Risers are an integral part of an offshore oil and gas exploration structures, and that they are prone to vortex-induced vibration (VIV). Although several studies are conducted to analyze the VIV characteristics of risers, the results of amalgamation of multiple parameters and also the relative significance of every parameter on the risers’ VIV characteristics have seldom been considered. Vortex elicited vibration is one of the main causes of the fatigue damage of the riser. In deep sea atmosphere currents sometimes vary in magnitude and direction with depth, thereby causing the likelihood of exciting multiple modes of the riser vibration because of VIV. Vortex elicited vibration analysis has been carried out of a long marine riser for different probable sea locations. To boost the understanding of deepwater environments, the behavior of a composite riser under these forces is investigated. 3D nonlinear dynamic analysis of riser is obtained in time domain using finite part software package ABAQUS/Aqua. The response histories thus obtained are utilized for fatigue safety analyses of the riser. Uncertainty modeling, particularly of crack growth parameters, is undertaken by means of the bi-linear crack growth relationship. Results concerning fatigue safety and crack size evolution are given using Monte Carlo Simulation. The bi-linear crack growth models are found to result in to higher fatigue life estimation. Sensitivity behavior pertinent to limit state adopted has also been investigated.

The underwater explosions due to an accident or other potential reasons cause severe shocks in the fluid medium. Safety assessment is an essential design requirement where the blast load concentration is expected due to smaller standoff distances. In the present work, the probabilistic analysis of structural response due to the underwater explosion is studied using NESSUS software. The study helps improve the concepts of improved blast mitigation strategies. The shock loading considered varies spatially and temporally. The fluid pressure is transformed into structural nodal forces. Taylor’s plate theory is used to estimate the shock loading. In the analysis, the material is considered to be rate-dependent elastoplastic with temperature softening. Maximum central deflection against shock factors is obtained to arrive at the safe thickness of the stiffened and unstiffened plates for the optimum design characteristics using LS-DYNA software. The finite element approach adopted efficiently captures expected failures for various combinations of structural properties leading to an optimized design to withstand the target explosion. Johnson-Cook dynamic failure model is employed as a limit state function for reliability assessment. Sensitivity analysis enables to achieve target reliability.

Random stress produces fatigue damage due to fluctuation in the presence of high bending stresses gets enhanced and leads to the primary failure of the production composite riser. The main problems with the fatigue deterministic model are related to the fact that not all waves have the same period. The second problem is due to the assumption that all waves are regular and do not into account the stochastic nature of the marine environment. The wave data is provided on a statistical basis. Work attributed to damage related stress distribution and interaction of composite along with integral difficulties in fatigue is challenging task. Resulting flexural stress history is random in nature. The purpose of this study is to present a methodology for assessing cumulative fatigue damage using MATLAB of the deep-water composite riser. The simulation of 12 random sea states is examined for the purpose of estimation of accumulated damage. Using the recommended method by DNV-OS-C501, fatigue life of composite riser is determined for different conditions. Damage induced by stress signals of varying amplitudes is calculated using Palmgren-Miner’s rule. Total fatigue life for stress history is estimated by the summation of damage fractions. Estimation of fatigue damage over riser’s entire service life is done taking into consideration the entire expected sea area. Extreme flexural stress random fluctuations are estimated with and without currents.

Composite materials have drawn considerable consideration from the offshore business, basically because of their high explicit quality. Notwithstanding weight decrease, composites offer extra advantages, for example, fatigue resistance, damping, and thermal (protection) properties, and high erosion resistance. As a part of design procedure there are requirements of mechanical strength based on criteria referring to failure modes, such as rupture by over loading, fatigue failures, buckling or an unstable fracture. Three dimensional nonlinear assessment of riser is carried out in time domain using ABAQUS/Aqua. The response time histories so obtained are used for the study of fatigue safety assessment of riser. It is based on a bi-linear approach to model fatigue crack growth and incorporates a failure limit to describe the interaction between rupture and plastic failure. Using Monte Carlo Simulation, tests of fatigue reliability and fatigue crack size evolution are obtained. It is observed that bilinear S-N curve and crack growth models leads to higher estimate of fatigue life. Sensitivity behavior pertinent to limit state adopted has been thoroughly examined. These outcomes implicate assessment of components of the marine structures to ensure minimization of the surprises due to wide scatter of the fatigue phenomenon in marine environment.

