Western offshore oil field in India has nearly 300 offshore platforms for oil and gas exploration, of which almost 50% of platforms have outlived their life. Life extension of these platforms has become essential for further production activities. In many cases, design level analyses combined with ultimate strength assessment, life extension has been granted. However, risk-based assessment based on the probability of failure based on available reserve strength linked to additional life extension could be a logical method. The Reserve Strength Ratio (RSR) is defined as a ratio of reserve capacity of the jacket structure and the design level environmental loads (1 year or 5year or 10 year or 100-year return period). The encounter probability of these design storms for the life extension period has been established probability for the extension period has been used as a threshold to determining the required RSR using the probability of collapse. For the present study, four typical aged wellhead platforms with different water depths are selected, and RSR is evaluated by carrying out push over analysis. The Monte Carlo Simulation method is used to generate the statistical values of RSR. The probability of failure is then calculated by First Order Reliability Method (FORM) using MATLAB for different RSR values. Reassessment criteria for the existing offshore platforms have been described from the reliability analysis results based on probability failure and encounter probability.

Fatigue is generally considered the most critical failure mode in mechanical and structural systems. Due to high-stress concentrations, welded joints represent the most common fatigue crack initiation in steel structures susceptible to fatigue. In India, especially in western offshore, there are about 300 platforms, and 50% of them have reached their design life but still operating due to existing oil and gas reserves. Fatigue prediction in offshore structures is an extremely complicated process involving many factors such as complicated geometry, material, loading, and environment. These uncertainties are modelled as random variables. The assessment of failure probabilities takes a basis to formulate a limit state function for the relevant failure mode and deterioration mechanisms. The fatigue failure assessment based on a simplified probabilistic approach using the application of reliability-based procedures such as the First Order Reliability Method (FORM) is a useful tool. In the simplified fatigue assessment method, the two-parameter Weibull distribution is used to model the long-term distribution of fatigue stresses. Reliability of tubular joint using known fatigue life is an important factor in decision making for life extension of aged platforms. The methodology adopted in this study uses the linear damage accumulation model of Palmgren-Miner, double slope S-N curve, and one-to-one transformation of the probability density functions of long-term stress range and uncertainties to obtain the probability of fatigue failure as a function of the service life from known fatigue life.

Over the years, with advancements in the shipping industry, there has been considerable increase in size, numbers and speed of ships. Ports and harbours have to handle these larger ships and increased ship traffic within the limited space especially in matured ports. Due to the above mentioned issues at the ports and harbours, passing ship interactions with a moored ship is eventual. Pressure variations around the passing ship may cause additional forces and moments on the ship moored in the vicinity. Though there is literature from past research available on the subject, only few parameters have been investigated. The passing ship forces on a moored ship has been evaluated using Computational Fluid Dynamics (CFD), based on a double-body flow model. Simulations were carried out for three-dimensional, unsteady, inviscid flow using overset meshing technique to facilitate the movement of the vessel. The results obtained from CFD simulations were compared with the existing results obtained by experiments (Remery, 1974) for the validation of the CFD model. Detailed parametric studies were carried out for parameters such as velocity ratio and displacement ratio, spacing ratio, and water depth ratio. Empirical equations for force and moment coefficients are derived based on the present study and recommendations for passing vessel distances from moored vessel has been made in the article.

Hydrodynamic response of spar with appendages such as heave plate has been investigated in the past, mostly attached at the bottom of the spar. The effect of geometry and appendages on the hydrodynamic response of spar has been investigated in this article. A curved neck form with a heave plate near the free surface is proposed as an energy dissipation device for both heave and pitch responses. Numerical simulation using Computational Fluid Dynamics (CFD) is used for capturing the flow around the curved neck with heave plate and corresponding damping characteristics. CFD free decay simulations have been carried out to obtain heave and pitch damping and were noted to be higher than the conventional spar with heave plate at the bottom. Comparison of the proposed geometry and heave plate at the free surface with a conventional heave plate at the bottom of the spar has been made, and significant changes to the response and hydrodynamic characteristics have been noted. It is observed that the buoy form spar combined with the heave plate located near the surface (within 10% of the draft) helps dissipate energy and thus reduce the heave response.

