strain path controlled test. In this paper, the authors carry out a conventional drained test and two kinds of drained test simulations for a remolded clay by using the strain path controlled mini-triaxial testing apparatus.
This paper presents the result of a series of experiments of forces on a fixed vertical truncated column due to Stokes 5th Lorder - like waves in wave tank. An effort was made to generate the waves as close as possible to the theoretical Stokes 5th-order waves. The horizontal and vertical velocities were measured under a typical wave to find the difference between the theory and experiment. The measured forces (Fx, Fz) increase almost linearly with the wave steepness and with wave period at the given steepness. The horizontal and vertical force TF (Fx/H/2 and Fz/11/2) due to a 2.0 s period wave at H/L =0.049, amount to 1.35 and 1.05 times the corresponding LTF. The theoretical LTF underestimates the measured horizontal force LTF, while overestimates the measured vertical force LTF.
Hogben and Standing (1975) compared linear and Stokes 5th-order theory predictions of the inertia force on a bottom-mounted column using Morison load model and concluded that the difference between the linear and nonlinear forces for typical north sea wave conditions was not large. The difference was attributed to the wave deformation due to the surface-piercing structure, which cannot be accounted for by the linear theory. The above nonlinear problem may formally be investigated by extending the diffraction theory to 5th order approximation on the basis of the Stokes expansion procedure. The 2nd-order approximation was done by Lighthill (1979), Molin (1979) and others. The 3rd-order diffraction forces were studied by Faltinsen et al. (1995) and Malenica and Molin (1995). There arc two other potential alternatives, both in timedomain simulation approach. One of them is numerical wave tank (NWT) and the other one is the universal linear system model (ULSM). for prediction of the particles horizontal velocity field including the crest region.
An instrumentation observation has been performed during construction of a rubble mound on soft ground improved by the deep soil mixing method. To establish the design criteria for the rubble mound on improved ground, two kinds of analyses for tile soil deformation behavior and the slope stability arc performed on various cases for rubble mounds, soil grounds and backfills with application of the finite element method and the Bishop simplified method. The horizontal displacements and settlements at the crest of rubble mounds are analyzed as a function of the safety [''actor of embankments. The analytical result shows that the soil movement increases considerably when the safety factor of rubble mounds is lower than 1.3.
The deep soil mixing method has been applied to improve the geotechnica[ engineering properties of soft alluvial and marine soils in Korea. Recently the deep soil mixing technique was used to stabilize soft seabed deposits during a port construction project in Pusan, Korea. For the shore protection works the deep soil deposits were improved by the deep soil mixing method and the rubble mound was constructed over the improved ground. Then. a backfill was executed behind the rubble mound and a landing pier was constructed in front of the rubble mound. After construction of rubble mounds, seabed grounds may undergo deformations such as settlements and horizontal movement'', due to the surcharge of embankments and backfills. Sometimes severe settlements and horizontal movements would induce lhilurcs of rubble mounds. To ensure the stability of rubble mounds and port structures during construction, such undesirable movements or 1hilures of both embankments and seabed grounds should be prevented by previous prediction during design (flamilton and flail, 1992). Design criteria or experiences, however, are insufficient lbr coastal development works.
Through the use of a combination of captive tests and some heuristic procedures, Sphaier, Fernandes, Pontes and Correa (1998, 1999, 2000) developed a quadratic model, that have been inherited from the ship maneuvering field, to account for the hydrodynamic lateral force and turn moment. The present paper incorporates some improvements to the model including a heuristic expression to determine the longitudinal force. It is derived from towing tests for different heading angles and takes into account the scale effects through the Reynolds number and the form effect, the viscous pressure resistance, through the Prohaska coefficient k. The process of determining the coefficients depends not only on the tests but also on the knowledge of the added masses. Results from the potential theory (WAMIT, 1995) and from experiments (Clarke, Gedling, and Hine, 1982) are used. Using the maneuvering model a dynamic stability analysis is carried out comparing the behavior of the ship and the model.
In the last forty years an extensive effort has been developed to define mathematical expressions to represent the forces and moments acting on ships traveling on the sea. Beginning with Abkowitz (1964) and Eda and Crane (1965) and followed by Norrbin (1970) and others the so-called maneuvering models for ships in normal speeds have been developed. Although a mathematical model has been used to establish the fundamentals of the theory, it is dependent on experimental tests. With the use of ships as stationary storage units some of these ideas have been imported and, in some case used directly. Using similar tests Wichers (1986), Molin and Bureau (1980), Obokata (1984) and others have developed different models based on the cross-flow model and also used on the maneuvering theory of ship.
Analytical studies of vortex-induced vibrations of structures have often focused on vibrations restricted to either the cross-flow direction or the in-line direction. Nevertheless, experimental evidence has confirmed that coupled cross-flow/in-line responses exist by virtue of the simultaneous excitation in these two directions. In this paper, we explore potential models for these coupled vibrations for elastic cables suspended in uniform cross flows. The coupling of cross-flow and in-line vibrations derives from two principal mechanisms: 1) structural nonlinearities, and 2) coupled fluid lift and drag. Wake-oscillator models for the fluctuating lift and drag forces are combined with the nonlinear partial differential equations of cable motion. Attention focuses on the resonant case when the natural frequencies for cable modes in the cross-flow and in-line directions are in the same 1:2 ratio as the excitation frequencies due to lift and drag. This case is used to investigate three limiting cases: i) the uncoupled response, ii) the coupled response due to structural nonlinearities, and iii) the coupled response due to coupled lift and drag. For each case, an analytical solution (based on asymptotics) is derived for the predicted periodic motions of the cable/fluid system. The addition of structural nonlinearities (mechanism 1) and coupled lift and drag (mechanism 2) lead to qualitatively different responses (number and stability of periodic motions differ) when compared to the response of the decoupled model.
