Experimental Study of the Motion Responses of a Large Mooring(LNG)Ship in the Waves with Grand Period

SHI Xian-ying,ZHANG Ning-chuan,YANG Yang

(1.State Key Laboratory of Coastal and Offshore Engineering,Dalian University of Technology,Dalian 116024,China;2.Dalian Neusoft Institute of Information,Dalian 116023,China)

1 Introduction

The movements of a mooring ship are not only significant indicators used to evaluate the operating conditions of a mooring ship,but also important parameters used to determine the strained condition of mooring lines and the effects a mooring ship has upon a pier.When the wave period upon it is equal or close to its natural period,the mooring ship may have harmonic rolling motions or even more violent motions.Excessive ship motions may considerably affect not only the normal loading and unloading operations[1-2],but also the security of a mooring system and the stability of a pier structure.Therefore,it is essential to make clear the characteristics of motion responses of mooring ships under different loading conditions,and to minimize the harmonic rolling motions under the effects of waves upon mooring ships in port design and operation management.

The characteristics of motion responses are the inherent nature of a mooring ship,which

has no relevance to the pattern and size of waves that acting on the ship.In fact,it depends on the factors like the types and loading conditions of a ship,the mechanical properties of mooring lines and fenders,the patterns of mooring,the layouts of fenders,and so on.As early as in 1989,Yang[3]has investigated the characteristics of frequency domain responses of mooring ships,ranging from 10 000～50 000 tons,by using transfer function and spectrum-type analyses.He pointed out that when changing the mooring lines and fenders on the premise that the lines are moored non-specifically and the fenders are used within the usual range,the ship’s responses to frequency will not be affected obviously.Nevertheless,due to the trend of upsizing on ships,the changes in a ship’s size and its hydrodynamic characteristics will inevitably lead to changes in the ship’s characteristics of motion responses.In recent years,many scholars have carried out researches on the motions of mooring ships under the effects of swells and long-period waves:a numerical simulation to a mooring ship’s motions induced by longperiod waves at Tomakomai Port in Japan has been studied by Van der MOLEN[4].The values of movements gained in the study were also compared with the measured ones of the prototype.Van der Molen[5]also employed the software named TERMSIM to simulate the movements and loads of an LNG mooring ship under the effects of swells at Withnell Bay of Australia.A conclusion was drawn from the study that the movement of yaw is a mooring ship’s main form of motions.Based on the analyses of a physical model test of mooring ships under the effects of long-period waves and the data obtained by measuring the movements of prototypes,Ligteringen[6]proposed an estimation formula used to calculate the movements of mooring ships under the effects of long-period waves.Shigeki[7]has studied the characteristics of low-frequency motions of mooring ships inside ports and harbors caused by the resonant effects of harbor oscillation.Uzakid[8]has explored the causes to mooring ships’drastic motions of surge and heave and advanced some relative countermeasures.Dr.Hidefumi IkedaDai[9]has pointed out in his paper that long-period waves are the primary cause to trigger long-period motions(surge motions)of a large ship at the Port of Ishinomaki in Japan.He has also developed a unique mooring system to effectively reduce long-period motions of a large ship.However,most those researches mentioned above focused mainly on the motions of a large mooring ship in each specific engineering project,but not on the characteristics of motion responses of a large mooring ship.In addition,domestic and foreign scholars have different understanding and awareness of the characteristics of motion responses of a mooring ship.For example,the movements of pitch and roll of a mooring ship are the only two taken for granted to have a natural period in China[10],while abroad four movements of a mooring ship including surge,sway,roll and yaw are taken for granted to have their own natural periods[11],which makes significant differences.Therefore,it is necessary to study the characteristics of motion responses of large mooring ships.In this paper,a physical model experiment was performed on a 266 000 m3mooring LNG ship for further study and discussion on the characteristics of motion responses under the effects of waves with grand period ranging from 10 s to 50 s.

