A Study on Common Ship Structural Failures

International Journal of Mechanical Engineering and Technology (IJMET) Volume 9, Issue 7, July 2018, pp. 746–754, Article ID: IJMET_09_07_078 Available online at http://iaeme.com/Home/issue/IJMET?Volume=9&Issue=7 ISSN Print: 0976-6340 and ISSN Online: 0976-6359

A STUDY ON COMMON SHIP STRUCTURAL FAILURES

MSP. Raju Associate Professor, Department of Naval Architecture & Offshore Engineering, AMET University, Chennai, India

A. PremAnandh Assistant Professor, Department of Naval Architecture & Offshore Engineering, AMET University, Chennai, India

ABSTRACT The present article delineates the common structural failures during the operation of ships. Fatigue cracks, buckling of panels, indents and corrosion are the most common failures in ship structures during operation. Out of these the fatigue failures play very critical part in the ship structures. Fatigue cracks occur due to cycling loading, specially, action of waves on ship structures. Buckling of panels occur due to the in plane compressive stresses. Indents are the damages due to the lateral loads on the ship structures. Corrosion causes thickness reduction of the plates and sections, thereby increases the stress in the plates and sections in ship structures. A 3C method, which was formulated to address the common ship structural failure, was explained in this paper. Keywords: Fatigue cracks, Buckling, Corrosion, Indents, Structural failure Cite this Article: MSP. Raju and A. PremAnandh, A Study on Common Ship Structural Failures, International Journal of Mechanical Engineering and Technology, 9(7), 2018, pp. 746–754. http://iaeme.com/Home/issue/IJMET?Volume=9&Issue=7

1. INTRODUCTION Ship maintenance and repairs are critical in the life cycle of Ship. One should understand the various failure modes that can occur during the ship operation for better ship hull maintenance and repairs. The various ship structural failure modes are: A. Fatigue Crack B. Buckling C. Indent D. Corrosion

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Cracks can occur due to fatigue and brittle fracture. Buckling is due to in plane compressive stresses during hogging and sagging of the ships. An Indent occurs during cargo handling. Any dent in the plate due to lateral hitting of the ship by external vessels like tugs is in Indent. Corrosion is one of the main failure modes that presents always in the maritime field.

2. CRACKS Most of the cracks that occur in ship are due to fatigue. Fatigue is the failure of a material by cyclic loading. A large number of complicated welded plate joints can be found in an ocean- going ship. During the service time of the vessel these plates are exposed to time-varying loads caused by the irregular seaway, the propulsion system, and changes in the loading conditions. Hard spots, such as connections between longitudinal stiffeners and web frames, where local rises in stress intensity can be found, may therefore be prone to fatigue cracking. Sometimes, the acceptable level of fatigue cracking is exceeded when significant structural changes are introduced in the ship designs, e.g. when high-tensile steel on a large scale was adopted in the building of the so-called second-generation of very large crude oil carriers (VLCCs). The higher structural strength of the high-tensile steel is not reflected in the same improvement of the fatigue strength of the material. Therefore, the reductions of the scantlings based on structural strength resulted in increased frequency and severity of fatigue cracking in many new tankers [1]. Dynamic stress variations experienced during the service period of a ship can initiate fatigue cracks in details which are inadequately designed, constructed or maintained. Subsequent crack propagation may cause failure of primary structural members leading to catastrophic consequences such as massive oil pollution or loss of the whole ship Thus, fatigue cracking should, if possible, be avoided or kept at an acceptable level.

2.1. Fatigue Evaluation Fatigue initiation is a localized phenomenon which strongly depends on the structural geometry and stress concentrations. In welded structures such as ships, cracks are known to initiate at stress concentrations caused by flaws from welding procedures and at cutouts and plate joints where abrupt geometrical transitions cause a local rise in stress intensity. Therefore, in cyclically loaded structures, the designer should evaluate the fatigue strength of details where high stress concentration can be found. Both simple and relatively complex approaches are available for such evaluations. They normally consist of  A load history description,  The response of the structure to the external load,  A model for combining the structural response and the material fatigue strength.  If the structure is exposed to a stochastic load, a fatigue accumulation hypothesis is also required Fatigue crack initiation and fatigue crack growth are important damage modes in ship structures. [2] Most approaches employed for fatigue life assessment are strongly related to the stress state at the crack tip. A definition of three stress measures often applied for fatigue analysis is therefore introduced. They are: 1) Nominal stress, 2) hot-spot stress, and 3) notch stress. Cracks may initiate at imperfections caused by welding and thermal cutting of edges. Residual stresses are self-equilibrating stresses existing in materials or structures under uniform temperature conditions Branched crack propagation can be found in structures

http://iaeme.com/Home/journal/IJMET 747 [email protected] MSP. Raju and A. PremAnandh containing residual stresses or in details exposed to biaxial loading, where curved crack propagation may have a significant influence on the crack growth rate. One of the major interests in residual stresses is their effect on the fatigue life of structures subjected to cyclic loading. There are two approaches for fatigue evaluation [3]: 1. S-N Curve approach and then applying the palmgren minor rule. This approach is used generally in the design stages [4]. 2. Fracture mechanics Approach. This approach is used for existing cracks propagation studies.

