1d Suspended Weights

7/24/2019 1d Suspended Weights

1/23

Suspended Weights (Rev. 03/09/

1

SUSPENDED WEIGHTS

Introduction

When a weight is suspended from a ships crane or derric its

effective centre of gravit! wi"" #e at the point of suspension of

the weight$ #eing at the derric or crane head.

%his means that as soon as the weight is piced up$ either from

the &ua! or from somewhere on #oard$ there wi"" #e an immediate

transfer of the weight to the derric (or crane) head with a

resu"ting upward vertica" movement of the ships centre of

gravit!$ '. %his causes a corresponding reduction in metacentric

height$ '. f the weight #eing "ifted is su#stantia"$ as in the case

of a heav! "ift$ the considera#"e upward movement of ' cannot #e

ignored$ re&uiring the maximum permissible KG$ or minimum

permissible GM$ during the "ifting operation to #e ca"cu"ated.

When the weight is p"um#ed over the side a "arger than norma"

"ist wi"" a"so occur and certain precautions wi"" have to #e taen

to ensure that the ma*imum "ist is restricted to an accepta#"e

"imit.


2/23


+

Effect on KG of lifting a weight using ships gear

,onsider the situation where a weight already on board is to #e

"ifted from a position in the "ower ho"d using the ships own

derric. nitia" -' is shown.

K

G

g


3/23


3

s soon as the weight is "ifted c"ear of the tan top the centre of

gravit! of the weight moves vertica""! up to its point of

suspension at g. %his resu"ts in a corresponding vertica"

movement of Gto G!$ causing an increasein -'.g

G

K

G!

d


4/23


GG!" w # d

W

where d is the distance through which the weight is effective"!

shifted upwards from its initia" stowage position to the derrichead.

Example

A ship has a displacement of 10516 t KG !"#0 m and KM $"00

m" A %ei&ht of !6 t in the lo%er hold K& '"(0 m is lifted by the

ships hea)y lift derric* the head of %hich is ##"00 m abo)e the*eel"

+a, -alculate the GM %hen the %ei&ht is suspended"

+b, -alculate the final GM %hen the %ei&ht is resto%ed in

the t%een dec* at K& !"50 m"


5/23


.olution

+a, GG / % d !6 +##"00 2 '"(0, 0"15# m

3 10516

4nitial KG !"#00 m

GG/ 0"15# m

KG %hen %ei&ht suspended !"'5# m

KM $"000 m

GM %hen %ei&ht suspended 0"6(! m

his is the minimum GM durin& the liftin& operation"

+b, o calculate the final GM %hen the %ei&ht has been

shifted treat as a normal sin&le %ei&ht problem 2

simply shift the %ei&ht from its initial sto%a&e

position +K& '"(0 m, to its final sto%a&e position +K&

!"50 m, i"e" i&nore the derric*"

GG/ % d !6 +!"50 2 '"(0, 0"0(# m

3 10516

4nitial KG !"#00 m

GG/ 0"0(# m

inal KG !"#(# m

KM $"000 m

47A8 GM 0"95! m


6/23


%he previous e*amp"e was ver! simp"e as there was no hori2onta"

shift of ' invo"ved as wou"d #e the case when "oading or

discharging a weight.

When "oading or discharging weights using ships "ifting gear thefo""owing must #e considered

+1, the increase in KG:decrease in GM %hen the %ei&ht is

suspended at the derric*:crane head;

+#, the maximum an&le of list that %ill occur %hen the

derric* or crane is plumbed o)er the ships side"


7/23


4

$oading a weight using ships lifting gear

,onsider the movement of the ships centre of gravit! (') when a

weight is "ifted off the &ua! on one side of the ship and stowed in

the "ower ho"d on the centre5"ine. (6or the purpose of thise*p"anation the ship is notshown to "ist.)

1"


8/23


7

#" %hich causes the ship to list"

%

G

g

G! GH&G'


9/23


9

t is at this stage that the maximum list wou"d occur which

coincides with the moment when the GM has its minimum )alue.

,onsider the "ist triang"e #e"ow

%

G

G! GH&G'

%()I%U% $IST

towards

g

Tan %()* $IST" GGH

G%%IN*


10/23


10

'"


11/23


11

(" 3ei&ht landed in the lo%er hold"

%he weight is removed from the derric head at g+as it

is "anded in the "ower ho"d and fina""! acts at g3. '+

moves to '3.

8ote that the weight is "oaded belo%the initia" centre of

gravit! of the ship$ hence the fina" position of '$ '3

must #e lo%er. -' is reducedovera"".

%

G

G+ G

gg+

g,

G,


12/23


1+

%he fo""owing e*amp"e i""ustrates a t!pica" "ist &uestion invo"ving

a heav! "ift #eing "oaded. %he ca"cu"ation ma! done #! one of two

methods

+1, ormula method"

+#, a*in& moments about the *eel and the centreline"

oth methods are shown$ however$ the moments method is much

simp"er and wi"" #e adopted for a"" other e*amp"es in this section.


