Ship operation in a seaway

L E C T U R E R S

SHIP OPERATION IN A SEAWAY

L E C T U R E R : T o r Svensen, Det Norske Veritas Classification A / S

1. I N T R O D U C T I O N

Seakeeping theories are finding increasing use in connection with ship operation. Rapid calculation methods combined with onboard computers present new opportunities. This section will present an overview o f the various factors affecting vessel

performance in service. The relative importance o f these various sources o f loss in performance are discussed. Examples o f voluntary and involuntary speed loss are presented and practical levels o f seakeeping criteria are discussed. The principles o f operational simulation and its role in the operation o f ships are dicussed. Advanced systems f o r weather routing are presented and the possible benefits are outlined. The use o f strategic weather routing in combination with a system for tactical routing (operational guidance) for damage avoidance is discussed. A n example o f the application o f such a system is presented. Future development trends are discussed.

2. S P E E D P E R F O R M A N C E I N S E R V I C E

It is often said that a new vessel will never again achieve the same level o f

speed/power performance, The loss o f performance in service can be attributed to three main effects:

1) - Deterioration in the underwater hull and propeller condition

2) - The effects o f t h e environment on the ship, including the wind and the sea

3) - The effects o f increasing ship resistance on the operation o f the propeller and the corresponding effect upon the main engine

Deteriorative effects are attributed to increasing hull and propeller roughness and possibly periodic fouling attacks. These are essentially a fijnction o f ship age, time out o f dock and quality o f maintenance strategy. Environmental effects are dependent upon

windspeed and direction and ttie waveheight/period and direction, all o f which are fijnctions o f vessel location and movement o f low pressure systems, monsoons and other climatic factors.

In the following the environmental effects will be discussed in some more detail.

Loss o f performance due to the effects o f wind and waves can be attributed to

involuntary and voluntary speed loss.

I N V O L U N T A R Y S P E E D L O S S includes:

- direct and indirect effects due to wind acting on the upper works o f the vessel

- added resistance due to wave induced ship motions in the seaway

- added resistance due to short waves (wave reflection)

- added resistance due to steering and manoeuvring in the seaway ( principally yawing and rudder motions)

V O L U N T A R Y S P E E D L O S S is a deliberate reduction o f speed and/or alteration o f

course by the ship's master to prevent damage due to excessive motions or hull structure loading. The most common effects are:

- excessive slamming, leading to structural damage

- shipping o f green water leading to loss or damage o f deck cargo or equipment and, in extreme cases, loss o f watertight integrity

= propeller emergence leading to unacceptable propeller shaft loadings

- extreme accelerations

Considering the operation o f a slow speed diesel engine, there will be an acceptable torque range for a given R P M within which the engine can normally operate. This can be an important limiting factor for involuntary speed reduction

Voluntary speed reduction or alteration o f course in service is in practice a highly subjective action by the ship's master based upon an observed degradation in habitability or operability. By describing seakeeping in terms o f physical parameters such as absolute or relative motions, accelerations, bow wetness and slamming, it is possible to quantify performance and to determine limiting values from an operability point o f view. These "seakeeping criteria" forms a fundamental part in design analysis as well as operational weather routing using environmental data.

The relative importance o f the various factors influencing vessel performance in service can be determined using some form o f voyage simulation model. A voyage simulation model is a computer based model for simulating the various in-service conditions and their effect upon the technical and economic performance o f t h e vessel. Probably the most important concept in the total voyage simulation model is that the factors affecting the vessel performance are modelled in combination rather than on an individual basis. This ensures that more realistic results are obtained. The degree o f sophistication required in a model will depend upon the type o f analysis to be performed.

2. " S E A W A Y " - A C O M P U T E R P R O G R A M M E F O R O P E R A T I O N A L S I M U L A T I O N

The computer programme S E A W A Y has been developed for evaluation o f the seakeeping performance o f a ship in a seaway. S E A W A Y is a practical tool for designers as well as operators. When used in combination with other computer programmes, such as O P T W A Y , the computer programme S E A W A Y may also be used as basis for advanced weather routing. This aspect is described in more detail in the following sections.

