In the last few years the public and governments of many nations have become increasingly aware of the need for improving oil tanker safety. The requirements for double hull tankers are an attempt to address this need through legislation. Even though a number of investigations on the mechanics of collisions have been done in the past, until recently very little research supported the development of structural improvements to reduce oil tanker damage during grounding and stranding accidents. The ability to predict damage from grounding accidents accurately is not currently available. CDINSWC has conducted stranding and grounding experiments as the first step in developing tools to predict the structural behavior of double hull tankers during grounding accidents. A series of large scale double hull grounding experiments have been conducted using the newly constructed Grounding Test Machine (the only one of its kind in the world) at Hi-Test Laboratories in Arvonia, Virginia. This unique test machine consists of: a 227-tonne test vehicle built on twin railcars to carry each grounding model down an inclined set of railroad tracks to an impact area at 12 knots actual speed; and an impact area, which includes a round-tipped steel cone, instrumented for force measurements, mounted to a 2-million pound reinforced concrete reaction mass to serve as the grounding "rock". Video and high speed photography are used to trace the detailed failure mechanisms that lead to the rupture of the inner shell of a double hull tanker during a grounding incident. The tanker bottom models (ranging in size from 7 x 9 ft to 8 x 24 ft) were developed for two different designs in the 30,000-40,000 dwt range: the baseline double hull (conventional transverse and longitudinal framing) and the advanced double hull (ADH: unidirectional cellular structure with no transverse frames).
An expression of the 3-D pulsating source of Michell type with forward speed for infinite water depth is derived in the present paper based on the Fourier transformation and contour integration technique. Emphases are laid on the discussion of the mechanical feature of the source and the radiation behavior far from the source. The relative importance of the each components of the pulsating source is discussed in detail. Compared with the commonly used pulsating source of Haskind type, the present expression has the merits as simplicity for numerical treatment and being more meaningful in its physical explanation. The Michell type expression of the 3-D pulsating source with forward speed can be widely used for the estimation of motions and wave forces for ships and offshore structures in waves with forward speed.
The Green''s function method has been widely used in the estimation of wave forces and motion characteristics of ships and offshore structure~ in waves. In general, there are two main approaches in the method, one of which adopts the simple Green''s function such as lnr in 2-D problems and 11r in 3-D problems (r denotes the distance from the singularity to the field point). Recently, this method develops fast due to its wide applicability. However, higher capacity for a computer is required to use this method and some further investigations are also necessary on the suitable finite region truncation of the infinite fluid domain and the treatment of the radiation condition. Another traditional approach is to introduce a special Green''s function, which satisfies certain boundary conditions according to the problem to let the velocity potential in the fluid field being determined by the distribution of the function on the body surface only. The so called 3-D pulsating source with or without forward speed belongs to that category.
A combined creep and yield model has been developed for the entire (primary, secondary and tertiary) creep and the long-term strength of frozen soil under multiaxial stress at both constant stresses and constant strain rates by a single (unified) constitutive equation. Secondary creep is assumed to be an inflection point of a creep curve defining time to failure. Secondary creep rate is described by a new flow law, the stress function of which includes the first invariant of the stress tensor. The model consists of four principal elements: a constitutive equation, a viscous flow equation and a yield criterion, all united by a time-to-failure function. The yield criterion is selected either in the form of a parabolic (extended) von Mises-Drucker-Prager model or a parabolic (extended) Mohr-Coulomb rupture model (Fish, 1991). The criteria take into account that, at a certain magnitude of the mean normal stress (σmax)'' the shear strength of frozen soil reaches a maximum. The yield criteria are included in the time-to-failure function, the shape parameters of which are independent of the loading regime. The model has been verified using test data on creep and the long-term strength of frozen soil under triaxial compression at -10°C.
It is well-known that the mechanical properties of frozen soil are similar to those of ice. Therefore, constitutive equations and strength criteria developed for one material are used interchangeably for the other. A considerable contribution in developing constitutive laws and failure criteria of frozen soils and ice under a complex stress-strain state was made by a number of researchers (Vyalov et al. 1963, Sayles 1973, Zaretsky and Vyalov 1971, Klein and Jessberger 1979, Vyalov and Slepak 1988, Domaschuk et al al. 1991, Zaretsky 1993, Puswewala and Rajapakse 1993, and many others).
