Ultrasonic Thickness Measurement of Ships

ULTRASONIC THICKNESS MEASUREMENT OF SHIPS

SEEN THROUGH

LEGISLATION

ACTION

REPORTING

ULTRASONIC THICKNESS MEASUREMENT OF SHIPS

LEGISLATION

REASON WHY?

• The reason for this legislation/requirement is simple and obvious .

• Ships, being of steel and operating in a marine environment ,are prone to corrosion.

• Apart from very obvious cases that corrosion has led to extreme thinning of material , sometimes to actual holes on the material, the extent of corrosion is not and has not always been easy to assess with the naked eye ,even at close-up distance, especially in case of general corrosion, when it is evenly distributed over a surface.

• There are three main types of corrosion, apart from the general corrosion: - pitting corrosion - grooving corrosion and - edge corrosion .

• Therefore, in addition to the basic visual examination of ships to assess the condition of their structural elements, an additional technical method of structural element thickness evaluation had to be adopted.

REASON WHY?

REASON

WHY?

REASON WHY?

REASON WHY?

PITTING

REASON WHY?

GROOVING-HOLES IN THE MATERIAL

REASON WHY?

EDGE CORROSION

REASON WHY?

HISTORY OF LEGISLATION

• ORIGINAL REQUIREMENT - DRILLING

• It was a requirement of the Classification Societies Rules, back in the 50s, that during Special Surveys of certain types of ships the thickness of structural elements at various locations of the hull be ascertained via drilling.

• This had cost implications associated with the practical problems that created, i.e. time consumption, opening holes and closing up.

HISTORY OF LEGISLATION (associated with IACS)

It was n’ t until 1992 that IACS introduced Unified Requirements for ultrasonic thickness measurement of ships’ structures during class surveys of Oil Tankers and Bulk Carriers under

U.R. Z10.1. and U.R. Z10.2 respectively.

Since then there have been 21 revisions to U.R. Z10.1,the latest being in Jan. 2014, and 31 revisions to U.R.Z10.2, the latest being also in Jan. 2014.

HISTORY OF LEGISLATION (IACS Formation)

• IACS can trace its origins back to the International Load Line Convention of 1930 and its recommendations. The Convention recommended collaboration between Classification Societies to secure “as much uniformity as possible in the application of the standards of strength upon which freeboard is based…".

• Following the Convention, RINA hosted the first conference of major Societies in 1939 -also attended by ABS, BV, DNV, GL, LR and NK -which agreed on further cooperation between the Societies.

• A second major Class Society conference, held in 1955, led to the creation of Working Parties on specific topics and, in 1968, to the formation of IACS by seven leading Societies.

• The value of their combined level of technical knowledge and experience was quickly recognized. • In 1969, IACS was given consultative status with the International Maritime Organization (IMO). • It remains the only non-governmental organization with Observer status which is able to develop

and apply Rules. • Today it comprises 12 member Classification Societies :

The current Members are: ABS ,KR ,BV ,LR ,CCS ,NK ,CRS ,PRS ,DNV –GL ,RINA , RS ,IRS ,

• Over the years certain Unified Requirements have been agreed by IACS Members and were transposed into the individual Class Rules


• An ultrasonic thickness gauge, actually a measuring instrument, is the basis of a nondestructive testing (NDT) method that evaluates the thickness of a component by using ultrasonic waves, i.e. by measuring the time it takes for sound to travel from the transducer through the material to the back end of a component, and then measuring the time of reflection back to the transducer. The gauge then determines the thickness based on the velocity of sound through the material being tested. The standard frequency used by an ultrasonic thickness gauge is 5MHz.

HISTORY OF LEGISLATION (associated with IACS)

CURRENT REQUIREMENTS

HISTORY & CURRENT LEGISLATION

(ADDITIONAL CURRENT IACS REQUIREMENTS)

• As it was said and it is known the original IACS requirements about thickness measurements during periodical surveys referred only to single-side skin Bulk Carriers and single hull Oil Tankers, the so called ESP ships, as the requirement for an enhanced survey program applied to these types of ships.

• The latest requirements refer not only to single-side skin Bulk Carriers ,and single hull Oil Tankers, but also to double side- skin Bulk Carriers , double hull Oil Tankers, Chemical Tankers , General Dry Cargo Ships , Liquefied Gas Carriers.

• However, not all of the aforementioned are ESP ships.ESP ships are those to which the ESP Code applies, i.e. Single-side and Double side skin Bulk Carriers, Double hull Oil Tankers and Oil Tankers other than Double hull , all 500 tons gross and above.


(ADDITIONAL CURRENT IACS REQUIREMENTS)

- Single Hull Oil Tankers U.R.Z10.1

- Single-side Skin Bulk Carriers U.R.Z10.2

- Double Hull Oil Tankers U.R.Z10.4

- Double-side Skin Bulk Carriers U.R.Z10.5

- Chemical Tankers U.R.Z10.3

- General Dry Cargo Ships U.R.Z7.1

- Liquefied Gas Carriers U.R.Z7.2

HISTORY OF LEGISLATION (associated with IMO)

• In parallel, IMO introduced IMO Res. A. 744(18) , which was adopted on 04 Nov. 1993, “GUIDELINES ON THE ENHANCED PROGRAMME OF INSPECTIONS DURING SURVEYS OF BULK CARRIERS & OIL TANKERS” emanating from Res. A. 713(17) on Safety of Ships carrying solid bulk cargoes, by which it specified interim measures to be taken to improve the Safety of Ships carrying solid bulk cargoes & Reg. 13 G of MARPOL 1973 which states that Crude Oil Tankers of 20,000 tonnes deadweight and above & Product Carriers of 30,000 tonnes deadweight and above shall be subject to the enhanced program of inspections the scope & frequency of which shall at least comply with the guidelines of the Regulation.

• On 30 Nov. 2011 the 2011 ESP Code was adopted by IMO Res.A.1049(27) and it was made mandatory via IMO Res. MSC.325(90) which amended SOLAS Reg. XI-1/2 accordingly. Thus it became mandatory through SOLAS on 01.01.2014, superseding IMO Res. A.744.(18).

