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to

GoFor an accurate analysis of the forces during ice-structure interaction it is necessary to use a correct material model for the Ice. An elasto-viscoplastic constitutive material model is able describe elastic, plastic and viscous deformations, which all are present during ice-structure interaction. The creep behavior of the material model is usually assumed to be purely viscous (i.e. secondary creep) and governed by Glen''s law. Neglecting primary creep will lead to an underestimation of creep strains in the initial stages of the deformation process. How this affects the indentation force is not known. In the present study, both primary and secondary creep deformations are considered. Different mathematical models for creep deformations in ice are investigated. These models are adopted in analysis of ice indentation problems using the finite element method. The numerical results show that the effect on the maximum indentation force of including primary creep in the material description is negligible, whereas the effect on the creep process itself is large.

Since ice is present in nature at a very high homologous temperature the mechanical behavior is rate and temperature dependent. Time-dependent deformations, usually referred to as creep, play an important role in a wide variety of ice mechanics problems. At low stresses and high homologous temperature creep strains usually dominate over other components of strain. Numerous publications have dealt With the experimental determination of creep deformations in ice under various loading and environmental conditions. One of the first constitutive models describing creep in ice was Glen''s law, Glen (1955), and is still widely used. Glen''s law neglects primary creep or delayed elasticity and only considers the viscous part of the creep process. Ashby and Duval (1985) reviewed several creep equations given in the literature and separated between phenomenological equations, where macroscopic variables are related, and constitutive equations which are more based on physical models.

ISOPE-I-94-152

The Fourth International Offshore and Polar Engineering Conference

This paper describes the geotechnical study on soft marine clay which is found at the seabed of Osaka Bay, Japan, where many large-scale man-made islands are being built. The geotechnical properties of the soft marine clays in the north half perimeter of Osaka Bay, namely Kobe and Osaka areas, are examined in detail because of their importance in the geotechnical work for constructing the man-made islands. From the study, it was found that there is a clear difference in the compression properties of the clay between the two areas while the strength properties may be uniquely evaluated by using the normalized water content which corresponds to the natural water content normalized by the liquid limit of the clay.

In Osaka Bay of Japan, a large number of water front developments are in progress and completed. The Kansai International Airport which is a man-made island 5 km off-shore of Sennan coast is to open in September, 1994 and many other large reclamation works are being executed. Because of the soft nature of seabed deposits m Osaka Bay, a large amount of geotechnical investigation is needed for each engineering work, and the total volume of geotechnical information from these engineering work are exhaustive. The Research Committee of Geotechnical Information of Osaka Seabed Deposits, of JSSMFE Kansai Branch has been collecting more than 3000 bore holes data of seabed explorations in the Osaka Bay. The data consist of m-depth soil testing data as well as geological test data, and a computer data-base system was built to make the retrieval and analysis of these data very easy and fast. In ISOPE-93, we have presented a geotechnical study of seabed deposits along the perimeter of Osaka Bay. The study was primarily to examine the stratigraphic features of the seabed deposits in Osaka Bay as well as the local variations in the geotechnical properties of soft seabed materials.

ISOPE-I-94-056

The Fourth International Offshore and Polar Engineering Conference

air transportation, clay, compression, difference, geotechnical property, Kobe, natural water, normalized water, Osaka, Osaka Bay, Reservoir Characterization, reservoir description and dynamics, seabed deposit, sedimentation compression, self-weight consolidation, soft marine clay, State, study, undramed strength

Industry:

- Transportation > Infrastructure & Services > Airport (0.54)
- Transportation > Air (0.54)

SPE Disciplines:

This paper describes the consolidation characteristics of aged clays in relation to geological history. As the results, it is shown that the consolidation yield stress of aged clays is influenced by the sedimentation curve of deposits, and the reference compression curve is closely related to index properties of aged clays. Also, pc-effect defined from the reference compression curve depends on the sedimentary age and the overburden pressure.

Along the shore Iine of Osaka Bay, Japan, many man-made islands have been constructed. The settlement of these islands is major concern for geotechnical engineers. It is difficult to accurately predict the consolidation settlement behavior of these islands, because there are such many clay deposits as a soft marine clay layer of Recent Deposit and underlying medium to stiff clay layers of Pleistocene Deposits. Especially, deeply deposited Pleistocene clays have been consolidated for such a long time as more than 10,000 years, which are called aged clays The authors have tried to elucidate consolidation behavior of the aged clays (Matsui and Sakagami (1992) This paper describes the consolidation characteristics of aged clays in relation to geological history. First, the effect of the geological history on the consolidatlOn behavior of aged clays is discussed The geological history for each clay layer is estimated, being based on the sedimentation curve of deposit that will be described later. Second, fundamental characteristics of compression curve of aged clays are discussed, followed by newly defining a reference compression curve, being based on laboratory consolidation test data of aged clays.

