Drydocking And Grounding s2l4g

DRYDOCKING AND GROUNDING

 When a ship enters a drydock she must have a positive initial GM, be upright, and trimmed slightly, usually by the stern. On entering the drydock the ship is lined up with her centre line vertically over the centre line of the keel blocks.  The rate of pumping is reduced as the ship's stern post nears the blocks. When the stern lands on the blocks the shores are hardened up commencing from aft and gradually working forward so that all of the shores will be hardened up in position by the time the ship takes the blocks overall. The dock gates are then closed and pumping out commences.

 The interval of time between the stern post landing on the blocks and the ship taking the blocks overall is referred to as the critical period.  During this period part of the weight of the ship is being borne by the blocks, and this creates an upthrust at the stern which increases as the water level falls in the drydock.  The upthrust causes a virtual loss in metacentric height and it is essential that positive effective metacentric height be maintained through-out the critical period, or the ship will heel over and perhaps slip off the blocks with disastrous results.

 The purpose of this chapter is to show the methods by which the effective metacentric height may be calculated for any instant during the drydocking process.

 A figure shows the longitudinal section of a ship during the critical period. `P' is the upthrust at the stern and `l' is the distance of the centre of flotation from aft. The trimming moment is given by Pl. But the trimming moment is also equal to MCTC Change of trim.

Therefore,

Where; P = the upthrust at the stern in tonnes, t = the change of trim since entering the drydock in centimetres, and l = the distance of the centre of flotation from aft in metres.

 Now consider Figure below which shows a transverse section of the ship during the critical period after she has been inclined to a small angle ( degrees) by a force external to the ship.  The weight of the ship (W) acts downwards through the centre of gravity (G). The force P acts upwards through the keel (K) and is equal to the weight being borne by the blocks. For equilibrium the force of buoyancy must now be (W - P) and will act upwards through the initial metacentre (M).

 There are three parallel forces to consider when calculating the effect of the force P on the ship's stability. Two of these forces may be replaced by their resultant in order to find the effective metacentric height and the moment of statical stability.

Method (a) In last figure consider the two parallel forces P and (W - P). Their resultant W will act upwards through M1 such that:

 There are now two forces to consider: W acting upwards through M1 and W acting downwards through G. These produce a righting moment of W x GM1 x sin . Note also that the original metacentric height was GM but has now been reduced to GM1. Therefore MM1 is the virtual loss of metacentric height due to drydocking.

Method (b) Now consider the two parallel forces W and P in above figure. Their resultant (W – P) acts downwards through G1 such that;

INTRODUCTION

It is a requirement that all ships be dry-docked for inspection and maintenance below the waterline. When a ship is being dry-docked additional forces acting at the keel take effect, being the reaction or upthrust afforded by the blocks onto which the ship is being landed. These forces can create undue loads on the stern structure and cause loss of stability of the ship. This section investigates these effects.

Learning Objectives On completion of this section, the learner will achieve the following: 1. Understand the sequence of events that takes place whilst a ship is being dry-docked. 2. Calculate the upthrust at the blocks (P force) at any stage during dry-docking of the ship. 3. Understand the loss of stability during dry-docking and calculate the loss of stability as either a rise of the ship’s centre of gravity (increase in KG) or as a fall of the metacentre (reduction in KM). 4. Conduct dry-docking calculations. 5. Understand the practical considerations during the dry-docking of a ship.

21.1

SEQUENCE OF EVENTS DURING DRY-DOCKING

Figures 21.1 to 21.3 illustrate what happens as the ship enters the dry dock and the water is pumped out of the dock. 1. The ship enters the dry dock with a small trim by the stern and is floated into position. 2. The gates are closed and water is pumped out of the dock until the ship touches the blocks aft. Immediately the ship touches the blocks aft this denotes the start of the critical period (it is now that the ship will start to experience a loss of stability, hence the term).

3. As more water is pumped out of the dock the true mean draught will start to reduce as the ship experiences more and more at the stern. The upthrust afforded by the blocks at the stern is termed the ‘P force’, this continues to increase as the buoyancy force reduces. Throughout the docking process the ship will displace a progressively lessening volume of water as the true mean draught reduces and the P force increases to provide more for the ship (in effect, the P force takes over ing the ship and the role of the buoyancy force in ing the ship reduces). At this stage the aft draught will be reducing at a greater rate than what the forward draught is increasing, the ship will be trimming by the head as the overall true mean draught reduces. For reasons discussed later, the loss of stability will also be increasing as the P force increases. 4. Eventually the ship will come to rest on the blocks along it’s entire length, this critical instant denotes the end of the critical period, since for a flat bottomed ship the problem of stability loss is no longer of concern.

