Hull Inspection, Damage & Repair - Part III

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

Stress Concentrations

Stress concentrations occur at any discontinuity in the structure with an intensity related to the abruptness of the change. For convenience, areas of stress can be divided into the following seven types.

Hard Point (more than 80mm from the structure)

The hard spot (or hard area) can be defined as:

• a point (or area) locally rigid in a flexible or less rigid structural element.
• also as a point (or area) where the deflection curve of a plate is
• abruptly interrupted by the effect of a very rigid member supported by the plate.
• as a point (or area) where there is an abrupt change in rigidity.

These abrupt changes in the deflection lines induce high local stresses. In those points (or areas) there is a great possibility to find cracks.

There are 3 types of hardpoints:

Hardpoint Nº 1. 

Bracket welded to a plate. Distance to the nearest stiffener >= 80 mm Consequences > Crack perpendicular to the bracket in the plate. Also, it can be caused by a “doubler”.

Hardpoint Nº 2. 

End of a face flat in a bracket Consequences > Crack in the bracket plating

Hardpoint Nº 3. 

Different deformation, under the same loads = Change of rigidity = Hardpoint 

The brackets should not end on unsupported plating.

The corners of trunks should not end on unsupported plating

SOLUTIONS:

The solution, although it may sound trivial, is to avoid or eliminate the hard points or to make a gradual change of rigidity

1) To place a small pad under the bracket toe. 

2) To extend the bracket to the next support.  

3) To place longitudinal support underneath the bracket.

4) To place transverse support underneath the bracket toe.

In other cases, we have to make the change of rigidity more gradual. Among the many possible solutions, we have to elect the best under the quality/cost criteria.

End of Bracket

The ends of brackets and of stiffeners are prone to fractures and therefore, should be examined carefully. 

We can consider three types of more frequent failures:

End of bracket Nº 1) - In this case, the welding connection of bracket or stiffener with the plating is fractured

End of bracket Nº 2) 

In this case, the bracket or stiffener breaks the support element

End of bracket Nº 3) 

In this case, the bracket or stiffener breaks the support plate.

In addition to three listed above there is also, Bracket Nº4) 

In this case, the bracket itself is broken.

SOLUTIONS for End of Bracket 

Change of Section

No1

No2

No3

In general, the solution is to modify the shape of the bracket (or to add a new one), although, sometimes, it is necessary to increase the size of the bracket.

In other cases, it will be necessary to extend the bracket to the adjacent stiffener or to incorporate a new stiffener to fix end the bracket.

Change of Section

Change of Section –Solution –Fit bracket

Change of Thickness

Solution – Insert an intermediate thickness 

Diminish the thickness difference and make the transition as gradual as possible, in the order of 3:1 or 4:1. 

Insert an intermediate thickness plate when the difference in thickness is more than about 1.5-2 times. Otherwise, improve the transition to 4-5: 1 by chamfering *

*A chamfer is a transitional edge between two faces of an object. A form of a bevel, it is created at 45° angle to two adjoining right-angled faces. A lark's tongue is a chamfer which ends short of a piece in a gradual upward curve, leaving the balance as a right angle.

Openings

Case 1

The solution for the photo above is to close the opening.

Case 2

The solution for case 2 – Increase the radius

If a flange is present on the opening ( as above) increasing the flange thickness in addition to increasing the radius will also reduce the stress.

Misalignments

Solution 1- Full penetration weld

Changing the fillet weld to a full penetration weld ( allows better continuity of the stresses through the misalignment.

Solution 2- Thicker insert

A thicker insert will also allow for better continuity of the stresses through the misalignment. Care should be taken to chamfer the ticker plate

to minimise the effect of the change of thickness.

Solution 3 Re –align

Re-alignment of structure in existing ships is never recommended as it will require a great deal of work to disconnect adjoining structure, and will induce other misalignments in the vicinity

Full penetration weld and a thicker insert will always be easier and cheaper.

Three Planes

(crack always on fillet weld)

When the three planes meet, such as at the intersection of a longitudinal and transverse bulkhead with a platform /stringer deck, there is always the possibility of the stress concentration arising at the point where they intersect. This can lead to fractures, although the type of damage depends on the size and type of the ship.

Solution 1 – Fit bracket (no scallop) in line with the plane on the other side

Solution 2 – Close scallops ( where the bracket is already fitted)

Difference Between Hard Points and End of Brackets

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.

Hull Inspection, Damage & Repair Part II

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

Considerations When Dealing With Damages

Fatigue

1. Alternate or cyclic loading
2. Loads lower than breaking load and yield stress

The material fatigue can be defined as the failure under alternating fatigue load or the propagation of a fracture due to a load of a cyclic nature.

Items of structure that form part of a ship may fail due to service loads and associated stresses, which are much smaller than the maximum breaking loads and stresses they were designed to withstand.

Fatigue

These loads are either cyclical or alternating in action.

In all types of fatigue fracture, crack start because of the concentration of localized stress in the component of the structure.

The fatigue curve

The fatigue curve shows the number of fatigue cycles at given stress before an item fails.

By reducing the stress in a location below the fatigue limit it should not fail again in the service life of the ship.

