international maritime organization e · pdf file1 a historical review of freeboard...

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For reasons of economy, this document is printed in a limited number. Delegates are kindly asked to bring their copies to meetings and not to request additional copies.

INTERNATIONAL MARITIME ORGANIZATION

IMO

E

SUB-COMMITTEE ON STABILITY AND LOAD LINES AND ON FISHING VESSELS SAFETY 45th session Agenda item 4

SLF 45/4/2 12 March 2002 Original: ENGLISH

REVISION OF TECHNICAL REGULATIONS OF THE 1966 LL CONVENTION

The influence of superstructures, sheer and tonnage on freeboard

Submitted by the United Kingdom

SUMMARY Executive summary:

This document reports a United Kingdom Maritime and Coastguard Agency commissioned study to review the freeboard corrections for superstructures and sheer, and to assess their relevance to ship types currently building. The study is attached as the annex to this paper

Action to be taken:

Paragraph 2

Related documents:

SLF 41/6/1, SLF 41/INF.2

Background 1 A historical review of freeboard calculations is conducted to establish the origin of the corrections for superstructures and sheer and consider how they have been adapted to suit the progress of ship design. The paper discusses the use of freeboard to minimise gross tonnage and the importance of the distribution of reserve buoyancy. The application of type A, B-60 and B-100 freeboards is considered and the conclusions of the study are presented in Section 6 of the study. Action requested of the Sub-Committee 2 The Sub-Committee is invited to consider the annexed report1 and take action as appropriate, noting the conclusions of Section 6.

***

1 Load Lines: The influence of superstructures, sheer and tonnage on freeboard.

SLF 45/4/2

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ANNEX

Load Lines: The influence of superstructures, sheer and tonnage on freeboard

Maritime and Coastguard Agency (MCA)

1 Introduction

A historical review of freeboard formulations was conducted to establish the origin of the corrections for superstructures and sheer and consider how they have been adapted to suit the progress of ship design since their introduction. The paper discusses the use of freeboard to minimise the gross tonnage and the importance of the distribution of reserve buoyancy. The application of B-60 freeboard to ships other than bulk carriers is considered and the conclusions of the study are presented in Section 6

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2 Background to the corrections for sheer, superstructure and trunks and their relevance

to today�s ships 2.1 The development for International Standards for Load Lines 2.1.1 Load Line Committee 1913-1915

Prior to a proposed International Conference on Load Lines, a Load Line Committee was established by the Board of Trade in 1913 and reported in 1915 (Reference 1). In this document the following are of interest:

• Length is measured on the Summer Load Waterline • The table of freeboards define freeboard for a range of lengths (72 feet to 600feet)

and block coefficients (0.66 to 0.80), with freeboard increasing for increase in block coefficient.

• The standard depth was L/12 compared with L/15 in ILLC 66 and the correction

applied for negative or positive values of depth. Where D is less than L/15 the correction is limited to that which would apply at L/15. A constant �r� based on effective length of superstructures to freeboard length was introduced to apply to ships with detached superstructures.

• Standard shear is defined as (3.3L+407) mm (originally in feet and inches), about 1/4

of the value in ILLC 66 for the forward end. The corrections applied for deficiency in sheer are similar in principle to ILLC 66

• The standard height of superstructures is defined as (0.055L+0.366) m with minimum

and maximum values of 0.915m and 2.29 m respectively (Original again in feet and inches). The correction for a partial superstructure is product of a type factor (based on type of vessel and percentage superstructure (i.e. S/L)), the effective length and the reduction in freeboard for complete superstructures.

• Corrections for longitudinal and transverse strength below a defined standard are also defined

2.1.2 The 1930 International Conference on Load Lines

The projected International Conference did not proceed due to the outbreak of war. A further review of load lines was carried out on behalf of the Board of Trade in preparation for the 1930 International Load Line Conference.

Reference 2 provides the text of the Convention and the minutes of the meetings. The 1930 Convention lays the foundation for the present rules. In particular, the following points are of interest: • Length remains measured on the summer load waterline, but the 0.96Lwl requirement

is introduced for ships with �cruiser sterns�

• The concept of calculating Cb at 0.85 depth is introduced, with a minimum value of 0.68, together with the present correction for block coefficient.

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• A single table of freeboard for steamers is introduced for lengths from 80 feet to 750 feet

• The depth correction is adjusted to produce corrections for depths in excess of L/15, and is essentially the same as that in ILLC 66.

• The standard height of superstructures is increased to 0.915m (3 ft) for less than 30.5 m (100ft) in length, 1.22m (4ft) for a length of 76m(250ft) and 1.83m(6ft) for lengths in excess of 122m(400ft). The corrections, both with respect to the deduction for a complete superstructure and the percentages for effective lengths less than 1.0L are essentially the same as ILLC 66, except that the latter has been adjusted to provide rounded metric units.

• The concept of a trunk is introduced with the same caveats as in ILLC66, except that the standard height of a trunk is defined to be the same as a superstructure, and the minimum breadth is not restricted to 60% of the breadth.

• The standard sheer fore and aft is increased to the present values, and the correction applied is the same as ILLC 66.

