I ACS

GENERAL CARGO SHIPS

Guidelines for Surveys, Assessment and Repair of Hull Structure

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General corrosion appears as a non-protective, friable rust which can occur uniformly on hold or tank internal surfaces that are uncoated. The rust scale continually breaks off. exposing fresh metal to corrosive attack. Thickness loss cannot usually be judged visually until excessive loss has occurred. Failure to remove mill scale during construction of the ship can accelerate corrosion experienced in service. Severe general corrosion in all types of ships, usually characterized by heavy scale accumulation, can lead to extensive steel renewals.

Grooving corrosion is often found in or beside welds, especially in the heat affected zone. The corrosion is caused by the galvanic current generated from the difference of the met allographs structure between the heat affected zone and base metal. Coating of the welds is generally less effective compared to other areas due to rough surfaces which exacerbate the corrosion. The grooving corrosion may lead to stress concentrations and further accelerate the corrosion. Grooving corrosion may be found in the base material where coating has been scratched or the metal itself has been mechanically damaged.

Pitting corrosion is often found in the bottom plating of ballast tanks. If there is a place which is liable to have corrosion due to local breakdown of coating, pitting corrosion starts. Once pitting corrosion starts, it is exacerbated by galvanic current between the pit.and other metal.

Erosion which is caused by the effect of liquid and abrasion caused by mechanical effect may also be responsible for material wastage.

3.3.3 Fractures

In most cases fractures are found at locations where stress concentrations occur. Weld defects, flaws, and where lifting fittings used during the construction of the ship are not properly removed are often recognized as areas of stress concentration when fractures are found. If fractures have occurred under repeated stresses which are below the yielding stress, the fractures are called fatigue fractures. In addition to the cyclic stresses caused by wave forces, fatigue fractures are also caused by vibration forces derived from main engine or propeller especially in the afterward part of the hull. If the initiation points of the fractures are not apparent, the structure on the other side of the plating should be examined.

Fractures may not be readily visible due to lack of cleanliness, difficulty of access, poor lighting or compression of the fracture surfaces at the time of inspection. It is therefore important to identify, clean, and closely inspect potential problem areas.

Fractui'e initiating at latent defects in welding more commonly appear at the beginning or end of a run of welding, or rounding corners at the end of a stiffener, or at an intersection. Special attention should be paid to welding at toes of brackets, cut-outs, and intersections of welds. Fractures may also be initiated by undercutting the weld in way of stress concentrations. Although now less common, intermittent welding may cause problems because of the introduction of st ress concent rations at the ends of each length of weld.

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It should be noted that fractures, particularly fatigue fractures due to repeated stresses, may lead to serious damage, e.g. a fatigue fracture in a frame may propagate into shell plating and affect the watertight integrity of the hull. In extreme weather conditions the shell fracture could extend further resulting in the loss of part of the shell plating and consequent flooding of cargo hold.

3.3.4 Deformations

Deformation of structure is caused by in-plane load, out-of -plane load or combined loads. Such deformation is often identified as local deformation, such as deformation of panel including stiffener, or global deformation; such as deformation of structure including plating, beam, frame, girder, floor, etc.

If in the process of the deformation large deformation is caused due to small increase of the load, the process is called buckling.

Deformations are often caused by impact loads/contact and inadvertent overloading. Damages due to bottom slamming and wave impact forces are, in general, found in the forward part of the hull, although stern seas (pooping) have resulted in damages in way of the after part of the hull.

In the case of damages due to contact with other objects, special attention should be drawn to the fact that although damages to the shell plating may look small from the outboard side, in many cases the internal members are heavily damaged.

Permanent buckling may arise as a result of overloading, overall reduction in thickness due to corrosion, or contact damage. Elastic buckling will not be directly obvious but may be detected by coating damage, stress lines or shedding of scale. Buckling damages are often found in webs of web frames or floors. In many cases this is due to corrosion of webs/floors, too wide a spacing of stiffeners or wrongly positioned lightening holes, man-holes or slots in webs/floors.

Finally, it should be noted that inadvertent overloading may cause significant damages. In general, however, major causes of damages are associated with excessive corrosion and contact damage.

3.4 Structural detail failures and repairs

3.4.1    For examples of st ructural defects which have occurred in service, at tention is drawn to Section 5 of these guidelines. It is suggested that Surveyors and inspectors should be familiar with the contents of Section 5 before undertaking a survey.

3.4.2    If replacement of defective parts must be postponed, the following temporary measures may be acceptable at the surveyor's discretion.

