Now showing 1 - 10 of 19
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    Hydrodynamic behavior of truss pontoon mobile offshore base platform
    (01-01-2016)
    Sakthivel, Somansundar
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    Srinivasan, Nagan
    Very Large Floating Structures (VLFS) are highly specialized floating structures with variety of applications ranging from airport strips to floating motels offshore ports etc. Their economic design is based on their hydro-elastic behavior due to wave environmental forces. VLFS are extra large in size and mostly extra long in span and for that reason they are mostly modularized into several smaller structures and integrated. VLFSs may be classified into two broad categories, namely the semi-submersible type and the pontoon-type. The former type of VLFSs having their platform raised above the sea level and supported by columns resting on submerged pontoons and can minimize the effects of wave actions. In open sea, where the wave heights are relatively large, the semi-submersible VLFSs are preferred. On the other hand, the pontoon-type VLFS is a simple flat box structure floating on the sea surface. It is very flexible compared to other kinds of offshore structures, and so its elastic deformations are more important than their rigid body motions. The critical problem is the longitudinal bending moment of the long floating vessel in waves/current environment. Most of the present available VLFS designs are not economical for applications in hostile ocean. This paper presents hydrodynamic analysis carried out on an innovative VLFS called truss pontoon Mobile Offshore Base (MOB) platform concept proposed by Srinivasan [1]. The concept uses a strong deck with strong longitudinal beams to take care of the needed bending moment of the vessel for the survival, standby and operational conditions of the wave. At the submerged bottom just above the keel-tank top, a simple open-frame trussstructure is used instead of a heavy shell type pontoon. Thus the truss-pontoon provides the necessary flow transparency for the reduction of the wave exciting forces and consequently the heave motions and the vertical acceleration. Numerical analysis of truss pontoon MOB platform is carried out using HYDroelastic Response ANalysis (HYDRAN). Responses of the isolated scaled module in waves are obtained from these numerical tools and compared with published literature. Unconnected two modules and three modules are analysed using HYDRAN and the responses are compared with the isolated module. The proposed concept yielded lesser responses as compared to semisubmersible conventional MOB platform.
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    Dynamic analysis of a J-lay pipeline
    (01-01-2015)
    Senthil, B.
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    Oil and gas reserves are moving towards deeper waters day by day and it has become increasingly important to construct structures and subsea pipelines in deeper waters to transport the hydrocarbons for the users. The J-lay technique has become a better alternative to the conventional S-lay technique for installing subsea pipelines in deep waters. Here the pipeline leaves the vessel in a near vertical position rather than the horizontal position and acquires the J-shape as it reaches the seabed. This method offers several advantages over the conventional S-lay method such as minimal bending and reduced suspended length of pipeline leading to reduced tension and reduced thruster power requirement, precise pipeline positioning and better vessel control.This paper considers a simplified J-lay pipeline numerical modelanalysed using ORCAFLEX. The model consists of 0.6 m diameter steel pipeline being laid at a water depth of 2000 m. Dynamic responses namely effective tension, bending moment and maximum von-Mises stress of the pipeline are studied under the action of waves with and without vessel interaction and under the combined action of waves and currents with vessel interaction. Vessel interaction and presence of currents induces additional stresses in the pipeline being laid and the increase in the maximum values of effective tension, bending moment and maximum von-Mises stress due to the dynamic effects is observed as 36%, 64% and 47.7% respectively.
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    Time and frequency domain analysis of self installing mono column wind float during operational phase
    (01-01-2015)
    Ramayan, Utkarsh
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    Srinivasan, Nagan
    Offshore wind power is relatively a new emergent and is proving to be a better and superior alternative for producing carbon neutral energy. However the huge investments mainly in the form of capital (CAPEX) and operational expenditure (OPEX) casts a great doubt over the economic feasibility and viability towards the expansion of offshore wind power into deep waters. The present paper proposes a novel floating substructure designed and conceived by Nagan Srinivasan, incorporating the transportation and self-installation capability integrated within the primary design itself. This reduces the total investment incurred by reducing the CAPEX involved and hence sustains the planned and well documented growth of floating offshore wind farmsto deeper waters.