• A longitudinal strength requirement is retained.

• A separate sub-committee, chaired by the United States, considered load lines for tankers and proposed reduced tabular freeboards and modified percentages for enclosed superstructures less than 1.0 L.

2.1.3 The 1966 International Conference on Load Lines

The 1930 Convention remained the basis for International Load Lines until they were modified by the 1966 Load line Convention (Reference 3). Reference 4 provides an account of the changes to the Convention and discusses possible future changes. The significant changes made include:

• Length defined as in 1930, but now at waterline at 0.85D • In view of experience in the preceding years, tabular freeboard for both tankers,

renamed Type A ships, and steamers, renamed Type B ships, were reduced and the concept of B-60 and B-100 freeboards introduced for Type B ships able to meet one and two compartment damage stability respectively.

• The block coefficient correction remained unchanged, in spite of some views to the

contrary. • No change was made to the depth correction • The standard height of superstructures was increased to reflect current practice, and

adjusted to round metric coefficients.

• The present limitation on trunk width and reduced standard height introduced

• The sheer correction remained unchanged apart from changing to rounded metric values.

• The longitudinal strength requirement was dis-continued.


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Ship A1 A2 A3 A4 A5 A6 A7 A8 A9Type of Ship Tanker Tanker Tanker Tanker Tanker Tanker Tanker Tanker TankerFreeboard Type A A A A A A A A ALpp 210 174.3 82.2 139.3 168 256.5 94.83 85 234B 36.8 32.2 16.5 23 30 42.5 16.5 15.5 42D 21 19.15 7.65 13.95 15.2 22 8.3 8 21T (design) 14.55 11.2 5.6 8.91 10 15 6.1 6 12.02T (scantling) 15.5 12.2 5.6 9.96 10.4 15.65 6.8 6 14.32D-T 5.5 6.95 2.05 3.99 4.8 6.35 1.5 2 6.68T(scantling)/D 0.738095 0.637076 0.732026 0.713978 0.684211 0.711364 0.819277 0.75 0.681905Displacement 104471 58220 5490 25697 42338 152972 8335 6200 116208Cb 0.850889 0.829536 0.705188 0.785634 0.78803 0.874777 0.764264 0.765184 0.805571Summary AverageTabular Freeboard 100.0% 100.0% 100.0% 100.0% 100.0% 100.0% 100.0% 100.0% 100.0% 100.0%Reduction per Sch 4 Para 5(4) & (5) 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0%Adjusted tabular freeboard 100.0% 100.0% 100.0% 100.0% 100.0% 100.0% 100.0% 100.0% 100.0% 100.0%Cb correction 1.131 1.124 1.030 1.088 1.092 1.149 1.064 1.070 1.104 1.095Freeboard corrected for Cb 113.1% 112.4% 103.0% 108.8% 109.2% 114.9% 106.4% 107.0% 110.4% 109.5%Depth Corrn 64.7% 81.0% 41.5% 64.0% 43.6% 40.2% 35.9% 45.4% 46.1% 51.4%Superstrucure corrn -3.9% -8.8% -21.0% -11.6% -8.7% -3.5% -19.2% -22.1% -2.0% -11.2%Sheer corn 24.5% 21.8% 27.4% 21.9% 21.1% 25.5% 21.1% 30.7% 24.0% 24.2%Sum 198.4% 206.4% 150.9% 183.1% 165.2% 177.0% 144.2% 161.1% 178.6% 173.9%Min bow height corrn 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 38.7% 4.3%Freeboard 198.4% 206.4% 150.9% 183.1% 165.2% 177.0% 144.2% 161.1% 217.3% 178.2%Difference from T scantling 0.9% 15.0% 11.4% 6.5% 9.4% 5.7% -0.6% 8.2% 2.5% 6.5%Difference from T design 6.9% 22.0% 11.4% 16.3% 12.9% 9.6% 9.8% 8.2% 18.1% 12.8%

• The concept of minimum bow height was introduced, with bow height increasing

with increase in length up to 250 metres in length and reducing with increase in block coefficient. A minimum value for block coefficient of 0.68 was defined, to be consistent with the block coefficient correction.

Since 1966 there have been a number of adjustments in the rules, but the essential means of evaluating minimum freeboard is unchanged.

2.2 The impact of the corrections to tabular freeboard on current ship designs.

There has been a significant change in shipping since 1966, both in terms of ship types (container ships, ro-ros, gas ships etc.) and in size. There is therefore a need to consider how well the load line rules serve today�s shipping. A brief review has been made of the ships described in one years issue of �Significant Ships of ��, published by the Royal Institution of Naval Architects (Reference 5). In making this review a number of ships were excluded for a variety of reasons;

• The displacement was not indicated • The freeboard of passenger ships and ferries is limited more by subdivision and

damage stability than by geometrical freeboard. • Gas carriers and large cellular container ships generally have freeboards well in

excess of geometrical freeboard, and will be largely unaffected by any change in rule changes.

