Thickness Measurement on Surface or Underwater

Thickness measurement for ships and platforms is fundamental to accredit annual, periodic or special inspections from Class. We as Techdiving offer you the planning, execution and report of thickness measurement of ships or platforms to attend the safety requirements from the Class above of underwater.

    • Thicknesses Measurement or gauging for Close up Survey
    • Technical advice in the Thicknesses Measurement.
    • Annual Inspection with thicknesses measurement or gauging.
    • Intermediate inspection with thicknesses measurement or gauging.
    • Special Inspections I, II, III, IV, with thicknesses measurement or gauging.
    • Dry dock survey or hull inspection with thickness measurement or gauging
    • Evaluation of areas in Substance Corrosion with thicknesses measurement
    • Delimitation of areas to renovate and calculate steel with thickness measurement.
    • Thicknesses Measurement or gauging for Close up Survey
    • Underwater thickness Measurement.
    • Special Periodical Surveys.
    • CAP & CAS Survey.
    • Thickness Measurement to Pressure Vessels & Pipelines 

    Thickness measurement realized by ultrasound, is used to calculate the real thickness of the materials in a broad range of industries, for quality control at the moment of the manufacture as for monitoring the wear during the service. Even though we can do thickness measurement services in any industrial, we specialize in the thickness measurement of ships, barges, drilling platforms, jack up platforms, semi-submersible platforms, holding tanks, conduction lines, drilling tools, cranes, etc.
    Underwater thickness measurement, follow the same physic principles of the thickness measurement technique, but in this case the waterproof equipment has to be used. The work must be done by technical diver’s inspectors N-II, specialized and familiarized with the ship`s nomenclature and the rules and regulations of the Class.

    Information for Hull Thickness Measurement

    Purpose
    The purpose of thickness measurement is to establish, in conjunction with a visual examination, that the condition of the existing structure is, or will be after the required repairs, fir for continued service during the subsequent survey interval. The gauging requirements include measurements which are used to verify remaining longitudinal strength, transverse sections, as well as measurements of areas known to be potential problem areas, main deck plates and wind and water strakes Thickness measurements are a confirmation of conditions sighted by the Surveyor. When a Surveyor confirms that a set of a thickness measurements are representative of the conditions found, he can only do so if he personally examined and selected the particular areas to be measured and is satisfied they actually represent the conditions of the structure involved and not the thickness in a way of isolated pits or localized corrosion. Local pits and corrosion are to be dealt with as necessary by the Surveyor.

    The Thickness Measurement Process

    When an Owner requests attendance for a survey with thickness measurements due, the Surveyor is to obtain scantling drawings, either from the Owner or from Class files. The Surveyor will advise the Owner of the thickness measurements.

    The Surveyor is to attend the vessel while thickness measurements are being taken in order to advise the Owner of the locations. The Surveyor is to regularly review the thickness measurement results, in order to promptly advise the Owner of any additional readings. When multiple readings are taken to confirm marginal or questionable readings, the Surveyor will determine and report the single reading which he feels, based on his observations of the structure from visual examination and the gauged readings.

    Thickness Measurement Review
    First, the Surveyor is to verify that all thickness measurement requirements for the applicable survey are met. Any required measurements which have not been taken, will be noted in his report as an item remaining to be completed before the survey can be credited. Next, the Surveyor is to check that all individual plate thickness measurement results are within Class allowances.
    For unconverted, conventional type vessels built to Class, wastage allowances are to be applied to the as-built thickness. For vessels built with reduced scantlings, the wastage allowances are to applied to the Class Rule-required thickness shown on the vessel’s approved drawings. Care must be taken to distinguish between the reduced as-built scantling and the Class Rule required scantling

    To request a thickness measurement review based on other than as-built thickness or a scantling reassessment, the Owner is to contact a Class local Technical Office.
    Finally, the Surveyor is to check the average wastage of the top and bottom transverse sections. If individual plates and internals are to be averaged separately. The top of the transverse section includes the deck plating, stringer plate and sheer strake. The bottom of the transverse section includes the flat keel plating, the bottom plating, and bilge plating. The internals are those longitudinal members attached to the afore mentioned plates.