With ocean engineering development, the problem of nonlinear interactions among offshore structure, waves and currents becomes more and more important. But so far, less has been known in its mechanism when the viscous effects is taken into consideration. As a part of the efforts to tackle the problem, a numerical model is presented in this paper for simulation of the flow field around a vertical circular cylindrical artificial island under the action of the co-existing skew waves and viscous currents based on the depth-averaged Reynolds equations and depth-averaged k-f turbulence model. Finite difference method is adopted in the numerical process with the structured block grids of body-fitted coordinate system. Flow fields and hydrodynamic coefficients are computed and compared with those for the cases of pure viscous currents and pure incident waves.
With the development of ocean petroleum and nature gas industries, artificial island has been widely constructed and used as the base of recovering, stockpiling and transportation of ocean petroleum and nature gas, and also as the supply depot for ship and aeroplane. The flow field around the artificial island under the action of skew waves and VISCOUS currents IS of primary importance for the scouring estimation and protection in the vicinity of the island. The nonlinear interactions among skew waves, viscous currents and structures form an extremely difficult problem, with less knowledge in its mechanism. So far, the problem has had a wide-ranging study in the field of potential theory(Cheng,1992, Isaacson,1981, Isaacson & Cheung,1992, Li, et.a1.1993, MacCamy,1954, Yoon,1989). As a part of efforts to tackle the problem, Wan, Liu and Miao(1994) made a numerical study on the flow field around a vertical circular cylindrical artificial island under the action of forward incident waves and viscous currents, and obtained some cognition of the problem.
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.
This paper describes in detail the methodology used for the reassessment of offshore installations on the Dutch Continental Shelf. The service life extension of four platforms, operated by Elf Petroland b.v., has formed the basis for the development and implementation of a rational method for reassessment in combination with an improved inspection planning. The total of mining installations on the Dutch Continental Shelf amounts approx. 94 at the end of 1993. Figure 1 shows a cumulative overview on the number of platforms related to the year of installation. This figure clearly indicates the ageing of the installations, of which 21 are installed before 1980. The design service life is in general 20 years. The ageing of platform structures will lead, in most cases, to an increased effort in mspection and repair, because of fatigue damage and/or corrosion. This effort i.e. costs for inspection and repair, can be minimised by the implementation of a rational approach for reassessment and inspection planning. The result of which should be regularly evaluated and fed back into the inspection programs as well as into the design methodology for new installations. The development and implementation of this rational approach, based on Bureau Veritas guidelines (ref.6) and UEG publication UR38 (ref.3), is desribed in section 3 and 4. Not only the technical feasibility but also the economics play an important role in this decision. In section 5 the economical aspects on underwater inspection are presented. This project showed the subject platforms are suitable for a service life extension, in addition the overall inspection costs can be reduced by 57%, due to reduced weld inspection and marine growth cleaning.
In February 1993 ElfPetroland b.v. in the Netherlands commissioned ABB Global Engineering to reassess three offshore installations; the L7P, L7Q and L7C platforms.
Sea ice is carried to the coast of Hokkaido by wind and water currents every year. In low pressure systems or when there is much sea ice, it drifts out toward the Pacific Ocean. When sea ice moves in shallow water areas, the sandy subgrade on the sea bottom is gouged by the sea ice. This phenomenon is generally called "ice gouging". Substantial damage to sea food i.e. (shellfish) and to structures embedded in the seabed is reported every year. However, the mechanism and behavior of ice gouging is not known sufficiently enough for discussion. Therefore, we have conducted a suitable experiment to clarify this phenomenon and have suggested formulas to measure ice gouging.
Ice gouging is a phenomenon in which sea ice attached to the sea bottom in a shallow sea area scrapes the sea bottom while it is pushed by the offshore ice field to'' a shallow sea area. During its movement from offshore, the sea ice gouges the sea bottom, creating strong friction on reef surfaces in reefy areas, and damaging sea urchins and sea weeds there. Conversely, the friction cleans up the breaker zone by eliminating sea weeds on the reef surface and directly exposes the surface. In addition, off sandy beaches pressure by the sea ice itself kills shellfish and carries them to the shore, thus typifying the great influence on shellfish in shallow sea areas. Kunimatsu et aI. (1993) and Kioka et aI. (1994) have been researching experimentally the effects of ice gouging and have suggested equations to compute ice forces created by ice gouging. As we discuss later, in our research new experimental conditions are added to the previous research conditions; the equations are strictly formulated, and they are verified by experiment.
