Heave motion minimization of semi submersible hulls is a subject of great importance in offshore engineering_ At certain wave periods, the total first order wave exciting heave force acting on this type of hull tends to zero_ These so-called cancellation periods are due to the phase angle difference between the forces acting on the vertical columns and on the horizontal pontoons _ Several approaches have been developed to control these periods , , , tuning them to fall within the wave period range the platform will encounter. The present paper investigates the use of lateral keels on the pontoons and blisters on the columns, as a further contribution to the motion minimization efforts. Detailed calculations and experimental results show that these devices can be advantageous in certain circumstances and specially interesting when converting drilling platforms into production units.
Minimization of semi submersible platforms heave motion on waves has been object of research all over the world, aiming at increasing operational performance and safety. Several studies analyse their hydrodynamic characteristics relating the hull''s principal dimensions with wave exci ting forces and motion responses , . Given the principal dimenSions, the hull geometry and the number of columns and pontoons, it is possible to "tune" the platform for best performance in the wave period range of the rig operating area_ This can be done by adjusting the aspect ratio of the pontoon cross section and/or the column geometry , . The volumetric distribution of each column and pontoon can also be elaborated to further reduce the vertical wave exciting force , . However, these procedures are duable only in a design phase. When working wi th platform conversion (e.g., from drilling to production), the basic hull geometry already exists and any major modifications can become very costly.
''This paper briefly describes a two-dimensional discrete element model which simulates the dynamics and interaction forces between distinct ice floes in a broken ice field. The broken ice field, is modelled as circular discs of different diameters, constrained in a domain with controlled displacements or periOdiC boundaries. The paper, utilizing the calibration of the visco-elastic-plastic rheology from L91set (l993a and 1993b), studies the full-scale interaction between a broken ice field and a boom used for ice management. The discrete element model has been used to simulate the forces exerted on the boom when it is pulled through a broken ice field. For an ice thickness of 1.0 m, floe diameters ranging from 20 m to 36 m and a boom speed of 0.2 mis, the average forces on a 250 m wide boom were 50 kN and 119 kN for ice concentrations of 0.4 and 0.6, respectively. These forces are well within the capacity of two anchor handling vessels supporting a boom to deflect ice. The force increases with increasing ice concentration, ice thickness, boom speed and boom width.
The forces exerted on a flexible boom when it is towed against a broken ice sheet have been studied in a series of experiments in the ice tank at the National Research Council Canada (L0set and Timco, 1992; Timco and Gagnon, 1992). Those investigations were motivated by the problems a floating and broken ice sheet can create when drilling or producing oil and gas in waters near the marginal ice zone (MIZ) in the Barents Sea. Broken ice is often dominant in the MIZ (Wadhams, 1980; Korsnes, 1991) and several problems may arise if an oil spill or blow-out occurs. Oil spill combatting techniques are vulnerable to the presence of sea ice.
This paper studies the dynamic interactions of the seabed motion with the free-surface flow by numerical calculation. The formulation of stream function and vorticity in an evolving boundary-fitted grid system supplies a comprehensive tool to analyze the transient flow phenomena with moving boundaries. The finite-analytic method with LSOR technique is used to calculate various configurations of fluid and sea-bed motion. The numerical result in the present study shows the interesting flow and wave characteristics caused by the sea-bed motion.
The instantaneous motion of the sea bed, caused by earthquakes, erosions or landslides, etc., sometimes generates complicate flow phenomena that are of great interests to both coastal engineers and scientific researchers. The strongly transient and nonlinear behaviors of fluid particles disturbed by the large and irregular sea-bed motion are usually difficult to be solved by analytical techniques. Moreover, the viscous effects on the nearby fluid flow may become significant at the initial stage of the bottom motion. The inclusion of viscosity in the formulation makes the problem more complicated but worthy to study. This paper intends to apply a numerical method to investigate the related flow phenomena due to the periodic sea-bed motion and analyzes their transient interactions with the nonlinearity and viscosity of fluid motion. The present study adapts the kinematic .formulation of stream function and vorticity to find out the flow solution. Since the bounding geometry of the flow domain may vary with time, the evolving boundary-fitted grids should construct a better coordinate system in the numerical application [1, 2]. Through this grid specification, one can not only handle the irregular and moving boundaries directly but also control the grid density to resolve some flow regions as required, for example, near the solid surface.
In this work, the effect of the three global warming scenarios on onshore permafrost and gas hydrates within permafrost and/or below the ocean floor is investigated. Temperature profiles, the time required for the onset of hydrate decomposition and the rate of permafrost melting are computed. It is found that while permafrost decomposition due to global warming is feasible, decomposition of the sub-oceanic hydrates is not foreseeable.
