Jun 25, 2010 - Alexander T. Bekker, Sergey G. Gomolskiy, Olga A. Sabodash, Roman G. Kovalenko, Tatyana E. Uvarova, Egor E. Pomnikov. Far Eastern ...
Proceedings of the Twentieth (2010) International Offshore and Polar Engineering Conference Beijing, China, June 2025, 2010 Copyright © 2010 by The International Society of Offshore and Polar Engineers (ISOPE) ISBN 978-1-880653-77-7 (Set); ISSN 1098-6189 (Set); www.isope.org
Physical and Mechanical Properties of Modeling Ice for Investigation of Abrasion Process on IceResistant Offshore Platforms Alexander T. Bekker, Sergey G. Gomolskiy, Olga A. Sabodash, Roman G. Kovalenko, Tatyana E. Uvarova, Egor E. Pomnikov Far Eastern National Technical University Vladivostok, Russia
Igor G. Prytkov, Pavel Anokhin Far Eastern Research Institute on Construction Vladivostok, Russia
ABSTRACT
ing, testing, strength, salinity, density, ice structure, concrete sample.
The sea oil&gas production offshore platforms exploited in the freezing seas with a severe ice regime are exposed to the considerable dynamic impacts from a drifting ice cover. One of difficult and little-studied problems in world practice of designing and exploration of such structures is abrasive action of ice on a material of the platform’s body in the area of variable water level. Thus physical and mechanical properties of ice are one of the important factors influencing abrasion process.
INTRODUCTION The problem of ice abrasion of construction structures is known for a long time in connection with bridge engineering, building of weirs, culverts, and transport facilities in the northern areas. In the last years it has attracted especial attention due to oil-and-gas-field construction offshore the freezing seas where the concrete is main material for the platform legs.
The purpose of the paper is the experimental studying of physical and mechanical properties of artificially grown modeling ice under various conditions of its fabrication in ice laboratory.
Owing to high dynamism, major cohesion and the significant strength of the ice cover to the north-east of Sakhalin, the greatest hazard is introduced by ice abrasion effect on legs of the ice-resistant concrete gravity-base platforms. All these stipulate increased requirements to selection of concrete mix of the bases for the legs to prevent their possible fracture over the expected 40-year life cycle (Vershinin et al, 2006).
In order to achieve this purpose, the authors solve the following tasks in the paper: - development of a technique and technology of fabrication of modeling ice blocks; - experimental research of physical and mechanical properties of modeling ice at various loading rates and temperatures; - analysis and statistical treatment of results of experimental investigations of physical and mechanical properties of modeling ice; - experimental determination of concrete resistance and intensity of an abrasive wear of an icebelt material.
The complicated and insufficiently studied process of ice abrasion effect on the offshore structures and rather small volume of field experiments on measuring physical-and-mechanical features of ice make this problem urgent to be studied in laboratory conditions. The purpose of this study is the experimental research of physicalmechanical properties of the ice influencing the process of the drifting ice cover abrasion effect on offshore structures for the Sea of Okhotsk.
As a result of the carried out tests, distribution of salinity, density and ice structure on a thickness of the block of modeling ice are received, and also strength on the uniaxial compression at various loading rates and temperatures are described. From results of experimental investigations of the abrading effect of modeling ice on concrete samples the sum polygon of specific abrasion was obtained and comparative calculations of the abrasion depth an of a structure material according to different authors were made.
Problems of experimental investigations of physical-mechanical properties of the modeling ice were as follows: - development of procedures of experimental investigations of physicalmechanical properties of the modeling ice and seawater; - strength testing of ice samples on axial compression; - obtaining of an empirical dependence of ice strength on axial compression from temperature; - study of the modeling ice salinity;
KEYWORDS: offshore gravity-based platform, ice abrasion, frost-
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- study of density of the ice samples; - study of the modeling ice structure.