If a fatigue failure occurs early in the life of a ship the remedial action must be comprehensive in order to reduce the actual stresses to blow the fatigue limit.

If a fatigue failure occurs in a location late in the ship life it may be appropriate simply to repair as per the original design.

Corrosion Under Stress

 Accelerated corrosion will occur where there are high levels of stress. Even within the same space on a ship, corrosion rates at locations of stress concentration will be greater than in those locations where the stresses are lower. This can be seen particularly where sea water is present, such as inside ballast tanks where coatings have broken down.

The “stress cycle”

The acceleration of corrosion is cyclical and self-perpetuating; it occurs because wastage from corrosion leads to more stress on the area and in turn corrosion.

An area suffering from corrosion can rapidly deteriorate if coatings are not repaired or modifications are not carried out on the structure, which will reduce the stresses present.

The stress cycle often produces fracture lines at the point of the highest initial stress. In other cases, the area around the initial stress concentration becomes corroded.

Corrosion Under Stress

Progress(ive) of Corrosion

Inspections would be simple if the rate of corrosion was linear, however, the real rate of corrosion tends to accelerate over time.

Progressive corrosion

The stress cycle applies here too and once corrosion is established, it will become progressively worse unless preventive action is taken. 

Corrosion will continue to develop when:

The coating is not in good condition

The load that the structure is carrying is high; and

There is greater humidity and or higher surrounding temperatures

Solving stress and progressive corrosion :

Recoat protective coatings to stop corrosion

Increase thickness of structural members to reduce stress

Increase the radius of openings to reduce stress

Increase the thickness of flanges used at the opening to reduce stress

Close opening to reduce stress.

Humidity and Heat

In general heat and humidity affects the tanks mostly above the load line. 

The only solution to prevent this corrosion is the adequate maintenance of the coating

Higher temperatures and humidity will increase corrosion rates in unprotected environments. 

If they are both present, the effect is compounded and the rate of corrosion accelerates rapidly.

Areas on the ship that are most prone to increased heat and humidity  ( and therefore to accelerated corrosion) are the tanks above the waterline and those next to heated spaces.

Within a tank, particular attention should be paid to its uppermost part, especially where the tank boundary is exposed to direct sunlight.

Vulnerable tanks:

• Fore and aft peak tanks
• Deep Tanks 
• Side tanks
• Tween deck tanks
• Topside tanks
• Tanks adjacent to heated fuel oil tanks
• Ballast tanks adjacent to heated cargo tanks.

Age of Ship

The age of the ship is one of the most important factors to be considered when selecting a suitable repair. The same defect may have different treatment depending on the ship’s age.

The age of a ship and how much longer a ship is excepted to stay in service, are factors that must sensibly be taken into account when assessing the seriousness of a defect and establishing an appropriate repair.

When defects are due to fatigue, then the age of the ship gives an indication of the actual levels of stress. 

Remember the fatigue curve; fewer cycles to failure usually indicate higher stresses are present in the area of the defect.

Fatigue curve occurring on a new or young ship most probably indicate that significantly higher levels of stress than were expected at the time of design are actually present.

This could possibly be due to poor workmanship at the time of build or because of poor design. 

Correcting poor workmanship, if that is established as the cause, might include correcting the misaligned structure, fitting missing brackets, or inserting the correct grade and thickness of the material.

If, however, stress concentration has occurred due to poor design, then significant modifications to arrangements may be needed such as fitting much larger and softer bracket, closing openings by fitting lugs or collars, increasing grades and thickness of the material used.

Fatigue defects occurring for the first time in an older ship, say after 20 years of service, show that although the structure has eventually failed ( such as a crack developing) there was a little wrong with either the initial design or workmanship.

The most appropriate action in such a case would probably be to repair as per the original design condition; in the case of a defect that took 20 years to develop, such repair per should ensure another 20 years of service.

When there is the damage to a ship the ship’s age gives us information about the actual levels of stress that has contributed to the failure.

Stress Concentration Factor

The stress concentration factor in a given area is measured by the ratio ( SCF) between the maximum stress and the nominal stress and the nominal stress in the surrounding structure. 

A WELD MUST NEVER BE LOCATED ON A

 PLACE OF STRESS CONCENTRATION

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.

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.

VGP – BWTS Sampling – PMS INTEGRATION

Vessel General Permit – BWTS Sampling – PMS INTEGRATION

The purpose of this article is to provide vessel owners/operators with tips and tools for meeting the sample collection and analysis monitoring requirements in the 2013 VGP by integrating the timeline into their existing PMS and reduce administrative burden to already burdened crew.

For vessels with keels laid after December 1st, 2013 we are required to carry out some additional testing as indicated in the table below. This is apart from various tests and calibrations, related to overboard discharge which is already done.