This has left a total of 22 vessels (9 Type A, 9 Type B Single deck ships, 2 Type B twin deck ships, 1 B-100 ship and 1 B-60 ship) for which approximate freeboard calculations have been made. Minor effects such as stringer plate thickness, keel thickness etc. have been ignored. 2.2.1 Type A ships.

The following table shows the results of the review for Type A ships.

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Ship B1 B2 B3 B4 B5 B6 B7 B8 B9Type of Ship General/Bulk General/Bulk General/Bulk General/ Bulk General General Multi purpose Container generalFreeboard Type B B B B B B B B BLpp 95 84.99 94.27 95 127.94 127.2 136.8 283.2 162.5B 17 13.17 17 20.4 22.8 16.5 22.7 32.2 27.5D 8.7 7.15 8.2 11.1 12.95 9.8 11.3 21.8 13.8T (design) 6.75 5.678 6.354 6.25 9.5 7.12 7.8 12 9.32T (scantling) 6.764 5.678 6.354 8.2 9.5 7.12 8.55 13 9.32D-T 1.936 1.472 1.846 2.9 3.45 2.68 2.75 8.8 4.48T(scantling)/D 0.777471264 0.794125874 0.774878049 0.738738739 0.733590734 0.726530612 0.756637168 0.596330275 0.675362319Displacement 0 5029.770088 8224 12023 21400 12164 17726 82496 31617Cb 0 0.772103064 0.787933558 0.738110454 0.753399121 0.794148092 0.65134159 0.678916796 0.740618954Summary AverageTabular Freeboard 100.0% 100.0% 100.0% 100.0% 100.0% 100.0% 100.0% 100.0% 100.0% 100.0%Reduction per Sch 4 Para 5(4) & (5) 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0%Adjusted tabular freeboard 100.0% 100.0% 100.0% 100.0% 100.0% 100.0% 100.0% 100.0% 100.0% 100.0%Cb correction 1.118 1.072 1.084 1.052 1.064 1.093 1.000 1.033 1.061 1.064Freeboard corrected for Cb 111.8% 107.2% 108.4% 105.2% 106.4% 109.3% 100.0% 103.3% 106.1% 106.4%Depth Corrn 39.6% 26.5% 32.3% 77.7% 59.4% 17.0% 26.5% 14.8% 28.4% 35.8%Superstrucure corrn -11.8% -12.8% -8.9% -14.1% -8.3% -4.5% -4.9% -0.9% -6.5% -8.1%Sheer corn 23.2% 27.2% 27.6% 27.0% 23.1% 22.8% 22.1% 21.0% 17.3% 23.5%Sum 162.9% 148.0% 159.4% 195.9% 180.6% 144.6% 143.7% 138.2% 145.3% 157.6%Min bow height corrn 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0%Freeboard 162.9% 148.0% 159.4% 195.9% 180.6% 144.6% 143.7% 138.2% 145.3% 157.6%Difference from T scantling 0.2% 0.1% -0.2% 6.1% 1.0% -0.1% -2.2% 16.2% 7.3% 3.2%Difference from T design 0.4% 0.1% -0.2% 28.5% 1.0% -0.1% 6.7% 22.7% 7.3% 7.4%

In the table, the tabular freeboard has been normalised to 100%, and the corrections are shown as a percentage increase or decrease of the tabular freeboard. The final freeboard varies between 144% for a small tanker up to over 200% for a particularly deep ship. The freeboard for only one ship is limited by minimum bow height. In 1966 tankers were deadweight limited designs, but subsequent legislation to introduce requirements for segregated ballast and double hulls has required a substantial increase in enclosed volume to the point at which tankers are now volume limited designs. This is confirmed by the fact that most ships in the table have scantling draft less than corresponding to the minimum freeboard and all have design drafts significantly less. Indeed 2/3 of the ships have freeboards in excess of Type B requirement. Clearly it is time to consider whether Type A freeboard remains relevant for today�s tankers.

2.2.2 Type B Ships

The following table presents the results for type B ships, all of which have the upper deck as the freeboard deck.

The results are presented as before and express corrections as percentages of the tabular freeboard. In general most of these ships have a forecastle or have excess freeboard to the extent that none are limited by minimum bow height. On average the final freeboard is about 58% greater than the tabular freeboard, with the corrections for depth and lack of sheer being the main sources of increase. For most ships the superstructure correction appears minimal.

The calculated drafts for scantling limited ships appear quite close to those quoted in reference 5.

2.2.3 Type B Ships with two decks

The following table shows the results of the review for two ships where the freeboard deck is the second deck and therefore the ships have full-length superstructures. In one case the minimum bow height is the limiting factor, due to the lack of a forecastle, the relative magnitude of the superstructure deduction and the tabular freeboard for a short length. In both cases the design draft is about 9% less than that equivalent to the minimum freeboard.


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2.2.4 B-100 and B-60 freeboards

The above table shows the results of the review of ships with B-100 freeboard and B-60 freeboard. The number of ships to which B-100 can be applied is minimal, but there are a lot of bulk carriers, Panamax size and above, with B-60 freeboard. The freeboard for both vessels is not all that different to those of the B freeboard ships in the previous table, due again to the high depth and sheer corrections, together with the effect of higher block coefficients. The superstructure correction is almost negligible in this instance.

