Wind loads on marine structures

*Corresponding author.Tel.:00351-1-841-7607;fax:00351-1-847-4015.Marine Structures 12(1999)199}209Wind loads on marine structuresM.R.Haddara ,C.Guedes Soares *Memorial Uni v ersity of Newfoundland,Faculty of Engineering and Applied Science,St.John 's,N y d,Canada A1B 3X5Unit of Marine Technology and Engineering,Technical Uni v ersity of Lisbon,Instituto Superior Te &cnico,A v .Ro v isco Pais,1049-001Lisboa,Codex,PortugalReceived 29January 1999;received in revised form 15March 1999;accepted 9April 1999AbstractA review of the available methods for the calculation of wind loads on ships and o !shore structures is presented.Based on the review,four methods were selected and implemented in order to carry out a comparative study of the wind loads on ly,the ahead force,the side force and the yawing moment were calculated and compared.A large tanker both in the loaded and ballast conditions was analysed using the four methods.In the second part,an expression for the estimation of wind loads on ships is proposed and compared with the experimental data.A neural network technique was used to obtain this expression. 1999Elsevier Science Ltd.All rights reserved.Keywords:Wind loads;Loads on ships1.GeneralThe wind loads do not usually play a major role in the structural design of ships and o !shore structures.The magnitude of the mean static forces and the moments caused by the wind amount to only a fraction of the total loading on the structure.However,there are instances when the e !ects of these forces and moments become critical.Cracks occurring in #are booms due to dynamic wind loading have been reported in the literature [1].Wind loads play important roles in the e $cient operation of a ship 's propulsion plant or on her manoeuvrability.Also in certain particular situations such as the towing of ships,moored ships and dynamic positioning,wind loads play an important role.200M.R.Haddara,C.Guedes Soares/Marine Structures12(1999)199}209Wind tunnel tests are the most accurate procedure to estimate the wind forces on ships and o!shore structures.However,these tests are time consuming and very expensive,thus the prediction methods are generally used complementary or alterna-tively to wind tunnel tests.Several numerical methods have been developed for the prediction of wind forces on#oating structures without recurring to direct model testing.Isherwood[2]proposed expressions for the forces and moment coe$cients,derived from multiple regression analysis of previously published experimental results.The wind coe$cients are given by equations,depending on the basic characteristics of the water above the hull.This method can be applied to a wide range of ship types and con"gurations and for all angles of the wind relative to the bow.Gould[3]presented a numerical procedure for the estimation of the wind forces and moment on the superstructures of ships.The e!ect of wind speed over the sea on the e!ective relative wind speed acting on the ship is discussed.In fact,the wind pro"le over the sea depends on the sea surface,which moves itself and presents increasing roughness with increasing wind speed.A logarithmic pro"le for wind speed is proposed by the author.The e!ective wind speed and the lateral centre of pressure for a gradient wind,is then estimated by subdividing the frontal and lateral projections into`universal elements a.The wind tunnel data for ships covering a wide range of designs are given with advice on their application to other ships.This data can be used to estimate the forces and moment coe$cients for a wide range of ship types.Blendermann[4]has presented a systematic collection of wind load data.Continu-ing this work,Blendermann[5,6]derived expressions for the coe$cients of the wind forces acting in the longitudinal and transverse directions and the yaw and rolling moments.These expressions are functions of a drag coe$cient,the angle of attack,the frontal and transverse areas subject to wind,the coordinates of the centre of area and a parameter called the cross-force parameter.This parameter is obtained statistically from the experimental data.Blendermann[7]also proposed a method for the prediction of wind loads on ships in a non-uniform air#ow using experimental data.The non-uniformity of the air#ow is accounted for an e!ective dynamic pressure.Brix[8,9]has presented a selection of the wind load data from Blendermann[4,5], together with methods for their use in manoeuvring problems.Lee and Low[10]obtained results of wind tunnel tests on models of rigid o!shore platforms.The pressure transducers located at141locations on the1:268scale model were used to measure wind loads.The wind velocity was measured using hot wire anemometer.The wind tunnel results show an overriding in#uence of the legs.They accounted for about70%of all drag and about80%of all over turning moments in the#oating mode.OCIMF[11]presented coe$cients and procedures for computing wind loads on very large crude carriers(VLCCs),i.e.tankers in the150,000}500,000DWT range. However,the coe$cients and procedures can be applied to smaller tankers with similar geometry.The wind coe$cients are based upon data obtained from windM.R.Haddara,C.Guedes Soares/Marine Structures12(1999)199}209201 A very long mat-like platform was used to study the wind forces on a#oating airport by Ohmatsu et al.[12].Experiments were conducted in a closed-return wind tunnel.A distorted model was used where only the fore and aft parts were accurately scaled.Wind pressure distribution,wind forces and wind velocity distribution around the model were measured.It has been found that the total drag force coe$cient increases with the increase in the model length and decreases with the increase in draft. Chen et al.