Actual Seas Research Articles

Study on the bow shapes above the waterline in view of the powering and greenhouse gas emissions in actual seas

In the shipping industry, as well as in other industries, a reduction in the greenhouse gas emissions is expected to help to reduce global warming. Shipbuilding companies are now developing ship hull forms to reduce greenhouse gas emissions with an emphasis on the vessel performance in actual seas where winds and waves are part of this environment. In this paper the bow shapes above the waterline, which may affect the added resistance in waves, are focused upon since the ship motion and wave reflection at the bow contribute to the added resistance in waves that can be reduced by an improvement in the bow shapes above the waterline. The relationship between the bow shapes above the waterline and the added wave resistance is investigated through tank tests in waves with model ships with different bow shapes. From the results of the investigation, the influences of the bow shape on the powering and greenhouse gas emissions is evaluated.

オンボードデータを用いた統計的船舶性能推定手法の応用 ‐オンボード計測指針取得及び船舶性能分布の予測‐

Global warming and other environmental problems are common threads in each industry. In the field of international maritime,for example, the development of low-resistance ship and energy-saving devices, the optimization of shipping schedule, and the development of new energy resources are done. It is important that the performance of a ship in actual seas is predicted accurately in those developments. The authors have proposed in the previous paper a statistical method to estimate the correlation between sea states and ship responses for a short time by using onboard measurement data. The correlation can be represented as the conditional probability density function (PDF) with Principal Component Analysis (PCA) which is one of the multi-variable analyses. We can accurately estimate the statistics of ship responses by using the conditional PDF. The onboard measurement data include various information such as nonlinearities and fluid mutual interferences. Therefore, there is a possibility that the statistical method based on the onboard measurement data will be applied to various purposes other than the estimation of ship responses.The present paper proposes two applications of the statistical method based on the onboard measurement data. The first is the application to the effective onboard measurement, the second is the application to the prediction of ship performance distribution.The ship performance distribution is defined as the probability distribution of ship responses corresponding to the voyage condition.In the first application, an attempt is made to find the onboard data with strong correlation to ship responses by analyzing the result of PCA in detail. If we can find those onboard data, the effective onboard measurement will become possible. In the second application, it is investigated whether the ship responses corresponding to the change of navigation can be estimated by using the correlation between sea states and ship responses obtained from the onboard measurement data. If we can estimate the ship responses corresponding to the change of navigation, the planning of the voyage will be made possible by using the correlation.

オンボードデータを用いた実海域船舶性能の統計的推定

It becomes increasingly important that the performance of a ship in actual seas is predicted accurately due to an increasing awareness about global warming and other environmental problems. The ship responses such as the ship motions, the stresses and the added resistances in actual seas vary with sea states and the ship speed. The onboard measurement data has this information,therefore the data is useful. However there is not an established analysis method of the onboard data because the data involves factors which are unexplained by the theoretical calculation. The ship responses which are caused by uncertainty sea states are stochastic variable. Therefore, the statistical analysis is useful in case of the prediction of ship responses with using the onboard data.The paper proposes a statistical method which estimates the correlation between sea states and ship responses for a short time as the conditional probability density function (PDF) by using the onboard measurement data. Two kinds of method are adopted to estimate the conditional PDF of ship speed loss for encounter sea states. On one hand it is parametric method, on the other it is nonparametric method. Using a time-series of sea states and ship speed loss during a one-year voyage obtained by an oceangoing simulation, the expectation and confidence interval of mean ship speed loss are estimated to validate these stochastic models based on each method. We conclude that the stochastic models are available. In particular, the stochastic model based on the nonparametric method offers an accurate prediction of the mean speed loss.

実船計測による実海域性能評価法の検証

For building and promoting low greenhouse gas emission ships, a design index of ship performance in actual seas is required to be developed. On a project for 10 mode at sea, a calculation method of ship performance in actual seas has been developed. The method can calculate added resistance in short waves with engineering accuracy in terms of correction factors based on tank test results.Onboard measurement is carried out for the purpose of verification of the calculation method. As for onboard measurement, it is difficult to observe encountered waves with accuracy. To avoid the difficulty, wave radar measurement system is applied and measured results are compared with those by other wave measurement systems and the accuracy of wave measurement is discussed.Thereafter, a comparison of calculated ship speeds is made with measured ones with correction by the displacement, engine output,and weather. A procedure for correcting the measured data is described at some length. As a result, it is clarified that the results of onboard measurement show effectiveness of the calculation method.

Dynamic Behaviour of Ship Propulsion Plant in Actual Seas

The paper consists of three main objectives: the mathematical model of diesel engine propulsion powerplant, hull-propeller interaction model including the effect of propeller emergence (propeller racing) and real-time dynamic behavior of the ship propulsion plant. One of the main advantages of the model is that it enables to investigate the dynamic behavior of diesel engine and speed control system at off-design and transient condition, allowing for real-time assessment of severity of propeller load demand fluctuation in order to improve model-based speed control schemes to ensure the safe operation of ship propulsion plant.

