In this study, we proposed a cable with ribbons attached in 120 degree apart. The ribbons can adjust themselves with the flow coming in any direction. Experiments were done to investigate the effects of the length of the ribbons, and the direction of flows for various uniform velocities. The experiments were conducted in a circulating water channel. Flow visualization has been carried out using laser sheet beam. Laser Doppler Velocimetry (LDV) was employed to measure the velocity field in the wake. This experiment, demonstrates that the attached ribbons can be used to reduce the drag force on a cylinder even when the direction of the flow is not constant.
Many researches were done on the reduction of vortex-induced vibration and the reduction of drag by adding vortex suppression devices (Zdravkovich(198l), Wong and Kokkalis (1982), Every et al. (1982), and Rogers(1983». Many devices were manufactured commercially. Figure 1 adopted from Blevins (1990) features the add-on devices for suppression of vortex-induced vibration of cylinders. They act by disrupting the formation of two-dimensional vortex sheet. Ribboned cable is one of the devices proposed by other researchers. The ribbons are attached on the surface of the cylinder to reduce the drag. Most of the proposed ribboned cables have the ribbons attached on one side of the cylinder. As a result, when the direction of flow changes, the attached ribbons cannot play their role properly. In this study, we proposed a cable with ribbons attached in 120 degree apart. Flow visualization has been carried out using laser sheet beam. An LDV measured the velocity field in the wake. The ribboned cable can be'' an effective vortex suppression device by adjusting the length of its ribbons even when the direction of mean current in the operating zone is not known in advance.
The work described in this paper is part of a research project conducted at the University of Texas at Austin in conjunction with the Offshore Technology Research Center. The research addresses the behavior of a spar in deep water. The spar is modeled as a rigid body with three degrees of freedom. It is connected to the sea floor by mooring lines which are attached to the spar at the fairleads. The inertia forces are calculated using a constant inertia coefficient, CM, as in the standard form of Morison''s equation or using a frequency dependent CM coefficient based on diffraction theory. The drag forces are computed using the ,nonlinear term of Morison''s equation in both cases. The analysis is performed in the time domain. The results of the two methods’ are discussed and compared. The effects of modeling the mooring lines as nonlinear springs neglecting the hydrodynamic forces acting on them are also investigated. Results including several second order effects are presented and compared with experimental data.
One of the new solutions for offshore structures in deep water is the spar buoy. Although the concept of a spar is not new (Rudnick, 1964 and 1967), it has been recently the subject of renewed interest. Spars can be more economical in deeper water than TLPs, they are insensitive to the deck load (Perryman and Beynet, 1994), and they can be relocated regardless of the number of wells or the water depth. The spar is simply a single cylinder with a moon pool in the middle which extends along its length. It consists mainly of two parts: hard tanks and soft tanks. Its center of buoyancy, CB, is always above the center of gravity, CG; therefore, it is inherently stable in pitch and roll motions.
This paper discusses experimentally the wave forces acting on a submerged sphere due to bichromatic waves. Discussion is centered on the drag and inertia coefficients of the Morison equation, determined by the least square method. In the analysis, two interpretations of the drag and inertia coefficients are determined. One set determined is for each wave determined by the zero-downcrossing method while the other set is for whole waves in a wave train. Based on the results, it is shown that the time variation of wave height plays an important role on the wave forces for zero-downcrossing waves. The maximum wave force and the inertia coefficient of the waves whose heights are increasing are larger than those corresponding to waves whose heights are decreasing. Differences of the drag coefficient between the waves in these two stages are also recognized, however, tendencies depend on flow separation. The differences of these quantities become large with increasing variation coefficient of wave height and decreasing correlation coefficient of wave height of successive waves, and it is pointed out that the effect of irregularity in the wave train should be taken into consideration for accurate estimation of wave forces.
A n accurate estimation of wave forces plays a very important role for designers of coastal and offshore structures; thus, much research has been conducted to describe the wave force characteristics and on methods for estimation of wave force. In particular, much knowledge has been accumulated on the wave force acting on a cylindrical body. Recently the present authors investigated the wave force acting on a submerged sphere and reported the characteristics and estimation scheme of wave forces on it (Mizutani and Iwata, 1993). In their work, however, only the wave force exerted by regular wave trains was considered.
Although Intensive exploration and mining and processing tests have been made at the end of the seventies, no commercial development of deep seabed mining has been considered since then. Such development needs the metal prices to be secured for a long period, In order to reduce the financial risks. It requires also a clear and reasonable legal system, to create a predictable and sufficiently stable environment. During the following decade, the structure of the world metal production will be modified, as a consequence of the changes of policy In major production countries. But nobody can presently predict the final Impact on the markets. The International legal framework will be consolidated following the entry Into force of the Convention of the Law of the Sea and the universal recognition of the new Agreement on the Implementation of the part XI of the Convention. This evolution could be crucial by determining more favourable conditions for a possible development of seabed mining.
