drydocking and grounding

8/18/2019 Drydocking and Grounding

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DRYDOCKING AND GROUNDING



When a ship enters a drydock she must have a positiveinitial G! "e upri#ht! and trimmed sli#htly! usually "y the

stern$ On enterin# the drydock the ship is lined up %ith

her centre line vertically over the centre line o& the keel

"locks$

'he rate o& pumpin# is reduced as the ship(s stern post

nears the "locks$ When the stern lands on the "locks the

shores are hardened up commencin# &rom a&t and

#radually %orkin# &or%ard so that all o& the shores %ill "e

hardened up in position "y the time the ship takes the

"locks overall$ The dock gates are then closed and pumping

out commences.



'he interval o& time "et%een the stern post landin# on

the "locks and the ship takin# the "locks overall isre&erred to as the critical period$

Durin# this period part o& the %ei#ht o& the ship is "ein#

"orne "y the "locks! and this creates an upthrust at thestern %hich increases as the %ater level &alls in the

drydock$

'he upthrust causes a virtual loss in metacentric hei#ht

and it is essential that positive e&&ective metacentric

hei#ht "e maintained throu#h)out the critical period! or

the ship %ill heel over and perhaps slip o&& the "locks

%ith disastrous results$



'he purpose o& this chapter is to show the methods bywhich the effective metacentric height may be calculated

for any instant during the drydocking process$

A &i#ure sho%s the lon#itudinal section o& a ship durin#

the critical period$ *P ( is the upthrust at the stern and *l (

is the distance o& the centre o& &lotation &rom a&t$ 'hetrimmin# moment is #iven "y Pl $ +ut the trimmin#

moment is also e,ual to C'C Chan#e o& trim$



'here&ore!

Where-

. / the upthrust at the stern in tonnes!t / the chan#e o& trim since enterin# the drydock in

centimetres! and

l / the distance o& the centre o& &lotation &rom a&t in metres$



No% consider 0i#ure "elo% %hich sho%s a transverse section

o& the ship durin# the critical period a&ter she has "een

inclined to a small an#le 1 de#rees2 "y a &orce e3ternal to the

ship$

'he %ei#ht o& the ship 1W2 acts do%n%ards throu#h the centreo& #ravity 1G2$ 'he &orce . acts up%ards throu#h the keel 1K2

and is e,ual to the %ei#ht "ein# "orne "y the "locks$ 0or

e,uili"rium the &orce o& "uoyancy must no% "e 1W ) .2 and

%ill act up%ards throu#h the initial metacentre 12$



'here are three parallel &orces to consider %hen calculatin# the

e&&ect o& the &orce . on the ship(s sta"ility$ '%o o& these &orces

may "e replaced "y their resultant in order to &ind the e&&ective

metacentric hei#ht and the moment o& statical sta"ility$



ethod 1a2In last &i#ure consider the t%o parallel &orces . and 1W ) .2$ 'heir

resultant W %ill act up%ards throu#h 4 such that5



'here are no% t%o &orces to consider5 W actin# up%ards

throu#h 4

and W actin# do%n%ards throu#h G$ 'hese

produce a ri#htin# moment o& W 3 G4 3 sin $

Note also that the original metacentric height was GM but has

now been reduced to GM 1

. Therefore MM 1

is the virtual loss of

metacentric height due to drydocking.



ethod 1"2No% consider the t%o parallel &orces W and . in a"ove &i#ure$

'heir resultant 1W 6 .2 acts do%n%ards throu#h G4 such that-



It is a re,uirement that all ships "e dry)docked &or inspection andmaintenance "elo% the %aterline$ When a ship is "ein# dry)docked

additional &orces actin# at the keel take e&&ect! "ein# the reaction or

upthrust a&&orded "y the "locks onto %hich the ship is "ein# landed$

'hese &orces can create undue loads on the stern structure and cause

loss o& sta"ility o& the ship$ 'his section investi#ates these e&&ects$

INTRODUTION



!earning Ob"ectives

On completion o& this section! the learner %ill achieve the &ollo%in#5

4$ Understand the se,uence o& events that takes place %hilst a ship

is "ein# dry)docked$

7$ Calculate the upthrust at the "locks 1. &orce2 at any sta#e durin#

dry)dockin# o& the ship$

8$ Understand the loss o& sta"ility durin# dry)dockin# and calculate

the loss o& sta"ility as either a rise o& the ship9s centre o& #ravity

1increase in KG2 or as a &all o& the metacentre 1reduction in K2$

:$ Conduct dry)dockin# calculations$

;$ Understand the practical considerations durin# the dry)dockin# o&

a ship$



74$4 <=>U=NC= O0 =?=N'< DURING DRY)DOCKING

0i#ures 74$4 to 74$8 illustrate %hat happens as the ship enters the dry

dock and the %ater is pumped out o& the dock$

4$ 'he ship enters the dry dock %ith a small trim "y the stern and is

&loated into position$

7$ 'he #ates are closed and %ater is pumped out o& the dock until the

ship touches the "locks a&t$ Immediately the ship touches the "locksa&t this denotes the start o& the critical period 1it is no% that the ship

