Extreme Environmental Loads Research Articles

Influence of pulse-like motions and extreme environmental loads on the seismic foundation response of offshore wind turbines on layered liquefiable soils

Monopile foundations are known as the most common foundation solution for offshore wind turbines (OWTs). However, the state of practice for designing monopile foundations in high seismicity areas is still limited. In particular, the impact of soil liquefaction on the seismic soil-foundation-OWT interaction is not yet well understood. In this paper, three-dimensional (3D), fully-coupled, nonlinear finite-element analyses performed in the OpenSees numerical platform were used to evaluate the seismic performance of a series of hypothetical 5 MW OWTs on monopile foundations in layered, liquefiable sites. A suite of earthquake recordings with and without strong velocity pulses (i.e., near fault, pulse-like and ordinary motions, respectively) was used to investigate the impact of ground motion characteristics on the seismic response of the OWT system. Also, the influence of soil-structure interaction and earthquake shaking coupled with extreme environmental loading (i.e., wind and wave loads) on the seismic performance of soil-OWT systems was evaluated. The numerical results showed pile movements induced by extreme climate loading led to a bias in permanent settlement accumulation across the foundation area and accumulation of soil deformations in the proximity of the pile. Ground motion velocity pulses increased the cyclic stress demand in soil and, therefore, the potential for the occurrence of soil liquefaction. A subsequent, limited numerical sensitivity study showed that the foundation rotations of the OWT system were influenced by ground motion characteristics such as polarity and velocity pulses, and the presence of the wind and wave loads. The cumulative absolute velocity (CAV) was identified as the optimum ground motion intensity measure for permanent foundation settlement and tilt as well as for peak transient foundation tilt of the OWT system under extreme environmental loadings. The net outcome of these factors determined the magnitude and orientation of the foundation rotations at the end of shaking. This study highlights the importance of considering the effects of extreme loadings and pulse-like motions in the design and performance of OWT systems.

Evaluating the Fatigue-Cracking Resistance of North Dakota’s Asphalt Mixtures

Fatigue cracking is a critical pavement distress caused by extreme environmental conditions and heavy repeated cyclic loading. Performance-based design methods that use mechanistic models focus on material performance properties, creating an opportunity to use engineered and recycled materials such as reclaimed asphalt pavement (RAP). The primary goal of this study was to use the simplified viscoelastic continuum damage (S-VECD) model to assess the fatigue behavior of North Dakota’s asphalt mixtures. Eight mixtures typically used in North Dakota were sampled and underwent | E*| testing to determine their linear viscoelastic (LVE) properties. The S-VECD tests were conducted on the same mixtures to determine their damage characteristics. The outputs from the two tests were exported to the FlexMATTM software to obtain their damage characteristic curves, DR failure criterion parameters, and Sapp-index values. CT-index values for the same mixtures were obtained from the North Dakota Department of Transportation (NDDOT). The results revealed that the mixture used for HWY 52, which does not contain RAP, was least susceptible to fatigue cracking. The mixture used for HWY 6, which contains the highest RAP content (25%), had the lowest fatigue resistance, revealing RAP’s stiffening effect on asphalt mixtures. The mixture used for HWY 1, which used the performance grade (PG) 58S-34 binder, had the second-highest fatigue-cracking resistance despite containing 15% RAP, indicating that binder grade affects an asphalt mixture’s fatigue behavior. A relatively low correlation was observed between the CT-index and Sapp-index.

A material-component-structure coupling damage model for the cyclic elastoplastic design of a large-span photovoltaic structure

To reduce structural deadweight without sacrificing stiffness and strength, a large-span offshore fixed truss is designed for bearing photovoltaic devices, and correspondingly, a material-component-structure coupling methodology of cross-scale damage evolution modelling is proposed for analysing the cyclic elastoplastic behaviours of this lightweight and high-strength structure under extreme environmental loads. Utilising this methodology, the mechanical intra-action among microscale defect propagation, mesoscale component failure and macroscale offshore-truss deterioration were characterised, in which a physical relation was established through the reciprocation of the material defect propagation model and refined bulk-member elements. The cyclic elastoplastic performance of the long-span offshore fixed truss was efficiently and accurately simulated. Moreover, this damage evolution modelling provides a reasonable framework for crack prediction by considering microstructure characteristics, stress redistribution and ductile damage, because the crack of the material is caused by void nucleation, growth, and coalescence. A cross-scale modelling approach for the offshore photovoltaic structure was presented and employed to assess the key design issue that affects typhoon performance, in which a coupled solution for the micro-defects in the material and large-scale structural bearing capacity was realised. This proposed methodology can be an alternative design technique for considering the structural features and material properties in practical engineering applications.