[13]carried out preliminary experiments to study the wind environment over the helideck of an o!shore platform in the Beihai sea,China.Tests were carried out in an atmospheric boundary layer wind tunnel using a1:100scale model.A hot-wire anemometer was used to measure the velocity over the helideck.The velocity was measured at equally spaced stations,ten in the horizontal direction and eight in the vertical direction.The results for"ve di!erent angles of attack were obtained.These experiments indicate that the distributions of the mean as well as the#uctuating velocities are quite sensitive to the direction of the wind.They also found that the separation occurs generally at the front edge of the helideck.In some cases,the mean velocity distribution at the centre of the deck has a turning point,which may suggest an instability.The vibration characteristics of several Norwegian o!shore#are booms have been discussed by Oppen and Kvitrud[13].A survey of the cracks produced by this resonant vibration was also given.The cross wind vortex-induced vibration and wake interaction with the chord members were the main causes for cracks.The methods for mitigating these e!ects were described and a recommended design practice was suggested.parative study on wind loadsThree methods available in the literature were used to estimate the wind loads on a351.0meter tanker,in the loaded and ballast conditions.Table1shows the principal dimensions of the tanker.The methods applied are those described in[2,3,11].The results of the calculations are shown in Figs.2}4for the loaded tanker and in Figs.5}7 for the tanker in ballast.The experimental data obtained by Blendermann[4]is plotted together for comparative purposes.Table1Main particulars of the tankerLoaded In ballastOverall length(m)351.40Beam(m)55.40Draft(m)23.5010.625Lateral projected area(m )3401.477839.63Height of centre of lateral area above 6.8312.34Distance of centre of lateral from midship24.45(forward)8.32(forward) Transverse area(m )1131.791803.93Fig.1.Convention for positivequantities.Fig.2.Longitudinal force coe $cient,loaded tanker.The convention for positive forces and moment are presented in Fig.1,where F V ,F W and N represent,respectively,the longitudinal force,side force and yaw moment and is the angle between the wind speed and the ship bow.Fig.2represents the longitudinal force coe $cient for the loaded tanker.Although one can observe some dispersion on the numerical results,in general the predictions compare well with the experimental results.For wind coming from the stern,all methods tend to overpredict the experimental results.Among the three methods,the OCIMF is the one which compares better with experiments.Fig.3,shows the results of the side force coe $cients for the loaded ship.One can observe that both Isherwood 's and the OCIMF methods agree with the experimental values obtained for the side force by Blendermann over the whole range of the wind angle.Values for the lateral force coe $cient obtained by Gould 's method are highly overestimated.In Fig.4the yaw moment coe $cients results are presented.One can "nd qualitative di !erences between Blendermann 's experimental results and OCIMF values for the yaw moment coe $cient on one side and estimates obtained by Gould 's and Isher-wood 's methods on the other side.The yaw moment coe $cients obtained using the 202M.R.Haddara,C.Guedes Soares /Marine Structures 12(1999)199}209Fig.3.Side force coe $cient,loadedtanker.Fig.4.Yaw moment coe $cient,loaded tanker.values obtained using Gould 's and Isherwood 's methods display a change in sign of the coe $cient around a wind angle of rge quantitative discrepancies exist between the results obtained using the three methods and Blendermann 's experi-mental results.Figs.5}7represent comparisons between the numerical predictions and experi-mental results,respectively of the longitudinal force,side force and yaw moment coe $cients for the tanker in the ballast condition.The conclusions for the longitudi-nal force coe $cients are similar to those obtained for the loaded condition.Again the OCIMF method tends to agree better with the experiments than the other methods.In the case of the side force coe $cients all methods overestimate the experimental data,the Isherwood results being closer to the experiments than the Gould and OCIMF results.The numerical results for the yaw moment coe $cients agree qualitat-ively with those obtained from Blendermann 's experimental values.For wind angles below 803,OCIMF is the closest to Blendermann 's experimental values.For angles M.R.Haddara,C.Guedes Soares /Marine Structures 12(1999)199}209203Fig.5.Longitudinal force coe $cient,tanker inballast.Fig.6.Side force coe $cient,tanker inballast.204M.R.Haddara,C.Guedes Soares /Marine Structures 12(1999)199}209In conclusion,it seems that this comparative study indicates that there is no general agreement between the four methods available for the estimation of wind forces on ships.The discrepancy between the estimates obtained using the other three methods and Blendermann 's experimental results may be caused by the limited range of experiments carried out in these studies and the statistical analysis carried on the data to obtain formulas for the estimation of the forces.3.An universal model for the estimation of wind loads on shipsThe trend of using neural network techniques in the parametric