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.

This paper explores the performance of a 10 MW offshore wind turbine (OWT) supported either on a large diameter monopile or a 4-legged jacket emphasizing on the nonlinear response of its belowseabed foundation. The seabed foundation alternatives, a monopile and a multipod foundation, are compared under monotonic, cyclic, and seismic loading. For all nonseismic scenarios considered, the monopile is more flexible than the jacket and transmits higher rotations at the OWT base. The differences between the two alternatives are amplified in the case of nonsymmetric cyclic loading; the monopile not only deforms more than the jacket but tends to accumulate irrecoverable rotation with increasing loading cycles. The seismic performance of the alternative support structures is assessed for a comprehensive set of earthquake motions. It is concluded that both systems are seismically robust especially when subjected to pure earthquake loading, neglecting the simultaneous action of wind and waves. Alarming issues for OWT performance may arise when a nonzero steady wind force is superimposed to the kinematically induced stressing of the seabed foundation due to the seismic wave action. Jacket legs settle unevenly, while monopiles are building up rotations at increasing rates. Assuming a design-level earthquake and a wind thrust of the order 60% of the NC wind loading amplitude, this seismically induced residual rotation for the monopile may often exceed the deformation tolerance criterion. For the same loading combination, the corresponding rotation of the Jacket installation remains safely within the prescribed limits.

Freeze-thaw action changes soil microstructure and thus has a great influence on physical and mechanical properties of soils, which is closely correlated to pore water pressure (PWP). Herein, the PWPs of sandy soil and silty clay were measured in laboratory during freeze-thaw cycles (FTC). Experimental results showed that PWP was influenced by temperature, freeze-thaw history (i.e., number of freeze-thaw cycle), soil type and others. The PWP experienced a periodical change as temperatures periodically changes during the FTC testing, the PWP decreased during freezing and increased during thawing. Soil type has a slight influence on the variation of PWP, both in character and extent. A theoretical analysis of PWP in frozen soil was given to explain the PWP changes. In addition, the PWP depression during freezing was a major driving force for water migration. The PWP variations are highly relevant to the changes in soil microstructure such as soil particle (grain size composition and mineral composition), pore structure, and particle arrangement, which will be the focus of further research.

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 winding and unwinding of a pipeline in the reeling installation process involve repeated excursions into the plastic range of the material, which induce ovality, elongation, and changes to the mechanical properties. The reeling/unreeling process involves some back tension required to safeguard the pipe from local buckling. This study examines the effects of winding/unwinding a pipe on a reel at different values of tension on the induced ovality and elongation and the resulting degradation in collapse pressure. In Part I, a model testing facility is used to simulate the reeling/unreeling process in the presence of tension. The combination of reel and tube diameters used induces a bending strain of 1.89%. A set of experiments involving three reeling/unreeling cycles at different levels of tension is performed on tubes with diameter-to-thickness ratios (D/t) of 20 and 15.5 in which the progressive changes in cross-sectional geometry and elongation are recorded. Both ovalization and elongation are shown to increase significantly as the back tension increases. A second set of experiments on the same two tube D/ts is performed in which following a reeling/unreeling cycle at a chosen level of tension, the tubes are collapsed under external pressure. The collapse pressure is shown to decrease significantly with tension, which is primarily caused by the reeling/unreeling-induced ovality. Part II presents models for simulating reeling and the induced structural degradation. The experimental results in Part I are used to evaluate the performance of the models.

Part II presents two modeling schemes for simulating the reeling/unreeling of a pipeline, with the aim of establishing the degrading effect of the process on the structural performance of the pipeline. A three-dimensional (3D) finite element model of the winding/unwinding of a long section of pipeline onto a rigid reel is presented first. The second model applies the curvature/tension loading history experienced at a point to a section of pipe in contact with a rigid surface of variable curvature. Both models use nonlinear kinematic hardening plasticity to model the loading/reverse loading of the material. The 3D model first demonstrates how the interaction of the problem nonlinearities influences the evolution of deformation and load parameters during reeling/unreeling. The two models are subsequently used to simulate the three-reeling/unreeling cycle experiments under different levels of back tension in Part I. The ovality-tension and axial elongation-tension results are reproduced by both models with accuracy for the first cycle, adequately for the second cycle, and are overpredicted for the third cycle. The two models are also used to simulate the reeling/unreeling followed by collapse of the tubes under external pressure experiments. Both models reproduce the measured ovality-tension results and the corresponding collapse pressures accurately. Since the two-dimensional (2D) model is computationally much more efficient, it is an attractive tool for estimating the effect of reeling on collapse pressure. Questions that require exact tracking of the winding/unwinding history and the interaction of the pipe with the reel are best answered using the 3D model.