GEOLOGICAL HISTORY OF AGED CLAYS

A comparison of the regional geological history and consolidation test data of deposits in Osaka Bay area is carried out The investigation sites are divided into such three areas as shown in Fig 1.

ISOPE-I-94-087

The Fourth International Offshore and Polar Engineering Conference

clay, compression, compression curve, consolidation, consolidation characteristic, consolidation yield stress, curve, deposit, geological history, layer, marine clay, Osaka, Pleistocene, Pleistocene clay, reconstituted clay, relation, reservoir description and dynamics, sedimentation, sedimentation curve, stress, Upstream Oil & Gas

This paper presents a new approach for remote repair grinding of cracks underwater. The approach uses Controlled Material Removal Rate (CMRR) grinding strategy which is based on a force controlled manipulator for handling the grinding machine. The CMRR-strategy requires an empirical model of the grinding process. Different grinding wheels were tested in underwater grinding, and experiments were carried out to derived an empirical model of the grinding process for a cylindrical grinding wheel. Finally, the CMRR- strategy was applied to grind a groove according to given specifications. The force control system was used to measure the groove geometry after grinding. The results were promising and demonstrate clearly that the CMRR-strategy is very useful in underwater grinding due to its high flexibility.

INTRODUCTION The establishment of underwater structures in a very tough marine environment creates problems in many areas, among these the maintenance area Many of the North Sea installations are large steel structures which are subject to large dynamic loads from wind and wave forces. These loads often lead to crack growth in areas close to welds on steel truss structures (Figure 1). The crack development on steel structures is a critical failure it creates a great need for detection and removal. The cracks often appear under water and the repair is mostly done by Divers. As the development is going toward deeper installations, it will be impossible to use divers this leads to remote repair grinding of cracks underwater. A stiff underwater manipulator with a force control system will be suitable for this application. A lot of work has been done on robotic force control (Whitney 1985) Ralbert and Craig (1981) introduced hybrid position and force control where position and force were controlled in parallel loops. This scheme was later refined by e g Hirzinger (1983) and DeScutter (1986) who placed the controllers in position and force controlled coordinates.

ISOPE-I-94-136

The Fourth International Offshore and Polar Engineering Conference

Industry:

- Machinery (0.74)
- Information Technology > Robotics & Automation (0.56)

This paper presents the result of a simulation of fully nonlinear irregular waves in a truncated three-dimensional wave tank. A Stokes third-order-like irregular wave is used as the incident input wave at the inflow boundary. The Sommerfield/Olanski open boundary condition is applied and the sponge layer beach is added to partly damp the wave near the down tank to ensure the nonreflection condition from the outflow boundary. The exact nonlinear free surface condition is satisfied while the simulation is continued for the repeating period of the incident irregular wave of 128 seconds. The wave elevation recorded at a few locations along the tank are used for the estimate of the amplitude spectrums, energies and skewnesses. It is found from the study that the energy dissipates and the skewness reduces as the wave progresses along the tank. In solving the boundary integral equation, a robust time domain higher-order boundary element method is employed.

The ultimate goal of the present work is to develop a numerical prediction technique of the nonlinear wave loading on offshore structures in severe sea conditions. As the first phase of the research we focussed our effort on the development of a robust and efficient simulation technique of fully nonlinear irregular waves which resemble the natural seas. It is thought that a simple way to simulate fully nonlinear irregular waves in a numerical wave tank is to employ a mathematical nonlinear Irregular wave that does not satisfy exactly the nonlinear free surface condition, as the incident irregular wave at the inflow boundary. There are two such theories i.e. Stokes 2nd-order-like irregular wave by Longuet Higgins (1963) and Stokes 3rd-orderlike by Pierson (1993). In the present work we employ the Stokes 3rd-order-like irregular wave that repeats every 128 seconds. The saw-tooth type noise in the wave elevation, with high frequencies, usually appears as the simulation time increases.