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    Parameter identification of a large moored floating body in random ocean waves by reverse miso method
    Dynamics of a large moored floating body in ocean waves involves frequency dependent added mass and radiation damping as well as the linear and nonlinear mooring line characteristics. Usually, the added mass and radiation damping matrices can be estimated either by potential theory-based calculations or by experiments. The nonlinear mooring line properties (usually cubic nonlinearity characterised by a constant) are usually quantified by experimental methods. In this paper, we attempt to use a nonlinear system identification approach, specifically the Reverse Multiple Inputs-Single Output (R-MISO) method, to a single degree of freedom system with linear and cubic nonlinear stiffnesses. The system mass is split into a frequency independent and a frequency dependent component and its damping is frequency dependent. This can serve as a model of a moored floating system with a dominant motion associated with the nonlinear stiffness. The wave diffraction force, the excitation to the system, is assumed known. This can either be calculated or obtained from experiments. For numerical illustration, the case of floating semi- ellipsoid is adopted with dominant sway motion. The motion as well as the loading are simulated assuming PM spectrum and these results have been analysed by the R-MISO method, yielding the frequency dependent added mass and radiation damping, linear as well as the nonlinear stiffness coefficients quite satisfactorily.
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    Analysis of deep sea umbilical in steep wave configuration
    (01-01-2016)
    Nair, Akash A.
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    Anbu, Gnanaraj A.
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    Kuttikrishnan, Gopakumar
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    Ananda, Ramadass Gidugu
    Deep sea mining is mineral retrieval process that takes place on the ocean floor wherein global industries are actively exploring and experimenting of different techniques in this relatively new concept of mining for extracting it economically from depths of 5000-5500 m below the ocean's surface. National Institute of Ocean Technology (NIOT), India has been working on a mining concept for ~6000 m water depth where a crawler based mining machine collects, crushes and pumps nodules to the mother ship using a positive displacement pump through a flexible riser (umbilical) system. The umbilical also serve as the weight supporting member for the miner and pump. In this paper, static and dynamic analysis of the umbilical system in steep wave configuration and the miner is carried out using ORCAFLEX for launching and touchdown conditions. Three different materials are considered and the best suitable material for umbilical is selected as the first step based on the tension. Then umbilical with Single Miner System is analyzed for the launching and touchdown conditions. Based on the analysis the optimum number and spacing of buoyancy tanks that will keep the stresses within the allowable limits in the umbilical cable are recommended.
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    Sea trials of a water jet propelled high speed craft
    (01-01-2014)
    RadhaKrishna, K. O.S.R.Ravisekhar
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    This paper presents the details of sea trials conducted on a water jet propelled high speed craft. This ship is an Inshore Patrol Vessel fitted with interceptors. The trials were conducted to establish the speed, manoeuvring performance and endurance of the ship. The speed trials were conducted with interceptors and without interceptors and the dynamic trim of the vessel was recorded along with speed. The interceptors contributed to a considerable increase of speed by dynamic reorientation of the ship. Under manoeuvring performance, the turning circle manoeuvre and the crash stop manoeuvre were conducted to establish the turning and stopping characteristics of the ship respectively. The turning characteristics of the ship were established by steering the jet to 30 degrees and completing one full turning circle at maximum continuous rating of the engine. The stopping characteristics were established by conducting crash stop manoeuvre (ahead to astern). For reversing the ship's direction of motion the water jet was reversed by deploying the buckets. The performance of the deck machinery, endurance of engines was also established during the sea trials. The obtained values from various sea trials are presented and compared with the recommendations of the standards.
