Research On Calculation Method Of Wave Load And Mooring .

JournaloOPENentgemnaefense MafDISSN: 2167-0374ACCESS Freely available onlineJournal ofDefense ManagementResearch ArticleResearch on Calculation Method of Wave Load and Mooring Force Basedon Time Domain Potential Flow TheoryZhang Bao-Ji*, Ning Ping-Bo and Zhang ChiCollege of Ocean Science and Engineering, Shanghai Maritime University, Shanghai 201306, P.R. ChinaABSTRACTIn order to predict the wave force of moored ships quickly and accurately, based on the unsteady time domain flowtheory, the calculation method of wave load and mooring force of LNG ship under full load and ballast conditionsis studied. Through short-term forecasting and long-term forecasting, the wave bending moment and wave force ofeach cross section of the LNG ship, as well as the motion response of LNG ship which have the greatest influence onthe wave load are obtained. Based on this, the wave force and motion response is performed, and the mooring pointis set according to the actual environmental conditions when the LNG ship is moored at the dock. The mooringforce of each cable that meets the specifications and the number of mooring lines required are finally obtained. Theresearch results in this paper can provide a technical support for dock mooring setup and mooring force calculationfor similar ship types.Keywords: Wave load; Time domain; Mooring calculation; Wave forceINTRODUCTIONShips or marine structures can be affected by the marineenvironment when they are moored at docks or at sea, generatingsome forces and reaction forces such as wind, waves, currents andtides. Under the influence of these external forces, the ship willproduce 6 Degrees of Freedom motion (6-DOF), such as: heave,pitch, sway, roll, yaw, and sway. In addition, it will be affected bypassing ships. Moreover, the nonlinearity of mooring lines andfender materials, as well as the tension and slack of the cables, canmake the calculation of mooring forces very difficult [1]. At present,the design and calculation of mooring mainly rely on methods suchas field observation, physical model test and numerical simulation.Although the model test is accurate, it is difficult to capture somecomplicated physical phenomena, especially the long experimentalperiod and high cost. It is difficult for some preliminary designersand researchers to accept. Therefore, numerical simulation shouldbe the main means to study the mooring calculation. Especiallyin recent years, some large-scale commercial software has comeout one after another. It is a trend to use software to calculatemooring. In foreign countries, as early as the 1970s, mathematicalmodels were used to predict the motion response and mooringforce of dock mooring vessels under wind and current. Natarrajanet al. used a combination of numerical simulation and physicalmodel tests to conduct a comprehensive numerical simulationand experiment on a certain wave height and different periodof the marine environment and obtained satisfactory results [2].Kreruzer proposed a cable dynamic analysis model [3]; Wicherset al. developed a fully coupled time-domain numerical model tostudy the coupling effects of deep-water mooring lines and riserdrag coefficients on the movement of floating bodies and thetension of mooring lines [4];Sphaier et al. solved the static problemof turret FPSO mooring system by using simplified equation ofship motion [5];Kim et al. studied the coupled motion of thehull and mooring of the turret FPSO by experimental methods.Domestic researchers' mooring calculations are based primarilyon potential flow theory and large commercial software [6]. Hu Yiused Seasam software to study the hydrodynamic performance oflarge LNG ships, and used AQWA software to study the mooringforce calculation of LNG ships [7]. Yu Yang from the static pointof view, does not consider the impact of cable tensile deformation,the cable tension is analyzed [8]; Xiang Yi used the Monte Carloalgorithm and the chaotic solution to simulate the tension of themooring ship ropes, and compared with the experimental values,and obtained consistent results [9,10];Li Wei studied the variationof the force of the cable and the fender with the wind and wavedirection and the wind direction by testing the large wind andwave mooring model of a 175,000-ton bulk carrier [11]. Jiang Qingtook the Dalian Ore Terminal as an example, and measured thecable pulling force in the 90-degree range and the 2.25 m intervalin all directions, and the ship was berthed by physical model test[12]. Liu Bi-Jin and Zhang Yi-Fei through theoretical analysis ofCorrespondence to: Zhang Bao-Ji, Professor, College of Ocean Science and Engineering, Shanghai Maritime University, Shanghai 201306, P.R. ChinaTelephone: 86-21-38282504; E-mail: zbj1979@163.comReceived: May 10, 2019; Accepted: May 21, 2019; Published: May 28, 2019Citation: Bao-Ji Z, Ping-Bo N, Chi Z (2019) Research on Calculation Method of Wave Load and Mooring Force Based on Time Domain Potential FlowTheory. J Def Manag. 9:180; doi: 10.35248/2167-0374.19.180Copyright: 2019 Bao-Ji Z, et al. This is an open access article distributed under the term of the Creative Commons Attribution License, whichpermits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.J Def Manag, Vol. 9 Iss. 1 No: 1801