In this study, freeze-thaw cycles were conducted on samples of a fine grained soil from the Qinghai–Tibetan plateau which had been prepared with different dry unit weights. During freeze-thaw cycles, electrical resistivity was measured. The soil samples were also scanned by X-ray computed tomography (CT) before and after freeze-thaw cycles. Unconsolidated and drained (UD) triaxial compression test was performed to obtain the apparent friction angle and cohesion. Changes in the arrangement and connections between soil particles were analyzed so as to investigate the mechanisms of changes in the strength parameters. The electrical resistivity increased in all samples, regardless of the different original dry unit weights, which implies that in all cases the arrangement of soil particles became more irregular and attached area between soil particles was increased. These changes contributed to the increase of apparent friction angle. On the other hand, the CT scans indicated that, depending upon the original dry unit weight, freeze-thaw cycles induced strengthening or deterioration in particle connections, and thus apparent cohesion was increased or decreased. With three freeze-thaw cycles, changes in microstructure of soil samples led to increases or decrease in both the apparent friction angle and cohesion.

Self-elevating mobile jack-up units have been employed in offshore exploration and development in shallow waters at depths of up to approximately 150 m. Jack-ups are designed to move to a new site after operations are completed. The spudcan footings, which can be embedded up to three diameters deep in soft soil, must therefore be extracted by jacking down the hull into the water and then floating it beyond the neutral draft. This provides the maximum pull-out force to overcome the soil resistance to the jack-ups, but this force may not be sufficient. Problematic cases of this offshore are reported to take up to 10 weeks to extract, a costly exercise for the industry. A method sometimes used offshore is to cycle the spudcans vertically in an attempt to free them. This can be achieved by pushing and pulling the leg by leaving the hull afloat in the water and allowing the impact of small amplitude waves on the hull to generate cyclic loads on the spudcan. This paper reports a series of centrifuge tests investigating the ability to extract a spudcan under regular and irregular cyclic loading. Spudcan extraction tests were performed from a depth of three spudcan diameters in normally consolidated clay in a geotechnical beam centrifuge. The results demonstrate that successful extraction is dependent on the combination of mean pull-out load and the amplitude of the cycling. It is also shown that insufficient tensile static loads and prolonged small cyclic loads result in the dissipation of the negative excess pore pressure at the spudcan invert caused by the buoyancy of the hull in excess of neutral draft. It results in consolidation of soil and changes in the shear strength of the soil and consequently either extraction of the spudcan after a long period of time or unsuccessful leg extraction.

This paper presents a numerical investigation of a two-dimensional (2D) oscillatory flow around a cylinder of different elliptic ratios, in order to study the effect of the elliptic form of the cylinder on the vorticity field and the hydrodynamic forces that act on it. The elliptic ratio ε was varied from 1 to 0.1, where the small axis is parallel to the flow direction, simulating cases ranging from a circular cylinder to the case of a cylinder with a profiled elliptic section. The investigations presented here are for Reynolds number Re = 100 and Keulegan number KC = 5. The numerical visualization of the flow for different elliptic ratios shows five different modes of vortex shedding (symmetric and asymmetric pairing of attached vortices, single-pair, double-pair, and chaotic), which depend on the range of the elliptic ratio. The results show that the longitudinal force increases with the reduction of the elliptic ratio. The transverse force appears from the elliptic ratio ε = 0.75 and increases with the reduction of this ratio in the range of 0.75 ≥ ε ≥ 0.4 , then decreases for ε < 0.4 . On the other hand, concerning the Morison coefficients the results show that the drag coefficient is sensitive to the swirling layout while the coefficient of inertia does not seem to be much affected by the geometry of the cylinder.

