Reliability of stiffened cylindrical shells
P. Radha and K. Rajagopalan
Department of Ocean Engineering, Indian Institute of Technology, Madras, Chennai 600 036.
Reliability analysis is needed for accounting both the qualitative and quantitative sources of uncertainties, which occur in the design of important structures. As such uncertainties can not be directly observed for anyone particular structure, there is a much greater level of abstraction and conceptualisation in reliability analysis than is conventionally the case for structural analysis or design. The stiffened cylindrical shell structures, which are commonly used in ocean engineering field, can fail through instability in many ways such as inter stiffener buckling, stiffener buckling, and overall buckling. All these modes of failures involve uncertainties and the combinations of these in probabilistic terms can become quite complex. Hence there is a desideratum for investigating the probabilities of various collapse modes. Monte-Carlo Simulation (MCS) technique can be used for this purpose. In this paper, the probabilities of occurrence of various collapse modes have been evaluated using MCS method. Then the total probability of failure is evaluated by incorporating these probabilities in the summation of the probability of different failure modes. A typical submarine pressure hull has been analysed using this method.
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Dynamic behaviour of an external turret moored FPSO system by VALM arrangement with 2 lines in regular waves
T. Rajesh Kannah and Dr. R.Natarajan
Department of Ocean Engineering, Indian Institute of Technology Madras, Chennai – 600 036
This paper deals with the dynamic response of an external turret moored floating production, storage and offloading (FPSO) system by vertical anchor leg mooring (VALM) arrangement under regular waves. A model study carried out on a 1:100 scale model of 140000 DWT turret moored FPSO system for three loading conditions, i.e., 40%DWT, 70%DWT and 100%DWT with a hawser length of 25% of the length of FPSO is presented. Experimental investigation was carried out in the 2 m wide wave flume at Department of Ocean Engineering, Indian Institute of Technology Madras, Chennai at a water depth of 1 m for the wave frequencies from 0.55 Hz to 1.25 Hz in steps of 0.04 Hz in head sea condition. Measurements taken during the model tests include the heave, surge and pitch motions, the mooring and hawser line forces. The proving ring type load cells were used to measure the mooring forces and hawser line forces. The motions of the FPSO model were measured by rotary type potentiometers. The experimental results of motions, mooring and hawser line forces were analyzed for different loading conditions at constant hawser length and presented with discussions.
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Performance of floating wave energy device with varying tidal conditions
S. Neelamani¹, R. Natarajan² and D.L. Prasanna²
¹Coastal Engg. and Air Pollution Dept, Environmental and Urban Development Division, Kuwait Institute for Scientific Research, P.O. Box : 24885, 13109 SAFAT,
²Dept. of Ocean Engg, Indian Institute of Technology Madras, Chennai 600 036.
Fixed oscillating water column (OWC) wave energy caisson is one of the promising devices for the extraction of power from ocean waves. However, this device is not suitable for larger tidal variation, as high tidal fluctuations results in excessive reflection of wave energy and loss of pneumatic efficiency. Fixed wave energy caisson will be very expensive in deeper waters and hence floating devices can be adopted. In the present work, an attempt is made to investigate the hydrodynamic characters like pneumatic efficiency, wave transmission, reflection and losses, dynamic response of the device and mooring forces of the OWC wave energy caisson in the moored condition under tidal effects, especially in deep water conditions. Physical model of an OWC wave energy caisson was tested for under different input conditions. It is found that the tidal variation of about 10% to 15% of the water depth does not change the pneumatic efficiency significantly, unlike fixed OWC device, whereas it is found to increase the wave transmission by about 15% and reduces the reflection by about 15%. The salient results of this study can be used in the design of floating OWC devices for wave power conversion and as a floating breakwater in a tide and wave dominating sea.