Tubular joints are common in offshore framed structures such as jackets. These joints are subjected to fatigue due to cyclic loads from waves. Stress concentration factor plays a major role in the estimation of fatigue life. Studies have been carried out in the past on stress concentration factors for these joints. However, literature on ring stiffened joints is limited. In the present study, numerical simulations have been carried out on ring stiffened tubular joints, especially the T joints, subjected to axial tension load using finite element method in linear analysis using ABAQUS software. A solid type 20-node quadratic brick element (C3D20R) has been used in the study. Stresses in hotspot locations at brace/chord intersection, ring/chord intersection and ring inner edge are examined. The numerical model has been validated using published experimental data and Lloyd’s register recommendations. Further parametric study has been carried out with 20 models, which includes geometric parameters of ring stiffener such as width of stiffener, thickness of stiffener and spacing between the ring stiffeners. The results of parametric study show a significant reduction of SCF at saddle location by placing ring stiffeners in unstiffened T joint. A set of new parametric equations are developed to calculate SCF for ring stiffened T joint at saddle location for axial tensile load.

The vortex induced vibration of slender cylindrical structures is common in offshore structures and marine applications such as risers, towing cables, etc. The VIV response of such slender elements in steady uniform current has been investigated in the past using numerical and experimental studies. Though few numerical studies exist for varying current (sheared flow), experimental studies are limited. Hence, the experimental studies are an essential part of VIV investigation, especially for sheared flow. The experiments were conducted using a specially fabricated circular steel tank of diameter 2.4 m with a central hinge to rotate the pipe horizontally in a water pool of depth 0.7 m. Shear current is simulated by rotating the pipe about the hinge. A pipe of diameter 25 mm (= D ) and length 1 m (= L ) was fixed at one end of the rotating cable support, and the other end was passed over a pulley inside a rotating cylinder. The rotating cylinder is provided with a pulley at the top to tension the pipe. A shear current with a maximum velocity of 1.3 m/s and a minimum velocity of 0.1 m/s is generated using the set up. The VIV response of the pipe was measured using electrical resistance-type strain gauges pasted along the length. The measured axial strain was used to obtain transverse displacements, which was used to determine the response frequency, amplitudes, and forces. The Strouhal number was calculated. The VIV response and the fluid force coefficients obtained from the experiments were compared with Shear7 results.

Vortex Induced Vibration (VIV) of slender cylindrical structures subjected to uniform flow is subject interest since the use of such elements is common in the offshore and naval industry. The numerical and experimental investigation of VIV for slender, flexible cable member in uniform current has been carried out in this study. The experiments were conducted in a towing tank of 85 m length and 2.8 m water depth mounted with the facility to control speed, The carriage has a range of velocity up to 5 m/s. A flexible cable of diameter 15 mm with an aspect ratio of approximately 100 has been used for the present study. The lateral displacement of cable has been measured using the strain gauges mounted along the length. The strain gauges measure the axial strain due to lateral bending of the cable and in turn, converted to lateral displacement. Experiments were carried out for a wide range of Reynolds number (Re) ranging from 3000 to 15000. From the measured responses, the Strouhal number (St) is back calculated. It is observed that the Strouhal number for flexible cable ranges from 0.13 to 0.17 for low Reynolds number from 3000 to 15000 and it falls within the expected range. The normalized RMS displacements (RMS A/D ) have been obtained for specified range of Re. The lift coefficient ( C L ) attains a stable value of around 0.10 to 0.25 for reduced velocity ( Vr ) greater than 5.33 for different locations. The lift coefficient follows a similar trend along the span for different Vr. Numerical simulation has been carried out using Shear7 software with results matching reasonably well.

The tubular members damaged by ship impact or falling objects require repair and rehabilitation in offshore structures. The repair of damaged underwater tubular member using welding is hazardous and expensive and hence alternative connection methods such as grouted clamp techniques have been in use for many decades. The existing guideline on the design of grouted connections especially under axial tension and moment is very limited and requires further study. The load transfer mechanism of grouted clamps depends on various geometric parameters and bond between clamp and parent member. A comprehensive study on split sleeve grouted connection for load transfer between two parts of tubular members has been investigated and presented. Numerical simulation of split sleeve grouted connection has been carried out using finite element model of tubular member – sleeve through bond strength of grout. The numerical model has been validated using existing guidelines and further parametric study has been carried out. The parametric study includes geometric parameters such as diameter to wall thickness ratio of split sleeve, sleeve friction length, grout strength and grout shrinkage. The simulations have been carried out for combination of axial tension and moment loading.

