ALTERNATIVE ASSESSMENT OF WEATHER CRITERION FOR SHIPS WITH LARGE BREADTH AND DRAUGHT RATIOS BY A MODEL EXPERIMENT: A CASE STUDY ON AN INDONESIAN RO-RO FERRY

The weather criterion is one of stability criteria to verify ability of a ships to withstand the combined effects of severe wind and rolling criteria in dead ship condition. An overestimated roll angle is obtained when the weather criterion is applied to ships with breadth and draught ratios larger than 3.50 and ratios between vertical centre of gravity and draught larger than 1.50. This paper discusses the assessment of weather criterion for an Indonesian ro-ro ferry by model experiments. The drift test is performed in four wave steepnesses with wave frequencies near the roll natural frequency. The maximum roll amplitude is used to calculate the effective wave slope coefficient correponding to the wave steepness, with Bertin’s coefficient obtained by the roll decay test. The damping factors correspond to the breadth and draught ratio as well as the bilge keel contribution are determined using the formula of weather criterion with the roll angle obtained by the Japanese formula with a correction factor of 0.70 due to the irregularity of waves. The obtained effective wave slope coefficient and the damping factors due to breadth and draught ratio and the bilge keel are smaller than those used in the weather criterion.

Investigation of the dynamic inclinations of vessels under the action squall within the conditions of the weather criterion

Рассматривается динамический крен судов, расположенных лагом к фронту волнения, под действием порывов ветра и шквалов. Путем имитационного моделирования пространственного движения оценивается выполнение основных допущений, используемых при нормировании остойчивости в рамках критерия погоды. Показано, что первое динамическое наклонение судна на подветренный борт может быть максимальным, если длительность порыва ветра или шквала не превышает полупериода собственных бортовых колебаний. При более длительных шквалах максимальным будет второе или третье наклонение. Оценивается влияние хода судна и постоянно действующего ветра на динамический крен. Подтверждено, что для малых судов, для которых волнение высокой балльности приближается к резонансному, опасность воздействия шквалистого ветра наиболее велика. Результаты показывают, что, несмотря на существенную упрощающую схематизацию реальных процессов, критерий погоды является вполне эффективным средством прогнозирования безопасности штормового плавания судов. Reviewed the dynamic heel of ships disposed broadside to the wave front under the influence of wind gusts and squalls. By means of simulation of spatial motion, the fulfillment of the basic assumptions used in the normalization of stability within the framework of the weather criterion is evaluated. It is shown that the first dynamic inclination of the vessel to the leeward side can be maximal if the duration of a wind gust or squall does not exceed the half-period of natural side vibrations. With longer squalls, the second or third inclination will be maximum. The influence of the ship's course and the constantly acting wind on the dynamic heel is estimated. It has been confirmed that for small vessels, for which high-intensity waves are approaching resonance, the danger of the impact of squally wind is greatest. The results show that, despite the significant simplifying schematization of real processes, the weather criterion is a quite effective means of predicting the safety of stormy navigation of ships.

Analysis of the ship's stability reserve for inland and mixed (river-sea) navigation

