Showing papers by "Nilanjan Saha published in 2022"

Assessment of coastal vulnerability for extreme events

Abstract: The necessity to protect coastal regions from sea-level rise (SLR) and extreme events demands a rigorous assessment of coastal vulnerability. The intergovernmental panel for climate change (IPCC) predicts that climate change will severely impact the coastal region, river systems, and urban infrastructures. The changing climate is observed to be increasing the frequency and magnitude of extreme hydrometeorological conditions, such as storm surges that adversely affect coastal areas by flood inundation. This study emphasizes the need to use extreme events and socio-economic data to evaluate the Coastal Vulnerability Index (CVI) for planning and adaptation measures. Therefore, the present study utilizes spatially varying surge heights for improved estimation of the CVI, unlike the constant surge heights considered in the previous studies. For coastal adaptation measures, synthetic cyclones of different return periods, such as 20-, 50- and 100-year, are simulated using a hydrodynamic model to represent a range of risk and vulnerability in the estimated CVI. The proposed methodology for calculating the CVI is demonstrated by considering the Chennai coast in the state of Tamil Nadu, India. The use of spatial distribution of storm surges from hydrodynamic simulations makes the CVI more realistic. Such thorough assessments utilizing high-resolution CVI maps can help policymakers suggest appropriate measures for specific coastal zones, which are more devastatingly affected by shoreline erosion, SLR and storm surge. The vulnerability assessment indicates that great care is needed in the planning and adaptation of the coastal ecosystem to extreme events for the safety and well-being of coastal populations and infrastructures.

Response control of fixed offshore structure with wind turbine using MR damper

Abstract: Due to the depletion of fossil fuels, offshore wind turbines have become significant contributors to energy production. The offshore wind turbines are subjected to severe environmental loads like wind, wave, and current. The combined effect of these loads induces significant vibrations in the structure, which may cause economic loss and reduce the power production capacity. Thus, the structure is designed to have a small response under environmental load, but it also increases the cost. Another alternative to reduce the structure’s response is implementing structural control devices. MR dampers are semi-active devices widely used in controlling onshore structures because of their adaptability to various loads. Since the environmental loads on the wind turbine have a wide range of spectral content, semi-active control is best suited due to their adaptability. The force generated by the MR damper is enormous for small input energy, and thus it is an attractive option for adaptive structural control in civil structures. Several semi-active control algorithms are available in the literature for nonlinear MR dampers. The semi-active algorithm obtains the optimal control force of the linearized system and smartly switches off the MR damper according to the system requirement. The offshore structure is nonlinear, so a nonlinear control algorithm is expected to have advantage over other techniques. One of the commonly used nonlinear control algorithms is the backstepping method. This method involves a systematic development of the Lyapunov function for a given nonlinear system. The nonlinearity in the MR damper arises due to the hysteretic property of the MR fluid. Many phenomenological models are studied in the literature to capture MR dampers' nonlinearities effectively. Bouc wen model is one such model that is widely used for modeling MR dampers.The backstepping controller requires full state feedback to produce the control signal but measuring many variables is difficult in real-time. Some controllers do not require an accurate structure model, and the control action is based on the measured response. This paper uses the feed-forward neural network controller (NNC) to control nonlinear dynamic systems. The controller is trained using the displacement time histories of the structure to predict the MR damper voltage. The NNC is trained with the output from the backstepping controller for various environmental conditions. In this study, a fixed offshore wind turbine in a water depth of 64.5m is controlled using NNC and backstepping controllers. Further, the control efficiency of the NNC is compared for different load cases.