• It was also made mandatory through MARPOL Annex I on 1 October 2014, by the adopted IMO Res. MEPC.236(65) which introduces amendments to CAS .

• The purpose of the CAS (Condition Assessment Scheme) is verification that the structural condition of single-hull oil tankers at the time of survey is acceptable and, provided subsequent periodical surveys are satisfactorily completed and effective maintenance is carried out by the ship’s operator, she will continue to be acceptable for a continued period of operation.

HISTORY OF LEGISLATION (associated with IMO)

• It has always been recognized that the best way of improving safety at sea is by developing international regulations that are followed by all shipping nations and from the mid-19th century onwards a number of such treaties were adopted. Several countries proposed that a permanent international body should be established to promote maritime safety more effectively, but it was not until the establishment of the United Nations itself that this was realized. In 1948 an international conference in Geneva adopted a convention formally establishing IMO (the original name was the Inter-Governmental Maritime Consultative Organization or IMCO, but the name was changed in 1982 to IMO).

• As a specialized agency of the United Nations, IMO is the global standard-setting authority for the safety, security and environmental performance of international shipping.

• IMO counts 170 Member States & 3 Associate Members. It is based in U.K.


(IMO Res.A.744(18))

HISTORY OF LEGISLATION (IMO Res.A.744(18))

HISTORY OF LEGISLATION ESP CODE

HISTORY OF LEGISLATION (IMO Res.A.744(18))

HISTORY OF LEGISLATION ESP CODE


The IACS / IMO requirements about ultrasonic thickness measurement of ships were incorporated into the Classification

Societies’ Rules as these are conventionally known, soon after their adoption by the two Bodies.

Later, however, the Common Structural Rules, separate for Tankers and Bulk Carriers (CSR) were formulated and adopted by IACS and were entered into force in April 2006. The Common Structural Rules for Double Hull Oil Tankers apply to Double Hull Oil Tankers of 150m length, L, and upward classed with the Society . The Common Structural Rules for Bulk Carriers apply to the hull structures of single side skin and double side skin bulk carriers with unrestricted worldwide navigation, having length L of 90 m or above.

Soon afterwards, in view of certain drawbacks that were ascertained on applying these Rules, new common Rules were developed by IACS in a single volume, The Harmonized Common Structural Rules for Bulk Carriers and Tankers (CSR-H), which are to be entered into force on 1 July, 2015.

Both of these sets of Rules omit a specific requirement for Surveys and refer the reader to the IACS Requirements Z.10.1 , Z.10.2, and Z.10.5 which shall apply to ships that have been / will be built under these Rules. However, both of these sets of Rules (one set not yet in force) specify the way that ultrasonic thickness measurements should be evaluated and assessed against the original scantlings of the ships. This method of evaluation is totally different to the conventionally known manner of assessment of existing scantlings during surveys (based on a percentage reduction limit tolerance of the as built scantlings), which is of course due to the different philosophy of the structural design and determination of scantlings of the types of ships in question adopted in accordance with these Rules (based on the renewal scantlings as determined by the relevant formulae).

CURRENT LEGISLATION

FUTURE LEGISLATION

ULTRASONIC THICKNESS MEASUREMENT OF SHIPS ACTION

PREPARATION FOR THICKNESS MEASUREMENTS

ACTION PREPARATION FOR THICKNESS MEASUREMENTS

• Ultrasonic thickness measurement of ships has to be carried out as a manner/means to detect, monitor and hence deal with corrosion.

• There are benefits from it: 1) Ensures local ship strength is retained 2) Ensures global ship strength is retained 3) Identifies areas with corrosion which may require more frequent inspection and assessment 4) Provides data for corrosion trend analysis for various types of ships


• UTM of ships is carried out with the involvement/ co-operation of three, or may say four parties: 1. Owners’ Representative 2. Class Surveyor 3. TM Company Representative 4. Dockyard or Shipyard Representative(Not always)

• Before commencement of any TMs taken the four or better say three parties shall gather into a meeting, usually on board the vessel and plan/decide the sequence and way that TMs are to be taken.


• Each party has certain obligations emanating from its role and position : Owners’ Rep. - Arrange the initial meeting with the remaining parties - Ensure that the TM Company that he has invited on board is approved by the ship’s Classification Society - As such, first to allow access to the ship, as no one can board the vessel without the Owners’ permission - Have readily available for the Surveyor all ship documentation that is required (Survey Programme, in case of a special survey, existing ESP documentation , all necessary structural plans) for the forthcoming survey in question , and advise him of any possible damage to the ship after the last survey has been held - Ensure cleanliness of spaces that have to be surveyed/thickness measured. - Provide means of access to all spaces /places that have to be close up surveyed and thickness measured - Ensure safety of access means The means of access to be provided are : - Staging - Cherry Picker - Rafting (IACS Recommendation 39 “Safe Use of rafts or Boats for Survey (1995) last revision Mar 2009”) - Lifts and Movable Platforms - Portable Ladders depending on the survey circumstances


Classification Surveyor - Shall be exclusive to the Classification Society that the ship is classed with

- In case of ESP ships two exclusive class Surveyors are required, one of whom shall be preferably a Senior Surveyor, to jointly conduct the survey as follows: -Single-skin side Bulk Carriers equal to or greater than 20,000 tons dwt at the 3rd special and the remaining intermediate and special surveys thereafter -Single-skin side Bulk carriers equal to or greater than 100,000 tons dwt at the intermediate survey after the 2nd special survey and at subsequent surveys -Double-side skin Bulk carriers, Double Hull Oil Tankers and Oil Tankers other than Double Hull equal to or greater than 20,000 tons dwt at the 3rd special survey and subsequent surveys

- Definitively be versatile with the Rule Requirements - Be aware of the ship’s history based on the information data available from the

ship’s classification records


Representative of the TM Company - The TM Company shall be approved by the ship’s Classification Society in accordance with IACS UR Z.17. ESP Code and IACS documentation refer to the requirement of the TM Company certification.

