15:30
Ship hydromechanics resistance III
Chair: Zheng Ma
15:30
30 mins
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A NEW PERFORMANCE PREDICTION PROCEDURE FOR PROPELLERS WITH UNCONVENTIONAL TIP SHAPE
Mariano Pérez-Sobrino Pérez-Sobrino, Juan Gonzalez-Adalid, Ramon Quereda, Amadeo Moran-Guerrero, Giulio Gennaro
Abstract: It is a fact that propellers with unconventional tip shape have proved in many cases at full scale that ship propulsion efficiency can be increased with this type of propellers; however at model scale this improvement is not clearly shown and therefore it is neither shown in the standard predictions of many experimental facilities. For conventional propellers ITTC’78 Performance Prediction Method [1] (ITTC’78-PPM) has been successfully applied, directly or with some variations, in many Institutions, Towing Tanks and specialized companies from 1978 when it was recommended to be used by ITTC. But it has been recognized by ITTC [2] that this method cannot be applied to Unconventional Tip Shape propellers. ITTC’78-PPM includes corrections to Open Water model tests results based on the different Reynolds number of the model tests compared to full scale, meaning that viscous effects in propeller blades are also different, and in consequence propeller open water parameters KT and KQ measured at model scale must be corrected to obtain appropriated values to be used for predictions at full scale. Correction method for conventional propellers implemented in ITTC’78-PPM was based in an extensive work made during several years before, analyzing a lot of correlation data for many types of ships for which model tests and sea trials data were available (for instance see [3]). For new types of unconventional propeller blades this is not the case; there are not so many published data to allow sea trials results to be correlated with model tests results.
The lack of a standard procedure to be applicable to this type of propellers is many times an obstacle to the possibility to implement energy saving propellers. For that reason a considerable amount of work has been made in recent years on this question. Three main approaches have been published trying to contribute to solve this problem: Semi-empirical Methods, Strip Methods and CFD based and/or Panel Methods.
In this paper a new procedure based in the strip method will be derived applying adequate expressions of the friction line more in accordance with the expected type of flow in each section of the propeller blade, depending of the section Reynolds number. Main conclusions will be the confirmation that ITTC’78-PPM is not useful to predict propulsion characteristics of unconventional tip shape propellers and that present method, going a step deeper in the basic principles of viscous effects, predicts with reasonable accuracy propulsion characteristics of conventional and unconventional propellers, monoblock or CPP type and it is easy to implement.
REFERENCES
[1] 1978 ITTC Performance Prediction Method. ITTC Recommended Procedures and Guidelines 7.5-02-03-01.4. Revised 2011.
[2] 27th ITTC-Copenhagen. Volume II, page 616. 2014.
[3] 14th ITTC. Report of the Performance Committee. 1975.
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16:00
30 mins
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A PRACTICAL CALCULATION METHOD FOR FORWARD-SPEED EFFECT NON CROSS-COUPLING RADIATION FORCES IN ENHANCED UNIFIED THEORY FOR SHIP MOTIONS
Masashi Kashiwagi, Jing Zhu
Abstract: The enhanced unified theory (EUT) developed by Kashiwagi has been in routine use for predicting seakeeping performance of a ship with forward speed. Although EUT takes into consideration the forward-speed and 3D effects on the inner solution through the matching procedure between the inner and outer solutions, the free-surface boundary condition to be satisfied in the inner problem is that essentially at zero speed. Thus the degree of agreement between experiments and computed results by EUT is not so good especially for cross-coupling terms in the radiation forces for instance between heave and pitch. Although several attempts have been done to incorporate the forward-speed effect in the inner free-surface boundary condition, no reliable calculation method has been established so far. In this paper, in order to develop a robust and practical calculation method with keeping the framework of EUT, we consider introduction of the concept in Ogilvie-Tuck's rational strip theory into the inner solution of EUT. That is to say, the advection term in proportion to the forward speed can be treated as an equivalent pressure applied on the free surface and its solution is incorporated into the inner solution. This practical method can improve the cross-coupling terms in the radiation forces, introducing forward-speed correction which plays an important role in improving the degree of agreement with experiments in ship motions. The improvement in ship-motion predictions is of crucial importance in the prediction of the added resistance.
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16:30
30 mins
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MOORING SYSTEM DESIGN AND ASSESSMENT FOR A MULTI-MODULE VERY LARGE FLOATING STRUCTURE
Bo Wu, Xiaoming Cheng, Chao Tian, Xinyun Ni
Abstract: Very Large Floating Structure (VLFS) has a great potential for various industrial applications. A VLFS usually has large dimensions and consists of multiple modules, each can be a normal-sized floater. The mooring system design for a multi-moduled VLFS is a challenging task for the reason that the huge dimensions and mass of the structure will incur extremely high environmental loads and inertia forces, and to constrain the movement of such a structure using a mooring system, in particular in shallow water, requires accurate assessment of the environmental loads on the structure and careful design and optimization of the mooring system, any under-design may lead to a failure of the mooring system and severe consequences; on the other hand, an over-design can mean a significant cost increase.
This paper presents the design, analysis and results of a mooring system for the positioning of a multi-moduled VLFS in shallow water. The mooring system is of a catenary type consisting of chains and steel wire ropes. The VLFS consists of five identical rigid modules with hinge-like connectors linking adjacent modules. The analysis was conducted numerically in both regular and irregular waves on the basis of linearized wave theory with the hydrodynamic interaction between modules taken into account. The numerical simulations were performed for the moored VLFS in various environmental conditions including wave, current and wind was carried out in time-domain. The mooring line tensions, VLFS motions in six degrees of freedom and connector forces were obtained for a number of wave, current and wind headings. The work presented in the paper can be used as a reference for the mooring system design and optimization of similar multi-moduled floating structures.
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