2.2.5 Sheer

None of the vessels reviewed employ sheer, other than to take up any centreline camber, to the extent envisaged in the rules. Some ships, such as large bulk carriers, employ straight line sheer forward of the aft end of No 1 hold to obtain the minimum bow height. Overall the absence of sheer results in an increase of 20-30 % of tabular freeboard and so one simplification might be to increase tabular freeboard to a level compatible with no sheer and then allow reduction where significant sheer is incorporated into the design.

2.2.6 Superstructures

In all cases the superstructures, whether significantly affecting the final load line or not, all have a height significantly greater than the standard height defined in the rules. The standard height was increased in 1966 to reflect current practice and a further increase to 2.5 m to reflect present practice could be considered.

2.2.7 Detached bridges.

None of the ships in the review had detached bridges. Indeed they have largely gone out of fashion, and consideration could be given to the removal of line II from regulation 37(2).

Ship B10 B11 B12 B13Type of Ship Reefer RoRo Ore BulkFreeboard Type B2 B2 B-100 B-60Lpp 91.5 249 320 216B 15.7 32.26 58 32.24D 7.3 14.72 30.2 19.1T (design) 5.7 10.7 23 12.2T (scantling) 6 11.75 23 13.85D-T 1.3 2.97 7.2 5.25T(scantling)/D 0.821917808 0.798233696 0.761589404 0.72513089Displacement 6210 96744.31238 359936 83746Cb 0.702903552 0.67 0.822613084 0.847113591Summary Average AverageTabular Freeboard 100.0% 100.0% 100.0% 100.0% 100.0% 100.0%Reduction per Sch 4 Para 5(4) & (5) 0.0% 0.0% 0.0% -31.2% -13.0% -22.1%Adjusted tabular freeboard 100.0% 100.0% 100.0% 68.8% 87.0% 77.9%Cb correction 1.019 1.000 1.010 1.110 1.129 1.120Freeboard corrected for Cb 101.9% 100.0% 101.0% 76.3% 98.3% 87.3%Depth Corrn 20.7% 0.0% 10.3% 45.8% 33.4% 39.6%Superstrucure corrn -81.1% -26.7% -53.9% -0.4% -1.3% -0.8%Sheer corn 4.4% -3.1% 0.6% 21.8% 15.9% 18.9%Sum 45.9% 70.2% 58.0% 143.5% 146.3% 144.9%Min bow height corrn 54.2% 0.0% 27.1% 0.0% 0.0% 0.0%Freeboard 100.1% 70.2% 85.1% 143.5% 146.3% 144.9%Difference from T scantling 3.1% 1.3% 2.2% 1.1% 0.7% 0.9%Difference from T design 8.0% 10.2% 9.1% 1.1% 12.5% 6.8%

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2.2.8 Trunks

None of the ships in the review had trunks. Although when they were introduced in 1930, their use was primarily intended for cargo vessels, they are also now commonly applied to small tankers. The potential benefits are discussed further in section 4 through application to a specific vessel. Appendix A summarises the requirements for trunks.

2.2.9 Raised Quarter Decks

None of the ships in the above analysis have a raised quarterdeck, and it was thought that these were no longer employed in modern designs. However a recent Conoship design, the mini-bulker MRS SONJA, reported in the 1999 edition of Significant Ships (reference 5) employs a raised quarterdeck instead of a trunk to reduce freeboard. (Figure 2.1) The accommodation is described as being in a �sunken poop�, in which the tween deck aft of the engine room bulkhead is at a lower level than the Main Deck. It is not clear whether such a significant step in the freeboard deck is strictly in compliance with the 1966 Convention.

Figure 2.1 Mini-bulker �Mrs Sonja� with raised quarterdeck

The principal particulars for the vessel are as follows:

Length OA 89.75m Length PP 84.98m Breadth 13.6m Depth to freeboard deck 7.2m Draft 6.36m Freeboard 0.84m Height of hatch coaming above main deck 2.265 m Height of RQD (estimated) 1.4m approx

The main difference between a raised quarterdeck (RQD) and a superstructure, with respect to freeboard, is the reduced standard height of the RQD. At the length of the vessel (85m), the standard heights of the RQD and superstructure are 1.32m and 1.92 respectively.


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2.3 Potential simplification of the Load Line Rules

From the above it is clearly evident that a major review of the Load Line Rules is now justified with the aim of simplifying the rules considerably. This might include: • Elimination of the Type A ships • Consolidation of the percentage deductions for superstructures less than 1.0L into a

single table for all ships • Review whether B-100 and B-60 freeboard should be discontinued.

3 The use of freeboard to minimise gross tonnage 3.1 Comparison of enclosed volume as a proportion of displacement for different ship types.

An analysis has been made of a number of ships listed in reference 5. The analysis has concentrated on ship types, whose dimensions are limited by deadweight, or marginally by volume. Representative spots were calculated for types with a large enclosed volume, such as ro-ro ships. The enclosed volume has been calculated from the quoted gross tonnage using the Microsoft Excel TM �goal seek� function on the following equation:

Gross tonnage = V x (0.2 + 0.02 Log10 V) where V s the enclosed volume in cubic metres.

