Hull Deformation Research Articles

Multi‐Objective Optimization and Experimental Research of Ship Form Based on Improved Bare‐Bones Multi‐Objective Particle Swarm Optimization Algorithm

ABSTRACTShip form optimization poses a complex and high‐dimensional engineering challenge. Therefore, when conducting multi‐objective optimization research of ship forms, traditional intelligent optimization algorithms are prone to falling into local optima solution and difficult to converge. In order to effectively improve the diversity and convergence performance of the algorithm, this paper improves the bare‐bones multi‐objective particle swarm optimization (BBMOPSO) algorithm by dynamically adjusting the local and global search step sizes, and verifies the algorithm's reliability through standard function testing. Then, a multi‐objective optimization design framework with high efficiency and high integration is constructed. Taking DTMB 5512 as the research case, Free Form Deformation (FFD) method is used for hull deformation, and the proposed algorithm is used for multi‐objective optimization of resistance performance and motion response. Ship model tests were conducted on the DTMB 5512's original hull. And the numerical simulations were compared with the ship model tests. Finally, under the constructed multi‐objective optimization design framework, satisfactory solutions were obtained through the improved algorithm, which confirms the effectiveness and practicality of the improved algorithm. The results show that the algorithm improved in this paper can provide some theoretical basis and technical support for green ship design and low‐carbon shipping.

Reinforcement Learning-Based Optimal Hull Form Design with Variations in Fore and Aft Parts

Abstract With recent advancements in artificial intelligence technology, various studies are being conducted in the shipbuilding industry. Traditionally, hull form variation methods have relied on the intuition and expertise of designers, leading to inconsistent results and unintended changes in the ship's main dimensions depending on the designer's competence. Moreover, the iterative process of design variation and analysis to derive the optimal hull form is both costly and time-consuming. To address these issues, this study proposes an optimal hull design technique utilizing reinforcement learning, a type of unsupervised learning in machine learning. Reinforcement learning allows the model to learn from past policies by recording and accumulating the rewards associated with various actions taken by an agent in a specific environment. In this study, after calculating the main parameters of the ship, the agent defines a state representing hull information and performs local transformations of the bow and stern. The reward of reinforcement learning is defined as the change in total resistance due to the hull deformation, constrained by limiting the tolerance of the ship's prismatic coefficient (CP) and longitudinal center of buoyancy (LCB). In this study, the problem is solved by comparing the proximal policy optimization (PPO) algorithm and the deep deterministic policy gradient (DDPG) algorithm to find the best deep reinforcement learning (DRL) model for the hull optimization problem. The results were compared with the genetic algorithm and speed-constrained multi-objective particle swarm optimization (SMPSO), and the optimal hull resistance values were less different, but the time of the reinforcement learning model was five times shorter.

Variable Structure Controller for Energy Savings in an Underwater Sensor Platform.

This paper introduces a new variable structure controller designed for depth control of an autonomous underwater sensor platform equipped with a variable buoyancy module. To that end, the prototype linear model is presented, and a finite element-based method is used to estimate one of its parameters, the hull deformation due to pressure. To manage potential internal disturbances like hull deformation or external disturbances like weight changes, a disturbance observer is developed. An analysis of the observer steady-state estimation error in relation to input disturbances and system parameter uncertainties is developed. The locations of the observer poles according to its parameters are also identified. The variable structure controller is developed, keeping energy savings in mind. The proposed controller engages when system dynamics are unfavorable, causing the vehicle to deviate from the desired reference, and disengages when dynamics are favorable, guiding the vehicle toward the target reference. A detailed analysis determines the necessary switching control actions to ensure the system reaches the desired reference. Finally, simulations are run to compare the proposed controller's performance with that of PID-based controllers recently developed in the literature, assessing dynamic response and energy consumption under various operating conditions. Both the VBM- and propeller-actuated vehicles were evaluated. The results demonstrate that the proposed controller achieves an average energy consumption reduction of 22% compared to the next most efficient PID-based controller for the VBM-actuated vehicle, though with some impact on control performance.

