To account for the effect of the stress concentration factor (SCF) at the weld toe on stress intensity factor range ΔK, a magnification factor M k is used. The existing M k solutions given in BS 7910 were developed for the fatigue assessment of flaws at the toes of fillet and butt welds and may not be suitable for assessing flaws at single-sided girth weld root toes, where fatigue cracking often initiates and the weld width is relatively small. Finite element (FE) modeling was performed to determine a 2D M k solution for engineering critical assessment (ECA) of a flaw at the weld root bead toe. The weld root bead profile was uniquely characterized by five variables including weld root bead width, weld root bead height, hi-lo, weld root bead angle, and weld root bead radius. Following a parametric sensitivity study, defect size, weld root bead height, and hi-lo were identified as the governing parameters. A total of 6000 FE simulations was performed, covering different combinations of weld root bead height and hi-lo. A series of automation scripts were developed in the python programming language and the M k solution for each type of defect model was developed and provided in a parametric equation. The accuracy of the 2D M k solutions was confirmed by the experimental data. It was found that the methods and M k solutions currently recommended in BS 7910 and DNVGL-RP-F108 are inappropriate for assessing a flaw at a girth weld root toe.

Fatigue design standards for offshore structures became needed with development of offshore structures in harsh environments like the North Sea during the 1970s. The need for fatigue design of ship structures became increased as more high strength steel was being used in these structures during the 1970s. New types of structures and structural components have been developed such as tension leg platforms and floating production platforms and support structures for wind turbines. These structures are subjected to significant dynamic loading such that fatigue design becomes the main issue and relevant fatigue design standards are needed. This paper gives an overview of the development of fatigue design standards for marine structures over the last 40 years.

Pipes, especially risers, pipelines, and umbilicals, are extensively used in the subsea production system. Umbilical, as a controlling component of subsea production system, as well as other pipes, will resist reeling, unreeling, and additional processing before on-site installation, which might lead to yielding and plastic deformation of the pipe. This plastic deformation often results in low cycle fatigue (LCF) issue of the pipes, and how to effectively estimate the corresponding fatigue life has become a topic of practical engineering interest. In the present paper, a structural strain method is applied to determine the elastic core of the pipe and to calculate the pseudo structural stress. The pseudo structural stress concept has been applied to analyze the pipe in LCF regime. Further, the results obtained have been compared with the experimental and other available data. It can be seen that the results coincide well with the experimental data. In addition to the demonstrated effectiveness, the key advantage of this pseudo structural stress approach is the simplicity in dealing with girth-welded pipe sections, since finite element stress analysis is unnecessary.

The current downturn of the oil and gas industry force managers to take hard decisions about the continuity of projects, resulting in delays, postponements, or even their cancellation. In order to keep with them, the rush for cost reduction is a reality and the industry is pushing the involved parties to be aligned with this objective. The Brazilian presalt region, characterized by ultra-deep waters, faces this scenario where flexible risers in lazy-wave configurations are usually adopted as a solution to safe transfer fluids from sea bed until the floating unit. The smaller the buoyancy length, the cheaper the project becomes, reducing the necessary amount of buoys and the time spent for its installation. This paper investigates the possibility of buoyancy length reduction of lazy-wave configurations by using structural reliability methods on fatigue failure mode. The application of the fatigue reliability approach considers four 6 in flexible riser configurations: an original lazy-wave, a lazy-wave with less 30% of buoyancy length, another one with less 50% of buoyancy length and a free-hanging. Failure probabilities and safety factor calibration curves are shown for each configuration and compared among themselves. The results indicate the possibility of defining a lazy-wave configuration with smaller buoyancy lengths, reaching 75% of reduction without changing the preconized high safety class. Structural reliability analysis is available to help engineers understand the driving random variables of the problem, supporting the actual scenario of cost reduction for better decision-making based on quantified risk.

This paper addresses the statistical uncertainty in long-term fatigue damage in offshore structures due to the short-term simulation length used in time domain analysis of stresses. The paper focuses on steel risers applications. A new simulation-based estimator for the variance of the short-term fatigue damage is presented. The proposed estimator is based on a variation of the original nonparametric bootstrap. It works with blocks of data instead of discrete values, in order to better account for the autocorrelation of the stress cycles in the stress time series. This versatile estimator can be applied in time-domain fatigue analyses to assess the variance of the fatigue damage using a single stress time series and does not require any previous assumptions on the stochastic stress process.

