Tanker StructureandHull Failure Strength
Hajime Kawano and Masaru Hirakata,National Maritime Research Institute, Japan.

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Contents;
１．Introduction
１−１. Large-scale oil spill accident by tankers
１−２. IMO rule movement on tanker structure
２．Aging effect of ship hull
２−１. Typical strength degradation by aging
２−２. Hull plate corrosion data properties
２−３. Degrading of longitudinal bending strength
３．Failure strength of aging tanker hull
３−１. Basic mechanism of large-scale hull failure３−２. Case study
４．Conclusion

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ＰＡＪ-２００３Symposium21-1．Large-scale oil spill accident by tankers

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31-2. IMO rule movement on tanker structure
History in VLCC structural changes (1)NMRI

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41-2. IMO rule movement on tanker structure
History in VLCC structural changes (2)NMRI

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51-2. IMO rule movement on tanker structure
Enhanced Survey Program on tanker structure

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61-2. IMO rule movement on tanker structure
Phase out of single hull tankers

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72. Aging effect on ship hull

2-1. Typical strength degradation by aging(1) Corrosion
a. Corrosion in frame member
b. Corrosion in platingc. Local corrosion(2) Fatigue crack(3) Degradation of paint coating

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82-1. Typical strength degradation by aging(1)(1) Corrosion
a. Frame corrosionb. Plating corrosionc. Local corrosion(2) Fatigue crack(3) Coating degradation

Corrosion wastage in deck longitudinal of WBT, with poor
fillet weld and sharp edge at depth end. (aged 15 years)NMRI

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92-1. Typical strength degradation by aging(2)(1) Corrosion
a. Frame corrosionb. Plating corrosionc. Local corrosion(2) Fatigue crack(3) Coating degradation

Corrosion in bottom plating ;1) horizontal/vertical plating2) splashed zone or not3) effect of fluid velocity4) effect of high temperature, etc
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102-1. Typical strength degradation by aging(3)(1) Corrosion
a. Frame corrosionb. Plating corrosionc. Local corrosion(2) Fatigue crack(3) Coating degradation＊ pitting corrosion＊ raised by high stresses＊ grooving corrosion , etc.
Typical local corrosion on stringer: below

grooving corrosion along
fillet weld of deck longl.
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112-1. Typical strength degradation by aging(4)(1) Corrosion
a. Frame corrosionb. Plating corrosionc. Local corrosion(2) Fatigue crack(3) Coating degradation

Fatigue crack at side longitudinal,
in 2nd generation VLCC damages.

Fatigue crack growthat fillet welded corner.
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122-1. Typical strength degradation by aging(5)Stress vs. strain curveof aging plate;a) Cut-out and flushedspecimen shows no lessability to virgin plate.
b) Apparent drop in S-S
curve for aging plate isby surface roughness
due to corrosion.

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132-1. Typical strength degradation by aging
Degradation tendency with increase of ship age(a) Trend in degradation modeNMRI(b) Trend in number of failures

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2. Aging effect of ship hull
2-2. Hull plate corrosion data properties
Corrosion rate analysis by using class NK database

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2-2. Hull plate corrosion data properties（２）−example for deck structure−

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16Allowable diminution Level by Class Society spec.

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172-2. Hull plate corrosion data properties
Schematic diagram on aging ship strength

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2. Aging effect of ship hull
2-3. Reduction in mid-ship section modulus
Estimated results on average tendency ofthe VLCC mid-ship section modulus;(1) IMO requirement :within 10% loss of ZNote:◆◇ analyzed---- imaginary scatter

1.02
1Z̲act./Z̲built(2) Average corrosion
damage is within
IMO requirement.

Z at Deck( Cumulative Prob. : 50% )

0.98
0.96
0.94

Double Hull Tanker
Single Hull Tanker

0.92
0.9
0510

15

2025

Service Year

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3. Failure strength of aging tanker hull
3-1. Basic mechanism of large-scale hull failure

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203-1. Basic mechanism of large-scale hull failure
As to hull break-up mode
Trigger element for tanker hull break-up；(1) Buckling/collapse at Deck structure in Sagging →◎(2) Crack propagation at Bottom structure in Sagging(3) Crack propagation at Deck structure in Hogging →○(multi-site damage)(4) Buckling/collapse at Bottom structure in Hogging∵ i) break-up occurs in high wave；Sagging M.> Hogging M.
ii) deck back surface is the most severe corrosive spacein hull circumstances, and so forth.

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213-2. Case study

Outline of the Nakhodka casualty
Date: 1977.1.02, 02:40am
Location: Okino-shima NNE 106km
wave condition: H1/3≒8m, Tave≒9 sec

Fr.137 Fr.153
Failed and broke in twoNMRI

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223-2. Case study

Loading pattern at the Nakhodka casualty
Loading pattern at the casualty ;excess to a standard loading patternvalues : Load (in kl) at the casualty（）indicates a standard condition.

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233-2. Case study

Corrosion wastage at the Nakhodka casualty
Measurement result ;20-35% of plate thickness reduced due to corrosion

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243-2. Case studyApplied force at the Nakhodka casualty
VBM and VSF were obtained by using non-linear ship motion
and response simulation software.

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3-2. Case study

Simulation cal. on ultimate collapse of Nakhodka
Simulation result showed ;the break-up started at the deck structure on about Fr.153.

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263-2. Case study

Estimated results on load and strength

Causes of the Nakhodka casualty;(1) Excessive corrosion made the Nakhodka’s vertical bending
strength about one half to that of as built.
(2) So, the most severe wave load in a year at Japan sea, let
her broke up.
(3) In addition to the above, the non-standard loading patternat the accident had enlarged the wave load.
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4. Conclusions(1) Large-scale oil spill from tankers were not yet exterminated.
And one critical factor must be hull excessive corrosion thatmight be overlooked, so that it should be strongly required
strict implementation of the ESP and excluding sub-standardtankers.
(2) From the analysis of corrosion measurement data at theclassNK inspections, not only average wastage rate but also
increase of standard deviation of the rate are key factors tounderstand the ship ageing and the influence.
(3) As to hull breaking up, it seems that excessive corrosion andsevere wave condition are two main players and a possible
trigger failure might be a buckling/collapse of deck structure
at the time of high wave of sagging.
In anyway more actions are necessitated, not only to prevent
casualties but also to mitigate the oil outflow and the damageof the ocean, to keep our global environment clean.
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