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    Hydrodynamic analysis of self-installing mono column wind float during transition phase
    (01-01-2014)
    Ramayan, Utkarsh
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    Srinivasan, Nagan
    Offshore wind energy has emerged as a major frontline competitor for producing sustainable and carbon free energy. With the recent advancement in engineering the researchers and developers are challenging the conventional wisdom of existing technology to improve the economics of wind power. Offshore wind turbines are rated in megawatts (MW) and costs approximately $1.5 million per MW for an installed capacity of 5 MW. On an average basis 5% to 15 % of this total capital investment goes into transportation and installation which is basically due to high day rates of jack up barges and other dynamic position vessels. An innovative floater conceived and designed by Nagan Srinivasan wherein the concept of self installation is introduced has been undertaken for hydrodynamic analysis during the transition phase in this study. Analysis has been carried out for different drafts and statistics of responses summarized for seastate 5.
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    Hydrodynamic analysis of Tension Based Tension Leg Platform
    (01-12-2012)
    Bhaskara Rao, D. S.
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    Srinivasan, Nagan
    Tension Leg Platforms (TLPs) are one of the best options for offshore industry in deep waters due to proven motion response characteristics. These are water depth sensitive structures and the motion responses in vertical plane motions (heave, roll and pitch) are critical for a TLP. Tension Based TLP (TBTLP) is a new concept and finds application in much deeper waters. A provision of a tension base at mid-depth results in an economical design of TLP. In fact, the TLP installed at a certain depth without any modifications can be made to be deployed in much deeper water depths by means of a tension base. In this paper, the concept of TBTLP is highlighted and hydrodynamic analysis of the chosen platform has been carried out using ANSYS AQWA package. The motion responses in terms of Response Amplitude Operators (RAOs) of TBTLP with one Tension Base in surge, heave and pitch have been obtained and compared with a TLP without a tension base. Copyright © 2012 by ASME.
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    Static and dynamic analysis of jacket substructure for offshore fixed wind turbines
    (01-01-2013)
    Ashish, C. B.
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    With the increase in demand of offshore wind energy, the need for offshore structures for supporting wind turbine is increasing. These structures can be floating and moored to the seabed or a fixed structure with its foundation on the seabed. In this paper, a jacket for supporting 5 MW NREL wind turbine is designed and analyzed for a site selected is in the Indian sea near Rameshwaram, in Tamilnadu.. The structure is designed based on the procedure followed in oil and gas industry which conforms to the API-RP2A WSD (2007). The structure includes the jacket, transition piece and the tower. The structure is modeled in SESAM Genie within which the static structural analysis is carried out. Dynamic analysis is carried out in USFOS for combination of wind and wave loadings. The preliminary analysis and design carried out meets the requirements specified in the codes API RP 2A and DNV-OS-J101.
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    Hydrodynamic responses of a single hinged and double hinged articulated towers
    (01-01-2015)
    Bithin, Ghorai
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    Numerical modelling and hydrodynamic analysis of singlehinged and double-hinged articulated tower are carried out. A single-hinged articulated tower (SHAT) is designed and modelled in AQWA and response in the time domain and frequency domain for the selected degrees of freedom has been studied. For double-hinged articulated tower (DHAT) an intermediate hinge is introduced and the response of the same is analysed. A comparative study between SHAT and DHAT response is carried out through the work. Also the influence of buoyancy chamber size on SHAT response has been investigated in the work. The entire analysis is done assuming the body is behaving rigidly. From the analysis, it is concluded that the maximum tip displacement of doublehinged articulated tower (DHAT) is increased by 40% compared to single-hinged articulated tower (SHAT) in random waves. The maximum tip displacement of doublehinged articulated tower increases by 2.3 times if the position of the intermediate hinge is raised by 5% (11 m above from the primary position). The natural frequency of SHAT increases by 22% and maximum hinge rotation by 14% if the outer diameter of buoyancy chamber is increased by 20% from primary dimension although in terms of its numerical value these are fairly small.