A Comparison of Direct Calculation Approaches Applied on the Fatigue Strength Assessment of a Panamax Container Ship

Today, it is common practice to carry out fatigue assessments of ship structures using direct calculation procedures. A direct calculation analysis of a ship’s fatigue strength involves hydrodynamic analysis, stress response evaluation followed by fatigue damage calculation. Many numerical codes are available for these types of analyses. They could yield different values in a fatigue life prediction because of the different degrees of complexity in the computation of the ship’s response. For example, hydrodynamic loads can be calculated using the strip theory or the panel method. The stress response to these loads can be computed using a beam theory or more advanced analyses, such as global and/or local finite element analyses. In a direct fatigue analysis for ship design, spectral methods have been dominating but there is a growing interest in time-domain fatigue damage calculation procedures. The objective of the current investigation is to compare four commonly used direct calculation methods against measurement data. The comparison is carried out by making a case study on a Panamax container ship on which full-scale measurements have been performed. The computational efforts involved in the application of the current direct calculation methods are compared and their applicability in ship fatigue design is discussed.

Response Based Identification of Critical Wave Scenarios

In the last years the identification and investigation of critical wave sequences regarding offshore structure responses became one of the main topics in the ocean engineering community. Thereby the area of interest covers the entire field of application spectra at sea — from efficient and economic offshore operations in moderate sea states to reliability as well as survival in extreme wave conditions. For most cases, the focus lies on limiting criteria for the design, such as maximum global loads, maximum relative motions between two or more vessels or maximum accelerations, at which the floating structure has to operate or to survive. These criteria are typically combined with a limiting characteristic sea state (Hs, Tp) or a rogue wave. For the investigation of offshore structures as well as the identification of critical wave sequences, different approaches are available — most of them are based on linear transfer functions as it is an efficient procedure for the fast holistic evaluation. But, for some cases the linear method approach implies uncertainties due to nonlinear response behavior, in particular in extreme wave conditions. This paper presents an approach to these challenges, a response based optimization tool for critical wave sequence detection. This tool, which has been successfully introduced for the evaluation of the applicability of a multi-body system based on the linear method approach, is adjusted to a nonlinear task — the vertical bending moment of a chemical tanker in extreme wave conditions. Therefore a nonlinear strip theory solver is introduced into the optimization routine to capture the nonlinear effects on the vertical bending moment due to steep waves acting on large bow flares. The goal of the procedure is to find a worst case wave sequence for a certain critical sea state. This includes intensive numerical investigation as well as model test validation.

Prediction of Fatigue Crack Initiation Life Based on the Extended Cyclic Plasticity Model

In order to simulate mechanical fatigue phenomena under macroscopically elastic condition, the plastic stretching within a yield surface has to be described, whilst the plastic strain is induced remarkably as the stress approaches the dominant yielding state. In this study, a phenomenological plasticity model, proposed for the description of the cyclic loading behavior observed for typical carbon steels during the high-cycle fatigue subjected to stresses lower than the yield stress, is applied for the prediction of fatigue initiation life. The model is formulated based on the unconventional plasticity model and is applied for materials obeying isotropic and kinematic hardening law. The mechanical responses under cyclic loading conditions are examined briefly. Finally, the initiation life of fatigue cracking is discussed based on the proposed model with the damage counting parameter.

Offshore Platform Fluid Structure Interaction Simulation

In this study a one-way coupled fluid-structure interaction (FSI) between ocean waves and a simplified offshore platform deck structure was modeled. The FSI model consists of a Volume of Fluid (VOF) based hydrodynamics model, a structural model and an interface to synchronize data between these two. A Computational Fluid Dynamics (CFD) analysis was used to capture the breaking wave and impact behavior of the fluid on the structure using commercially available software STAR-CCM+. A 3D Finite Element (FE) model of the platform deck developed in ABAQUS was used to determine the deflection of the structure due to hydrodynamic loads. Nonlinear material behavior was used for all structural parts in the FE model. Transient dynamic structural analysis and CFD analysis were coupled by transferring the CFD-predicted pressure distribution to the structural part in each time step using the co-simulation capabilities of STAR-CCM+ and ABAQUS. The one-way FSI model was applied to investigate the possible physical causes of observed wave damage of an offshore platform deck during a hurricane. It was demonstrated that with proper physical conditions/configurations, the FSI model could reproduce a structural deformation comparable to field measurement and provide valuable insight for forensic analysis.

