Voyage Acceleration Climate: A Comprehensive Statistical Method for the. Evaluation of Design ... the fatigue life of structural parts or cargo, Io this purpose it is.
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Voyage Acceleration Climate: A Comprehensive Statistical Method for the Evaluation of Design Loads for Offshore Transport A.B.. Aalbers*) , C E.J.. Leenaars**) and F.HHA Quadvlieg*) *) Maritime Research Institute Netherlands ") Dockwise NV, Copyright 1996 Offshore Technology Conference This paper was prepared for presentation at the Offshore Conference held in Houston Texas, 6,·9 May 1996
This paper was selected for presentation by the OTe Program Committee following review of information contained in an abstract submltted by the author(s).. Contents of the paper
as presented have not been reviewed by the Offshore Technology as are subject to correction by the author(s). The material. as presented, does not necessarily reflect any position of the Offshore Technology Conference or its officers Permission to copy is restricted to an abstract of no! more than 300 words. Illustrations may not be copied. The abstractshould contain ccnspiclous acknowledgement of where and by whom the paper was presented
Abstract: The paper presents a new procedure for the calculation of realistic values for the design loads for heavy lift and vulnerable cargo transports The presented work is the result of a joint effort of 6 surveying companies, 5 heavy transport companies and MARIN Traditionally, design accelerations for ocean transports were calculated by the use of a so called design wave Now, a tool is produced which takes the motion climate during a complete voyage into account. DUling a voyage, a ship meets a variety of wind and wave conditions called the wind and wave climate The ship and the crew react on these wind and wave conditions This results in a motion climate for that particular voyage, The most probable maximum load during a transport is derived from the motion climate and on the time actually spent at sea The final goal of the participating companies is that the method will develop into an international industry standard Introduction A user friendly computer program called VAC (Voyage Acceleration Climate) was developed The ship motion calculations are based on striptheory, using 2-D diffraction theory to solve the hydrodynamic potential per section This makes the method applicable to all monohull ship shapes In this paper, the general methods used in the program to derive the motion climate on arbitrary voyages of the ship are treated Sever al aspects are elaborated: the voyage
modelling, the wave climate, the calculated motion behaviour, the scenarios for ship operation, the sustained speed, voyage duration and acceleration statistics, In the second part of the paper, the results of the program are demonstrated in a number of different cases, some of which have been used for validation The application of the method is not restricted to heavy overseas transport design, but also for offshore operations like dredging and installation workability.. As many of these type of operations make use of weather forecasts, to select a weather window, the VAC computer program allows to apply a wave height limit Furthermore, for the design of ships, the program can be used to assess design values for e g . container lashings or to assess the fatigue life of structural parts or cargo, Io this purpose it is possible to define a linear combination of signals in a user interface This can be used to compute loads- or stresses at locations of interest New in this method are the following aspects: The probability of all wave heights on the route with their range of different periods are taken into account So, it is no longer necessary to select a design wave 2, The effect of the actions of the crew can be taken into acount by applying various scenarios, In this way, a much better fit is obtained between the actual motion climate as experienced and the calculated motion climate 3 The effect on roll of water on deck can be taken into account This is applicable for lIat top barges only. The effect on the roll motions was already demonstrated in the Noble Denton Barge Research, Ref (I) The program takes into account the non-linearity due to: a Non-linear roll damping b. Involuntary speed loss in waves
Program structure 10 make the calculation method for the motion climate as versatile and flexible as ship operators would require, a structure was developed as shown in Figure 1 The number crunching of the ship motion computation has been made as elaborate as feasible on a modern Pc. A huge database of motions, accelerations, relative motions
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VOYAGE ACCELERATION CLIMATE: A COMPREHENSIVE STATISTICAL METHOD
