In the transitional waters of 30 to 90 m, jacket foundation has great application potential due to its advantages of light weight, high structural stiffness and good stability. In addition to the long-term normal wind and waves, the wind turbines will suffer from typhoons and waves in extreme bad weather. Currently, research on the dynamic response of jacket supported OWTs in clay under severe typhoons is very rare. The study develops a numerical method to calculate the dynamic response and fatigue damage of jacket supported OWTs under typhoon loads by incorporating a simplified single bounding surface model of clays. Through three-dimensional numerical analysis across various scenarios, this study investigates the dynamic response characteristics of jacket supported OWTs on clay soil. It also examines the impact of wind-wave coupling effects on the fatigue damage experienced by these structures. It was found that severe typhoons can lead to notable permanent tilting of the jacket foundation, thereby failing to meet the requirements of normal serviceability limits. The most critical nodes of the OWT are situated at the mudline of the pile foundations, followed closely by the bottom of the tower structure. The most significant fatigue damage occurs for wind-wave co-directional coupling loading along the orthogonal direction of the OWT. The research outcomes provide valuable guidance for enhancing the typhoon-resistant design of jacket supported OWTs.

In this study, the fatigue damage to a power takeoff (PTO) shaft was evaluated under various operating conditions in rotary-tillage operations, considering soil strength and texture. Pearson correlation analysis was conducted to identify the significant variables influencing PTO shaft fatigue damage, and a prediction formula was derived through regression analysis using these variables. The PTO shaft exhibited increased shear stress with higher transmission gear stages, PTO gear stages, or soil properties, including strength and texture. The fatigue damage increased with higher transmission gear stages and soil strength while decreasing with higher PTO gear stages. Notably, as the PTO gear stage increased, the mean stress increased; however, the stress amplitude and equivalent completely reversed stress significantly reduced fatigue damage. Statistical analyses revealed a strong correlation between PTO shaft fatigue damage and factors such as tractor travel speed, PTO shaft power consumption, PTO shaft rotational speed properties, including strength and texture. The developed prediction equation, incorporating all significant variables, demonstrated, with a coefficient of determination (R2) of 0.93 and a root mean square error (RMSE) of 2.94x10-9. This equation effectively identifies trends in PTO shaft fatigue damage based on key operational variables. Furthermore, the findings emphasize the critical role of soil texture in assessing PTO shaft fatigue damage.

The stable protection of the walls of high-temperature geothermal wells is a challenging issue for sustainably exploiting geothermal resources. However, the cement stone filling layer of the cemented portion of the well deteriorates gradually during geothermal mining due to the dry-wet cycles of the saline geothermal water, reducing the service life of the geothermal well. For this, this paper presented five groups of cement stone cylinders with salt contents of 0%, 1%, 6%, and 11%, which were subjected to heating to 300 degrees C and 1-5 dry-wet cycles. Nuclear magnetic resonance (NMR) and nonmetallic detection were used to test and analyze the porosity and wave velocity. Additionally, the damage evolution induced by dry-wet cycles was captured based on acoustic emission (AE) data. The experimental results indicated that the heating process primarily resulted in mineral and salt crystal expansion, which in turn caused damage. The damage threshold due to the salt content was found to be 6%. The sudden increase in the thermal stress caused by cooling and deterioration of the tensile strength of the cement column were the key factors in the damage during the cooling process. As the number of cycles increased, the accumulated AE energy moved forward and backward, with decreasing and increasing temperature, respectively. The threshold of signal mutation in the heating process is 200 degrees C, and the accumulated AE energy decreases by 11.7%. When the salt content was 0%, 1%, 6% and 11%, the wave velocity decreased by 19%, 27.3%, 35.5% and 35.9%, respectively. This study also proposed a damage model, which could provide theoretical support for long-term health monitoring and safety protection of geothermal wells. (c) 2024 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/ 4.0/).

Jacket foundation is typically the preferred choice for Offshore Wind Turbines (OWTs) erected in water depth varying from 40 m to 80 m. In this paper, an integrated dynamic analysis model is designed to study the coupling between aerodynamics, servodynamics, hydrodynamics, soil-structure interaction for piled jacket OWTs. The performances of the AeroDyn and ServoDyn modules are verified by FAST, showcasing their applicability under deterministic and stochastic environmental conditions. The OWT dynamic responses, especially for t-z modeling, stress-transfer mechanism and structural fatigue damage, are subsequently studied. The overall deformation of the jacket calculated by the nonlinear elastic t-z curve in the API guideline, is overwhelmed by the t-z curve formulated using bounding surface plasticity framework, due to the ignorance of the loading history effect. Accompanied by a compressed-released-recompressed stress-transfer process, the downwind tube would experience high stress level, hence necessitating more attention in the ultimate limit state design of piled jacket structure. Otherwise, the upwind tube seems to be more decisive to the fatigue limit state design of piled jacket structure, owing to severe fluctuation in structural stress caused by a tensed-released-re-tensed stress-transfer tendency.

Compressive cyclic loads induce a progressive failure in rock materials, and the long-term stability can not be guaranteed by the strength under monotonic load. To this end, the present study aims at establishing an elastoplastic fractional fatigue damage model for predicting the accumulative deformation of rock materials in a unified framework. A fractional-order plastic flow rule is introduced to describe volume transformation of rock sample from compression to expansion, eliminating the need for plastic potential functions. And a hardening function with an equivalent plastic shear strain is adopted. Concerning the fatigue effects, the progressive deterioration of material due to cyclic loads is intricately linked to microstructural degradation, depicted by a convolution law. In the context of creep deformation, loading cycle serves as an equivalent time measure, connecting the plastic deformation with the fatigue damage. In order to verify the accuracy, the proposed model is numerically implemented by a returning mapping procedure simulate the mechanical responses of three types of rocks in both uniaxial and triaxial cyclic tests. Comparative analysis with associated fatigue model is also provided to evaluate the accumulative deformation and damage evolution of concerned rocks.