Figure 3.1 shows a plot of enclosed volume/displacement against length for a number of ship types. The equivalent ratio for ro-ros is between 3.5 and 4.

In carrying out the above analysis it was noted that many of the smaller vessels had only 1 or 2 holds, and must have a lower degree of survivability than ships with larger ships having 4 or 5 holds. Figure 3.2 plots the same ratio data against number of holds. This shows that small cargo ships, with only 1 or 2 holds, have a similar ratio to bulk carriers with 7 and 9 holds, which have B-60 freeboard, and therefore have single compartment subdivision.

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1

1.5

2

2.5

3

0 1 2 3 4 5 6 7 8 9 10

No of Holds

Bulk Cargo Container Tanker

Area of concern

Figure 3.1 Ratio of enclosed volume/displacement as a function of length

Figure 3.2 Ratio of enclosed volume/displacement as a function of number of holds

The smaller ships, with lengths in excess of 100 metres should comply with probabilistic damage stability requirements, and those with length between 80m and 100m and built after 1st July 1998 should also comply. However the validity of this standard is questionable, as will be discussed later.

1

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Length



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0 50 100 150 200 250 300Length

No

of H

olds


NB No of Bulkheads = Holds +3

Self unloader

Figure 3.3 shows a plot of number of holds against length. There are two distinct groups, tankers that have much greater subdivision to limit outflow, and bulk carriers and cargo ships where the aim is minimise the number of hatches to optimise cargo handling.

Figure 3.3 - No of holds as a function of ship length.

The minimum number of bulkheads to be fitted is defined in the classification rules, but the classification societies do not seem to take a consistent approach. LR. DNV and BV have a tabular statement, whereas GL and ABS state that the minimum number of bulkheads required is that to meet the relevant subdivision requirements for each ship type. For lengths less than 145 metres the LR, BV and DNV tabular requirements are somewhat inconsistent, as shown in figure 3.4, below.

3.2 Reserve buoyancy of small cargo/bulk/container ships

In order to get a better comparison a more detailed calculation was made to compare the reserve buoyancy to the upper deck of small general cargo ships with bulk carriers. However, unlike 7 and 9 hold bulk carriers, all the smaller vessels have poops and forecastles with weather tight closures, and this increases the reserve buoyancy. The result of this comparison is shown in Figure 3.5, plotted on the basis of draft/depth ratio.


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0

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6

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50 100 150 200Length

No

of B

ulkh

eads

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All

LR

BV, DNV

LR DNVBV

30%

35%

40%

45%

50%

55%

60%

65%

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0.66 0.68 0.7 0.72 0.74 0.76 0.78 0.8 0.82Draught/Depth Ratio

Res

erve

Buo

yanc

y %

Bulk CarriersCargo to Main DeckCargo incl Superstructures

Figure 3.4 Class requirements for number of bulkheads are inconsistent

Figure 3.5 � Comparison of reserve buoyancy of cargo ships and bulk carriers


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20%

25%

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45%

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0 1 2 3 4 5 6 7 8 9 10No of Holds

Res

erve

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y

Bulk CarrierCargo incl. superstructures

The small cargo vessels tend to have an increased draft/depth ratio in comparison with bulk carriers and corresponding lower reserve buoyancy to the main deck. This is due to the effect of the greater effective length of superstructures. The poop and forecastle increase the reserve buoyancy significantly. Figure 3.6 compares the reserve buoyancy of the two ship types plotted on the basis of no of holds, and shows that the reserve buoyancy of small cargo ships is of a similar magnitude to bulk carriers with single compartment subdivision.

Figure 3.6 - Reserve buoyancy of small cargo ships appears inadequate in comparison with bulk carriers

It appears therefore that the subdivision standard for these smaller vessels should be the subject of greater concern.

3.3 Does probabilistic damage stability provide an illusory standard of safety?

The probabilistic stability requirements as specified in reference 6 have to be met by cargo ships over 100 metres in length built after 1st February 1992 and cargo ships of 80-100 metres in length built after 1st July 1998. The regulations require all vessels to achieve a required subdivision index, which is a function of length, as shown in figure 3.6. The achieved index is the summation of the product of the probability that the compartment or group of compartments will be flooded and the corresponding probability of survival from damage stability calculations. The regulations allow account to be taken of subdivided side tanks to be used singly, and in combinations to achieve the required index, and also the range of drafts at which the ship operates.


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Small ships covered by these regulations are predominantly of double skin construction. Experience to date indicates that, within the cargo hold length, single hold ships depend solely on the side tank subdivision to achieve the required index, and that two hold ships can obtain a contribution from flooding in the centre hold in the light condition only. For each type of ship, hold flooding when at the load draft is seldom survivable. Indeed it is obvious that some builders are designing vessels to just satisfy the required subdivision index without any margin. They are the modern equivalent of the �paragraph� ships of yesteryear. Commonly to increase the inherent reserve buoyancy and achieve a higher working platform, these vessels include deep hatch coamings to form trunks, with access to forward working areas at an appropriate level. Unfortunately as yet there is insufficient published data for ships in the 80-100 metres length, meeting the probabilistic damage stability requirements, to assess the means by which the regulations can be met.