    Thickness Measurement Reports
    Survey reports for vessels subject to the Enhanced Survey Program (ESP) are to be prepared following the format contained in “Reports on Thickness Measurement of Bulk Carriers”,” Reports on Thickness measurement of Oil Carriers, Ore/Oil Ships and Chemical Carriers- Non-Double Hull”, “Reports on Thickness Measurement of Oil Carriers, Ore/Oil Ships and Chemical Carriers- Double Hull” “Report on Thickness Measurement of Bulk Carriers- Double Skin”.
    Reports for other vessels are to clearly present the location, original thickness, measured thickness and percentage wastage.
    The attending Surveyor will review the results for accuracy and completeness and endorse the report with the vessel’s name, Class ID, date and his signature to indicate the results are considered representative of the actual condition of the vessel.

    Thickness Measurement Company Approval Requirements
    Persons or companies not employed by Class who act at the request of the Owner to take ultrasonic thickness measurement on Enhanced Survey Program (ESP) vessels; and non-ESP/ESPDC vessels 90 meters in length and over for classification or statutory surveys, must be approved by ABS in accordance with Class procedures for approval of External Specialists.
    The local Class office can provide the Owner with a list of approved companies in that office’s area.

    Thickness Measurement Requirements and Locations
    The thickness measurement requirements for Annual and Intermediate Surveys are based on the conditions found at the time of survey, as well as conditions documented at the previous Special Periodical Survey. Thickness measurement requirements for Special Periodical Survey are based on age, size and type of vessel. The minimum thickness measurement requirements are contained in the applicable Class Rules and Guides. For vessels subject to the ESP, thickness measurements for Special Periodical Survey are to be dealt with in the required Survey Planning Document.
    In all cases, the Surveyors have the discretion to require thickness measurements to supplement their visual examinations where there is any doubt of the structure’s fitness for continued service.
    The Surveyor will indicate the locations to be measured. Transverse sections should be in one section, not over several frame spaces. The sections should be located where the largest reductions are suspected to occur or are revealed from deck plating thickness measurements. Where possible, locations different from those measured at previous Special Periodical Surveys should be selected.

    Thickness Measurements of Structural Members Subject to Close-up Survey
    Close-up Surveys are required for Special Periodical Survey of vessels subject to ESP and may also be required at Annual and Intermediate Surveys. At Special Periodical Survey, thickness measurements of structural members subject to Close-up Survey are required for general assessment and recording of corrosion patterns.

    Modification of Thickness Measurement Requirements
    In general, thickness measurement requirements of internals may be modified for vessels with acceptable, corrosion-resistant, hard-type coatings, such as epoxy or zinc, providing that after a careful examination, the surveyor can verify the continued effectiveness of same. No consideration for reduced thickness measurements will be given for soft type coatings.
    Where there is evidence that the coating is no longer intact (such as heavy staining, blistering, cracking, peeling or bare spots), the Surveyor must require sufficient confirmatory thickness measurement to clearly establish the condition of the internals. Please note that in the initial stages of coating breakdown, the corrosion may proceed at a very high rate in the exposed spots due to the abnormal area ratio between the protected and unprotected surfaces.
    For vessels subject to ESP, thickness measurements for general assessment for corrosion patterns of structural members subject to Close-up Surveys are required at Special Periodical Survey. Up to and including Special Periodical Survey No. 2, the Surveyor may modify the extent of thickness measurements of these structural members where the coating is found in “GOOD” condition, as defined in Class Rules. The Surveyor will specifically examine and report on the coating condition and take a number of confirmatory thickness measurements to support his recommendation to modify the extent of thickness measurement of these structural members.
    After Special Periodical Survey No. 2, modification of the extent of thickness measurements of these structural members is to be specially considered. The Surveyor will submit his findings on the coating condition and the confirmatory thickness measurements, along with sufficient photos to support his recommendation, for special consideration. Normally, the Divisional Assistant Chief Surveyor will examine the Surveyor’s submittal and either accept or reject the Surveyor’s recommendation to modify the extent of thickness measurements.
    For converted vessels, the Special Periodical Survey and the thickness measurement requirements are based on the age of the original, retained sections. However, the thickness measurement requirements for the new body sections may be in accordance with Special Periodical Survey requirements for a vessel of corresponding age.