VGP required tests	Test	Period	Remarks
1. Bilgewater monitoring *	Oil and grease content	Yearly	-    The test must be done by a laboratory. -    OCM readings during sampling must be compared with laboratory results -   Maintain record as per VGP 2013 – Page 189 -    If no discharge occurs then no sampling is necessary.
2. BWTS effluent biological organism monitoring	Measure: - Total heterotrophic bacteria - E. coli - Enterococci	2 times per year (from two separate discharge events at least 14 days apart)	-    For the first year, the first test can be considered the one commissioning test from SY -    Maintain record as required by VGP 2013 – Table on Page 185
3. BWTS residual biocide monitoring	TRO (chlorite) – less than 100 µg/l (0.1 ppm)	Initial: 3 times in the first 10 discharge events (within 180 days)	-    The life of the sample is 15 minutes. The test must be done on board. Vessel needs to be provided with the necessary equipment -    Maintain record as required by VGP 2013 – Table on Page 185
		Normal Maintenance: 2 times per year
4. Greywater*	Measure: - BOD – biochemical oxygen demand - Faecal coliform or E. coli - Suspended solids - pH - Total residual chlorine	2 times per year (from two separate discharge events at least 14 days apart)	-   Maintain record as per VGP 2013 – Page 188 -   If no discharge in VGP waters occurred within the year, the monitoring tests are not necessary

*As it is not normal to discharge greywater and bilge water within 3 Nm of US Coast, the tests for this will not normally be required and so are not required to be included in your maintenance program.

Depending on your PMS type and set up it would be good to incorporate the ballast water treatment tests within it, following the above table.

Example:

Ballast Water Management System

PERIODICAL CHECKS OF SYSTEM’S APPLIANCES

JOB: EFFLUENT BIO.ORGANISM RES. BIOCIDE MON

INSTRUCTIONS:

a)      RISK MANAGEMENT:

The risk assessment must be carried out as per operating procedures if deemed necessary for the specific job.

b)      PERMITS: NORMAL WORK

c)      MAN-HOURS: 2 person x 3 hours = 6 Man-hours   

d)     MANUAL: 

EPA -VESSEL DISCHARGE SAMPLE COLLECTION & ANALYTICAL MONITORING - A How-To Reference for EPA'S 2013 VGP

e)      SAFETY PRECAUTION:

SAFETY PROCEDURE TO BE FOLLOWED AS PER COMPANY AND INDUSTRY STANDARDS. PPE TO BE USED AS PER COMPANY MATRIX. SAMPLING WILL BE DONE BY ATTENDING CHEMIST. SAMPLING AND TESTING SHALL BE CONDUCTED ACCORDING TO 40 CFR PART 136.

f)       TOOLS NEEDED:

TEST AND SAMPLING KITS WILL BE PROVIDED BY ATTENDING CHEMIST

g)      PROCEDURE:

Request attendance by an approved chemist for carrying following sampling/testing on ballast water treatment system:

1.      BWTS effluent biological organism monitoring

Measure: Total heterotrophic bacteria, E. coli, Enterococci.                              

Maintain record as required by VGP 2013 Table on Page 185

2.      BWTS residual biocide monitoring

Measure: TRO (chlorite) - less than 100 μg/l (0.1 ppm)

Maintain record as required by VGP 2013 - Table on Page 185

Note:

Prior arranging for attendance identify required sampling points for BWTS present on board.

Preferred ports are: list the preferred ports

As an alternative to attendance on board, it may be possible to provide sampling equipment for samples to be landed ashore. “

For the initial testing for BWTS effluent biological organisms, the vessels testing should be initiated manually by a person in charge but always recorded within the system. When initial testing is done vessel hast to follow PMS schedule normally.

Initially, it is good for companies to choose one company worldwide to attend the vessel to take samples and carry out the required testing.

As an alternative to attendance on board, it may be possible to provide sampling equipment for samples to be landed ashore. However, due to strict sampling procedure and short lifespan of samples that will not be practical.

For a better understanding of entire sampling and testing process refer to 

“VESSEL DISCHARGE SAMPLE COLLECTION & ANALYTICAL MONITORING A How-To Reference for EPA’s 2013 Vessel General Permit (VGP)”, 

which can be easily found online.

Bibliography:

1.      Stienstra O. & Van Der Vlies A. 2015. THE VESSEL GENERAL PERMIT IN A NUTSHELL - www.lagersmit.com [Online]. [3 October 2018]. Available from: https://www.lagersmit.com/the-vgp-in-a-nutshell/

2.      US EPA. 2013 “March 2013 - Final 2013 Vessel General Permit” - www3.epa.gov - [Online]. Available from: < https://www3.epa.gov/npdes/pubs/vgp_overview2013. > [Accessed: 3 October 2018].

3.      US EPA (2014). VESSEL DISCHARGE SAMPLE COLLECTION & ANALYTICAL MONITORING - A How-To Reference for EPA's 2013 Vessel General Permit (VGP). (Final Report ed.). Washington DC: United States Environmental Protection Agency.

       Disclaimer:

     “ Out of Box Maritime Thinker” © 2018 and Aleksandar Pudar assumes no responsibility or liability for any errors or omissions in the content of this paper. The information contained in this paper is provided on an “as is” basis with no guarantees of completeness, accuracy, usefulness or timeliness or of the results obtained from the use of this information. The ideas and strategies should never be used without first assessing your own company situation or system, or without consulting a consultancy professional. The content of this paper is intended to be used and must be used for informational purposes only.