R. Sundaravadivelu

Mooring forces and motion responses of pontoon-type floating breakwaters

The experimental and theoretical investigations on the behaviour of pontoon-type floating breakwaters are presented. A two-dimensional finite element model is adopted to study the behaviour of pontoon-type floating breakwaters in beam waves. The stiffness coefficients of the slack mooring lines are idealized as the linear stiffness coefficients, which can be derived from the basic catenary equations of the cable. The theoretical model is supported by an experimental programme conducted in a wave flume. The motion responses and mooring forces are measured for three different mooring configurations, and the results are reported and discussed in detail in this paper. The wave attenuation characteristics are presented for the configurations studied.

Diffraction–radiation of multiple floating structures in directional waves

Abstract The dynamics of multiple floating structures have been studied using the finite element method. The emphasis is on the hydrodynamic behaviour of multiple bodies under a multi-directional wave field. A two-dimensional numerical model has been adopted to evaluate hydrodynamic coefficients and forces in an oblique wave field. The responses in sway, heave and roll modes are reported. The linear filter technique is then used to extrapolate the responses under directional waves. The effect of mean wave direction and directional homogeneity on the hydrodynamic behaviour of the structure is studied. Based on the present study, it is found that the two-dimensional model is applicable to investigate the wave-structure interaction problems of the type herein considered.

Dynamic pressures and forces exerted on impermeable and seaside perforated semicircular breakwaters due to regular waves

The semicircular breakwater (SBW) is a composite breakwater consisting of a semicircular caisson resting on a rubble mound. The SBW function as a barrier dissipates the incident wave energy and creates tranquillity on its leeside. The dynamic pressures due to regular waves exerted on seaside perforated SBWs with 7 and 11% of exposed surface area with perforations were measured. The measured pressures are compared with those exerted on impermeable SBWs. In addition, the forces exerted on the caisson alone are measured. The reflection coefficient, measured total forces on the caisson of the models, and the pressures are presented as functions of relative water depth. The effect of the water depth and the percentage of perforations on the above stated variables are examined, details of which are reported in this paper.

Dynamics of a moored barge under regular and random waves

Developments in the study of wave forces and construction techniques in deep water by the offshore oil industry have increased the use of marine terminals at deep water locations. A thorough understanding of moored ship dynamics when subjected to waves, wind and current combined with the use of flexible mooring lines would help to design berthing terminals for exposed areas. In this paper, the three dimensional problem of wave interactions with a barge moored to a single point is dealt with, based on the finite element method. The effect of flexibility of the mooring line and the point of mooring on the response of the barge as well as the mooring line tension is investigated. The paper compares the numerical results with model tests carried out on a barge moored to a fixed support under regular and random waves in head sea. The effect of stiffness of the mooring line on the barge response for different mooring points is discussed, which would be useful for the designers. The effect of viscous damping is also considered. The analytical results are in good agreement with the experimental results in both regular and random waves.

A Novel Setup for Testing of Ship Deck Stiffened Panels under Axial and Lateral Loads

A novel setup for testing of stiffened panels in ship decks subjected to axial and lateral loads is developed in the Department of Ocean Engineering, IITMadras. The entire setup consists of self-straining test rig, imperfection measuring device, displacement controlled hydraulic jacks, rigid grillage, inflatable air balloon with and without opening, and a data acquisition system. The fabricated deformable test rig replicates similar boundary conditions along the loaded and unloaded edges of stiffened panels in between transverse stiffeners of a ship deck. An imperfection measuring device located on the test rig is designed and fabricated to measure geometrical initial imperfections on the plate and stiffener. A unique displacement controlled twin hydraulic jack system is developed to apply axial load on the panel. A rigid grillage connected to the bottom of the test rig to act as reactive support for the application of lateral load on the panel is fabricated by performing static nonlinear analysis using ANSYS. An inflatable air balloon with and without opening is fabricated to simulate constant cargo (lateral) load acting on the panels. The applied axial load and produced axial deformation measured by load cells and LVDT, respectively, is plotted simultaneously while testing using an integrated data acquisition system. Nonlinear finite element analysis of tested specimens is performed using ANSYS to compare the ultimate load obtained from experiments. A close agreement between the experimental data and finite element analysis results indicates realistic simulation of truly combined axial and lateral loads with deformable supports acting on stiffened panels as in ship decks achieved in the laboratory and numerically.

