Dynamic behaviors of a submerged tension-moored structure with a pressurized air-chamber under wave action and wave transformations are investigated theoretically and experimentally. The rolling, heaving and swaying of the floating structure, wave reflection and transmission, and air pressure in the chamber are theoretically formulated with consideration of the compressibility of air. The validity of the newly developed theory is verified by two-dimensional hydraulic experiments.
So far, a lot of researches have been conducted on the dynamic responses of semi-submerged moored floating structures such as semi-submersible platforms (DeBoom,et al.,1983), floating breakwaters (Fugazza and Natale, 1988) and floating storage tanks (Naftzger and Chakrabarti, 1980), and wave transformations in the wave field. A lot of the useful information and knowledge for designing and constructing the structures have been accumulated. In contrast, very few researches on dynamic behaviors of submerged moored structure and wave transformation have been carried out and many problems are left to be solved. The submerged moored floating structure such as a submerged moored breakwater can control the wave reflection and transmission better than semi-submerged ones(Iwata,et al.,1988). In addition, the submerged structure is pointed out to be better than the semisubmerged structure in seascaping in the near shore zone, since the crown height of the former is bellow the sea level. With this background, this paper is intended to ,deal with a submerged tension-moored structure having a pressurized air-chamber and to investigate theoretically and experimentally the dynamic responses of the structure to waves and wave transmission and reflection. The major advantage of the structure with the pressurized air-chamber lies in easy control of the natural frequency of the structure only by adjusting the air pressure in the air-chamber. Secondary, two-dimensional laboratory experiments are performed to discuss the validity of the proposed theory.
The variability of pressures resulting from plunging wave impact on a smooth vertical cylinder is examined. Based on laboratory measurements the fluctuations in the pressure characteristics are found to be associated with two main factors; one is the significant shift in the wave breaking location relative to the cylinder location and the other is the randomness of the wave breaking kinematics and the trapped air dynamics. The variation of the peak pressure magnitudes associated with the latter is presented. Both the mean characteristics and the probability distributions are examined.
In the design of offshore structures one is often seeking an estimate of the extreme wave loads. It is long known that such a loading can result from waves breaking onto the structure. Research in recent years (e.g. Sawaragi and Nocmno 1984, Kjeldsen et.al. 1986. Chan and Melville 1987, 1988, Basco and Niedzwecki 1989, and Tan et.al. 1989) have in fact shown that wave impact forces can be more than two times higher than non-impact forces from waves of comparable amplitudes. Moreover, the corresponding impact pressures can be more than ten times higher compared to non-impact pressures. These impact loads are highly impulsive and transient in nature and the physics of the impact process is complex. In a hostile ocean environment, particularly during store weather the chances of encountering such wave impacts are very high. Consequently, a good knowledge of the mechanics and dynamics of wave impact is essential especially when the overall objective is to produce a safe and economical structure. Also, plunging wave impact occurs not at one critical structure location relative to the wave breaking location, but over a range of locations within the region of wave breaking Chan and Melville 1987,1988, Tan et.al. (1989).
With 14 samples of various chemical compositions of 12% Cr steel and one 1% CrMoV steel, the effect of back stress on creep rate was studied in relation with activation energy and stress exponent. Back stress is depended on applied stress and temperature. The equation can be formulated. Effective stress exponent, ηο is not fixed value for all materials as reported but strong dependence of temperature and microstructure ηο value represents the stability of a material at the elevated temperature and no value is expected to be a criterion of stability of material in the other atmospheres. Elements of Nb, Zr, Ti, W, La, Pd and Ru are found to be effective to form very stable microstructure in 12% Cr steel and the elements are expected to improve creep rupture strength.
In order to improve the thermal efficiency of a steam turbine in the power generation system, there is a trend towards increasing the operating temperature and pressure. 1% Cr-Mo-V steel which has high creep rupture strength and good toughness up to 550''C of steam temperature has been successfully applied for the material of turbine rotor. However, to meet higher creep rupture strength above 600t, 12% Cr steel is considered a promising material to replace the conventional 1% Cr-Mo-V steel. An intensive study of 12% Cr steel for the application of turbine rotor was made by General Electric, Climax Molybdenum Company" in U. S. A in 1960s and Mitshubishi, Tokyo Electric PowerCo", and Tokyo University in Japan in 1980s. Hitachi studied" mould design for ingot making to reduce segreagation in ingot. The high creep rupture strength of 12% Cr steel was reported due to the secondary hardening by increasing coherent strains or increasing the volume fraction of precipitates such as M2X or MX carbides.
This paper presents a method of static analysis of a marine cable spanning two fixed points. The top tension of the cable is given, while the total arc length, equilibrium configuration, and top and bottom angles are to be determined. In the analysis, a functional is Introduced in which the potential energy of stretching and virtual work done by other forces are included The stationary condition of the functional and one of two equilibrium equations are used to solve the problem. Application of the finite element method yields a system of nonlinear equations, which is solved using the Newton-Raphson iterative procedure. Accuracy of the method is demonstrated by a comparison of the numerical results obtained with those of a catenary and a neutrally-buoyant cable.
