1

The Effect of Tonnage Measurement Regulation

on Ship Design and Safety

Capt.Ahmed Hamdy

Collage of Maritime Transport and Technology

مستخلص

اتفق المجتمع البحري ان يكون هناك معيار موحد لتحصيل لذلك ابععالم و تالتجارة عبر البحار تنقل من ميناء الى الاخر حول ال

رسوم الميناء والرسوم الادارية من السفن طبقا لحمولة السفينة. ففي القرن التاسع عشر اقرت المنظمة البحرية الدولية نظام

حمولة كلية تين للسفنحمول دقيق وموحد لحساب حجم السفن وفراغاتها ونتج عن ذلك المعاهدة الدولية للحمولة والتي اقرت

مصمموا السفن في تغيير تصميم السفينة أيناء والرسوم الادارية. لذلك بدتحسب رسوم المتطبقا للحمولة الكلية وحمولة صافية و

لتقليل الحمولة الكلية بغرض تخفيض الرسوم وزيادة فراغات السفينة المخصصة للبضاعة لتحقيق ربح اكبرلملاك السفن عن

ش طريق اختزال بعض المكونات الهامة فى بناء السفن التي تؤثر علي اتزان السفينة في أعالى البحار. فمن هذا المنطلق تناق

السفن الكبيرة فى الحجم ةن علي سلامة واتزان السفن و خاصتصميم السفثار المترتبة عن هذا التغيير في هذة الورقة البحثية الأ

.مثل سفن الحاويات و سفن ناقلات النفط

Abstract

Shipping industry is running from port to port all around the world. So ships have to pay

charges such as: port fee, administrative charges and other dues, so the maritime community

decided to have one scale to collect these charges upon the tonnage measurement. Finally, in

1959 the IMO started to discuss adopting international standards for tonnage measurement

system to have an accurate measurement for the volume of the ship and its size. As a result, the

International Convention on Tonnage Measurement of Ships was adopted by IMO in London in

1969.

According to this convention, any ship must have gross and net tonnage, which the ship

has to pay the port dues depending on the gross tonnage and the bigger the gross, the higher the

port fees. Therefore, the navel architects changed in the design of the ships to maximise the

profit and minimise the gross tonnage for the benefit of the ship-owners. For such changes, this

paper will discuss the effect of tonnage measurement regulation on ship design and safety, as

well as the effect on the longitudinal strength, stability of the ship, freeboard, reserve

displacement and free ports.

2

1. Introduction

IMO has expressed concern about the adverse effect of gross tonnage (GT) reduction

that enables ship designers to minimize the volume of enclosed spaces above the

waterline such as freeboard, superstructures, deckhouses, sheer, hatch coamings and

hatch covers (IMO, 2007a) and crew accommodation (IMO, 2003b). Moreover, open

top container (IMO, 2007b) and smaller depth design (Grey, 2002) are possible ways to

reduce GT. Consequently, they will influence the design and ship safety which cause

loss of life and property. This paper will discuss the impact of the above methods to

reduce GT and how to lessen their negative effects.

2. The design and ship safety parameters

2.1 Longitudinal strength and ship safety parameters There are at least two factors that influence longitudinal strength: Length/Depth ratio

(L/D) and superstructures. L/D ratio contributes to longitudinal strength (Dokkum, 2008),

when a ship experiences hogging, sagging forces regularly and cargo weight. Depth has significant

effect on ship’s stress. Stress distribution in a beam can be calculated as follows (Smith, 1981).

P = M x y P = Stress at distance y from neutral axis, N/m2

I

M= Bending moment, N.m

I = Second moment of area, m4

Figure 1 illustrates the relationship of the transverse bending stress among the depth of the ship.

As a result any increase in number of stress along the ship will increase deflection and lessen

longitudinal strength which may damage the propeller shafts, pipes, ceilings and other structures

may collapse. Accordingly, the largest L/D ratio will increase deflection and decrease

the longitudinal strength.

3

D

Figure 1. Relationship among D, y and h

to calculate stress

Source: (Rawson & Tupper, 2001)

y1

y2

The adverse effect of lower depth can be minimized by strengthening flanges to

withstand stress especially at amidships such as sheer strake, bilge, bottom and shell

plates which suffer maximum bending moment than other parts. It will maintain the

section modulus which resists bending moment and enables proper transmission of the

shear forces (Schenekluth & Bertram, 1998).

Non-effective superstructures, for example reduced

superstructures, will not support longitudinal strength

because they are not only inadequate in length, but also

they have smaller scantlings than main hull and use

light materials to decrease LWT.

However, superstructures extending at least 0.15L

within 0.4L amidships considered to contribute to

the longitudinal strength (Bureau Veritas, 2006).