The increase in the concentration of trace atmospheric gases (TAG) in the atmosphere (Brackley, 1990; Crutzen, 1991) has caused the current concern that the global climate will undergo drastic changes. The physics of the phenomenon known as the greenhouse effect are now well validated (Taylor, 1991). This phenomenon is responsible for sustaining the current average temperature on the surface of the earth. The temperature would be 33 K lower in the absence of the greenhouse effect. The increased concentration of TAG will disturb this climate equilibrium because more infrared energy will be trapped and subsequently re-radiated causing global warming near the earth''s surface. Although the magnitude of the global warming can not be easily calculated, three possible global temperature rise scenarios for the next century were suggested by a group of scientists (Schneider, 1990). A catastrophic (0.08 °C/year), a moderate (0.003 °C/year) and a low impact annual temperature rise (0.006 °C/year) scenarios have been suggested. Among the consequences of this global temperature rise is the possible decomposition of the earth''s methane gas hydrates. Lachenbruch and Marshall (1986) and MacDonald (1990) employed the heat conduction equation without phase change to compute temperature profiles in the earth. The stability of the methane hydrate in the hydrate stability zone is examined by a superposition of the temperature prolile on the temperaturedepth (geothermal gradient) diagram.
This paper assesses the suitability of different types of anchor systems as foundations of tension leg platforms in soft soils taking into account (a) installation feasibility, (b) ability to withstand static and cyclic uplift loads, and (c) sinking under self weight. The merits and demerits of six types of systems are considered, namely, piles with template, gravity anchors, gravity anchors with piles, gravity anchors with skirt walls, superpile anchors, and gravity anchors with skirt walls having a sealed top. Keeping in view the soft nature of the soil, integrated gravity anchors with skirt walls having a sealed top appear to be the most suitable system.
In India, a large number of steel jacket platforms have been erected off the west coast in the Arabian Sea in water depths of 70 to 80 m for recovery of petroleum resources. Exploration work is currently in progress off the east coast. The continental shelf off the east coast is very narrow and the water depths where offshore structures may have to be constructed exceed 200 m. The soil beneath the seabed consists of soft clay, which at many locations, is underconsolidated up to large depths. Jacket type fixed structures are not likely to be suitable for such conditions. Tension leg platforms, anchored to the seabed appear to be a promising alternative. Design of foundations of TLPs in soft clay is a subjected of current research in India. This paper assesses the suitability of these five anchors systems in fulfilling the three requirements mentioned above which are critical for their satisfactory performance as foundations of TLPs in soft clays. Only vertical loads have been considered in the present study. Horizontal loads and moments are also important but their influence on anchor design is beyond the scope of this paper.
The impact of dropped objects has been considered as one of the major accidental loads for the secondary structural design of offshore platform, especially for the design of the deck within the reach of cranes. Among all dropped items in offshore engineering, drill-collars have been identified as the most critical item due to their high impact energy and small contact area which may result in a penetration through the deck. Theoretically, a dropped drill-collar can be modelled as a circular cylindrical projectile. Some theoretical models have been proposed for small or medium projectile mass with high impact velocity which, however, may not be directly applicable to the deck structure exposed to the impact of drill-collars. Some experimental studies have been carried at the Norwegian Institute of Technology by use of scaled specimens (1 :4) and some formulae have been proposed regarding to the punching shear resistance of the plate subjected to the impact of dropped drill collars (M. Langseth, 1988). These formulae have been, in practice, applied to the design of platform deck although some more verification is still desired for some details. In the previously proposed formula, participating factor a seems to be a very important parameter in determining the kinetic energy imparted into the plate from the projectile. Longseth (1988) has found, from his experimental research, that the partiCipating factor is nearly a constant (0.=0.48) and independent of other characteristic parameters. However, this conclusion is obtained only from experimental observation. In the present paper, some theoretical studies have been done on the participating factor. The obtained theoretical results show that the participating factor varies from 1.0 to 0.33 when the hinge (which may not be plastic hinge) travels from the centre to the edges of the plate provided the projectile vertically impacts the centre of the plate.
Although the information relating to the dynamic analysis of structures subjected to various moving loads is numerous, the literature regarding the dynamnic behavior of a freely floating structure due to a moving load is not found yet and the purpose of this paper is to present some relevant information in this area. For a ship hull floating on still water surface and subjecting to the action of a moving load, the dynamic response is a combination of rigidbody (heave and pitch) motions and elastic vibrations of the ship hull and is determined by means of Newmark direct integration method incorporated with the finite element method in this paper. In order to realize the contribution of each component on the combined (coupled) dynamic response, the response component due to rigidbody motion was determined through application of strip theory. The difference between the foregoing two kinds of response gives the response component due to elastic deflection of the ship hull. The influence of some key factors on the dynamic behavior of the ship hull was also studied and some significant results were obtained.
