Roberto Ojeda

Pitting Degradation Modeling of Ocean Steel Structures Using Bayesian Network

Modelling depth of long-term pitting corrosion is of interest for engineers in predicting the structural longevity of ocean infrastructures. Conventional models demonstrate poor quality in predicting the long-term pitting corrosion depth. Recently developed phenomenological models provide a strong understanding of the pitting process however they have limited engineering applications. In this study, a novel probabilistic model is developed for predicting the long-term pitting corrosion depth of steel structures in marine environment using Bayesian Network. The proposed Bayesian Network model combines an understanding of corrosion phenomenological model and empirical model calibrated using real-world data. A case study, which exemplifies the application of methodology to predict the pit depth of structural steel in long-term marine environment, is presented. The result shows that the proposed methodology succeeds in predicting the time dependent, long-term anaerobic pitting corrosion depth of structural steel in different environmental and operational conditions.

Wave-in-deck forces on fixed horizontal decks of offshore platforms

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Experimental investigation of extreme wave impacts on a rigid TLP model in cyclonic conditions

This paper describes a series of model tests of a rigidly mounted tension leg platform (TLP) subjected to extreme wave events corresponding to long-crested irregular wave trains of a 10,000-year cyclonic sea state. The experimental setup was instrumented to simultaneously measure wave surface elevations in the vicinity of the model, global wave impact forces and local pressure distribution on the underside of the models topside deck. Model accelerations were also monitored for each wave impact event so that the inertial force due to structural dynamic response could be identified. The deck–column intersection areas were found to experience large wave-in-deck slamming pressures, in particular around the aft columns. A reduction of the deck clearance was found to increase the magnitude of the global horizontal forces; however, the global vertical forces and local wave-in-deck slamming pressures did not follow this trend.

Measurements of global and local effects of wave impact on a fixed platform deck

This article describes a series of model tests conducted to examine extreme wave events associated with tropical cyclonic conditions and their impacts on an offshore deck structure. Extreme waves of a representative cyclonic sea state were examined in a towing tank within long-crested irregular wave trains. Experimental results presented include global forces and localised slamming pressures acting on a rigidly mounted box-shaped deck, which represents a simplified topside structure of a tension leg platform. The effect of static set-down on the still-water air gap was investigated by applying an equivalent reduction for the deck clearance. It was found that a small reduction of 20 mm (2.5 m full scale) in the original deck clearance can lead to a doubling of the magnitude of the horizontal force and the vertical upward-directed force components, as well as significantly increased slamming pressures in many locations on the deck underside.

ANFIS method for ultimate strength prediction of unstiffened plates with pitting corrosion

ABSTRACT Increasing attention has recently been paid to the effects of localised pitting corrosion on the ultimate strength of marine structures. In this paper, an adaptive neuro-fuzzy inference system (ANFIS) method was developed to predict the ultimate strength reduction of steel plates with pitting corrosion subjected to uniaxial in-plane compressive loads. Published ultimate strength data-sets for unstiffened plates affected by pitting corrosion were used to train and test a series of ANFIS models composed of several input variables. To develop the best accurate model, rule-based fuzzy sets were used for mapping the inputs to the output using seven different types of membership functions. The two-sided Gaussian-type function was found to be more effective and less sensitive to the sample size than other functions tested. The developed method provided good estimates (maximum RMSE of 0.019) in comparison with published results obtained using the finite element and artificial neural network methods.

Simulating Loads and Motions of Full-Scale Offshore Platforms in Cyclonic Wave Conditions

The commercial Computational Fluid Dynamics (CFD) code STAR-CCM+ was used to simulate the dynamic behaviour of a tension leg platform in extreme weather conditions. The numerical results of surge motions and tendon tensions were compared against the measurements acquired in model tests. The full-scale CFD simulations were then conducted on the basis of the settings performed in modelscale simulations. Both model- and full-scale surge motions and tendon tensions predicted by CFD were in good agreement with the measurements. Using CFD results, it was revealed that the component of the vertical wave-in-deck force caused a slam force on the platform followed by tendon slack situations in the down-wave tendons.

Localized Fatigue Failure Analysis of Aluminium Crewboats due to Propeller Pulse Loads: A Case Study

Marine grade alloys are extensively being used in high speed vessels such as patrol crafts, ferries and crew boats, where a reduction of the structural weight is critical to achieve higher speeds [1]. The use of aluminium has forced marine industry engineers to develop methods to design against fatigue failure. This has largely been addressed by the development of design standards, analysis techniques and the improvement of quality control and construction methods [2]. Nevertheless, even with these advancements there is a continued need for the development and improvement of aluminium analysis methods and guidelines [3].