2 Design of the experiment

2.1 Experiment equipments and measuring instruments

The experiment was conducted in an ocean environmental flume of the State Key Lab of Coastal and Offshore Engineering(SLCOE),Dalian University of Technology,China.The flume is 40 meters long,24 meters wide and 1.2 meters deep.A piston type wave maker system designed and constructed by SLCOE is installed at one end of the flume,which can generate multidirectional complex waves of both low-frequency and high-frequency according to different test requirements.Wave absorbers are arranged at the other end of the flume to absorb incoming waves to avoid wave reflection.

In the experiment,the wave data were collected by adopting the DS30 system developed by Beijing Research Institute of Water Conservancy Technology(BRIWT).The system can handle up multi-points of wave surface simultaneously and then process data analyses,the wave measurement instrument spans the range of 35 cm,and the proportional error is less than 0.5%.The measurement of a mooring ship’s movements employs the system dedicated to model ship tests with twin CCD optical six-component movement measurement,which is also developed by BRIWT.Such system employs non-contact measurement method to avoid added mass and friction that generated by using the traditional contact one.The system can also be used to simultaneously measure six-component movements,namely sway,surge,heave,yaw,roll and pitch.Besides,the relative error of angular surveying is less than 5%,and the relative error of line displacement is less than 2%.

2.2 Simulations of the mooring ship

The model scale was set 1:60 in accordance with the requirements of Wave Model Test Regulation[12].The experiment was performed on a moored 266 000 m3LNG ship to an island berth.The dimensions of the LNG ship are given in Tab.1.The model ship was built based on the 3D hull shape definition of a prototype LNG ship at a geometric scale of 1:60,the weight balance method was used to meet different requirements of load and weight distribution,the LNG ship’s main particulars such as its center of gravity,the periods of roll and pitch,etc were consistent with similar dynamic conditions.

Tab.1 Dimensions of the 266 000 m3LNG ship

Continued Tab.1

2.3 Simulations of the structure of island berth

In the same way,the simulations of the structure of island berth were fulfilled by reducing the prototype on the geometric scale 1:60.The simulations of island berth structure can ensure both the geometric similarity and the similarity of the location of caisson piers,as well as the stability of caisson pier.In addition,the outer shells of the caisson piers are made of wood,filled with gravels and small lead weights inside.The top of each caisson pier was connected with the upper wooden part of the berth,which formed a unity.A number of weights were evenly added to the surface of the upper structure to make the overall structure of the berth rigid and stable enough.The layout of the berth is shown in Fig.1.

Fig.1 Berth layout and diagrammatic illustration of mooring patterns of the LNG ship

2.4 Simulations of mooring lines and fenders

The mooring lines used for mooring the LNG ship were arranged in a symmetrical way by the number of 3:2:3:2.That is to say,there were 3 ropes for both head line and stern line,2 for both additional head line and stern line,3 for both forward breast line and after breast line,and 2 for head spring line and stern spring line(See Fig.1).When the locations of mooring dolphins and positions of the ship’s mooring pipes were fixed,the length of lines would automatically satisfy geometric similarity.In the experiment,the nylon ropes with a diameter of Ф=75 mm were chosen for use.Two strands of the ropes were twisted into one mooring line,and two of three strands of the ropes were twisted into one mooring line and the other strand was directly used as another line.The lines used for simulations are made of cotton ropes with sufficient spare length for use,and their mass of per unit length can satisfy gravity similar rule.Before simulating,the lines were hung heavy weights in advance to make them lose elasticity completely.When simulating,the elastic similar rules of lines had to be taken into consideration,and the elastic pieces of steel were adopted to simulate the elasticity of the lines.The force-deformation of the simulating lines can be calculated by the Wilson formula given by

where T is the model line force(kg),λ is the model scale,and Ceis the elasticity modulus of lines(when the lines are nylon ones Ce=1.26×105 kg/cm2,d is the diameter of lines,ΔS/S is the relative elongation of lines,and n is the exponent(when the lines are nylon ones n＝3).In the experiment,an initial tension of 100 kN was loaded on each line in accordance with the prototype lines.Fig.2 shows the curve graphs of force-deformation of forward breast and after breast lines,which manifests good simulation results.