- The TM operator shall be Grade 1 for ESP ships

- Dockyard or yard representative (as per Rep.’s decision and depending on the type of the survey and the age of vessel)


Conclusion of Meeting Time and sequence of survey, access arrangements , sequence of availability of spaces required to be surveyed and thickness measured, agreement between the TM Company and Surveyor as to time and sequence of close-up surveys , bearing in mind that when close-up surveys are required the thickness measurements shall be taken at the same time as both require the availability of access means, agreement that any deficiencies revealed by the TM operator in absence of the Surveyor during thickness measurement process shall immediately be brought to the attention of the Surveyor and Owners’ Rep., so that necessary and timely action be decided in order to avoid problems and delay of the ship , number of Surveyors and TM operators , & may be the most important ensure that all necessary plans of the ship are available.


As aforesaid during ESP surveys, and following the agreement during the initial meeting, the class Surveyor shall be in attendance during thickness measurement procedure to the time extent that is necessary for him to control the process. Ideally two TM operators shall collaborate during TM process, one taking measurements and the other recording the results .

ACTION

• Thickness measurements shall be taken as per the particular survey requirements and under the guidance of the Surveyor, and the locations that are selected shall provide the best representative sampling of areas likely to be most exposed to corrosion, considering cargo and ballast history and arrangement, and condition of protective coatings about all of which at least the class Surveyor shall be informed by the ship’s Master or the Owners’ Representative.

• In certain instances, in view of the ship’s commitments and in order to shorten the time required for the survey, or to ascertain the condition of the ship for themselves, the Owners may decide to arrange for thickness measurements to be taken at sea and not necessarily under the supervision of the Surveyor, in which case, mainly for ships not having problems, the thickness measurements may be accepted PROVIDED these are subsequently verified by spot thickness measurements and accepted by a Surveyor during the required survey.

ACTION

• Four thickness measurements are required to be taken for each plate, two forward and two aft, or two starboard and two port depending on plate orientation.

ACTION

• General practical difficulties encountered during the thickness measurement process:

-availability of spaces}Problems -cleanliness of spaces}requiring solution -access means} by Owners -coating/scaling/rusting} Surface material problems

-pitting} requiring a solution -grooving} by Surveyor/TM

operator, Owner may also be involved

ACTION

• Availability of spaces problem is actually minimized on ESP ships during special surveys by the Survey Programme proposing the sequence of survey as well as the means of access to be provided, which is required to be submitted to the ship’s class Society for agreement. This is agreed with or without amendments and returned to the Owners for follow up and it is taken into consideration during the initial meeting of the three/four parties. It has to be submitted at least six months before the commencement of the survey.

• Dependent on the above comes the cleanliness of spaces problem which has to be dealt with by the ship’s Master/crew and/or shipyard/dockyard in co-ordination with Owners’ Rep. Spaces offered for survey/TMs shall be sufficiently clean & free from water, scale, dirt, oil residues e.t.c. to reveal corrosion and any other structural problems as well as to enable TMs to be taken. Ideally , if necessary, they shall be sand or hydro blasted, and certainly dry and clean with all sediments removed, all loose coating and rust flakes removed. And definitely of paramount importance is that all spaces shall be gas freed, ventilated as applicable, and illuminated before any member of crew or any of the aforementioned parties enters any space.

• Safety of access means provided shall be ensured. No party shall make use of any means that appear unsafe.

ACTION

• Scaling; Heavy scaling may be removed by hydro or sand blasting and this has to be agreed between the Surveyor and the Owner (and the dockyard). In most cases this is done upon Owners’ initiative.

• Rusting; If not heavy the area may be cleaned by the TM operator at locations where TMs are required to be taken

• Coating; to avoid impairing a good condition of the coating , coating thickness gauges may be used.

ACTION

EVALUATION OF RESULTS GENERAL CORROSION Pre-CSR Ships built under the Classification Rules

ACTION EVALUATION OF RESULTS

GENERAL CORROSION

• On pre-CSR ships thickness measurements shall be assessed on the basis of a maximum acceptable percentage diminution of the steel material which reflects the way the Rules were compiled which has also been empirical, apart from scientific. This acceptable diminution is given as a percentage of the original thickness of the structural member in question and it differs from structural member to structural member and from location to location on board ships as well as from ship type to ship type. To this end ships are categorized in three categories, 1, 2 and 3,depending on the ship type and length . The above refers to a relatively even corrosion pattern over a surface.

• Category 1: Non CSR Oil tankers, chemical tankers, dry bulk cargo ships, combination carriers and liquefied gas ships having a length L equal to or greater than 90 metres.

• Category 2: • All remaining ship types not included in Category 1 and having a length L equal to or greater than

90 metres. • Category 3: • All ship types having a length L less than 90 metres

• Attention should be given to Suspect Areas which are locations showing Substantial Corrosion

and/or are considered by the Surveyor to be prone to rapid wastage. Also to Critical Structural Areas which are locations which have been identified from calculations to require monitoring or from the service history of the subject ship or from similar or sister ships (if available) to be sensitive to cracking, buckling or corrosion which would impair the structural integrity of the ship.


GENERAL CORROSION

Areas which are found with substantial corrosion (75% of acceptable diminution) during surveys shall be under observation/re measured during annual surveys and re-assessed. IACS documents include all details for performing measurements of areas with substantial corrosion. Areas which are found with excessive diminution, i.e. greater than the allowable values, shall be renewed with material of original thickness and grade. In case a surface area/structural member is of a high grade , say high tensile steel 32 and the ship is in a port where such material grade is not available, steel of a higher or lower grade may be used (i.e steel H36 or mild steel) of scantlings equivalent to the original. Similarly, in case of stiffeners, if the same profile and grade of steel is not available, equivalent profiles and lower or higher grade of steel may be used in lieu provided the renewal stiffeners have the same properties of section modulus and inertia as the originally built ones, all to the Surveyor’s satisfaction.