Hydrodynamics of a SPAR-Type FPSO Concept for Application as a Production Platform

A state of the art non-ship-shaped FPSO vessel for the applications to the frontier fields, including arctic, deepwater and harsh environment, is introduced in this paper. Its application towards arctic field is described more elaborately. The need for such a vessel and its feasibility are emphasized herein. As a first phase of the study, extensive physical large scale model test is performed with dis-connectable turret mooring for application in the arctic summer environment in shallow water. The arctic environment with ice conditions require turret mooring system for the vessel with the capability that the platform is removable in emergency conditions e.g., approaching icebergs. Here, the motion behavior of the vessel in waves with 100% turret mooring is determined physically in the tank test. For wave conditions in another field location in the world with clear water, the vessel is also needed to be designed with a 100% conventional mooring system. Hence, the effect of different kinds of mooring system, including 50% vessel and 50% turret mooring, on the vessel behavior is studied. Key results on the comparison of test vs. analysis results are presented in this paper. These results are considered of immense interest and add value to the Oil and Gas industry.Copyright

Ocean Space Utilization Using Very Large Floating Semi-Submersible

Very Large Floating Structures (VLFS) are highly specialized floating structures with variety of applications in the civil engineering of ocean. 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 in the design feature for their applications. For that reason they are mostly modularized into several smaller structures and joined together in the site. The critical problem is the longitudinal bending moment of the long floating vessel in severe wave environment. With the result of that the present available VLFS designs become not economical for applications in hostile-ocean.This paper presents ocean space utilization using an innovative VLFS with truss pontoon concept. 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 ocean environment. At the submerged bottom just above the keel-tank top, a simple open-frame truss-structure is used instead of a heavy shell type pontoon. The truss-pontoon provides the necessary flow transparency for the reduction of the wave exciting forces and consequently reduces heave amplitude of motions and the vertical acceleration. Each individual columns of the truss pontoon semi-submersible is tuned to have heave-period over 22 sec, independently, such that minimum hydrodynamic-motions are obtained for the overall structure.The VLFS is designed with minimum heave for the extreme storms unlike the conventional column stabilized semi-submersible unit with conventional pontoon. The paper proposes a new VLFS concept which is feasible for applications in harsh environment. Most importantly cost effective VLFS is achieved. This paper presents the details of the VLFS design, stability, motion, and experimental verification from the physical wave-tank with the scaled-down model. At the end of the paper, a few comprehensive example applications are illustrated.Copyright

Transportation Analysis of Dry Tree Semisubmersible

With the advancement of oil & gas into ultra deepwater, the need for drilling and production platforms has becomes more acute. The dry tree semisubmersible can be used for direct vertical access into reservoirs from deepwater since it offers small in-place motions, large open deck areas, dockside commissioning and minimum offshore hookup. The direct access allows the operator to drill, complete and work over the well directly from the same platform. The drytree semisubmersible consists of a keel tank which can be telescoped up and down based on the requirement of the platform. The keel tank is fully telescoped down while in operating condition and telescoped up while in transportation condition. The transportation analysis of a drytree semisubmersible using the linear radiation, diffraction panel program WAMIT is presented in this paper. Parametric studies were carried out in WAMIT by varying the size of the keel tank by considering the keel tank size as 80X80m, 91.5X91.5m and 100X100m. The Case 2 in which the keel tank dimensions were 91.5×91.5 m was found to be most appropriate of all, as the response for it was lower compared to other cases and also the structure was good from stability point of view. It was found that the heave and response were high only beyond 15 seconds wave period thus making transportation operations safe, as transportation is carried out in sea states 5 and 6 which have wave periods less than 14 seconds. Also the pitch RAO was found to be very low thus posing no threat in transportation mode.

ULTIMATE STRENGTH OF STEEL AND GLASS FIBRE REINFORCED COMPOSITE PLATES WITH SQUARE OPENING UNDER AXIAL AND OUT OF PLANE LOADS

The weight of glass fiber reinforced polymer composite (GFRP) plate is about one fourth of the steel plate and can be used in ship and offshore structures, so that the payload can be increased. However comparative studies on the behaviour of steel and GFRP composite plates with square opening have not been studied in detail. The experimental studies on steel and GFRP plates with and without openings are carried out for the combined loading of axial compression and out-of-plane loads. The in-plane and out-of-plane deflections are measured. The reduction in the axial load carrying capacity of the plates due to out-of-plane load is quantified. The effect of column slenderness ratio and plate slenderness ratio on the collapse load of simply supported stiffened plates is presented. Two sets of interaction equations are developed, one for the steel plate and another for the GFRP composite plate.Copyright

Innovative Harsh Environment Dry-Tree Support Semi-Submersible for Ultra Deepwater Applications

Natural heave period exceeding over 21 sec is not the only important design criteria for the design of an ultra deepwater semi-submersible for dry-tree support application. The key design features that would enhance the applications of the semi-submersible design for ultra deepwater are vessel motion characteristic, heave natural period, high air-gap, large deck-area, large deck pay-load, reduced mooring load, large riser pull-down load, feasible for dynamic positioning, feasible for dry-tow, feasible for wet-tow, feasible for self-installation, feasible for quay-side top side integration, feasible for float-over installation. This paper presents the design innovations of the semi-submersible that has reached the above listed desired key factors. The designed vessel supports dry-tree with large riser loads resulted from ultra deepwater which is decoupled and is also independent of the designed top side pay load. The design is streamlined to withstand harsh environment. The vessel is designed to favor both options with quay-side deck integration or float-over deck installation. Extreme low accelerations in extreme wave environment on top of the above desired features make this semi-submersible suitable for harsh environment and ultra deepwater applications. A brief summary of hydrodynamic response of the vessel is added in this paper.Copyright