При проверке остойчивости по основному критерию или критерию погоды, кренящий и восстанавливающий момент мо гут быть равны между собой. С одной стороны этого достаточно, что бы судно было признано остойчивым при действии динамического ветра, с другой стороны, минимальные отклонения различных элементов и характеристик судна от расчётных значений, могут вызвать рост кренящего и снижение восстанавливающего моментов, который при низком уровне самой остойчивости судна может привести к чрезвычайной ситуации, и в худшем случае потере судна. В настоящей работе представлен общий подход к определению фактически необходимого коэффициента запаса остойчивости, проанализированы факторы влияющие на изменение действующих на судно моментов при его наклонении на конечные углы. Авторами так же рассмотрены минимальные приращения данных факторов, которые допускаются действующими Правилами Российского Речного Регистра и фактически приводят, к незначительной ошибке. В качестве примера рассчитан фактический коэффициент запаса остойчивости, для сухогрузного судна смешанного (река-море) плавания. When checking the stability of the main criterion or the weather criterion, the heeling and restoring moment can be equal to each other. On the one hand, this is sufficient for the ship to be considered stable under the influence of dynamic wind, on the other hand, the minimum deviations of various elements and characteristics of the vessel from the calculated values can cause an increase in the heeling and reducing the restoring moments, which, if the ship's stability is low, can lead to an emergency situation, and in the worst case, the loss of the vessel. This paper presents a General approach to determining the actual required coefficient of stability margin, and analyzes the factors that affect the change in the moments acting on the ship when it tilts to the final angles. The authors also consider the minimum increments of these factors, which are confirmed by the current Rules of the Russian River Register and actually lead to a minor error. As an example, the actual coefficient of responsiveness is calculated for a dry cargo ship of mixed (river-sea) navigation.

Alternative Assessment of Weather Criterion for Ships with Large Breadth and Draught Ratios By a Model Experiment: A Case Study on an Indonesian Ro-Ro Ferry

The weather criterion is one of stability criteria to verify ability of a ships to withstand the combined effects of severe wind and rolling criteria in dead ship condition. An overestimated roll angle is obtained when the weather criterion is applied to ships with breadth and draught ratios larger than 3.50 and ratios between vertical centre of gravity and draught larger than 1.50. This paper discusses the assessment of weather criterion for an Indonesian ro-ro ferry by model experiments. The drift test is performed in four wave steepnesses with wave frequencies near the roll natural frequency. The maximum roll amplitude is used to calculate the effective wave slope coefficient corresponding to the wave steepness, with Bertin’s coefficient obtained by the roll decay test. The damping factors correspond to the breadth and draught ratio as well as the bilge keel contribution are determined using the formula of weather criterion with the roll angle obtained by the Japanese formula with a correction factor of 0.70 due to the irregularity of waves. The obtained effective wave slope coefficient and the damping factors due to breadth and draught ratio and the bilge keel are smaller than those used in the weather criterion.

Estimation of Effective Wave Slope Coefficient of Ships with Large Breadth and Draught Ratio

One of parameters to estimate heel angle of a ship in beam seas is effective wave slope coefficient. In the weather criterion of IMO, the effective wave slope coefficient is determined as function of ratio between distance of center of gravity from the sea surface and the ship draught. The others methods could be used to estimate the effective wave slope coefficient are simplified strip theory and model experiment. A ship with shallow draught and large vertical center of gravity can have an effective wave slope coefficient larger than 1.0 if the coefficient is calculated by using the formulae of weather criterion. Therefore, an alternative method to estimate the coefficient is necessary when it is applied to ships with geometry characteristics different with those used to develop the formulae. This research conducts to estimate the effective wave slope coefficient using three different methods, namely the formulae of weather criterion, the simplified strip theory and model experiment. Results of the three methods may provide enough evidence about suitable method to estimate the effective wave slope coefficient of ships with breadth and draught ratio larger than 3.5 like the Indonesian ro-ro ferries. Results and discussion show that the effective wave slope coefficient obtained by using the formulae of weather criterion is larger compared to that obtained by using the simplified strip theory and the model experiment. Here, the result of simplified strip theory for wave frequency the same as the roll natural frequency of subject ship is similar with the result of model experiment. This results show that the simplified strip theory can be used as an alternative method to determine the effective wave slope of a ship with breadth and draught ratio larger than 3.5 if the result of model experiment does not available.