GENERAL CORROSION-NON CSR SHIPS

• Permissible Diminution Levels for Category 1 non-CSR Oil Tankers, Chemical Tankers and Liquefied Gas Carriers

• Hull Envelope

• Strength deck plating 20%

• Side shell plating 20%

• Bottom shell plating 20%

• Forecastle deck plating 25%

• Poop deck plating 25%

• Superstructure deck plating 25%


GENERAL CORROSION-NON CSR SHIPS • General – Internal Structure

• Transverse bulkhead plating 25% • Transverse bulkhead stiffeners 25% • Transverse bulkhead horizontal

stringer plating and face plates 25% • Longitudinal bulkhead plating 20% • Longitudinal bulkhead longitudinals 25% • Strength deck longitudinals 25% • Side shell longitudinals 25% • Bottom shell longitudinals 25% • Inner bottom plating 20% • Inner bottom longitudinals 25% • Horizontal girder (fabricated) 20% • Horizontal girder face plate 25% • Horizontal girder rolled section 25%


GENERAL CORROSION-NON CSR SHIPS • Deck girder plating (fabricated) 30% • Deck girder face plates 25% • Deck girder (rolled section) 25% • Bottom girder plating (fabricated) 30% • Bottom girder plating (rolled section) 25% • Bottom girder face plates 25% • SWBT* & COT web frame plating 25% • SWBT* & COT web frame face plates 25% • SWBT* & COT web frame stiffeners 25% • SWBT* & COT web frame secondary structure 30% • *SWBT includes any tanks (including peak tanks, wing tanks, centre

tanks, double bottom tanks, side tanks and deep tanks) designated for the use of salt water ballast.


GENERAL CORROSION-NON CSR SHIPS • Centre tank deck transverse plating 25% • Centre tank deck transverse face plate 25% • Centre tank deck transverse stiffeners 25% • Centre tank deck transverse secondary structure 30% • Centre tank bottom transverse plating 25% • Centre tank bottom transverse face plates 25% • Centre tank bottom transverse stiffeners 25% • Centre tank bottom transverse secondary structure 30% • Peak tank longitudinal bulkhead plating 30% • Peak tank longitudinal bulkhead stiffeners 25% • Peak tank stringer plating 30% • Plating of seachests 30% • Shell plating in way of overboard discharges 30%


GENERAL CORROSION-NON CSR SHIPS

• Permissible Diminution Levels for General Dry Cargo Ship s and All Other Category 2 & 3 Type Ships

• Hull Envelope

• Strength deck plating 30% • Side shell plating 30% • Bottom shell plating 30% • Deck plating inside line of openings, where fitted 30% • Forecastle deck plating 30% • Poop deck plating 30% • Superstructure deck plating 30% • Miscellaneous & Internal Structure Strength deck longitudinals 25% • Side shell longitudinals 25% • Bottom shell longitudinals 25% • Transverse bulkhead plain plating 30% • Transverse bulkhead corrugated plating 25% • Transverse bulkhead stiffeners 25%


GENERAL CORROSION • Longitudinal bulkhead plating 30%

• Longitudinal bulkhead stiffeners 25%

• Inner bottom plating 30%

• Inner bottom longitudinals 25%

• Hopper sloping plating 30%

• Hopper sloping longitudinals 25%

• Topside sloping plating 30%

• Topside sloping longitudinals 25%

• SWBT frames or diaphragms 25%

• Cargo hold shell frame s and end brackets 25%

• Cargo hold hatch cover plating 30%

• Cargo hold hatch cover stiffeners 25%

• Cargo hold hatch coaming plating 30%

• Cargo hold hatch coaming stiffeners 25%

• SWDBT Floors 25%

• Web frame plating 25%

• Web frame face plates 25%

• Web frame secondary structure 30%

• Other miscellaneous plating 30%

• Other miscellaneous longitudinals or stiffeners 25%

• Plating of seachests 30%

• Shell plating in way of overboard discharges 30%


GENERAL CORROSION

• Pre - CSR Tankers For tankers having a length L of 90 metres or above, the thickness measurements of upper deck, apart from satisfying the local diminution acceptable criteria ,shall be checked for compliance with the deck plating residual buckling thickness requirements according to the following equations: 1) tr = (to-1.5)mm 2) tr = s / Jr mm The residual buckling thickness shall be the smaller of the above two values, where : tr = the residual buckling thickness to = the original thickness s = the spacing of the deck longitudinals Jr a factor dependent on location and steel type, as given in the following table. It is worth mentioning that in certain instances the residual buckling thickness may be the limiting factor for tanker upper deck renewals.


GENERAL CORROSION


GENERAL CORROSION • CSR SHIPS

• On CSR ships the minimum allowable thickness diminution is linked to the net thickness philosophy that has been adopted on developing CSR Rules. The strength calculations carried out during the design stage are based on the so called net scantlings. New building gross scantlings requirements are calculated by the addition of an allowance for the expected wastage during the design life of the ship to the required net scantlings determined in accordance with the Rule requirements. The net scantlings are meant to remain / not to be reduced throughout the life of the ship.

ACTION

EVALUATION OF RESULTS CSR SHIPS


GENERAL CORROSION

CSR – Double Hull Oil Tankers For evenly distributed local corrosion the evaluation of thickness measurement results shall be based on the Renewal thickness, where: tren = tas-built-twas-town-tcorr-2.5 tren :renewal thickness tas-built :as built thickness twas : wastage thickness, given by the Rules, depending on the member and space in question town : any voluntary Owners’ thickness that has been added during construction tcorr-2.5 :=0.5 mm, wastage allowance in reserve for corrosion occurring in the 2 1/2 years between Intermediate and Special Survey If tmeasured > trenewal no action or other assessment is required if tmeasured <trenewal area concerned shall be renewed with material of a repair thickness and same grade as the original trepair=tas-built-town Re-examination and additional thickness measurements at annual and intermediate surveys are required where the measured thickness, tmeasured ,is less than the allowable thickness at annual survey, tannual, i.e tmeasured < tannual ,where tannual = tas-built – town - twas


GENERAL CORROSION • CSR – Single - side skin and Double – side skin Bulk Carriers

