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The framework of a "wind farm" as an offshore structure with multiple wind power generation units is introduced. In Japan, these offshore structures are designed as floating structures owing to the bathymetry of the deep seafloor. They are situated in open seas, such as the Pacific Ocean or the Sea of Japan, where strong winds are frequent. However, these floating structures are occasionally damaged in rough seas. This can lead to the wind power generation being interrupted for a longer time period. This implies that maintenance and repair work needs to be performed regularly and quickly by specialist engineers. This study aimed to minimize the duration of wind power interruptions and examine various aspects. First, the wave height, wave period, and wave direction were predicted using a machine learning method with the coastal wave database in Japan as training data. Second, the movements of the working ship were reproduced based on the predicted results of the wave parameters to evaluate the safety and practicability of the engineers' transfer from the working ship to the offshore structure for maintenance operations. Finally, an optimal maintenance support system for Japanese offshore structures was devised based on the study results. • Constructing machine learning to predict wave information one week in advance. • Estimating the ratio of significant wave heights below 1.5 m as an indication of calm seas. • The motion response of the working ship is calculated based on predicted wave information. • Safety criteria is different if the feasibility of transfer is evaluated by surge and sway motions. • Number of consecutive calm days is estimated and compared to evaluate maintenance scheduling. [ABSTRACT FROM AUTHOR]

Safety evaluation of a lashed trailer on the cargo deck of ships is essential for ferry operations. Although ferry accidents have occurred worldwide, few studies have focused on the dynamic analysis of trailer motion in irregular sea states. To date, effective counterapproaches have not been established. Thus, first, ferry masters were interviewed to summarize the difficulty in cargo management during rough sea voyages. Second, a modified simulation model of lashed trailer motions was constructed to evaluate the safety of cargo management, considering wave, seakeeping, and trailer motion simulations. Few studies apply the model to the irregular sea state in actual seas, using a combination of three different kinds of models. A simulation was conducted to analyze the collapse of a trailer loaded on a ferry in high waves owing to rapidly developing atmospheric pressures. Essentially, the model can reproduce the collapse of trailers, especially when a shift in the center of gravity is considered when accelerations occur in loaded cargo inside trailers. The model enables the estimation of trailer motions and lashing wire tensions in ferry operations, considering suitable lashing arrangements. • Dynamic analysis of lashed trailer motions in irregular sea state is focused. • A complete simulation model of lashed trailer motions was newly constructed. • The model can reproduce the collapse of 100 trailers for the accident in 2009. • Lashing arrangement with longer breast lines can be effective to reduce motions. • A shift in the center of gravity for cargo in large motions could be occurred. [ABSTRACT FROM AUTHOR]

Accurate estimation of ship motions is inevitable for safe, economical, and environmentally friendly ship routing. Onboard measurements have been conducted for a bulk carrier in 2010–2016, including eight rough seas in the Northern and Southern Hemispheres. Rough sea conditions were reproduced using a wave model, WaveWATCH III, with wind inputs from meteorological organizations. Ship performances were evaluated using traditional seakeeping theories, with the main focus being on transverse motion. The 3D frequency-domain Rankine Panel Method (RPM), New Strip Method (NSM), and Enhanced Unified Theory (EUT) were compared. First, roll decay tests and simplified Ikeda's methods of roll damping were applied in each method. Second, the results of the model tests in oblique seas in regular waves were additionally considered for the accurate estimation of ship motions in actual seas. These were validated with measured ship motions in eight rough sea voyages. It was found that errors are smaller in the RPM than those in the EUT and NSM for measured roll and pitch motions in actual seas. The favorable results obtained using the proposed methods may provide a new alternative for optimal ship routing to predict ship roll and pitch motions in rough seas with reliable accuracy and computational efficiency. • Ship motions in actual seas are analyzed for 28,000DWT bulk carrier. • Frequency-domain Rankine panel method is used to estimate ship motions. • New strip method and enhanced unified theory are used for comparison. • Roll damping is estimated by decay tests and simplified Ikeda's methods. • Ship motion computations are validated by model tests and actual seas. [ABSTRACT FROM AUTHOR]