Before water delivery to the precooling tanks, the seawater is desalinated down to S=20‰ salinity. In the pre-cooling tanks, the water is cooled down to plus (1÷2) °С and then it moves to the forms. In the process of ice accretion and with the related augmentation of salinity, the water in the precooling tanks is diluted by the water from the tanks of the secondary container.
The tests were carried out on modeling ice samples with different salinity S (fresh ice, 5, 16, 20 ‰) obtained at miscellaneous conditions of the ice freezing in forms. The required level of salinity was obtained by dilution of the seawater at appropriate ratios according to the procedure, described by (Yakovlev, 1971).
LABORATORY FOR THE MODELING ICE The laboratory is introduced by two refrigerator containers: the main container and the secondary one (Fig. 1). The main container is divided by walls into three areas: 1 is the freezing area, 2 is the abrasion testing area, and 3 is the engine room. In the freezing area the forms for ice samples are disposed. In the abrasion testing area, the devices for abrasion testing with prepared ice blocks are disposed. In the secondary area, there is equipment which supports the operation testing device (the lubricator, etc.). The secondary container is also divided by walls into three areas: 1 is the air precooling area, 2 is the water-treating area and the 3 is the sample-grinding area which includes the secondary equipment. The water treating area includes disposed water tanks (seawater, fresh water and 2 predetermined salinity water tanks). The sample grinding area with the secondary equipment includes a sample grinding device and racks with the secondary equipment. The ice-making device includes ice forms, precooling water tanks and piping. Ice forms are installed inside the main container, and the tanks are placed on the container roof. Eight ice forms are installed on four decks, and they may be considered conventionally as “left-handed” and “right-handed”, respectively. The left-handed and right-handed forms have similar supporting systems independent from each other. From the precooling tank, the supercooled water moves with gravity flow into a distribution tank, and further, through independent pipes, through inlet elbows, into every form. Through the outlet pipes, the water from the forms is collected by a drainage riser pipe to a drainage container. When the drainage container is full, the down-pump installed inside it pumps out the water back to the precooling tank.
Fig. 1. Plan of laboratory. The water temperature in the precooling tanks is kept plus (0.6÷0.8) °С in order to eliminate frosting-up of the cooling system heat sink.
In every tank the cooling system is installed, which includes a couple of heat sinks and the antifreezing agent pump connected as a closed network. One heat sink is installed inside the tank, and the other one is mounted under the ceiling in freezing area of the main container. In order to improve air circulation through the heat sinks, the fan is installed in the main container.
The height of water in the forms is set by vertical displacement of the input and exit pipes on the forms. Ice thickness in the forms is monitored at regular intervals. The degree of water freezing in the forms is also inspected at regular intervals by measuring of temperature inside the forms. If the water temperature falls down to minus (1.2÷1.4)°С, then the ice thickness in the form is sufficient for the further application of the ice block.
In order to provide continuing air circulation over the forms, the special box is installed over the exhaust outlets of air vents of the refrigerator container which forwards the cool air over every form.
When the block holder completely freezes through, the remained water drains from the form and the ice block is left for refrigerating to the required strength and safety extraction. After the ice-block cooling for a sufficient time, a cap of the form is disassembled. The ice which has frozen between the block holder and the form is chopped off, and the off-the-shelf ice block is delivered to the testing area of the main container in order to achieve the temperature of ice required for the tests.
In this experiment it is necessary to create conditions associated with full scale conditions as much as possible. With that end in view a special system of ice preparation has been constructed. This system ensures water circulation in the forms, provides indispensable salinity and temperature of water and continuous air circulation over the forms, as well.
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characterized of different phases of elasto-plastic fracture of ice at different temperatures (Bogorodsky, 1980).The ice samples were loaded perpendicularly to top surface of the ice block which was marked on the side of sample by special paint. In all cases it was observed that the ice strength had the complex nonlinear dependence from the loading rate. Also it has been noted that the nature of ice fracture greatly depended on salinity: the fresher was ice, the more brittle failure it had, and the more saline ice had the more strongly expressed creeping phase. However in all cases it was noted that the modeling ice had the complex mechanism of elastic-brittle-plastic fracture, and specificity of each phase depended on conditions of ice growth.