At general corrosion,i.e. evenly distributed over the surface, tmeasured < trenewal where, trenewal = tas-built-tc-tvoluntary addition tc :corrosion addition tvol.add. :Owners ’voluntary addition Corrosion addition is as per CSR relevant table Substantial corrosion in this case is an extent of corrosion such that assessment of the corrosion pattern indicates a measured thickness such as trenewal <tmeasured < trenewal + treserve treserve ;is the reserve thickness to account for anticipated thickness diminution that may occur during a survey interval of 2 ½ years and it is equal to 0.5 mm In the above case coating may be applied over the affected area or annual gauging may be adopted as an alternative to steel renewal


GENERAL CORROSION


GENERAL CORROSION

• CSR – Single - side skin and Double – side skin Bulk Carriers and Double Hull Oil Tankers – Harmonized The corrosion approach and the corrosion protection of the ships have not been significantly changed from CSR-OT or CSR-BC, but additional studies were performed to confirm the corrosion addition values and the issue of protection against corrosion in CSR-Harmonized. The Harmonized CSR has been developed by blending the requirements and philosophies of the existing CSR-OT and CSR-BC and have benefitted from extensive review by industry. The net thickness philosophy remains.


GENERAL CORROSION

tdm is the Design Production margin, in mm, which is the difference between offered gross thickness & required gross thickness


GENERAL CORROSION • CSR-H Bulk Carriers and Tankers

At general corrosion evenly distributed over a surface: trenewal = tas-built – tc –t voluntary addition tc = corrosion addition = Round up 0.5(tc1+tc2) + t reserve ,where tc1,tc2 ; corrosion addition in mm on one side of the considered structural member. Round up 0.5(t) means that the corrosion addition should be rounded to the upper half millimetre t reserve = 0.5 m Total corrosion addition tc shall not be taken less than 2mm except for web and face plate of stiffeners or in way of internals in way of dry spaces where 1.5 mm is applicable trepair = tas-built –tvol.add. Alternative solutions may be adopted if trenewal < or = tmeasured < trenewal+treserve The as-built , the renewal, and any voluntary addition thicknesses shall be clearly stated on the plans .


GENERAL CORROSION


LOCAL CORROSION - PITTING • Pitting is a problem mostly encountered on tankers. It is in fact heavy localized corrosion,

which forms the so called pits over a generally corroded area and it shall be differently dealt with on ships built according to class Rules and ships built according to CSR or CSR-H.

• To this effect a pit gauge may be required to measure the extent of pitting at pits, or alternatively a miniature transducer (3-5mm dia.) – a conventional transducer is of 10mm dia- .

• In case the thickness of the remaining (sound) material at heavy pits is acceptable, depending on the ship age, if the condition of the material area surrounding the pits in question is acceptable, the pitted area may be repaired/filled in by - welding - coating with special material and not renewed.

• However, the condition of the surrounding area shall be carefully examined and verified by numerous thickness measurements

• INSERT SKETCH IF OBTAINED


LOCAL CORROSION - PITTING • Pre-CSR ships

On ships built in accordance with the Classification Rules a pitted area may be repaired and not renewed provided the thickness of the remaining material in way of all pits is not less than a certain value. For LR ships this thickness value is confirmed at 6 mm. If the depth of pits at any location is such that the thickness of remaining material is less than , say, 6mm, the plate in question has to be renewed.


LOCAL CORROSION - PITTING • CSR Double Hull Tankers (allowable pitting)

• “Pitting corrosion is defined as scattered corrosion spots/areas with local

material reductions which are greater than the general corrosion in the surrounding area. “ (defn as per CSR) On tankers built in accordance with the Common Structural Rules a pitted area may be accepted and not renewed provided the thickness of the remaining material in way of all pits is not less than the lesser of the following values : tmeasured >=0,7(tas-built – t Owners) tmeasured >=trenewal-1 mm and the pitting intensity of the considered area is less than 20%.


LOCAL CORROSION – PITTING CSR_ DOUBLE HULL TANKERS


LOCAL CORROSION - PITTING • CSR Bulk Carriers

For pitted areas the following shall apply: - where the pitting intensity of the considered surface area is less than 15%, the area in question may be coated without any thickness measurements to be taken. - where the pitting intensity of the concerned area is greater than 15%, pit measurements shall be taken of the pitted area , about 5 over an area of 300mm diameter or more and the minimum remaining thickness in pits shall be greater than: 75% of the as-built thickness in the frame and end brackets webs and flanges 70% of the as-built thickness in the side shell , hopper side and topside tank plating attached to the each side frame over a width up to 30mm each side without being greater than trenewal The average thickness across any cross section in the plating is not to be less than the renewal criteria for general corrosion previously defined


LOCAL CORROSION - PITTING

• CSR Bulk Carriers


LOCAL CORROSION - GROOVING

“Grooving corrosion is typically local material loss adjacent to weld joints along abutting stiffeners and at stiffener or plate butts or seams.”

ACTION EVALUATION OF RESULT

GROOVING – EDGE CORROSION • Grooving/edge corrosion form normal material

deterioration on pre-CSR ships.

• Depending on the severity of grooving this may be repaired by filling the grooves with welding material.

• Edge corrosion around openings of girders / floor may be dealt with by enlarging the opening in question down to sound material by removing deteriorated material and possibly stiffening it with a flat bar or a material ring all around. Depending on the location of the structural member in question the opening may be completely closed by new material.


LOCAL CORROSION - GROOVING • CSR – Double Hull Oil Tankers and Bulk Carriers – acceptance criteria

Where the groove breadth is a maximum of 15% of the web height, but not more than 30mm, the measured thickness, tm, in the grooved area is to meet the lesser of the following criteria: t m ≥ 0.75 (tas-built - tvol add) mm tm ≥ tren - 0.5 mm but is not to be less than tm = 6 mm where, tm : measured thickness tas-built: as built thickness tvol add : voluntary added corrosion tren : renewal thickness as defined for general corrosion Structural members with areas of grooving greater than those described above are to be assessed based on the criteria for general corrosion of CSR using the average measured thickness across the plating/stiffener.