Ship shafting alignment technology and hull deformation based on improved genetic algorithm and shipbed calibration

As science and technology develop in recent years, the center of ship shafting has received high attention from the ship industry. The traditional ship shafting calibration mostly focuses on the issue that hull deformation cannot be accurately estimated. The ship is floating after entering the water, and this method is not conducive to the long-term stable operation of the ship shafting. To solve the above problems, the study establishes the optimization model of ship axis alignment based on slide alignment and finite element method. This model can be optimized by adopting the non-dominant sequencing genetic algorithm improved by elite strategy. The study verified the performance of the optimization model of ship axis alignment. The results showed that the adaptive value, super-volume value, and inverse generation distance of the improved genetic algorithm were 74.57, 0.38, and 0.03, respectively. In the application of a ship, the intermediate bearing position could be adjusted by the ship shafting optimization model based on the improved non-dominant sorting genetic algorithm. As a result, the shaft reaction under the ballast condition was reduced by 24019 N than before, making the bearing load of the ship shafting more uniform. To sum up, the proposed optimal model is robust, which can effectively reduce the impact of hull deformation, improve the optimal effect of ship shafting alignment and ensure the safe navigation of the ship.

A dynamic-static combination risk analysis framework for berthing/unberthing operations of maritime autonomous surface ships considering temporal correlation

The autonomy of the ship is bound to bring new risk challenges to its berthing/unberthing operations process. Potential damages resulting from these risks can range from minor hull deformations to substantial losses of assets and environmental harm. To address these risks in the process of berthing/unberthing operations of MASSs or mitigate them to an acceptable level, this paper aims to propose a risk analysis framework combining dynamic and static Bayesian network models, and which can consider the temporal correlation of the values of risk factors, to help ships avoid the occurrence of operational accidents. In this framework, a set of probabilistic model is included to calculate the real-time risk value of autonomous berthing operation, so as to analyze and solve the time-dependent problem of risk change with time during berthing/unberthing of autonomous ships. In addition, the developed model can determine the key influencing factors and possible key situations during the berthing/unberthing of autonomous ships, which can support the risk prevention and mitigation measures in the process of autonomous ship berthing/unberthing. Finally, the applicability of the proposed risk analysis framework is analyzed through a virtual simulation test case of autonomous berthing/unberthing.

Experimental Study on the Effect of Hull Deformation on the Relative Attitude between Shaft and Bearing

The unclear change laws of bearing offset and rotation, both of which influence the condition of shaft alignment during hull deformation, make it difficult to optimize shafting design. In this paper, an integrated hull-bearing-shaft model is designed and built for a cantilever beam loading test. Displacement sensors are utilized to determine the change in displacement of the hull, bearings, and shaft. The pressure distribution at the bow and stern ends of the bearing is measured using a new type of thin-film pressure sensor. The test results show that the rotation angle of the shaft and bearing varied differentially during hull deformation, and the magnitude of the shaft-bearing angle was comparable to the rotation angle. The measured rotation angles of the front and rear ends of the stern tube bearings are opposite to the theoretical value of a cantilever beam, indicating that the stern tube has a non-negligible effect on local deformation, and it is recommended to measure the bearings directly as opposed to the alternative structure to obtain the rotation. The change pattern of the shaft and bearing attitude does not change with the different initial state of the shaft, which indicates that the initial error of installation will be retained during the hull deformation process. The change pattern of the shaft and bearing attitude is unaffected by the initial state of the shaft, indicating that the initial installation error will persist during hull deformation. In some instances, the bearing reaction force remained unchanged, but the shaft-bearing angle and bearing pressure altered, indicating that the bearing condition cannot be determined solely by the bearing reaction force. The results of bearing pressure and the shaft-bearing angle can be compared, indicating that the thin-film pressure sensor can be used to determine the status of the shaft-bearing angle, particularly during the installation phase.

Study of the Hull Structural Deformation Calculation Using the Matrix Displacement Method and Its Influence on the Shaft Alignment

The analysis of the influence of hull deformation on shaft alignment is predominately conducted using the finite element method (FEM), which is time-consuming, labor-intensive, and challenging to use for iterative hull design optimization. In this paper, hull deformation is separated into two parts—global deformation and local deformation, simplified to a single-span beam model and a grillage beam model, respectively—then solved using the matrix displacement method (MDM). Compared to FEM, the proposed method has a small calculation error, proving its correctness, while the calculation time is greatly reduced. The proposed method has been used to calculate the hull deformation of a ship under various conditions and evaluate its influence on shaft alignment. The results indicate that under certain conditions, the bearing reaction forces are constant, whereas the bearing pressure changes as a consequence of the change in shaft-to-bearing angle. The comparison between local rotation and shaft-to-bearing angle reveals that bearings in various positions follow distinct laws. We suggest that the shaft-to-bearing angle be used as an additional parameter in the evaluation of shaft alignment calculations. Moreover, when optimizing bearing pressure, bearings in different positions are affected differently by global and local deformation, and their optimization priorities are distinct.