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Experimental p-y curves for caissons in marine clay
N. Darga Kumar, S. Narasimha Rao, V. Sundar
Department of Ocean Engineering, Indian Institute of Technology, Madras, Chennai- 600036
Caissons can be used in marine environment for various applications. These caissons could experience lateral forces due to water waves and currents. There is a necessity to predict the behaviour of caissons under lateral loading. In general, p-y curves are to be developed for understanding the behaviour of caissons under lateral loads. In this investigation model tests were carried out on caissons embedded in clay. The lateral earth pressure, ground level deflection and tilt of model caissons were measured using the instrumentation. From the measured data deflection has been established along the depth of embedded caisson. The p-y behaviour of caissons is established along the depth. Also, the ultimate passive earth pressure is proposed for laterally loaded caissons with the measured earth pressure.
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Reliability of offshore jacket structure with rigorous modeling of soil pile system
G. Arunakumari, and K. Rajagopalan
Department of Ocean Engineering, IIT-Madras, Chennai-600036, Tamil Nadu
Due to the growing realization of the fact that, many uncertainties affect the performance of any structure, the probability theory is becoming more popular now a days. Reliability analysis is performed taking into account randomness in ultimate strength and loading intensity. Uncertainties in material characteristics and geometrical data introduce randomness in ultimate strength. Uncertainties in design wave force estimation, variations in cohesion of top layers of seabed, currents, topside loads etc. introduce randomness in loading intensity. In this paper, a rigorous model of soil-pile system had been developed to study the randomness in ultimate strength of combined jacket-pile system by repeated applications of finite element software package i.e. NISA. The piles are embedded in seabed and the surrounding soil is modeled as a series of non-linear springs using guidelines from API-RP2A (LRFD) code. A constant vertical force and an incremental lateral force are applied simultaneously. The skin friction offered by soil-pile system resists the vertical load and the horizontal forces by the passive resistance offered by the soil. The behavior of vertical, horizontal, and tip springs are predicted by drawing T-Z, P-Y, Q-Z curves respectively. These are then incorporated into the discrete, non-linear springs modeling the soil system. Then reliabilities of pile alone, fixed jacket alone and the combined pile-jacket system are obtained assuming normal distribution for ultimate strength and loading intensity. Conclusions are advanced regarding the effect of rigorous soil-pile modeling on the reliability.
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Influence of bulbous bow on the resistance characteristics of a Catamaran
Y. Balasubramanyam and R. Natarajan
Department of Ocean Engineering, Indian Institute of Technology Madras, Chennai – 600 036
Presently, a various types of marine vehicles are being developed to meet the increased demand for inland and coastal transportation services. Since the propulsive power of any marine vehicle mainly depends on its resistance characteristics, it is essential to develop a suitable hull form of the marine vehicle in order to obtain better resistance characteristics, which contemplate primarily on fuel economy. In the initial design stage, the propulsive power is usually predicted analytically by any one of the existing power prediction methods. But, however, it is necessary to conduct model tests to obtain the final hull form. Hence, for the present study, a 1:12 scale catamaran model with typical bulbous bow was tested for three hull spacings in the towing tank at Department of Ocean Engineering, Indian institute of technology Madras, Chennai. The model test results are analyzed and presented with discussions in this paper.
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Slamming forces on an open sea mooring dolphin with battered piles
S. Neelamani¹ and P. Baskaran²
¹Coastal Engineering and Air Pollution Dept, EUD, Kuwait Institute for Scientific Research, P.O. Box : 24885, 13109 Safat, KUWAIT,
²Chennai Port Trust, Chennai 600 001, INDIA.
