Floating offshore wind turbines are subjected to higher tower fatigue loads than their fixed-to-seabed counterparts, which could lead to reductions in turbine life. The worst increases are generally seen in the tower axial fatigue, associated with the tower fore-aft bending moment. For a spar type platform this has been shown to increase by up to x2.5 and, for a semi-submersible platform, by up to x1.8. Reducing these loads would be beneficial, as the alternative of strengthening the towers leads to increases in cost. Here, two offshore floating wind turbine systems, of the spar type, are analysed and selected responses and tower fatigue compared: one incorporates a variable speed, variable pitch-to-stall blade control system and a back twisted blade, and the other a conventional pitch-to-feather control. The results are then compared to those obtained in an earlier study, where the same turbine configurations were coupled to a semi-submersible platform. A weighted wind frequency analysis at three mean turbulent wind speeds highlights that the impact of the back twist angle magnitude and initiation point on tower axial fatigue life extension was the same for both platform types. Compared to their respective feather base models, an increase in the tower axial fatigue life of 18.8% was seen with a spar platform and 10.2% with a semi-submersible platform, when a back twist angle to the tip of −6° was imposed along with the variable speed, variable pitch-to-stall control.

Design of a steel catenary riser (SCR) requires the use of connection hardware to decouple the large bending moments induced by the host floater at the hang-off location. Reliability of this connection hardware is essential, particularly in applications involving high pressure and high temperature fluids. One option for this connection hardware is the metallic tapered stress joint. Titanium (Ti) Grade 29 has been identified as an attractive material candidate for demanding stress joint applications due to its “high-strength, low weight, superior fatigue performance and innate corrosion resistance” 2 . Titanium stress joints for deep-water applications are typically not fabricated as a single piece due to titanium ingot volume limitations, thus making an intermediate girth weld necessary to satisfy length requirements. As with steel, the potential effect of hydrogen embrittlement induced by cathodic and galvanic potentials must be assessed to ensure long term weld integrity. This paper describes testing from a joint industry project (JIP) conducted to qualify titanium stress joint (TSJ) welds for ultra-deepwater applications under harsh service and environmental conditions. Corrosion-fatigue crack growth rate (CFCGR) results for Ti Grade 29 1G/PA gas tungsten arc welding (GTAW) specimens in seawater under cathodic potential and sour brine under galvanic potentials are presented and compared to vendor recommended design curves.

The development is presented of an analytical model for the prediction of the stochastic nonlinear wave loads on the support structure of bottom-mounted and floating offshore wind turbines. Explicit expressions are derived for the time-domain and frequency-domain nonlinear exciting forces in a seastate with a significant wave height comparable to the diameter of the support structure based on the fluid impulse theory. The higher-order moments of the nonlinear load are evaluated from simulated force records, and the derivation of analytical expressions for the nonlinear load statistics for their efficient use in design is addressed.

It is well known that material properties undergo significant changes with temperature. In order to meet extreme environmental requirements for ships and offshore structures operating in Arctic regions, the effect of temperature on material behavior needs to be considered. In recent studies, significantly higher fatigue strength was observed for base materials and welded joints in comparison to room temperature. Fatigue strength increased even for temperatures far below the allowed service temperature based on fracture toughness results; however, sub-zero temperatures fatigue data are scarce and the effects of steel strength and welding type on fatigue strength changes are unknown. Material selection for ships and offshore structures is typically based on empirical Charpy and fracture toughness relations at the design temperature, minus a safety margin. Thus, this study presents material test results including fatigue tests of butt-welded joints, tensile test, and Charpy impact toughness tests at room and sub-zero temperatures of different structural steel types. Additionally, the effect of welding techniques and steel strength are discussed. The results can be used to extend design approaches for ships and offshore structures subject to sub-zero temperatures and to improve material selection for ships and offshore structures operating in Arctic regions.

A study was conducted to investigate the effects of surface microhardness on different phases of fatigue damage. This helps to estimate the evolution of the material resistance from microplastic distortions and gives pertinent data about cumulated fatigue damage. The objective of this work is to propose a damage criterion, associated with microstructural changes, to predict the fatigue life of steel structures submitted to cyclic loads before macroscopic cracking. Instrumented indentation tests (IIT) were conducted on test samples submitted to high cycle fatigue (HCF) loads. To evaluate the role of the microstructure initial state, the material was considered in two different conditions: as-received and annealed. It was observed that significant changes in the microhardness values happened at the surface and subsurface of the material, up to 2 µm of indentation depth, and around 21% and 7% of the fatigue life for as-received and annealed conditions, respectively. These percentages were identified as a critical period for microstructural changes, which was taken as a reference in a damage criterion to predict the number of cycles to fatigue failure (N f ) of a steel structure.

A mooring fatigue assessment for mooring chain links of a semi-submersible in Offshore West Africa (OWA) is presented. Three cases that mooring chain links are subjected to pure tension, out-of-plane bending (OPB), and torque are considered in the assessment. For the case that mooring chain links are subjected to pure tension, a comparative study on S–N curves, T–N curves, and fracture mechanics (FM)-based mooring system fatigue analyses is made, and the results show that the fatigue lives predicted by these three approaches are generally comparable if the safety factors suggested by API and DNVGL are applied to T–N curves and S–N curves based approaches. For the cases that mooring chain links are subjected to the OPB and torque, the investigation shows that fatigue lives of mooring chain links are decreased significantly due to the OPB effects, while the decline of fatigue lives of mooring chain links happens when the twist angles are more than 10 deg.

A simplified analysis method was developed to evaluate the fatigue damage of an ice-going ship under broken ice conditions. The ice resistance, which is available in the early design stages of an ice-going vessel, and the hull form information were used to estimate the local ice load acting on the outer-shell of the ship. The local ice load was applied to a finite element analysis model, and the Weibull parameters of the local stress range at the target fatigue point were derived. Finally, the fatigue damage was evaluated by applying the S–N curve and Palmgren–Miner rule. To verify the proposed method, numerical analyses using the direct approach, which takes into account the detailed interaction model among the fluid, structure, and ice, were performed under the same conditions. Fatigue analyses of the Baltic Sea for actual ice conditions were performed to check the validity and applicability of the proposed method, and the resulting fatigue damage was compared with that obtained by the classification society’s recommended practice.

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.

This paper gives an overview of the factors that affect the strength and structural design of advanced thin-walled marine structures with reduced plate thickness or alternative topologies to those used today in the marine industry. Due to production-induced initial deformations and resulting geometrical non-linearity, the classical division between primary, secondary, and tertiary responses becomes strongly coupled. Volume-averaged, non-linear response of structural element can be used to define the structural stress–strain relation that enables analysis at the next, larger, length scale. This, today’s standard homogenization process, needs to be complemented with localization, where the stresses are assessed at the details, such as welds for fatigue analysis. Due to this, the production-induced initial distortions need to be considered with high accuracy. Another key question is the length-scale interaction in terms of continuum description. Non-classical continuum mechanics are needed when consecutive scales are close. Strain-gradients are used to increase the accuracy of the kinematical description of beams, plates, and shells. The paper presents examples of stiffened and sandwich panels covering limit states such as fatigue, non-linear buckling, and fracture.

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