S E A W A Y calculates:

- added resistance due to waves, wind and current - power setting

- actual ship speed in a seaway (short and long term predictions)

- probability distribution o f ship responses such as : vertical and horizontal velocities and accelerations, rolling, slamming, deck wetness etc.

- ranges o f ship speed and courses to be avoided due to exceedance o f seakeeping criteria for a given environmental condition

I n combination with the computer programme O P T W A Y the optimum route can be calculated.

The enclosures provided in Appendix 1 and Appendix 2 describe the principles and calculation method employed in S E A W A Y in more detail.

3. S T R A T E G I C W E A T H E R R O U T I N G

General principles. The avoidance o f bad weather is the most effective way o f

minimizing both operational delays due to voluntary and involuntary speed losses and occasional heavy weather damage. Weather routing is a method o f strategic route selection using weather forecasts to calculate the optimum route. Criteria for use in advanced routing procedures will typically be relative motions, accelerations, time and fuel consumption.

Routing services have been provided on a commercial basis for many years. These have been relatively primitive services where the main emphasis has been on

meteorology and with litde or no consideration for the ship motions and seakeeping criteria f o r the individual ship in question. Advances in seakeeping theory, computer technology, meteorology and communications technology now makes it possible to introduce advanced routing services where criteria for speed loss and voluntary speed reduction are tailored individually to the ship and route optimization can be performed directly by the ship's master on the bridge o f the vessel. Such systems are under

development and are in the process o f being introduced on a commercial basis in 1993.

In practice, weather routing is only usefijl in ocean areas with changing weather patterns. I t is also neccessary that the voyage is o f sufficient length to permit feasible alternative routes. Probably the most important weather routing decision is the strategic choice made at the start o f the voyage. This will typically involve a decision about following a northeriy, great circle or southerly route. Once a decision is made and the vessel is underway, the economic penalty associated with an alteration o f route becomes large. The need for an accurate prognosis covering the complete length and duration o f the intended voyage is therefore obvious.

Storms have three spacial dimensions and in addition a fourth dimension in time. Therefore a four dimensional consideration o f the atmosphere is necessary when planning to avoid the worst effects o f middle latitude depressions

The need f o r accurate prognosis cover both wind and waves. Between the two, waves are far more important for ship performance, although accurate wind prognosis is the basis f o r accurate wave predictions.

Wind and wave predictions.

AJl weatlier forecasts extending more than a few hours ahead in time are based upon numerical models o f the atmosphere. The models are run to predict the development o f the atmosphere in space and time. Prior to running the models, all available

meteorological observations are input to the model as basis for setting the initial conditions. The improvements in numerical models combined with advanced

supercomputer technology has resulted in considerable improvements in both accuracy and length o f forecasts. The latest models are capable o f producing forecasts extending about 10 days out in time and the accuracy o f the forecasts has improved considerably over the past few years.

The atmospheric models provide information about the wind fields at the sea surface. This information serve as input to the wave models. Global wave models can today provide forecasts o f waveheight as well as wave period and direction for both wind generated seas and swell. The latest wave model run by the European Centre f o r Medium Range Weather Forecasts ( E C M W F ) is capable o f wave forecasts as far ahead in time as 10 days. The new model only became operational in July 1992 and represents a major step forward when it comes to weather routing o f ships. The level o f accuracy is not yet well documented. A comparison with actual waveheight measurments made at the Snorre Field showed a prognosis error less than I m on significant waveheight for the 1st day in 75% o f the cases. For the subsequent days f r o m 2 to 7 the deviation was less than I m in 67, 58, 59, 48, 46 and 49 percent o f the cases respectively.