The dynamic response of a structure subjected to a moving load is an important problem in engineering, therefore, a lot of materials concerned may be found. By means of analytical approaches the vibration problem of a single-span Euler-Bernoulli beam under a moving load or many moving loads has been solved by Fryba (1971), Shimogo and Kurihara (1978), Saigal (1986), Suzuki (1977), Timoshenko, Young, et al.(1974), Steele (1967), etc. The ends of the beam are either simply-supported or fixed. By means of numerical methods, more problems about the moving-load-induced vibration of various structures, such as beams (Sridharan, 1979 and Wu, 1987a)'' plates (Gupta, 1980 and Wu, 1987c) and cable (Wu, 1989)'' were studied.
High density polyethylene (HOPE) geogrids are being utilized for a variety of shoreline protection applications including wavebreaks, jetties, dikes and revetments. Al though available in a variety of forms, the most widely used configuration of geogrids is termed Geocell~. Geocells are a patented cellular confinement system for hydraulic fill that is comprised of a cylindrical hoop of geogrid contained by a geogrid base and lid. A variety of forms is available from repeating circular cofferdams to box like mattresses. This paper describes the features and benefits of the geocells and highlights the practicality of their construction, even below water.
Marine applications for shoreline protection have and will continue to play an increasingly important role in engineering practice. The use of such systems has become more common as sea levels rise in certain areas while growth in development in these same areas continues. citing the recent United states Supreme Court decision in Lucas v. South Carolina Coastal Council, 112 S. ct. 2886 (1992) as an example. This case demonstrates the legal tension between beachfront development private property rights and public concerns to prevent erosion of natural shorelines. In this case the Supreme Court determined that South Carolina must compensate the owner of two residential lots if it wishes to prevent construction, regardless of the environmental necessi ty. Internationally one must suspect that similar findings will also hold in this fashion because of the long standing traditions of property rights being much more firmly entrenched in legal precedent than the more recent concerns over the environment. In light of these legal findings, the ability to protect environmental interests at a reasonable or low cost without sacrificing private property in-shore development rights will be a primary focus within the engineering community..
Some statistical properties relevant to the local hydrodynamic forces acting, below the mean water level, on a unit length section of an isolated smooth vertical circular cylinder in directional seas on deep water are examined. The flow separation regime is considered and the random force fields are deduced by the Morison equation after specifying the wave kinematic fields through the results obtained from linear numerical simulations and for some aspects, from a spectral approach too. '' The local drag, inertia and total forces in the inline and transverse directions arc taken into account and, with reference to these forces, the behaviour of some properties is highlighted. To sum up: a convenient procedure to obtain the standard deviations of the local forces is given and relationships for the statistical distributions of these forces are suggested; the characteristic oscillations of the local forces are introduced and relationships supplying their amplitudes and periods are proposed; finally, the local force grouping is quantified in terms of partial, gap and total extensions of the groups referred to different threshold amplitudes.
The prediction of the hydrodynamic forces on offshore isolated structures is an area of active research, owing to the importance in the design. The first equation which combines the drag and inertia inline forces acting on a unit length section of a vertical circular cylinder in regular waves was given by Morison et al. in 1950. In 1967, Borgman extended the Morison equation to random unidirectional waves by introducing in Eq.l the time series of the random wave velocities and accelerations. Assuming the surface elevations form a random stationary Gaussian process, the same is true for the velocities, accelerations and inertia forces, whereas the drag, and thus the total forces, lose the Gaussianity, owing to the non linear dependence on the velocity.
The influence of direct and alternating voltages on the density, weight and visual aspects of ice deposits at an intensity of 2g m-2 s·l on H. V. conductors was investigated in a laboratory. Numerical simulation employing a simple model was developed, enabling uS to explain the influences of electric parameters such as corona current, ionic wind, voltage polarity and. bombardments of the ice surface by positive ions and electrons. It was found, under our experimental conditions, that the above electric phenomena and parameters had significant effects on the amount and the mean density of ice deposits on H.V. conductors.
In cold regions, such as the northern area of the United States and Canada, conductors· and insulators of transmission lines energized by ac or dc, are frequently covered with ice deposits accreted from supercooled droplets, such as those encountered in cloud riming, drizzle and freezing rain. These deposits consist of three main types of ice: glaze, a clear and semitransparent ice with a density between 0.85 and 0.92 g.cm-3; hard rime, an opaque, and white-colored ice with a density between 0.6 to 0.9 g.cm-3 and soft rime, an ice feathery in appearance, with a density lower than 0.5 g.cm-3. The amount and the characteristics of these atmospheric ice accretions grown on power lines depend not only on meteorological and environmental conditions, but also on the polarity (Farzaneh et aI, 1992; Farzaneh and Laforte, 1992) and the electric strength at the surface of the conductors submitted to icing. However, experiments in cold laboratories involving cylindrical collectors submitted to high voltage electric field strengths equivalent to those on real conductors, have shown that ice density properties can be altered under energized conditions, especially at low intensities of icing (Phan and Laforte, 1981; Laforte et al., 1982).