A critical review of literature on the mooring forces on the berthed ship together with the passing vessel effects indicates limited data, thus requiring additional studies. Four methods have been found in the literature namely (a) Seelig’s, (b) Flory’s, (c) Kreibel’s, and (d) Modified Seelig’s methods. A comprehensive review and comparison of existing methods has been carried out to verify the applicability of each method to certain conditions and also to establish the parameters for study. It has been found that displacement of the vessels, separation distance between the vessels and velocity of the passing vessel greatly influence the passing vessel forces on moored ship. The software OPTIMOOR has been used to verify Flory’s and Seelig’s methods with the experimental data of Remery and also to study the combined effect of environmental loads and passing vessel effects on mooring lines. The results indicate that the interaction of waves and passing vessel effects are too onerous. Conclusions have been arrived based on the results that for separation distance of 100 m or less between vessels has a greater influence on mooring line forces, leading to breakage of lines.

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.

The prediction of nonlinear roll motion of ships depends highly on the accurate estimation of roll damping. The nonlinear nature of roll damping arises from the viscous flow and the associated phenomenon of flow separation around the ship hull. Roll damping changes noticeably with a slight change in the ship hull geometry and appendages. The estimation methods employed in industry are highly empirical in nature. These empirical methods were derived from combinations of model tests conducted in wave flumes and basins, and some selected formulae used in fluid dynamics. These methods have limitations and the roll damping prediction show large variation with change in the ship parameters. The advances made in Computational Fluid Dynamics (CFD) in recent times, and validation of the CFD results using experimental studies, can help in predicting roll motion and damping more accurately. The present work uses CFD as a tool to estimate roll damping of a ship mid-section with bilge keel with validation using published experimental results.

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.

Damping elements are used in ships and offshore structures as response reduction devices for maintaining the hydrodynamic response within acceptable limits. The use of such elements so far has been based on past experience or using empirical based design approach. An investigation of the wave excitation forces acting on these structures with and without damping elements may give an insight into the damping effects of such elements. Also understanding the flow pattern and the magnitude of flow variation around these elements may be useful to improve the design methods. In the present study, measurement of damping forces and response were carried out to establish the relationship between wave frequency amplitude and damping ratio. It was observed that the damping varies with wave period non-linearly and linearly with wave height. Also it was observed that increase in added mass combined with viscous damping is a key factor for the effectiveness of heave plates in reducing motion response of spar platforms. Of the various configurations tested, the spar with double heave plate at the bottom has higher damping compared to other configurations.

Spar technology has been used in offshore oil and gas exploration successfully for number of years for drilling, production and storage in deepwater. The motion response of floating structures especially the heave response in particular, is very important to the selection of suitable drilling and production equipments. Reduction of heave response can be achieved by attaching a heave damping plate to the keel of a Spar. This has been used in the past. Experimental and numerical studies on such devices of various diameters under regular waves has been carried out and presented. The experiments were conducted on a 1: 100 scale model of Spar designed for a water depth of 245m with a payload of 10000 tonnes and the numerical analysis was carried out using ANSYS AQWA software. Numerical and experimental values of RAOs for surge, pitch and heave compares reasonably well. Measured and numerical results of RAOs for surge, pitch and heave and the effect of diameter ratio, wave steepness, and mooring line pretension were presented and discussed. An optimum heave response is achieved when the heave plate diameter is 20% to 30% larger than the diameter of the spar.

The offshore platforms shall be designed with deck elevation that maintains sufficient air-gap with maximum predicted wave crest elevations. However, due to compelling reasons of hydrocarbon process and changes to water levels/seabed subsidence may have structures in the close vicinity of the sea level or submerged. Hence the assessment of wave induced forces on the deck structures especially the old platforms become very essential. Review of literature indicates that the previous studies focused mainly on the circular sections and not much information is available on the non-circular sections. The current study focused on measurement of wave induced slam/slap loads in a laboratory wave flume and estimation of force coefficients based on drag based empirical equations. Further, it is of interest to focus on the flow around the open sections (non-circular) using a flow visualization technique (Particle Image Velocimetry) to understand the flow pattern around the elements. The results indicate that the slam and slap coefficients for I sections are smaller than the channel sections and the values of coefficients are few times higher than the circular sections.