%ill start to e3perience a loss o& sta"ility! hence the term2$



8$ As more %ater is pumped out o& the dock the true mean draught

%ill start to reduce as the ship e3periences more and more support

at the stern$ 'he upthrust a&&orded "y the "locks at the stern is

termed the #P force$% this continues to increase as the

"uoyancy &orce reduces$ 'hrou#hout the dockin# process the ship

%ill displace a pro#ressively lessenin# volume o& %ater as the true

mean drau#ht reduces and the . &orce increases to provide more

support &or the ship 1in e&&ect! the . &orce takes over supportin#

the ship and the role o& the "uoyancy &orce in supportin# the

ship reduces2$ At this sta#e the a&t drau#ht %ill "e reducin# at a#reater rate than %hat the &or%ard drau#ht is increasin#! the ship

%ill "e trimmin# "y the head as the overall true mean drau#ht

reduces$ 0or reasons discussed later! the loss o& sta"ility %ill also

"e increasin# as the . &orce increases$

:$ =ventually the ship %ill come to rest on the "locks alon# it9s entire

len#th! this critical instant denotes the end o& the critical period!

since &or a &lat "ottomed ship the pro"lem o& sta"ility loss is no

lon#er o& concern$



;$A&ter settlin# on the "locks &or%ard and a&t %ater continues to "e

pumped &rom the dock and the drau#ht reduces at the same rate

&or%ard and a&t$ 'he upthrust . "ecomes uni&ormly distri"uted alon#

the ship9s len#th and continues to increase as the e&&ective "uoyancy

&orce reduces$

@$ When the dock "ecomes nearly empty and the ship is &ully dry the

upthrust . %ill "e e,ual to the ship9s displacement havin# no%

replaced all the upthrust a&&orded "y the "uoyancy &orce$



&'(&(' alculation of P force at any stage during dry)docking

'hrou#hout the dry)dockin# procedure the true mean drau#ht

reduces as it %ould i& the ship %ere risin# out o& the %ater due to%ei#hts "ein# dischar#ed$

Consider the &ormula5

'he . &orce may "e considered to have the same e&&ect on true meandrau#ht as i& a %ei#ht had actually "een dischar#ed! there&ore5

'his &ormula may "e used to calculate the upthrust at the "locks at

any sta#e in the dockin# process since the true mean drau#ht is

al%ays reducin# as %ater is taken out o& the dock$

74$7 CACUA'ING 'B= . 0ORC=



&'(&(& alculation of P force during the critical period when dry)

docking

'hrou#hout the dry)dockin# procedure the true mean drau#ht reduces

as it %ould i& the ship %ere risin# out o& the %ater due to %ei#hts"ein# dischar#ed$

Consider the &ormula5

'he . &orce may "e considered to have the same e&&ect on true mean

drau#ht as i& a %ei#ht had actually "een dischar#ed! there&ore5

'his &ormula may "e used to calculate the upthrust at the "locks at

any sta#e in the dockin# process since the true mean drau#ht is

al%ays reducin# as %ater is taken out o& the dock$



74$8 O<< O0 <'A+II'Y WB=N DRY)DOCKING

oss o& sta"ility commences as soon as the ship touches the "locks a&t

and continues to %orsen as the value o& the . &orce increases$ 'he

ma3imum loss o& G o& concern occurs the instant immediately prior to

the ship settlin# on the "locks &or%ard and a&t 6 this time "ein# termed

the critical instant$ Once the ship is &lat on the "locks it %ill "e in a sa&e

condition as the risk o& heelin# over as a result o& "ecomin# unsta"le

%ill have passed 1most ship9s havin# a su"stantial area o& &lat "ottom2$

0or ships that have a relatively small percenta#e o& &lat "ottom areaadditional measures must also "e taken such as usin# side shores to

support the ship in the upri#ht condition %hen in the dry dock$ =ither o&

t%o methods o& calculation o& the loss o& G may "e used$



&'(*(' !oss of +, as a result of a rise in + -increase in .+/ Consider the up%ard movement o& G that %ould occur i& a %ei#ht %9 is

