Ivar Langen

Cost optimal reliability based inspection and replacement planning of piping subjected to CO2 corrosion

Abstract A methodology for cost optimal reliability based inspection and replacement planning of piping subjected to CO 2 corrosion is described. Both initial (design phase) and in-service planning are dealt with. The methodology is based on the application of methods for structural reliability analysis within the framework of Bayesian decision theory. The planning problem is formulated as an optimization problem where the expected lifetime costs are minimized with a constraint on the minimum acceptable reliability level. The optimization parameters are the number of inspections in the expected lifetime, the inspection times and methods. In the design phase the nominal design wall thickness is also treated as an optimization parameter. The most important benefits gained through the application of the methodology are consistent evaluation of the consequences of different inspection and replacement plans, consistent incorporation and handling of uncertainties, and consistent updating of inspection and replacement plans based on inspection results. The latter is achieved through application of Bayesian statistics for updating in combination with structural reliability methods.

On stochastic dynamics of floating bridges

Abstract A theoretical analysis of the dynamic behaviour of floating bridges is presented. The main emphasis is placed on the wave-induced response. The waves on the sea surface are idealized as a three-dimensional zero mean covariance stationary Gaussian process and the wave loading processes are derived using the linearized probabilistic potential theory. The structural idealization is based on the finite element technique. Alternative solution techniques of the equations of motion are discussed, including a sample treatment based on Monte Carlo simulation of time-space realizations of the wave loading processes combined with step-by-step integration. Results from numerical analysis of a continuous curved floating bridge are presented. This includes a demonstration of the influences of the shortcrestedness of waves on the structural response.

Measured and predicted dynamic behaviour of an offshore gravity platform

The paper discusses the dynamic characteristics and dynamic behaviour of the Gullfaks C platform as evaluated from full scale measurements and numerical predictions. Two issues are emphasized: identification of a dynamic model for the platform, and a discussion of the dynamic response. In the identification procedure both natural frequencies and mode shapes are used. These are estimated using a multichannel ARMA model. One important parameter is the stiffness of the soil. The identified value for this stiffness is compared with stiffness values estimated from displacement at mudline and calculated base shear. The identified model is furthermore compared with the design model, and the predicted response using the model and a theoretical load model, is compared with measured response. In the discussion of the measured dynamic response the paper features response composition, possible nonlinearities in the soil structure interaction and wave loading. Special attention is given to observed ringing response in the platform.

Application of Nonlinear Stochastic Mechanics in Offshore Engineering

Nonlinear stochastic mechanics are widely used in the field of offshore engineering. In this paper we survey four applications: nonlinear collapse and fatigue analysis of jacket structures, dynamic analysis of jack-up structures, springing and slow-drift response of a tension leg platform, and hysteretic foundation behavior of a concrete gravity platform.

An accurate fatigue damage model for welded joints subjected to variable amplitude loading

Researchers in the past have proposed several fatigue damage models to overcome the shortcomings of the commonly used Miners rule. However, requirements of material parameters or S-N curve modifications restricts their practical applications. Also, application of most of these models under variable amplitude loading conditions have not been found. To overcome these restrictions, a new fatigue damage model is proposed in this paper. The proposed model can be applied by practicing engineers using only the S-N curve given in the standard codes of practice. The model is verified with experimentally derived damage evolution curves for C 45 and 16 Mn and gives better agreement compared to previous models. The model predicted fatigue lives are also in better correlation with experimental results compared to previous models as shown in earlier published work by the authors. The proposed model is applied to welded joints subjected to variable amplitude loadings in this paper. The model given around 8% shorter fatigue lives compared to Eurocode given Miners rule. This shows the importance of applying accurate fatigue damage models for welded joints.