Ship Stability for Masters and Mates 5 Episode 10 pot

Interaction occurs when a ship comes too close to another ship or too close to, say, a river or canal bank.. 2 When they are directly in line each ship will develop an angle of heel andt

304 Ship Stability for Masters and Mates

Fig 35.1 Deadweight Scale

Chapter 36

Interaction

What exactly is interaction?

Interaction occurs when a ship comes too close to another ship or too close

to, say, a river or canal bank As ships have increased in size (especially inBreadth Moulded), Interaction has become very important to consider InFebruary 1998, the Marine Safety Agency (MSA) issued a Marine Guidancenote `Dangers of Interaction', alerting Owners, Masters, Pilots, and Tug-Masters on this topic

Interaction can result in one or more of the following characteristics:

1 If two ships are on a passing or overtaking situation in a river the squats

of both vessels could be doubled when their amidships are directly inline

2 When they are directly in line each ship will develop an angle of heel andthe smaller ship will be drawn bodily towards the larger vessel

3 Both ships could lose steerage ef®ciency and alter course without change

in rudder helm

4 The smaller ship may suddenly veer off course and head into the adjacentriver bank

5 The smaller ship could veer into the side of the larger ship or worse still

be drawn across the bows of the larger vessel, bowled over and capsized

In other words there is:

(a) a ship to ground interaction;

(b) a ship to ship interaction;

(c) a ship to shore interaction

What causes these effects of interaction? The answer lies in the pressurebulbs that exist around the hull form of a moving ship model or a movingship See Figure 36.1 As soon as a vessel moves from rest, hydrodynamicsproduce the shown positive and negative pressure bulbs For ships withgreater parallel body such as tankers these negative bulbs will be

comparatively longer in length When a ship is stationary in water of zerocurrent speed these bulbs disappear.

Note the elliptical Domain that encloses the vessel and these pressurebulbs This Domain is very important When the Domain of one vesselinterfaces with the Domain of another vessel then interaction effects willoccur Effects of interaction are increased when ships are operating inshallow waters

Ship to ground (squat) interaction

In a report on measured ship squats in the St Lawrence seaway, A D Wattstated: `meeting and passing in a channel also has an effect on squat It wasfound that when two ships were moving at the low speed of ®ve knots thatsquat increased up to double the normal value At higher speeds the squatwhen passing was in the region of one and a half times the normal value.'Unfortunately, no data relating to ship types, gaps between ships, blockagefactors etc accompanied this statement

Thus, at speeds of the order of ®ve knots the squat increase is ‡ 100 percent whilst at higher speeds, say ten knots, this increase is ‡ 50 per cent.Figure 36.2 illustrates this passing manoeuvre Figure 36.3 interprets thepercentages given in the previous paragraph

How may these squat increases be explained? It has been shown in thechapter on Ship Squat that its value depends on the ratio of the ship's cross-section to the cross-section of the river This is the blockage factor `S' Thepresence of a second ship meeting and crossing will of course increase theblockage factor Consequently the squat on each ship will increase.Maximum squat is calculated by using the equation:

dmax ˆCb S0:81 V2:08k

Consider the following example

306 Ship Stability for Masters and Mates

Fig 36.1 Pressure distribution around ship's hull (not drawn to scale).

Example 1

A supertanker has a breadth of 50 m with a static even-keel draft of 12.75 m She is proceeding along a river of 250 m and 16 m depth rectangular cross- section If her speed is 5 kts and her CBis 0.825, calculate her maximum squat when she is on the centre line of this river.

S ˆB  Hb  T ˆ50  12:75250  16 ˆ 0:159

d max ˆ0:825  0:159200:81 52:08ˆ 0:26 m Example 2

Assume now that this supertanker meets an oncoming container ship also travelling at 5 kts See Figure 36.4 If this container ship has a breadth of 32 m a

Cb of 0.580, and a static even-keel draft of 11.58 m calculate the maximum squats of both vessels when they are transversely in line as shown.