dischar#ed &rom a position at the keel 1K# / m2$ When dischar#in# a

%ei#ht the centre o& #ravity o& the ship! G! %ill move directly a%ay &rom

the centre o& #ravity o& the dischar#ed %ei#ht to G? as sho%n in &i#ure74$:$

GGv %ill "e e,ual to the loss o& G %here5



d9 is the distance "et%een the centre o& #ravity o& the ship 1G2 and the

centre o& #ravity o& the dischar#ed %ei#ht %hich %as at the keel K9$

'here&ore distance d9 is the initial KG o& the ship$

1It should "e noted also that K chan#es as a result o& a reduction in the

ship9s drau#ht$2

I& the . &orce is considered to have the same e&&ect as dischar#in# an

e,uivalent %ei#ht &rom the keel then5

'he e&&ect on the ship9s sta"ility is made clearer i& the availa"le ri#htin#

moment at a particular an#le o& heel is considered$ 0i#ure 74$; sho%s a

ship durin# the critical period %here it has taken the "locks at the a&tend only$ Durin# dockin# the ship "ecomes heeled to a small an#le o&

inclination "y an e3ternal &orce such as the %ind$



'he &orces actin# are as &ollo%s5

0f is the total %ei#ht &orce actin# do%n%ards throu#h the centre o&

#ravity at G-

-0 1 P/ is the remainin#! or residual! "uoyancy &orce actin# up%ardsthrou#h the #eometric centre o& the under%ater volume at +4-

P is the upthrust o& the "locks e3erted at the keel a&t$

1W 6 .2 GE represents a righting moment -

. GE4 represents a capsi2ing moment $



'here&ore the availa"le ri#htin# moment is #iven "y5

It is essential that the ri#htin# moment a&&orded "y the up%ard actin#

1remainin#2 "uoyancy &orce remains #reater than the capsiFin# moment

a&&orded "y the upthrust o& the . &orce actin# at the keel at all times

prior to the ship touchin# the "locks &or%ard and a&t$ I& the ship should

"ecome unsta"le durin# the critical period it %ill lurch o&& the "locks toone side resultin# in structural dama#e to the ship! movement o& the

"locks and #reat em"arrassment

It is &or this reason that the loss o& G is calculated &or the critical

instant 1%hen the ship touches the "locks &or%ard and a&t2 to ensure

that ade,uate sta"ility is maintained prior to the ship takin# the "locksoverall$



&'(*(& !oss of +, as a result of a fall in , -decrease in .,/

Consider &i#ure 74$@ that illustrates the ship heeled "y an e3ternal &orce

such as the %ind durin# the critical period %here the ship has taken

the "locks at the a&t end only.

'h t t l i ht & & th hi t d d th h G



'he total %ei#ht &orce o& the ship acts do%n%ards throu#h G$

Counteractin# this are the t%o up%ard &orces- the . &orce actin#

up%ards at the keel and the residual "uoyancy &orce 1W 6 .2 actin#

up%ards throu#h the centre o& "uoyancy 1+42$ 'he resultant o& the t%o

up%ard actin# &orces acts throu#h the ne% metacentre 142 such that5

. 3 / 1W 6 .2 y 142

4 represents the resultin# &all o& the transverse metacentre 1or loss

o& G2$

Consider the t%o similar trian#les5

Com"inin# &ormulae 4! 7 and 8 a"ove #ives5

1W 6 .2 <ine 4 / . <ine K4

Divide "oth sides "y <ine 5

1W 6 .2 4 / . K4



NoteIn this &ormula the K value is that %hich corresponds to the true mean

drau#ht &or the instant that the loss o& G is "ein# calculated and not

that &or the initial true mean drau#ht that the ship has prior to dockin#$ It

is &ound "y enterin# the hydrostatic data %ith a displacement value that

corresponds to that #iven "y 1W 6 .2$

W in this &ormula is the ship9s initial displacement$



74$: 'Y.ICA DRY)DOCKING .RO+=<

In the &ollo%in# e3ample "oth methods o& calculatin# the loss o& G %ill

"e used$ An e3planation is included to prove that "oth methods are

e,ually valid$

34ample '

Prior to entering dry dock ,(5( 6lmar has draughts 7 8(9: m 6 ;(&8 m

and an effective .+ of <(':m(alculate=

-a/ the +, when the ship takes the blocks forward and aft -at the critical

instant/>

-b/ the draughts at the same instant>



?oth answers are different but are both valid since a true measure of

a ship$s stability is it$s righting moment value at any given angle of

heel(



0ithin small angles of heel the righting moment is given by=

?y method '