The accurate estimation of the slamming force on an open sea mooring dolphin built in a wave and tide dominating region is essential for safe and economic design of such structures. Physical model investigations with water waves on a typical open sea mooring dolphin model containing a deck slab and nine piles (One vertical and 8 inclined) is carried out to estimate the uplift wave forces for different tidal conditions. The water depths are varied such that the deck slab encounters both fully submerged and fully exposed conditions. It is found that the uplift force is critical when the deck is just fully submerged for relatively shortwaves. Relatively long waves are critical, when the deck is just emerged. The slamming coefficient on the open sea mooring dolphin varies from 2 to 8. An average slamming coefficient, C s of 5 can be used for this structure for the estimation of average uplift force. For deck submerged case, the maximum uplift force is nearly 5 times that of the maximum in-line force. It is found from the study that the open sea mooring dolphin receives highest in-line and uplift forces when the deck is completely submerged and not when the bottom of the deck is touching the still water level. The coefficient of correlation between the uplift and in-line forces is closer to 1.0 for the submerged deck case of the mooring dolphin. When the deck is emerged, the coefficient of correlation is closer to zero. Design of open sea mooring dolphin should take care of all these important points both for safety and economics.
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Computation of wave loads on floating structures
S.P. Singh and S. Dhavalikar
Indian Register of Shipping, 52-A, Adi Shankaracharya Marg, Powai, Mumbai – 400072
In the present paper, computation results for both first and second order forces are presented. The method for computation is based upon 3-D zero speed frequency domain Green's function (Garrison et al., 1972). The second order forces are computed based on the near field approach proposed by Pinkster, 1980. The computations are carried out for various shapes and results are compared with other available results. The results of the first order heave response and the second order mean drift surge forces for the hemisphere are presented. Also the first order motion response of a semi-submersible is presented.
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Sea-keeping characteristics during initial design
Debabrata Sen, Subhendu Maity and Saumya Sengupta
Dept. of Ocean Engineering & Naval Architecture, Indian Institute of Technology, Kharagpur-721 302
During initial design stage of a vessel, usually detailed sea-keeping computations are not performed while evolving the principal dimensions of the hull. Sea-keeping is considered based on some elementary design guidelines. In this paper we propose a scheme wherein during preliminary design itself, the choice of the principal dimensions, particularly length and breadth, the two most influencing factors in this regard, can be made considering the sea-keeping performance predicted from an industry-standard method of computation. An example case is shown where length and breadth are systematically varied. For this particular hull it is seen that the detailed calculation is likely to lead the designer to a choice, which may be contrary to the choice he/she would have made based on the simplified design guidelines.
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Reliability assessment of TLP tethers under impulsive loading
R. A. Khan¹, N.A. Siddiqui¹, S.Q.A. Naqvi¹ and S. Ahmad²
¹Department of Civil Engineering, Aligarh Muslim University, Aligarh-202002
²Department of Applied Mechanics, Indian Institute of Technology, Delhi-110016
In the present study, reliability assessment of Tension Leg Platform (TLP) tethers against maximum tension (i.e. tension exceeding yield) has been carried out under combined action of extreme wave and impulsive forces. For this purpose, a nonlinear dynamic analysis of TLP has been carried out in time domain. Using responses obtained after dynamic analysis under sinusoidal, half-triangular and triangular impulsive forces and employing Von-Mises theory of failure-based limit state function, reliability assessment of TLP tethers has been carried out. Effect of angle of impact of various impulsive forces on tether reliability has also been investigated on parametric basis.
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Experimental investigation on the take-off performance of a wig craft
J. Vimal Doss and R. Natarajan
Department of Ocean Engineering, Indian Institute of Technology Madras, Chennai – 600 036
Wing in Ground effect (WIG) craft is emerging as one of the most promising mode of transport in the 21 st century. The greatest hurdle faced by WIG craft designers is the assessment of the hump drag associated with the take-off mode of marine WIG crafts. During the initial stage of a WIG craft design, the take-off speed can, however be analytically determined using Savitsky's method. But, it is essential to conduct model tests to ascertain the take-off performance for obtaining the final hull form of the WIG craft. Hence, this paper deals with the model tests of a typical WIG craft conducted in the towing tank, at Department of Ocean Engineering, Indian Institute of Technology Madras, Chennai. The test results are analyzed and presented with discussions. The present study gives insight knowledge of the drag characteristics associated with take-off mode of a WIG craft and thereby suggests a suitable methodology to predict the take-off speed.
