Against the backdrop of global warming, maritime activities in the Arctic region are increasing, and polar vessels inevitably encounter ice loads during navigation. A precise understanding of the deformation characteristics and failure mechanisms of hull plate structures under ice loads is crucial for ensuring the safe navigation of polar vessels. However, significant gaps remain in existing research, particularly regarding initial hull damage and the mechanisms of repeated ice loads. Based on this, this study aims to deeply explore the mechanical response characteristics of hull plate structures under ice loads, focusing on key scientific issues such as initial damage effect assessment and repeated ice load mechanisms, providing theoretical and technical support for the design of anti-ice structures and the safety assessment of polar vessels.

A repeated compression test on steel hull plates and ice was conducted using a simplified plate model based on the actual bow shoulder structure, with some specimens pre-fabricated with initial defects to simulate real service damage. During the test, triangular pyramid ice models were prepared using the filling-freezing method. A total of 12 test cases were designed to systematically examine the effects of variables such as plate thickness, stiffener arrangement, defect direction, and position offset. The plate was pressed vertically at a constant speed of 6 mm/min to repeatedly compress the ice. High-precision sensors were used to measure displacement, force, and strain in real time, while the plastic deformation of the structure was recorded after each loading cycle to fully characterize the structural response under ice loads.

The results show that plate thickness has a significant impact on structural strength. As plate thickness increases, the slope of the compression force-displacement curve increases, while the loading displacement at ice failure significantly decreases. Although the arrangement of stiffeners can effectively enhance anti-ice compression strength, their ability to compensate for strength reduction caused by longitudinal defects is limited. Notably, when the defect location aligns with the ice load area, especially for longitudinal defects, significant stress concentration occurs at the root, leading to defect propagation. A comparative analysis showed that under the same load conditions, transverse defects exhibit higher load-bearing capacity than longitudinal defects. Additionally, offsetting defects significantly alters stress distribution: a 200 mm longitudinal offset of a longitudinal defect can reduce root stress by about 74%, while a 75 mm transverse offset of a transverse defect can increase root stress to 202 MPa at one end while reducing it to 66 MPa at the other end. After the first loading, plastic deformation generally occurred in the plates, with plates of lower initial strength exhibiting greater strength improvement during the second loading, even potentially surpassing those with higher initial strength.

The study reveals the structural response characteristics of plate structures with initial damage under repeated ship−ice compression scenarios, providing valuable references for the design and evaluation of ice-resistant structures polar vessel structures.

The sloshing load is one of the most important and complex loads in the LNG carrier cargo containment system, but it is difficult to calculate or measure accurately due to the simplifications in numerical simulations and model experiments. To obtain accurate time-history characteristics of the sloshing load and enable real-time health monitoring of the LNG carrier cargo containment system, the inverse impulse-space superposition method is used to measure the local response of the structure and deduce the sloshing load and the response of high-stress regions (hotspots).

Based on the improved inverse impulse-space superposition method, an inverse mathematical model of measuring point positions and sloshing load positions is established to predict loads in multi-regions. Using the time-shift property of the convolution integral, the Duhamel integral is reformulated and discretized into a matrix equation to predict sloshing loads at different time steps. The matrix equation is solved by least-squares. To address the instability due to noise interference and small singular values in the unit impulse load response matrix, the Tikhonov regularization method is adopted, with the optimal regularization parameter selected by the L-curve method. Based on the improved impulse-space superposition method, a response prediction mathematical model for sloshing load positions and hotspot positions is established to predict multi-hotspot stress, such as shear stress in the secondary plywood and vertical stress in the secondary polyurethane foam.

The algorithm's performance is systematically evaluated under both triangular and random load conditions. The application of multiple triangular sloshing loads with randomly generated characteristic shows that the predicted values agree well with actual measurements, indicating the method's ability to accurately predict multi-region triangular sloshing loads from any starting moment. For random loads, the investigation focuses on three prediction step sizes (0.5 ms, 0.25 ms, and 0.05 ms). The analysis shows a strong correlation between prediction accuracy and step-size reduction. At the finest resolution of 0.05 ms, the predicted load curve successfully captures all peak features of the actual load profile. While minor fluctuations occur in zero-value regions without prominent peaks, primarily due to noise interference and small singular values, comprehensive error analysis across all regions demonstrates that step-size reduction effectively minimizes prediction errors. Specifically, the maximum load peak pressure error for individual regions decreases from 21.861% to 9.530%, with corresponding average errors decreasing from 10.081% to 4.023%. Similarly, temporal accuracy improves significantly, with maximum load peak time errors decreasing from 0.900 ms to 0.050 ms and average errors decreasing from 0.256 ms to 0.022 ms. Stress prediction shows equally promising results. For both loading scenarios, the predicted curves for plywood shear stress and foam vertical stress agree well with actual measurements, particularly at smaller step sizes. The maximum stress peak prediction error remains within 1% for selected hotspots. In the most challenging cases, the maximum peak stress error reaches 5.267% for plywood shear stress and 2.644% for foam vertical compressive stress, with peak time errors not exceeding 0.15 ms.