LOCAL CORROSION – EDGE CORROSION

• CSR – Double Hull Tankers and Bulk Carriers – “Edge corrosion is defined as local corrosion at the free edges of plates, stiffeners, primary support members and around openings. An example of edge corrosion is shown in the following Figure.”


LOCAL CORROSION – EDGE CORROSION

• CSR – Double Hull Tankers and Bulk Carriers – acceptance criteria Provided that the overall corroded height of the edge corrosion of the flange, or web in the case of flat bar stiffeners, is less than 25%, of the stiffener flange breadth or web height, as applicable, the measured thickness, tm, is to meet the lesser of the following criteria : tmeas >= 0.70 (tas-built – town) tmeas>= tren – 1 mm The average measured thickness across the breadth or height of the stiffener is not to be less than tren. Plate edges at openings for manholes, lightening holes etc. may be below the minimum renewal thickness provided that: (a) the maximum extent of the reduced plate thickness, below the minimum tren, from the opening edge is not more than 20% of the smallest dimension of the opening and does not exceed 100mm (b) rough or uneven edges may be cropped-back provided that the maximum dimension of the opening is not increased by more than 10%.

ACTION

• It may be of interest to show how thickness measurements shall be taken on side shell frames of single-side skin Bulk Carriers in order to ensure compliance with URS31. The aim of URS 31 was to reinforce single-side skin Bulk Carriers in respect of the aforesaid structural members and it applies to ships built before 01 July 1998.

ACTION GAUGING OF SIDE SHELL FRAMES AND BRACKETS IN SINGLE SIDE

SKIN BULK CARRIERS REQUIRED TO COMPLY WITH UR S31

ACTION GAUGING OF SIDE SHELL FRAMES AND BRACKETS IN SINGLE SIDE SKIN BULK

CARRIERS REQUIRED TO COMPLY WITH UR S31

For the purpose of steel renewal, sand blasting and coating, four zones A, B, C and D are defined. Zones A & B are considered to be the most critical zones. Pitting and grooving are the usual corrosion phenomena that may be found on side shell frames, both at webs and face plates. To ascertain compliance with the requirements of URS 31 pitting has to be measured, and the remaining thickness material in way to be calculated and assessed.

If pitting intensity is higher than 15% in an area , then thickness measurements are to be taken to check the extent of the pitting corrosion. The 15% is based upon pitting or grooving on only one side of the plate.

In cases where pitting is evidently exceeding 15 % then the thickness measurements shall be taken as in the case of CSR Bulk Carriers

ACTION GAUGING OF SIDE SHELL FRAMES AND BRACKETS IN SINGLE SIDE SKIN BULK

CARRIERS REQUIRED TO COMPLY WITH UR S31

• The number of side frames required to be measured are equivalent

to those of Special Survey or Intermediate Survey corresponding to the ship’s age. Representative thickness measurements are to be taken for each zone as specified in the relevant sketch.

• Special consideration to the extent of the thickness measurements may be given by the Class Surveyor, if the structural members show no thickness diminution with respect to the as built thicknesses and the coating is found in "as-new" condition (i.e., without breakdown or rusting).

• Where gauging readings close to the criteria are found, the number of hold frames to be measured is to be increased.

• If renewal or other measures according to S31 are to be applied on individual frames in a hold, then all frames in that hold are to be gauged.

ACTION GAUGING OF SIDE SHELL FRAMES AND BRACKETS IN SINGLE SIDE SKIN BULK CARRIERS REQUIRED

TO COMPLY WITH UR S31

For Zones A, B & D five measurements shall be taken. The five measurements shall be within an area across the full depth of the web and vertically along the same distance. The average reading shall be included in the thickness measurement report. Depending upon the condition of the web in way of Zone C, the web may be measured by taking three measurements over the length of Zone C and considering the average. The average reading is to be compared with the allowable thickness. If the web plating is found with general corrosion then five measurements shall be taken and the average value assessed.

Additional measurements shall be taken at the lower bracket in case the lower bracket length or depth does not meet the requirements in UR S12(Rev.3), at sections a) and b) to calculate the actual section modulus required in UR S31. At least 2 readings on the flange or faceplate shall be taken in way of each section. Also, at least one reading of the attached shell plating is to be taken on each side of the frame (i.e. fore and aft) in way of section a) and section b).

ACTION GAUGING OF SIDE SHELL FRAMES AND BRACKETS IN SINGLE SIDE

SKIN BULK CARRIERS REQUIRED TO COMPLY WITH UR S31

ACTION EVALUATION OF LONGITUDINAL STRENGTH

• In addition to the local strength assessment requirements during the service life of ships, Classification Societies had also requirements about global strength assessment.

• The longitudinal strength assessment requirements were applicable to certain type of ships only and at certain classification surveys.

• Evaluation of longitudinal strength of ships during their service life is actually evaluation of sectional properties (section modulus and inertia)of the ship’s transverse sections within 0.4L mid ships.


• Classification Societies carried out this evaluation not by calculating the sectional properties of transverse sections within 0.4L , but for the selected number of transverse sections (depending on the type of ship and survey), by defining the Topside and Bottom areas and by adding up the measured thickness of each strake and each longitudinal member included therein and assessing these sums as a percentage of the sum of the original thicknesses of the same areas.

• Top side and Bottom areas were defined as all continuous longitudinal material from the deck to a distance 0.1Dmld meters below the molded deck line at side and from the bottom to a distance 0.1Dmld meters above the base line respectively.

• If the sum of measured thicknesses was more than 90% of the sum of the original thicknesses, the properties of the section were considered satisfactory and no further action was required.

• Otherwise steel renewal or reinforcement was required, as in the case of local corrosion, and the aforementioned sums were reevaluated so as to ensure compliance with the percentage criterion.


• Insert TM8 form EXAMPLE


• In 1992 Annex 12 to IMO Res. A.744(18) was entered into force which required that on oil tankers of 130 meters in length and upward (as defined in the Load Line Convention in force)and of over 10 years of age, the longitudinal strength of the ship’s hull girder should be evaluated in compliance with the requirements of the Annex on the basis of the thickness measured, renewed or reinforced, as appropriate, during the renewal Survey of the Cargo Ship Safety Construction Certificate.

