F. Silva-González

Reliability Analysis of Marine Platforms Subject to Fatigue Damage for Risk Based Inspection Planning

The probability of failure of steel jacket platforms subjected to fatigue damage is computed by means of Monte Carlo simulations using limit state functions in which wave, wind, and deck loadings are expressed in terms of empirical functions of uncertain maximum wave height. Limit state functions associated with the base shear capacity of the jacket and the shear capacity of the deck legs were used. The sensitivity of the probability of failure to the coefficient of variation of resistance, of wave height, of resistance and loading biases, and to parameters in empirical loading functions, as well as the influence of the reserve strength ratio is analyzed using a simplified limit state function. Results from simulations are compared to those obtained with a formulation that relates the reserve strength ratio to the reliability index. An application to risk based inspection planning for extension of the service life of a platform is given.

Effect of Uncertainties on the Reliability of Fatigue Damaged Systems

Fluctuating stresses and strains due to wave forces cause accumulated fatigue damage in tubular joints of marine platforms. Considering the uncertainties in the loads, material properties, initial crack sizes, and stress intensity factors, etc., may affect significantly the reliability assessment of marine jacket platforms. In this paper, we assessed the effect of uncertainties about such fatigue variables on the time evolution of the reliability of series and parallel systems considering correlation between failure modes. The fracture mechanics Paris-Erdogan model is used to model crack growth and a FORM method is used for computing the safety index. The uncertain variables analyzed are: initial crack size, material parameters C and m in the fracture mechanics model and the shape and scale parameters of the Weibull density function used for the long-term distribution of stress range.Copyright

Non-Gaussian Stochastic Equivalent Linearization Method for Inelastic Nonlinear Systems with Softening Behaviour, under Seismic Ground Motions

A non-Gaussian stochastic equivalent linearization (NSEL) method for estimating the non-Gaussian response of inelastic non-linear structural systems subjected to seismic ground motions represented as nonstationary random processes is presented. Based on a model that represents the time evolution of the joint probability density function (PDF) of the structural response, mathematical expressions of equivalent linearization coefficients are derived. The displacement and velocity are assumed jointly Gaussian and the marginal PDF of the hysteretic component of the displacement is modeled by a mixed PDF which is Gaussian when the structural behavior is linear and turns into a bimodal PDF when the structural behavior is hysteretic. The proposed NSEL method is applied to calculate the response of hysteretic single-degree-of-freedom systems with different vibration periods and different design displacement ductility values. The results corresponding to the proposed method are compared with those calculated by means of Monte Carlo simulation, as well as by a Gaussian equivalent linearization method. It is verified that the NSEL approach proposed herein leads to maximum structural response standard deviations similar to those obtained with Monte Carlo technique. In addition, a brief discussion about the extension of the method to muti-degree-of-freedom systems is presented.

Reliability Analyses of Clay-Embedded Offshore Pipelines Under Vertical Buckling Considering Lower-Bound Pipe-Soil Capacity

This paper presents the reliability formulation and analyses for studying and quantifying probabilistically the impact of the main parameters involved in the upheaval buckling of offshore buried pipes due to high pressure high temperature conditions (HPHT) on the reliability of the pipeline. Pipelines are considered installed in a clayey trench and naturally covered. The limit state function is established in terms of the vertical pipe-soil capacity and vertical loading. A lower-bound capacity of the pipe-clayey soil system is included in the reliability analysis in order to represent more realistic conditions with regards to the uncertainty in the capacity. The lower-bound capacity is the smallest possible physical limit of the pipe-soil capacity. Reliability assessments using the main parameters that control the vertical buckling in terms of loading and capacity were performed. Consequently, the variations of the reliability index (β) with the vertical imperfection (δ) and the ratio of the cover height (depth of pipe embedment) to the pipe diameter (H/D) were quantified. The reliability index was evaluated by means of Monte Carlo simulations. The inclusion of the lower-bound capacity by means of a left-censored lognormal distribution was found to increase in some cases the values of β.Copyright

