A review is provided of various approaches that have been adopted recently to assess the fatigue of ships and offshore structures. The relevant fatigue loading is reviewed first, focusing on the successive loading and unloading of the cargo and the transient loadings. The factors influencing fatigue strength are discussed, including the geometrical parameters, material, residual stress, and ones related to the environment. Different approaches for fatigue analyses of seam-welded joints are covered, i.e., the structural stress or strain approach, the notch stress or strain approach, notch intensity approach, and the crack propagation approach.

The primary objective of the present literature review is to provide a constructive and systematical discussion based on the relevant development, unsolved issues, gaps, and misconceptions in the literature regarding the fields of study that are building blocks of artificial intelligence-aided life extension assessment for offshore wind turbine support structures. The present review aims to set up the needed guidelines to develop a multi-disciplinary framework for life extension management and certification of the support structures for offshore wind turbines using artificial intelligence. The main focus of the literature review centres around the intelligent risk-based life extension management of offshore wind turbine support structures. In this regard, big data analytics, advanced signal processing techniques, supervised and unsupervised machine learning methods are discussed within the structural health monitoring and condition-based maintenance planning, the development of digital twins. Furthermore, the present review discusses the critical failure mechanisms affecting the structural condition, such as high-cycle fatigue, low-cycle fatigue, fracture, ultimate strength, and corrosion, considering deterministic and probabilistic approaches.

Offshore wind substations are subjected to uncertain loads from waves, wind and currents. Sea states are composed of irregular waves which statistics are usually characterized. Irregular loads may induce fatigue failure of some structural components of the structures. By combining fatigue damage computed through numerical simulations for each sea state endured by the structure, it is possible to assess fatigue failure of the structure over the whole deployment duration. Yet, the influence of the discretization error on the fatigue damage is rarely addressed. It is possible to estimate the discretization error on the quantity of interest computed at the structural detail suspected to fail. However, the relation between this local quantity of interest and the fatigue damage is complex. In this paper, a method that allows propagating error bounds towards fatigue damage is proposed. While increasing computational burden, computing discretization error bounds is a useful output of finite element analysis. It can be utilized to either validate mesh choice or guide remeshing in case where potential error on the fatigue damage is too large. This method is applied to an offshore wind substation developped by Chantiers de l’Atlantique using two discretization error estimators in a single sea state.

Cruciform joints in ships are prone to fatigue damage and the determination of type of weld plays a significant role in the fatigue design of the joint. In this paper, the effect of weld geometry on fatigue failure of load carrying cruciform joints in ships is investigated using Effective Notch Stress (ENS) approach. A fictitious notch of 1 mm radius is introduced at the weld root and toe and fatigue stress is evaluated. The effect of weld leg length (l) and weld penetration depth (p) on ENS at weld root and toe are determined. The critical weld leg length (l_cr) at which fatigue failure transitions from weld root to weld toe is investigated. An approximation formula for determination of the critical weld leg length considering weld penetration depth (p) is proposed.

The aim of this work is to study the stress distributions and the location of hot spots stress in the vicinity of the intersection lines of the tubular elements of the tubular TY-joints. Using the finite element models, we analyze the effects of geometrical parameters on the stress concentration factor in the case of in-plane bending and out-of-plane bending loads, around the weld toe of the tubular joints. Our results reveal the location of the maximum stress concentration factor at the heel or toe in the case of in-plane bending loads and at the saddle point in the case of out-of-plane bending loads. Six parametric equations are established and used to calculate the stress concentration factor at critical locations using the non-linear regression method. The results obtained from the finite element analysis are close to the results of the parametric equations and the experimental data from the previous work.