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RBSO of stiffened cylindrical shell with ring buckling mode of failure
V. Manorama
Department of Ocean Engineering, Indian Institute of Technology, Madras, Chennai-600036.
Optimization based on structural reliability is a relatively new research subject. It includes several uncertainty factors, such as the strength of the structure and applied loads, which are considered as random variables, and studies an optimum design procedure for minimizing the structural weight and thus satisfying the reliability requirement. The Reliability Based Structural Optimization (RBSO) of stiffened cylindrical shell with ring buckling mode of failure used as pressure hulls in submarines and in submersibles is described in this paper. The problem has been formulated via the Selective Active Constraint Technique with the buckling of the stiffener along with an effective width of the shell as the active constraint. The stiffener is modeled in a reduced basis, using the depth of the stiffener as the variable. The reliability-based optimization has been formulated as a minimum weight problem for a target reliability index, in terms of two variables viz., the depth of the stiffener and the thickness of the shell. The results of RBSO were plotted / tabulated for mean values of Young's modulus, radius of the ring stiffener, Reliability index, Optimal weight and Coefficient of Variation of loading, spacing of the stiffener, plating thickness and depth of the stiffener.
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Reliability of offshore jackets for different bracing configurations
M. Ramesh Babu and K. Rajagopalan
Department of Ocean Engineering, Indian Institute of Technology-Madras, Chennai - 600036.
We demonstrate a methodology for finding out which type of bracing is most reliable. The risk of failure was calculated for a typical model of Bombay high jacket in India subjected to environmental loading. Pushover analysis was used to evaluate the ultimate strength of the structure. The reliability of the structure is evaluated for different bracing patterns and then reliability per tonne of jacket weight is calculated and then compared.
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Dynamic behaviour of coupled OTEC platform -cold water pipe system
A. Venkata Subbaiah, V. Anantha Subramanian and R. Sundaravadivelu¹
Department of Ocean Engineering, Indian Institute of Technology, Madras, Chennai-600 036
Ocean Thermal Energy Conversion (OTEC) system uses thermal gradient in the ocean to generate electrical power. The sea water temperature on the surface layer is warm at 20 o -30 o while, it is cold at 2 o -7 o in the deep layer at about 1000 m depth. OTEC obtains the thermal energy associated with the temperature difference between the warm sea water on the surface layer and the cold seawater at greater depth. The spar platform is one of the best options for consideration of OTEC system in deep waters. By proper design, the platformmotions in stationary conditions can be minimized. In view of this, a 25 MW OTEC spar platform is analyzed for motions and the response of a Multiple Anchor Leg (MAL) moor under dynamic excited conditions at different levels are estimated.
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Offshore triangular TLP behavior under random sea using iterative frequency domain method
S.Chandrasekaran¹, A. K. Jain² and N. R. Chandak¹
¹Dept. of Civil Engg (Structures Group), Institute of Technology, Banaras Hindu University, Varanasi-221005
²Dept. of Civil Engg, Indian Institute of Technology, Hauz Khas, New Delhi-16
The present study deals with the investigation of response of Offshore Triangular TLP under random sea using Iterative Frequency Domain (IFD) method. The random wave has been synthetically generated considering a large number of linear small amplitude harmonic waves with randomly varying phase components. Wave forces are considered to be acting in the surge direction. Hydrodynamic forces are evaluated using modified Morison's equation with water particle kinematics evaluated using Airy's linear wave theory. Chakrabarti's nonlinear stretching approximation in evaluating the wave kinematics up to variable level of submergence is considered in the analysis. Two types of analyses are performed namely, (i) evaluation of PSDF and (ii) statistical analysis of the extrema of the structural response and the distribution of the response ranges. The chi-square tests are performed on the sample extreme values of the response to determine their fitness to Gumbel Type-I and two parameter Weibull distributions. Results show that the extrema of the displacements fit adequately well with the Gumbel Type-I distribution and that of the range of the extreme value of displacements with the Weibull distribution. The study was also carried out in time domain and the results thus obtained are compared with that obtained using Iterative Frequency Domain method. It is seen that both the set of results closely agree with each other in most of the hydrodynamic cases taken for the study.