Foldable Torpedo Anchor: A Novel Anchoring System for Deepwater Floaters

Abstract: Ultra-deepwater energy resources are explored very little to till date. Depletion of oil and gas in shallow water depth made the exploration into the deep and ultra-deepwater resources. The offshore floating energy extractors like wind turbines, oscillating water column, offshore oil exploration production and drilling platforms often needs an efficient anchoring system. The development in offshore gas exploration and platform construction has made use of several anchor types like vertically loaded drag anchor (VLA), Omni-Max anchor, suction embedded plate anchor (SEPLA) and dynamically embedded plate anchor (DEPLA) to withstand heavy mooring loads. In the late 1990s, torpedo anchors were made, which were inexpensive and less time-consuming to install due to the simple installation technique. The anchors penetrate the target embedment depth by the kinetic energy acquired in the process of free fall, and the installed anchor is moored to the floating structure. Even though the torpedo anchors are able to withstand ample horizontal loads, in the case of vertical load-carrying ability, their performance is poor. This is one of the main reasons the concept has not become very popular in practice despite its simplicity. Hence, there is a need to improve the conventional anchor suffering from low vertical load-carrying ability. The objective of the present study is to depict the novel foldable torpedo anchor (FOTOAN) concept, design developments and installation mechanism. The FOTOAN is similar to the conventional torpedo anchors but comprises of foldable wings and fins which extend laterally to increase the pull-out resistance. Thus, the FOTOAN can be an efficient technique for deepwater anchoring applications.

Analysis of Spar with Floating Dock

Abstract: The installation of wind turbines at offshores is a complex and arduous task. The floating dock concept is the current method for installing an offshore wind turbine which is discussed in details in this paper. This paper investigates a floating dock that provides shielding to the spar foundation while installing a 5 MW wind turbine at the offshore site. The floating dock of 60 m inner diameter and 85 m length is analyzed using ANSYS AQWA software for calculating the Heave Response Amplitude Operator (RAO). Viscous damping is provided as an external damping matrix in the hydrodynamic diffraction analysis of ANSYS AQWA. It is noticed that viscous damping plays a major role in the analysis. The behavior of Spar with and without dock is analyzed. It can be observed that the floating dock helps in reducing the heave RAO of the Spar.

Design of Gabion Reinforced Railway Embankment Connecting Indian Ocean and Bay of Bengal

Abstract: The southeastern tip of Pamban Island, Dhanushkodi, is one of the unique attractions of Rameswaram. It is located around 20 km away from Rameswaram. The town of Dhanushkodi is surrounded by the Bay of Bengal on one side and the Indian Ocean on the other side. The railway line from Pamban Station to Dhanushkodi was destroyed in the 1964 cyclone, so by then, the rail transportation facility for Dhanushkodi was stopped. A new railway line and an embankment between Rameswaram to Dhanushkodi are proposed to increase better connectivity. The proposed railway line is highly environmentally sensitive to ocean waves and the geotechnical properties of soil. The conventional earthen embankment is not a viable solution due to the non-availability of usable soil nearby. At the same time, an elevated railway corridor is quite expensive. Considering the extreme waves, soil characteristics, and usable landmass, a Gabion reinforced railway embankment is proposed for a height of +8.5 m from Cart datum. This embankment is exposed to both the Indian Ocean and the Bay of Bengal region. Gabion reinforced embankments are designed to encounter the extreme wave forces on both facings as well such structures require minimal foundation width for feasible construction. Gabion reinforced embankments are economical, good in wave energy dissipation, quicker construction. Gabions with facia tails are designed for extreme wave forces, and the geotechnical stability of the same is also checked numerically. The combined hydrodynamic forces and monolithic stability of the structure are satisfactory.

Peridynamic Analysis of Floating Ice Under Transverse Pressure

Abstract: Offshore structures frequently interact with floating ice during its operational conditions in the arctic region. The structures are mainly designed in a slopping manner such that vertical loads are exerted, leading to out-of-plane failure of floating ice. In [1], the fracture pattern and failure process of ice floe with different dimensional conditions are studied through the classical approach. In the present study, a peridynamic approach has been adopted to analyze the fracture pattern of floating ice. Peridynamics [2] is a modified continuum mechanics that reformulates classical governing equations by replacing spatial derivatives with integrals. The main advantage of the novel approach is the incorporation of damage within governing equation. Unlike the classical method, it treats crack as an internal part of the body, eliminating the need for external kinetic relations to analyze crack growth. In [3], the peridynamic model of floating ice has been presented. Also, the crack growth pattern under transverse load with a pre-existing crack is studied. The current study validates the peridynamic model of floating ice, modeled as a peridynamic Mindlin plate [4] resting on Winkler elastic foundation [5] (representing sea surface), under static transverse pressure with classical theory. Then, the fracture pattern of the floating ice with and without pre-existing crack is studied and validated with the available literature.