The industry consensus would appear that the effect of currents on wave-induced fatigue damage accumulation is assumed as insignificant and can be ignored. Only when dealing with stability, ultimate limit state design, and vortex-induced vibration (VIV), is the recommended industry practice to consider both currents and waves simultaneously, except for fatigue design. This paper presents a study on how environmental loads should be considered in terms of currents and waves for the fatigue life design of offshore pipelines and risers. The study is intended as a spur to redress the misapprehension by focusing on the coupling effect of direct waves and currents in the context of fatigue damage assessment. It is demonstrated unequivocally that waves and currents cannot be decoupled for fatigue design assessments. Wave-induced fatigue with the inclusion of currents is manifested twofold, not only the increased mean stress correction effect but also higher total damage accumulation due to elevated stress ranges. The practice of using wave histograms while ignoring currents is shown to result in an unacceptable nonconservative fatigue design. Both effects should be accounted for in the engineering assessment. A first-order correction factor involving the ratio of current and wave velocities is introduced to evaluating the environmental load coupling effect. It is recognized that fatigue associated specifically with VIV phenomena is well understood and documented elsewhere, its discussion is thus out with the aims of this paper.

Structural safety is one of the most important issues associated with liquefied natural gas (LNG) storage systems, such as LNG carriers, LNG Floating Production Storage Offloading (FPSO), and Floating Storage Regasification Unit (FSRU). One of the most common materials for the LNG storage systems has been 9% nickel steel over the last 50 years as it has excellent mechanical properties under cryogenic temperature. Recently, there have been efforts for lowering the nickel content due to the increased nickel price as well as the high price of nickel based welding consumables. In this respect, 7% nickel steels are recently developed for reducing the associated costs mainly for cryogenic applications. The newly developed 7% nickel steels are known to have improved toughness comparable to that of 9% nickel steels by thermomechanical control process (TMCP) and micro-alloying technology. The main objective of this study is to evaluate the fatigue performance of 7% nickel steels with a special attention to type B LNG carrier applications. Cyclic fatigue and fatigue crack growth rate (FCGR) tests for 7% nickel steels were conducted at room and cryogenic temperatures. Fatigue tests were carried out with three different types of specimens such as base metal, butt weld, and fillet weld to characterize the fatigue properties at various locations. In addition, FCGR tests were carried out using compact tension (C(T)) specimens. The difference of FCGR characteristics among base, weld, and heat affected zone (HAZ) is investigated. The fatigue and FCGR test results of 7% nickel steels are evaluated and compared with reference data of 9% nickel steel. Based on this study, it is observed that the 7% nickel steel exhibits similar fatigue performance in comparison with that of 9% nickel steel.

Multiplanar tubular KK-joints are one of the most popular joint types in offshore structures, which are exposed to cyclic loads and fatigue damages. In this paper, a fatigue prediction method based on scaled model test is proposed. First, a scaled KK-joint, including deviations for model simplification, was designed based on sensitive analysis and similarity analysis. Then, static and fatigue tests on the scaled model under axial loading were performed, by which hot spot stress (HSS) distributions and the maximum HSS were recorded. From the test, the fatigue crack initiates from the location of the maximum HSS and propagates along the weld toe. Finally, the maximum HSS of original KK-joint was deduced by finite element analysis (FEA), and then, the fatigue life was predicted accordingly and compared with the rule-based result.