ISOPE-I-94-184

The Fourth International Offshore and Polar Engineering Conference

Long cylindrical marine structures such as risers, TLP tendons, ocean pipes are subjected to several kinds of excitations of forcing, parametric, combined, self-excited etc. In this work, the response characteristics of the slender marine structures is investigated for the first three excitations. The slender structure is reduced to a non-linear differential equation with an integrand. The non-linear equation is soled numerically. The time histories of response amplitudes of the three excitations are obtained for actual TLP tethers, Hutton, Jollie and Snorre TLPs. The response of combined excitation is most dominant for all three tethers. The total displacement is largest in the case of the Jolliet tether which corresponds to the second instability region. The response curves of combined excitation are also obtained. When the strength of forcing excitation is increased, the response amplitude of combined excitation is increased, the response amplitude of combined execution evenly increases. However, when the strength of parametric excitation increases strongly and slightly in the even and odd numbers of instability regions, respectively.

Long cylindrical marine structures such as rigid risers, TLP tendons and ocean mining pipes play an important role in offshore structural systems. Figure 1 shows a typical connection mechanism of a vertical slender marine structure with a superstructure. In the dynamic analysis of these structures, in addition to finding its natural frequencies, obtaining the response characteristics of the system to excitations is also important. Slender marine structures are subjected to several sources of excitations such as forcing excitation, parametric excitation, combined excitation, impulse loads, self-excitation etc in this study, the relative importance of the first three excitations is compared and the necessity of each excitation in the dynamic analysis of slender marine structures is examined. The research into forcing excitation problem has been most actively carried out (de Boom et al, 1984; Triantafyllou et al, 1983).

ISOPE-I-94-122

The Fourth International Offshore and Polar Engineering Conference

The authors propose an estimation method for the abrasions of concrete structures constructed in cold regions when a moving ice sheet breaks against the concrete structure surface near the water level. At least seven different laboratory abrasion apparatuses for determining the ice abrasion resistance of rather high. strength concrete have been proposed to investigate the mechanism of ice impact and abrasion damage. Some conclusions from different types of tests do not coincide with each other. This paper describes physical considerations through the comparisons between our experimental results obtained by our original test method and other results. Summarizing our systematical ice-sliding laboratory experiments and the field observations, we present an experimental formula for design purpose of arctic offshore structures, where each important factor can obtained based on a scenario of ice impact and abrasion process.

In rivers, lakes and oceans where periodic ice floes occur, concrete structures such as bridge piers, sea walls, wharves, docks, lighthouses and oil production platforms have serious damage. It has been reported by Janson (1986) and Huovinen (1990) that field inspections have been made of 26 Swedish concrete lighthouses and 4 Finnish concrete lighthouses in the Gulf of Bothnia, respectively, to examine a belt of deterioration and loss of concrete at the waterline. Past research works and field observations suggested that the concrete deterioration at or near the waterline is due to a combination of cyclic freezing thawing plus abrasion and/or impact loading from the moving ice sheet. With the recent development of high strength concrete, the concrete material durable against freezing and thawing has already developed, therefore it is important and useful for the design of concrete offshore structures in arctic regions to accurately predict the amount of the ice-sliding abrasion. Hoff (1989) has evaluated five different abrasion test methods for determining the ice abrasion resistance of high-strength lightweight concrete through the three phase joint industry project organized by ABAM Engineers Inc.

ISOPE-I-94-171

The Fourth International Offshore and Polar Engineering Conference

Ooi, Tadashi (Ishikawajima-Harima Heavy Industries Co., Ltd.) | Yoshida, Yuki (Ishikawajima-Harima Heavy Industries Co., Ltd.) | Takahashi, Yoshiaki (Ishikawajima-Harima Heavy Industries Co., Ltd.) | Kidoushi, Hideki (Ishikawajima-Harima Heavy Industries Co., Ltd.)

An attempt is made to combine the classical controller with the concept of neural network, the result of which is a control system that we have named the Robust Adaptive Neural-net Controller (RANC). The RANC identifies the dynamic characteristics of the remotelv operated vehicle (ROV) including its ambient environment involving cyclic disturbances such as forces induced by waves, and organizes automatically an optimized controller. A tank experiment is described in which the RANC is set to maintain a model ROV at a prescribed depth of water under artificially generated wave disturbance.

INRODUCTION

Remotely Operated Vehicles (ROVs) are cable-tethered unmanned submersibles for use in underwater operations and surveys. Remote manual control of these vehicles is an intricate exercise for the operator stationed on the surface tender vessel. To alleviate the burden imposed on the operator, all ROVs are provided with some degree of automation in their controlling systems. Such control systems have in the past been based on classical algorithms, which require the parameters of the control system to be adjusted and set beforehand, based on pre-estimated environmental conditions. In practice, however, the underwater environment surrounding an ROV involves factors-such as waves and currents-that are difficult to determine in advance, and this has so far stood in the way of developing an automated control system that would adapt itself automatically to a wide range of underwater conditions. What was needed was a control system that would automatically adjust its control system parameters so as to match it to whatever environment the ROV would be encountering. The concept of neural network provides precisely such a learning function to adjust the system parameters that determine the manipulated variables, and the present study covers an attempt to apply this neural network concept to ROV control.