identi "cation of mathematical models using measured input and output of the system is growing.One of the main advantages of this method is that no functional relationship between the dependent and the independent variables has to be assumed a priori.This technique was used to obtain a universal expression for the wind loads.Blendermann 's [4]experimental data were used for training a feed-forward neural network.Optimisation of the network was obtained using the back-propagation method.The data for a tanker,which were not used in the training of the network,was used to test the ability of the network to predict wind loads for a new ship.This approach yields a mathematical expression which can be used to estimate loads on any ship.This means that this is a universal mathematical expression which does not depend on the type of ship.The following expressions are used to calculate the coe $cients of the wind forces:C I "K GIG H IG ,k "1,2,3,(1)where k "1,2,3refer to the longitudinal force,transverse force and yaw moment,respectively,andH IG "[1!e \%IG ][1#e \%IG ],G IG " H Jw IGH x H ,k "1,2,3(2)andx "A *¸ ,x "A P B ,x "¸B,x "S ¸,x " ,x "1,(3)where A *is the lateral projected area,A 2is the transverse projected area,s is the distance between the centre of the lateral projected area and the midship section of M.R.Haddara,C.Guedes Soares /Marine Structures 12(1999)199}209205Table 2Ships used in the training of the neural networkShipLoad (m)B (m)D (m)Container ship (full)210.7530.5011.6Container ship (empty)210.7530.509.6Container ship (loaded)210.7530.509.6Container ship (empty)216.4023.77 6.94Drill ship150.121.357.00Cruise ship143.9017.35 5.90Cruise ship161.0029.00 6.05Cutter25.05 5.80 2.50Cargo ship (loaded)141.118.507.32Cargo ship (empty)141.118.50 4.43Cargo ship (loaded)155.4523.108.69Cargo ship (container on deck)155.4523.108.69Research vessel:Wind from Port55.0012.50 3.95Research vessel:Wind from Starboard55.0012.50 3.95Speed boat53.609.20 2.50O !shore supply vessel61.9513.00 4.85O !shore supply vessel61.0013.00 4.85Gas tanker (loaded)274.0047.2010.95Gas tanker (ballast)274.0047.208.04the ship 's length and B is the ship 's beam.The values for the weights, IG and w IGH ,are calculated by the neural network.A neural network having one hidden layer was used.The input layer has six inputs as shown in Eq.(3).The hidden layer has 11nodes.The experimental data for 19ships were used to train the network.Table 2shows a list of the ships used in the training of the network.The weights,which were calculated by the neural network,are used to predict the wind forces acting on the same tanker that has been used in the compara-tive study in the previous section.This ship was not included in the data used to train the network.It should be noted that the training data did not include any tankers.Furthermore,all the ships used in the training of the network were smaller than the tanker used to test the generalisation ability of the network.Figs.8}13show a comparison between the experimental and the predicted wind forces coe $cients for the tanker.The predicted data agreed qualitatively with the experimental data in all cases.The errors in estimating the maximum forces on the tanker using the neural network are much smaller than those produced by the other methods.These results show that this method produces much better predictions than that given by the methods currently available in the literature.4.ConclusionsA review of the available methods for the calculation of wind loads on ships and o !shore structures is presented.A comparative study was carried out,where the206M.R.Haddara,C.Guedes Soares /Marine Structures 12(1999)199}209Fig.8.Predicted and measured longitudinal force coe $cient (loadedtanker).Fig.9.Predicted and measured side force coe $cient (loadedtanker).M.R.Haddara,C.Guedes Soares /Marine Structures 12(1999)199}209207Fig.11.Predicted and measured longitudinal force coe$cient(tanker inballast).Fig.12.Predicted and measured side force coe$cient(tanker inballast).208M.R.Haddara,C.Guedes Soares/Marine Structures12(1999)199}209M.R.Haddara,C.Guedes Soares/Marine Structures12(1999)199}209209 ahead force,side force and yawing moment were calculated for a tanker in loaded and ballast conditions using four methods.The quality of the predictions is assessed by comparing the numerical results with experimentally obtained results.Generally speaking the comparative study indicates that there is no general agree-ment between the methods used for the estimation of wind forces on ships.The experimental results obtained by Blendermann are the most comprehensive and reliable among the four methods.The discrepancy between the estimates obtained using the other three methods and Blendermann's experimental results may be caused by the limited range of experiments carried out in these studies and the statistical analysis carried on the data to obtain formulas for the estimation of the forces.In the second part of the paper,a neural network technique was used to obtain a universal expression for the estimation of wind loads on ships.The tanker,which was used for the comparative study,was chosen to test the ability of the network to predict wind loads on ships.It should be noted that this tanker was not used in the training of the network.It was found that the numerical predictions agree well with the experimental results,suggesting that the neural network technique can be used to generate expressions,which produce better results than those given by the methods 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