In recent years, the number of waiting ships in offshore anchorage has increased owing to several reasons, such as the port lockdown and increased cargo volumes in maritime transportations. Moreover, port terminals have ordered ships to stay outside the harbor to prevent mooring accidents when rough waves are forecasted. Anchored ships have been exposed to dangers owing to dragging anchors under rough sea conditions, especially those facing the open seas. In this study, we perform a numerical simulation of anchored ship motions to reproduce the dragging anchor. Additionally, we further evaluated the anchored ship motions based on underestimated wave conditions. Lastly, we constructed a novel risk assessment technique for anchored ships to assess the stranding risk, damage to marine structures, and risk of collision. The stranding risk was evaluated based on the relationship between the vertical displacement and Under Keel Clearance (UKC). Damaging risk can be identified from the information of harbor charts. The risk of collision was quantitatively assessed considering the main influential factors such as Closest Point of Approach (CPA), and the Ship Domain Overlapping Index (SDOI). Results showed that the proposed methodology can contribute to port safety and ship operation in terms of optimal ship routing. • Anchored ship motions are simulated for 28,000-DWT bulk carrier. • Drifted distance of anchored ship can be estimated with underestimated waves. • Risk of stranding is assessed by the UKC and vertical displacements. • Risk of collision can be quantitatively estimated with relative ship's positions. [ABSTRACT FROM AUTHOR]

Restrictions on greenhouse gas emissions and the existence of developing countries has diversified ship routes and ship types, making the ship operation more complex. Many types of simulation methods have come up in the last decade due to the enforcement of the energy efficiency design index. However, few studies that validated their simulations with actual data. This study performs the optimal ship routing of a 28,000-DWT-class bulk carrier to evaluate the optimal ship route with the speed loss analysis. The simulation results are compared with the measurement data of the bulk carrier in conditions including rough sea voyage in the Pacific Ocean. For comparison, a modified version of the speed loss analysis algorithm is developed. These algorithms are combined with the isochrone method. The differences in the simulation results with the American and European weather databases without higher wave avoidance and deliberate speed reduction are obvious. The simulated routes are closer to the measured ship trajectory if higher wave avoidance is considered. There are a couple of remarkable speed drops in the measured result, and the deliberate speed reduction is considered. Finally, some factors to control the reproducibility of ship routing including in rough sea voyages are summarized. • The analytical model with time-domain speed loss evaluation is constructed. • Optimal route is simulated for 28,000-DWT bulk carrier with measured data. • Differences of simulations are clarified in each condition with actual voyage. • Speed loss is evaluated by introducing precise seakeeping and propulsion theories. [ABSTRACT FROM AUTHOR]

The increasing demand for efficient, safe, and economic operation of ships has drawn attention to practical maneuvering behaviors for developing autonomous ships. Actual sailing conditions are reproduced to determine the relationship between environmental factors and ship steering records in rough seas. First, we generate realistic ocean environmental fields and analyze actual sea data. Then, we derive a modular maneuvering model reflecting environmental disturbances for further simulations. The correlation and multi regression analyses are performed based on measured data and environmental factors, which illustrate that the abnormal rudder angles are caused by reduced steering effectiveness. Finally, an attenuation function acting on the rudder normal forces is proposed to simulate this type of reduction. The time histories of maneuvering difficulties are selected as verification datasets. The maneuvers are simulated by adopting the attenuation function, and the simulation results show fair agreement with the measured data. The significant wave height, wind speed, mean wave period, current speed, wind apparent direction, and wave encounter angles are found to be the most statistically significant factors of rudder attenuation in the studied cases. The results and conclusions obtained from this study are of great significance for the further exploration of actual ship maneuvering behaviors in seas. • Problems to maintain course in actual sailing are investigated. • Data from spatiotemporally varying environmental fields are collected. • Measured data are utilized to reproduce realistic fields. • Rudder attenuation under severe weather is found by a statistical analysis. [ABSTRACT FROM AUTHOR]

