S. Surendran

Numerical simulation of ship stability for dynamic environment

Abstract The prediction of ship stability during the early stages of design is very important from the point of vessel’s safety. Out of the six motions of a ship, the critical motion leading to capsize of a vessel is the rolling motion. In the present study, particular attention is paid to the performance of a ship in beam sea. The linear ship response in waves is evaluated using strip theory. Critical condition in the rolling motion of a ship is when it is subjected to synchronous beam waves. In this paper, a nonlinear approach has been tried to predict the roll response of a vessel. Various representations of damping and restoring terms found in the literature are investigated. A parametric investigation is undertaken to identify the effect of a number of key parameters like wave amplitude, wave frequency, metacentric height, etc.

Reduction in the dynamic amplitudes of moored cable systems

Cable-body systems are intelligent and cost-effective structures used in ocean engineering and also for many oceanographic missions. The demand for such systems is increasing steadily because of new types of systems like wind farms, ocean energy converters and floating airports. Mooring of such platforms to a fixed point on the earth makes the system secure. Dynamic behaviour of a moored system with a subsurface buoy is studied here. The optimum position of the buoy is found using an analytical approach. The location of the buoy along the cable is a critical aspect in tension reduction. An analytical method is developed for the solution of a typical mooring problem taking into account the presence of a subsurface buoy. The method originally proposed by Walten and Polachek (1959) is modified here to suit the present problem. The nodal accelerations and velocities at the points of lumped mass of subsequent time steps can be expressed as per the Houbolt scheme. Integration of the accelerations and velocities lead to the solutions of the mooring-line dynamic problems subject to constraints to achieve the numerical convergence of solution. Dynamic amplitudes vary with the frequency of excitation at the surface due to external disturbances like waves and current. Wave, current and water depths are considered for the dynamic load built up along the mooring line. A number of cases are worked out, and some important and significant results are presented in this paper. Dynamic tension variations with and without a subsurface buoy are determined. Effect of pre-tension is determined and presented. Dynamic amplitude for different frequency ratios and different amplitudes of motions are determined. The method is useful for deepwater systems in which tension reduction provides better efficiency for the performance of the anchor by providing better seakeeping ability in the case of a ship or a platform.

Dynamic fracture toughness of aluminium 6063 with multilayer composite patching at lower temperatures

Aluminium alloys provide many benefits and challenges for use in marine environment. Attention on lightweight hulls leads to power saving, better handling and easy transportation. The stiffness and mass of the structure or member is responsible for its natural vibration. The hull panels and stiffeners vibrate at different frequencies when their stiffness is reduced by formation of a crack or any structural problems. If there is a monitoring system on board and by using a sensor detecting the modified vibrations, the maintenance crew senses the spot where urgent repair is needed. Application of composite laminates is convenient in view of portability and simplicity in procedure such as hand lay-up. In this study, the edge-cracked aluminium plates were repaired with one-sided composite patches and their temperature were varied to both low and high temperature levels. Dynamic fracture toughness was determined using Charpy impact test. Three different types of materials were used for composite patching: GFRP (glass-fibre-reinforced plastic), CFRP (carbon-fibre-reinforced plastic) and Kevlar. For each temperature level, two lay-ups were used, viz. three layered and five layered. Aluminium specimens with edge cracks of two different lengths, keeping the width of the crack constant, were prepared. Impact tests were performed for the prepared specimens at different temperatures. The fracture toughness of aluminium strips reinforced with patches was determined and compared with the energy absorbed during the impact of the corresponding specimen without the patch. It was observed that the specimen with patches gave a higher fracture toughness value when compared with that without the patch; corresponding observation on aluminium has been made along with patch at these temperatures. The patch using Kevlar was found to be the best among the three composites. Followed by these experiments, simulations using FRANC 2D software were carried out for finding stress intensity factor at different crack lengths with and without patches. The research has a spin-off application in the selection of materials used for patching and hence in the maintenance of LPG (liquefied petroleum gas) and LNG (liquefied natural gas) storage tanks under cryogenic conditions.