    Some important definitions related to thickness measurement at barges, ship`s and platforms are the following:

    Active Corrosion:
    Active Corrosion means loose scale and evidence of moisture penetration to the steel.

    Air Pipe Head:
    Air Pipe Head installed on the exposed decks are those extending above the freeboard deck or superstructure decks.

    Allowable Corrosion or Wastage Limit
    Allowable Corrosion or Wastage Limit is the acceptable corrosion limit for the vessel’s structure in a given area. Also known as Allowable Limit.

    Bay
    Bay is the are between adjacent transverse frames from longitudinal bulkhead to longitudinal bulkhead (or side shell)

    Ballas Tank
    A Ballast Tank is a tank which is used primarily for the carriage of salt water ballast.

    Ballas Tank-Tankers ESP
    A Ballast Tank is a tank which is used solely for the carriage of salt water ballast.

    Ballast Tank-Tankers ESP
    A Ballast Tank which is used solely for salt water ballast, or where applicable, a space which is used for both cargo and salt water ballast wil be trated as a ballast tank when substantial corrosion has been found in that space. A double side tank is to be considered as a separate tank even if it is in connection to either the topside tank or the hopper side tank.

    Bulk Carrier
    A Bulk Carrier is a ship which is constructed generally with single deck, double bottom, topside tanks and hopper side tanks in cargo spaces, and is intended primarily to carry dry cargo in bulk. Combination Carriers are included. Ore and combination carriers are not subject to the IACS Common Structural Rules.

    A Bulk Carrier- Double Skin
    A Double Skin Bulk Carrier is a ship which is constructed generally with single deck, topside tanks and hopper side tanks in cargo spaces, and is intended primarily to carry dry cargo in bulk, including such types as Ore Carrier and Combination Carrier, in which all cargo holds are bounded by a double-side skin (regardless of the width of the wings space).

     

    Cargo Area- Tankers
    Cargo Area is that part of the ship that contains cargo tanks, slop tanks and cargo/ballast pump rooms, cofferdams, ballast tanks, fuel tanksand void spaces adjacent to cargo tanks or slop tanks; and also deck areas throughout the entire length and breadth of the part of the ship pver the above-mentioned spaces. Where independent tanks are installed in hold spaces, cofferdams, ballast or void spaces at the after end of the aftermost hold space or at the forward end of the forwardmost hold space are excluded from the cargo area.

    Cargo Length Area- All Vessels
    Cargo Length Area is that part of the vessel which contains cargo holds and adjacent areas including fuel tanks, cofferdams, ballast tanks and void spaces.

    Chemical Carrier
    A Chemical Carrier is a ship which is constructed or adapted and used for the carriage in bulk of any liquid product.

    Close-up Survey
    A Close-up Survey is a survey where the details of structural components are within the close visual inspection range of the Surveyor, i.e., normally within hand’s reach.

    Coating Condition
    Coating Condition of hard coating is defined as follows:

    • GOOD is a condition with only minor spot rusting.
    • FAIR is a condition with local breakdown at deges of stiffeners and weld connections and/or light rusting over 20% or more of areas under consideration, but less than as defined for POOR condition.
    • POOR is a condition with general breakdown of coating over 20% or more of areas, or hard scale at 10% or more of areas under consideration.

    Combined Cargo/Ballast Tank-All Vessels
    A Combined Cargo/Ballast Tank is a tank which is used for the carriage of carg or ballast water as a routine part of the vessel’s operation and will be treated as a ballast tank. Cargo tanks in which water ballast might be carried only on exceptional cases per MARPOL I/18(3) are to be treated as cargo tanks.