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Application of geotextile and gabions for shore protection against erosion along south west coast of India
R. Sundaravadivelu, V. Sundar, N. Nagabhushan and K. Kamesh Arun
Dept. of Ocean Engineering, IIT Madras, Chennai-600036
A stretch of the coast along Tamil Nadu in the southwest of India for a length of about 60 km has been experiencing continuous erosion. One of the serious problems along this coast is the acceleration of erosion particularly during the southwest monsoon (May to September). During, the said season, the sea is rough and the waves break on the landmass resulting in significant erosion. To protect the coast from erosion, a seawall with geotextile as filter and gabions for the toe mound is contemplated as a possible coastal protection measure considering the environmental conditions. The seawall is proposed for 28 sites, which were severely affected. The details of the design, laying of the geotextile and gabion boxes along with the placing of the armour layers are discussed and reported in this paper.
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Strength characteristics of low calcium flyash concrete exposed to marine environment
Vijaykumar V* and S. Jayakumar
Ocean Engineering Division, National Institute of Oceanography, Dona Paula, Goa. *Software group, ITG, NIO, Goa.
Concrete is the most used construction material for Port and Harbour structures as well as for shore protection works. Due to its ability to resist weathering action, chemical attack and other processes of deterioration, it is an excellent material for coastal construction. Studies were carried out with replacement of cement with flyash at different percentages by weight and subjected to the marine environment. The experimental investigations were carried out on the strength characteristics from early ages, i.e., at 1, 7, 14 and 28 days and afterwards up to 30 months at intervals of 3 months. The results of the studies and the advantages of addition of flyash in concrete for marine applications in the strength characteristics are presented in this paper.
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Experimental study on wave slamming forces on horizontal circular members in intertidal zone
V. Hariprasad¹, S. Neelamani² and R. Sundaravadivelu
¹Ocean Engineering Department, Indian Institute of Technology Madras, Chennai–600036
²Coastal Engg. and Air Pollution Dept, Environmental and Urban Development Division, Kuwait Institute for Scientific Research, P.O. Box : 24885, 13109 SAFAT, KUWAIT
Wave forces on slender horizontal and vertical cylinders are normally estimated using Morison equation (1950). When a structural member is near the free water surface, it experiences the slamming force. Wave slamming on horizontal members of any ocean structure is crucial to its design. API Recommended Practice 2A-WSD (2000) recommends slamming coefficient C S equal to p for circular cross sectional members near the still water level. The shape of the member (circular, rectangular or square) is also expected to alter the values of C S . The horizontal cylinders in the inter-tidal zone for Port craft jetties are subjected not onlyto slamming force in the vertical direction and horizontal direction but also berthing force in the horizontal direction. If two cylinders are kept in close spacing, then the load on each members will be different compared to force on a single member kept alone. In this paper, the results of investigations of the effect of tidal variation on slamming forces are reported. The comparison of single circular member and twin circular members with c/c spacing varying from two to six times the diameter of cylindrical member in regular waves is also reported in this paper.
