S Nallayarasu

The offshore platforms are generally designed with sufficient vertical clearance from the maximum predicted wave crest elevations. This vertical clearance is termed as 'air gap'. However, due to compelling reasons of hydrocarbon processes and also due to the increase in water levels due to climatic changes or seabed subsidence due to reservoir subsidence, the lower decks may become vulnerable to wave action due to reduction in air gap. The structural elements in the lower decks may become prone to wave-induced loads caused by high wave crests. The assessment of such wave-induced loads on the lower deck and supporting structures, especially in the existing platforms, becomes very essential for the continued safe operations. In the case of wave slamming on a structure, these supporting structural members will also be prone to large loads. Circular sections are commonly used for substructures due to their hydrodynamic efficiency; however, for deck structure, it is very common to use non-circular sections for supporting superstructure loads. The non-circular sections used in the lower decks get exposed to wave-induced loads. The imposed loads could be of higher magnitude compared with the circular sections due to the flat surface of the sections. A review of the literature indicates that the previous studies focused mainly on the circular sections and not much information is available on the non-circular shapes, especially in the case of wave slam and slap. The current study was focused on the measurement of wave-induced slam and slap loads on an array of non-circular sections in a laboratory wave flume and the estimation of force coefficient based on drag-based empirical equations. The studies revealed that the slam and slap coefficients for non-circular shapes are greater than those for circular shapes. © 2013 Copyright Taylor and Francis Group, LLC.

Spar platforms are used for drilling, production, storage and offloading of oil in deep water. Spar with its deep draft and large inertia experience low heave and pitch motions in operating conditions. However, the heave motions can be large when encountered by long-period swells or near resonant period. The heave motion of the Spar platforms can be reduced by decreasing the water plane area, increasing the draft, the added mass and/or damping. Various alternatives to reduce the heave motion in long-period swells have been in the fore front of research for the last two decades. An alternate hull form of a shape similar to a buoy with deep draft has been proposed in this study. The buoy form Spar is a cylindrical floating vessel with curved surface near the water plane. The present study focus on the efficiency of the buoy form Spar in reducing the heave motion and increasing the heave natural period. A classic Spar of 31 m diameter and deep draft buoy form Spars with 25 and 20 m diameter at the water plane area has been considered. The moon pool diameter of 12.5 m and the displacement of 63,205 tonnes are maintained for all Spars. The experimental investigations are conducted using 1:100 scale models in the wave flume. The natural period and the damping ratio for the heave and pitch motions were obtained by conducting free decay tests. Numerical simulations have been carried out using panel method. Based on the study, it is concluded that the reduction in water plane area is effective in reducing the hydrodynamic response of the buoy form Spar and increase in the heave natural period is noted.

Linear damping models have been used in the past for solving floating body dynamics, especially for simple geometries such as spar. However, due to the addition of heave damping elements to spar such as heave plate, complex flow around these elements may change the relationship between damping and velocity of the body to nonlinear. The damping plays a major role in accurate determination of motion response of spars, especially the heave. Free decay tests have been carried out for spar with and without heave plate in calm water condition. The Computational Fluid Dynamics (CFD) simulation of heave decay is carried out using ANSYS FLUENT and validated by free decay test results using scale models. Mesh convergence study has been conducted to determine the optimum mesh size. The heave motion obtained from CFD are used to derive the damping terms by matching the heave motion obtained using equation of motion by changing the damping term with linear, quadratic and the combination of linear and quadratic. The heave motion obtained from linear damping model matches well with that obtained from measured motion and CFD simulation for spar without heave plate. However, the linear / quadratic damping models alone are not suitable for spar with heave plate. Hence a combination of linear and quadratic damping model is proposed for spar with heave plate. The heave motion computed using a combination of linear and quadratic damping model matches well with that obtained from experimental studies and CFD simulations thus indicating the complexity of flow around heave plate in comparison to the spar alone. Further, the vortices around the spar models obtained from CFD simulations are also presented and discussed with regard to the higher order damping.

Fatigue-induced cracks of tubular joints are common in aged offshore structures which can be strengthened using grouted clamp connections. However, only limited guidance is available for design of these clamps. Hence, a comprehensive experimental and numerical investigation on the behavior and capacity of the grouted clamped joints has been carried out. In order to obtain the deformation and ultimate capacity of joints experiments were conducted using scaled model (1:8) in laboratory and nonlinear FE analysis was performed using modified RIKS method. The numerical model has been validated using the results obtained from experiments and found to be in good agreement. Parametric studies have been performed for the parameters such as crack length ratio, grout properties, sleeve friction length ratio and slenderness. The capacity of grouted clamp joint increases minimum by a factor of 2 and 1.6 for compression and tension loads compared to unclamped cracked joint for the tested range.

The ships passing close to each other in narrow channels or in the vicinity of existing berths causes forces on the moored ships. A detailed study on these effects has been conducted using StarCCM + CFD software. The results obtained from the CFD simulations were compared with the existing results from experiments (Remery, 1974). Parametric studies were carried out for wider range of parameters such as displacement ratio, Froude number based on water depth, separation ratio, water depth to draft ratio and the ratio of the length of the moored and passing ships. It is observed from the results that the surge force due to the passing ship effects was proportional to the velocity of the passing ship to the power of 2.1 while the sway force and the yaw moment were proportional to the square of the velocity of the passing ship. The empirical equations to compute non-dimensional peak surge force, sway force and yaw moment on the moored ship due to a passing ship were formulated from the proportionality relationships and the empirical factors. The validity of the derived empirical equations was evaluated and compared with the experimental results obtained by Remery's (1974) and found to match reasonably well.