Conceptual design and analysis of a two-stage underwater glider for ultra-long voyage

An underwater glider (UG) designed for ultra-long voyage undoubtedly requires additional battery energy, which is generally realized by increasing the volume. However, a larger volume of UG results in a larger net buoyancy loss caused by seawater density variation and hull deformation, which will lead to a larger energy consumption. This study proposes the conceptual design of a novel two-stage UG (TUG) for enhancing the voyage range. To reduce the volume of UG, the first stage will be separated from the glider and discarded after its battery energy is drained. Mathematical models for the TUG are established, including the dynamic model, the energy consumption model and the performance evaluation models. To better evaluate this novel design, a multi-objective optimization model considering the energy utilization rate and voyage velocity is developed based on the proposed models, and implemented by surrogate model and non-dominated sorting genetic algorithm II. The results demonstrate that the designed TUG has a better motion performance than conventional UGs, and the two-stage design is more suitable for UGs with low hotel loads, which can theoretically improve the voyage range by 16.6% at a 0.25 W hotel load. This work can provide theoretical guidance for the optimization design of UGs applied for ultra-long voyage.

Improved Accuracy of Prediction of Thermal Deformation of the Stern Shaft Block Considering the Erection Process

Thermo-elasto-plastic analysis using nonlinear finite element analysis is used for predicting welding-induced structural deformation; however, its application in the prediction of welding deformation in large structures, such as ships and offshore plants, is practically impossible. Therefore, in this study, an inherent strain-based thermal deformation analysis method, which can be applied to large structures, was developed; furthermore, the inherent strain-based analysis was used for predicting the welding deformation occurring during block erection welding. However, although the tendency is somewhat similar in terms of quality, it still differs from the actual measured value in terms of quantity. In this study, a regression equation was derived by analyzing the sensitivity of the variables used in the existing method, and, subsequently, the results corresponding to the measured values were obtained; finally, the accuracy improvement for the three ships was confirmed. Therefore, using the proposed method, it is possible to improve the technology of predicting and controlling hull deformation.

Theory and practice of hull dynamic deformation law and heading angle transfer error correction

In order to meet the demand of high-precision heading angle transmission in the transfer alignment of inertial navigation system on moving base, the analytical function relationship between the hull deformation and the turning angular velocity and angular acceleration was derived by using the classical beam theory based on the analysis of the equivalent load exerted by the hydrodynamic force and inertia force on the hull structure during the turning process under the combined action of the steering rudder moment and wave force. The objective law between the angular motion and the azimuth deformation angle of the hull under the combined action of maneuvering and sea waves was revealed. Finally, the correction coefficients were determined according to the left turn and right turn motions of the hull by using the measured data of the ship in the sea trial during the S-shape maneuvering navigation, and the azimuth deformation angle correction was completed. The results indicated that the application of the Qu's bending deformation correction formula could greatly reduce the influence of the hull flexural deformation on the heading angle accuracy, meet the needs of high-precision heading angle transmission, and fully verify the correctness of the hull azimuth deformation law and the heading angle transmission error correction theory. This theory and method provided technical support for establishing high-precision distributed digital reference in the field of transfer alignment of inertial navigation on moving base and the application of heading angle transfer of other shipborne equipment.

A real-time shaft alignment monitoring method adapting to ship hull deformation for marine propulsion system

Shaft alignment states are strictly stipulated in the marine propulsion system. However, there are relatively limited studies on shaft alignment monitoring and control considering the effects of ship hull deformation (SHD). SHD can be caused by wave load, loading, depth, speed, etc., and leads to various relative displacements at different shaft supporting points between the ship foundation and the support bearings of the propulsion system. It leads to shaft misalignment, increases shaft vibration and noise, and even endangers the safety of shaft system during the operation stages. To overcome this problem, a real-time shaft alignment monitoring method based on laser displacement sensors (SAMM/LDS) is proposed. Firstly, the defects of a present shaft alignment monitoring method based on eddy current displacement sensors (SAMM/ECDS) are analyzed when considering the effects of SHD. The structure and working principle of a shaft alignment monitoring system adapted to SHD (SAMS/SHD) are proposed. It consists of three sub-systems: the shaft alignment monitoring sub-system adapting to SHD (SAMSS/SHD), the present shaft alignment monitoring sub-system (SAMSS/P), and the shaft alignment correcting sub-system (SACSS). Based on SHD data-driven, SAMS/SHD can not only take advantage of SAMSS/P for long-time operations but also reflect SHD by exploiting the advantages of SAMSS/SHD. Secondly, SAMM/LDS is deduced. Two laser displacement sensors (LDS) are placed on the driving shaft and the driven shaft to reflect the spatial relative displacement and attitude information affected by SHD. Therefore, two reference coordinate systems are fixed at the center of LDS, and the coordinate expressions of their imaging points are derived using a 4 × 4 homogeneous coordinate transformation matrix. For reducing computing complexity in engineering applications, a simplified monitoring model is proposed for very small rigid-body rotational angularities. Finally, the effectiveness and testing accuracy of the SAMM/LDS is verified on a five-DOF platform which can simulate the effects of SHD. The SAMM/LDS can be used to realize real-time and high-precision shaft alignment monitoring for marine propulsion system which can adapt to the effects of SHD and so on. Based on the foundation of this research, compensation control algorithms to solve the misalignment effects deduced by SHD will be studied for marine propulsion system with resilient mounts in the next few years.