5.After settling on the blocks forward and aft water continues to be pumped from the dock and the draught reduces at the same rate forward and aft. The upthrust P becomes uniformly distributed along the ship’s length and continues to increase as the effective buoyancy force reduces. 6. When the dock becomes nearly empty and the ship is fully dry the upthrust P will be equal to the ship’s displacement having now replaced all the upthrust afforded by the buoyancy force.

21.2

CALCULATING THE P FORCE

21.2.1 Calculation of P force at any stage during dry-docking Throughout the dry-docking procedure the true mean draught reduces as it would if the ship were rising out of the water due to weights being discharged. Consider the formula:

The P force may be considered to have the same effect on true mean draught as if a weight had actually been discharged, therefore:

This formula may be used to calculate the upthrust at the blocks at any stage in the docking process since the true mean draught is always reducing as water is taken out of the dock.

21.2.2 Calculation of P force during the critical period when drydocking Throughout the dry-docking procedure the true mean draught reduces as it would if the ship were rising out of the water due to weights being discharged. Consider the formula:

The P force may be considered to have the same effect on true mean draught as if a weight had actually been discharged, therefore:

This formula may be used to calculate the upthrust at the blocks at any stage in the docking process since the true mean draught is always reducing as water is taken out of the dock.

21.3

LOSS OF STABILITY WHEN DRY-DOCKING

Loss of stability commences as soon as the ship touches the blocks aft and continues to worsen as the value of the P force increases. The maximum loss of GM of concern occurs the instant immediately prior to the ship settling on the blocks forward and aft – this time being termed the critical instant. Once the ship is flat on the blocks it will be in a safe condition as the risk of heeling over as a result of becoming unstable will have ed (most ship’s having a substantial area of flat bottom). For ships that have a relatively small percentage of flat bottom area additional measures must also be taken such as using side shores to the ship in the upright condition when in the dry dock. Either of two methods of calculation of the loss of GM may be used.

21.3.1 Loss of GM as a result of a rise in G (increase in KG) Consider the upward movement of G that would occur if a weight ‘w’ is discharged from a position at the keel (Kg = 0 m). When discharging a weight the centre of gravity of the ship, G, will move directly away from the centre of gravity of the discharged weight to G V as shown in figure 21.4.

GGv will be equal to the loss of GM where:

‘d’ is the distance between the centre of gravity of the ship (G) and the centre of gravity of the discharged weight which was at the keel ‘K’. Therefore distance ‘d’ is the initial KG of the ship. (It should be noted also that KM changes as a result of a reduction in the ship’s draught.) If the P force is considered to have the same effect as discharging an equivalent weight from the keel then:

The effect on the ship’s stability is made clearer if the available righting moment at a particular angle of heel is considered. Figure 21.5 shows a ship during the critical period where it has taken the blocks at the aft end only. During docking the ship becomes heeled to a small angle of inclination by an external force such as the wind.

The forces acting are as follows: Wf is the total weight force acting downwards through the centre of gravity at G; (W – P) is the remaining, or residual, buoyancy force acting upwards through the geometric centre of the underwater volume at B1; P is the upthrust of the blocks exerted at the keel aft. (W – P)  GZ represents a righting moment; P  GZ1 represents a capsizing moment.

Therefore the available righting moment is given by:

It is essential that the righting moment afforded by the upward acting (remaining) buoyancy force remains greater than the capsizing moment afforded by the upthrust of the P force acting at the keel at all times prior to the ship touching the blocks forward and aft. If the ship should become unstable during the critical period it will lurch off the blocks to one side resulting in structural damage to the ship, movement of the blocks and great embarrassment! It is for this reason that the loss of GM is calculated for the critical instant (when the ship touches the blocks forward and aft) to ensure that adequate stability is maintained prior to the ship taking the blocks overall.

21.3.2 Loss of GM as a result of a fall in M (decrease in KM) Consider figure 21.6 that illustrates the ship heeled by an external force such as the wind during the critical period where the ship has taken the blocks at the aft end only.

The total weight force of the ship acts downwards through G. Counteracting this are the two upward forces; the P force acting upwards at the keel and the residual buoyancy force (W – P) acting upwards through the centre of buoyancy (B 1). The resultant of the two upward acting forces acts through the new metacentre (M 1) such that: P  x = (W – P)  y

(1)

MM1 represents the resulting fall of the transverse metacentre (or loss of GM). Consider the two similar triangles:

Combining formulae 1, 2 and 3 above gives: (W – P)  Sine   MM1 = P  Sine   KM1 Divide both sides by Sine : (W – P)  MM1 = P  KM1

Note In this formula the KM value is that which corresponds to the true mean draught for the instant that the loss of GM is being calculated and not that for the initial true mean draught that the ship has prior to docking. It is found by entering the hydrostatic data with a displacement value that corresponds to that given by (W – P). W in this formula is the ship’s initial displacement.