Probabilistic Condition Assessment of Aging Equipment Considering the Effect of Operating Conditions and Maintenance

Assessing the technical condition and remaining useful life of aging equipment is crucial for the life extension of O&G facilities. In order to perform a reliable assessment, models describing the degradation of the equipment are necessary. However, the use of accurate physical models for this purpose may be challenging. Some reasons are that the equipment can be exposed to various degradation mechanisms, which may be influenced by different operating conditions, and that the operational data may be scarce. This paper presents a systematic approach for modelling different degradation mechanisms, assessing the technical condition of a component, and quantifying the expected remaining useful life. The quantification is performed using a Bayesian network. Finally, the application of the proposed model is illustrated with the analysis of a fire water pump.

Investigating the Use of the Hilbert-Huang Transform for the Analysis of Freak Waves

This paper presents the Hilbert-Huang Transform (HHT) with Empirical Mode Decomposition (EMD), which is an opportune method to analyse non-stationary traces of nonlinear waves, with freak waves as a case in point. Comparison to conventional methods such as the Fourier, Wavelet and Stockwell Transform is undertaken. Furthermore, an investigation of different EMD schemes (Ensemble EMD or Smoothed EMD) is undertaken to improve the interpretability of the results. Ultimately the Draupner New Year Wave is examined by using the HHT. After all, on successfully applying the HHT, a new perspective on analysing waves is suggested.

Numerical Investigation on the Plastic Response of a Small-Scale Laterally Impacted Tanker Double Hull Structure

Numerical simulations are presented, on the dynamic response of a one-tenth scaled tanker double hull structure struck laterally by a knife edge indenter. The small stiffeners of the full-scale prototype are smeared in the small-scale model by increasing the thicknesses of the corresponding plates. The dynamic response is evaluated at an impact velocity of 7.22 m/s and the impact point is chosen between two frames to assure damage to the outer shell plating and stringers. The simulations are performed by LS-DYNA finite element solver. They aim at evaluating the influence of strain hardening and strain rate hardening on the global impact response of the structure, following different models proposed in the literature. Moreover, the numerical model is scaled to its full-scale prototype, summarizing the governing scaling laws for collision analysis and evaluating the effect of the material strain rate on the plastic response of large scaled numerical models.

Development of a Relationship Between Residual Ultimate Longitudinal Strength Versus Grounding Damage Index Diagram for Container Ships

Various accidents such as grounding, collision, fire, and explosion commonly occur on operating ships. The structural damage caused by such accidents is often accompanied by casualties and serious pollution. Therefore, an accidental risk-based approach that is in line with the goal-based standard of the International Maritime Organization is being developed in the literature. In the present paper, the residual ultimate longitudinal strength versus grounding damage diagram (R-D diagram) for container ships is established as per the method of Paik et al. [1]. The proposed R-D diagram should be useful for defining acceptance damage criteria and making rapid salvage plans or rescue schemes for container ships that have sustained a grounding accident.

The Fatigue Crack Propagation of the Ship Structures in Random Sea States

The traditional method for the fatigue strength assessment of ship structures is based on S-N curves and Miner linear cumulative damage rules. However, with the development of the ship mechanics, the fracture mechanics method has aroused people’s attention. Some researchers have begun to use the fracture mechanics method to perform the fatigue strength assessment of ship structures. A fracture mechanics based approach for the fatigue assessment of ship structures in random sea states is presented. First, the fatigue stress history of the ship structures in random sea states is simulated. Then, the stress intensity factor in random sea states is calculated through the weight function and the fatigue stress of the ship structures in random sea states. Finally, the crack growth is calculated using Pairs equation for each stress cycle throughout the fatigue stress history of the ship structures in random sea states.

Study on the Influence of Bow Shape to the Sidestructure During Ship Collision

The problem about ship collision is the hot point of the ship mechanics. However, the research work in the past always ignored the influence of the shapes of different striking bows. The quantitative comparison analysis was performed for striking with different bow curvatures and bow angles. The results show that different shapes of the striking bows have an obvious impact on the collision capability. The smaller of the striking bulb curvatures parameter or the bigger of the striking bow angles, the deeper of the limited penetration, the higher of the collision force, the more of the energy absorption at the limited penetration.