自動車運搬船及び大型油槽船の実船計測による速力低下計算法の検証

For reduction of Greenhouse Gas (GHG) emission from shipping sector, low GHG emission ships are required to be built and promoted. On a project for 10mode at sea, a calculation method on decrease of ship speed has been developed. The calculation method has confirmed by tank tests.Full-scale onboard measurements have been conducted for four kinds of ocean-going ships in operation; pure car carrier, very large crude oil carrier, container ship and bulk carrier, with a view to attesting the reliability of the method for evaluating ship performance in actual seas. In this paper, results of onboard measurements for a pure car carrier and a very large crude oil carrier which data accumulate for whole four seasons are reported. The measured and calculated data matched well, showing sufficient reliability of performance estimation for both a fine and a blunt ship type.The results of onboard measurement for the two ships show good agreement with those of the calculation. These results confirmed that the calculation method can be accurately applied to both a fine and a blunt ship type.

バラスト状態での波浪中抵抗増加の推定

As for bulk carriers and tankers it is important to calculate added resistance in waves in a ballasted condition with accuracy,because they are operated in ballasted conditions for half of their operation time. However, it is difficult to estimate the resistance with accuracy by the numerical calculation since wave making phenomena in a ballasted condition are a complex phenomena due to emersion of the bulbous bow.In this paper, we examine the adaptability of the hybrid method to the estimation of added resistance for the ballasted condition,which is practically corrected the added resistance in waves using tank tests. The comparison between calculation and tank test are presented here. Then we investigate the effect of trim which features the ballasted condition, and the effect of draft and frequency as well. Thereafter the influence of the present method on decrease of ship speed in actual seas is evaluated by contrast that derived from the tank tests.

波浪中抵抗増加と船首水面上フレア形状の関係について

Added resistance in waves is a predominant component for the estimation of ship speed in actual seas, so that it is necessary to be calculated accurately. The added resistance in waves is influenced by the hull form above water line. However it is difficult to take into consideration the form to the calculation theoretically. As a solution to this matter, the hybrid calculation method which combines a result of tank tests with the calculation was developed. The method includes the hull form above water line with the result of tank tests.In order to investigate the effects of the hull form above water line on the basis of the hybrid calculation method, the tank tests was carried out using three types of container ships with the different form above water line. For the expression of the effect of the hull form above water line on the added resistance in waves, a flare bluntness coefficient is installed as a parameter in the hybrid calculation method. The application for the hull form design is studied. The results of the tank tests and the calculation are compared.

海の１０ モード指標計算法

In order to increase ship performance in actual navigation conditions, a definite evaluation method of the performance should be established. For that purpose, a design index, 10 mode Index for Ships, is developed. The index consists of ship speed in ten kinds of weather conditions as well as ship speed in calm seas.The present paper prescribes a calculation method for the 10 mode Index for Ships. To obtain the ship speed in actual seas,equilibrium equations of external forces acting on a ship are calculated. Selected as the external forces acting on a ship are resistance in still water, propeller thrust, and resistance due to drift, steering, winds and waves. In calculating these forces, accuracy should be kept for resistance due to waves. Therefore, the calculation is developed based on the hybrid type procedure, which combines the model tests with the numerical calculation. In addition to this, a package program to calculate the index is developed.In the paper following verifications are carried out; 1) comparing calculations on added resistance in regular waves with other programs, 2) comparing tests of a container ship model in several ship model basins and 3) full scale measurement of a pure car carrier. From the verifications it is clarified that the hybrid calculation is robust to evaluate ship speed in actual seas.

On a Calculation of Decrease of Ship Speed in Actual Seas

On a Calculation of Decrease of Ship Speed in Actual Seas

実海域中推進性能を考慮した船型設計

実海域中推進性能を考慮した船型設計

Strip method for a laterally drifting ship in waves

Lateral drift occurs due to the effects of wind forces, wave drifting forces, or both on ships sailing in actual seas. It is important therefore to investigate the influence of lateral drift on seakeeping performance for improved ship operation. The velocity potential was expanded as an asymptotic power series in terms of the lateral speed parameter, τ, defined as ω e V 0/g, where ω e is the frequency of wave encounter; V 0 denotes the lateral velocity, which is assumed to be sufficiently small; and g is the acceleration due to gravity. By combining this technique with the strip method, two sets of motion equations of all the hydrodynamic force coefficients for ship seakeeping were derived. The first set is for ships without lateral drift and is the same as the equations in the new strip method, and the second set is for the additional motions induced by lateral drift. It was found that all ship motion modes except surge are coupled when a ship drifts laterally in waves.