A Numerical Study of Brace-Viscous Damper System of Fixed Offshore Jacket Platforms Under Extreme Environmental Loads

The Persian Gulf is one of the most common regions where offshore platforms exist due to the presence of oil and natural gas.Wind, current, and wave loading affect the dynamic response of offshore structures, increasing performance uncertainty and catastrophic failure probability. Thus, this study investigates energy dissipation systems, particularly viscous dampers, to solve design and rehabilitation problems of fixed offshore structures. Viscos dampers improve vibrational behaviour and reduce structural response to optimise platform performance. Thus, eliminating costly structural repairs and strengthening components under extreme environmental loads extends structure lifetime. Thus, different viscous damper system configurations are tested to reduce dynamic response under extreme environmental loads in the Persian Gulf. Diagonal and inverted V-shaped brace viscous dampers with ninedifferent arrangements are compared to find the best configuration. The study found that the brace viscous dampers with only three applied dampers at the top three levels are most efficient at mitigating dynamic response. It reduced displacements from level 1 to level 5 by 52% to 64% and connection stresses by 38% to 54%. Finally, viscous dampers reduce structural vibration and provide uniform and constant structural dynamic response, so the oil and gas industry should use them for offshore structure design and rehabilitation.

Development of design typhoon profile for offshore wind turbine foundation design in Southern China

For offshore wind development in Southern China, typhoon represents the extreme environmental loading condition to be considered in the design of the turbine structures. For the geotechnical design of the foundations, it is specified by the design code that the effect of cyclic loading on the soil strength and stiffness must be considered. This necessitates the complete time history of loads on the foundation experienced during a design typhoon. Idealised storm profiles with staged mean wind speeds and significant wave heights are commonly used as input to time-domain simulations to generate the foundation load history. However, no such design typhoon profile is provided in the Chinese design standard which creates considerable difficulty and uncertainty in the design process. In this paper, the methodology and the rationale that were applied to derive the design typhoon profiles for an offshore wind project in Guangdong province are presented. The wind and wave characteristics generated by typhoon are described in detail and the implications for the foundation design are discussed. To demonstrate application of the proposed typhoon profile, a case study for the ULS design of a suction bucket jacket is presented.

A ReCon-BCALoD clustering algorithm for field monitoring data of marine structures

Monitoring of the extreme marine environmental loads and hazardous dynamic responses of marine structures is critical to maintaining safe field operations. Field monitoring data from marine structures are high-dimensional with high aggregation and weak modes. Furthermore, monitoring data accumulates rapidly over time, which makes it difficult for conventional clustering algorithms to accurately identify the extreme environmental loads and hazardous response conditions experienced by marine structures during their service life. Cluster analysis is an unsupervised learning algorithm widely used in the fields of machine learning and information recognition. Currently, clustering algorithms are mainly used to calculate and analyze large data clusters, but few studies have focused on small clusters. Therefore, this study proposes a ReCon-BCALoD clustering algorithm suitable for long-term monitoring data of marine structures. The ReCon-BCALoD algorithm consists of up and down processes in which the data are clustered based on the ReCon local densities of data points. Applied to field monitoring data, the ReCon-BCALoD clustering algorithm proved capable of automatically determining cluster quantity and effectively identifying small data clusters. This showed that the ReCon-BCALoD clustering algorithm can be used to identify extreme current profile environment conditions and hazardous response conditions of a soft yoke mooring system. These clustering results can be used to improve the safety of in-service structures and provide valid references for future designs of marine structures.