Quantitative reliability and integrity analysis of steel catenary risers (SCRs) can provide important information about their safety and toward their cost-effective and optimal design. SCRs are one of the commonly used riser systems in offshore production stations. The consequence of an SCR failure is significant; however, the overall safety of the riser is typically not quantified. Especially, because of the uncertainties associated with environmental conditions and structural capacities, quantitative reliability methods can take advantage of available data and developments in computing technology to provide a strong basis for their reliable engineering decision making. This paper presents a simplified approach for assessing the strength and fatigue reliability of SCRs, accounting for the uncertainties with their yield strength and fatigue capacities as well as the environmental conditions. Moreover, the integrity-based optimal design of riser strength limit state for a target annual probability of failure is discussed. The fatigue reliability of the SCR system is also assessed in component and system levels. The proposed method is then applied to a typical SCR attached to a semisubmersible vessel under Gulf of Mexico (GOM) conditions. Results of dynamic (time-domain) analyses under various environmental conditions are used to quantify the SCR safety and integrity and to optimize its design for a target annual probability of strength failure. By estimating the riser system probability of strength and fatigue failure in its lifetime, the strength and fatigue integrity indices, and the optimality factors of the riser sections for the strength limit state, suggestions are provided to improve the riser design. For example, it was found that considering the two main limit states of strength and fatigue failure of the SCR system, a strength failure at the taper stress joint (TSJ) is the likely mode of failure in this riser system, which has a probability of 0.0035 in its 25 year lifetime.

Semisubmersible floating platforms used in offshore deep or ultradeep water environments have hull structures that are comprised of vertical cylinders (columns) connected by braces, pontoons, etc. Several of the connections between these various members are susceptible to fatigue damage. In fatigue damage assessment or fatigue reliability analysis, a global structural response analysis is typically carried out using a finite element (FE) model where internal forces or stresses in the various members are evaluated for specified sea states measured at the site. Of specific interest in the present study is the fatigue reliability analysis of brace-column connection details in a semisubmersible hull unit for selected Brazilian environmental conditions. Stress concentration factors (SCFs) for the selected critical hot spots are applied to the nominal component stresses due to axial forces and biaxial bending. The hot-spot stress response spectra are used with various spectral methods—referred to as Rayleigh, modified Rayleigh (with bandwidth correction), and Dirlik—to estimate fatigue damage using Miner's rule. Uncertainties in some parameters used in the fatigue life assessment are considered and the probability of fatigue failure in the last operational year of the structure is estimated.

Buried linepipe can be exposed to various external interferences and corrosive environment and as a result, damage in the form of dent or corrosion or crack or gouge or combination of any of these damages can form in the pipe wall. A defect combining dent and crack, often known as dent–crack defect, which may lead to a rupture or leak in the pipe wall and hence, the pipeline operator becomes concerned about the performance and safety of the pipeline. A research was recently completed at the Centre for Engineering Research in Pipelines (CERP), University of Windsor to study the influence of dent depth and operating line pressure on the pressure capacity (burst strength) of 30 in. diameter and X70 grade linepipe. This study found that the dent depth of 12% with crack depth of 4 mm or more can reduce the pressure capacity by 38%. This paper discusses the test specimens, test setup, test procedure, test results, and data obtained from finite element analyses.

The design of welded structures for the fatigue limit state is normally carried out by means of either linear or bilinear S-N curves, which have been found adequate to predict crack initiation only. To properly assess the effects of the design, fabrication, inspection, and repair strategy for structure degradation due to crack growth, fracture mechanics (FM) models need to be applied. In this paper, alternative S-N and FM formulations of fatigue are investigated. The probabilistic fracture mechanics approach predicts the fatigue life of welded steel structures in the presence of cracks under random spectrum loading. It is based on a recently proposed bi-linear relationship to model fatigue crack growth. Uncertainty modeling, especially on fatigue crack growth parameters, is undertaken with the aid of recently published data in support of the bilinear crack growth relationship. Results pertaining to the fatigue reliability and fatigue crack size evolution are presented using the Monte Carlo simulation technique and the emphasis is placed on a comparison between the linear and bilinear crack growth models. Variations in the system configuration, service life, and coefficients of crack growth laws have been studied on the parametric basis

The development of a vortex induced vibration (VIV) fatigue factor of safety (FoS) consistent with state-of-the-art industry design practice is cast within the coherent framework of reliability analysis. The proposed methodology consists of the following steps: (i) define the failure criteria or limit-state function (ii) setup a deterministic analysis model (iii) characterize the uncertainties involved in the problem (iv) propagate the uncertainties through the deterministic model and assess the probability of failure due to VIV fatigue and (v) calculate the FoS required to achieve a given failure probability. The proposed methodology is demonstrated by determining the FoS associated with using state-of-the-art VIV prediction models to attain varying reliability levels (probabilities of failure) in a hypothetical design scenario. Prediction uncertainty is based herein on measured flow and response data for several full-scale drilling risers working in the field. Results indicate that depending on the reliability level required of a particular design, different FoS than those that currently appear in guidance may be appropriate. Results also indicate the sensitivity of the FoS to the riser and prevailing current type, analysis program and input parameters, and accumulation of conservatism in aggregate versus single-event damage predictions.