Figure 3.6 - Required index of subdivision for probabilistic stability assessment

As part of the bulk carrier FSA study, the Cyprus Bureau of Shipping have been looking the application of SOLAS Chapter XII and other risk control options to bulk carriers less than 150metres in length and over 500 tons gross. Reference 7 provides a hazard analysis and risk assessment of these small ships and identifies inadequate stability as a major cause of fatalities, whether due to collision, foundering, or structural failure.

It is therefore questionable whether the current probabilistic damage stability requirement is an adequate standard for this type of ship. It is noticeable that the method does not take into account the probability that a particular vessel type is involved in a collision, as the data was not readily available when the method was being developed. The HARDER Project is reviewing the robustness of the method and the validity of the probabilistic assumptions and involves detailed calculations on a large number of ships, validated by model tests on a number of ships. It is understood that the starting point for the project is the assumption that a collision has occurred and that the output will be updated and validated assumptions of the distribution and extent of damage. Early results, reported in reference 8, confirm that the original assumptions for damage location, length and penetration are reasonable.

0.3

0.35

0.4

0.45

0.5

0.55

0.6

80 100 120 140 160 180 200Length

R


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However reference 7 shows clearly that collision is not the main initiating event leading to fatalities, and the impact of water ingress from other initiating events needs to be considered when framing regulations. It is suggested that any revised regulations be subjected to FSA to assess the reduction in fatalities from its introduction.

An alternative approach might be to identify those damage scenarios, which have a probability of occurrence greater than a specified value, which a vessel must be able to survive in order to avoid an unacceptable level of casualties, and then use a deterministic approach to evaluate whether the vessel will survive.

4 The importance of the distribution of reserve buoyancy

As mentioned in an earlier section the inclusion of sheer, superstructures and trunks are a means of enhancing the reserve buoyancy of ships operating near minimum freeboard. Recent work by the Chinese has highlighted the importance of buoyancy distribution by bulk carriers (reference 9), and the same is true of other ship types where freeboard is reduced to achieve minimum tonnage.

Detailed calculations have therefore been carried out to assess the distribution of buoyancy of four ships of with 1 or 2 holds, follows:

• Ship A - a two hold ship of 128 metres in length, for which seakeeping calculations

have been reported (Reference 10)

• Ship B � a two hold mini bulker built by a UK shipyard for an overseas owner.

• Ship C � a typical feeder containership, 100 metres in length, with minimal freeboard and several tiers of containers on deck, the type particularly criticised by the Vossnack group.

In addition to generating buoyancy distribution, an assessment was made of the impact of the Chinese proposals for freeboard (reference 11).

4.1 Ship A � a 9000 tdw multi purpose cargo vessel.

This design was used as a basis for a series of experiments at MARIN to determine the impact of various design changes on green water incidence and loads. A ship with identical dimensions, displacement and arrangement was identified in Reference 5, from where the general arrangement is included as Figure 4.1 below.

Figure 4.1 - Ship A �General Arrangement


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The ship has the following principal particulars:

Length OA 134.5 m Length pp 127.2 m Breadth 16.5 m Depth to main deck 9.8m Draught SWL 7.12m Displacement 12,164 tonnes No of Holds 2 Sheer Nil Superstructure Forecastle, Poop

Figure 4.2 shows the reserve buoyancy distribution for the vessel, as a proportion of the reserve buoyancy area at midships, as shown by the following relationship:

Reserve buoyancy area = {breadth x (depth-draught)}

Figure 4.2 � Ship A Reserve Buoyancy Distribution.

The benefit achieved from the poop and forecastle is self evident, and the effect of substituting standard sheer for the superstructures is also shown. Note that standard sheer actually reduces the freeboard amidships, but the overall effect appears less effective at protecting the ends of the vessel. Figure 4.3 presents the residual buoyancy in the forward half in the same format as reference 10, and indicate that the standard sheer apparently has the same benefit forward as a forecastle, but figure 4.2 clearly shows that it is the distribution of buoyancy within the forward 0.25L rather than the total reserve buoyancy, which is the real benefit.


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Figure 4.3 Reserve buoyancy forward

The application of the Chinese proposal for freeboard in reference 10 results in an increase in freeboard of 126mm (4.6%) and a loss of deadweight of about 237 tonnes.

4.2 Ship B - a 7200 dwt mini bulker

Figure 4.4 Ship B � General Arrangement

Figure 4.4 shows the general arrangement of the vessel from reference 5. The ship has the following principal particulars.

Length OA 99.95 m

Length pp 95 m

Breadth 17 m

Depth to main deck 8.70 m

Draught SWL 6.75 m

Displacement 9502 tonnes

No of Holds 2

Sheer Nil

Superstructure Forecastle, Poop, Continuous hatch coaming

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This ship has a continuous hatch coaming, which could be used as a trunk. Discussion with the shipbuilder indicated that they considered this as an option when the ship was designed, and had identified a route for the forward access, but it was not implemented for various reasons. Calculations have been made to confirm the original freeboard and to investigate the impact of designating the deep hatch coaming as a trunk of 1.676m in depth plus a 0.6m deep hatch coaming. The resulting reduction in freeboard could then be taken as an increase in deadweight or the depth could be reduced to reduce gross tonnage. The results of the various calculations are as follows.