The values of shear strength of the ice rubble in the ridge keels and rubble fields are important in force computations. To increase the data on ridge properties, the shear strength of partially consolidated ice rubble was studied by making tests in a shear box. The only variable was the consolidation time. The tests were done in a simple shear box in which the top and bottom of the ice are'' not confined. 1be dimensions of the box were 0.96 m in length, 0.8 m in width and 0.8 m in height. The upper part of the box was pulled with the carriage. The ice field was sawn in blocks in order to get a controlled block size distribution. The size distribution for the ice blocks was selected to correspond published data on full-scale ice ridges. The measured mean values of shear strength varied from I kPa to 17 kPa depending on the freezing conditions.
The internal properties of ice ridges and rubble fields are quite poorly known even though these features are very common in Arctic and Subarctic seas and create often the major loading case in the design of offshore structures. The main dimensions of these features can be measured with a good accuracy by the traditional drilling methods. However, the internal structure and the strength properties should be studied in more detail. Field tests are usually expensive and time consuming but some properties can be measured in model scale in the laboratory where the amount of consolidation and the degree of deterioration in the individual ice blocks can be varied. Most studies attempting to elucidate the shear-deformation behaviour of ice rubble have made use of a classical soil theory where a linear Mohr-Coulomb failure criterion is assumed to exist throughout the material.
Size and thickness of drift ice belong to basic parameters which determine design and operation of oil and gas producing facilities offshore of the northern Sakhalin. Drift ice appear~ on the northern Sakhalin offshore in November -December and disappears in May - June, Maximum development of ice cover falls on March - April. Under the influence of constant East-Sakhalin current sea ice drifts southwards and covers the distance from northern regions of Sakhalin (Yelizaveta Cape) to the Kuril Islands within 20 -30 Days. Multiyear aerial ice reconnaissance data were analyzed which involved winter seasons of the entire severity range. The most severe winter seasons within the analyzed time period occurred in 1957, 1966. 198) and 1988. Hence, the results given in this paper describe basic regularities for seasonal variability of sea ice size and thickness and may be considered as reliable climatic characteristics for the region under discussion.
The analysis of seasonal ice variability was based on aerial ice survey maps obtained for every 10 days in 1958-1986 and on satellite imageries. The north-eastern Sakhalin offshore was divided into 26 squares (1ºx 1º) (Fig. 1). The paper compares the data on ice conditions obtained in SIX squares (1,220.127.116.11 and 25). Three of these squires (1,9 and 24) are located in shallow waters along the coastline and the rest three ones are in deeper waters in parallel to the first three Squares. Monthly variations for the Ice size distribution function (ISDF) and ice thickness distribution function (ITDF) were studied ISDF plots were based on calculations of recurrence for each sea ice feature.
SEA ICE SIZE VARIABILITY
Seasonal variability for ISDF in all the squares of the region exhibits the following trend: predominance of small ice cake in December and big ice floes in January - March.
Fatigue tests with frequency of 0, 1, 5, 10, and 20 Hz were carried out on three types of cracked alumina ceramics. A specimen surface was replicated at the maximum stress during one cycle. The crack length was measured on plastic- replica with optical microscope and the crack opening displacement was observed on metal-replica with scanning electron microscope. The fatigue crack propagation rate, da/dN, was not controlled by the stress intensity factor. However an equivalent crack propagation rate da/dtl eq was controlled by the effective stress intensity factor at crack tip obtained from COD, independent of the frequency with some percent scatter.
As fracture strength of ceramics is dependent on a crack propagation characteristics from a pre-existing defect in ceramics, its value has a large scatter. Therefore an artificial crack has been introduced and many researches have been carried on its behavior. However the difficulty on research of cracked ceramics is based on (1 )difficulty of crack introduction, (2)difficulty of observation, (3)difficulty of loading, (4)vagueness of driving force for crack extension and (5)scatter of strength and so OIL Therefore a long crack, which is comparatively easy on the above respect, has been used for research: for example, DT or CT specimen. As results, the crack propagation rate is represented in the form of da/dt = C . Kn (for example, Wiederhorn, 1969 and Evans,1972). However, it is well known recently that the fracture toughness and R-curve of a small crack are different from those of a long crack (for example, Steinbrech,1983 and Mai-Lawn, 1987). Moreover It is pointed out that the crack propagation rate of a small crack is different from that of a long through crack (for example, Kishimoto, 1991 and Mutoh-Takahashi,1933). And dynamic fatigue mechanism in ceramics has been investigated.