13/23


13

Example

A ship displaces !!50 t KG 9"15 m and KM 9"$! m" A %ei&ht of

(0 t is to be loaded from the =uay %hich is 15"0 m to starboard

of the ships centreline" 4f the head of the derric* is #9"0 m

abo)e the *eel %hen topped to its maximum extent for the

liftin& operation calculate>

+a, the GM %hen the %ei&ht is suspended;

+b, the maximum an&le of list;

+c, the final an&le of list if the %ei&ht is placed on dec* at

K& 10"(0 m 5"0 m off the centreline to starboard"

+d, the %ei&ht of ballast to transfer bet%een t%o double

bottom tan*s each ha)in& its centre of &ra)ity ("0 m

off the centreline to brin& the ship upri&ht"

+Assume KM remains constant,


14/23


1

.olution +Method 1,

+a, o calculate the GM %hen the %ei&ht is suspended"

8oad the %ei&ht at the derric* head"

GG/ % d GG / (0 +#9"00 2 9"15, 0"0!$ m

3@% !!50 @ (0

4nitial KG 9"150 m

GG/ 0"0!$ m

Maximum KG 9"#'$ m

KM 9"$!0 m

M474MM GM 0"9(1 m +GM %hen the %ei&ht is suspended,

+b, -alculate the distance that G is off the centreline

%hen the %ei&ht is suspended o)er the =uay +GG?,"

GG? % d GG ? (0 15 0"069 m

3@% !!50 @ (0

an MAB" 84. GG? an MAB" 84. 0"069 0"0$0(#

GMM47"

0"9(1

Maximum list 5"#C .tbd"


15/23


1

+c, -alculate the final an&le of list %hen the %ei&ht is

placed on dec*"

+.imply load the %ei&ht on dec* i&norin& the derric*

as in a normal sin&le %ei&ht problem",

GG/ % d GG / (0 +10"(0 2 9"15, 0"015 m

3@% !!50 @ (0

4nitial KG 9"150 m

GG/ 0"015 m

inal KG 9"165 mKM 9"$!0 m

inal GM 0"!15 m

GG? % d GG ? (0 5 0"0## m

3@% !!50 @ (0

an 84. GG? an 84. 0"0## 0"0#900

GM47A8" 0"!15

inal list 1"5C .tbd"


16/23


1

+d, -alculate %ei&ht of ballast to transfer from .tbd"


17/23


14

.olution +Method #,

+a, o calculate the GM %hen the %ei&ht is suspended"

8oad the %ei&ht at the derric* head" a*e moments

about the *eel"

GM %hen %ei&ht suspended 0"9(1 m +Minimum GM,

+b, a*e moments about the centreline to determine GG?"

an MAB" 84. GG? an MAB" 84. 0"069 0"0$0(#

GMM47" 0"9(1

Maximum list 5"#C .tbd"

weight (t) -g (m) moments (t5m)

770.0 4.1 3+44.

/0.0 +4.00 1070.0

7790.0 4.+39 /34.

- 4.970

-' 4.+39

' 0.41

:ist off ;ort St#d

weight (t) ,< (m) moments (t5m) moments (t5m)

770.0 0.00 0.0 0.0

/0.0 1.00 00.0

7790.0 0.04 0.0 00.0


18/23


17

+c, -alculate the final an&le of list %hen the %ei&ht is

placed on dec*"

a*e moments about the *eel"

a*e moments about the centreline to determine GG?"

an 84. GG? an 84. 0"0## 0"0#900

GM47A8" 0"!15

inal list 1"5C .tbd"


770.0 4.1 3+44./0.0 10./0 /1.0

7790.0 4.1 393.

- 4.970-' 4.1

' 0.71

:ist off ;ort St#d

weight (t) ,< (m) moments (t5m) moments (t5m)770.0 0.00 0.0 0.0

/0.0 .00 +00.0

7790.0 0.0++ 0.0 +00.0


19/23


19

+d, -alculate %ei&ht of ballast to transfer from .tbd"


20/23


+0

To calculate the -a.i-u- per-issi/le KG re0uired prior to

loading or discharging a weight to ensure that a certain list

li-it is not e.ceeded*

%he e! point here is to first"! identif! the situation during the"ifting operation which wi"" create the ma*imum "ist. :rawing a

diagram wi"" he"p.

,onsider the fo""owing e*amp"e.

Example

A ship displacin& 16#00 t is upri&ht and has a $0 t %ei&ht on

dec* at K& 1'"0 m 6"00 m to port of the centre2line" his %ei&ht

is to be dischar&ed into a li&hter on the port side 1("00 m from

the centre2line usin& the ships hea)y lift derric*" 4f the an&le of

list is not to exceed !C at any time durin& the operation

calculate the maximum allo%able KG prior to dischar&e &i)en

that KM is $"60 m"


21/23


+1

a*imum "ist wi"" occur when the weight is suspended at the

derric head and the derric is p"um#ed over the port side.

.olution

' wi"" have its minimum va"ue.

Tan %()* $IST" GGH

G%%IN*

G

=.


22/23


++

-alculate the distance that G %ill be off the centre2line +GG?,

%hen the ship is at its maximum an&le of list"

GG? % d GG? $0 +1( 2 6, 0"0(( m

3 16#00

Maximum allo%ed list is !C" GG? is 0"0(( m" his allo%s the

minimum GM to be calculated"

an MAB" 84. GG?

GMM47

an !C 0"0((

GMM47

herefore> GMM47

0"0(( 0"'1' m

an !C

A minimum GM of 0"'1' m is re=uired

%hen the %ei&ht is plumbed o)er the side

at K& #9"0 m"

KM $"600 m

Minimum GM 0"'1' m

Maximum KG $"#!9 m

G

7>

GH

%


23/23

?a)in& calculated the maximum allo%ed KG %hen the %ei&ht is

suspended from the derric* head +K& #9"0 m, shift the %ei&ht

bac* to its ori&inal sto%a&e position on dec* +K& 1'"0, to

calculate the maximum initial KG re=uired for the operation"

a*e moments about the *eel"

he initial KG must not be &reater than $"#0$ m in order to

limit the list to !C"


1+00 9.+74 10//9./

590.0 +4.00 5+/30.0

90.0 13.00 1140.0

1+00.0 9.+09 1/9179./

Documents

1d Suspended Weights