    Corrosion Prevention System
    A Corrosion Prevention System is a normally considered a full hard protective coating. Hard Protective Coating is usually to be epoxy coating and equivalent. Other coating systems, which are neither soft nor semihard coatings, may be considered acceptable as alternatives, provided that they are applied and maintained in compliance with the manufacturer’s specification.

     

    Critical Structural Areas
    Critical Structural Areas 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 to be sensitive to cracking, buckling or corrosion that could impair the structural integrity of the ship.

    Edge Corrosion
    Edge Corrosion is defined as local corrosion at the free edges of plates, stiffeners, primary support members and around openings.

    Excessive Corrosion
    Excessive Corrosion is corrosion that exceeds the allowable limit.

    Extensive Area of Corrosion
    Extensive Area of Corrosion is corrosion of hard and/or loose scale, including pitting, over 70% or more of the plating surface in question, accompanied by evidence of thinning.

    General Dry Cargo Vessel (ESDC)
    A General Dry Cargo Vessel (ESDC) is a vessel carrying solid cargoes other than:

    • Bulk Carriers, Double Skin or Non-Double Skin, subject to ESP
    • Dedicated containers carriers
    • Ro-ro cargo vessels
    • Refrigerated cargo vessels
    • Dedicated wood chip carriers
    • Dedicated cement carriers
    • Livestock carriers
    • Deck cargo ships (A deck cargo ship is a ship that is designed to carry cargo exclusively above deck without any access for cargo below deck.)
    • General dry cargo ships of double side-skin construction, with double side-skin extending for the entire length of the cargo area, and for the entire height of the cargo hold to the upper deck.

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

    Lightering Service
    Lightering Service is defined as the side-by-side morring of two vessels, either while underway or stationary, for the purpose of transferring petroleum cargo, excluding bunkers, from a ship to be lightered to a service vessel. Both the lightered vessel and the service vessel are to be considered in lightering service.

     

    Localized Corrosion
    Localized Corrosion is by name in nature and may be caused by a local breakdown in coating from contact damage, insufficient preparation or at areas of stress concentration.

    Oil
    Oil, for the purpose of the Rules, means petroleum in any form including crude oil, fuel oil, sludge, oil refuse and refined products other than petrochemicals.

    Oil Carrier
    An Oil Carrier is a ship which is constructed primarily to carry oil in bulk and includes a ship of similar types such as a Combination Carrier (Ore/Oil), etc.

    Oil Carrier-Double Hull
    An Oil Carrier-Double Hull is a ship which is constructed primarily to carry oil in bulk and includes similar types of ships such as a Combination Carrier (Ore/Oil), etc., which have the cargo tanks protected by a double hull which extends for the entire length of the cargo area, consisting of double sides and double bottom spaces for the carriage of water ballast or void spaces.

    Overall Corrosion
    Overall Corrosion appears as a non-protective rust which can uniformly occur on tank internal surfaces that are uncoated, or where coating has totally deteriorated. The rust scale continues to break off, exposing fresh metal to corrosive attack. Thickness cannot be judged visually until excessive loss has occurred.

    Overall Survey
    An Overall Survey is a survey intended to report on the overall condition of the hull structure and to determine the extent of additional Close-up Surveys.

    Panel
    Panel is the area between adjacent transverse frames from longitudinal stiffener to longitudinal stiffener.

    Pitting Corrosion
    Pitting Corrosion is defined as scattered corrosion spots/areas with local material reductions which are greater than the general corrosion in the surrounding area.

    Prompt and Thorough Repair
    A Prompt and Thorough Repair is defined as a permanent repair completed at the time of the survey to the satisfaction of the Surveyor.

    Renewal Thickness
    Renewal Thickness (tren) is the minimum allowable thickness, in mm, below which renewal of structural members is to be carried out.
    Representative Spaces/Tanks
    Representative Spaces/Tanks are those which are expected to reflect the condition of other spaces of similar type and service and with similar corrosion prevention systems. When selecting representative spaces, account should be taken of the service and repair history onboard and identifiable Critical Structural Areas and/or Suspect Areas.