The low velocity impact (LVI) result of laminated targets is an imperative study to evaluate its failure for applications to inconsequential structures. The work has extensive applications to off shore and naval industry. Safety and reliability assessment as per the international standards is one of the basic objectives of the study. LVI on composite plates is performed taking the material parameters and loading condition as random variables. Graphite fiber reinforced laminated plates are vulnerable to damage due to impact by foreign objects and in plane loading. In order to evaluate the safe load carrying capacity and the reliability under impact, dynamic analysis of composite plate subjected to LVI is carried out. Probabilistic finite element method (PFEM) is performed to determine the stochastic response. During impact, the in-plane damage modes such as matrix cracking, fiber failure and shear cracking are modeled using a failure criterion. The out of plane de-lamination is modeled using cohesive surfaces. The randomness associated with the system properties due to the inherent scatter in the geometric and material properties and input loads are modeled in a stochastic fashion. Random parameters represent various characteristics appearing in the performance function. The stochastic response and reliability forecast of the system is determined by Gaussian processes response surface method (GPRSM) and validity of method for the present problem is establish using Monte Carlo simulation (MCS) procedure. The safety level qualification is achieved in terms of reliability level targeted.

Welding is one of the most used joining methods in the ship industry. However, residual stresses are induced in the welded joints due to the rapid heating and cooling leading to inhomogenously distributed dimensional changes and non-uniform plastic and thermal strains. A number of factors, such as welding speed, boundary conditions, weld geometry, weld thickness, welding current/voltage, number of weld passes, pre-/post-heating etc, influence the residual stress distribution. The main aim of this work is to estimate the residual stresses in welded joints through finite element analysis and to investigate the effects of boundary conditions, welding speed and plate thickness on through the thickness/surface distributions of residual stresses. The welding process is simulated using 3D Finite element model in ABAQUS FE software in two steps: 1. Transient thermal analysis and 2. Quasi-static thermo-elasto-plastic analysis. The normal residual stresses along and across the weld in the weld tow region are found to be significant with nonlinear distribution. The residual stresses increase with the increase in the thickness of the plates being welded. The nature of the normal residual stress along the weld is found to be tensile-compressive-tensile and the nature of normal residual stress across the weld is found to be tensile along the thickness direction.

The low velocity impact on composites has been studied as it leads to serious damage. The damage initiates as an intra ply matrix crack due to shear or bending which propagates further into the interface causing de-lamination between dissimilar plies and fiber breakage. This damage evolves with time and adversely affects the mechanical properties and strength of the composite. Since, multiple cracks in the ply are difficult to track, a progressive damage mechanics approach is used to model this failure. The inter ply failure is modeled using cohesive surfaces between the plies. The low velocity impact on composite plate is studied using finite element method. Impact parameters like velocity of impactor, the mass of the impactor and elastic properties of the material etc. are considered. An explicit central difference integration scheme is used to solve for displacements and impact forces. Progressive damage and failure in composites is modeled; an efficient algorithm has been developed and implemented in the FE code ABAQUS through a user-defined subroutine (VUMAT). Reduced integration yields satisfactory results for the impactor velocity less than or equal to 3 m/s for larger mass impact. However, full integration is recommended to obtain the satisfactory results for the (impactor velocity beyond 3 m/s), high velocity impact involving small masses. For the low velocity impact, the peak contact force and displacement are linear functions of impactor velocity for a constant mass. However, a nonlinear behavior is observed for the variation of mass with a constant striking velocity.

Low velocity impact on composite plates is studied taking material properties and initial velocity as random parameters. Graphite fiber reinforced composite plates are susceptible to damage due to impact by foreign objects and in plane loading. In order to assess the safe load carrying capacity and the probability of failure under impact, dynamic analysis of composite plate subjected to low velocity impact is carried out. Finite element method is used to study impact. During impact, the in-plane damage modes such as matrix cracking, fiber failure and shear cracking are modeled using a failure criterion. The out of plane de-lamination is modeled using cohesive surfaces. The uncertainties associated with the system properties due to the inherent scatter in the geometric and material properties and input loads are modeled in a probabilistic fashion. Random parameters represent various characteristics appearing in the limit state function. The probabilistic analysis and reliability prediction of the system is carried out using Gaussian response surface method and validity of method for the present problem is establish using Monte Carlo simulation (MCS) procedure. Sensitivity analysis of the probability of failure with respect to random parameters considered is an important study for design optimization. The safety level qualification is achieved in terms of reliability level targeted. The mean and standard deviations of random variables show an appreciable influence on the probabilistic failure. Systematic changes in the input parameters are governed by the probabilistic sensitivity tools to achieve target reliability.