Bao-Ji Z, et al.OPENthe ship under mooring conditions summarized the main factorsaffecting the mooring force of the ship [13]. Under differentworking conditions, the cable of each position of the ship wasanalyzed under different wave height and periodic environment.The variation of the tether force also considers the influence ofthe wave angle on the tether force. Zou Zhi-Li introduced the basiccharacteristics of soft steel arm mooring and the importance ofaccurately calculating its mooring force [14]. Taking FPSO in BohaiSea as an example, analyzing its force and establishing the equationof the force and moment, after various calculations and analysis,verifies the reliability and practicability of its dynamic analysismethod. Based on the previous research results and experience,this paper studies the wave load of LNG ships under full loadand ballast conditions based on the unsteady time domain flowtheory. Through short-term forecasting and long-term forecasting,the wave bending moment and wave force are obtained, and thenthe 6-DOF motion performance is performed to obtain the swayacceleration and the surging acceleration which have the greatestinfluence on the wave load. Based on this, the calculation of thewave force is carried out. According to the actual environmentalconditions of the LNG ship dock mooring, the mooring point issteed. After many calculations and modifications of the mooringpoint setting, each of the specifications is finally obtained. Themooring force of the root cable and the number of mooring cablesrequired. The research results in this paper can provide theoreticalcalculation methods for the dock mooring setting and mooringforce calculation of similar ship types.BASIC THEORYSolution of unsteady disturbance potentialDefine zero-speed radiation potential ф0j(x, y, z) and speedadditional velocity potential k0j(x, y, z), so that it satisfies thecontinuous equation and the bottom condition, free surfacecondition, remote radiation condition and the surface conditiondefined by the following formula [15]: 0φj (x ,y ,z ) nj ,(j 1,2, ,6) n Uφj (x ,y ,z ) m j ,(j 1,2, ,6) n(1)Then the radiation potential фj(x, y, z) can be decomposed into thesum of the radiation potential ф0j(x, y, z) without speed and theadditional velocity potential фUj (x, y, z) with speed, namely:φ j ( x, y, z ) φ 0j ( x, y, z ) U Uφ j ( x, y, z ), j 1,2, ,6iω(2)where,0j １,2,3,4 0φ ( x, y, z ) φ3 ( x, y, z )j 5 φ 0 ( x, y , z )j 62 Uj(3)For the above solution problem, each disturbance potential фj(x, y,z) can be determined by an appropriate numerical solution method.After disturbance potential фj(x, y, z) defined, the generalizedadditional mass coefficient and the generalized wave-dampingcoefficient can be obtained by the following formula:mij ρ Re(φ j )ni dssHN ij ρω Im(ϕ j )ni ds(4)ACCESS Freely available onlineThe resilience coefficient can be obtained by the following formula:C33 ρ gA C35 C53 ρ gS y C ρg h ρg(J z z )44xxBG C55 ρ g hy ρ g ( J y z B zG ) (5)where, A is the waterline area of the ship at equilibrium;Sy is thestatic moment for the y-axis;Jx and Jy is the moment of inertia for thex and y axes; is the ship's drainage volume; zB and zG is the verticalcoordinates of the ship's center of gravity and center of gravity;hxand hy is the ship's horizontal stability and vertical stability arehigh. From the linearized Bernoulli equation, the hydrodynamicpressure after the hydrostatic pressure change is subtracted is: P( x, y, z , t ) Re { p ( x, y, z ) eiωt } ρ (iω Up ( x, y , z , ) )[ϕ I ( x, y, z ) ϕ D ( x, y, z ) ϕ R ( x, y, z )] x(6)The hydrodynamic forces and moments acting on the hull can beintegrated by the above-mentioned hydrodynamic pressure alongits average wet surface S:F(t ) nPdSS(t )m 0 (r n)PdS(7)SCatenary modelThe catenary mooring cable has a standard quasi-static modelequation, which is based on the vertical gravity action of themooring cable to resist the resilience of the environmental load ofthe platform, whose equation is [16]:I w h(h 2THTW) l ' H sinh 1 (WWTHh(h 2TH) 0W(8)In Eq. (8), lw is the working length of cable when not stretched l’is the length of cable after stretching, h is the water depth, TH isthe cable tension, w is the gravity of the unit catenary line in thewater. Catenary model is very effective in shallow water mooringcalculation, so it has been widely used. This paper studies themooring calculation of shallow water in a water depth of 30 m, sothe catenary model is used.CALCULATION OF THE WAVE LOAD ANDMOTION RESPONSETaking a LNG ship as an example, the ship's wave load is predictedby time domain potential flow theory, and the safety of the ship isanalyzed and verified. The dock mooring design is carried out tocalculate the mooring force of each cable based on the wave loadand compare it with the specification value to verify the reliabilityof the mooring method. The main dimensions of the LNG ship areshown in Table 1. Different wave directions and wave frequencieshave different effects on the radiation and diffraction of the ship,which affects the amplitude response of the RAO. The ship has atotal of 0 , 30 , 60 , 90 , 120 , 150 , and 180 wave directions. The0 direction is the ship's corresponding wave condition, and the90 direction is the ship's starboard side wave. The 180 direction isthe case where the ship is facing the waves. The wave frequency isset from 0.1 rad/s to 1.8 rad/s for a total of 35 frequencies with aninterval of 0.05 rad/s. The wave uses JONSWAP (γ 1).sHJ Plant Pathol Microbiol, Vol. 10 Iss. 4 No: 4802