1. SHORT HISTORY OF DEEP SEABED MINING
At the middle of the sixties, when John Mero was trying to convince the American companies to consider the deep seabed nodules, the "Club de Rome" warned that the rate of growth of our raw material consumption was driving the world economy to be in shortage of most commodities before the end of the century. Even with a linear trend, the growth is high. In response, the mining companies started to invest meaningfully in the exploration of new deposits. During the seventies, while the newly formed consortia were exploring the seabed and testing mining and processing technology, the efforts on-land of the mining and metallurgical companies continued. The world-wide installed production capacity of the metals contained in the nodules reached a pick.
The hydroelastic-response of a box-like floating airport of shallow draft, subjected to wave action, is studied. A linear potential theory and three-dimensional structural dynamic theory are used in the hydrodynamic and structural analyses, respectively. The problem is numerically solved using the source distribution method for the velocity potentials, in which the simplified Green function for a zero draft floating body is employed, and the finite-element method is used for the response of the airport. The geometric symmetries of a body are exploited in the calculations to further increase the computational efficiency. Present predictions are compared with the available results to verify the application of the method. The dynamic response of a 3000m long airport is analyzed.
Floating airports have been proposed as a possible solution to the problem of lack of adequate land space near coastlines where there are densely populated metropolitan areas (see for example, Lemke, 1987, Webster, 1991, Ertekin et al., 1994). The dimensions of a floating airport should meet the requirements for the size of the runway, taxiway, administrative. and storage structures, navigational requirements, etc. For a Boeing 747 airplane, for example, the distance for taking off and landing is about 28QOm. Therefore, a floating airport for a Boeing 747 airplane should be at least 3000m long. The dimensions of the airport studied in this paper are 3000m in length, 300m in width and 25m in depth. The draft is 15m. Such an airport, when constructed, will. be the largest and the most expensive floating structure ever built. Therefore, many problems associated with its design, construction and operation need to be seriously studied. In comparison with a conventional floating structure, the proposed airport will relatively be more flexible because of its large dimensions in length and width and small dimension in depth.
Interannual modes of ice cover variation across the Chukchi and the Beaufort Seas during the period 1969-1992 are analyzed using two different procedures. The first procedure is based on the estimation of the sea-ice extent over a given region and the analysis of :its time series. The second procedure is based on a principal component analysis of the annual average total sea-ice and multi-year ice concentration coverage over a grid of sites in a given region. Both procedures appear to validate the previously observed five year cycle in ice severity for the region. The first six principal components explain more than 90% of the observed variance during the period of record and identify the most important patterns of departures from mean annual ice conditions. These patterns are stable both in space and time, and appear to be related to fluctuations in the sea surface temperature, persistent easterly or westerly wind patterns, and the discharge from the Mackenzie River.
Forecasting of long term ice conditions in the Arctic is of interest for planning navigation and exploration activities during the relatively short summer season. The study of long term fluctuations in ice conditions go beyond forecasting however, and are useful in the study of high-altitude atmospheric processes, environmental protection, and fisheries. Principal component analysis has been used to characterize the spatial patterns of short-term (weekly) ice cover variations during the summer season in the Arctic and during the winter season in the Gulf of St- Lawrence (Chouinard and Garrigues, 1995). It was demonstrated that they can be used in multivariate regressions for improving predictions of ice conditions at one week intervals. The focus of this article is to explore the possibility of using this procedure at much longer time intervals.
A complete theory to obtain semi-analytical solutions of the wave drift damping of vertical circular cylinders is developed. It includes both interaction between cylinders and radiation effects of wave frequency motions. The contribution of the second order steady potential in terms of wave slope is also considered. Special treatment is suggested to deal with the singularity around the sharp comer of the body. Numerical examples are presented. The theoretical approach gives an effective mean to calculate wave drift damping.
The prediction of the slow-drift motion of moored floating structures still remains a challenge for marine hydrodynamicists. Although there are several numerical models, Herfjord and Nielsen (1992) showed that they can come out with over 100% differences on predicted values, and that the main reason for their discrepancies was the damping estimation. Slow-drift damping may consist of three main components : I. ''wave drift damping'', associated with the variation of the drift force with the slow-drift velocity of the structure; 2. ''mooring damping'', due to reactions of the mooring system; 3. ''viscous damping'', due to friction and drag on the hull itself. Wave radiation damping is usually negligible. This paper is concerned with the first one, i.e.''wave drift damping''. It is a topic which has received much attention during the last 15 years. It is now widely accepted that if the instantaneous slow-drift motion is assumed to have the same influence on the wave frequency flow as a steady forward motion, the problem reduces to the one in case where a current is superimposed on incident waves. This is not the only plausible assumption, but a number of proposals have been made as to the form of perturbation expansion appropriate to the problem of wave drift damping.
