DRY D O C K I N G
Trim Effect on G Simpson Rules Dry Docking Simplified Stab Statical Stab Revisions Ex. Inclining Test
LEARNING OBJECTIVES • To understand the virtual loss of GM and the calculations. • To calculate the maximum trim allowed to maintain a minimum stated GM. • To understand the safe requirements for a ship prior enter into dry dock.
LEARNING OBJECTIVES • To understand the critical period during dry docking process. • To calculate the ship’s drafts after the water level has fallen and after the ship has taken the block overall. • Effect to stability when vessel has run aground (single point).
SHARING EXPERIENCES... Anybody would like to share their experience during dry docking….? Graving Dock Floating Dock Marine Railway
Before enter into dry dock, vessel must have… • Positive initial GM (GM fluid) • Upright • Trim - if possible even-keel or slight trim by stern • Double bottom tank kept either dry or pressed up - reduced FSE • If initial GM is small - D.B. tank to be pressed up to increase GM
When coming into Dry Dock: • The vessel will line-up with her centerline vertically over the keel blocks • Dock gate will be closed and commence pumping out water
F No effect on ship’s Initial Stability…
When coming into Dry Dock: • The rate of pumping will be reduced as the ship's sternpost near the block.
Sueing Point F Commence touching the ground… ‘Sueing Point’
When coming into Dry Dock: • Once the sternpost is touching the block, the UP-THRUST forces start to act against the sternpost. • At this moment part of ship's weight gets transferred to the keel blocks.
P F P is the Upthrust Force acting at first point of touching the ground. Commence Critical Period…
Sueing Point at ‘AP’
P K P is the Upthrust Force acting at first point of touching the ground. Commence Critical Period…
When coming into Dry Dock: • When ship's weight gets transferred to the keel blocks, vessel will suffer loss on her GM. • The time interval between the sternpost landing on the blocks and the ship taking the blocks overall is referred to as the CRITICAL PERIOD.
P F P force is increasing gradually as the trim change by Head…Vessel is still in Critical Period…
When coming into Dry Dock: • Vessel must have positive effective GM that to be maintained throughout the critical period. • If not vessel may heel over, slip off the blocks when there is an external force acting and heel the ship.
P F Vessel is fully rest on the blocks… End of Critical Period
M G1 Initial GM loss by GG1 after completed the Critical Period… This is due to Upthrust Force or ‘P’ Force… G B
P F What is the total P Force during Critical Period __?___ tonnes “How much weight to be discharged in order to bring the ship from trim by stern to even-keel…”
CALCULATION OF UPTHRUST FORCE AT THE STERNPOST - 'P' FORCE
Trimming Moment w d F Weight discharged to even keel the draft… Trimming Moment = w x d t-m by Head
Even Keel Draft F After weight discharged… T M By Head = T M By Stern
Trimming Moment P d F P is the Upthrust Force or weight dischargedto the blocks… T.M = w x d = P xd t-m by Head
P F Vessel is fully rest on the blocks, Change of Trim by Head and finally vessel at even keel drafts… End of Critical Period…
Change of Trim = Trimming Moment (TM) MCTC = P x d Whereby TM = w x d Change of Trim = P x d MCTC P = COT x MCTCtonnes d
Exercise in classroom • MV OneSuch, LBP 120m is going to dry dock at the following condition in sea water • Draft forward is 3.5m and aft is 4.0m, distance sueing point (AP) to F is 57.5m. • Her displacement is 4600 tonnes, MCTC is 86 t-m and TPC 15.45 • Calculate • The amount of up-thrust force (P) at the end of Critical Period? • Final drafts forward and aft?
Calculation of P force… P = COT x MCTC d = 50 x 86 57.5 P = 74.8 tonnes
Body rise = P TPC = 74.8 15.45 = 4.8cm = 0.048m COT = P x d MCTC = 74.8 x 57.5 86 = 50cm CODA = 57.5 x 50 120 = 24cm CODF = 50 – 24 = 26cm
Forward Aft Initial draft 3.500m 4.000m Body rise 0.048m - 0.048m – COD 0.260m + 0.240m – Final draft 3.712m 3.712m
P d F P is the Upthrust Force or weight dischargedto the blocks… T.M = w x d = P x d t-m by Stern
P F Vessel is fully rest on the blocks, Change of Trim by Stern and finally vessel at even keel drafts… End of Critical Period…
Change of Trim = Trimming Moment (TM) MCTC Whereby TM = w x d = P x d Change of Trim = P x d MCTC P = COT x MCTCtonnes d
Virtual Loss Of GM During Critical Period
Method 1 – GG1 • Method 2 – MM1
Method 1 • When the vessel comes in contact with the blocks, it is assumed that there is a transfer of weight 'P' from the keel to the blocks. • Hence there is a virtual rise of ship's G (discharged of weight below G)
P d F
P d F Trimming Moment by… Head
P K P is the Upthrust Force acting at first point of touching the ground. Commence Critical Period…”weight discharged from the ship”
M G1 G K Reduction or Loss of GM = GG1
M G1 Initial GM loss by GG1 during the Critical Period… This is due to Upthrust Force or ‘P’ Force… G B
Method 1 GG1 = P x KG in metres W - P
P M G1 G B W During Critical Period… part of ship body is still floating
P W - P External Force M G B B1 W Vessel is inclined to a small angle by an external force…
P External Force M G1 G K W - P Method 1 Discharged of weight, shift of GG1
P M X G1 X = KG1 Sin G K W - P Method 1 Discharged of weight, shift of GG1
P M Y G1 G K W - P W Method 1 Discharged of weight, shift of GG1
Y G1 Y = GG1 Sin G Method 1… Discharged of weight, shift of GG1…
P M Y G1 X G K W - P W Method 1 Discharged of weight, shift of GG1