The improved inverse impulse-space superposition method based on the Duhamel integral successfully inverts the sloshing load and the predicts hotspot stresses in the LNG carrier cargo containment system. This method combines the accuracy of experimental with the cost-effectiveness of numerical simulations, minimizing the negative effects of experimental measurement errors and numerical model simplifications. It provides a novel and reliable approach for assessing the safety of LNG carriers and other ship-ocean structures. Although the current study adopts a uniform load model and does not fully account for the internal non-uniformity of actual sloshing loads in different regions, it still serves as a valuable reference for future research in this field.

In view of the fact that the structural performance assessment method of the internal-pressure-resistant square cabin is not clear and the general specifications for naval ships are not fully applicable, the stress analysis method and strength assessment criteria applicable to the internal-pressure-resistant square cabin are studied.

Based on the theory of elasticity, the two yield criteria Mises and Tresca commonly applied in ASME BPVC were analyzed. According to the principle of safety, Tresca was determined to be the analysis criterion applicable to the internal-pressure-resistant square cabin. By taking the bulkhead grillage as the basic unit, the stress classification of the internal-pressure-resistant square cabin was carried out based on ASME BPVC, and four typical assessment locations were obtained: the center of the plate panel, the midpoint of the short side of the plate panel, the midpoint of the long side of the plate panel, and the corner of the plate panel. In order to reduce the amount of engineering calculation, the theoretical formula and numerical calculation method of stress components for plate element finite element analysis were proposed based on the stress linearization theory, and the solid finite element model of grillage was established for comparing the difference between the structural assessment results of two models.

Compared with the accurate results of the solid element model, the error of plate element stress analysis result is basically about 3%, and the results of plate elements are generally larger. Considering the safety conservative assessment principle of ships and nuclear structures, it can be considered that the strength assessment method of internal-pressure-resistant square cabins based on the plate element finite element model and ASME BPVC meets the engineering requirements.

This study can provide a reference for the stress analysis and strength assessment of the internal-pressure-resistant square cabin, and is of great significance for tackling the technical bottleneck faced by the ships using nuclear power plants.

With the continuous development of larger scale and more complex ships, the number of finite element model elements required to model hull structures at the cabin structure level and above is increasing dramatically, resulting in collision, impact, contact and other large-scale non-linear mechanical problems which are difficult to solve. To this end, a simplified method for the deformation of two-way stiffened plate structures based on orthotropic equivalent theory is proposed in order to simplify the modeling of ship structures.

First, the current well-established simplification method for the plane stress of one-way stiffened plates is extended to the more complex plane bending problem of two-way stiffened plates. The ratio of the total moment of inertia of stiffened plates to the moment of inertia of plates in the orthogonal direction is introduced to reflect the structural orthotropism. Next, the moment of inertia ratios are substituted into the equivalent constitutive equation for the plane bending of stiffened plates to achieve the transformation to physical orthotropism, thereby taking into account both the deformation resistance of the structure and the influence of the membrane forces generated by shifting the neutral surface at the mechanical level. Finally, finite element calculations are used to classify the deformation modes of the four-sided fixed stiffened plates according to different displacement distributions, and the actual results of the stiffened plates are analyzed in terms of error comparisons with the equivalent results of this method and the traditional method.

The result comparison shows that the proposed method can reduce the number of elements in two-way stiffened plates by up to 84%, and the equivalent errors in all three deformation modes can be controlled within 6%, which is much lower than those of the two traditional methods.

With high precision, a wide application range and greatly reduced calculation resources, the proposed method can provide a direct modeling and simulation calculation solution to address the nonlinear mechanical problems of large hull structures for practical engineering applications.

To identify the change in structural strength after an interaction between hull girders and ice, this paper puts forward an analytical technique which can determine the structural strength of hull girders after an ice riding accident and disclose the related structural strength features.

First, the hull girder structural strength analysis model and associated load analysis model are constructed in accordance with the characteristics of each section. To account for the buoyancy variations brought on by ice riding, the buoyancy distribution of the ice riding conditions is retrieved from the load analysis model and inserted into the structural strength analysis model. Gravity and the reaction force of the ice are then used to determine the structural strength. Finally, an analysis is carried out on the impact of lifting position and height on the buoyancy, shear force, bending moment and local stress distribution of the hull girders.

The results show that the sites of buoyancy and shear force of the hull girders are situated in the stern part and middle part, and unaffected by changes in the bow lifting height. The shell is closer to the vertical when the lifting position is at the bow of the bulbous bow, making it harder to resist the reaction force of the ice and resulting in the high stress areas becoming comparably larger and more dangerous.

The proposed method has high computation efficiency and can be used to estimate how hull girders will respond to a significant bow lift and make preliminary judgments on hull girder structural strength under risky ice riding conditions.