The analytical solutions by Wu (1987), Lee, Yates & Wu (1989) and Shen (1996) for predicting the amplitude and period of resonantly forced long waves by submerged moving disturbances are further investigated by comparing the analytical solutions with both numerical results based on the KdV model and with data from the towing tank experiments: The objective is to examine the validity of the analytical solutions for predicting waves generated by disturbances of different lengths. Our results show that the analytical solutions by Wu (1987) and Lee, Yates and Wu (1989) are valid for a broad range of disturbance length, while the recent solution by Shen (1996) gives excellent prediction for forced waves generated by relatively shorter disturbances such as those with length shorter than five times the water depth. INTRODUCTION It is known that, when being resonantly forced by a submerged moving object in shallow water, a series of solitary waves are generated periodically in front of the disturbance and move upstream with supercritical speed. A sketch of this phenomenon is shown in figure 1, where a. is the amplitude of the solitary waves, , the wave elevation, h the unperturbed water depth, d the depth of the depressed region behind the disturbance, and U and L the speed and length of the disturbance, respectively. The dominant forcing factor that affects the wave amplitude a. and wave period T. is the blockage ratio b which is defined as the ratio of the blockage area Ad of the disturbance over the unperturbed channel cross-sectional area A as shown in figure 1. The blockage ratio b generally varies in the longitudinal direction. The maximum value of b is called the blockage coefficient and its effect was discussed in Ertekin (1984), Ertekin, Webster and Wehausen (1984, 1986), Mei (1986), Ertekin and Qian (1989), and Teng and Wu (1990, 1997a,b,c).
A force allocation strategy used in connection with dynamic positioning of ships and semi-submersibels is presented. The strategy dynamically allocates force, and in case of azimuth thrusters - force directions, to a set of thrusters. The strategy handles n thrusters (n≥2) in an arbitrary configuration. The decision making of the strategy is based on minimum power consumption taking into account effects of reversing thrusters, thruster/hull interaction and partly the effects of interaction between thrusters. The functions of the full allocation strategy is illustrated through a number of examples. Finally the strategy is compared with a more classical allocation strategy to show that significant reductions in energy consumption can be achieved. INTRODUCTION Dynamic positioning interpreted as an automatic control of the horizontal position and the heading of a vessel may be applicable in a number of fields within the maritime industry. One such example is the docking of a ferry and more directly related to the present development is positioning of a drilling platform/vessel. In both cases the horizontal position and heading of the platform or vessel shall be controlled. The precision required to the positioning of an offshore structure yields quite a challenge. It is therefore necessary to be assured that the most efficient use of the thruster capacity is acquired. In general terms the problem has three constraints, the desired position and heading. With a ship or semi- submersible platform utilising more than two thrusters or combinations of propeller/rudder and thrusters more than three parameters are available to adjust the control of the position resulting in an mathematically underdetermined problem. Often this problem of having an under-determined system is solved by allocating individual units to perform a certain task i.e. azimuthing thruster in the bow of a FPSO are allocated for heading control and consequently provides only a side-force near the bow.
This paper describes a recent technical innovation in design, called ''''Design Through Analysis" led by the authors_ In the new approach, finite element methods are used to simulate global behavior and detailed structural strength. The global analysis and local analysis are integrated to determine the governing limit-states and optimize the design. The advantage of using such advanced engineering is a substantial reduction of project CAPEX and OPEX. The paper presents the following technical developments: (1) Design Through Analysis (DTA) concepts, (2) non-linear finite element simulations of global behavior and detailed local strength, (3)limit-state design criteria, (4) vortex-induced vibrations and fatigue (5) design for trawling loads, (6) wall-thickness design based on LCC (Life-Cycle-Cost) optimization, (7) reliability-based calibration of safety factors and inspection planning, (8) design of in-field flowlines. INTRODUCTION A recent technical revolution in the design process has taken place in the Offshore and Marine industries. Advanced methods and analysis tools allow a more sophisticated approach to design that takes advantage of modem materials and revised design codes supporting limit state concepts and reliability methods. At JP Kenny we call the new approach "Design Through Analysis" where the finite element method is used to simulate global behavior of pipelines as well as detail structural strength. The two-step process is used in a complementary way to determine the governing limit states and to optimize a particular design. The advantage of using advanced engineering is a substantial reduction of project CAPEX and OPEX by minimizing unnecessary conservatism in the design through a more accurate determination of the effects of local loading conditions on the structure. Rules and design codes have to cover the general design context where there are often many uncertainties in the input parameters and the application of analysis methods. Where the structure and loading conditions can be accurately modeled, realistic simulations reveal aspects of the design codes,
In the paper the modifications of a wave field due to a non-linear interaction with an opposing jet-current are analysed. Hydrodynamic governing relations are represented by the mass conservation and 2DH momentum equations. Wave refraction is obtained by the irrotationality of wave number, the energy conservation and Doppler effect equations. A numerical solution based on a "two-level iteration" method, is proposed and some applicative examples are also reported. INTRODUCTION The influence of a current on wave motion is a very common physical phenomenon, characterised by the generation of geometric and kinematic modifications of the flow field (such as wave front deformation, wave height and length reduction or increase, tidal current development etc.). In coastal areas this subject assumes considerable interest in the examination of the effects of an opposing jet-current on waves. This situation can be, in fact, the basic scheme of some important technical applications (e.g. controlled sea discharges from plants or sewage outfalls, natural discharges, river mouths etc.). The hydrodynamic study of this problem has been treated by several researchers with mathematical models, where some appropriately simplified schemes are considered. A good synthesis of these researches is reported by Peregrine (1976) and Jonsson (1990). Ismail (1981) and Ismail and Wiegel (1983) treated the problem analytically, evaluating only the spreading of a surface jet due to an opposing wave and also reported some experimental results. This aspect is very interesting because the laboratory studies on the subject request large experimental facilities (wave basin). More recently, Yoon (1987) and then Yoon and Liu (1989) studied the problem considering only the long waves (e.g. tidal flows near an inlet or estuary entrance). By applying the parabolic approximation criterion, the Authors obtained a particular linearised expression of the Boussinesq equations.
A finite difference model based on potential flow theory is proposed for estimating the equilibrium scour hole underneath offshore pipelines. The model solves the Laplace equation for velocity potential in a curvilinear coordinate system to cope with the irregular and free boundaries involved in this problem. A local method is used to determine the free boundary formed by the eroded seabed via the equilibrium of all forces acting on a sediment particle on a slope bed. The major features of the present model are: 1) that it takes into account the nonlinear interactions between the flow, pipe and the changing bed topography in calculating the equilibrium scour hole, 2) that the shear stress on sediment particles is represented in terms of a characteristic near-bed velocity, creating a dynamic link between the flow and the sediment movement, and 3) that the model predicts the equilibrium scour hole without using any sediment transport formula, which usually contains many empirical parameters or constants. The maximum scour depth and the upstream part of the scour hole predicted by the model compare well with the experimental data published in the literature. INTRODUCTION Many offshore pipelines are directly laid on the erodible sea bed and are exposed to currents, waves, and storms. Even with moderately strong currents, local scour can occur, and this will cause the pipelines to be suspended in water. As the scour hole develops, the flow around the pipe will exert a downward force on the pipe instead of an uplifting force. This force together with the self-weight of the pipe will tend to sink the pipe section into the scour hole and cause additional stress on the pipe joints. Consequently, the excessive deflection of the pipeline may result in damage and failure. Therefore, the prediction of local scour around pipelines is of great importance to pipeline design.