Composite production risers in deep sea condition operate in harsh environment induced by wind and waves together with large axial tension fluctuations. Therefore, a suitable fatigue analysis of the riser stress joints must be undertaken at the structural design stage to ensure their adequate lifespan of 25–30 years. The composite riser, under study, is considered to be a part of Tension Leg Platform (TLP) to be installed at a depth of 2000 m. The probabilistic records of twelve sea states are used to simulate the expected loads for its entire service life. The response time histories are obtained by implicit time domain analysis of the composite riser for each sea state of random nature, duly considering the nonlinearities involved. Each stress time history is decomposed into equivalent stress blocks. A stress range histogram is generated in terms of the number of cycles in each block. The rain flow cycle counting method is hence used to estimate the cumulative fatigue damage. Palmgren-Miner rule is used to calculate the damage caused by stress signals of variable amplitudes. The damage fractions are then summed linearly to give an estimate of the total fatigue life for a particular stress history. The cumulative estimate for its entire service life is hence estimated considering all expected sea states.

Deep water composite risers are subjected to randomly fluctuating loads, induced by wind and waves in the presence of fluctuating axial tension which may be critical in deep sea conditions. Therefore, risers experience the extreme bending and randomly fluctuating stresses throughout their service life. Cumulative fatigue damage is a critical assessment of riser life in the presence of large dynamic stresses. Probabilistic analysis and risk assessment of composite risers for cumulative fatigue is a vital design requirement for its satisfactory service and survival for stipulated period. Without addressing the reliability assessment, composite risers may not be recommended for deep water oil and gas exploration and production. Hence, the reliability assessment is a critical issue that is to be addressed for the safety of the deep water composite riser. It is studied for the entire system for all possible sea states occurring in the exploration region. Unlike conventional risers, the wall structure of a composite riser is more complicated. Therefore, multiple failure mechanisms are used jointly to assess the safety of the composite riser. Fatigue reliability is a challenging task due to complex nature of dynamic response and associated uncertainties caused by the material and external loads. The present study is focused on reliability assessment using stochastic finite element analysis. Response time histories for random sea plus current have been obtained. Requisite numbers of sea states are considered for the simulation of a wide range of off-shore environment and estimation of accumulated damage. By using the S-N data, damage fractions are calculated then summed linearly using Miner-Palmgren rule. The total damage has been obtained by summing the accumulated damages over all the sea states under consideration. Non-linear limit state function is derived based upon the above given approach to calculate the fatigue life. Important uncertainties associated with random variables are considered while deriving the limit state function. Numerical methods, such as Monte Carlo simulation and Advanced First Order Reliability Method, are used for the calculation of the reliability. The sensitivities of various random variables on overall probability of failure have been studied and design points have been located on failure surface. Probabilities of failure for important parameters are investigated.

The neck portion of the human femur is the most vulnerable region to attract stress-induced fractures. The loads of the human body act on the hip joint and on the greater trochanter region through abductor muscles. The bone is a natural composite and a good example of functionally graded material (FGM). This study considers a probabilistic finite element approach to assess the critical stresses in the femur under static loads. Material properties assigned to the bone model are linearly elastic, isotropic and orthotropic. Material characterization in terms of bone density is established by Computed Tomography (CT) data. The strength reliability and safety margin are obtained using relevant limit state function. Sensitivity analysis with respect to random parameters provides basis for a possible implant material characterization.