In many designs, the top tension of a marine cable or mooring line spanning two fixed points is specified. The specified tension may be governed by the strength of the cable, the capacity of the Installation equipment, or the desired top force for operation. The problem of static analysis for the cable, therefore, is to find the cable configuration and the total arc length when the top tension is given. In contrast, an alternative problem is to find the configuration and maximum top tension with a given total arc length. This paper focuses on the former problem. Although the governing differential equations of a cable segment are simple, explicit solutions can be obtained only for simple cases due to the nonlinearity In the problem and, in general, solutions are obtained by numerical techniques. Literature of the analysis of mooring lines and cables can be found In several text books, for example Berteaux (1976), Irivne (1981), and Leonard (1988). Extensive reviews are given by Migliore (1979) and Webster (1982).
Longuet-Higgins (1977) observed in a two-dimensional shallow tank that a sandbar model moves with mean speed as high as 1.2 cm/sec. towards the weatherside, while incident waves break on the leeside of the sandbar. In this paper we intended to examine the problem numerically by using boundary integral method. Computations are made based on a mixed EulerianLagrangian solution scheme under the assumptions of potential flow. Wave forces acting on the sandbar including hydrostatic contributions are evaluated and assessed to quantify the experimental- finding of Longuet-Higgins. It is found that the time-derivative and hydrostatic components in Bernoulli''s equation dominate the total force acting on the sandbar. The force becomes negative, i.e. directs the weatherside, mainly due to the resulting hydrostatic unbalance, when waves break on the leeside of the sandbar.
The semi-Lagrangian time-stepping procedure initiated by Longuet-Higgins and Cokelet (1976) provided a sound basis for investigating non-linear water wave problems, because it can be implemented to exactly satisfy both the dynamic and the kinematic conditions on the free surface in numerical sense. Faltin- sen (1977) reformulated the method on the physical plane and successfully applied to wave-body interaction problems. Vinje and Brevig (1980a) incorporated Cauchy''s theorem into the method in order to derive boundary integral equations for the complex potential. They employed it to simulate plunger-type breaking waves in two dimensions with the assumption of spatial periodicity. In their method, the stream function is prescribed on the boundaries, where the normal velocity is known, while the velocity potential on the free surface. In extending their approach to the case of a floating body, Vinje and Brevig (1980b) regarded the intersection points between the body and the free surface as a part of kinematic boundary, where the normal velocity is to be prescribed.
The number of offshore platforms in Indonesia has been increasing since 1969 when the first offshore platform was built. At the moment there are a total of approximately 330 offshore platforms with various types. Planning, design and construction of the first platforms were carried out by American contractors, however, at present there are already a few Indonesian construction companies that build offshore platforms. Standards and regulations used for offshore platforms in Indonesia are according to API RP2A, API RP2L and to the regulation of the Minister of Mining, Republic of Indonesia. There are several problems in offshore platform practice in Indonesia, one of them is the very rapid tropical marine growth. Care should be taken for offshore platform leg which has hard marine growth with thickness of 2 inches. Underwater marine growth surveys and measurements should be conducted during offshore platform inspections.
OFFSHORE OILFIELDS IN INDONESIA
The producing offshore oilfields in Indonesia are located on the Java sea, north of the island of Java, and the Natuna Sea, the sea between the Malayan peninsula and the island of Kalimantan. Offshore platforms started to be built in 1969. Planning, design and construction of the first platforms were carried out by American contractors for American oil companies. The platforms were, hence, built according to American Petroleum Institute recommended practice which continued to be used as design and construction bases until today. With the presence of offshore platforms on Indonesian seas, Indonesian engineers started to become familiar and to have experiences with them. In 1980 an Indonesian consulting company won a platform design tender in an international competition. API recommended design standard was used. Until mid 1980, approximately 30 Indonesian designed platforms were in successful operation. At present there are a few Indonesian construction companies that build offshore platforms.
Fatigue behavior of 2024-T3 aluminum alloy is caracterized for the role of compressive portion of constant or variable amplitude loading conditions which include single or periodic single peak compression overloads. It is found that the compressive load cycles have a remarkable effect on crack propagation as well as crack initiation. Particularly, it is noticeable that, under high baseline load ratio without crack closure, the non-propagation crack arrested by single peak tension overload recommenced to propagate by following single peak compression overload. Such results are discussed in terms of crack opening level, reversed plastic zone size, residual deformation on the crack surfaces and so on.