Change in Freeboard Effect Fit trunk/ increase draft -735mm Increase displacement by 1153 tonnes Fit trunk/reduce depth Nil Reduce depth by 1.066m and GT by

52.4(1.096%) Apply Chinese proposal +269mm Reduce displacement by 422 tonnes

The resulting reserve buoyancy distribution is shown in Figure 4.5 below. The beneficial effect of the superstructures in comparison with standard sheer is again shown. In this case the figure also shows that part the buoyancy, which is derived from a poop and forecastle at the standard superstructure height and that part which is derived from the full height. It is noted that the standard height of the superstructure is significantly less than current practice, and it is suggested that, as in 1966, an increase in the standard height of superstructure is also justified.

Figure 4.5 � Ship B Reserve Buoyancy distribution

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The beneficial effect of the trunk/hatch coaming is also shown, and this seems justified, unless there are any down flooding points at a lesser angle of heel.

Figure 4.6 � Ship B � Reserve buoyancy forward

Figure 4.6 shows the reserve buoyancy of Ship B in same format as Reference 10, and demonstrates again the benefit, which arises from the deep hatch coaming.

4.3 Ship C � A single hold bulk/general cargo ship

Figure 4.7 Ship C General Arrangement

Ship C is a single hold general cargo/container ship, with minimum under deck volume and capable of carrying up to seven tiers of containers on deck. The vessels principal particulars are as follows:

Length OA 100.5 m

Length pp 95 m

Breadth 20.4 m

Depth to main deck 11.1 m

Draught SWL 8.2 m

Draught Design 6.25m

Displacement 12,023 tonnes

No of Holds 1

Sheer Nil

Superstructure Forecastle, Poop

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The vessel also has a second deck at a depth of 8m forward of the engine room bulkhead and a depth of 8.2m aft of this bulkhead. The side tanks are 2.55m wide (0.125B) and the double bottom depth is 1.40m. The vessel has two 150 tonne cranes on the starboard side, and the accommodation is arranged at the extreme aft end to maximise the on deck container capacity. Although more sophisticated than the typical single hold short seas trader, the basic hold and side tank arrangement is the same, and therefore poses similar problems with regard to the consequences of the hold flooding.

The calculated minimum geometric freeboard, based on the main deck as the freeboard deck, gives a draft about 0.44 metres deeper than the quoted SWL. Theoretically it would have been possible to use the t'ween deck as the freeboard deck and taking advantage of a full superstructure correction, a draft somewhere between the existing SL and design drafts would have been obtained. The Chinese proposal would have resulted in a freeboard some 500mm greater than that calculated from ILLC66, giving a draft about 0.1m less than the existing SWL. Figure 4.8 shows the distribution of reserve buoyancy along the ships length, and Figure 4.9 shows the reserve buoyancy in the same format as reference 10.

Figure 4.8 � Ship C � Distribution of Reserve Buoyancy.

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Figure 4.9 � Ship C Reserve Buoyancy Forward. 4.4 Results

The following table summarises the resulting reserve buoyancy of the three vessels in comparison with the large bulk carriers previously studied (reference 12) and with the Chinese data from Reference 9.

Reserve Buoyancy Ship Freeboard No of Holds

1.0L 0.5L 0.25L

Ship A B 2 58.00 28.44 14.97

Ship B B 2 45.30 22.00 11.72

Ship C B 1 58.67 28.92 14.64

Ref 14 � A Capesized B-60 9 45.75 22.55 10.56

Ref 14 � B Panamax B-60 7 46.5 23.76 12.12

Ref 14 � D Handy sized B 5 53.68 26.87 12.89

Ref 17 Capesized B-60 9 42.05 22.44 9.68

This comparison is shown graphically in Figure 4.10, from which it can be seen that Ships A and C plus the Handy sized bulk carrier have reserve buoyancy above the B-60 freeboard ships. Ship B seems to have a comparatively low level of reserve buoyancy, and it compliance with any damage stability requirement is doubtful.


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Figure 4.10 Comparison of reserve buoyancy between small and large bulk carriers.

These results confirm the concerns raised in the previous section. 5 The application of B-60 Freeboard to ships other than bulk carriers.