and selected linear combinations threreof is computed for a range of headings (24), ship speeds (5), wave spectra (100) With this database available, the design value computation is a fast operation with probabillity values and interpolation routines In principle two methods can be used to analyze the problem of design loads for transports: Monte Carlo type simulations and methods based on a pure statistical approach, see Ref (2, 3) The statistical model is selected, because the voyage scenario is relatively simple In this way. a practical and robust tool has become available. In the following paragraphs the calculation method for the 'base case scenario' will be described It is the default calculation procedure for a ship at maximum sustained speed on a given percentage of MeR Voyage modelling" For ship routing purposes the world seas on the globe are divided into several wave regions, the so called Marsden ~quares, For each of the squares, wave statistics are given, see Ref (4), in the Global Wave Statistics published by Hogben, Dacunha and Olliver These local wave statistics are needed for the calculation of the long term statistics For the description of the voyage, a list of way-points must be supplied These are defined in Figure 2 The distance travelled between each way point is called a leg A voyage can exist of a series of one or more legs A preprocessing program (MARPLOT) is used the graphically define the voyage The Wave Climate. The wave scatter diagrams as presented in GWS, are the result of a fit of coefficients The program uses the scatter diagrams to make a new fit of the GWS data, so that a more detailed subdivision could be made with regard to the wave heights and directions Steps of 05 m wave height and 15 in direction are generated These headings are not defined in the global direction system, but in ship fixed directions 0
Calculated Motion Behaviour. Much emphasis has been
paid to the fact that the correct motion behaviour in waves could be calculated. The strip theory was modified in several ways to accommodate the calculation of motions for extreme conditions Changes are made on the following aspects 1 Two-dimensional diffraction calculations are used to calculate the hydrodynamic properties per strip 2 A method was derived for the calculation of the surge motions, which was validated against three dimensional
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diffraction theory 3 The roll motion is calculated in a non-linear iterative procedure Especially for transverse accelerations, considerable differences were found 4. For flat top barges, the effect of green water on deck on the roll motion is taken into account. The twodimensional diffraction strip theory is able to calculate when water comes on deck A series of model tests have been carried out to study this aspect and to quantify the roll damping induced by water on deck 5. To be able to calculate not only the accelerations, but also the seafastening loads, special calculation templates are introduced according a method presented by Dallinga, Ref (5). Sustained Speed pel' Seastate.. For the calculation of the sustained speed, 3 aspects are taken into account with respect to the calm water resistance I The wind resistance, 2 The wave added resistance 3. The loss of propulsive efficiency due to the increased propeller loading. Other aspects of speed loss are neglected For offshore constructions on transport, the wind added resistance can be very high, the need to take this into account is therefore evident The wind resistance is calculated as: (I)
where: AT frontal cross sectional wind area ex wind coefficients, depending on the upper geometry and O:wind rel the relative wind direction (angle between the apparent wind and the sailing direction) V wind rel relative wind velocity, a function of the actual wind speed and the ship speed The actual wind speed is calculated with the Kruseman relation as a function of the seastate (H~ and T2) For the irregular wave spectra with normal Hs-Tp relations, the following formula is used:
v.:wmd ~
372 {Hs I"'/T 2M} ' p
(2)
The well known method according to Genitsma and Beukelman, Ref (6) is used for the calculation of the added resistance at larger speeds and at head waves For beam waves and stem waves, the method of Boese, Ref (7) is selected for giving better results in stern waves The
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A B. AALBERS, C E J LEENAARS AND F.H.HA QUADVLlEG
method of Jinkine and Ferdinande, Ref (8), is used to evaluate the wave added resistance at lower speeds For towed barges the method according to a publication of Bureau Veritas, Ref (9) was implemented With the aid of Kt-Kq curves of propellers, the efficiency loss of a propeller in off-design conditions is modelled. Ihis method was also used to predict the delivered thrust with reduced power settings, as is needed in the seamanship scenario (see the next section), The method is based on the Wageningen Bvscries propellers Acceleration Statistics.. The Rayleigh probability function is generally accepted as a good approximation to find the probability density function for the maxima of responses in stationary conditions By combining the Rayleigh distribution with a joint frequency table for wave heights and wave periods, the long-term probability for a signal can be obtained For Voyage Acceleration Climate, we have to sum over the number of voyage legs, the wave direction with respect to the ship and the seastates of the scatter diagram The long term probability (frequency) of exceedance is then given by equation 4, given in Appendix I at the end of the paper The equation describes the probability of exceedance on basis of the number of oscillations Still unknown in these equations are: the total voyage duration and the duration per leg The used root mean square values are obtained from the responses at the correct sustained speed, wave direction, wave height and wave period Voyage Duration The total duration of the voyage is based on the principle of the progress made per seastate For each seastate, a different sustained speed is calculated, which even may be negative For sea areas containing a considerable probability of heavy seastates, the average progress is low (but always positive) and therefore the duration of the voyage in the leg will be large