STRENGTH TESTS In order to study strength of the modeling ice on axial compression from an ice block prepared in the same conditions, as the blocks prepared for the abrasion test of concrete samples, the cubic ice samples were prepared. Dimensions of the samples were as follows: 110÷150 mm length and width; 110÷150 mm height. The dimensions of the samples have been selected from the average thickness of the ice block. From the freezing conditions the ice block in the form has a symmetry direct-axis, and because of such conditions it was sawn in samples symmetrically from its left and right sides. Thus, results from the investigations of the ice strength make the unified data array defining fluctuation of the ice strength against the thickness of the ice block. Samples were taken from different blocks and they were kept in the climate chamber at given temperature minimum 1÷2 days. The samples were tested at ice temperatures between -5С and 22 С (SNiP, 1989).
Table 2 - Relation of strength to temperature of modeling ice and water salinity Ice temperaSalinity of Strength of ice Rc, MPa ture t, °С water S, ‰ average standard value deviation -5 20 1.387 0.185 -10 20 1.543 0.278 -20 20 2.352 0.332 -5 16 2.295 0.26 -15 16 2.479 0.197 -20 16 2.862 0.727 -16 5 2.992 0.739 -18.8 5 2.97 0.8 -25 5 3.164 0.443 -7 0 1.857 0.633 -11 0 1.904 0.598 -21 0 1.634 0.379
For the axial compression testing of the ice samples the press was used with 3000 kPa force (Fig. 2). Prior to the testing of the ice samples, loading rate values were set on the press control panel. 52 ice samples were tested at five values of the loading rate, namely 5.5 kN/s, 4.0 kN/s, 2.5 kN/s, 1.5 kN/s and 0.5 kN/s (Bogorodsky, 1980).
Fig.2. Process of the ice sample loading.
Fig.4. Ice samples strength range depending on temperature of testing. Fig.3. Function of normal distribution of the ice strength (S=5 ‰, t = –18.8 °С). Loading rates have been selected according to the strain rates of ice
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Fig. 6. Relation between ice strength (S=20 ‰, t = –20С) and loading rate.
Fig. 5. Fracture of the modeling ice samples (S=20 ‰). The test results have shown that in all cases distribution of ice strength is well-described by the normal law (Fig. 3). Table 2 shows data from statistical analysis of the relation between strength and temperature of the modeling ice and water salinity. Fig. 4 shows the relation between strength of modeling ice and temperature of tests. The graphic chart shows clearly that there is a linear dependence between ice strength and temperature, and such results match with the investigations of other researchers (Bogorodsky, 1980). The test results have displayed that modeling ice samples fracture along-the-grain in the direction of load application with deep longitudinal cracks (Fig. 5). Ice with lower salinity has smaller dimensions and thickness of cracks. Depending on a method of freezing on and storage conditions the modeling ice strength varies within wide range of 1.0÷3.7 MPa, and sometimes strength of the samples exceeded 4.5 MPa, while the minimum strength in the test made 0.98 MPa.
Fig. 7. Relation between ice strength (S=16 ‰, t= –15С) and loading rate.
RELATION BETWEEN STRENGTH AND THE LOADING RATE The relation between strength and loading rate (Fig. 6-9) at constant temperature has the complex nonlinear dependence in some cases. Usually the greatest strength was observed at low (0.5 kN/s) and average (2.5 kN/s) loading rates.
Fig. 8. Relation between ice strength (S=5 ‰, t= –25С) and loading rate.
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From 12 cm thickness of the block, and also from the sample dimensions, results on salinity definition concern horizons-2 cm,-6 cm and 10 cm in relation to the upper top surface of the ice block. As far as the ice block has a symmetry direct-axis by conditions of freezing then left and right sides of the block were used to study the density of water samples. The selected areas mutually supplement each other, and the results from density investigations make the unified data array defining ice density variation against thickness of the block. Prior to weighting, the modeling ice samples were kept in the thermal chamber for 24 hours at -5C. During weighting of the samples, temperature and density of kerosene were measured by areometer. Results of modeling ice density measuring, namely ice density variations against thickness of the ice block, are shown in Fig. 11.