21.4

TYPICAL DRY-DOCKING PROBLEMS

In the following example both methods of calculating the loss of GM will be used. An explanation is included to prove that both methods are equally valid. Example 1 Prior to entering dry dock M.V. Almar has draughts F 4.86 m A 5.24 m and an effective KG of 9.16m. Calculate: (a) the GM when the ship takes the blocks forward and aft (at the critical instant); (b) the draughts at the same instant;

Both 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.

Within small angles of heel the righting moment is given by:

By method 1 At the critical instant the effective displacement = W – P = 15126 t since the P force acts as a weight being discharged from the keel. RM = Displacement  GM  Sine  RM = 15126  1.454  Sine  = 21993Sine t-m By method 2 By considering the loss of GM as a result of the fall of the metacentre: RM = Displacement  GM  Sine  RM = 15264  1.441  Sine  = 21995 Sine t-m (The slight difference arises due to rounding up of values in the calculation

Solution (b) At 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. Method 1 The initial TMD has already been calculated as being 5.045 m. Entering the data with this obtain the TPC value.

Method 2 If the effective displacement at the critical instant is (W – P) Effective displacement = W – P = 15264 – 138 = 15126 tonnes Enter the data with this displacement value to obtain the TMD at the critical instant

Therefore the draught at the critical instant = 5.002 m (Ans) (Clearly method 2 is much easier!)  During the docking operation it is essential that the ‘critical instant’ draught is determined as both draughts forward and aft will be constantly being read.  As the ship’s draught approaches that as calculated for the critical instant, also evidenced by the fact that the ship will be in a near even keel condition at that time, the rate at which the water is pumped out of the dock will be slowed down to allow final adjustment of the ship’s fore and aft alignment prior to the ship taking the blocks overall. Once on the blocks the rate of pumping will be increased again.

21.5 (2)

PRACTICAL CONSIDERATIONS 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 GM during the critical period.

21.5.1 The requirement to limit the P force During the critical period prior to taking the blocks fully forward and aft the P force will be acting at a single point on the stern frame of the ship. The stern frame is specially strengthened to accept the forces exerted on it during dry- docking but there will be a maximum limit that must not be exceeded. If the P force becomes too great structural damage will occur. It is usual to have acceptable near-light conditions of loading for dry-docking specified in the ship’s stability data book. If an actual P-force value is not quoted then it may be approximated from the recommended condition(s) given by rearranging the drydocking formulae and calculating it. Under normal circumstances the ship’s classification society will investigate any proposed dry-docking condition and that it is appropriate. Under exceptional circumstances a ship may be dry-docked in a part-loaded condition but this will only ever be done after taking classification society advice. It would often be more prudent to discharge any cargo on board prior to entering dry dock.

An obvious method to limit the P force during the critical period is to keep the initial trim by the stern small, consider the formula for calculating the P force during the critical period:

It is clear from the above that P force is directly proportional to the change of trim that the ship will undergo. Limiting the trim will therefore limit the maximum loads that will be experienced by the stern frame. The greater the displacement of a given ship, the more important will be the need to limit the docking trim.

21.5.2 Limiting the loss of GM Consideration of the formulae will indicate that the greater the trim of the ship when docking, the greater will be the loss of GM.

It is clear from the above that P force is directly proportional to the change of trim that the ship will undergo. Limiting the trim will therefore limit the maximum loads that will be experienced by the stern frame. The greater the displacement of a given ship, the more important will be the need to limit the docking trim. Clearly, the greater the trim, the greater the P force; the greater the P force, the greater the loss of GM!

Alternatively, the ship should dry-dock with a greater effective GM that will ensure that stability is maintained. Improving the ship’s initial GM will be achieved by: (1) Lowering the effective KG by lowering weights within the vessel, discharging weights from high up or taking on an acceptable amount of ballast in double bottom tanks, or; (2) Minimising free surface effects by topping up slack tanks wherever possible.

Example 2 M.V. Almar about to dry dock requires a minimum GM of 0.3 m at the time the ship takes the blocks forward and aft. Current draughts are F 6.89 m and A 8.47 m. KG is 8.86 m. Calculate the maximum permissible trim by the stern on entering the dry dock.

Calculate the maximum allowed P force and hence the maximum intial trim

To ensure that a GM of 0.3 m is maintained at the critical instant the trim of the ship must not exceed 1.30 m by the stern.