When structures are exposed to a flowing fluid, the flow may act as an energy source that drives structural vibrations. Such flow induced vibrations occur in diverse applications including heat exchanger tubes, electric power lines, airplane wings, and underwater risers and cables and result from fluid vortex-shedding. Alternate vortex shedding leads to time and space dependent pressure excitation on the structure.
Tubular sections have become increasingly popular in recent years due to their superior performance as structural members (especially in compression and torsion) as well as their aesthetic appearance. However, if exposed to high local shear forces, e.g. at supports of long lattice girders, tubular joints perform less favourably due to punching shear. To increase the joint capacity of these joints ring-stiffeners can be used. So far, ring-stiffened joints can mostly be found in large scale buildings. Yet, theoretical research on ring-stiffened T joints with compression loading of the brace (Willihald et aL 1999) indicate that ring-stiffeners can also be useful for small chord diameters. An experimental study is presented and the results of the earlier work are verified. In addition, further Finite Element modelling has been carried out, to study the influence of ring-stiffeners in tubular Y joints with compression loading of the brace.
In recent years, the use of tubular sections as structural elements has steadily increased. A comanon application is found in lance girders. The efficiency of tubular sections in such structures is mainly influenced by the design of the brace to chord joints. At supports, problems can arise due to high forces transverse to the chord from the braces. Internal ring-stiffeners (flat plates) placed at the intersection of the chord and brace members are a possible simple solution (see Figure 1). Design guides and Codes (ClDECT/Wardenier et al., 1991, EUROCODE 3, 1994) do not include methods for dealing with this type of reinforcement. So far, ring-stiffeners can mainly be found in offshore structures, where they are used to reduce stress concentrations and therefore improve fatigue performance. Most research has therefore been concentrated in determining Stress Concentration Factors (SCF) for such joints.
Several very large ocean structures have been proposed as part of the Office of Naval Research feasibility study of a Mobile Offshore Base (MOB). The MOB platform nominally is about 1500m (lmile) by 129m (400ft), which is unprecedented in size and operations compared to any floating structure to date. The objective of this study was to provide a risk-informedconstruction feasibility assessment for five proposed MOB concepts and quantify their construction cost and schedule. The risk associated with cost and schedule were established by comparing resource requirements to build a MOB with the US industrial capacity. These risks were then modeled and simulated using commercial simulation software to provide cost and schedule estimates that accounted for uncertainty and risks. A decision analysis process is demonstrated that allows a decisionmaker to minimize identified risks. As a risk communication tool constructability guidelines particular to the construction of such a large structure have been developed as a useful tool for future designers and builders of a MOB. The guidelines are developed within the framework of risk management and specific applications are applied to general conditions, design for production, construction, and special topics in construction management that control or reduce construction risk. The scope of this study was limited to the construction of the hull.
Aself powered military platform called a Mobile Offshore Base (MOB) has been proposed by the US Navy for long term deployments and force projection. The MOB is a revolutionary structure due to its extremely large size and unique functions, thus building such a platform is a high risk venture. The objective of this study is to assess the construction feasibility of a MOB by applying risk analysis techniques to obtain optimum costs and schedules for the following five proposed MOB concepts:
The current demand for hydrocarbon products and the recent advances in technology have meant that reservoirs previously thought to be uneconomical are now being considered for development. As part of the recent advances in technology, Pipe-In-Pipe systems are being considered and adopted in High Pressure / High Temperature (HP/HT) pipeline in order to provide the required thermal insulation which cannot be achieved with conventional single pipe insulation coatings. The structural force transfer mechanism of Pipe-ln-Pipe systems during operation is such that the outer pipe will be subjected to very high tensile load that balance part of the axial compressive forces induced in the HP/HT inner pipe (Sriskandarajah 1998). In addition, pressure and temperature fluctuations during operation introduce tensile stress variations in the outer pipe which increase the risk of fatigue and fracture induced failure modes. Also, pipeline installation loads, and planned or un-planned shut downs and restarts, add very large tensile stress variations to the outer pipe, and thus contribute to fatigue damage accumulation. Structural failure due to fracture is mainly due to tensile loads acting through stress concentration locations, such as those at the outer pipe field joint welds, where weld defects could also Field joint weld details represent one of the weak links in the Pipe-In-Pipe systems and need to be scrutinised in detail design stage. Due to practical difficulties associated with the joints during offshore installation where clearly inner pipe segments need to be welded prior to the outer pipes, "Half Split Sleeve" or "Sliding Collar" outer pipe field joint arrangements have to be devised to complete the final connections. In the Sliding Collar arrangement, the collar is attached to the sleeve pipe via a single fillet weld.