実海域中推進性能の推定と評価

実海域中推進性能の推定と評価

実海域中推進性能の推定と評価

実海域中推進性能の推定と評価

斜航する船の波浪動揺と定常波力に関する水槽実験

Lateral drift occurs due to the effects of wind forces and/or wave drifting forces in ships sailing in actual seas. It is important therefore to investigate the influence of lateral drift on seakeeping performance to attain a rational design for ships. This paper describes the experimental results with respect to ship motions in regular waves and steady drifting forces for an obliquely moving ship. The experiments were carried out for SR108, container ship at seakeeping and maneuvering basin, Nagasaki R & D Center, MHI. Due to the effect of the hull drift, the lateral motions such as sway, roll and yaw are induced even in pure heading waves, which never appear in case without hull drift. Their amplitudes become larger with increase of absolute value of the drift angle. In beam waves, also, amplitudes of sway, roll and yaw become larger with increase of the drift angle. On the other hands, the influence of the hull drift on the motions of surge, heave and pitch is not remarkable. The influence of the hull drift on wave drifting lateral force and yawing moment is considerable large. The lateral drifting force acts on the hull so as to damp the lateral motion. The influence of the hull drift on the added resistance is small.

On a Setting Method of Sea Margin

At the design stage of a ship, the engine output is determined by taking into consideration of a lowering of performance. The lowering is mainly caused by winds, waves, aging deterioration and biological fouling. To keep the speed in actual seas, the output is appended to a proper ratio, which is called sea margin.The component of sea margin divides into two groups. One is a weather effect and the other is a deterioration effect which progresses with time. For the estimation of the weather effect, weather forecasting data of 8 years were applied. Also, for the estimation of the deterioration effect, roughness increase of the hull was taken into account.In this paper, the desirable sea margin is derived rationally. As an example, for a coastal chemical tanker, Shinko Maru No.28, performance estimation and voyage data analysis were carried out.By comparison with the estimated and analyzed sea margin, it made clear that the estimated sea margin agrees with the analyzed one. Consequently the physical meaning of sea margin is explained as a function of wave, wind and time.

2-9 実船データ解析による推進性能評価法の検討

2-9 実船データ解析による推進性能評価法の検討

Markov Description of Wave-Crest Statistics

Abstract Markov description of wave-crest statistics is considered for the case of severe random seas where Stokes nonlinearities require crests to rise higher than troughs fall. Of interest are the average number of consecutive high waves in runs of high waves and the average number of waves between beginnings of such runs, with high waves being defined in terms of a threshold level above mean water level. Two basic parameters are involved in the description: 1) the probability P that a high wave will occur at a fixed location, and 2) a coefficient C relating P to the probabilities associated with transitions from high to low waves and low to high waves. Comparison with laboratory measurements from scaled Bretschneider seas having a range of nonlinearities indicates that the basic Stokes nonlinearity is contained in the probability P, with no sensible dependence exhibited by the coefficient C. Additional comparison with scaled Jonswap seas illustrates the dependence of this coefficient on spectral shape. A final consideration of results from full-scale hurricane sea conditions, and from corresponding scaled laboratory data, demonstrates the general applicability of the laboratory results presented here to actual wind-driven seas.

Estimation of Directional Frequency Response Functions of a Running Ship Based on Experiments in Directional Spectrum Waves

Estimation of directional frequency transfer functions of a running ship is important to estimate their behaviours in realistic short crested irregular waves (such waves are called as directional spectrum waves)Of course, there are some theoretical calculation methods for such directional response functions, but it seems that there are not so many experimental results confirming such theoretical ones. For that purpose, square basins are needed.On the other hand, recently the authors developed a wave maker which can generate directional spectrum waves in a long towing tank, and many experiments of running ships were conducted in such waves.In those experiments, obtained results are one dimensional motion spectra and direct comparison with theoretical ones is impossible. So we tried to make inverse estimation of directional transfer functions of a running ship using both one dimensional response spectrum measured in such waves and measured two dimensional directional wave spectrum.Estimated results by present method on heave, pitch, roll and vertical acceleration response functions are reported here, but this method will be also applicable to other situations like motions of a running ship in actual seas by measuring its motions and incoming directional waves.

Estimation of directional frequency response functions of a floating offshore structure based on experiments in directional spectrum waves

Estimation of directional frequency transfer functions of floating offshore structures is important to estimate their behaviours in realistic short crested irregular waves (such waves are called as directional spectrum waves) Of course, there are some theoretical calculation methods for such directional response functions, but it seems that there are not so many experimental results confirming such theoretical ones. For that purpose, square basins are needed.On the other hand, recently the authors developed a wave maker which can generate directional spectrum waves in a long towing tank, and many experiments of offshore structures are conducted in such waves.So we tried to make inverse estimation of directional transfer functions of a floating offshore structure using both one dimensional response spectrum measumred in such waves, and measured two dimensional directional wave spectrum.Estimated results by present method on heave, pitch, and roll response functions are reported here, but this method will be applicable to other situations like motions of floating structures in actual seas by measuring its motions and incoming waves.