    Ro-Ro Definitions

    • Ro-Ro Ship
      A ship which utilizes a loading ramp to enable wheeled vehicles to be rolled-on and rolled-off the ship.
    • Ro-Ro Passenger Ship (Ro-Pax)
      A passenger ship with Ro-Ro spaces or special category spaces.
    • Ro-Ro Spaces
      Spaces not normally sub-divided in any way and normally extending to either a substantial length or the entire length of the ship, in which motor vehicles with fuel in their tanks for their own propulsion and/r goods (packaged or in a bulk, in or on a rail or road cars, vehicles (including road or rail tankers), trailers, containers, pallets, demountable tanks or in or on similar stowage units or, other receptacles) can be loaded and unloaded normally in a horizontal direction.
    • Special Category Spaces
      Those enclosed vehicle spaces above or below the bulkhead deck, into and from which vehicles can be driven and to which passengers have access. Special category spaces may be accommodated on more than one deck provided that the total overall clear height for vehicles does not exceed 10m.
    • Securing Device
      A device used to keep the door closed by preventing it from rotating about its hinges.
    • Supporting Device
      A device used to transmit external or internal loads from the door to a securing device and from the securing device to the ship’s structure, or a device other than a securing device, such as a hinge, stopper or other fixed device, that transmits loads from the door to the ship’s structure.
    • Locking Device
      A device that locks a securing device in the closed position.

     

    Serious/Significant Corrosion
    Serious/Significant Corrosion means more than 30% corrosion, and active scale is present. Active scale that is loose or has fallen off the structure.
    Spaces
    Spaces are separate compartments including holds, tanks, cofferdams, and void spaces bounding cargo holds, decks, and the outer hulls.

    Special Consideration
    Special Consideration or  Specially Considered (In connection with close-up surveys and thickness measurements) means sufficient close-up inspection and thickness measurements are to be taken to confirm the actual average condition of the structure under the coating.

    Substantial Corrosion
    Substantial Corrosion is an extent of corrosion such that assessment of the corrosion pattern indicates a wastage in excess of 75% of the allowable margins, but within the acceptable limits.

    Superstructure Deck
    For gauging purposes, a Superstructure Deck is a deck over an enclosed superstructure i.e., the first-tier deck.

    Suspect Areas
    Suspect Areas are locations showing substantial corrosion and/or are considered by the Surveyor to be prone to rapid wastage.

    Tanker
    A Tanker is a ship which is constructed primarily to carry liquid cargo in bulk. Oil Carriers, Combination Carriers, Chemical Carriers and Liquefied Gas Carriers are included in this category.

    Topside Ballast Tanks
    Topside Ballast Tanks are ballast tanks in bulk carriers that normally extend along the length of the vessel’s side and occupy the upper corners of the cargo hold.

    Transverse Section (Girth Belt/Belt)
    A Transverse Section includes all longitudinal members such as plating, longitudinals and girders at the deck, sides, bottom, inner bottom, longitudinal bulkheads, hopper sides, inner sides and bottoms of topside tanks. For transversely framed vessel, a transverse section includes adjacent frames and their end connections in a way of transverse sections.

    Weld Metal Corrosion
    Weld Metal Corrosion is defined as preferential corrosion of the weld deposit. The most likely reason for this attack is galvanic action with the base metal which may start as pitting and often occurs on hand welds as opposed to machine welds.

    Wind and Water Strakes
    Wind and Water Strakes are the two (2) strakes located in the vicinity of the load waterline. Due to vessel’s trim, the strakes may very over the length of the vessel.

     

     

     

    IACS: Association Of Classification Societies Ltd 

    ABS: American Bureau Of Shipping.
    BV:   Bureau Veritas.
    DNV: Det Norske Veritas
    GL: Germanischer Lloyd
    LR:    Lloyd’s Register of Shipping.
    NKK: Nippon Kaiji Kyokai

    RINA: Registro Italiano Navale