2.2. Fatigue cracks due to Global and Local loads Global vertical and horizontal wave bending moments give rise to longitudinal dynamic stresses in deck and bottom. Highest global dynamic loads occur for all longitudinal members in the mid ship area. The consequence of hogging/sagging movement of the hull girder may be fatigue cracks in the deck or bottom longitudinal. Figure 1 shows the sagging and hogging modes of the ship structure.

Figure 1 Sagging and Hogging Local wave loads give dynamic stress amplitudes in side and bottom. Local dynamic loads are highest in the waterline area. Consequence of the local wave loads may be fatigue cracks in side longitudinal. Fatigue life of a detail can be calculated by using formula given below: ³      Where, N= Fatigue Life = Average stress in the detail K= Stress Concentration factor C= Constant The fatigue life is inversely proportional to cubic times the average stress in the detail. If stress is increased the fatigue will be reduced. Figure 2 and 3 shows the cracks in bottom longitudinal and side longitudinal stiffeners.

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Figure 2 Fatigue crack on bottom longitudinal stiffener

Figure 3 Fatigue cracks in side longitudinal Workmanship plays very important role in fatigue crack initiation. Lack of trained welders may result to poor workmanship and leads to fatigue crack initiation [5]. A good workmanship during welding will reduce fatigue cracks.

3. BUCKLING Buckling is permanent deformation that occurs when structure is subjected to excessive compressive forces. Shear buckling occurs when structure is subjected to excessive shear forces. Hull Girder buckling occurs when the load gets too high or the steel is worn too thin due to corrosion, the deck or bottom may buckle across the entire breadth of the ship. Hull girder buckling is a sudden failure mode and may lead to total loss of the ship. When the vessel is in sagging mode, buckling occurs in the deck panels. When the vessel is in hogging mode, buckling occurs in bottom panels. Figure 5 shows the buckling failure in the bottom structure of the ship.

Figure 5 Buckling of hull bottom

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Buckling occurs in the longitudinal stiffener webs of deck and bottom due to hogging and sagging. The study of the web buckling is done by considering the elastic buckling principles [6]. Closed form methods using empirical formulations and semi analytical buckling models are used to evaluate the buckling capacity of the ship panels [7].

4. INDENTS Indents are caused by lateral forces on the structure. Yield strength of the material exceeded resulting in permanent deformation of the structure. Figure 6 shows the indents on the tank top of cargo hold due to cargo cranes operation.

Figure 6 Indents on the tank top of cargo hold Consequences of indents depend on the extent of damage caused. The permissible indent is S/12 times as per the classification regulations [8]. Where, S is the frame spacing. Figure 7 shows the severe indents caused due to lateral hits on the ship side shell structure. Figure 8 shows the permissible indent as per the classification society regulations

Figure 7 Severe consequence of indent.

Figure 8 Maximum indent depth allowed for local plates is S/12, where S= Stiffener spacing.

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5. CORROSION Most of the ship hull related repairs are caused by corrosion. Corrosion rates increases with increasing the following: I. Salinity II. Temperature III. Oxygen Content IV. Water Velocity

5.1. Types of Corrosion Corrosion may occur in several forms [9]. a. General Corrosion b. Local Corrosion c. Pitting Corrosion d. Edge Corrosion e. Grooving Corrosion f. Bacterial Corrosion g. Galvanic Corrosion

5.1.1. General Corrosion This is the most form of corrosion that occurs in holds and tanks. It takes place in all unprotected tanks in marine environments. It is relatively even distribution of corrosion. Unprotected segregated ballast tanks are particularly likely to show this form of corrosion. Figure 9 shows the general corrosion.

Figure 9 General Corrosion

5.1.2. Local Corrosion This sort of corrosion is common in coated structure with local breakdown of coating. This may have minor impact on the global stress level, but have severe impact on local strength. It may cause local cracks and buckling.

5.1.3. Pitting Corrosion It is a part of local corrosion in which limited areas are attacked. Pitting is particularly likely to occur in the bottom of cargo and ballast tanks, or other horizontal surfaces, along welds and on passive metals such as stainless steel or aluminum. The pits may be very deep and the rate of attack may be high.

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Figure 10 Pitting Corrosion

5.1.4. Edge Corrosion This is a local corrosion at edges of internal structure, normally an effect on local strength only. In areas with higher shear stress level, this corrosion might give too high shear stress level which will lead to shear buckling.

Figure 11 Edge Corrosion

5.1.5. Grooving Corrosion This is a local corrosion which occurs in Heat affected zones (HAZ) of fillet weld of side/deck plate/longitudinals. This will reduce the shear capacity for the profile.