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Diffraction analysis of a semi-submersible platform
A.P. Shashikala
Assistant Professor, Department of Civil Engineering, National Institute of Technology, Calicut 673 601
Semisubmersibles are large floating structures used for drilling or production of oil and gas particularly suitable for deep-water conditions. The design of semisubmersibles requires an understanding of static and dynamic behavior of the structure due to waves. Usually, wave forces on the semisubmersibles are calculated based on Morison's approach considering the structural elements as slender. But, the modern semisubmersibles are very large structures in which it is necessary to take the diffraction effects into account such that the hydrodynamic forces can be estimated accurately. The present paper describes the evaluation of hydrodynamic forces using linear diffraction radiation theory. The diffracted and radiated potentials are calculated based on finite element approach. The coupled terms are determined which are neglected in the Morison's approach. The responses of the semi submersible are found to be slightly higher than that calculated using Morison's approach. The hydrodynamic forces obtained by this method give a better understanding of the interaction of the structure with the waves.
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Dynamic analysis of a taut mooring system
A. Umar¹, S.Q.A. Naqvi¹, Seema¹ and T.K. Datta²
¹Department of Civil Engineering, Aligarh Muslim University, Aligarh- 202002
²Department of Civil Engineering, Indian Institute of Technology, Delhi-110016
In the present study, the dynamic analysis of a multi-point taut moored buoy is performed under first and second order wave forces. The analysis is done using time domain integration scheme. Results are presented which illustrate the effects of the geometric nonlinearity of the mooring lines on the response of the buoy.
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Some durability studies on silicafume-admixed marine concrete mixes
Mattur C. Narasimhan, M. Nazeer, B. Ramesh, S.V. Rajeeva and K.R.C. Murthy
Department of Civil Engineering, National Institute of Technology Karnataka Suratkal, Srinivasnagar PO, Mangalore – 575025, Karnataka
Concrete is the widely used construction material for different types of marine structures, offering number of advantages. However, due to higher concentration of sulphate salts in seawater, concrete gets deteriorated and looses its strength and thus the service life of the structure is reduced. Reinforced concrete in marine environment undergoes severe problem of corrosion of embedded steel reinforcement due to higher concentration of chlorides in seawater, upto 20000 ppm. HPC mixes, incorporating mineral and chemical admixtures, are proved to be much durable than ordinary concrete. The present study investigates the performance of a few HPC mixes in sulphate and chloride solutions at ordinary temperature. Silicafume, a mineral admixture, is used as a partial replacement of cement in the preparation of the concrete test specimens. A representative HPC mix designed for a target compressive strength of 50 MPa is considered. Durability behavior of HPC mixes with partial replacement of cement by Silica fume with increasing percentages (up to 12%) has been investigated in detail. To make the mixes workable, appropriate dosages of a commercially available chemical admixture also have been used in the preparation of mixes.
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Submergence of Poompuhar – study based on underwater explorations and coastal processes
Sundaresh¹, S. Jayakumar¹, A.S. Gaur¹, P. Chandramohan² and B.K. Jena³
¹National Institute of Oceanography, Dona Paula, Goa.
²Indomer Coastal Hydraulics, Rangamalika Apartments26, Kumarana colony main road, Vadapalani, Chennai.
³NIOT, Pallikaranai, Velachery-Tambaram Main Road, Narayanpuram, Chennai
Poompuhar, a flourishing port in the beginning of Christian era played a major role in maritime activities and cultural expansion in the history of India. Sangam literature vividly describe about its location, habitation and the town planning of Poompuhar. The marine archaeological explorations around Poompuhar brought to light the remains of terracotta ring wells, brick structures, storage jars in the inter tidal zone and brick structures, stone structures, pottery from offshore explorations strongly supports thehabitation sites. There are several references suggesting the shift of shoreline at Poompuhar, which may be oneof the reasons of its submergence. The studies on waves were conducted in the region for a period of one year to understand the local phenomena responsible for the coastline changes. The paper deals with the archaeological findings around Poompuhar and the impact coastal process in the shifting of shoreline in the area, which was responsible for the submergence ancient port town and other monuments in the region. The data collected during underwater investigations at Poompuhar and the study of coastal processes are discussed to prove the causes of its submergence.
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