Hull Inspection, Damage & Repair -Part I

Hull Inspection, Damage and Repair ( Reporting and Assessing Structural Defects )  Part I

Hull defects are in many cases preventable with timely maintenance and understanding of their causes.

Understanding the typical causes of defects is equally important in finding them efficiently and repairing them in the most appropriate manner;

in other words, using a repair that will both stop the re-occurrence of the defect and is at the same time cost-effective.

Hull Damages and Defects

Overload - In case of overload the repairs will consist of crop and renew according to original plans. In the case of overload due to heavy weather, the redesign should be considered.

1. Grounding
2. Collision
3. Contact
4. Operational failure  (loading, ballasting)

Collision.

Inappropriate Design - Crack initiated at the change of section of the faceplate. In cases of inappropriate design, the structure should be re-designed to reduce the load.

1. Non-compliance with the standard
2. Ignorance of actual loads
3. Inadequate initial design

Inappropriate design.

Poor Workmanship

Poor workmanship.

1. Poor welding
2. Poor alignment
3. Sub-standard material
4. Poor finish omission

Fatigue by Vibration

1. Mechanical Source
2. Hydrodynamic Source
3. Stiffeners span too big
4. Sometimes weak superstructure

Extensive Wear and Tear (Corrosion)

1. General wastage
2. Localised corrosion
3. Localised pitting

Pitting.

Pitting corrosion, or pitting, is a form of extremely localized corrosion that leads to the creation of small holes in the metal.

The driving power for pitting corrosion is the O2 depassivisation of a small area, which becomes anodic while an unknown but potentially vast area becomes cathodic,

leading to very localised galvanic corrosion.

The corrosion penetrates the mass of the metal, with a limited diffusion of ions.

The mechanism of pitting corrosion is probably the same as crevice corrosion.

Pitting defects are generally caused by corrosion. When localised pitting is confined to the bottom of a tank, and the depth of the pitting is less than 50% of the original plate thickness, you can repair the isolated pitting's with a suitable epoxy compound according to the manufacturer's recommendations.

Isolated pitting with a depth of less than 50% of the original plate thickness can also be repaired by welding, provided that residuals thickness of remaining plate exceeds 6 mm.

 The following rules should be observed:

•The pitting must be adequately prepared for welding ( usually by grinding)

•The electrodes used must be appropriate low hydrogen grade for the steel of the bottom plating.

•No less than four runs must be deposited in each pit

 The affected plate must always be renewed when :

•The intensity  of the pitting is excessive ( above 30%)

•The pitting is deeper than 50% of the original thickness of the plate; and

•The residual thickness is less than 6 mm

Pitting intensity - diagrams

References :

1. Bureau Veritas (2006). Mini Survey Handbook Part A - Ships in Service. (First ed.). France: Marine Division Ships in Service Management ( DNS).
2. Gutierrez, M. – Retired Senior Surveyor LR, 2018. Conversation/Lecture to/with Aleksandar Pudar from 18 to 21/9/2018.
3. Lloyd's Register Marine (2014). Hull Inspection, Damage and Repair. (Third Edition ed.). United Kingdom: First published by Lloyd's Register ,2009. 
4. Lloyd's register marine (2015). Hull Inspection, Damage and Repair. (First ed.). United Kingdom: Lloyds Register.
5. Researchgate.net. 2018. “ Pitting intensity diagrams” www.ResearchGate.com [Online]. Available from: <https://www.researchgate.net/figure/Pitting-intensity-diagrams-Figure-taken-from-2_fig4_320716318 > [Accessed on : 2 October 2018]. 
6. Marchant , T, Dr.Lomas, J & Dr.Callow, L (2009). WHITE PAPER:THE FEASIBILITY OF A CORROSION RESISTANT SHIP. [Online]. (1 ed.). United Kingdom: © BMT Defence Services Limited 2009 © Amtec Consultants Ltd 2009. Available from: <https://www.bmtdsl.co.uk/media/6098421/BMTDSL-Corrosion-Resistant-Ship-Whitepaper.pdf >[Accessed on : 2 October 2018]. 
7. Tanker Structure Co-operative Forum (1995). GUIDELINES FOR THE INSPECTION AND MAINTENANCE OF DOUBLE HULL TANKER STRUCTURES. (1st  ed.). England: Witherby & Co.Ltd.
8. Tanker Structure Co-operative forum (2014). Guidance Manual for Tanker Structures - Consolidated Edition 2014. (2nd ed.). Great Britain: Witherby Seamanship International.
9. Tanker Structure Co-operative forum (1986). Guidance Manual for the Inspection and Condition Assessment of Tanker Structures. (1st  ed.). England: Witherby Marine Publishing.
10. Tanker structure co-operative forum (1992). Condition Evaluation and Maintenance of Tanker Structures . (1st ed.). England: Witherby & Co.Ltd.