The data f r o m E C M W F is today available in a resolution matrix o f 3x3 degrees on a global basis. This is sufficient for weather routing o f ships on major routes such as the North Atlantic and the Pacific.

Weather routine principles.

The role o f seakeeping in weather routing is to provide a rational basis for estimating the voluntary and involuntary speed losses for alternative route selection. Using the computer programme S E A W A Y , a database containing all the relevant information can be tailored to the individual vessel. The route optimization programme O P T W A Y can thus be used in combination with the forecasts for wind and waves to evaluate and select the optimum route. The basic principles o f route optimization can be described as follows:

In the absence o f any form o f route optimization, the ship will normally follow a Great Circle Route, which in principle is the shortest distance route. Many different

optimization methods have been developed. The modified isochrome method is based upon a least time route. In this method a number o f lanes parallel to the great circle route are defined. The time step dt is defined as the interval between two calculations in the optimization procedure. This interval may correspond to the time between successive weather forecasts. The optimization is carried out as follows;

Consider the possible positions o f a ship after navigating from point A following different headings during time dt. These arrival points represent a line called the isochrome. By selecting one point on the isochrome inside o f each lane we define a set o f departure points X(i) for the next time step. From each departure point X(i) a number o f different ship courses may be chosen. I f the ship will follow these headings over the next dt time, the different arrival points may be computed. They represent the isochrome after 2 x dt time. For the next step only one point inside o f each isochrome is selected using the shortest great circle distance to the destination point as the selection criterion. Repeating this procedure until the ship reaches the destination point, the optimum route with the shortest voyage time will be determined.

Other criteria may be used instead o f the least time criterion explained here. These may, for example, be lowest fuel consumption or lowest ship responses. The accuracy o f the route optimization is dependent upon the quality o f the wind and wave forecasts as wll as selection o f seakeeping criteria and how well the vessel is described in the model. A very fijndamental parameter in the route optimization process is the numerical values o f the individual seakeeping criteria parameters used. These will normally be selected based upon subjective experience from previous vessel operation, measurments or published results. Actual measurments will provide the best basis for determining relevant seakeeping criteria. This may be combined with a system for tactical routing as described below.

4. T A C T I C A L R O U T I N G

Background

Even when vessels are using active weather routing, weather pattems can change unexpectedly and the ship may fmd itself encountering severe weather necessitating speed reduction or course alteration. Such tactical weather routing actions are taken by the ship's master in order to minimize the risk o f damage to ship or cargo.

For large vessels, such as containership, potentially dangerous situations are typically water on deck or bow flare slamming. The identification o f dangerous situations f r o m the bridge can be difficult, partly due to the physical distance f r o m the bow and partly due to the fact that the view o f the forward part o f the vessel is often obstructed by containers. Installation o f instruments capable o f observing and measuring parameters, such as motions, accelerations or green water on deck, can significantly improve the situation by providing relevant and accurate information to the ship's master. Such a system may be used in combination with a system for strategic routing o f ship.

The following describes in detail an example o f such a development. In this

development a single sensor in the form o f a vertical accelerometer was used. Model tests in combination with calculations were carried out in order to determine both vessel motion characteristics and limiting criteria for operation in various seastates. B y measuring the ship response spectrum, this can be compared with the criteria database to detect possible dangerous situations. The ship's master can f r o m the computer display select the optimum speed and heading, thus minimizing the risk o f heavy weather damage to ship or cargo. Shortcomings o f this relatively simple system are that the wave heading have to be input manually and the system is not capable o f handling combined wind generated waves and swell f r o m a different direction.

Model tests. The model tests were carried out using a free running model. The model

was equipped for measurment o f 6 d.o.f motions, relative motions at the bow and a special force panel on the deck for measuring the forces due to green seas on the breakwater and the first tier o f deck containers. Initial tests were performed in regular waves to determine the critical periods and wave heights with respect to green water on the forecastle deck. Another objective o f the tests in regular waves was to establish transferflinctions and relative phase for the actual ship motions for comparison with the theoretical calculations. Irregular tests were performed for two different speeds and for

headings O and 30 degrees. The purpose o f the irregular wave tests was to obtain statistical information on the vessel motion, relative bow motion and forces acting on the breakwater structure. By gradually increasing the waveheight at critical wave periods, it was possible to establish limiting criteria for green water on deck.