• It was then that Classification Societies commenced evaluating the ship’s longitudinal strength by evaluating also the sectional properties of three selected transverse sections during her service life for survey purposes.

• To this end they developed a suitable software, namely Lloyd’s Register developed TMCalc ,via the required sectional properties were evaluated.

• The three transverse sections selected for the evaluation of longitudinal strength of tankers which were built under the classification Rules shall lie within 0.4L and shall preferably include a section in way of a ballast tank, which is always the case for a Double hull tanker but not necessarily for a conventional crude oil carrier.

• Upon entry into force of the CSR Rules, slightly different global strength requirements were entered into force, with different acceptance criteria and new definitions:


• IACS formulates the current requirements as follows for Single Hull Tankers: 1. Ships regardless of the date of construction: Transverse sectional areas of deck flange (deck plating and deck longitudinals) and bottom flange (bottom shell plating and bottom longitudinals) of the ship’s hull girder have been calculated by using the thickness measured, renewed or reinforced, as appropriate, during the special survey most recently conducted after the ship reached 10 years of age, and found that the diminution of the transverse sectional area does not exceed 10% of the as-built area. 2. Ships constructed on or after 1 July 2002: Section moduli of transverse section of the ship’s hull girder have been calculated by using the thickness of structural members measured, renewed or reinforced, as appropriate, during the special survey most recently conducted after the ship reached 10 years of age , and are found to be within their diminution limits determined by the Classification Society.


3. Ships constructed before 1 July 2002: Section moduli of transverse section of the ship’s hull girder have been calculated by using the thickness of structural members measured, renewed or reinforced, as appropriate, during the special survey most recently conducted after the ship reached 10 years of age in accordance with the provisions of paragraph 2.2.1.2 of Annex III, and found to meet the criteria required by the Classification Society and that the actual modulus, Zact, is not less than a minimum value,Zmc: Zmc = cL2B (Cb + 0.7)k (cm3) where L = Length of ships. L is the distance, in metres, on the summer load waterline from the fore side of stem to the after side of the rudder post, or the centre of the rudder stock if there is no rudder post. L is not to be less than 96%, and need not be greater than 97%, of the extreme length on the summer load waterline. In ships with unusual stern and bow arrangement the length L may be specially considered. B = Greatest moulded breadth in metres. Cb = Moulded block coefficient at draught d corresponding to summer load waterline, based on L and B. Cb is not to be taken less than 0.60.


Acceptance Criteria for the Hull Girder Strength a) CSR and non CSR Deck and Bottom Zones: The measured sectional areas of the Deck Zone and of the Bottom Zone which are the sum of the measured item areas of the considered zones are not to be less than 90% of the sectional area of the corresponding zones determined with the gross offered thicknesses (original Rule sectional area). b) Non CSR Neutral axis Zone: The measured sectional area of the Neutral Axis Zone, which is the sum of the measured plating areas of this zone, is not to be less than 85% of the sectional area of the corresponding zone determined with the gross offered thicknesses (original Rule sectional area). c) CSR Neutral Axis Zone: The current sectional area of the neutral axis zone, which is the sum of the measured plating areas of this zone, is not to be less than the sectional area of the neutral axis zone calculated with the gross offered thickness minus 0.5 tc. (Gauged Thickness) x (Plate/Profile Width/Height) >(Plate/Profile Width/Height) x (((As-Built Thickness) – (Owners Voluntary Addition) – (Renewal Thickness)) / 2) + (Renewal Thickness))


For the purposes of assessment, each Transverse Section is therefore divided into 3 zones:- ‐ Deck Zone For Bulk Carriers, Ore Carriers and other ESP ships of similar configuration: strength deck plating deck stringer sheer strake side shell plating top side tank sloped plating, including horizontal and vertical strakes longitudinal stiffeners connected to the above mentioned plating For Oil Tankers (including Chemical Tankers, Oil/Bulk/Ore and Ore/Oil Ships): strength deck plating deck longitudinals

For all other ship types: continuous longitudinal elements above the level corresponding to 0.9Dmld (where Dmld = Depth of ship) above the base line.


- Bottom zone For Bulk Carriers, Ore Carriers (and other ESP ships of similar structural configuration): elements up to the upper level of the hopper sloping plating or up to and including the inner bottom plating if there is no hopper tank: Keel plate Bottom plating Bilge plating Bottom girders Inner bottom plating Hopper tank sloping plating, and horizontal plating, if any Side shell plating Longitudinal stiffeners and girders connected to the above mentioned plating. For Oil Tankers (including Chemical Tankers, Oil/Bulk/Ore and Ore/Oil Ships): Keel plate Bottom plating Bottom longitudinals and double bottom girders in way. For all other ship types: elements below the level corresponding to 0.1D (where D = Depth of ship) above the base line.


-The neutral axis zone For all ship types includes longitudinal material between the deck zone and the bottom zone, for example: Side shell plating Inner hull plating (longitudinal bulkheads and attached longitudinals (if any) Inner bottom plating and attached longitudinals (for Tankers only)

ACTION

• Following the above it is clear that it is essential for the TM operator to advise timely the Surveyor (if not present at the time the measurements are taken) and the Owners’ Rep. of any findings requiring rectification. Depending on the seriousness of the material deterioration and following an agreement between the Surveyor and the Owners’ Rep. the affected material area may be rectified by any of the following methods: - Steel Coating - Steel Reinforcement - Steel Renewal - Welding - Coating with special material

ACTION

• It is also essential that the longitudinal strength results are evaluated soon after the three section measurements are taken so as to confirm whether there are any problems that require to be dealt with. That is: - Classification Rules Ships : In case the deck area is found more than 7.5 per cent deteriorated from the original deck area, two additional transverse sections shall be selected and thickness measured in order to ascertain the condition of the ship. In case the deck area is found above 10 per cent deteriorated , steel renewal or steel reinforcement may be required as it will be agreed between the Surveyor (class) and the Owners’ Rep. - CSR and CSR-Harmonized ships are to be longitudinal strength wise evaluated on a different basis .