6t the critical instant the effective displacement @ 0 1 P @ ';'&: t since

the P force acts as a weight being discharged from the keel(

R, @ Displacement +, Aine

R, @ ';'&: '(8;8 Aine @ &'<<*Aine t)m

?y method &

?y considering the loss of +, as a result of the fall of the metacentre=

R, @ Displacement +, Aine

R, @ ';&:8 '(88' Aine @ &'<<; Aine t)m

-The slight difference arises due to rounding up of values in the

calculation



Aolution -b/

6t the critical instant the ship will be on an even keel( The draught at the

same instant may be calculated by one of two methods(

,ethod 'The initial T,D has already been calculated as being ;(B8; m(

3ntering the data with this obtain the TP value(

, th d &



,ethod &

If the effective displacement at the critical instant is -0 1 P/

3ffective displacement @ 0 1 P @ ';&:8 1 '*9 @ ';'&: tonnes

3nter the data with this displacement value to obtain the T,D at the

critical instant

Therefore the draught at the critical instant @ ;(BB& m -6ns/ -learly

method & is much easierC/

Durin# the dockin# operation it is essential that the critical instant9

drau#ht is determined as "oth drau#hts &or%ard and a&t %ill "e

constantly "ein# read$

As the ship9s drau#ht approaches that as calculated &or the critical

instant! also evidenced "y the &act that the ship %ill "e in a near even

keel condition at that time! the rate at %hich the %ater is pumped out o&

the dock %ill "e slo%ed do%n to allo% &inal adHustment o& the ship9s &ore

and a&t ali#nment prior to the ship takin# the "locks overall$ Once on the

"locks the rate o& pumpin# %ill "e increased a#ain$



74$; .RAC'ICA CON<ID=RA'ION< DURING DRY)DOCKING

The major considerations that should be borne in mind are:

(1) that the P force is kept to an acceptable level, and;

() that the ship maintains an acceptable positive !" during the critical period.

&' ' Th i li i h P f



&'(;(' The reuirement to limit the P forceDurin# the critical period prior to takin# the "locks &ully &or%ard and

a&t the . &orce %ill "e actin# at a sin#le point on the stern &rame o& the

ship$ 'he stern &rame is specially stren#thened to accept the &orces

e3erted on it durin# dry) dockin# "ut there %ill "e a ma3imum limit thatmust not "e e3ceeded$ I& the . &orce "ecomes too #reat structural

dama#e %ill occur$ It is usual to have accepta"le near)li#ht conditions

o& loadin# &or dry)dockin# speci&ied in the ship9s sta"ility data "ook$ I&

an actual .)&orce value is not ,uoted then it may "e appro3imated

&rom the recommended condition1s2 #iven "y rearran#in# the dry)dockin# &ormulae and calculatin# it$ Under normal circumstances the

ship9s classi&ication society %ill investi#ate any proposed dry)dockin#

condition and veri&y that it is appropriate$ Under e3ceptional

circumstances a ship may "e dry)docked in a part)loaded condition

"ut this %ill only ever "e done a&ter takin# classi&ication society

advice$ It %ould o&ten "e more prudent to dischar#e any car#o on

"oard prior to enterin# dry dock$



An o"vious method to limit the . &orce durin# the critical period is to

keep the initial trim "y the stern small! consider the &ormula &or

calculatin# the . &orce durin# the critical period5

It is clear &rom the a"ove that . &orce is directly proportional to the

chan#e o& trim that the ship %ill under#o$ imitin# the trim %ill there&orelimit the ma3imum loads that %ill "e e3perienced "y the stern &rame$

'he #reater the displacement o& a #iven ship! the more important %ill "e

the need to limit the dockin# trim$



&'(;(& !imiting the loss of +,Consideration o& the &ormulae %ill indicate that the #reater the trim o&

the ship %hen dockin#! the #reater %ill "e the loss o& G$

It is clear &rom the a"ove that . &orce is directly proportional to the

chan#e o& trim that the ship %ill under#o$ imitin# the trim %ill there&ore

limit the ma3imum loads that %ill "e e3perienced "y the stern &rame$ 'he#reater the displacement o& a #iven ship! the more important %ill "e the

need to limit the dockin# trim$

Clearly! the #reater the trim! the #reater the . &orce- the #reater the .

&orce! the #reater the loss o& G



Alternatively! the ship should dry)dock %ith a #reater e&&ective G that

%ill ensure that sta"ility is maintained$ Improvin# the ship9s initial G

%ill "e achieved "y5

142 o%erin# the e&&ective KG "y lo%erin# %ei#hts %ithin the vessel!

dischar#in# %ei#hts &rom hi#h up or takin# on an accepta"le amount o&

"allast in dou"le "ottom tanks! or-

172 inimisin# &ree sur&ace e&&ects "y toppin# up slack tanks %herever

possi"le$

34ample &



34ample &

,(5( 6lmar about to dry dock reuires a minimum +, of B(* m at the

time the ship takes the blocks forward and aft( urrent draughts are 7

:(9< m and 6 9(8E m( .+ is 9(9: m(alculate the ma4imum permissible trim by the stern on entering the

dry dock(



alculate the ma4imum allowed P force and hence the ma4imum intial trim



To ensure that a +, of B(* m is maintained at the critical instant the

trim of the ship must not e4ceed '(*B m by the stern(

drydocking and grounding

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