The oil and gas exploration has moved from shallow water to much deeper water far off the continental shelf. Spar platforms under deep water conditions are found to be the most economical and efficient type of offshore platform. Several Spar platforms installed in the Gulf of Mexico and North Sea proves its suitability for deep water exploration. Accurate prediction of motions of a Spar hull is very important for the integrity and associated costs of the riser/mooring line. The most common approach for solving the dynamics of Spar platform is to employ a decoupled quasi-static method, which ignores all or part of the interaction effects between the platform, mooring lines and risers. Coupled analysis, which includes the mooring lines, risers and platform in a single model, is the only way to capture the damping from mooring lines and risers in a consistent manner. The present coupling is capable in matching the forces, displacement, velocities and acceleration for mooring line with Spar hull at the fairlead position and riser with Spar hull at the riser keel connection. It can handle possible significant nonlinearities. The output from such analyses will be platform motions as well as a detailed mooring line and riser responses. In actual field problems hydrodynamic and aerodynamic loads act simultaneously on Spar platform, mooring lines and risers. In finite element model, the entire structure acts as a continuum. This model can handle all nonlinearities, loading and boundary conditions. The selected configuration of Spar platform is analysed under wave force together with wind loading and its structural response behaviour in steady state is studied. An automatic Newmark-β time incremental approach in ABAQUS/AQUA environment has been implemented to conduct the analysis in time domain. The wind force acting on the exposed part of the platform encompasses mean and fluctuating wind components. The frontal region includes the topside assembly and the spar hull portion above the sea level. High degree of nonlinearities makes the solutions convergence sensitive and it requires large number of iterations, at each time station. Spar responses in surge, heave and pitch along with top tension in moorings are computed. The coupled Spar experiences significant lateral shift along wave direction due to wind loading. Increase in standard deviation shows the participation of wind loading giving higher fluctuations. The CML tension increases for wind loading but the extent of the tension fluctuations under wind loading is not much due to high pretension of mooring line.

The advantage of composite riser is more pronounced due to its light weight and reduced axial tension and bending moment. Its characteristics provide more flexibility in the design of riser system such as stress joint, tensioner or even the platform hull. The composite riser under study consists of alternative hoop and axial layers. There is a marginal coupling between these two orientations hence, the riser is spatially orthotropic. The pressure capabilities of riser are governed by the performance of hoop layers under combined fluctuating axial tension and bending moment. The internal steel liner provided to ensure no leakage often limits the composite riser capabilities. The steel liner provided for production inevitably carries major share of loads. An attempt has been made to carry out a detailed local analysis of a segmental length under critical loads. Damage analysis for various combinations of axial tension and bending moment are performed. The results of global analysis are used to act as boundary/initial condition for the local and detailed dynamic analysis of the segmental length modeled as finite element assemblage of shell elements. The critical stress time histories obtained by global analysis are applied at the local level. The composite layers sustained the random stresses that lead to the failure of the composite. Some realistic failure criteria are chosen to check the damage at local level.

Due to depleting sources of oil and gas reserves in shallow water depths, exploration and production activities have moved into ultra-deep offshore oil fields. Risers are an essential part of any offshore drilling facility. A riser tensioner located on the drilling platform has to provide an adequate vertical tension to maintain the stability of the riser. It is essential for a successful operation. Composite risers in deep sea conditions require much lower top vertical forces due to their high strength to weight ratio. Carbon/epoxy composite has been considered in the present study to carry out the burst analysis and to assess the safety of the composite riser under internal and external pressures and other environmental loads due to random sea currents. In order to ensure the permissible pressure and no fluid leakage, composite risers are provided with an internal steel liner. Initiation and propagation of debonding between the liner and composite has been studied and probability of failure is obtained. In burst analysis, maximum internal pressure is applied to a riser section and the stresses in all (hoop and longitudinal layers) the composite layers are checked against the failure. High pressures are incremented in small steps until fiber rupture occurs due to bursting. Maximum normal stress theory is employed for checking the failure. The same theory provides the limit-state to assess the safe pressure considering uncertainties associated with random input parameters involved. A finite element analysis has been carried out in ABAQUS/AQUA for random sea motion and fluctuating axial tension considering salient non-linearities. A small riser section modelled as a hybrid beam element (for global analysis) has been considered to study the bursting and debonding behavior. It is further discretized into thin shell elements (S4R). Steel liner and composite pipes are modeled separately and assembled together to ensure the overlapping various layers and sharing nodes. The composite body sustains the major stresses in the inner layers that diminish on moving outwards radially. An implicit time domain analysis has been carried out to obtain the response. The debonding through circumference and length are studied. The stresses obtained are compared with their ultimate strength.