Mooring Analysis of Ocean FarmITS (OFITS)

Ocean Farm ITS (OFITS) is a pilot project of offshore aquaculture designed by Institut Teknologi Sepuluh Nopember (ITS) which will be installed at Sidoasri Beach, Malang Districts. OFITS structure is an integration of fish cages with ecotourism facilities. This paper describes the mooring analysis for OFITS installation. The mooring configuration uses a rectangle grid consisting of 4 mooring buoys, 8 top inner mooring grids, and 14 mooring lines arranged symmetrically. The mooring line uses a type of nylon line with a size of 42 mm in diameter and 70 m in length. Five mooring lines on the front side and two mooring lines on the back side secure the moored buoy to the seafloor. The maximum capacity of buoy is 3 tons. The anchor for mooring uses an anchor with a weight of 0.25 tons for each anchor. The purpose of this analysis is to obtain the maximum tension of the mooring rope at OFITS based on extreme environmental loads. This analysis uses the time domain analysis method.

The Effect of geometric parameters on stress concentration factor of a multiplanar DKT tubular joint through finite element model

It is important to consider geometric parameters on Stress Concentration Factor (SCF) at the tubular joint, especially multi-planar joint, when designing a jacket platform. Thus, a jacket platform capable to withstand the extreme environmental loads. Many researchers use the Finite Element (FE) method to analyze stress at joints and estimate the SCF with the variation of geometric parameters. That studies obtained the result that geometric parameters have significant effect on decreasing or increasing the SCF. Therefore, this study will delve the effect of geometric parameters (β, τ, and γ) on maximum SCF caused by axial loading, in-plane bending moment, and out-of-plane bending moment at DKT tubular joint. The increase of β results in the decrease in maximum SCF at brace-side and chord-side weld toe, respectively. The increase of τ and γ results in the increase in maximum SCF at brace-side and chord-side weld toe, respectively.

Guest Editorial: Special Section on Sustainable and Innovative Use of Emerging Ocean Structural Systems

Near-shore and open-ocean areas offer abundant energy resources. There is an ever-increasing demand to harness these resources in a safe, cost-effective, and sustainable manner. Innovative ocean structural systems can play an important role to meet this demand. Along with advances in engineering design methods and computational power, the development of such structural systems appears to be more efficient than ever before. Still, the design process is nontrivial and often requires physical insights combined with practical considerations. On the one hand, the structural system can be highly complex and may consist of multiple components, some of which require active control. On the other hand, the system can be subjected to extreme environmental loads from wind, waves, and ocean currents, and the structural integrity of the system then becomes a great concern. This Special Section aims to highlight recent efforts in addressing the challenges of next-generation ocean structural systems.There is a wide range of papers included in this Special Section. One study by Li and co-authors proposes a modular floating structure (MFS) that consists of inner hexagonal tension leg platform modules and outer floating artificial reef modules. Relative motions between adjacent modules of the MFS are utilized by wave energy converters (WECs) in the form of ball connectors. Through dynamic response analysis of this novel modular system, the authors demonstrate its technical feasibility in operational and extreme sea states.The subject of WECs is further pursued by Husain and co-authors. In their work, important design parameters are studied of a variable-geometry oscillating WEC. This system resembles a paddle pitching about a fixed hinge, and its geometry variation is achieved by opening controllable flaps on the paddle. By numerical simulations in the time domain, the authors gain insights into structural loads and dynamics, power generation, and the design of such systems.Another study by Ma and co-authors deals with the station-keeping control of an interesting subsea shuttle tanker concept. This tanker was originally proposed for efficient underwater transport of liquid carbon dioxide; ocean currents pose maneuvering challenges during the offloading process. The authors propose and develop a comprehensive maneuvering model and a linear quadratic controller for hovering the tanker in stochastic currents.Additionally in this Special Section, the concept of using a spar floating wind turbine to power a floating production storage and offloading unit is addressed by Schnepf and co-authors. Through numerical simulations and by comparing the dynamics of the structural systems, insights are provided into the design of suspended power cables. In another study, an improved actuator line method was developed by Fan and co-authors and then used to study the wake characteristics and power performance of two wind turbines in tandem.