Fig. 9. Relation between ice strength (S=0 ‰, t= –21С) and loading rate. It is necessary to note that conditions of freezing and storage of ice samples significantly affect the test results. These conditions include availability or lack of water circulation during freezing, temperature drops in the process of storage and shelf-life, availability of the insulation which may prevent ice contact with air and some other. Also the ice structural degradation is possible during storage that may change ice strength.
ICE DENSITY Fig. 11. Ice density variations against ice block thickness (S=20 ‰).
Density of the modeling ice was investigated by a method of hydrostatic weighing of ice samples (Yakovlev, 1971). In order to study the ice density, the samples were made in the form of parallelepiped of 90÷160 grammas weight from the block prepared in the same conditions as the blocks intended for the concrete samples abrasion tests.
According to Fig. 11 ice density in the middle layer of the block is a little higher than in the top and bottom layers. The average ice density value at t =-20С made 0.909 g/cm3 that match well the results of other researchers.
The weight of the ice samples has been selected from capabilities of electronic scales АUW220D made by SHIMADZU Corporation and from weight of the bin intended for weighting of ice samples in special fluid.
ICE SALINITY In order to study salinity, the ice block prepared under the same conditions as the blocks for abrasion testing of concrete samples was used for making parallelepiped samples which gave water samples (when melted) of V=250÷330 ml by volume. The volume of the water sample was chosen from the minimum volume applicable for the tester (when entirely submerged) of the conductometer. From 12 cm thickness of the block and also from the dimensions of a sample made for the water sample, the results from salinity definition concerned horizons-2 cm, -6 cm and -10 cm in relation to the top surface of the ice block.
These electronic scales have below-weigh hook that allows weighing the ice samples submerged into fluid in the thermopot below the scales. Kerosene was applied as fluid during the tests. As far as kerosene may change its density during testing, special experiments were carried out to reveal the relation between its density and temperature when the temperature is rising (Fig. 10).
As well as in case of gravity test, symmetric areas of the left and right sides of the block were investigated. The selected areas mutually supplement each other, and the results from salinity study make the unified data array defining ice salinity variation against thickness of the block (Fig. 12). When the ice samples melted each water sample was poured into a special pot which one was previously treated by distilled water. The water sample temperature (С) was measured by the electronic thermometer made by EBRA Corporation, and the electric conductivity (mS) was measured by HANNA conductometer. Fig. 10. Relation between density and temperature of kerosene.
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Fig. 13. Structure of ice (S=20 ‰) in a direction perpendicular to the top surface of the block.
Fig.12. Salinity variation against the thickness of ice block. Prior to test on definition of the ice samples salinity for the conductometer, the interaction between electric conductivity and salinity at different temperatures of water samples was revealed experimentally. Thus salinity of specially prepared water samples was determined according to the known argentometric method (Yakovlev, 1971). Results of salinity distribution against thickness of the modeling ice block are shown in Fig. 12.
The structure of fresh ice was investigated as well, and it greatly differed from the modeling ice structure. The dimensions of chips were fuzzy, and the structure as a whole represented a single crystal with defects in the form of cracks and blisters (Fig. 14). Distribution of these defects by quantity and dimensions was unequal as the defects in the upper layer of the block were small in size and quantity and in the lower layer such defects increased in quantity and dimensions sharply since the midpoint of the block height. If to consider a lower level of microsection in a parallel direction, the chips would be needle-shaped, 0.5÷2.0 mm thick and 2÷10 mm long.
From Fig. 12 we can see that range of salinity values of the upper and lower layers is a little more than range of salinity values of the middle layer, and its salinity is 3% more than salinity of the upper and lower layers. The same distribution of salinity in the layers was watched in all ice blocks irrespective of initial salinity of water and availability of circulation.