Principal Component Analysis is used to analyse weekly ice charts in the Gulf of St Lawrence (Canada) and develop weekly forecasts of ice conditions. The procedure can be used to identify the most important spatial patterns in weekly ice cover variations and to define predictor in a forecasting model. The procedure is applied to a grid of 83 locations in the Gulf of St Lawrence for ice cover observations in consecutive weeks. Results indicate that the most important patterns can be used to improve forecasts on a weekly basis.
1.1 Objective of the study
Ice cover prediction in terms of concentration, thickness and type is of crucial importance for shipping in Canadian waters. Forecasting ice conditions represents a difficult task due to the complexity of mechanisms involved in ice cover formation and evolution. Usually, forecasts are obtained either by numerical or empirical-statistical models. Numerical models (Hibler, 1979) require a large amount of input data but still cannot account for all the processes involved. The second category of models is based on the analysis of historical ice cover data (Barnett, 1979) to identify recurring patterns which can be used as a basis for forecasting. The latter type of model is developed in the present paper.
Since the beginning of the century, people involved in marine activities in cold regions, have been reporting ice conditions. At first, only visual observations from ships or the coastline were available, but quickly planes were used to survey much larger areas, and in the last few years, remote sensing has become an important instrument for ice observation and forecasting. PCA extracts uncorrelated modes of variation between a specified group of random variables for a given data set, and provides spatial and temporal representations of those variations .
The parabolic yield function proposed by Ralston (1977) for the description of ice strength under multiaxial loading has been applied many times for ice load evaluation within the plasticity limit theorems. Therefore, this criterion seems to be well developed ill the sense that dependencies of the principal strength parameters on temperature, salinity, strain-rate and soon have been studied relatively well. On the other hand, it was applied for computer-based simulation only once. The existing computer-based model, "SHELF-M", which originally incorporated the Mohr-Coulomb criterion as a yield function for ice was modified. At present, either parabolic Yield function or the Mohr-Coulomb criterion can be used to simulate ice fracture against wide offshore structures due to the horizontal motion of an ice field. A series of numerical experiments with wide vertical-faced structures of various cross-sectional forms were carried out for 15 sets of ice strength parameters described in the literature. Three ice structure interaction scenarios were studied within this research work: structure frozen-in to an ice cover (stiff boundary), structure surrounded by ice field but with no adfreezing bond between the ice and structure surface (free boundary), and structure penetrating an ice sheet making a slot or cut in the ice cover behind itself. The results of these experiments are analysed and discussed in the paper.
One of the basic points for any mathematical model of the icestructure interaction process is the ice strength criterion. A number of them have already been used to describe ice behaviour under uniaxial or multiaxial loading, but only a few seem to be well enough developed for use in the systematic analysis of the phenomena. The first one is the well-known Mohr-Coulomb criterion which has been used in the mechanics of deformable solid bodies since the 18th century.
The offshore Russian Arctic offers new challenges for the pipeline engineer, which if not properly approached can lead to astronomical development costs. With all the associated problems of life support and logistics, an unrealistic approach to the construction, and its associated excessive costs, hydrocarbon developments could well be stifled. Considerable experience with field developments in Alaska and the Canadian Arctic has shown that with innovative engineering approaches, projects can be completed which are technically viable and commercially sound. This paper provides a historical background of the offshore pipeline research and developments and discusses in detail the pipeline installation in the Canadian Arctic for Panarctic Drake 76 and the Polargas thirty-six inch main gas transmission system. With the changing of the location to the Russian Arctic, this paper discusses how these successfully developed technologies can be applied to the Russian scene.
The Russian Federation has one of the largest land areas on the Eurasian continent with various European and Mid. East countries and China OJi their western and southern borders. Most of the Northern land mass is on the Arctic Ocean from Murmansk eastward to the U.S. Alaskan border. The Arctic and offshore areas probably have the largest untapped oil and gas reserves in the world. With the fall of communism and the free enterprise system awakening in the former Soviet Union, the development of these reserves will transform Russia into one of the very powerful and rich democracies. The Russian people, in time, will prosper beyond any of their present dreams. The expertise developed to date in Alaska and Canada will go a long way in these Arctic areas development programs.
Since the methods of pipeline installation discussed in this paper are primarily the tow or pull method, a brief history of this technique is appropriate.