Conversely, the effect on fatigue crack propagation is generally considered to be minimal, since the creek is expected to be fully closed during the compressive portion of the cycle, necessitating a stress intensity of zero at the crack tip (Hudson et aI, 1974). Similarly, it is found that compressive overload cycles In general have little effect under variable amplitude loading, except when they immediately follow a tensile overload where they act to reduce the post-overload retardation (Schijve et aI, 1962; Stephens et al,1976). In such tests, however, the crack growth rates were analyzed with respect to not the effective stress Intensity range (11 K. If) but the nominal stress intensity range (11 K) which rely on only the tensile portion of the load cycle, so that one of the most Important effects of compressive portion of the cycle, I.e., reduction of crack opening level were neglected. On the other hand, some studies show compression stresses have a strong effect on the crack opening level and increase crack growth rate in both stages of near- threshold and steady- state crack propagation (Newman, 1984; Yu et al. 1984; Kurihara, 1986).
A performance analysis of an OTEC plant using an integrated hybrid cycle (I -H OTEC Cycle) is conducted. The I-H OTEC cycle has a combination of a closed cycle OTEC plant and a spray flash desalination plant in which a flash evaporator is installed after the OTEC evaporator. The I-H OTEC cycle is an improved version of a joint hybrid cycle(J-H OTEC Cycle) in which the flash evaporator is installed before the OTEC evaporator. The results of a numerical analysis are reported for a 10 MW DTEC plant using the I -H DTEC cycle with plate-type heat exchangers and ammonia as working fluid.
Ocean thermal energy conversion (OTEC) is the system converting heat energy into electricity by using the temperature difference between the warm sea water of the surface and cold sea water of the depth (Uehara et al., 1987). Since the temperature difference in the OTEC plant is 15 ''" 23°C available, the thermal efficiency is only 3 ''" 5 %. Many countries and small islands in the tropical and subtropical zone where are in need of the electric power as well as drinking water expect to construct the OTEC and desalination'' plant. Therefore, a hybrid cycle system of the OTEC and desalination plants will be considered. Multi-stage flash, (MSF) desalination plants are typically used for producing fresh water. However, there are disadvantages in using MSF desalination plants. These are a suppression of the evaporation at a lower part due to the liquid level and a temperature drop caused by the mixture of the cold sea water after evaporation and hot sea water before evaporation. In the plant using the J-H OTEC cycle, as the electric power increases, the desalination rate decreases, conversely, as the desalination rate increases, the electric power decreases.
In technique there is a fairly Wide Class of devices, among them systems of automatic control, regulation and monitoring which require applying certain dynamic loads at the gear shafts. The necessity in these technical devices which automatically simulate some given regime becomes especially acute when complex lines of testing equipment and instruments for various applications are developed. This paper considers the feasibility of creating simulators of dynamic loads, as well as mathematical models describing them in different regimes (static or dynamic) and their versatile technical implementations based in the results of their Studies. In the process on their elaboration, design and final tests the requirements for compactness, high efficiency and possibility of using accessory equipment available in the USSR had been taken Into consideration. The D.C. electric gear motors fed by thyristor converters and monitored by a special automatic line were used as loading mechanises (benches). The appropriate monitoring signal (sinusoidal, triangular, rectangular, casual is sent by a personal computer or by a special generator.
The System was designed for the loads in the range of 0-25Nm, 25-250Nm, 250-500Nm. For the second and third modifications of electric gear motors permanent magnets fabricated of magnetic materials of high coercive force are used Instead of bus winding. This makes It possible to increase the specific power and obtain a high ratio of the moment and the current in relation to their nominal values. Application of high moment electric gear motors appears to be the lost prospective. The above motors retain the advantages of ordinary D.C. electric motors, such as the high thermal time constant (possibility of considerable electric overloads All this likes it possible to use motors of a relatively small power In the loading capacity for the sake of obtaining great moments.
This paper briefly reviews results of the current work undertaken by Universiti Teknologi Malaysia in the field of fluid structure- seabed interactions, in particular around pipelines laid horizontally on a bed of sand with shell fragments. The approach adopted in the study was directed mainly towards the collection of data to establish the influence of several variables on the development of local scour. It is evident from the study that the equilibrium scour depth decreases due to the pronounced effect of the armouring phenomenon when sand with shell fragments were used in the experiments.
Increasing activities offshore with regards to the petroleum industry has created a widening interest in research in this field lately. Since it is anticipated that a continued increase of offshore pipelines will be laid in future in several parts of the world, relevant studies pertaining to these pipelines have been undertaken by various researchers. As a direct consequence of this, is the study of the local scour beneath submarine pipelines under current only or wave superimposed on current flow conditions. A practical method which have been exploited by a number of investigators was the application of analytical methods to deal with the real problem in which comparisons were made with experimental results and observations. Many such attempts have been made in recent years to evaluate and describe the local scour process beneath pipelines subjected to a current or wave field or a combination of both. This is apparent from reports written by researchers such as Herbich (1981), Bijker & Leeuwestein (i983), and Ibrahim & Nalluri (1981). Basically, they studied local scour beneath pipelines laid in granular and fine sediments that have been subjected to either this current/wave field or both.