WiCrew: Gait-Based Crew Identification for Cruise Ships Using Commodity WiFi

Security check-in life-support areas, e.g., bridge and engine room are crucial for cruise ships due to numerous and diverse passenger identities. Instead of conventional security check approaches, such as facial recognition and fingerprint identification, device-free approaches enabled by WiFi-based gait recognition have attracted considerable attention owing to their low cost, nonintrusiveness, and privacy protection. Despite the excellent performance of existing indoor methods, they cannot be trivially extended to cruise ships because of the unique characteristics of hull deformation caused by vibrating engines and waves. This stems from the flexible structure of cruise ships, which introduces additional noise to the WiFi signals. To address this challenge, we propose WiCrew, a device-free gait recognition system that detects crew identity anomalies in cruise ships. WiCrew consists of two components: 1) a spatial separation algorithm that separates the signal components from ship vibration and human activity and 2) a speed-independent adversarial learning framework that identifies the ship’s crew using human gaits at an arbitrary walking speed. Extensive experiments on a cruise ship demonstrate the effectiveness of WiCrew. While the crew members walk at speed of 0.7 to 1.8 m/s, the average recognition accuracy reaches 82%, which is similar to vision-based approaches.

Research on the Self-recovery and Control Method of Hull Deformation Alignment Offset Based on Structure Adaptation

Research on the Self-recovery and Control Method of Hull Deformation Alignment Offset Based on Structure Adaptation

Improved spatio-temporal alignment measurement method for hull deformation

Improved spatio-temporal alignment measurement method for hull deformation

Experimental study on the distributed lubrication characteristics of full-size water-lubricated stern bearings under hull deformation

A full-size test rig of water-lubricated stern bearing (WSB) with an elevation adjustment device was built. High-precision wireless telemetry device obtained four film pressure signals. Six eddy current sensors were adopted to measure the axis orbit. The effect of bias load, rotational speed, and hull deformation on bearing lubrication characteristics are systematically studied. Experimental results showed the distribution lubrication characteristic of WSB at the axial direction. The WSB lubrication region can be divided into the three sub-regions in the middle and both ends of the bearing. Three lubrication sub-regions have different take-off speeds. Under the bias load, the negative pressure was found in the film pressure distribution. The hull deformation affects the position and area of each lubrication sub-region, changing the loads carried by the water film and asperity contact. Research results have a great contribution to WSB optimization structure design.

Deep-Learning-Based Wireless Human Motion Tracking for Mobile Ship Environments

Being able to track passengers’ movement without invasion of their privacy plays an important role in cruise ships; it enables crucial location-based services, such as maritime search and rescue, tourist services, and epidemic prevention. The past few years have witnessed commodity WiFi holding great potential that provides such services available thanks to its ubiquitous in indoor scenarios. However, existing WiFi-based tracking methods suffer from huge performance degradation in sailing ships due to their complex metal structures and dynamic hull deformation caused by engines and waves/payloads pressure. In this article, we present CRLoc, a deep learning-based passive human tracking system that can overcome the practical limitations of traditional WiFi-based localization approaches applied in a multipath-rich and mobile ship environment, and provide decimeter-level tracking accuracy in cruise ships. Specifically, we make two contributions, i.e., we propose a super-resolution parameter estimation algorithm that better characterizes ship indoor environments, and a deep neural network-based end-to-end solution to remove the impact of noise, interference, and mobility in ships. The real-world implementation and extensive experiments in several passenger ships demonstrate that CRLoc tracks human motions with a median error of 92 cm, better than state-of-the-art localization methods. To our knowledge, this is one of the first WiFi-based passive human motion tracking system in a cruise ship environment.