S ˆ…b1 T1B  H† ‡ …b2 T2†

S ˆ…50  12:75† ‡ …32  11:58†250  16 ˆ 0:252 Supertanker:

d max ˆ0:825  0:252200:81 52:08

ˆ 0:38 m at the bow Container ship:

dmaxˆ0:580  0:252200:81 52:08

ˆ 0:27 m at the stern The maximum squat of 0.38 m for the supertanker will be at the bow because her Cbis greater than 0.700 Maximum squat for the container ship will be at the stern, because her Cbis less than 0.700 As shown this will be 0.27 m.

If this container ship had travelled alone on the centre line of the river then her maximum squat at the stern would have only been 0.12 m Thus the presence of the other vessel has more than doubled her squat.

Clearly, these results show that the presence of a second ship does increase ship squat Passing a moored vessel would also make blockage effect and squat greater These values are not qualitative but only illustrative of this phenom- enon of interaction in a ship to ground (squat) situation Nevertheless, they are supportive of A D Watt's statement.

Ship to ship Interaction

Consider Figure 36.5 where a tug is overtaking a large ship in a narrowriver Three cases have been considered:

308 Ship Stability for Masters and Mates

Fig 36.5 Ship to ship interaction in a narrow river during an overtaking manoeuvre.

Case 1 The tug has just come up to aft port quarter of the ship TheDomains have become in contact Interaction occurs The positive bulb ofthe ship reacts with the positive bulb of the tug Both vessels veer to portside Rate of turn is greater on the tug There is a possibility of the tugveering off into the adjacent river bank as shown in Figure 36.5.

Case 2 The tug is in danger of being drawn bodily towards the shipbecause the negative pressure (suction) bulbs have interfaced The biggerthe differences between the two deadweights of these ships the greater will

be this transverse attraction Each ship develops an angle of heel as shown.There is a danger of the ship losing a bilge keel or indeed fracture of thebilge strakes occurring This is `transverse squat', the loss of underkeelclearance at forward speed Figure 36.4 shows this happening with thetanker and the container ship

Case 3 The tug is positioned at the ship's forward port quarter TheDomains have become in contact via the positive pressure bulbs Bothvessels veer to the starboard side Rate of turn is greater on the tug There

is great danger of the tug being drawn across the path of the ship's headingand bowled over This has actually occurred with resulting loss of life.Note how in these three cases that it is the smaller vessel, be it a tug, apleasure craft or a local ferry involved, that ends up being the casualty!!Figures 36.6 and 36.7 give further examples of ship to ship Interactioneffects in a river

Methods for reducing the effects of Interaction in Cases 1 to 5

Reduce speed of both ships and then if safe increase speeds after themeeting crossing manoeuvre time slot has passed Resist the temptation to

go for the order `increase revs' This is because the forces involved withInteraction vary as the speed squared However, too much a reduction inspeed produces a loss of steerage because rudder effectiveness is decreased.This is even more so in shallow waters, where the propeller rpm decreasefor similar input of deep water power Care and vigilance are required.Keep the distance between the vessels as large as practicable bearing inmind the remaining gaps between each ship side and nearby river bank.Keep the vessels from entering another ship's Domain, for examplecrossing in wider parts of the river

Cross in deeper parts of the river rather than in shallow waters, bearing inmind those increases in squat

Make use of rudder helm In Case 1, starboard rudder helm could berequested to counteract loss of steerage In Case 3, port rudder helm wouldconteract loss of steerage

Ship to shore interaction

Figures 36.8 and 36.9 show the ship to shore Interaction effects Figure 36.8shows the forward positive pressure bulb being used as a pivot to bring aship alongside a river bank

Interaction 311

Fig 36.6 Case 4 Ship to ship interaction Both sterns swing towards river banks The approach situation.

Fig 36.7 Case 5 Ship to ship interaction Both bows swing towards river banks The leaving situation.