Experimental and numerical investigations on the hydrodynamic response of a spar and semi-submersible linked system under regular waves (Part I) and random waves (Part II) are presented. These studies were performed for two cases: the first case with the spar alone and the second case with a semi-submersible linked to the spar. The influence of the semi-submersible on the motion response of the spar and its mooring system are evaluated and presented together with acceptance criteria used in the industry. Experimental results are compared with numerical results and found to be acceptable. © 2012 Copyright Taylor and Francis Group, LLC.

Fixed offshore platforms have been used for extraction of oil and gas. These platforms were primarily constructed using steel frames made of tubular members welded at joint or specially fabricated joints. The tubular joints are vulnerable to fatigue-induced cracks which initiate at joints and may propagate through its design life. If the platform life is extended depending upon oil and gas availability, the initial cracks may extend beyond acceptable limits. In recent times, the research on evaluation of residual strength of cracked tubular connection has been considerably increasing since the platforms in various oil and gas fields are ageing. To determine the residual capacity of cracked T-tubular joints, a nonlinear finite element analysis has been carried out. The FEM model of uncracked T joint was validated with experimental result available in literature. The benchmark study has also been made on uncracked T-joints with a specific d/D, t/T and D/2T and compared with the results obtained from empirical equations (API RP 2A). The possible crack locations have been identified using the maximum SCF at crown and saddle points for axial loads. The cracks are introduced in the maximum SCF locations of tubular joint. The study has been extended to range of d/D and D/2T. A correlation has been established between lengths of crack to the residual strength for various crack locations investigated. The residual strength obtained has been compared with reduction factor (BS 7910). It was also found that the residual strength of joints decreases with increase in D/2T.

The design procedure based on empirical equation for computing the chord strength of tubular joints in offshore structures has been established by various researchers and adopted by American Petroleum Institute and other organizations. Local stiffening of chord may be required to carry the loads from braces from various directions. One such method is to introduce internal circular rings at the brace-chord interface. In the present study experimental and numerical investigation on a ring-stiffened tubular joint have been carried out using 1:8 scale model of commonly adopted configuration used in the industry. Experiments have been conducted to measure the failure loads of stiffened and unstiffened tubular joints of (T-geometry) with plain and flanged ring-stiffeners fitted inside the chord. Models were fabricated using steel material and tests were conducted to measure the chord deformation using displacement transducers. First peak in load-displacement relationship has been used to determine the ultimate capacity. Numerical simulations have been carried out using ABAQUS software based on nonlinear stress-strain relationship of steel. The load-displacement characteristics for stiffened and unstiffened joints have been established. Results obtained from numerical simulation have been compared with that obtained from the experimental studies. Strength enhancement factors for stiffened joints have been calculated and compared with that of the unstiffened joints. The enhancement factors for plain and flanged internal ring-stiffener are noted to be 1.66 and 2.03 respectively.

The buoy form spar, in combination with the heave plate, offers a reasonable reduction in heave response, and it has been reported that the heave plate location has a certain influence on damping, added mass, and response characteristics. A classic spar and buoy form spar with a heave plate attached at the keel and near the free surface has been investigated for its effectiveness in reducing the overall response by conducting experimental studies in the wave basin and parametric studies using numerical simulations. The experimental investigations have been carried out using 1:100 scale models in the laboratory wave basin and compared with numerical simulations. It is observed that the buoy form spar with a heave plate attached near the free surface offers a reduction of peak heave motion response by about 75% and peak pitch response reduction by about 30% compared to classic spar without heave plate. The motion response of buoy form spar with different heave plate positions and diameters are compared, and the bouyform with heave plate near the free surface has been found to be effective in reducing the motion response considrably and hence recommended for use in practical design.

Spar platforms have been used for oil and gas exploration in deep water for the past two decades. Spar experience low heave and pitch motions in operating conditions with its deep draft and large inertia. The heave motions can be large when encountered by long period swells. These resonant response leads to unstable motions due to heave-pitch coupling in spar platforms when the heave/pitch natural period ratio is 0.5, 1.0, 1.5 and 2.0, referred to as Mathieu-type instability. This instability can be avoided by changing heave or pitch natural periods, so that the heave-pitch coupling can be avoided. The buoy form Spar proposed in this study is a cylindrical hull with curved surface near the water plane. A classic Spar of 31 m diameter and deep draft buoy form Spars with 25 m and 20 m diameter at the water plane area have been considered. The moon pool diameter of 12.5 m and the displacement of 63000 tonnes are maintained for all Spars. The experimental investigations are conducted using 1:100 scale models in the wave flume. Numerical simulations have been carried out using panel method. The classic Spar experiences Mathieu-type instability, since the heave/pitch natural period ratio is 0.5. The heave natural period of the buoy form Spar is higher than the classic Spar by 24% and 72%. The heave/pitch natural period ratio of the first buoy form Spar with 25 m diameter at the water plane area is 0.667; hence the heave-pitch coupling is avoided. The second buoy form Spar with 20 m diameter at the water plane area does not experience Mathieu-type instability, even though the heave/pitch natural period ratio is 1.0. Also the heave natural period of the second buoy form Spar is 36s (3.6 s in scale model) which is much above the design wave period. The possibility of Mathieu-type instability is avoided in the Spar by varying the hull shape near the water plane.