5.1.6. Bacterial Corrosion This generally occurs in oil tanks, ballast tanks, etc. This often appears as cluster of pits. The conditions that encourage bacterial corrosion are:  Stagnant water  Adequate nutrition for bacteria  Sulphate Suitable temperature ( 20-40o C)

Figure 12 Bacterial Corrosion

5.1.7. Galvanic Corrosion When two materials are electrically coupled in seawater, the more negative (active) material in the couple will act as the anode, and will have an increased corrosion rate, the more positive component, the cathode, will have a reduced corrosion.

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Table 1 Galvanic Series Reference Electrode Ag/AgCl Graphite +0.25 Titanium 0 St.S (316) -0.05 Monel (400) -0.08 Copper Nickel (90: 10) -0.23 St.S (416) -0.28 Copper -0.33 Naval Brass -0.34 Low Alloy Steel -0.61 Mild Steel -0.66 Aluminum -0.80 Zinc -1.03 Magnesium -1.60 Due to corrosion, the plate thickness is reduced and hence, the stresses in the plate are increased. There by, causing to the failure of the structure. Corrosion health monitoring is done throughout the life time for maintain the structural rigidity [10].

5.2. Corrosion control methods Corrosion can be controlled by following one or few methods given below [11]:  Using the appropriate paints  Using Impressed current cathodic protection system ( ICCP)  Using Sacrificial anodes  By using corrosion inhibitors make the metal passive  By chemical dosing to change the local PH environment.

6. FAILURE ADDRESSING METHOD: A 3C (Cause- Consequence – Cure) method was proposed by the authors to address any failure in ship structures during operation. 1. Cause: To address any structural failure, cause of the structural failure is to be identified first. 2. Consequence: If a particular failure occurs, it has to be studied that what are all its consequences of the failure on the structure. 3. Cure: What are all the methods that can be applied to cure the failure? If we apply this 3C method to address any structural failure, a quick solution can be achieved.

7. CONCLUSION Various ship structural failure modes were discussed which should be understood thoroughly by shipbuilding and ship repair engineers before attempting to repair or suggesting a solution for the problem. Fatigue cracks occur due to cyclic loading. Proper workmanship during welding reduces the fatigue crack initiation. Clear distinction between the buckling and indent was discussed. Buckling is caused by in plane compressive stresses and indents are caused due to lateral loads. Corrosion types and their controlling methods were explained.

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REFERENCES

[1] Michael Rye Andersen Lyng, Fatigue Crack Initiation and Growth in Ship Structures, January 1998. [2] Y. Sumi et al, Computational prediction of fatigue crack paths in ship structural details, Fatigue and Fracture of Engineering Materials and structures, 28(1-2), January 2005 [3] Fatigue assessment of Ship Structures, Det Norske Veritas Classification Notes No.30.7, April 2014. [4] Espen H.Cramer et al, Fatigue assessment of ship structures, Marine Structures, 8(4), 1995 [5] A. Prem Anandh and MSP Raju, An Analysis of Indian Shipbuilding and Repair Industry, International Journal of Civil Engineering and Technology, 9(7) 2018, pp 715-723. [6] J K Paik eta al, Local buckling of stiffeners in ship plating, Journal of Ship research, March 1998. [7] DNVGL class guideline on Buckling, October 2015. [8] DNVGL Rules for classification of ships, October 2015. [9] Corrosion protection of Ships, Det Norke Veritas Recommended Practice, 2000 [10] P K Satheesh Babu eta al, Corrosion Health Monitoring system for steel ship structures, International journal of Environmental Science and Development, 5(5), October 2014. [11] P K Satheesh Babu eta al, integrated corrosion protection system for ship structures, International journal of Design and Manufacturing Technology 5(3), Sep-Dec 2014 [12] Viktor Vasilevich Ushakov, Vladimir Apolenarevich Yarmolinsky, Eduard Mikhailovich Dobrov, Mikhail Gennadjevich Goryachev, Sergey Vladimirovich Lugov, Revision of Descriptions and Calculated Properties of Road Bed Soils and Asphalt Concrete Materials upon Designing of Highway Pavements in Terms of Criteria of Residual Deformations and Fatigue Cracking of Asphalt Concretes. International Journal of Civil Engineering and Technology, 8(9), 2017, pp. 1074–1083. [13] Viktor Vasilevich Ushakov, Vladimir Apolenarevich Yarmolinsky, Eduard Mikhailovich Dobrov, Mikhail Gennadjevich Goryachev, Sergey Vladimirovich Lugov, Revision of Descriptions and Calculated Properties of Road Bed Soils and Asphalt Concrete Materials upon Designing of Highway Pavements in Terms of Criteria of Residual Deformations and Fatigue Cracking of Asphalt Concretes. International Journal of Civil Engineering and Technology, 8(9), 2017, pp. 1074–1083.

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