Numerical calculations. Calculations o f vessel motions were performed using a linear

strip theory program. The purpose o f the calculations was to obtain transferfijctions for a wider range o f headings and speeds than covered by the model tests. The calculated responses in regular waves showed good agreement with test results. However, f o r the extreme responses in irregular waves, there are clear limitations in the linear theory assumtion as discussed below.

Criteria development. The model tests showed a clear relationship between the

relative motion and the amount o f green water on deck. A simplified relationship between vertical motion and relative vertical motion in the bow was developed based upon the results o f the model tests. This simplified relationship is based upon the assumption that there is only one wave spectrum (i.e. not a combined situation with wind generated waves as well as swell) and that the critical situations o f interest are the bow to beam headings. Using vertical motios instead o f relative vertical motions as the parameter for limiting the vessel operafion makes the issue o f instrumentafion considerably simpler.

It is important to note that a significant difference was observed between the calculated relative bow motion and the measured values from the model tests. The difference was principally speed dependent and was due to the stationary wave field and dynamic swell-up. When developing the reladonship between absolute and relative vertical motion, the theoretically calculated values were corrected for this effect.

Database development. Based upon the results o f the model tests and calculations, a

comprehensive database was developed containing vertical and relative bow motion for a range o f wave headings, periods and ship speeds.

Instrumentation and analvsis programs. A n instrumentation pack consisting o f a

vertical accelerometer and a dedicated PC for data analysis was developed. On this system the analogue signals f r o m the accelerometer measuring the vertical

acceleration in the bow, are sampled and converted to vertical motion by two times integration o f the signal. A t fixed intervals a spectral analysis o f the vertical motion time trace is performed. The results o f this analysis are transmitted to a second

operational PC on the Bridge o f the vessel where the operational and advisory programs are implemented.

The most important part o f the advisory programs on PC no. 2, is the theoretically calculated model o f the vertical bow motion and the relative bow motion o f the ship for various speeds and headings to the waves. Based upon the data received f r o m PC n o . l , the operational PC no.2 will present a trend plot with development o f t h e relative bow motion. A consequence graph is also presented on a semicircle where the effect o f heading and speed changes with respect to relative motion is presented.

Implementation. The system was first implemented on a single vessel in a fleet o f

sisterships and tested during a winter season. The system was run in combination with a system f o r strategic weather routing based upon the S E A W A Y and O P T W A Y suite of programs as described previously. The onboard system was implemented with a data recording facility and a manual log was maintained o f observed seastate and speed/heading where voluntary speed reduction was put into effect. This procedure allowed the initial criteria f o r water on deck and voluntary speed reduction as derived f r o m the model tests to be evaluated and subsequent corrections made based upon operational experience. The system was subsequently fitted to the complete fleet o f sistervessels.

Conclusions. The peoject demonstrated the effectiveness o f model tests in

combination with theoretical calculations in determining realistic operational criteria for a particular type o f vessel. The project also demonstrated that the information obtained through a systematic set o f model tests and calculations can be used in combination with relatively simple onboard instrumentation and data analysis system for providing operational guidance about possible dangerous situations.

Other parameters such as accelerations or vertical bending moment could be used in a similar manner for other vessel types. I t is believed that this type o f operational guidance systems will gain increasing acceptance in the ftiture as costs are no longer a

Screen display - Heavy weather avoidance system.

r

Weather Routing

Strategic

Tactical

. Route selection using

. Onboard system for

imme-weather forecasts to select

diate selection of

speed/-optimum route

heading.