Consequently, they must have continuous structures

from main hull with equal scantling and strength

(Figure 2). Figure 2.Good and bad superstructures arrangement.

Source :( Chalmers, D.W.1993)

4

2.2 Stability and ship safety parameters

Hatch coverless container is designed to reduce GT because cargo holds are not

enclosed spaces, therefore they exempt from TM 1969 regulation. Ship carries

containers with several layers on deck may arise two problems at least: centre of gravity will rise

and the influence of wind on the ship stability. When the ship is in rolling motion, sea water

will reach cargo hold easier because of coverless. The possibility of hatch coverless container

experience free surface higher than hatch cover container.

Ship carries heavy containers at top layers will rise G position which makes ship unstable. The

arrangement of containers is essential to lower G position, therefore metacentric height (GM)

will reach optimum position. GM affect on GZ (righting lever length) which will

produce stability moment to return ship at upward right position after rolling. Lower GM,

which is caused by rising G position, will decrease stability moment, therefore ship

capsizes at small heel angle. GM should large enough to minimize the possibility of a

serious list under pressure from strong beam winds (Goldberg, 1988). Indeed,

combination between strong wind force which pressures containers above decks and

the lack of securing arrangement causes container loss overboard and influence ship

stability. Consequently IMO (2002) recommended initial GM for all ships should not be

less than 0.15 and the maximum GZ should be at least 0.042 cm for container ship

with L > 100 m.

2.3 Reserve Displacement and ship safety parameters

Reserve displacement or reserve buoyancy is “the volume of watertight hull above the

load waterline” (Cleary & Ritola, 1980) (Figure 3). Enclosed superstructures, trunks,

freeboard and sheer are categorized into reserve displacement. Reserve buoyancy supplies

extra buoyancy required when extra weights are carried. When weights are loaded on board,

the displacement and draft will increase but reserve buoyancy and freeboard will decrease.

It means that the amount of reserve buoyancy will be replaced by the same amounts

of weights are loaded.

5

Figure 3. Reserve buoyancy.

Source: (Jِnsson, J.A. 2008) However, ship designers tend to make many openings in those spaces to reduce

tonnage that will lead to unsafe condition. Inadequate reserve buoyancy will causes

ship cannot carry weights more than her own weight. IMO (2008) reported that the

largest deficiencies, which found for capsized bulk carriers, was 25%-30% less reserve

buoyancy in their spaces. Consequently International Convention on Load Lines 66

reg. 18. (1) and 36.1.b stipulate watertight cover of manholes and flush scuttles

in position within superstructures other than enclosed superstructures and watertight cover

of trunk on freeboard deck (IMO. 2003a).

Omitting forecastle or sheer is not only reduce tonnage but also reserve buoyancy.

When ship in pitching motion, water reach over deck easier because of lower bow

height . No reserve buoyancy at the bow leads to negative effect on pitching motion of

ship because no additional buoyancy to lift bow upwards when it submerges.

Accordingly, International Convention on Load Lines 66 reg. 39. (5) requires all ships

type B other than oil, chemical tankers and gas carriers shall have additional reserve buoyancy

in the fore end (IMO, 2003a).

6

2.4 Freeboard and ship safety parameters

Freeboard plays important roles in ship safety as follows:

2.4.1 Allow dryness of deck

Higher freeboard will prevent sea water shipped on deck, therefore, deck in dry

condition. Although, water could reach on deck, the rise of chamber enables

water flows to both sides of ships and pour out through freeing ports. Dry deck

will prevent crew slip on deck and minimize water ingress through openings on

deck and superstructures. Consequently, It is not adequate to reduce the

freeboard further because a freeboard has much effect on the magnitude of

impact pressure owing to the deck wetness (IMO, 2005). Accordingly, freeboard

design has been regulated in International Convention on Load Lines 66 reg.27

to protect crews adequately from head sea (IMO, 2003a).

2.4.2 Reserve buoyancy in damaged condition

Reserve buoyancy of freeboard is provided by several watertight compartments

of hull. In casualty case, adequate reserve buoyancy enables ship afloat when

water filled a compartment. Ship may comply with one or two compartment

damage to keep afloat depends on regulation. In worst case, crews have time to

lower lifeboats or freefall boat before ship sink. Reduced freeboard will decrease

reserve buoyancy capacity which affect on ship and crew safety but the International

Convention on Load Lines 66 reg. 27 allows a reduced freeboard for Type B ships

over 100 m in length, unless the Administration is satisfied that a number of conditions

including the ship shall be able to withstand the flooding of any compartments and remain

afloat in a satisfactory condition of equilibrium (IMO, 2003a).