Figure 36.9 shows how the positive and negative pressure bulbs havecaused the ship to come alongside and then to veer away from the jetty.Interaction could in this case cause the stern to swing and collide with thewall of this jetty.

Summary

An understanding of the phenomenon of Interaction can avert a possiblemarine accident Generally a reduction in speed is the best preventiveprocedure This could prevent on incident leading to loss of sea worthiness,loss of income for the shipowner, cost of repairs, compensation claims andmaybe loss of life

Interaction 313

Fig 36.8 Ship to bank interaction Ship approaches slowly and pivots on

for-ward positive pressure bulb.

Fig 36.9 Ship to bank interaction Ship comes in at too fast a speed Interaction

causes stern to swing towards river bank and then hits it.

314 Ship Stability for Masters and Mates

Let the ship in Figure 37.1 ¯oat in salt water at the waterline WL Brepresents the position of the centre of buoyancy and G the centre ofgravity For equilibrium, B and G must lie in the same vertical line.

If the ship now passes into fresh water, the mean draft will increase Let

W1L1 represent the new waterline and b the centre of gravity of the extravolume of the water displaced The centre of buoyancy of the ship, beingthe centre of gravity of the displaced water, will move from B to B1 in adirection directly towards b The force of buoyancy now acts verticallyupwards through B1 and the ship's weight acts vertically downwardsthrough G, giving a trimming moment equal to the product of thedisplacement and the longitudinal distance between the centres of gravityand buoyancy The ship will then change trim to bring the centres ofgravity and buoyancy back in to the same vertical line

Example

A box-shaped pontoon is 36 m long, 4 m wide and ¯oats in salt water at drafts

F 2.00 m A 4.00 m Find the new drafts if the pontoon now passes into fresh

Fig 37.1

water Assume salt water density is 1.025 t/m 3 Assume fresh water density ˆ 1.000 t/m 3

(a) To Find the Position of B1

Now let the pontoon enter fresh water, i.e from rSWinto rFW Pontoon will develop mean bodily sinkage.

(b) To Find the New Draft

In Salt Water

Mass ˆ Volume  Density

ˆ 36  4  3  1:025

In Fresh Water.

Mass ˆ Volume  Density

; Volume ˆDensityMass

ˆ36  4  3  1:0251:000 cu m (Mass in Salt water ˆ Mass in Fresh Water)

316 Ship Stability for Masters and Mates

Fig 37.2

; MBS ˆ 0:075 m

Original mean draft ˆ 3:000 m

New mean draft ˆ 3:075 m say draft d2

(c) Find the Change of Trim

Let B1B2be the horizontal component of the shift of the centre of buoyancy Then

ˆ442:8  35:12100  36

ˆ 4:32 tonnes metres Change of Trim ˆTrimming MomentMCTC

ˆ21:564:32 ˆ 5 cm

Effect of change of density on draft and trim 317

Change of Trim ˆ 5 cm by the stern

ˆ 0:05 m by the stern Drafts before Trimming A 4.075 m F 2.075 m

Change due to trim ‡0:025 m ÿ0:025 m

New Drafts A 4.100 m F 2.050 m

In practice the trimming effects are so small that they are often ignored by shipboard personnel Note in the above example the trim ratio forward and aft was only 2 1

1 A box-shaped vessel is 72 m long, 8 m wide and ¯oats in salt water at drafts

F 4.00 m A 8.00 m Find the new drafts if the vessel now passes into fresh water.

2 A box-shaped vessel is 36 m long, 5 m wide and ¯oats in fresh water at drafts F 2.50 m A 4.50 m Find the new drafts if the vessel now passes into salt water.

3 A ship has a displacement of 9100 tonnes, LBP of 120 m, even-keel draft of

7 m in fresh water of density of 1.000 t/m 3

From her Hydrostatic Curves it was found that:

MCTCSW is 130 t m/cm TPCSW is 17.3 t LCB is 2 m forward †e… and LCF is 1.0 aft †e…:

Calculate the new end drafts when this vessel moves into water having a density of 1.02 t/m 3 without any change in the ship's displacement of 9100 tonnes.