Multiaxial fatigue analysis on reeled steel tube umbilical is proposed based on finite element analysis (FEA) results. Due to the complexity of the umbilical geometry, the stresses and strains field becomes multiaxial, worsening the fatigue resistance. The reel-lay method is one of the most efficient and cost-effective methods of umbilical installation. However, if installed by the reel-lay method, material properties of the umbilical change may have an influence in fatigue performance of the line. Finite element analysis is applied with contact formulation, and the load history through the reel, aligner, and straightener is simulated. The strain history of the whole modeling process in the critical element is given, and the properties' change after the load history is discussed. Then, the principal stress criterion and the Brown–Miller criterion (both combined with the critical plane approach) based on FEA results are used to do fatigue life prediction of reeled and nonreeled models. The results indicated that a reel-lay method has a great influence on the fatigue life of steel tube umbilical, so the influence of installation on the steel tube umbilical fatigue must be considered when the reel-lay method is adopted.

This paper develops the models for the structural performance of the loading and probabilistic characterization for the fatigue and the serviceability limit states for the support structure of offshore wind energy converters. These models and a sensitivity study are part of a risk based assessment and monitoring framework and will be applied for establishing the “as designed and constructed” reliability as prior information for the assessment and the design of monitoring systems. The constitutive physical equations are introduced in combination with the fatigue and serviceability limit state requirements as the starting point for the development of the structural performance and loading models. With these models introduced in detail, several modeling aspects for both limit states are analyzed. This includes analyses of the influence on the hot spot stresses by applying a contact formulation for the pile guide brace connection and the application of a finite element formulation using solid elements. Further, the comparison of the natural frequencies of a discrete rotor model with a continuous rotor model is documented. To account for uncertainties associated with the structural and loading models, a probabilistic model is derived on the basis of literature review and measurement data from a prototype Multibrid M5000 support structure. The sensitivity study is based on the calculation of a nonlinear coefficient of correlation in conjunction with predetermined designs of experiments. This is conducted by a systematic analysis of the influence of the random variables on limit state responses and hence on the structural reliability. Integrating the analyses and sensitivity studies of the fatigue and serviceability limit state models developed in this paper as well as the ultimate limit state models in Thöns et al. (“Ultimate Limit State Model Basis for Assessment of Offshore Wind Energy Converters,” ASME J. Offshore Mech. Arct. Eng.), the model basis for the assessment is completed. The process of establishing and analyzing such a model basis contributes to a detailed understanding of the deterministic and probabilistic characteristics of the structure and provides valuable insights in regard to the significance of available data.

T-joint connections are commonly encountered in many machine components and load carrying members. The T-joint connections can be fabricated using traditional cutting (machine or flame) or by laser cutting techniques. The present study investigates the feasibility of using laser cutting to produce welded square hollow-section T-joints with similar or higher fatigue strengths than their conventional mechanical cut counterparts. A total of 21 full-scale T-joint samples, typical of those found in the agricultural industry, were included in this study. Nineteen of these samples were examined with the intention of forming a fatigue crack of approximately 3.8 cm (1.5 in.es) in length, and two samples with strain gauges attached for strain measurements. The experimental results of the full-scale T-joints subjected to cyclic loads showed that the fatigue strength of the samples that were manufactured with laser cutting were higher than those fabricated with mechanical cutting. A finite element analysis (FEA) of the T-joints was also performed, and the FEA results were verified with the experimental strain measurements.