It has been reported that a European administration has recently assigned B-60 freeboard to a container vessel. However it is uncertain what assumptions have been made when reviewing the associated damage stability results. The regulations (Reference 3) themselves do not indicate the types of ships to which B-60 freeboard can be applied, but Murray Smith (Reference 4) indicates that �ore carriers and bulk carriers� were �chiefly in mind� when the rules for B-60 and B-100 were agreed. The Convention lays down certain conditions for B-60 ships regarding access forward and the method on which damage stability is to be assessed, namely that the permeability in the holds is to be assumed to be 0.95 when the ship is loaded to a waterline corresponding to the B-60 freeboard. Provided that all the conditions attached to the requirement for B-60 freeboard have been applied, it is suggested that this is a permissible application of the rules. However if, as has been suggested, that credit has been given for the buoyancy of containers within the hold, then this would not in accordance with the wording or the spirit of the rules. Presumably also, the administration would have to satisfy themselves that the structure is capable of withstanding the hydrostatic loads associated with one compartment flooded. It is not understood why any operator would wish to obtain B-60 freeboard for a container vessel, as they rarely get down to their marks without the addition of a considerable amount of water ballast. However it is conceivable that a multi purpose container/general cargo/bulk ship might require B-60 freeboard when it is operating in a bulk cargo mode, but this must be assigned on the assumption of a hold permeability of 0.95.


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6 Conclusions and recommendations. 6.1 This study is concerned with changes to the method of calculating freeboard only

The 1966 International Load Line Convention covers much more than the calculation of freeboard, and in considering the suggested changes below, account needs to taken of the impact on other aspects of the convention, such as access forward, closing devices, hatch cover loads etc.

6.2 Some aspects of the current freeboard calculations are no longer relevant to new ship

designs

It is shown quite clearly that a number of simplifications could be made to the existing method of calculating freeboard, such as:

1. Type A freeboard could be reconsidered � tankers are no longer limited by deadweight.

2. The tabular freeboard could be increased to apply to vessels with zero sheer. 3. The corrections for block coefficient and depth could remain unchanged. 4. The revised sheer correction would give reductions where significant sheer is

adopted. 5. The standard height of superstructures might be increased to

around 2.5 metres to reflect current practice, and consideration could be given to increasing the standard height of a raised quarterdeck to be the same as that for superstructures in general. Alternatively a lower single height reflecting modern single quarterdeck height could be considered. The effect of length on the standard height should also be reviewed to reflect current practice. A single table of percentages for E/L less than 1 (line BI) could be produced.

6. Reduction in the freeboard could offered to ships meeting the current one and

two compartment subdivision standard currently applied to B-60 and B-100 ships, except that the residual freeboard and dynamic stability after damage could be increased to more realistic levels to reflect recent changes elsewhere. This reduction would be applied after the basic freeboard has been calculated from steps 1-5 above.

7. Minimum bow height can be checked using existing or revised formula, as

appropriate.

The above assumes that the basic philosophy behind the freeboard regulations remains unchanged and does not take into account recent work undertaken by the Load Line Working Group at SLF.


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6.3 The need for changes to the tonnage convention should be kept separate from changes

to the load line convention

Whilst there is evidence that designers are exploiting the existing load line rules to minimise gross tonnage, it is suggested that other changes to achieve a more direct link between freeboard, subdivision and survivability are likely to have more effect in reducing fatalities. It is noted that such changes are likely to increase reserve buoyancy, leading to an increase in enclosed volume and gross tonnage. This may lead to a demand to discontinue with tonnage as an undoubtedly inappropriate means of charging port dues. In passing it is noted that the recent changes to regulations for tankers must have led to an increase in enclosed volume and hence gross tonnage. This appears to be accepted by the industry as a side effect of the increase demand for safety and environmental protection. It is possible that ship designers are over emphasising the importance of reduced tonnage, or using it as a means of reducing shipbuilding costs.


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References

1 Report of the Committee appointed by the Board of Trade to advise on the Load Lines of Merchant Ships and the Carriage of Wood Goods. 1915

2 International Load Line Conference 1930 � Report of Committees and Proceedings, Volumes I and II

3 International Convention on Load Lines, 1966. IMO Sales number IMO-710E

4 Murray Smith, D.R.: �The 1966 International Conference on Load Lines� Trans RINA Vol. 111 1969 p1

5 Significant Ships of 1993 - 2000, Published annually by the Royal Institution of Naval Architects, London

6 Subdivision and Damage Stability of Cargo Ships of 80m in Length and Over � MSN1715 (M) � MCA 1999.

7 Work Package 6A � Hazard Analysis and Risk Assessment � CBS Marine and BMT RCL November 2001

8 SLF44/INF.11 Development of revised SOLAS Chapter II-1 Parts A, B and B-1 � Updated Statistics for extent of damage. Report from the research project HARDER. Submitted by Denmark, Germany, Norway and the United Kingdom, 13 July 2001

9 SLF44/4/4 Revision of the Technical Regulations of the 1966 LL Convention � Analyses and Suggestions for Reserve Buoyancy of Forward Parts of Bulk Carriers, Submitted by China 31 May 2001.

10 Kapsenberg GK & de Kat JO �Effects of Freeboard and bow height on green water loads for a general purpose cargo ship�, Osaka Colloquium on Seakeeping, OC2000, Osaka Oct 2000

11 SLF 41/6/1 and SLF 41/INF.2 Revision of Technical Regulations of the 1966 LL Convention- Proposals based on the results of study on reviewing freeboards of ICLL

12 Annex to UK Briefing paper on Chinese proposals in SLF 44/4/4, August 2001

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international maritime organization e · pdf file1 a historical review of freeboard...

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