An improved radial basis function for marine vehicle hull form representation and optimization

The marine vehicle is one of the most important platforms in oceanic research and development. During the evolution of marine vehicle design, hull optimization is a key topic consisting of various steps such as hull deformation analysis, optimization algorithm selection and hydrodynamic performance evaluation. For hull deformation analysis, radial based function interpolation technique is usually applied with Wendland’s ψ3,1 function, which requires a great number of fixed points. To improve the optimization effect, this paper proposes a novel radial basis function for better support radius condition. According to the support radius condition, the effect of different deformation functions acting on hull form is studied. To verify the effectiveness of the improved radial basis function in marine vehicle hull optimization, a Series 60 hull is taken as the initial hull, and the Rankine source method is utilized as the solver to calculate the wave resistance coefficient. The Non-dominated Sorting Genetic Algorithm (NSGA) is applied to determine the optimal hull form at given speeds. By comparing the optimization results, the benefits and drawbacks of the improved radial basis function are analyzed. The results show that the number of fixed control points can be reduced by applying the improved radial basis function. When different deformation functions are acting on the marine vehicle hull, the improved radial basis function makes the gradient transition smoother. In the process of hull optimization, the wave resistance coefficient obtained by utilizing the improved function to deform the hull is smaller than that of using Wendland’s ψ3,1 function under different support radii.

Numerical Investigation of Plastic Deformation of Flat Plate for Slamming Impact by Coupled Eulerian–Lagrangian Method

Ships and offshore structures are subjected to impact loads, such as slamming and sloshing. High impact pressures can cause permanent hull deformation by a single impact event. In addition, significant fatigue damage can be accumulated via repeated impact pressures. In this study, the plastic deformation behavior of flat plates under slamming impact is numerically investigated using a coupled Eulerian–Lagrangian method. The dynamic impact pressure of the flat plates by weight and drop height is investigated under the assumption of viscous and compressible fluids. To evaluate the plastic deformation of the plate, contact between water and the plate is removed after a certain duration after dropping, and then the remaining deformation is measured. Optimized finite element models for drop simulations are selected via a mesh sensitivity study, and the simulation results are calibrated and compared with experimental data. Results of the simulation and the experiment show good agreement in general in terms of deflection range. However, because the initial condition of the plate is not reflected in the simulation, some discrepancy is observed in maximum deflections. Finally, a discussion is presented for a more accurate fluid impact analysis model based on the comparison results with the experimental data.

One-Step Initial Alignment Algorithm for SINS in the ECI Frame Based on the Inertial Attitude Measurement of the CNS.

To solve the problem of high-precision and fast initial alignment for the Strapdown Inertial Navigation System (SINS) under both dynamic and static conditions, the high-precision attitude measured by the celestial navigation system (CNS) is used as the reference information for the initial alignment. The alignment algorithm is derived in the Earth-centered inertial (ECI) frame. Compared with the alignment algorithm in the navigation frame, it is independent of position parameters and avoids the influence of the approximate error caused by the dynamic deflection angle. In addition, hull deformation is considered in attitude optimal estimation, which can realize initial the alignment of the SINS installed in various parts of the carrier. On this basis, the velocity measurement information is added to the alignment process, which further improves the accuracy and speed of the initial alignment under static conditions. The experimental results show that the algorithms proposed in this paper have better performance in alignment accuracy, speed, and stability. The attitude and velocity matching algorithm in the ECI frame can achieve alignment accuracy better than 0.6′. The attitude matching algorithm in the ECI frame has better robustness and can be used for both dynamic and static conditions, which can achieve alignment accuracy better than 1.3′.

Dynamic behavior analysis and bio-inspired improvement of underwater glider with passive buoyancy compensation gas

To deal with the buoyancy loss caused by hull deformation and seawater density variation, low-density gases, such as nitrogen, have been adopted in underwater vehicles for passive buoyancy compensation. Therefore, the accurate understanding of passive buoyancy compensation gas (PBCG) is critical to the design and operation of underwater glider. In this paper, a theoretical model for the Petrel-L underwater glider with PBCG is established, which consists of environment model, net buoyancy model and dynamic model. The above models are verified by sea trial, and then the feasibility and long-term reliability of PBCG are demonstrated by a long-range voyage. Co-simulation of Petrel-L with the models shows that pre-charge pressure, distribution and volume of PBCG have significant effects on the motion performance in terms of attitude stability and velocity stability. Besides, to achieve better buoyancy compensation and motion stability, an improvement design of PBCG inspired by swim bladder is proposed. Deviation of pitch angle from its target is less than ±2° over the depth range from 50 m to 950 m by optimizing the improvement design.