Studi Koefisien N Pada Prediksi Roll Kapal Sarat Rendah Dalam IMO A.749(18)

Perkembangan penggunaan kapal dengan sarat rendah atau rasio B/T besar (deck cargo) di Indonesia menjadi suatu pilihan dalam rangka efektifitas dan efisiensi dibandingkan membangun kapal tipe hatch cargo. Perairan Indonesia yang sangat bervariasi kedalaman lautnya serta terhubung ke jalur sungai-sungai besar yang masuk hingga pedalaman merupakan suatu pertimbangan pemilihan tipe kapal sarat rendah. Seiring dengan itu peraturan keselamatan kapal semakin ketat merupakan tantangan dalam perancangan dan sistim operasional kapal. Karakteristik lengan stabilitas pada kapal sarat rendah yang berbeda pada kapal umumnya seperti yang disyaratkan dalam kriteria kestabilan minimum yang dikeluarkan oleh International Maritime Organization (IMO) Resolusi A.749(18) menjadikan kapal sarat rendah perlu kajian tersendiri. Dari hasil kajian dan uji model didapatkan bahwa untuk kapal sarat rendah (B/T ³ 4) pada prediksi roll dalam weather criterion direkomendasikan untuk adanya koreksi nilai N dari sebelumnya yang dipakai yaitu 0.02 menjadi 0.035, dengan nilai tersebut menunjukkan hasil prediksi maksimum roll yang realistik bagi kapal sarat rendah. Hal ini merupakan masukkan dalam penyusunan aturan keselamatan kapal sarat rendah.

Preliminary Study on Stability Parameters of Semi-Displacement Ships

Semi-displacement passenger ships operate at the speed range of 19 to 23 knots or Froude number Fn of 0.50 to 0.75. Those ships need higher engine power to maintain their speeds. The efforts should be made to minimize their higher engine powers. Most experts worked to minimize the engine power for most merchant ships. In their works, the initial metacentric (GMO) at departure condition was taken as constraint to represent stability parameters. However, this is not a general case for semi-displacement passenger ships. In this study, a parent semi-displacement passenger was designed and modified. Stability parameters were computed and evaluated as required by the rules IMO-HSC. In addition, one existing high speed passenger ship was evaluated for stability evaluation as reference also. It was found that the stability parameters at large angles of inclination such as: weather criterion, angle of GZmaxand others are more critical compared to GM0. Also, the stability parameters when ship is at arrival condition are more critical compared to those at departure condition. The study resumed that the application of criterion GM0≥ 0.15 m at optimization process of semi-displacement passenger ships should be considered again for the future.

Design Aspects of a High Speed Monohull RoPax Ferry

This paper describes the design process of a high speed mono hull RoPax ferry which operates at a Froude number of 0.4. The design task was quite challenging, as two possible transport concepts were in principle possible: Two ships were needed with a total speed of 50knots, which could result in a combination of a 30kn high speed Catamaran plus a conventional 20kn RoPax Ferry or alternatively in two identical sister vessels of 25kn each. The solution with the high speed catamaran plus the conventional RoPax-Ferry defined the total cost budget, which must not be exceeded by the design of the two sister vessels. This resulted in a tough boundary condition and made life cycle cost evaluations necessary. Due to harbor restrictions, the length of the ships was limited by abt. 110m, resulting in a Froude number of abt. 0.4. This resulted in high costs for the propulsion system. The ferries should initially have open RoRo-Cargo spaces for cost reasons, which made the stability requirements (weather criterion plus Stockholm Agreement) quite challenging. This also strongly influenced the design of the final hull form. As the ship is very sensitive to weight, detailed steel structure optimizations had to be carried out to optimize the main grillage systems of the vehicle decks. The hull form and the appendage design required careful optimization to guarantee the required service speed with the engine power which was available in the price budget. As no vessel of comparison was available, the speed power estimation as well as all design tasks had fully to rely on numerical predictions. As the ship had further demanding requirements for course keeping and comfort in waves, the optimization of the hull form must include also these issues. The paper shows that the design of complex ships is actually a holistic task which includes many engineering disciplines. The paper also shows that 1st principle based design methods can support the design process of specialized vessels significantly.