Operational Aspects: Scenario's In the program, three major different scenario's have been analyzed and modelled, These are alterations on the 'base case scenario' in which the ship will always sail at the maximum sustained speed that can be obtained with a given MCR setting 1 Seamanship scenario The policy of the captain of a heavy transport would be that of safety, Resonant roll i or example is avoided as well as too many shipping of
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water or too high acceleration levels, 2 F!xed heading scenario: As a worst case scenario. the fixed heading scenario is implemented and can be used to make comparisons with the traditional design methods, where the assumption is made that during the complete voyage the worst possible heading would be encountered 3 Short voyage scenario When short routes are evaluated, the operator can choose to sail only when the weather forecast is appropriate, The scenario allows to work with a wave height limit Ihis is modelled through an adapted scatter diagram where the probabilities on wave heights above the certain wave limit are decreased The duration of a short voyage is restricted to 3 days The truncated scatter diagram of the 3rd scenario can also be applied in combination with the 1st and 2nd scenario,
Cases A number of cases will be presented in the following to demonstrate the VAC program Ihe results of the computations is presented in the form of Wei bull distribution plots and tables with voyage particulars The voyage particulars are used for interpretation of the results and cover data as the most probable maximum sea state per Marsden Square on the route, travel time per leg and total travel time, number of oscillations per leg and for the total voyage, per signal that is calculated Case 1: Validation on Dock Express Voyages In 1983,· 1985 the Dock Express vessels have carried out a large number of voyages between Japan and the Arabian Gulf The vessels were instrumented with accelerometers, see Fig 3, and the long term statistics (covering almost 25 years) of these signals has been analysed by the authors, Ref (10) The voyages have been calculated with the VAC program and the results are presented in Table 1 Two data bases for the wave climate have been considered: the GWS of Ref (4) and the older database from voluntary ship observations, the 'Ocean Wave Statistics' (OWS). Note that GWS has been derived from OWS, but is corrected with measurements and hindcast wave data, for wave heights and wave periods. It is observed that GWS scatter diagrams give at the same probability level higher waves than the OWS scatter diagr ams The results in Table 1 show that the agreement between the VAC predictions based on the OWS wave data base and the measurements is quite good A maximum
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VOYAGE ACCELERATION CLIMATE: A COMPREHENSIVE STATISTICAL METHOD
difference of about 15% or about 0.04 g is found for most signals An exception is surge, but that may be explained from the fact that: 1 The computed surge motion IS based on an approximative approach 2 The measured surge acceleration maximum values may show the influence of non-linear effects when the bow flare of the vessel dives into a steep wave The difference in the prediction based on OWS and GWS can mainly be explained by the higher waves in the GWS data base, as is demonstrated in Table 2, The agreement between measured accelerations and those predicted by VAC on basis of OWS leads to the conclusion that due to weather routing of ships the actually encountered wave climate is close to OWS In I able 3 a comparison is made between the VAC program and various design methods existing in 1983 (see Ref (10) for details) This shows that the design methods give a reasonable outcome for the vertical accelerations only; for transverse accelerations (being affected by nonlinear roll) the agreement with VAC computations and actual measurements is poor Partly this is caused by the conservatism in the existing design methods Case 2: Lranspurt of modules on GIANI 3" In 1990 a module transport was canied out on one of Smit Intemationale's Giant barges, from the Philippines to Thailand The design accelerations of the operator's strip theory based analysis werevcompared with those of the VAC program in Table 4 It appears that the differences are significant, mainly due to selection of a sea state for design The operator used OWS and selected a design sea state on a probability level of 96,8 % non-exceedance, resulting in Hs=314m for the South Chinese Sea The VAC analysis on basis of GWS shows that the most probable wave crest amplitude for the voyage duration is 695 m, which would correspond (according to the Saddle Point Method of Gran, Ref (10» to a design significant wave height of Hs=46m Furthermore, according to standard practice a 1/20 steepness defined the wave period of the beam sea I zero speed design condition (1,=75s), while a longer period of Tp=9.6s was selected for the 2 kn speed head sea condition VAC considers all possible wave periods at maximum sustained forward speed Hence, the accelerations from the VAC program computations are higher:
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Due to considering much higher wave conditions Due to considering the maximum sustained speed, which leads to an average tow speed of 5.2 kn Due to considering all wave periods, thereby including those which give the maximum pitch response In Fig 4 long term distribution plots of the accelerations aft are given on Wei bull scale, for the base case scenario and for the fixed heading scenario (beam, and head waves respectivaly) Case 3: Hibernia Modules on Mighty SeI vant 2, In 1995 the MS2 transported Module No. 10 from Korea to Newfoundland, and the design calculations were carried out with the highest standards of safety, using Wijsmuller Engineering's design programs and OWS A comparison with the VAC results is given in Table 5 and the agreement is good The design wave height for the computations by DOCKWISE was the 10 Year storm in the worst area on the route, i e a sea with H s=15,1 m, for which in a 3 hrs storm the most probable maximum wave is 27..4 m (crest to trough) According to VAC and the Saddle point method, the 10 year storm maximum wave height is 27 7 m, fairly close to the maximum wave for design So, it may be concluded that waves for the DOCKWISE design were based on the GWS database The voyage simulations were carried out using the sustained speed scenario and the seamanship scenario In Fig 5 is shown that the seamanship scenario yields about 10% reduction Jn the longitudinal accelerations, and about 15% on the vertical accelerations on the bow, However, the transverse and vertical accelerations near midship increase, The reason is that the seamanship criterions for applying course changes or speed reduction are based on motions at the bow, Since vertical accelerations on the bow are highest in waves from forward, the course changes are to wave directions more beam-en, leading to higher transverse accelerations and higher vertical accelerations at midship Case 4: Installation of subsea manifolds at Ninian Field The Dock Express 20 vessel was used for the installation of 6 units and the job suffered from delays due to waiting for weather The heave compensator required a maximum heave below 30m, for a period of 12 hours The VAC program can be used to predict what level of sea state limit should be taken in to account when waiting
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for a weather window In Ref 11 the procedure of the 'short voyage scenario' is described. Basically, the sea state limit is used to truncate the wave scatter diagram The V AC program takes into account uncertainty in the weather forecast as a function of time, see Fig, 6, to gradually shift the truncation level and allow higher sea states to enter the wave climate (with very low probability) In Fig 7 the result is given: the probability of exceeding the heave compensator stroke as a function of sea state limit for a weather window of 12 hOUIS, In the actual performance of the installations a sea state limit of 2 m was applied, which is confirmed by the VAC prediction In Fig 8 is shown that truncation of the wave scatter diagram has great influence on the wave height distribution after 12 hours Conclusions A practical design method for transport of heavy or vulnerable cargo, and for offshore operations was developed With modem hardware it is feasible to compute the full wave climate for a given voyage or project, as a standard engineering procedure The advantage is that no assumptions have to be made as to wave height&period, speed, and phase relation between motions First validation results of the VAC program show that the predicted acceleration climate is in good agreement with the measured results and in line with industry standar~ds Differences can be explained from simplifications applied Two operational aspects have shown to be important: 1 Ships tend to avoid bad weather areas Ihis is clearly shown by the difference in OWS and GWS wave databases Comparison with the measurements show that the wave climate encountered on ships is close to OWS 2 Confronted with heavy sea states, seamanship behaviour tends to further reduce the probability of excessive motions By means of the truncated scatter diagrams generated in the short voyage senario, the VAC program is able to predict workability and operational limits for offshore operations Future developments may be to incorporate fatigue analysis and the probability of slamming with the associated damage in the program The importance of weather routing and seamanship have led to an initiative to develop an on-board decision support system for