ISOPE-I-94-143

The Fourth International Offshore and Polar Engineering Conference

SPE Disciplines: Management and Information > Information Management and Systems > Neural networks (1.00)

Technology: Information Technology > Artificial Intelligence > Machine Learning > Neural Networks (1.00)

Tomita, Yasumitsu (Osaka University) | Kawabe, Hiroshi (National Defense Academy) | Sasaki, Tsukasa (National Defense Academy) | Fukuoka, Tetsuji (Mitsui Engineering and Shipbuilding Co., Ltd.)

The purpose of this study is to clarify the time history of wave-induced stress by analyzing data collected from 6 naval ships over a period of 3 years. The time history of wave-induced stress at midship deck plate is compared with encountered wave height condition. By the data, wave-induced stress is linear to wave height in the elastic stress region. The property of the time history of wave-induced stress, therefore, is considered to be the same as that of encountered wave condition. The proposed random fatigue loading model based on the encountered wave conditions can be verified by the data from time history of wave-induced stress. Based on the above result, the random fatigue loading model for ship structural members is discussed.

The fatigue strength is one of the most important factors taken into consideration in the design of ships. On the analysis of fatigue strength of ship hull structure, it is necessary to obtain both the long-term distribution and the time history of wave-induced load/stress. Recently, we investigated data collected from 38 ships that sailed in the North Pacific and 11 ships that sailed in the Japan-Indian route over a period of 14 years ( 1976 - 1989 ) (Tomita 1991,1993 ). It is concluded that the tlme history of wave conditions during real sailing can be divided into two types. One is the calm sea condition, and the other consists of various storm conditions. These two kinds of conditions appear alternately m random order. Based on the results, we propose the random fatigue loading model for ship structural members. At this point, it is assumed that the response of wave induced load is linear to wave height. In this paper, we collected data measured by Ship Motion Analyzing Computer System installed on 6 Japan Maritime Self Defense Force destroyers that sailed m North Pacific near Japan over a period of 3 years.

ISOPE-I-94-285

The Fourth International Offshore and Polar Engineering Conference

A two-dimensional material nonlinear consolidation theory which is the generalized version of one-dimensional one provided by Gibson in 1967 and 1984 has been developed. Void ratio remains the primary unknown for describing the consolidation processes. The relationship between the horizontal permeability and void ratio is newly needed and a new method for that has been developed. An application for the simulation of a model test is performed. The results are presented in a nondimensional form.

Consolidation is the time-dependent process in which the pore water is gradually squeezed out from the soil. The most famous two- or three-dimensional consolidation theory is that proposed by Biot (1941). One of the assumptions in this theory states that the coefficients of compressibility and permeability of soil remain constants during the consolidation processes. But in some circumstances, for example, soil improvement or deposition of hydraulic fill, the coefficients of compressibility and permeability of soil may change substantially and the assumption will not be true. Gibson (1967,1981) developed an one-dimensional consolidation theory which is called finite strain consolidation theory for this situation. This theory has been used in practice in many ways including the estimation of capacities for dredge-fill and tailings impoundments, settlement estimation for design of reclamation covers, and pore pressure and undrained shear strength estimation for stability analysis. In this paper, the theory will be extended into two-~imensiona1 condition. Void ratio remains the main unknown for describing the consolidation process. Self-weight effect is not included here. The relationship for horizontal permeability will be needed. It will be obtained by isotropic or one-dimensional consolidation tests.

COORDINATE SYSTEMS

In discussing the problems of fluids, there is an approach which is referred to as the Lagrange approach. It considers an individual fluid particles at all time (Roberson and Crowe, 1985).

ISOPE-I-94-091

The Fourth International Offshore and Polar Engineering Conference

consolidation, coordinate, datum plane, direction, equation, flow in porous media, Fluid Dynamics, Horizontal, horizontal permeability, one-dimensional consolidation, permeability, plane, Pore Fluid, reservoir description and dynamics, SOIL, theory, two-dimensional finite strain consolidation theory, Upstream Oil & Gas, void

SPE Disciplines: Reservoir Description and Dynamics > Reservoir Fluid Dynamics > Flow in porous media (1.00)

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