Predicting erosion in wet gas pipelines/elbows by mathematical formulations and computational fluid dynamics modeling:

Sand erosion has been identified as a potential damage and failure mechanism in pipelines/elbows employed to transport gas from wells to terminals. Erosion can cause localized material loss decreasing the structural integrity of pipelines/elbows leading to failure. As a result, sand erosion has been the object of much research work in the oil and gas industry. The prediction of erosion caused by sand transported by hydrocarbons flow is a difficult task due to the large number of variables involved. At present, a great number of empirical models have been developed to predict sand erosion in smaller diameter pipelines under laboratory conditions. Therefore, such formulations generally present uncertainties for their application in larger diameter pipelines employed to transport oil and gas because there is no fundamental basis showing how the empirical formulations can be extrapolated to large diameters pipelines as most of the models have been developed on the basis of elementary laboratory experiments, which may not represent the real sand erosion conditions. Furthermore, most of the analytical/empirical models were developed for specific pipeline/elbows diameters and cannot be employed to predict erosion in different engineering structures. Hence, in the present work a computational fluid dynamic modeling strategy is proposed, which incorporated fundamental physically erosion parameters to predict erosion in larger diameter pipelines/elbows. The methodology was applied to different elbows/pipelines diameters in order to investigate how pipelines diameter, sand production rate, and sand particles sizes affect the erosion mechanism and the erosion rate. The results showed the importance of including fluid and flow conditions, sand particles trajectory, and self-particles movement. The computational fluid dynaimcs results were compared with those obtained with the most employed empirical models to predict sand erosion in the oil and gas industry models published in the literature, and it was shown that the proposed modeling strategy can be used to predict erosion in larger diameters pipelines/elbows with good results.

Calibration of partial safety factors for suction caissons in connection to catenary and taut-leg mooring systems

A risk and reliability based calibration of partial safety factors for the ultimate limit state of suction caissons subjected to inclined tension loads from floating production systems is presented. The formulation is for the case of normally consolidated clay deposits with undrained loading conditions.The load capacity analysis is carried out using the plastic limit model proposed by Aubeny et al. [1–3]. A procedure to calibrate the plastic limit model based on finite element numerical results is described. Line tensions from two mooring systems of an FPSO designed for two sites in deep water at the Bay of Campeche, Gulf of Mexico, are used.A procedure to characterize probabilistically the load capacity of the caisson at mudline is presented. The physical lower limit of the load capacity and soil-chain interaction are taking into account. The mean and dynamic tensions of mooring lines are modeled through response surfaces in terms of uncertain metocean variables describing extreme sea-states. Reliability analyses are performed using FORM and considering both mooring line tensions and load capacity of the soil-caisson system at the mudline, rather than at the padeye. Partial safety factors for the design of caissons in connection to catenary and taut-leg mooring systems are calibrated separately. This is considered to be most appropriate taking into account their differences in terms of relative contributions of the mean and dynamic tension components, failure mechanisms controlling the loading capacity, and lower bound capacity. Calibration of safety factors is performed for a target reliability index equal to 4.2.Copyright

Calibration of Partial Safety Factors Through the Inverse FORM Method

A calibration method which uses the so-called inverse FORM approach is proposed. The objective is to find the design point on a hypersphere in U-space, whose radius is the target reliability index, which maximizes the response that the structural system must withstand. Once the design point is determined, the safety factors can be calculated. It is demonstrated that the computational cost of the proposed method is less than the computational cost of traditional calibration techniques. The proposed method is illustrated by means of three examples: parabolic, Eurocode and suction caisson design equations.Copyright

Seismic Reliability of Jacket Platforms

The accumulation of fatigue damage produces a deterioration of the structural capacity of offshore marine platforms which increases the risk of a potential failure during a strong earthquake. In this paper a method to calculate the global probability of failure of marine platforms with accumulated fatigue damage subjected to strong earthquakes, is presented.Copyright