Furthermore, SOLAS reg. 25.1-7 requires subdivision and damaged stability arrangement

for cargo ships(IMO, 2004).

7

2.4.3 Improving intact stability

Reduced freeboard enables water on deck may cause free surface, added

weight on deck, corrosion intensively and deck wetness. The major problem of

reduced freeboard is stability performance will decrease. Comparing both higher

and lower freeboards, although they have GM, higher freeboard (47o) has

stability range larger than lower ones (34o) (Dokkum, 2008). The lower stability

range causes GZ becomes smaller, therefore stability moment will zero at 34o

and ship capsizes faster than higher freeboard (Figure 4 & 5).

Figure 4. Effect of freeboard on stability.

Source: (Dokkum, 2008)

Figure 5. Effect of freeboard on stability moment.

Source: (Dokkum, 2008)

8

IMO (2002) recommended the maximum righting arm (GZ) should occur at an

angle of heel preferably 30o but not less than 25

o. Figure 6 above, showing

lower freeboard reaches maximum GZ at 22o, therefore it does not comply with

IMO stability criteria. This case shows that higher freeboard is better than lower

freeboard in terms of stability.

2.5. Freeing port and ship safety parameters

Fishing vessels and hatch coverless containers are good example of tonnage reduction

effect on freeing ports. In general, water pours out from deck through freeing ports. In

rolling motion, reduced freeboard causes water shipped on deck leads to free surface

effect if freeing port arrangement does not appropriate. Freeing ports of fishing vessels

may blocked because of bad design such as sliding cover slips down and hinged

covers locked which prevent water goes out (MNZ, 2007).

Figure 6. Effect of reduced freeboard and free surface caused by blocking freeing ports on stability.

Source: (MNZ, 2007)

9

Free surface effect, where water trapped on deck, will rise the centre of gravity (G).

Consequently metacentric height (GM) becomes smaller then negative GM will result in

capsize (Dokkum, 2008). In figure 6 above, the curve shows the combination between

lower freeboard and free surface causes ship capsizes at smaller heel angle than ship

with original righting arm curve. At this angle, GZ = GM = negative (below x-axis) which

means ships have not stability moment to return ships to upright position. Consequently,

lower freeboard ship with trapped water on deck will reduce intact stability.

Sliding cover of freeing ports should be replaced to prevent freeing port from being

blocking. ICLL 66 reg.24. (6) requires freeing ports have bars/rails arrangement or

hinged flaps with ample clearance to prevent jamming when water penetrates (IMO,

2003a). Hinged flaps should be fitted to outside bulwark, therefore they become non

return flaps. Non return flaps enable water pours out but block water goes inside deck.

Hatch coverless container is vulnerable design because rain and sea water will enter

holds easily and create free surface effect. In general, free surface in container holds

are similar to cargo tanks in oil tankers. It will reduce intact stability as above stated in

fishing vessel case.

Several arrangements should be made to reduce free surface effect. First, ship has

freeing ports to drain water out of cargo holds above the level of ports. Consequently,

they are designed to have adequate numbers, position, and dimension which will be

assessed during sea keeping test (Hoogenboom, 1994). To prevent sea water comes

inside holds, freeing ports must have non returning flooding valves both ship sides.

Accordingly, freeing ports will reduce trim and equalizes the differences in loading when

operated during a case of unequal flooding (Hoogenboom, 1994).

Second, in ballast condition without cargoes, empty containers should be placed entire

cargo holds to minimize water accumulation in holds. Third, using longitudinal vertical

plates with face bars positioned between each row of container (Bendall & Stent, 1995).

It is not only to reduce free surface effect but also to support longitudinal strength

(Figure 7). Finally, powerful bilge pumping systems have to be installed by duplicat

10

automatic bilge pumping arrangement (Bendall & Stent, 1995), with ejectors to enable

the emergency fire fighting pumps to power these (Hoogenboom, 1994).

Indeed, SOLAS reg.21.(1.1) & (1.4) specifies bilge pumping requirement to prevent flood that

will maintain stability and survival condition (IMO, 2004).

Figure 7. Hatchcoverless of cargo hold container ship arrangement

Source: (Bendall & Stent, 1995)

11

3. Conclusion

Today there is a great trend to reduce the depth of the ships to minimise the gross tonnage

in the same way to reduce the chargers of the ports, which leads to minimise the freeboard

of the ships. As a result, when the sea situation becomes worse, some ships in this case or

small freeboard especially the container ship lose its reserve buoyancy and stability,

that raising problems. Moreover, the calculation of tonnage is very complex and

naval architects normally try hard to design the ship that will offer large deadweight

and small gross tonnage , but that will be achieved by affecting the safety and design

of the ships and make it less seaworthy.