ICE STRUCTURE INVESTIGATIONS The modeling ice structure was investigated on the microsections prepared from samples sawed out of the block in directions perpendicular or parallel to its top surface. From samples, ice microsections were prepared of 1.5÷1.8 mm thickness and 95×95 mm dimensions. The micro-sections were photographed highlighted by a special bulb. The camera was equipped by a microfilm lens and the screen of the polarized light. Fig.14. Fragment of the fresh ice structure in direction perpendicular to a top surface of the block.
The ice structure depends greatly on its salinity (Bogorodsky, 1980). The modeling ice prepared from water with various salinity (S=5÷20‰) has a fine-grain structure with the dimension of chips approximately 0.4÷1.0 mm, at the same time in some cross-sections the dimension of chips varied between 1.0÷3.0 mm (Fig. 13). Irrespective of water salinity and freezing conditions, the ice had a uniform structure with minor impurities.
Fig.15. Fragment of the fresh ice structure (lower layer) in direction parallel to a top surface of the block.
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RESULTS FROM EXPERIMENTAL TESTS ON THE CONCRETE ABRASION
Analysis of outcomes shows that intensity of the abrasion depends not only on the parameters of ice effect but on the properties of concrete, as well.
The modeling ice obtained in the ice laboratory with characteristics very close to full-scale parameters was applied in experimental investigations of the ice abrasion of concrete samples. Main objective of the laboratory experiments on abrasion was to adjust the procedure of tests and to develop a program of experiments for the Sea of Okhotsk conditions. Concrete with different characteristics was applied for the tests. Concrete samples were prepared for testing according to special technology.
CONCLUSIONS 1. The developed technology of modeling ice preparation allows obtaining the artificial ice close to the real sea ice by parameters that allows to apply it in experiments on ice abrasion of various materials. 2. The technology of concrete testing on ice abrasion is adjusted. The obtained outcomes of the concrete abrasion depth are comparable to outcomes of the other authors.
An experimental unit especially designed and made for the investigations was applied ensuring maximum 1800 mm amplitude of concrete sample reciprocation against an ice block at the rate of 0.5 m/s and normal force component of 2 tons.
ACNOWLEGMENTS
The main controlled variables of the experiment were as follows: ice strength and temperature, air temperature, normal pressure on “iceconcrete” contact, a distance of interaction (route of the comparative displacement on interaction between a concrete sample and the ice block within a test period), and the shape and depth of abrasion. These parameters were registered automatically and were introduced in the form of numerical serieses analysed statistically for the further analysis. As a result of analysis, the intensity of concrete abrasion was determined in millimeters per one km of the route of interaction.
Authors of the study appreciate highly assistance and support rendered by Hydrotex Co. Ltd in carrying out of the experimental investigations and in data processing.
REFERENCE Bogorodsky, VV, Gavrilo, VP (1980). “Ice. Physical Properties. Stateof-the-art Methods of Glaciology”, Leningrad, Gidrometeoizdat, 384 p. (in Russian).
Some ice abrasion test results of three different concrete mixes are shown in Fig. 16. The same figure shows Itoh’s test results (Itoh et al, 1994) for comparison.
Itoh, Y, Tanaka, Y, and Saeki, H (1994). “Estimation Method for Abrasion of Concrete Structures Due to Sea Ice Movement”, Proc 4th International Offshore and Polar Engineering Conference, Osaka, Vol. II, pp. 545-552. SNiP 2.06.04-82* (1995). “Loads and Actions of Hydroengineering Structures (wave, ice and from vessels)”, Moscow, Stroyizdat, 45 p. (in Russian). Vershinin, SA, Truskov, PA, Kouzmitchev, KV (2006). “Sakhalin Island – Offshore Platform Structures – the Impact and Influence of Ice”, Institute Giprostroymost, Moscow, 205 p. Yakovlev, GJ (1971). “Guide on Physical-Mechanical Properties of Ice”, Offset duplicator AARI, Leningrad, 45 p. (in Russian).
Fig. 16. Results from concrete samples tests on ice abrasion
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