Peridynamic Analysis of Offshore Jacket Structures

Abstract: Peridynamics [1] is a new continuum nonlocal theory which reformulates classical governing equation by replacing spatial derivatives with integrals. The motive behind the derivation of novel theory is to incorporate damage in governing equation, thereby treating crack as an integral part of the body. The present study focuses on validating peridynamic theory by implementing it on the OC4 jacket [15]. The beam formulation in the peridynamic framework is derived by Nguyen et al. [12] with all six degrees of freedom. By considering the jacket as a combination of beams, the static and dynamic response of the jacket under static and dynamic point loads are evaluated and compared with classical results.

Offshore Wind Turbine Support Structures Along Indian Coast - Multi Criteria Analysis

Abstract: The selection of a feasible support structure for the site-specific environmental conditions helps in reducing the capital cost of the offshore wind turbines. As the study location falls under shallow water depth, the bottom fixed structures such as monopile, tripile and jacket have been considered as alternatives. The analysis of the alternatives has been carried out using API and DNVGL codal provisions in a finite element based framework, accounting for hydrodynamic and soil structure interaction effects. OWTs are simultaneously subjected to aerodynamic loads from the turbine blade rotations, estimated using blade element momentum theory based software and are coupled analysis is carried out. Multi criteria decision analysis (MCDA) is adopted to aid in the decision making regarding the choice of the feasible support structure. Multi-criteria evaluation problem consists of a viable set of alternatives for a given problem. MCDA explicitly evaluates the conflicting criteria involved in the decision making. It helps analyzing a complex problem using a predefined set of variables/criteria to derive at a logical solution. Among the various methods of MCDA for evaluating the alternatives against the criteria, the Analytical Hierarchy Process (AHP) has been adopted. In this, the pairwise comparison is made for the given alternatives using predefined criterion. The criteria considered for the study are structural configuration, fabrication, transportation and installation of the structure. The criterion again are classified into different sub criteria in order to sort out the priority of all the alternatives. The support structure with the highest priority has been identified as the optimal and feasible structure for the location under study.

Effect of Monopile Installation on Nearby Breakwater

Abstract: Wind energy generation (both onshore and offshore) has gained wide popularity across the globe because of the emerging need for renewable energy. Monopile is a reliable foundation that can be adopted for wind turbines at deeper water depths, usually up to 25m. Monopiles are massive structures weighing around 1000t and 6m in diameter. The proximity of the port facility ensures better connectivity to the turbine location, helping faster construction and hence providing economic benefits to the wind farm project. This also safeguards and helps immediate repair the wind turbine, if necessary. The monopile located close to the port enables easy installation of the wind turbine and its sophisticated system. The main goal of this work is to study the effect of monopile installation on the breakwater located closer. The three-dimensional finite element software Plaxis 3D is used to model monopile and the adjacent breakwater using the soil and structural details obtained from the site. The water depth at the site is 20m. The influence of underlying soil is studied by conducting numerical analysis with varying soil strata- clay and sand. The soil-pile interaction is reproduced via contact elements. The soil properties are described via the nonlinear Mohr-Coulomb model. The steel pile having 6m diameter and 65mm thickness is driven by Hydrohammer S-2300. The 115t hammer can produce maximum blow energy of 2,300kNm when dropped at its maximum stroke of 2.018m. The hammer drop is set to 200mm height, and the frequency varies from 10 to 30 blows per minute. The breakwater that borders the port consists of the inner core and outer armour layers. The inner core is made of quarry run with a weight ranging 10-500kg, and the armour layer consists of rock weighing 1-3t. The core and armour layer permits the lateral flow of water and is created in the numerical model using a drained Mohr-Coulomb model with horizontal permeability. The safe distance of monopile is determined by placing it at 10m from the breakwater, and consecutive monopiles are placed at 10m spacing each. The vibration on the nearby breakwater is analyzed using the peak particle velocities (PPV) along with the three directions. The peak vibration at the breakwater toe closer to the monopile, top of the breakwater, and toe away from the monopile is measured simultaneous to pile driving. The vertical PPV is found to dominate and hence plays a significant role in determining the optimum hammer loading. The study is conducted with varying monopile distance and hammer frequency, ensuring the vibration limit suggested by the British standard (BS 7385-2:1993) is not exceeded. The distance from the breakwater is varied as 10, 20,30,40&50m and the safe distance to restrict the vibration at the breakwater is within the limit suggested by the Indian standard DGMS (Circular, 1997) is reported.