Long-term shipboard measurements of vessel performance in actual seas and practical guidance for proper ship operations in rough sea conditions are of great importance for a safe, economical and environment-friendly optimum ship routing. Data was collected from a bulk carrier during 2010–2016 covering both the Southern and Northern Hemispheres with high seasonal variability. Using the meteorological and wave models, the Weather Research and Forecasting Model (WRF) and WAVEWATCH III (WW3), numerical simulations of the weather and sea state were conducted for eight rough sea cases using the grid point value (GPV) datasets NCEP-FNL and ERA-Interim. Model results were validated by shipboard measurements and ship motion calculations. Results show a better performance of NCEP-FNL on generating high ocean surface wind speed of heavy weather than that of the ERA-Interim, while the ERA-Interim tends to perform better for generation of wind direction. Besides, utilization of a high-resolution WRF model tends to increase the accuracy of wind simulations for extremely high and low wind speed cases when compared to the linear interpolation of GPV datasets, which tends to perform better for relatively modest wind speed conditions. Results also suggest that, to achieve an optimum ship routing considering safety, cost and emission reduction; it should use the high-resolution WRF model with NCEP-FNL initial and boundary conditions to generate ocean surface winds, followed by the WW3 wave model to generate ocean surface waves for the most accurate performance in heavy weather. [ABSTRACT FROM AUTHOR]

In addition to other studies focused on the WRF performance of rainfall, temperature, and precipitation, the present study provides practical suggestions for ship operators on choosing grid point value (GPV) datasets and WRF modeling approaches to help make better decisions on weather routing by estimating strong wind with a higher accuracy. Two different GPV datasets, the NCEP-FNL and ERA-Interim, are used for WRF modeling. Various modeling approaches, which include applying different physics options and choosing whether to adopt the four-dimensional-data-assimilation, are employed for numerical simulation. Then, on-board measurements of a bulk carrier has been used to validate the calculated wind of eight rough-sea navigational cases. This occurs over a wide spatial range, covering ocean regions such as the Northwest Pacific Ocean, South of Australia, Tasman Sea, Southeast of Africa, and Southeast of America, as well as a wide temporal range of four years. Results of 64 WRF simulations show that ERA-Interim is suitable for longer periods, while NCEP-FNL can analyze extremely high winds. Additionally, grid nudging can also reduce the wind strength due to its smoothing effect. Finally, selections of these different GPV datasets and modeling approaches for ship weather routing are discussed, particularly for rough-sea navigations. • Ocean surface wind over rough seas of several seasons and ocean regions have been generated. • GPV datasets including both the NCEP-FNL and ERA-Interim have been utilized for wind simulation. • On-board observations of weather and ship motion has been done for a long term. • Modelling approaches can add significant value to optimum ship routing. [ABSTRACT FROM AUTHOR]

The evaluation of speed loss is the most important studies in the optimal ship routing. However, the mode of speed loss during rough sea voyages is complicated because of the involvement of the human factor. A speed loss analysis is conducted to improve the accuracy of evaluation of measured data extracted from actual marine conditions. First, the conventional criteria to judge the deliberate speed reduction is summarized with the conventional ship maneuvering techniques. Second, a nationwide questionnaire for marine engineers implemented for engine safety during rough sea voyages is detailed. Third, these conventional criteria are validated by analyzing the collated data on accelerations, ship motions, navigation, and engine parameters. The limiting value of vertical acceleration agrees with that obtained via practical measurements. On the contrary, the limiting values of lateral acceleration, roll motions, probabilities of deck wetness, and slamming do not agree with that obtained via measurements. The probabilities of deck wetness, slamming, and propeller racing start to increase at approximately the same instant where the deliberate speed reduction occurs. Some relations between the deck and engine parameters detailed in this paper were found to be highly correlated. Finally, the results of the analysis were used to model simpler estimations. • This study makes clear the detailed situations on the speed loss in rough sea voyages. • Related parameters are validated with the measured database for 28,000-DWT bulk carrier. • Deliberate speed reduction occurs at the starting point of deck wetness or propeller racing. • Speed loss has a boundary region at Δ V = 3–4 knots, and are correlated with pitch motion. [ABSTRACT FROM AUTHOR]