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seamanship and routing decisions Acknowledgement The participating companies: Heerema Engineering B V IDockwise NV, MARIN, International Towing Contractors (IT C), Smit Engineering, Mammoet Transport, Kahn Shipping (Jumbo), Breeman Engineering Services, Noble Denton and Associates, Matthews Daniel/HMC, Det norske Veritas, _london Offshore Consultants and The Salvage Association References 1 Noble Denton &Associates,: "Barge Motion Research Project, Summary Rept No. L12140/NDNJBW, 1984 2 Aalbers, A B, Dallinga, R P and Nienhuis, U: "Computer Prediction Methods for the Workability of a Dynamically Positioned Vessel", PRADS '87 Conference, Trondheim 1987 3 Pinkster, L and van Oostveen, D,C" : "Seakeeping Optimization in Preliminary Ship Design", 5 th International Marine Design Conference, Delft, 1994 4 Hogben, N, Dacunha, NC, and Olliver, GF: Global Wave Statistics British Maritime Technology, 1986 5 Dallinga, R P .; Safe securing of trailers and deck cargo, Ro-Ro and Ferry Symposium, London, 1995 6 Gerritsma J and Beukelman, W,: "Analysis of the resistance increase in waves of a fast cargo ship", International Shipbuilding Progress, 1972 7 Boese, P : "Eine Einfache methode ZUI' Berechnung dcr Widerstanderhoehung eines Schiffes in Seegang", Journal of Fluid Dynamics, 1964 8 Jinkine, V and Ferdinande, V:"A method for predicting the added resistance of fast cargo ships in head waves" ,International Shipbuilding Progress V21, 1974 9 Bureau Ver itas: "Towage at sea of vessels or floating units", Guidance Note Nl l83-eNl, 1982 10 Leenaars, C EJ and Aalbers, AB : "Two years of acceleration measurements on the Dock Express Heavy Lift Ships compared with predicted values from several methods",RINA Spring Meeting, London,1987 II Quadvlieg, F H H A, Aalbers, A B , Dallinga, RP D , and Leenaars, CE T,: "Voyage Acceleration Climate: A New Method to come to Realistic Design Values for Ship Motions based on the full Motion Climate for a Particular Transport", 5th International Conference The Jack-Up Platform', London, 1995
VOYAGE ACCELERATION CLIMATE: A COMPREHENSIVE STATISTICAL METHOD
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Appendix 1 The Rayleigh probability of exceedance of motions or accelerations is given by the following formula: 1 A'
Piu » R) = e
2
(1)
Cf~
where: the long term probability/frequency that the value of u does exceed R the standard deviation of the response of signal x for a seastate
P(u>R) (J"
For Voyage Acceleration Climate, we have to sum over the number of voyage legs, the wave direction with respect to the ship and the seastates of the scatter diagram
nlegs
P (u > R)
Ppi
nsea ndir nlegs T3 j p i
Hi ndir
Dvo)'age
"'] ] o;
-"2 -,-.-.
,
'
Cf~I~'
p" ,
(2)
0 voyage
nsea
=:E :E I.l "'1
o;
N, TX ) "
e
the long term probability (frequency) that the value of u exceeds the criterion value R the root mean square of the response of signal x for a mean H1/3 and a wave period 1 2 in scatter diagram point j for wave direction J..I for leg number i This root mean square value is calculated non-linear for each irregular seastate the joint frequency probability for a significant wave height and a modal wave period to be in point of the scatter diagram j for wave direction Jl for leg number i the probability (frequency) of occurrence of direction fl along leg number i number of seastates in scatter diagram number of directions per leg number of legs per voyage mean period of motion x for scatter point j for wave direction Jl for leg number i the average number of oscillations pel second, calculated for each scatter diagram j " is given by:
P(u>R)
fIJi
:E
p)" ,
(3)
j=1
the average duration of the ship travelling on this leg the average duration of the ship travelling on the total voyage
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AS AALBERS, C E J LEENAARS AND F H.H A QUADVLlEG
7
TABLE 1: 1-Year Most Probable Maximum (single extreme)
II
Signal
Measured loaded
Measured ballast
VAG gws loaded
VAC gws ballast
VAG ows loaded
VAC ows ballast
Ax1 (g)
0.24
0.24
0.26
0.27
0.16
0.16
Ayl (g)
0.21
0.39
0.37
0.63
0.21
0.42
Az1 (9)
0.22
0.22
0.47
0.47
0.26
0.26
Ax2 (9)
0.09
0.09
0.11
0.11
0.06
0.06
Ay2 (g)
0.19
0.28
0.40
0.50
0.23
0.31
Az2. (9)
0.37
0.74
0.40
I
I
0.47
I
0.68
I
~I
0.43
TABLE 2: Comparison of voyage particulars (GWS vs. OWS, Travel time 641 Hrs)
II
I
QUan;ily
i·Year Max. Wave No. 01 oscillations
"
tt.am
6.7 m
332.000
330.000
Ay2
363.000
353.000
A1 , !
)",~
,p,",
-'-.:;'b:""d:::~:--
o o
-
";:;;","" )-,- ,.' I II :..--
:i
I
ci
"• ;;
Fig.. 5 - Weibull distributions of accelerations on Mighty Servant, Korea to Newfoundland
ASSUM DEVOLUTION Fore-Cast
DESIGN LIMIT
Error
aFH
\
TRIP START
INFINITY
TRIP END
Fig.6 - Truncation of wave height distribution of scatter diagram in Short voyage scenario c
z
"o
~
T
I
1 0
Fig.8 - Effect of truncation on wave climate
2.0
I
3..0
4.0
Truncation level (m)
Fig.7 - Max 3 m vertical motion at heave compensator: predicted probabilities for a range of wave limits
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