R Panner Selvam

Tension Leg Platforms (TLPs) are one of the reliable structures in the offshore industry in deep waters because of its motion characteristics in heave, roll and pitch degrees of freedom (dof). Heave motion is very important in offshore facilities and have to be kept as minimum as possible. As the water depth increases TLPs suffers from some limitations and hence has to be modified to cater to deeper waters. One such concept proposed is Tension Based Tension Leg Platform (TBTLP). In this paper, experimental investigations carried out on a 1:150 scaled model of a Tension Based Tension Leg Platform in regular waves in 3 m water depth is reported. These are the first ever experiments which were carried out on a scaled model of the new concept. To investigate the effects of Tension Base, experiments were also conducted on the TLP (without Tension Base). Responses have been compared in terms of Response Amplitude Operators (RAOs) for surge, heave and pitch dof for TBTLP and TLP. Numerical modeling of the TLP and TBTLP responses using ANSYS® AQWA™ software is included as well for comparisons.

India has one of the fastest growing economies in the world and has an increasing energy demand, which is expected to double in 2020 compared to the present demand. Wind energy has gained wide acceptance across the globe and presently the focus is toward development of offshore wind farms. The offshore wind farm technology faces a number of technical challenges due to the harsh installation and operation conditions. Foundations supporting offshore wind turbines/wind data collection platforms are subject to constant wave loads. Offshore work involves increased risks of strong winds which affect the amount of time available for installation and maintenance which in turn influence capital and operation costs. Hence, this work is focused on development and analysis of economic and safe installation methodology in high tidal and current environment. A monopile has been designed suitable for high tidal environment at Gulf of Khambhat and Gulf of Kutch, Gujarat. Monopile static analysis, pile–soil interaction studies, and free vibration analyses have been carried out using finite element method. Developed safe and economic installation methodology through detailed lowering analysis for monopile in regular and irregular wave conditions and recommended appropriate vessel with hydraulic gripper as attachment to restrict the lateral displacements.

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.

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.

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 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 with and without noise assuming PM spectrum and these results have been analyzed by the R-MISO method, yielding the frequency dependent added mass and radiation damping, linear as well as the nonlinear stiffness coefficients quite satisfactorily.

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.

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 are usually quantified by experimental methods. In this paper, we attempt to use a nonlinear system identification approach, speciakally the reverse multiple input-single output (R-MISO) method, to coupled surge-pitch response (two-degrees-of-freedom) of a large floating system in random ocean waves with linear and cubic nonlinear mooring line stiffnesses. The system mass matrix has both frequency independent and frequency dependent components whereas its damping matrix has only frequency dependent components. The excitation force and moment due to linear monochromatic waves which act on the system are assumed to be known that can either be calculated or obtained from experiments. For numerical illustration, a floating half-spheroid is adopted. The motion as well as the loading are simulated assuming Pierson-Moskowitz (PM) spectrum and these results have been analyzed by the R-MISO method yielding frequency dependent coupled added mass and radiation damping coefficients, as well as linear and nonlinear stiffness coefficients of mooring lines satisfactorily. Copyright © 2006 by ASME.

This paper examines the results in case of breaking wave groups reported in Sriram et al. (2015) against a wave breaking model called Tian–Barthelemy model (Seiffert and Ducrozet, 2017). In this paper, Tian–Barthelemy model is extended to incorporate (Schäffer, 1996) corrections to linear wave making signals for generating focused wave groups and implemented in an open source computer code for high order spectral (HOS) method based numerical wave tank (NWT) viz., HOS-NWT. The developed model is then studied in the context of wave breaking in dispersively focused intermediate water wave groups with particular focus on multiple wave breaking events. The adapted model performs well in conformity with experimental observations in Sriram et al. (2015). A range of kinematic thresholds are studied with this model to calibrate the right threshold for the model against the experiments. We find strong evidence of a non-breaking wave group in intermediate depths crossing the kinematic threshold of 0.85 (stated in Seiffert and Ducrozet (2017)), implying that this threshold is not universal for intermediate depths. It is also shown that the model does not interfere with the spurious free wave suppression. However, various dynamic and kinematical aspects of this study reveal that suppression of spurious free wave makes the short waves dominant in the lead up to breaking. Hence, the model detects numerous threshold exceedences implying plausibly more wave breaking instances. It is also shown that this model in general violates the spectral signature of breaking in a dispersively focused wave.

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.

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.