A series of well-known tubular joints tested in UKSORP II have been re-evaluated using the mesh-insensitive structural stress method as a part of the on-going Battelle Structural Stress JIP efforts. In this report, the structural stress based analysis procedure is first presented for applications in tubular joints varying from simple T joints, double T Joints, YT joints with overlap, and K joints with various internal stiffening configurations. The structural stress based SCFs are then compared with those obtained using traditional surface extrapolation based hot spot stress methods. Their abilities in effectively correlating the fatigue data collected from these tubular joints are demonstrated. These tests are also compared with the T curve typically used for fatigue design of tubular joints as well as the structural stress based master S-N curve adopted by ASME Section VIII Div 2. Finally, some of the implications on fracture mechanics based remaining life assessment for tubular joints are discussed in light of the results obtained in this investigation.

The fatigue assessment of support structures is one of the most significant challenges in the design of offshore wind turbines (OWT). Fatigue analysis can be conducted in either the time-domain or the frequency-domain. The advantage of frequency-domain analysis is its time efficiency. This paper shows how the frequency domain method can be used to dimension lattice-type OWT towers such that they meet the fatigue criteria in the preliminary design stage. Two types of lattice towers, a three-legged and four-legged version, were redesigned in the fatigue limit state for the NREL 5 MW baseline wind turbine sited at a water depth of 35 m. The wall thickness of the members was chosen as the only variable and varied during the design process until the towers could survive for at least 20 years. In comparison with designs based upon ultimate strength, the mass of both types of towers increased no more than 30% when the fatigue limit state was considered. It is concluded that the lattice type structure requires only half as much material as its tubular counterpart. The three-legged tower is promising because of its simple geometry, even though it displayed a lower torsional stiffness than the four-legged tower. All the analyses in this paper were performed by an in-house FE code, intended for the early design stage of lattice towers. Once the optimum configuration is found in the early design stage, integrated time-domain analyses for the entire OWT system might be required to refine the design, taking all the nonlinear parameters into account.

Residual stress may have a significant effect on the fatigue strength of welded joints. As a nonfluctuating stress, it has an effect similar to that of the mean stress. Recently the International Association of Ship Classification Societies (IACS) has issued Common Structural Rules (CSR) for respectively tankers and bulk carriers. The effect of mean stress in fatigue design is taken into account in both sets of rules. However, the treatment is quite different, in particular with regard to residual stress and shakedown effects. In the present paper a comparative study of fatigue design procedures of the IACS rules is reported, with emphasis on residual stress effects. Testing was carried out with longitudinal attachment welds in the as-welded condition. The initial residual stress was measured by a sectioning method using strain gauges. Hot spot stress was determined experimentally by strain gauges and numerically by finite element analysis using different types of elements. Fatigue testing was carried out and SN-curves were plotted according to the relevant stress as specified by the rules. In order to investigate the shakedown effect of residual stress, testing was performed for several preload conditions, which could be taken to represent maximum load levels in a load history. The aim of the study is to contribute towards a better understanding of the effect of residual stress and shakedown on fatigue strength of welded joints.

The objective of this study is to investigate the typical failure mode and to obtain the stress range versus number of cycles to failure (S-N) data of MARK-III type liquefied natural gas (LNG) insulation system under the fatigue loading at actual cryogenic environment. A systematic experimental research is carried out for the assessment of the fatigue strength of MARK-III insulation system at cryogenic temperature. Three different types of test specimens are tested for the evaluation of fatigue performance of MARK-III insulation system. Test specimens are determined considering the fatigue vulnerable locations such as mastic area, slit area, and top bridge pad area inside the actual LNG cargo tanks. All test specimens are fabricated as close as possible to the actual yard practice. A series of fatigue test results is represented as S-N curves. Cyclic fatigue loadings were carefully considered similar to the actual sloshing loads. The effect of sloshing impacts is considered by selecting the stress ratio ( R = − 10 ) . The load levels have been determined based on the ultimate strength of reinforced polyurethane foam as 12.2 bars. Different cryogenic temperatures are employed according to the test locations in consideration of temperature gradient within the insulation system. All test results including fatigue life, as well as failure locations of MARK-III insulation system at cryogenic temperatures, are reported and compared with those at room temperature. Consistent S-N curves of MARK-III insulation system at both room and cryogenic temperatures are obtained and compared. The slopes of S-N curves from both fatigue test results are observed to be almost identical, and the fatigue strengths are found to exhibit similar trend. The results from this research can be used for the fatigue assessment of the LNGC insulation system, as well as a design guideline of LNG CCS at cryogenic temperature.