RIASSUNTO
ABSTRACT
Seakeeping is a key standard to examine whether a ship has good performance in waves. Ship hull forms can significantly affect the motion behavior in waves and the influences vary a lot under different wave conditions. Therefore, it is necessary to conduct the multiobjective optimization design of ship hull based on seakeeping performance in different waves. In the present work, the KRISO Container Ship (KCS) is considered as a parent ship. The hull form is globally deformed. The main principal parameters are constrained within a certain range. The heave and pitch motions in the typical head wave condition are used as objective functions. The whole optimization process is implemented based on in-house solver OPTShip-SJTU. It turns out that OPTShip-SJTU has a very practical application in the aspect of multi-objective optimization of ship seakeeping performance based on CFD.
INTRODUCTION
The pros and cons of seakeeping is one of the important indicators to measure the performance of a ship. In most cases, ships sail in the waves. In particular, some offshore engineering vessels, such as marine drilling vessels often sail in heavy sea condition. Seakeeping directly affects the navigation, safety and comfort performances of a ship. Ships' severe motions have a series of detrimental impact on the ship, including the safety and efficiency of crew and various equipment and so on. Therefore, ship preliminary design must consider the seakeeping performance. Seakeeping is determined not only by the external marine environment, but also closely related with the ship's main particulars and shape. Under a given marine environment, it may be considered to guide ship design optimization with the aim of improving seakeeping performance.
Ship optimization design is mostly based on the rapidity performance of ships in still water (Campana et al, 2006; Zhang et al, 2015; Huang & Yang, 2015; Liu et al, 2016; Wu et al, 2017). In recent years, some scholars also consider seakeeping performance in ship optimization design. At first, some seakeeping performance indicators is only as constraints (Harries, 2001), and later also taken as objective functions. Peri and Campana (2003a) optimized the hull shape line with the total resistance and ship's heave and pitch peaks as three objective functions. D. Peri and Campana (2003b) established a ship optimization model to minimize the total resistance and the maximum velocity and acceleration of different locations at two different speeds and three different sea states. Peri et al (2004) improved wave-making resistance, heave and pitch peak and the vortex around the top of the sonar at a single speed. Boulougouris et al (2006) considered the total resistance and maximum vertical motion at the center of gravity to create a multiobjective optimization problem. Diez et al (2015) presented recent research conducted within the NATO RTO Task Group AVT-204 "Assess the Ability to Optimize Hull forms of Sea Vehicles for Best Performance in a Sea Environment ", with the goal of reducing drag and improving seakeeping performance. The parent ship was the DTMB 5415. Six research teams including ECN, TUHH, NTUA, INSEAN, ITU and UI were involved in this ship optimization problem considering the two speeds, the head and oblique waves, the vertical acceleration and the rolling angle of the bow and the total resistance. The results were compared and analyzed. The most promising hull was with the resistance improved by nearly 10% and the seakeeping 9%, respectively. Qiu et al (2011) implemented integrated optimization design on the iSight platform for the ship drag/seakeeping performance of the 46000 DWT oil tanker. The total resistance per displacement of the unit and peak pitching and pitch peak were taken as the objective functions of ship optimization; Yang et al (2011) selected four objective functions of significant pitch angle, the acceleration, the number of slamming per hour and significant roll angle, improving seakeeping performance of a fishing vessel. The four objective functions of the optimized hull were increased by 2.32%, 7.17%, (-0.13%) and 9% respectively. Bagheri et al (2014) optimized the Wigley ship model with the heave and pitch peaks as the objective functions. Seakeeping in researches above was almost evaluated by strip theory. Also, some scholars (Kim et al, 2010; Wu et al, 2016) optimized ship seakeeping just evaluated by simple empirical formulas. These evaluation methods are fast but low-fidelity.