The study aims to calibrate parameters of two-phase fatigue prediction model based on the results of the small-scale fatigue test experiments for zero stress ratio and without residual stresses, and then to investigate their applicability for different stress ratios and in the presence of residual stresses. Total fatigue life using the two-phase model consists of crack initiation phase, calculated by strain-life approach, and crack propagation phase, calculated by fracture mechanic’s approach. Calibration of the fatigue parameters is performed for each phase by fitting numerical to the experimental results. Comparative analysis of calculated and measured fatigue lives is then conducted for different stress ratios, in both stress-relieved and as-welded conditions. Given that calculation parameters are calibrated for the basic case, uncertainty of predictions is large, showing that application of the method for real-life complex marine structures is challenging.

It is essential to precisely predict the crack growth, especially the near-threshold regime crack growth under different stress ratios, for most engineering structures consume their fatigue lives in this regime under random loading. In this paper, an improved unique curve model is proposed based on the unique curve model, and the determination of the shape exponents of this model is provided. The crack growth rate curves of some materials taken from the literature are evaluated using the improved model, and the results indicate that the improved model can accurately predict the crack growth rate in the near-threshold and Paris regimes. The improved unique curve model can solve the problems about the shape exponents determination and weak ability around the near-threshold regime meet in the unique curve model. In addition, the shape exponents in the improved model at negative stress ratios are discussed, which can directly adopt that in the unique curve model.

The main purpose of this paper is to provide a summarized general guideline to aid decision making of choosing the type of fatigue analysis approach, best suited for modelling and evaluating high-cycle fatigue damage in welded structural joints. It describes how addition of stress concentration and stress direction information into fatigue assessment methodology affect simulated fatigue damage accumulation results and when it is beneficial or necessary to use a particular fatigue damage estimation approach. The focus is on stress-life curve based approaches, particularly when deciding between variants of nominal, hot-spot or multiaxial fatigue assessment approaches for evaluating fatigue damage within welded joint structures. Evaluation is illustrated through application of proposed methodology to choose and perform fatigue assessment for a non-conventional load-bearing tubular joint structure within a floating lemniscate crane upper arm, which has been observed of being prone to aggressive crack propagation within its welds. Damage within the structure is estimated using two non-optimal fatigue analysis approaches to verify applicability of proposed selection methodology. Results are then summarized through comparative assessment and findings are discussed based on what leads to result changes within each fatigue damage analysis approach.

The Local Joint Flexibility (LJF) of steel K-joints reinforced with external plates under axial loads is investigated in this paper. For this aim, firstly, a finite element (FE) model was produced and verified with the results of several experimental tests. In the next step, a set of 150 FE models was generated to assess the effect of the brace angle (θ), the stiffener plate size (η and λ), and the joint geometry (γ, τ, ξ, and β) on the LJF factor (f_LJF). The results showed that using the external plates can decrease 81% of the f_LJF. Moreover, the reinforcing effect of the reinforcing plate on the f_LJF is more remarkable in the joints with smaller β. Also, the effect of the γ on the f_LJFratio can be ignored. Despite the important effect of the f_LJF on the behavior of tubular joints, there is not available any study or equation on the f_LJF in any reinforced K-joints under axial load. Consequently, using the present FE results, a design parametric equation is proposed. The equation can reasonably predict the f_LJF in the reinforced K-joints under axial load.

In the present research, results of buckling analysis of 384 finite element models, verified using three different test results obtained from three separate experimental investigations, were used to study the effects of five parameters such as D/t, L/D, imperfection, mesh size and mesh size ratio. Moreover, proposed equations by offshore structural standards concerning global and local buckling capacity of tubular members including former API RP 2A WSD and recent API RP 2A LRFD, ISO 19902, and NORSOK N-004 have been compared to FE and experimental results. One of the most crucial parts in the estimation of the capacity curve of offshore jacket structures is the correct modeling of compressive members to properly investigate the interaction of global and local buckling which leads to the correct estimation of performance levels and ductility. Achievement of the proper compressive behavior of tubular members validated by experimental data is the main purpose of this paper. Modeling of compressive braces of offshore jacket platforms by 3D shell or solid elements can consider buckling modes and deformations due to local buckling. ABAQUS FE software is selected for FE modeling. The scope of action of each of elastic buckling, plastic buckling, and compressive yielding for various L/r ratios is described. Furthermore, the most affected part of each parameter on the buckling capacity curve is specified. The pushover results of the Resalat Jacket with proper versus improper modeling of compressive members have been compared as a case study. According to the results, applying improper mesh size for compressive members can under-predict the ductility by 33% and under-estimate the lateral loading capacity by up to 8%. Regarding elastic stiffness and post-buckling strength, the mesh size ratio is introduced as the most effective parameter. Besides, imperfection is significantly the most important parameter in terms of critical buckling load.