The main similarity conditions of fenders refer to the similarity of the curves of force-deformation and energy-deformation of fenders between the prototype and the model.The standard SUC-2250H cell rubber fenders were taken into use with a layout of one in one row.When simulating,the curve of force-deformation of fenders was focused on in the experiment.The simulation results shown in Fig.3 manifest that the use of rubber fenders has achieved better results.

Fig.2 The modeling results of force-deformation curves of breast lines

Fig.3 The modeling results of force-deformation curves of fenders

2.5 Experiment conditions and simulations of waves

In the experiment,some dynamic factors such as currents and wind loads,etc.,which affect the motion of a mooring ship,were not taken into account.The water in the flume is 0.24 meters deep.The direction of waves is transverse which is the most unfavorable to a mooring ship.The characteristic parameters of the mixed waves in the experiment can be referred to in Tab.2.The irregular waves used in the experiment are the ones simulated by the internationally recognized JONSWAP spectrum.

Tab.2 Characteristic parameters of waves in the experiment

3 Results and discussion

3.1 Surge

As shown in Figs.4 and 5,under the effects of transverse waves with different wave heights,the mooring ship’s movement of surge observes a very regular pattern:it gradually grows with the increase of wave period.When the ship is half loaded at the wave period of 32 s,the harmonic rolling motions of surge appear,and the value of surge increases significantly,the movement of surge experiences attenuation both before and after the peak value.It is about 50%of the peak value when the maximum attenuation appears.When the mooring ship is fully loaded,no peak value appears under the experimental working conditions.However,when the wave period is greater than 40 s,the movement of surge has rapidly increased with the increase of wave period.

Fig.4 Motion responses of the LNG ship under the ballasted condition affected by waves of different heights

Fig.5 Motion responses of the LNG ship under the laden condition affected by waves of different heights

3.2 Sway

Under the effects of transverse waves,the mooring ship’s movement of sway has periodic intermittent motions with the increase of wave period:under the ballasted condition,the harmonic rolling motions of sway appear when the wave period is 12 s,24 s and 40 s,respectively.The ratio of the natural rolling period of the mooring ship and the natural periods of sway is 1:1.11:2.22:3.69,the peak value of sway grows with the increase of wave period.Under the mooring ship’s laden condition,when the harmonic rolling motions of sway appear,the wave period will grow with the increasing amount of loading.The harmonic rolling motions of sway appear twice at the wave period of 18(20)s and 38 s respectively.The ratio of the natural rolling period of the mooring ship and the natural periods of sway is 1:1.11(1.23):2.34,the sway’s second peak value is significantly greater than the first one.As seen from Figs.4 and 5,the movement of sway attenuates when the mooring ship’s harmonic rolling motions appear for the second and third time,and the value of the maximum attenuation is about 35%of its peak.

3.3 Heave

Along with the periodic changes of waves,the regularities of the mooring ship’s movement of heave are basically the same under different loading conditions.The peak values of heave appear at different wave periods:at 40 s under the ballasted condition and at 32 s under the laden condition.Under the effects of waves with the period less than 6 s,the movement of heave is less than one third of wave height[13],while under the effects of waves with grand period,the peak values of heave are basically equal or even greater than the wave heights.

3.4 Pitch

From Figs.4 and 5,it can be seen that the ship’s movement of pitch is affected little by the changes of wave periods under the effects of transverse waves.

3.5 Roll

The mooring ship’s movement of roll has harmonic rolling motions for twice under each of the loading conditions-the ballasted and the laden:when ballasted the peak values of roll appear at 14s and 32s respectively.The ratio of the natural rolling period of the mooring ship and the natural periods of roll is 1:1.29:2.95,with the increasing loading of the mooring ship,the wave period grows at the peak values of roll.The values of wave period at the peak values of roll are 20 s and 40 s respectively.The ratio of the natural rolling period of the mooring ship and the natural periods of roll is 1:1.23:2.46,the peak values of roll grows along with the increase of wave period:it increases by about 15%when the mooring ship is ballasted and increases by about 30%when laden.The reason for the phenomenon is that the parameter of stabilizing height under the laden condition of the mooring ship is less than that under the ballasted one.When the mooring ship is fully loaded,the rough waves are more likely to roll the ship along.