. Only useful in oceans with

changing weather patterns

Main purpose is to reduce

the probability of damage

to ship or cargo.

. Passage length need to be

4 days or more

. Advisory system based

upon sensors for

measure-. Optimization based upon

ment of ship behaviour

time or fuel consumption

and the environment

. 1

METEOROLOGICAL FORECAST SKILL

S A M P L E S O F DEVELOPMENTS IN F O R E C A S T QUALITY

A n o m a l y c o r r a l a t i o n

Days

ECMWF FORECAST SKILL (NORTHERN HEMSPHERE) Janua/y 1980 - Deceniber 1990

Forecasl skill

1980 1981 1982 1983 1984-1985 1986 1987 1988 1989 1990 1991 ECMWF FORECAST S K I L L (TROPICS)

Forecast day on which the 850 hPa vedof wind correlalion reaches 0.6 Forecast skill Januaiy 1980 - December 1990

12 MOKTH MOVING AVERAGE

1 9 8 0 1981 1982 1983 1984 1985 1 9 8 6 1987 1988 1989 1990 1991

ECMWF F O R E C A S T SKILL (SOUTHERN HEMISPHERE) January 1980 - Deceniber 1990 Forecast skill 1 9 8 0 1981 1982 1 9 8 3 1984 1985 1 9 8 6 1987 198S 1989 1 9 9 0 1991 ECMHf f o r e c a s t s k i l l J a n u a r y 1 9 8 0 to D e c e m b o r 1 9 9 0 . G ü t e d e r V o r h e r s a g e n d e s E Z M W v o n J a n u a r A D C O J O S p r e v i s i o n n e l i e o e s p r e v i s i o n s d u 1 9 8 0 bis D e z e m b e r 19&0 C E ° M . i . r o o ) a n v i e - 1 9 S 0 a o e c s n o r c 1 9 9 :

SEAWAY

A S Y S T E M F O R C A L C U L A T I O N O F P E R F O R M A N C E IN A S E A W A Y S E A K E E P I N G TESTS S H I P HULL CHARACTERISTICS PROPELLER CHARACTERISTICS CALCULATIONL M E T H O D S E N G I N E 1 CHARACTERISTICS 1

SEAWAY

W E A T H E R D A T A ( W A V E S , W I N D A N D CURRENT) CRITERIA FOR VOLUNTARY S P E E D LOSS CRITERIA FOR VOLUNTARY S P E E D LOSS E N G I N E / PROPELLER CHARACTERISTICS H P , Q , R P M

OPTWAY

A S Y S T E M F O R V E S S E L R O U T E O P T I M I Z A T I O N S H I P A T T A I N A B L E S P E E D ( S E A W A Y / M A R I N T E K ) WEATHER F O R E C A S T S (DMI ) C U R R E N T DATA ( DMI ) U S E R I N P U T 1^ O P T W A Y R E S U L T S TO P A P E R DATA F I L E • FOR P L O T T I N G R O U T E P L O T D I G I T A L I Z E D MAPS OF P L O T O P T I M A L ROUTE

Waves Wind Current

7 ;

/

/

Bow

A

stern

C O O R D I N A T E S Y S T E M a n d S I G N COt^VENTION 2At o I s o c h r o n e M O D I F I E D I S O C H R O N E METHOD

RoRo (Lpp = 190m) Design speed L P G carrier (Lpp = 80m) Design speed 1m 2m 3m 4m 5m 6m 7m H Container Vessel (Lpp = 185 m) Design speed LNG carrier (Lpp = 273m)

Speed loss curves for 4 Ship types

Design speed

I m 2m 3m 4m 5m 6m 7m H .