Tether response of offshore Triceratops under hurricane conditions

The form-dominant approach is increasingly adopted in offshore platform design; triceratops is one of the very recent examples. These platforms are position-restrained by taut-moored tethers, which govern the dynamic response under extreme environmental loads. Due to the partial isolation of the deck from the buoyant legs in the presence of ball joints, sub-structure analysis is more critical. The present study examines the tether response of triceratops in 2400 m water-depth under hurricane conditions caused by 10-yr, 100-yr, and 1000-yr Gulf of Mexico (GoM) storms. The numerical studies show that the Peak-Wave case governs the dynamic tension variation of tethers for the chosen hurricane conditions. For the 100-yr and 1000-yr storms, they are about 15% and 45% higher than 10-yr storms, respectively. However, the maximum axial tensile stresses are about 8% of the yield, ensuring a no-rupture condition of the tethers. The service lives of the tethers for 10-yr storms are about one-third that of 100-yr and 1000-yr storms. A positive minimum tension for all load cases confirms no slackening of the tethers.

Experimental and numerical study of a novel cold-formed steel T-Stub connection subjected to tension force

Cold-formed steel (CFS) is a popular form of construction in New Zealand and the developed world because it can be cost-effective, durable, sustainable, resilient to extreme environmental and seismic loads and easy to install. When used in domestic and medium-rise buildings, T-Stub connection between CFS channel sections is formed by using self-drilling screws which connect the flanges. In the literature, no prior investigation on the tension capacity of such connection has been reported. Therefore, this paper describes a novel test procedure for determining the tension capacity of such CFS T-Stub connection and the associated mechanism of failure. An experimental test program that comprises thirty experimental tests is reported in this paper. Three channel section thicknesses were examined: 0.55 mm, 0.75 mm, and 0.95 mm. All specimens failed as a result of tilting of the screws and bearing failure. Non-linear elasto-plastic finite element (FE) models were developed, and the predictions were validated against the experimental test results. It was adjudged that there was reasonable agreement in terms of connection strength and failure mode prediction. Finally, the design strengths derived from current design standards were compared with the experimental failure loads. It was found that the American Iron and Steel Institute (AISI 2016) and Australian and New Zealand Standard (AS/NZS 2018) are conservative by 6% and unconservative by 10% for the initial and final failure loads of T-Stub CFS connections, respectively.

Eliminating the influence of measuring point failure in ice load identification of polar ship structures

Ice loads are extreme environmental loads for polar ships sailing in ice regions. Field measurement of polar ship structures is the most direct approach for obtaining ice loads. This study investigates the influence of measuring point failure in ice load identification of polar ship structures and consequently proposes effective practices to eliminate this influence considerably. Based on the simulated strain of a typical polar ship's side grillage, the influence is studied in detail, and the spatial distribution characteristics of the identification error in the monitoring area are subsequently determined. According to the systematic analysis of the strain data at measuring points along frames, the spatial distribution characteristics of the strains are determined and further verified considering typical load scenarios. Consequently, the least squares fitting (LSF) method for the estimation of the strains at failed points is proposed. Practical schemes for ice load identification using the influence coefficient matrix (ICM) method under the influence of measuring point failure are presented and discussed as well. It is found that the LSF method decreases the identification error in the loading area, thereby significantly eliminating the influence. Thus, the scheme associating subarea combination with strain fitting results in higher fineness and better accuracy of ice load identification, which is recommended for adoption and has broad application prospects in the future.

Bivariate statistics of floating offshore wind turbine dynamic response under operational conditions

FOWT (Floating offshore wind turbines) belong to the modern offshore wind energy industry generating green renewable energy. Accurate extreme loads and response prediction during power generation is an important design concern. More accurate and reliable estimations of extreme responses are significant for the offshore wind industry as it advances the design, manufacturing and deployment of large FOWTs in the coming decade. In this study, the OpenFAST code was used to model offshore wind turbine mooring line tension force and blade bending moment due to environmental loads, acting on a site-specific FOWT under realistic local environmental conditions. This paper presents an efficient Monte Carlo based method to study bivariate extreme dynamic response statistics. The ACER2D (bivariate average conditional exceedance rate) method is briefly discussed. The ACER2D method enables robust estimation of bivariate statistics, efficiently utilizing available data. Large return period 2D (two-dimensional) probability contours were obtained using the ACER2D method. Based on the studied performance of the presented methodology, it was seen that ACER2D provides accurate and efficient predictions of extreme return period contours.The described approach may be utilized at the design stage, defining optimal FOWT design values to minimize potential structural damage due to extreme environmental loads. It should be noted that the bivariate design point is less conservative than the classic univariate one; therefore, this study advocates a design method leading to lower structural production costs.