3.6 Yaw

The general changing trend of the mooring ship’s movement of yaw is on gradual rise along with the increase of wave period.Under the experimental working conditions with different loadings,the harmonic rolling motions of yaw appear just once:at 30 s when ballasted and at 32 s when laden.

4 Conclusions

Under different loading conditions,a large mooring LNG ship’s characteristics of motion responses investigated experimentally under the effects of transverse waves with grand period.The conclusions can be drawn as follows:

(1)Apart from the movements of pitch and roll,which have their own natural periods respectively,the mooring ship’s movements of surge,sway and yaw also have their own natural periods.

(2)The movement of sway is a kind of periodic intermittent motion.The ratio of the natural period of sway and the natural rolling period of the mooring ship is within 1.11～1.23,which roughly grows exponentially along with the increase of wave periods,the peak value of sway grows with the increase of wave period.

(3)Along with the periodic changes of waves,the regularities of the LNG ship’s movement of heave are basically the same under different loading conditions.The peak values of heave differ when the wave period changes due to the ship’s different loading conditions,under the effects of waves with different heights,the peak value of heave is greater than the wave height.

(4)Under the action of transverse waves,the ship’s movement of pitch is affected little by the changes of wave period.

(5)The movement of roll has periodic motions.When the movement of roll is at peak values,the ratio of the natural period of roll and the natural rolling period of the mooring ship is within 1.23～1.48,which roughly grows exponentially along with the increase of wave period,the peak value of roll grows with the increase of wave period.

[1]BS 6349-1:2000 Maritime Structures-Part 1:Code of Practice for General Criteria[S].UK,2000.

[2]PIANC.Criteria for movements of moored ships in harbours:A practical guide[R].Report of Working Group NO.24 of the Permanent Technical CommitteeⅡ.Brussel:PIANC,1995.

[3]Yang Xianzhang.Characteristics of frequency reponses of a mooring ship[J].Harbour Engineering,1989(2):14-23.(in Chinese)

[4]Van der Molen W,Monardes P,Van Dongeren A R.Numerical simulation of long-period waves and ship motions in Tomakomai Port Japan[J].Coastal Engineering Journal,2006,48(1):59-79.

[5]Van der Molen W,Ligteringen H J,Van der Lem C,et al.Behavior of a moored LNG ship in swell waves[J].Journal of Waterway,Port,Coastal,and Ocean Engineering,ASCE,2003,129(1):15-21.

[6]Ligteringen H,Moes J.Motions of moored ships in long waves[C]//International Conference on Port and Maritime and Technology.Singapore,2001.

[7]Shigeki Sakakibara,Kubo Masayoshi.Characteristics of low-frequency motions of ships moored inside ports and harbors on the basis of field observations[J].Marine Structures,2008,21:196-223.

[8]Uzaki Ken-ichi,Nobuhiro Matsunaga,Yasuhiro Nishii,et al.Cause and countermeasure of long-period oscillations of moored ships and the quantification of surge and heave amplitudes[J].Ocean Engineering,2010,37:155-163.

[9]Hidefumi IkedaDai,ki Yasuda,Haruo Yoneyama,et al.Development of mooring system to reduce long-period motions of a large ship[C]//Proceedings of the 21st International Offshore and Polar Engineering Conference.Hawaii,USA.ISOPE,2011:1214-1221.

[10]Zhang Ningchuan.A research report on the physical model experiment of mooring and berthing ships at the LNG terminal in Dalian[R].Dalian:Dalian University of Technology,2008.(in Chinese)

[11]Ausenco Sandwell.Berth availability assessment at lumina copper S.A.C.Port Eten[R].Ausenco Sandwell,Queensland,Australia,2009.

[12]JTJ/T 234-2001.Wave model test regulation[S].China,2001.(in Chinese)

[13]JTJ211-99.Design code of general layout for sea port[S].China,1999.(in Chinese)