Depleting oil reserves in shallow waters, off-shore oil fields are opening the avenues of new ventures in deep sea conditions. A marine riser is a major component of an offshore drilling and production system that are either fixed or floating in nature. Deepwater risers are quite long and significant currents usually excite natural bending mode that is much higher than the fundamental bending mode. In ultra deep environment currents usually change in magnitude and direction with depth, thereby inducing possibility of exciting multiple modes of the riser vibration due to VIV. Vortex induced vibration analysis has been carried out of a long marine riser in a probable deep sea location. To improve the understanding under deepwater harsh environments, the behavior of the riser under these forces is thoroughly investigated. 3D Nonlinear dynamic analysis of riser is obtained in the time domain using finite element software package ABAQUS/Aqua. The response histories so obtained are employed for the study of fatigue reliability analyses of riser. Uncertainty modeling, especially of fatigue crack growth parameters, is undertaken using bi-linear crack growth relationship. Results pertaining to fatigue reliability and fatigue crack size evolution are presented using Monte Carlo Simulation. The bi-linear crack growth models are found to lead to higher fatigue life estimation. Sensitivity behavior pertinent to limit state adopted has been thoroughly investigated. These findings implicate inspection schemes for components of the marine structures to ensure minimization of the surprises due to wide scatter of the fatigue phenomenon in marine environment.

The oil and gas industry has moved towards the offshore deep water regions due to depletion of these resources in shallow and intermediate water depths. Conventional fixed jacket type platforms and bottom supported compliant platforms have been found to be inefficient and uneconomical for exploring these resources in deep water regions. In view of deep water conditions, Spar platforms have been seen to be the most economical and suitable alternative offshore platforms. Several operational Spar platforms such as SB-1, Shell’s ESSCO, Brent Spar, Oryx Neptune Spar, Chevron Genesis Spar and Exxon’s Diana Spar in the Gulf of Mexico and North Sea have shown the effectiveness and success of such platforms in deep-ocean. In deep water conditions, the severity of sea states has substantial effects on the spar platform. The mooring lines contribute significant inertia and damping because of their longer lengths, larger sizes, and heavier weights. Precise motion investigation of platforms should consider these actions in deep waters. However, proper dynamics cannot be assessed by the commonly used decoupled quasi-static method that ignores all or part of the interaction effects between the mooring lines and platform. Coupled analysis, which includes the platform and mooring lines in a single model, is the only way to capture the damping from mooring lines in a consistent manner. In the present study, coupled analysis of integrated Spar-mooring system has been performed. Cylindrical spar hull is treated as a rigid beam element and catenary mooring line as hybrid beam element. Nonlinear dynamic responses have been evaluated under several severe sea states of dissimilar wave heights and wave periods. Damping due to mooring lines has been assessed. An automatic Newmark-β time incremental approach has been implemented to conduct the analysis in time domain. Wave induced spar hull motion in surge, heave and pitch direction along with maximum tension in mooring line has been assessed for different wave conditions with and without current in 1018 m water depth. The time histories of spar responses follow substantial alteration for larger wave heights and wave periods. Maximum tensions in mooring line are very sensitive with momentous value for extreme sea loading. Mooring tension responses are significantly different reflecting the damping effect of mooring lines.

Reliability analysis of mooring lines requires an accurate prediction of extreme responses for large number of sea states even for a short-term based approach. In deep water, the interactions between the floater motions and the large number of risers and mooring lines become significant and must be considered for accurate prediction of floater motions as well as line dynamics. Time-domain coupled dynamic analysis procedures have been shown to give more accurate results but at a higher computational expense. Therefore, efficient computational tools are required for reliability analysis of mooring lines for deep water floating systems. Enhanced decoupled dynamic analysis method, in which the floater motions are computed by coupled analysis considering a coarse finite element model of the mooring line, is an efficient method and provides results comparable in accuracy with the fully coupled dynamic analysis procedures. This paper presents the application of enhanced de-coupled dynamic analysis method for reliability assessment of mooring lines for deep water floating systems. For reliability analysis of mooring lines, the methodology presented in Ding et al. [5] is adopted. Reliability analysis of a critically loaded mooring line for a deep water classical spar floater under extreme environmental loads is performed using environmental contour approach. Mooring line tension time histories under various storm conditions are calculated using enhanced de-coupled dynamic analysis. The uncertainty in the predicted maximum mooring line load due to different storm events, variability in met-ocean conditions and numerical models is considered. Probability of failure and the corresponding reliability index of the mooring line are calculated. The impact of variability in predicted mooring line load, line capacities and factors of safety on mooring line reliability are studied. It is seen that enhanced de-coupled dynamic analysis, which predicts the mooring line loads as accurately as coupled dynamic analysis with lesser CPU time, can be used more efficiently for reliability assessment of mooring lines for deep water floating systems.