The adverse effect of GT reduction will impact on design and ship safety including

longitudinal strength and stability. Longitudinal strength is influenced by several factors

such as L/D ratio and effective length of superstructures. Inadequate these factors lead

to larger deflection and lessen longitudinal strength. Stability performance involves

freeboard, reserve displacement and freeing ports arrangement. In many cases, GM

will lower and stability moment will reduce effectively. Reducing GT could be acceptable,

if construction and ship safety are designed adequately complied with rules and regulation.

Therefore, there is a strong demand that the port dues don’t be calculated on the tonnage

of the ship, but using another measurement such as the length of the ship, the breadth and

the draft beside the port facilities which the port provided to the ship and this idea has already

been done in some French and polish ports have started to charge the ships to LOA x B x draught

as Grey mentioned (Grey, 2002).

12

References

1. Bendall, H.B. & Stent, A.F. (1995). Hatchcoverless container ships: productivity gains

from a new technology. Maritime Policy & Management. 22 (2), 103-114

2. Bureau Veritas. (2006). Common Structural Rules for Bulk Carriers. Paris: Author.

Chalmers, D.W. (1993). Design of ship’s structures. London: HMSO

3. Clark, I. C. (200) The management of merchant ship stability, trim and strength. London:

The Nautical institute

4. Cleary, W.A., & Ritola, A.P. (1980). Load lines. In Robert Taggart (Ed), Ship design and

construction (pp-173-194). New York: The Society of Naval Architects and Marine Engineers

5. Dokkum, V.K. (2008).Ship knowledge ship design, construction and operation. Enkhuizen, The

Netherland: DOKMAR

6. Goldberg, L. (1988). Intact stability. In E.V. Lewis (Ed), Principles of Naval Architecture

(pp.63-138). Jersey City: The Society of Naval Architects and Marine Engineers.

7. Gray, M. (2002). Time to kill off gross tonnage. The Sea, (159), p.5.

8. Hoogenboom, R. (1994). The ultimate feeder? The technical feasibility of hatch

coverless feeder. SWZ maritime magazine, (1), 20-23. Retrieved February 24, 2008 from world wide web www.knvts.nl/S&W%20archief/Hatch%20coverless%20container%20feeder.pdf

9. International Maritime Organization. (2002).Code on Intact Stability for all types of ships covered by

IMO instruments . London: Author.

10. International Maritime Organization. (2003a, June 5). Adoption of amendments to the

protocol of 1988 relating to the international convention on load lines, 1966 (MSC77/26/Add.1). London: Author.

11. International Maritime Organization. (2003b, July 18). Open-top containerships

International Convention on Tonnage Measurement of Ships, 1969: submitted by The Netherlands (SLF 46/15/2). London: Author.

12. International Maritime Organization. (2004). SOLAS consolidated edition 2004. London: Author.

13

13. International Maritime Organization. (2005, July 8). Revision of the technical regulations

of the 1966 LL convention a methodology for the revision of the freeboard tables and corrections:

submitted by Japan (SLF 48/9/1). London: Author.

14. International Maritime Organization. (2007a, February 16). Development of options to

improve effect on ship design and safety of the 1969 tonnage measurement convention: submitted by Australia (SLF 50/6/1). London: Author.

15. International Maritime Organization. (2007b, February 22). Development of options to

improve effect on ship design and safety of the 1969 tonnage measurement convention: submitted by ICFTU (SLF 50/6/2). London: Author.

16. International Maritime Organization. (2008). Sub-Committee on Stability and Load Lines

and on Fishing Vessels' Safety (SLF). Retrieved February 23, 2008 from world wide web http://www.imo.org/About/mainframe.asp?topic_id=246&doc_id=1759

Lon

17. J ِ onsson, J.A. (2008). Load line survey. Unpublished lecture handout, World Maritime

University, Malmoِ, Sweden

18. Maritime New Zeeland. (2007). Freeeing ports cover on fishing vessel. Maritime New

Zeeland Guidelines. (7). Retrieved February 24, 2008 from world wide web www.msa.govt.nz/publications/safety_bulletins/issue7-mnz-safety-bulletin-february-

2007.pdf - New Zeeland: Author

19. Rawson, K.J. & Tupper, E.C. (2001). Basic ship theory. Oxford: Butterworth-Heinemann

20. Schenekluth, H. & Bertram, V. (1998). Ship Design for Efficiency and Economy. Oxford:

Butterworth-Heinemann

21. Smith, R.M. (1981). Ship & Naval Architecture. London: The Chameleon Press Limited