ACTION

• Upon completion of the thickness measurements process and timely communication of the results to the Surveyor and the Owners’ Rep., the TM Operator shall prepare a Draught Survey Report and submit same to the Surveyor for his assessment. On satisfactory assessment the Surveyor shall advise the TM operator accordingly so that he can proceed with the preparation of the Final Thickness Measurement Report.

REPORTING

REPORTING

• Upon satisfactory acceptance of the Draught Survey Report by the Surveyor, the Final Thickness Measurement Report shall be compiled by the TM Operator for submission to the Classification Society.

• The report is prepared using the Thickness Measurement software developed by the Classification Society with which the ship is classed and it includes and is based on the Thickness Measurements Forms developed by IACS.

• It would be a serious omission not to mention that Lloyd’s Register just recently (October 2014) launched its new thickness measurement software under the name “Argonaut” to be used for pre-CSR and CSR ships.

REPORTING

• Main things that have to be considered when compiling the Report but also beforehand , during the survey, upon evaluation of a particular ship area/structural member are: - c.c. Notation; In case the ship has a c.c. Notation, care shall be taken so that the thickness measurements are compared and evaluated against the ship scantlings before allowance for c.c. notation, and not against the “as built” scantlings. - Owners’ extra shall be ignored when assessing thickness measurements - Re-assessment of scantlings. In case a ship has been re-assessed thickness measurements shall be assessed against the re-assessed scantlings and not the “as built” scantlings. The above apply to pre-CSR existing ships. In case of CSR and CSR-H it is worth stressing that the original/as built scantlings are clearly shown on the structural plans.

REPORTING

• Other items to be given attention by the TM Company/operator during compilation of the Report are :

1. Correctness of original scantlings 2. Correctness of the thickness measurements given in the Report against those included in the Draught Report which have been verified/accepted by the attending Surveyor

3. Correctness of the type of Form used to include the readings for a particular structural member (all the above three aim at correct assessment of the thickness measurements taken)

4. Completeness of the Report, i.e. ensure that all structural members required to be thickness measured at that particular survey have been included 5. All Ship and Survey identification particulars to be correctly included in the Report

REPORTING

• Non CSR TM Forms • TM1 is to be used for reporting the thickness measurement of deck plating, bottom shell plating and side shell plating within the cargo length area. • TM2~3 is to be used for reporting the thickness measurement of the transverse section longitudinal plating, girders and longitudinal frames and

stiffeners under the deck, bottom or neutral axis zones. • TM4 is to be used for reporting the thickness measurement of transverse structural members and all attached structure in water ballast tanks, deep

tanks, cargo tanks and void spaces. This excludes reporting of W.T. transverse bulkheads of any type in any location. • TM5 is to be used for reporting the thickness measurement of W.T. transverse bulkheads where appropriate. This excludes reporting of bulk carrier

W.T. transverse bulkheads subject to assessment derived by Unified Requirements Strength 18 and 19 (UR S18, UR S19). • TM5 UR S18 is to be used for reporting the thickness measurement of bulk carrier W.T. transverse bulkheads subject to assessment in accordance

with Unified Requirements Strength 18 (UR S18). • TM5 UR S19% is to be used for reporting the thickness measurement of bulk carrier W.T. transverse bulkheads subject to assessment in accordance

with Unified Requirements Strength 19 (UR S19).This form is to be used in case the approved bulkhead upgrade plan does not provide any additional thickness measurement requirements, but requires applicability of Class Rules (diminution criteria) in order to assess the bulkhead during periodical surveys.

• TM5 UR S19 is to be used for reporting the thickness measurement of bulk carrier W.T. transverse bulkheads subject to assessment in accordance with Unified Requirements Strength 19 (UR S19) and the approved bulkhead upgrade plan, with additional thickness measurement requirements to conventional Class Rules(diminution criteria).

• TM6 is to be used for reporting the thickness measurement of miscellaneous structural members. Also this form is to be used for reporting of any critical areas or any additional survey areas outside the normal scope of survey that would require to be thickness measured. This form is also to be used for reporting of the deck, shell and bottom plating outside the cargo length area.

• TM6 UR S21 is to be used for reporting of Scantlings of Hatch Covers / Coamings of Bulk Carrier cargo holds assessed in accordance with Unified Requirements Strength 21 (UR S21).

• TM6 UR S21A is to be used for reporting of Scantlings of Hatch Covers / Coamings of cargo holds for all ships except Bulk Carriers, Ore Carriers and Combination Carriers, assessed in accordance with Unified Requirements Strength 21A (UR S21A).

• TM7 is to be used for reporting the thickness measurement of cargo hold/tank transverse frames where appropriate. The form may also be used for reporting of any attached structure to the cargo hold/tank transverse frames, unless it has been fully reported onto an additional TM form e.g. TM6. Insert forms TM from IACS

• TM7 UR S31 is to be used for reporting the thickness measurement of cargo hold/tank transverse frames of bulk carriers assessed in accordance with Unified Requirements Strength 31 (UR S31).

• TM8 is to be used for reporting the transverse sectional area of the hull girder strength for deck, bottom and neutral axis zone. This form is automatically generated in the LR software and cannot be generated by the user individually. This form will get updated by data reported onto TM2~3 form, for each zone under consideration.

REPORTING

• On receipt of the Final Thickness Measurement Report at the Classification Society, the attending Surveyor examines the Report for correctness of the thickness measurements included therein against the Draught report, the correctness of the corresponding original scantlings and upon satisfactory completion, he signs the report as the attending Surveyor.

REPORTING

• Finally the Report is examined by the so called authorizing surveyor and upon satisfactory completion it is vetted and ready for record. The Owners are advised accordingly upon receipt of the ship’s Executive Summary which has to be issued by the Classification Society for ESP ships upon satisfactory completion of the Survey , as reported by the Surveyor, and the vetting of the Final Thickness Measurement Report.

REPORTING

Ultrasonic Thickness Measurement of Ships

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