The horizontal component of the shift of the ship's centre of gravity isequal to GZ and the horizontal component of the shift of the centre of

Fig 38.1

gravity of the weight is equal to d cos y.

; w  dEcos y ˆ W  GZThe length GZ, although not a righting lever in this case can be foundusing the `Wall-sided' formula

BMEWˆ tan3yor

tan y ˆ



2EwEdBMEW

KM ˆ 8:0 m

KB ˆ 3:8 m

BM ˆ 4:2 m The ship has zero GM

; tan list ˆ



2EwEd WEBM

320 Ship Stability for Masters and Mates

List with zero metacentric height 321

Exercise 38

1 Find the list when a mass of 10 tonnes is shifted transversely through a horizontal distance of 14 m in a ship of 8000 tonnes displacement which has zero initial metacentric height BM ˆ 2 m.

2 A ship of 8000 tonnes displacement has zero initial metacentric height.

BM ˆ 4 m Find the list if a weight of 20 tonnes is shifted transversely across the deck through a horizontal distance of 10 m.

3 A ship of 10 000 tonnes displacement is ¯oating in water of density 1.025 kg/cu m and has a zero initial metacentric height Calculate the list when a mass of 15 tonnes is moved transversely across the deck through a horizontal distance of 18 m The second moment of the waterplane area about the centre line is 10 5 m 4 :

4 A ship of 12 250 tonnes displacement is ¯oating upright KB ˆ 3.8 m,

KM ˆ 8 m and KG ˆ 8 m Assuming that the ship is wall-sided, ®nd the list

if a mass of 2 tonnes, already on board, is shifted transversely through a horizontal distance of 12 m.

1 General particulars of the ship.

2 Inclining experiment report and its results

3 Capacity, VCG, LCG particulars for all holds, compartments, tanks etc

4 Cross curves of stability These may be GZ curves or KN curves

5 Deadweight scale data May be in diagram form or in tabular form

6 Hydrostatic curves May be in graphical form or in tabular form

7 Example conditions of loading such as:

Lightweight (empty vessel) condition

Full-loaded Departure and Arrival conditions

Heavy-ballast Departure and Arrival conditions

Medium-ballast Departure and Arrival conditions

Light-ballast Departure and Arrival conditions

On each condition of loading there is a pro®le and plan view (at upperdeck level usually) A colour scheme is adopted for each item ofdeadweight Examples could be red for cargo, blue for fresh water,green for water ballast, brown for oil Hatched lines for this Dwtdistribution signify wing tanks P and S

For each loaded condition, in the interests of safety, it is necessary toshow:

Deadweight

End draughts, thereby signifying a satisfactory and safe trim situation

KG with no Free Surface Effects (FSE), and KG with FSE taken intoaccount

Final transverse metacentric height (GM) This informs the of®cer if theship is in stable, unstable or neutral equilibrium It can also indicate ifthe ship's stability is approaching a dangerous state.

Total Free Surface Effects of all slack tanks in this condition ofloading

A statical stability curve relevant to the actual loaded condition with theimportant characteristics clearly indicated For each S/S curve it isimportant to observe the following:

Maximum GZ and the angle of heel at which it occurs

Range of stability

Area enclosed from zero degrees to thirty degrees (A1) and the areaenclosed from thirty degrees to forty degrees (A2) as shown in Figure39.1

Shear force and bending moment curves, with upper limit lines clearlysuperimposed as shown in Figure 39.2

8 Metric equivalents For example, TPI00 to TPC, MCTI00 to MCTC, ortons to tonnes

All of this information is sent to a D.Tp surveyor for appraisal On manyships today this data is part of a computer package The deadweight itemsare keyed in by the of®cer prior to any weights actually being moved Thecomputer screen will then indicate if the prescribed condition of loading isindeed safe from the point of view of stability and strength

The Trim and Stability book 323

Fig 39.1 Enclosed areas on a statical stability curve.