The dynamic response of the supporting structure is critical for the in-service stability and safety of offshore wind turbines (OWTs). The aim of this paper is to first illustrate the complexity of environmental loads acting on an OWT and reveal the significance of its structural dynamic response for the OWT safety. Second, it is aimed to investigate the long-term performance of the OWT founded on a monopile in dense sand. Therefore, a series of well-scaled model tests have been carried out, in which an innovative balance gear system was proposed and used to apply different types of dynamic loadings on a model OWT. Test results indicated that the natural frequency of the OWT in sand would increase as the number of applied cyclic loading went up, but the increasing rate of the frequency gradually decreases with the strain accumulation of soil around the monopile. This kind of the frequency change of OWT is thought to be dependent on the way how the OWT is cyclically loaded and the shear strain level of soil in the area adjacent to the pile foundation. In this paper, all test results were plotted in a nondimensional manner in order to be scaled up to predict the consequences for prototype OWT in sandy seabed.

The results of a four points bending test on a box girder are presented. The experiment is part of series of tests with similar configuration but with different thickness and span between frames. The present work refers to the slenderest plate box girder with a plate's thickness of 2 mm but with a short span between frames. The experiment includes initial loading cycles allowing for partial relief of residual stresses. The moment curvature relationship is established for a large range of curvature. The ultimate bending moment (UM) of the box is evaluated and compared with the first yield moment and the plastic moment allowing the evaluation of the efficiency of the structure. The postbuckling behavior and collapse mode are characterized. Comparison of the experiment with a progressive collapse analysis method is made taking into consideration the effect of residual stresses on envelop of the moment curvature curve of the structure.

In the fatigue design of steel catenary risers, there are concerns regarding the fatigue damage to girth welds from low stresses, below the constant amplitude fatigue limit, in the loading spectrum and the validity of Miner's cumulative damage rule. These fundamental issues were addressed in a recent joint-industrial project (JIP). A key feature was development of the resonance fatigue testing rigs to enable them to test full-scale pipes under variable amplitude loading. Such tests were performed under a loading spectrum representative of that experienced by some risers, with many tests lasting over 100 million cycles to investigate the fatigue damage due to small stresses as well as the validity of Miner's rule. However, the resonance rigs are only capable of producing spectrum loading by gradually increasing or decreasing the applied load whereas more “spiky” random load sequences may be relevant in practice. Therefore, the program also included fatigue tests in conventional testing machines on strip specimens cut from pipes to compare the two types of loading sequence. This paper presents the results of these tests, conclusions drawn, and recommendations for changes to current fatigue design guidance for girth welded pipes regarding the definition of the fatigue limit, allowance for the damaging effect of low stresses, and the validity of Miner's rule.

The results of five tests on narrow stiffened panels under axial compression until collapse and beyond are presented to investigate the collapse behaviors of stiffened panels. Tension tests were used to evaluate the material properties of the stiffened panels. The tests were made on panels with two half bays plus one full bay in the longitudinal direction. Initial loading cycles were used to eliminate the residual stresses of the stiffener panels. The strain gauges were set on the plates and the stiffeners to record the strain histories. The displacement load relationship was established. The collapse behavior, modes of failure and load-carrying capacity of the stiffened panels are investigated with the experiment.

Modal decomposition and reconstruction (MDR) of marine riser vortex induced vibration (VIV) is a technique where vibration is measured using accelerometers and/or angular rate sensors, the modal displacements are solved for and the stress and fatigue damage is reconstructed along the riser. Recent developments have greatly increased the accuracy and reliability of the method. However the computational burden is onerous due to stress time history reconstruction and rainflow cycle counting at every desired location along the riser. In addition, fully synchronous data are required to reconstruct the stress histories. Dirlik’s method for obtaining rainflow damage for Gaussian random stress using only spectral information (four spectral automoments) has proven to be quite accurate with a significant reduction in computational effort. In this paper two spectral formulations of MDR are introduced. The first method is applicable when all the measured data are synchronous. In this method, spectral cross moments of the modal displacements are solved from the spectral cross moments of the measured data using basis vectors consisting of normal mode shapes. The spectral automoments of stress are obtained from the modal displacement cross moments and analytical stress mode shapes. Dirlik’s method is then applied to obtain rainflow damage. The second method is a generalization of the first, where the measured data cross moments are only partially known. This method is applicable when measured data are partially synchronous or asynchronous. A numerical root-finding technique is employed to solve for the modal response cross moments. The method then proceeds in the same manner as the first. The spectral methods are applied to simulated VIV data of a full-scale deepwater riser and to Norwegian Deepwater Program (NDP) scale-model test data on a 38 m long slender riser. Comparisons of reconstructed fatigue damage versus simulated or measured damage indicate that the method is capable of estimating fatigue damage accurately for Gaussian VIV even when data are not fully synchronous. It is also shown that computational cost is greatly reduced.