Efficient Numerical Solution Method for Large Deformation Analyses of Structures Based on the Updated Lagrangian Formulation

AbstractLarge deformation analyses of structures are of great importance to the evaluation of structural performance under extreme environmental loads, but currently available methods are time-cons...

Promising methods for predicting the residual life of polymer composites in refined variational statements under extreme environmental influences

Modifications of the methods for finding the global extremum of multi-parametric performance indicators are developed, taking into account the structural features of the quality assessment criteria used in predicting the defining characteristics of composites. The article analyzes promising ways to increase the potential opportunities associated with the improvement of methods for constructing optimal directions for finding the absolute minimum of efficiency indicators associated with solving problems of predicting the residual life, reliability, and durability of polymer composites under the influence of extreme environmental factors and operational loads. The comparative analysis of existing approaches and their possible modifications allowed us to conclude that the most promising opportunities for the effective construction of globally optimal solutions in refined variational formulations of inverse forecasting problems are associated with the development of a methodology for effectively narrowing the search areas containing globally optimal solutions. The results of comparative computational experiments are presented.

Assessing Extreme Environmental Loads on Offshore Structures in the North Sea from High-Resolution Ocean Currents, Waves and Wind Forecasting

The fast development of the offshore energy industry becomes an essential component of resilient economies in most of the countries around the North Sea, addressing an increasing demand for cost-efficient and environmentally safe energy sources. Offshore wind farms are planned to be installed further away from the coasts to ensure stronger and more stable wind resources in this region. Oil and gas extraction infrastructures are also planned to move into deeper areas of the continental shelf and continental shelf slopes to explore new fields. These deeper areas of the ocean are characterised by harsh environmental conditions: stronger winds, larger waves and strong shelf slope currents, inducing considerably larger loads on offshore structures. This study brings together operational physical oceanography and the mathematics of fluid-structure interactions to estimate the likelihood of extreme environmental loads on offshore structures in the North Sea. We use the state-of-the-art Met Office high resolution ocean forecasting system, which provides high-frequency data on ocean and tidal currents, wave heights and periods and winds at a ~7 km horizontal resolution grid, spanning the North–West European Shelf. The Morison equation framework is used to calculate environmental loads on various types of offshore structures that are typically employed by the offshore industries in the North Sea. We use hourly data for a 2-year period to analyse the spatio-temporal variability of mean and extreme hydrodynamic loads and derive the relative contributions of currents, waves and winds in the region. The results indicate that waves dominate extreme hydrodynamic forces on the shallow shelf, whereas the current contribution is important at the shelf break and in the English Channel.

Seismic analysis of a model tension leg supported wind turbine under seabed liquefaction

The paper presents numerical analyses of a pile supported Tension Leg Platform (TLP) wind turbine, during seismic loading and seabed liquefaction, taking consistently into account the pile-tendon-platform interaction. The emphasis is on the system response when subsoil liquefaction is extensive, leading to degradation of the pseudo-static safety factor against pile pullout well below unity. It is shown that the pile resistance to pullout failure decreases drastically during shaking, but fully recovers during the following dissipation of earthquake-induced excess pore pressures and even exceeds the initial (pre-shaking) resistance. Pile head displacements develop steadily both during shaking and dissipation, but only while the static pullout safety factor remains less than unity. Due to the high tensional stiffness of the tendons relative to the buoyancy stiffness of the platform considered in this study, the pile head pullout is mostly transmitted to the platform with a relatively small part corresponding to reduction of tendon elongation. The resulting loss of buoy stability and tendon pretension may prove detrimental for the short-term operation of the platform. However, except possibly from the rare scenario of concurrent extreme environmental loads, both effects are recoverable, as they are unlikely to cause irreparable damage to the foundation and the superstructure.