Design and analysis of composite panel for impact loads in marine environment

Impact loads from incoming waves always challenge the strength of structural parts of a ship. Modern high-speed crafts including naval vessels are subjected to impact loads during high speed and sudden manoeuvre. Wave slamming may damage structural parts and cause flooding of compartments due to impact. Hydrodynamic impact load results in structural damage in conventional marine vehicles. Loads during cargo loading, unloading, shipping of green water, impact with icebergs etc. come under low-velocity impact loads. The structural parts of a ship can also be damaged due to the impact of collision with icebergs as in the case of the Titanic accident. Weapon discharge, flight operation, bulk cargo operation and collision with other structures are other examples of impact load acting on marine vehicles. Severe hydrodynamic impact load may cause damage of shell plating, collapse of bow part, damage of hatch covers, collapse of hull girder etc. Such structural parts should possess sufficient impact strength. Composite shells can be used for hull, deck or hatch covers of bulk carriers to reduce the weight of vehicle and to indirectly increase the pay load capacity. In the present work, cost-effective composites are prepared by adding fillers to improve impact strengths and tested to verify the same. The main objective of the study is to develop a cost-effective composite to be used as part of hull, deck and hatch cover of bulk carrier. Feasibility and selection of fillers to improve impact strength are established. Prior to this, different sizes of fillers and the chemical treatment of filler with acid were considered. Calcium carbonate (CaCO3), silicon carbide (SiC) and alumina (Al2O3) are considered with glass epoxy composite for the above analysis. The additional impact strength due to the application of fillers is determined using Izod impact strength tests. Contact force is considered as a scale parameter for the impact response for composite laminates. Contact force is the force against impact load on composite structures. Contact force and deflection history of composite panels are affected by a number of parameters such as fibres angle, laminate geometry, impactor energy, stiffeners, surface area of impactor and loading eccentricity. Effects of all possible parameters are included and their influence on the response is determined using the well-known package based on finite element method (FEM), ABAQUS. The FEM analysis of Khalili et al. (2011 Compos Struct 93:1363–1375) is used for comparison of results of the present numerical analysis. The study leads to many new insights to designers of cost-effective composite structures.

Computer and experimental simulations on the fin effect on ship resistance

Container ships move at a higher speed compared to other merchant ship types. A fin attached to the ship hull proves to be more efficient in controlling the moving ship. However, such attachments on the naked hull attract additional problems such as slamming, requirement of continuous maintenance, etc. The main objective of this study is to find the influence of fin action at various angles of attack with the incoming flow and recommend the best possible fin position for the least resistance. While experiments were performed for different angles of attacks of the fin with respect to the flow, a reduction in resistance was observed for an Fn range of 0.13–0.26. The fin was fitted at the lowest possible location of the hull surface at the bow part of the ship. Experimental investigation was done using model tests in a towing tank to determine the resistance of a scaled down model and it was compared with computer simulation. The interaction of a bow fin fitted to a container ship with its own generated and encountering waves are discussed in this paper. It was observed that at certain angles of attack of the fins favourable resistance characteristics were observed. Modifications from the expected resistance due to fin effect are paid attention in this study. Various resistance values for different angles of attack of the fin were compared and an angle of attack of 5° is found to be the best.

Handling and launching of heavy concrete caissons in a marine environment

Handling of heavy structures demand unusual crane capacity. Availability of such cranes may be limited in developing countries. An innovative way to construct and launch heavy concrete structures in a developing country, where crane availability is rare, to lift, transport and handle heavy structures, is discussed in this paper. Heavy caissons are used in the marine environment as breakwaters, as berthing facilities for vessels, as gravity structures to absorb energy from waves, etc. The methodology for constructing multi-caissons on-board a barge and then on-board a floating dock is the subject of this paper. The respective feasibilities are discussed.The free floating condition of the caisson and its righting arm stability are described. At first the caisson was considered as its lip wall opened to the sea. The case of lip wall closed for more buoyancy is also considered. The stability of the barge and floating docks is shown for various stages of completion of construction. After investigating many factors, a safe procedure for construction and launching thereafter is described in this paper. The problem of handling a heavy concrete structure from the view point of a naval architect is highlighted.

Fish induced yaw

Abstract Cable-body systems for underwater purposes are widely used in many occasions. The problem of fish induced yaw and that due to bare cable on a tow system is subjected to analysis. A remedy to get rid of the fish induced yaw is done and satisfactory results are obtained. The data presented give some idea to attack the problem of yaw of the total system in a sea operation.

Numerical and experimental study on varying cross-section of moonpool for a drill ship

ABSTRACT The mass of water in the moonpool of a drill ship will be in linear, angular, and coupled motions influencing the moving ship. The water mass in the moonpool is subjected to sloshing and piston modes. These modes of movement of water mass sometimes lead to a larger quantity of green water on the weather deck causing downtime of the vessel in critical conditions. This paper discusses experimental and numerical study on moonpool with different cut-out angles facing the head on side of incoming waves. The study also aims at minimizing the water motion inside the moonpool under the response to the incident waves. It is concluded that the moonpool with cut-out angle configuration reduces the free surface elevation inside it in a considerable level. With the presence of moonpool on board ship, it is inferred that there is modification in damping, inertial and restoring components of loads and moments.