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.

Design S-N curves in design codes are based on fatigue test data, where the stress cycle is under external tension load. It is observed that during pile driving most of the stress cycle is compressive and the design procedure used for fatigue analysis of piles might therefore be conservative. In order to investigate this further, it was proposed to perform laboratory fatigue testing of specimens that are representative for butt welds in piles under relevant loading conditions. In the present project 30 test specimens made from welded plates were fatigue tested at different loading conditions to assess effect of compressive stress cycles as compared with tensile stress cycles. In 2006, the Edda tripod in block 2/7 was taken ashore. This platform has been in service since 1976 and the piles are considered to be representative for the piles installed in the North Sea jacket structures during the 1970s. Therefore it was suggested to investigate the pile weld at the sea bed in detail to assess the stress due to fabrication and 30 years of in-service life and the residual fatigue life of the pile. Six test specimens made from the Edda pile were fatigue tested. The results from the assessment and the fatigue testing are presented in this paper.

In China, the oil and natural gas resources in Bohai Bay are mainly marginal oil fields, which freeze in the winter. It is necessary to build both ice-resistant and economical offshore platforms. However, risk is involved in the design, construction, utilization, and maintenance of offshore platforms as uncertain events may occur within the life-cycle of a platform. In this paper, the optimum design model of the expected life-cycle cost for ice-resistant platforms based on the cost-effectiveness criterion is proposed. Multiple performance demands of the structure, facilities and crew members, associated with the failure assessment criteria and evaluation functions of costs of construction, consequences of structural failure modes including damage, revenue loss, death, and injury, as well as discounting cost over time are considered. Different reliability analysis approaches involved in life-cycle cost evaluation, such as the global reliability under the extreme ice load, the dynamic reliability, and fatigue life induced by ice vibration, are studied. The proposed life-cycle optimum design formulas are applied to a typical ice-resistant platform in Bohai Bay, and the results demonstrate that the life-cycle cost-effective optimum design model is more rational compared with the conventional static design and the optimum dynamic design.

As the offshore industry moves towards deeper water developments and continues to embrace harsh environments, unbonded flexible pipes are increasingly being utilized as a cost effective riser solution. Furthermore, with the advent of issues such as nonpristine annuli environments, the fatigue performance of these flexible risers is becoming a critical issue. This paper presents an overview of the comparisons between deterministic and stochastic global fatigue analysis techniques. Methods used to perform both deterministic and stochastic analyses are outlined, from performing the global analyses to using local models to generate armor wire stresses and subsequent fatigue damage. The paper identifies the key issues in the analysis performed and presents key results and conclusions with regard to the characterization of the wave environment in the global fatigue analysis of flexible risers.

The optimal design of offshore structures is formulated as a decision theoretical problem. The objective is to maximize the expected net present value of the life cycle benefit. The general optimization problem is simplified by taking into account the cost impacts of a possible reconstruction of the structure. The analytical solution to this problem has been derived for the case, where failure events follow a stationary Poisson process. The life cycle benefit is formulated in terms of the production profile, the design and construction costs, failure costs and reconstruction costs. In order to assess the effect of potential loss of lives, the costs of fatalities are included applying the concept of the Implied Costs of Averting a Fatality ICAF . The suggested approach to optimal design, which can be applied for any type of offshore structure, is exemplified considering the special case of steel structures. Here, it is standard to represent the ultimate structural capacity in terms of the Reserve Strength Ratio RSR . For the purpose of illustration, the relation between material usage and RSR , which is valid for monopod structures, is applied. Optimal RSR ’ s and corresponding annual failure rates are assessed for both manned and unmanned structures covering a wide range of different realistic ratios between the potential revenues and costs for construction, failure and reconstruction.

The air gap response of a specific semi-submersible platform subjected to irregular waves is considered. Detailed model tests for this structure are studied in depth. Using time-histories of both motions and air gap, statistical analyses both for the absolute near-structure wave elevation (with respect to a fixed observer), and the relative wave elevation (with respect to the moving structure) are performed. Statistics of wave crest amplification, due to diffraction, are established. Corresponding amplification factors are derived from linear diffraction theory, and the results of theory and observations are critically compared.