1d suspended weights
TRANSCRIPT
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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.
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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
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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
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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"
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.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 $"000 m
47A8 GM 0"95! m
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%he previous e*amp"e was ver! simp"e as there was no hori2onta"
shift of ' invo"ved as wou"d #e the case when "oading or
discharging a weight.
When "oading or discharging weights using ships "ifting gear thefo""owing must #e considered
+1, the increase in KG:decrease in GM %hen the %ei&ht is
suspended at the derric*:crane head;
+#, the maximum an&le of list that %ill occur %hen the
derric* or crane is plumbed o)er the ships side"
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$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"
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#" %hich causes the ship to list"
%
G
g
G! GH&G'
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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 %()* $IST" GGH
G%%IN*
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'"
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(" 3ei&ht landed in the lo%er hold"
%he weight is removed from the derric head at g+as it
is "anded in the "ower ho"d and fina""! acts at g3. '+
moves to '3.
8ote that the weight is "oaded belo%the initia" centre of
gravit! of the ship$ hence the fina" position of '$ '3
must #e lo%er. -' is reducedovera"".
%
G
G+ G
gg+
g,
G,
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%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.
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Example
A ship displaces !!50 t KG 9"15 m and KM 9"$! m" A %ei&ht of
(0 t is to be loaded from the =uay %hich is 15"0 m to starboard
of the ships centreline" 4f the head of the derric* is #9"0 m
abo)e the *eel %hen topped to its maximum extent for the
liftin& operation calculate>
+a, the GM %hen the %ei&ht is suspended;
+b, the maximum an&le of list;
+c, the final an&le of list if the %ei&ht is placed on dec* at
K& 10"(0 m 5"0 m off the centreline to starboard"
+d, the %ei&ht of ballast to transfer bet%een t%o double
bottom tan*s each ha)in& its centre of &ra)ity ("0 m
off the centreline to brin& the ship upri&ht"
+Assume KM remains constant,
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.olution +Method 1,
+a, o calculate the GM %hen the %ei&ht is suspended"
8oad the %ei&ht at the derric* head"
GG/ % d GG / (0 +#9"00 2 9"15, 0"0!$ 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$0(#
GMM47"
0"9(1
Maximum list 5"#C .tbd"
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+c, -alculate the final an&le of list %hen the %ei&ht is
placed on dec*"
+.imply load the %ei&ht on dec* i&norin& the derric*
as in a normal sin&le %ei&ht problem",
GG/ % d GG / (0 +10"(0 2 9"15, 0"015 m
3@% !!50 @ (0
4nitial KG 9"150 m
GG/ 0"015 m
inal KG 9"165 mKM 9"$!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"
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+d, -alculate %ei&ht of ballast to transfer from .tbd"
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.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
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+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"
weight (t) -g (m) moments (t5m)
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
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+d, -alculate %ei&ht of ballast to transfer from .tbd"
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+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"
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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
=.
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++
-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
%
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?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"
weight (t) -g (m) moments (t5m)
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./