EXAMPLE OF WEATHER ROUTING

NUMBER OF GREEN SEAS AND FX-LOADS ON BREAKWATER = 1 0 m, T = 14 s e c . s p V E D L . End. \^^Z^iy aiNTCF-ORUPPCN

NUMBER OF GREEN SEAS AND FX-LOADS ON BREAKWATER

= 1 0 m, T = 14 s e c . s p

R A P P O R T Report NUMBER OF GREEN SEAS AND

FX-LOADS ON BREAKWATER = 1 0 m, T = 14 s e c . s p

D A T O Date NUMBER OF GREEN SEAS AND

FX-LOADS ON BREAKWATER = 1 0 m, T = 14 s e c . s p R E F . Ref. HEADING = 3 0 d e g TEST NO 240 Vg - 12 kts Fx load Number o f

breakwater green seas

(kN) <1000 3 1 . 0 0 0 - 2 . 0 0 0 6 3 . 0 0 0 - 4 . 0 0 0 1 TOTAL 10 Encounter waves: 162

Percentage green seas: 6.2%

TEST NO 2 3 0 , Vs - 17 kts

Fx load Number of

breakwater green seas

(kN) <1000 3 1 . 0 0 0 - 2 . 0 0 0 3 2 . 0 0 0 - 3 . 0 0 0 1 3 . 0 0 0 - 4 . 0 0 0 3 4 . 0 0 0 - 5 . 0 0 0 2 5 . 0 0 0 - 6 . 0 0 0 1 7 . 0 0 0 - 8 . 0 0 0 1 9 . 0 0 0 - 1 0 . 0 0 0 1 1 4 . 0 0 0 - 1 5 . 0 0 0 1 TOTAL 16 Encounter waves: 190

Percentage green seas: 8 . 4 % ^

5 H = 17 k n o t s " 1/1 ro OJ QJ CU cn 4 1 • H I E 1 1 10 12 I 14 16 FX _{10-^ ( k N )} BJm:P"jM gra'is^e as

HISTOGRAM OF PROBABILIPr' OF EXCEEDING

SEAKEEPING CRITERIA FOR SHIP 1 AND SHIP 2

ON NORTH ATLANTIC ROUTE

P r o b a b i l i t y

DECK

WETNESS

CONTAINER VESEL SHIP 1 SHIP 2

LPP B Draught (operating) C B " Speed (operating) 210 m 30.5 m 11.58 m 0.616 18 knots 202 m 32.24 m 10.02 m 0.634 18 knots

VERTICAL

BOW ACCELERATION

Q n 3 CQ O H m H I cn

^ <

UJ C J - ur

PROPELLER

EMERGENCE

SAMPLE RESULTS FROM MODEL TESTS

AND CALCULATIONS

15 k n o l s ' 15 k n o l s

O Heas. v e r U c o L bou n o U o n C o L c . v e r L L C o l bou moUon

O Meos. r e L o U v e bou moUon — C o l e . r e l o U v e bou « o L i o n 5.S 5.0 2.5 2 . 0 1.5 (.0 0 . 5 0.0 15 20 25 50 Encounler perLod ( s e e l

i

f

^ 0 5 • 0 5 ; ?0 S 5 E n c o u i l e r p«rLod ( s e c I

Measured and calculated responses in regular waves at 15 knots

VerLLcaL bow moUon, 12.0 k n o l s

Peok uovs period I sec I

R e l o U v e bow raoUon, 12.0 knols

5,0 2.5 2.0 5,0 2.5 2.0 -e — S I . • - - S I . > ^ - - S I . <3 - S I .

dev Im/ml 0 . deg i dov Im/ml 50. deg dev Im/ml 60. deg 5,0 2.5 2.0 -e — S I . • - - S I . > ^ - - S I . <3 - S I . dev (m/ml 90. deg : - ^ y ^ ^ \/ fc ** ^ " .J 5,0 2.5 2.0 -X" I / ^ : - ^ y ^ ^ \/ fc ** ^ " .J (.5 -• > ^ - - ^ — ' ' c t ^ 'h— ' '••c. - 0 . . 1.0

-«

— e 5 - = — = ^ 0.5 0.0 -0 5 0 15 0 25 'eok wave per cod I s e c l