This paper aims to investigate the effects of soil penetration resistance, tillage depth, and operating speeds on the deformation and fatigue of the subsoiling shovel based on the real-time measurement of tractor-operating conditions data. Various types of sensors, such as force, displacement, and angle, were integrated. The software and hardware architectures of the monitoring system were designed to develop a field operation condition parameter monitoring system, which can measure the tractor's traction force of the lower tie-bar, the real-time speed, the latitude and longitude, tillage depth, and the strain of the subsoiling shovel and other condition parameters in real-time. The time domain extrapolation method was used to process the measured data to obtain the load spectrum. The linear damage accumulation theory was used to calculate the load damage of the subsoiling shovel. The magnitude of the damage value was used to characterize the severity of the operation. The signal acquisition test and typical parameter test were conducted for the monitoring system, and the test results showed that the reliability and accuracy of the monitoring system met the requirements. The subsoiling operation test of the system was carried out, which mainly included two kinds of soil penetration resistances (1750 kPa and 2750 kPa), three kinds of tillage depth (250 mm, 300 mm, and 350 mm), and three kinds of operation speed (4 km/h low speed, 6 km/h medium speed, and 8 km/h high speed), totaling 18 kinds of test conditions. Eventually, the effects of changes in working condition parameters of the subsoiling operation on the overall damage of subsoiling shovels and the differences in damage occurring between the front and rear rows of subsoiling shovels under the same test conditions were analyzed. The test results show that under the same soil penetration resistance, the overall damage sustained by the subsoiling shovels increases regardless of the increase in the tillage depth or operating speed. In particular, the increase in the tillage depth increased the severity of subsoiling shovel damage by 19.73%, which was higher than the 17.48% increase due to soil penetration resistance and the 13.07% increase due to the operating speed. It should be noted that the front subsoiling shovels consistently sustained more damage than the rear, and the difference was able to reach 16.86%. This paper may provide useful information for subsoiling operations, i.e., the operational efficiency and the damage level of subsoiling shovels should be considered.

A numerical model for computing the vortex-induced vibration (VIV) and fatigue damage of steel catenary risers (SCRs) was developed. The structural dynamics were accurately simulated using an absolute nodal coordinate formulation (ANCF). The Van der Pol wake oscillator is applied to generate the fluctuating lift, which is further transformed into the cross-flow direction by considering the structural deformation. The Randolph-Quiggin (RQ) model and the Coulomb friction 'bilinear' model are employed to simulate the vertical and lateral riser-soil interactions, respectively. After case validations, the effects of riser-soil interaction on the VIV amplitude, frequency, mode, and fatigue of the SCR at different current angles are investigated, and a sensitivity study of different seabed model parameters is discussed. The bands of significant VIV frequencies were broadened by riser-soil interactions, accompanied by more frequency components of disturbance and more abundant vibration modes. Severe fatigue damage cannot be captured by the truncated model, and seabed models that require improvement are ignored. It is suggested that vertical and lateral riser-soil interactions should be considered in the evaluation of VIV fatigue damage for SCRs.

The Steel Catenary Riser (SCR) is a vital component for transporting oil and gas from the seabed to the floating platform. The harsh environmental conditions and complex platform motion make the SCR's girth-weld prone to fatigue failure. The structural stress fatigue theory and Master S-N curve method provide accurate predictions for the fatigue damage on the welded joints, which demonstrate significant potential and compatibility in multi-axial and random fatigue evaluation. Here, we propose a new frequency fatigue model subjected to welded joints of SCR under multi-axial stress, which fully integrates the mesh-insensitive structural stress and frequency domain random process and transforms the conventional welding fatigue technique of SCR into a spectrum analysis technique utilizing structural stress. Besides, a full-scale FE model of SCR with welds is established to obtain the modal structural stress of the girth weld and the frequency response function (FRF) of modal coordinate, and a biaxial fatigue evaluation about the girth weld of the SCR can be achieved by taking the effects of multi-load correlation and pipe-soil interaction into account. The research results indicate that the frequency-domain fatigue results are aligned with the time-domain results, meeting the fatigue evaluation requirements of the SCR.

The steel catenary riser (SCR) serves as a primary solution for deep-water oil and gas field development, but it encounters complex dynamics due to forced oscillations induced by wave-driven floater motions, especially at the touch-down zone (TDZ). Traditional pipe-soil models often fail to address these challenges, as they do not account for soil remoulding and the impact of irregular motion. This is particularly relevant in real sea states characterized by irregular waves, where the floater's movements have a significant impact on seabed trenching, thus complicating the dynamic responses of the SCR. To address these issues, this study integrates an innovative effective-stress-based pipe-soil interaction model into a global SCR analysis to explore its dynamic response and fatigue damage under irregular waves. The irregular movements of the floater, derived from response amplitude operator (RAO) data and wave spectra, are applied to the SCR's top end after a translation to the hang off location. This allows for dynamic simulations that consider the evolution of the seabed and the process of trenching. The study focuses on deriving the dynamic stresses experienced by the SCR at the TDZ and evaluating fatigue damage using the S-N curve method. It also examines the seabed interaction, including the evolution of trenching, changes in seabed stiffness, and soil resistance at various SCR locations. By considering real sea conditions, this study yields insights into trenching and seabed-SCR interactions, promising to enhance design methodologies and bolster offshore infrastructure performance and safety.