A combined experimental and numerical simulation approach for burst pressure analysis of fiber-reinforced thermoplastic pipes

Reinforced thermoplastic pipes (RTP) are widely used in the offshore oil and gas production industry for their excellent flexibility, corrosion resistance and internal pressure resistance. RTPs inevitably experience extreme environmental loads including internal pressure and tensile load in the procedures of installation and operation, which influence the structural integrity and safety. Therefore, reinforced layers of RTP are designed to bear internal pressure and tensile load. Herein, the mechanical behavior of fiber-reinforced thermoplastic pipe that consisting of aramid fiber braid reinforced layers is investigated by combination of numerical simulation and experimental methods. The full-scale internal pressure bursting experiments of 1-inch aramid fiber RTPs were carried out to study the burst pressure and failure behavior. A finite element model of aramid fiber RTP is established using ABAQUS, which considers the material nonlinearities as well as frictional interactions between layers. Based on the finite element model which validated by the experimental results, the mechanical behavior of fiber reinforced thermoplastic pipe subjected to internal pressure and tensile load is investigated in detail. The current study compares the influence of braided angle of fiber-reinforced layers, friction coefficient between each layer and load conditions on the mechanical behavior of RTPs. The experiment results reveal that the bursting pressure of 1-inch aramid fiber RTPs is about 76.1 MPa. The failure of aramid fiber RTP under internal pressure is dictated by leakage due to the fracture of fiber-reinforced layers and the fracture of the internal layer at the end fitting. It is worth noting that the braiding angle renders an obvious influence on mechanical properties of fiber-reinforced RTPs, whereas the mechanical behavior remains insensitive to friction coefficient between layers. Moreover, the synergistic influence of internal pressure and tensile load significantly affects the mechanical behavior of fiber-reinforced RTPs. The current study shall serve as a reference for the design and application of fiber-reinforced thermoplastic pipes in marine oil and gas production.

Directional Bending Performance of 4-Leg Jacket Substructure Supporting a 3MW Offshore Wind Turbine

A comprehensive investigation of the directional bending performance of a 4-leg jacket substructure, supporting a 3 MW offshore wind turbine, has been carried out in this study. The jacket substructure with a Pratt bracing system which is already installed in the southwest offshore wind farm in South Korea has been chosen as a reference support structure. A numerical model of the 3MW support structure (i.e., tower, transition piece, and jacket structure) is configured, and its structural performances are evaluated under the conditions of (1) extreme environmental loads (Env), (2) critical Design Load Cases (DLCs), and (3) a total of 288 combined load cases (CBs). For the case of Env (i.e., wind, wave, and current loads), loading directions varying from 0° to 360° at intervals of 15° are considered. The DLCs are provided from the 3 MW wind turbine manufacturer, in a 6 × 12 matrix format. The selected 4-leg jacket substructure in this study showed the smallest bending stiffness at the loading angles of 135° and 315° under the condition of Env, and at the loading angles between 105° and 150° under the CBs. From these results, critical bending directionality of the 4-leg jacket substructure is identified. This study also found that the effects of Env loads are not small compared to the total structural responses of the 4-leg jacket substructure which is supporting a 3 MW offshore wind turbine.

Reliability assessment of marine structures considering multidimensional dependency of the variables

For an improved estimation of marine structural reliability, a consideration of random variable dependency is essential. With a limited study on dependence modelling of marine structures, this study proposes a framework for reliability assessment of ocean structural systems with multidimensional variables. This framework captures possible nonlinearity and tail dependence in the variables using vine copula. The proposed method develops a graphical structure of random variables consisting of nodes, edges, and trees using the Drawable Vine (D-vine) approach. This study demonstrates the developed framework on a jacket support structure subjected to the extreme environmental load conditions at Jeanne D′ Arc basin on Canada's east coast. The structure's reliability is evaluated with optimally selected copulas in the D-vine trees and associated marginal distributions. A comparison between the reliability result using the D-vine copula method, Gaussian coupling assumption, and statistical independence between variables proved its superiority in modelling variable dependence of complex marine systems. The probability of failure (Pf) using D-vine copula was closer to the reference Importance Sampling (IS) results than other methods.