In this paper, the dynamic response of a fixed offshore platform subjected to the underwater explosion (UNDEX) and probable events following it have been investigated. The pressure load due to UNDEX in a specified depth has been applied with a model that considers the effect of blast bubble fluctuations into account. The effect of water on the natural frequency and Fluid-Structure interaction has been modeled as equivalent added mass formulation. The effect of explosion distance on platform response is studied. In this regard, three cases of near, medium, and far-distance explosions are considered. For a case study, a real fixed offshore jacket platform, installed in the Persian Gulf, has been examined. Only the UNDEX pressure load is considered and other dynamic loads such as surface water waves and winds have been neglected. Dead loads, live loads and hydrostatic pressure has been considered in the static case based on the design codes. The results indicated that in near-distance explosions, the UNDEX pressure load can locally damage parts of the platform that are located at the same level as that of explosive material and it can destabilize the platform. In the medium to far distance explosion, a very large base shear was applied to the platform because more elements were exposed to the UNDEX load compared to the near-distance explosion. Therefore, precautionary measures against UNDEX such as risk assessment according to design codes are necessary. As a result of this, member strengthening against explosion may be required.

In this study, the dynamics of the tendon/top tension riser (TTR) system of a tension-leg platform (TLP) are investigated through an experiment and by using absolute nodal coordinate formulation (ANCF). First, the model test of the TLP system is conducted in the water tank of Harbin Engineering University to examine the motion response of the TLP and the dynamic response characteristics of the tendon and TTR. The test scale ratio is set to 1: 66.3. Then, on the basis of the ANCF, the stiffness, external load, and mass matrices of the element are deduced to establish the motion equation of the tendon/riser. Finally, the static and dynamic characteristics of the tendon/TTR system of TLP are analyzed systematically by using the ANCF method. The results are compared with commercial software and test results. The motion response of tendon/TTR is affected by the TLP movement and environmental load simultaneously. The analysis proves the effectiveness and accuracy of the ANCF method despite the low number of riser units, suggesting the superiority of the ANCF method for calculating the dynamics of tendon/riser in the field of ocean engineering.

For the tripod bucket jacket foundations used in offshore wind turbines, the probable critical tilt angles should be avoided during tilt adjustment operation. Thus, these critical values must be identified by engineers, and remedial techniques must be established prior to the occurrence of the problem. Model tests were carried out for typical tilting conditions of tripod bucket foundations, which were allowed to tilt freely at various penetration depths without interruption by manual operation. After the foundation ceased its tilting, some measures, such as water pumping, water injection, air injection, or a combination of the above methods, were enabled for adjustment. The research results showed two critical values in the tilting state of the tripod bucket jacket foundation, namely the terminal and allowable angles. In the installation condition, the terminal angle was negatively correlated with the initial penetration depth, but the opposite was observed with the removal condition. The allowable angle was less than or equal to the terminal angle. The allowable angle in the installation was related to the terminal angle. The critical angles all varied linearly with the initial penetration depth. When tilting during installation, adjustment measures can be used in the order of high drum pumping, low drum water injection, high drum pumping and low drum water injection, air injection, and exhaust. When tilting during removal, the sequential use of low drum water injection, air, and exhaust was applied. For buckets that were sensitive to angle changes, adjustment measures of the “point injection” mode can be selected.