(a)    The affected area may be sandblasted and painted in order to reduce corrosion rate.

(b)    Doubler may be applied over the affected area. In case of bucking under compression, however, special consideration should be paid.

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(c)    Stronger members may support weakened stiffeners by applying temporarily connecting elements.

(d)    Cement box may be applied over the affected area.

A suitable condition of class should be imposed when temporary measures are accepted.

3.5 IACS Early Warning Scheme (EWS) for reporting of significant hull damage

3.5.1    IACS has organised and set up a system to permit the collection, and dissemination amongst Member Societies of information (while excluding a ship's identity) on major hull damages.

3.5.2    The principal purpose of the IACS Early Warning Scheme is to enable a Classification Society with experience of a specific damage to make this information available to the other societies so that action can be implemented to avoid repetition of damage to hulls where similar structural arrangements are employed.

3.5.3    These guidelines have incorporated the experience gained from IACS EWS reporting.

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4 Survey planning, preparation and execution

4.1    General

4.1.1    The owner should be aware of the scope of the forth coming survey and instruct those responsible, such as the master or the superintendent, to prepare necessary arrangements. If there is any doubt, the Classification Society concerned is to be consulted.

4.1.2    Survey execution will naturally be heavily influenced by the type of survey to be carried out. The scope of survey will have to be determined prior to the execution.

4.1.3    When deemed prudent and/or required by virtue of the periodic classification survey conducted, the surveyor should study the ship's structural arrangements and review the ship’s operating and survey history and those of sister ships, where possible, to determine any known potential problem areas particular to the class of the ship. Sketches of typical structural elements should be prepared in advance so that any defects and/or ultrasonic thickness measurements can be recorded rapidly and accurately.

4.2    Conditions for survey

4.2.1    The owner is to provide the necessary facilities for a safe execution of the survey.

4.2.2    Tanks and spaces are to be safe for access, i.e. gas freed (marine chemist certificate), ventilated, etc.

4.2.3    Tanks and spaces are to be sufficiently clean and free from water, scale, dirt, oil residues, etc. and sufficient illumination is to be provided, to reveal corrosion, deformation, fractures, damages or other structural deterioration. In particular this applies to areas which are subject to thickness measurement.

4.3    Access arrangement and safety

4.3.1    In accordance with the intended survey, measures are to be provided to enable the hull structure to be examined in a safe and practical way.

4.3.2    In accordance with the intended survey in cargo holds and salt water ballast tanks a secure and acceptable means of access is to be provided. This can consist of permanent staging, temporary staging or ladders, lifts and movable platforms, or other equivalent means.

4.3.3    In addition, particular attention should be given to the following guidance:

(a) Prior to entering tanks and other enclosed spaces, e.g. chain lockers, void

spaces, it is necessary to ensure that the oxygen content is to be tested and confirmed as safe. A responsible member of the crew should remain at the entrance to the space and if possible communication links should be established with both the bridge and engine room. Adequate lighting should be provided in addition to a hand held torch (flashlight).

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Contents

1 Introduction

2    Class survey requirements

2.1    Periodical classification surveys

2.1.1    General

2.1.2    Special Survey

2.1.3    Annual Survey

2.1.4    Intermediate Survey

2.1.5    Drydock Survey

2.2    Damage and repair surveys

3    Technical background for surveys

3.1    General

3.2    Definitions

3.3    Structural damages and deterioration

3.3.1    General

3.3.2    Material wastage

3.3.3    Fractures

3.3.4    Deformations

3.4    Structural detail failures and repairs

3.5    IACS Early Warning Scheme (EWS) for reporting of significant damage

4    Survey planning, preparation and execution

4.1    General

4.2    Conditions for survey

4.3    Access arrangement and safety

4.4    Equipment and tools

4.4.1    Personnel protective equipment

4.4.2    Personnel survey equipment

4.4.3    Thickness measurement and fracture detection

4.5    Survey at sea or at anchorage

4.6    Documentation on board

5    Structural detail failures and repairs

5.1    General

5.2    Catalogue of structural detail failures and repairs Part 1 Cargo hold region

Area 1 Upper deck structure

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Area 2 Side structure

Area 3 Transverse bulkhead structure

Area 4 Tween deck structure

Area 5 Double bottom structure

Part 2 Fore and aft end regions

Area 1 Fore end structure Area 2 Aft end structure

Area 3 Stern frame, rudder arrangement and propeller shaft support

Part 3 Machinery and accommodation spaces

Area 1 Engine room structure Area 2 Accommodation structure

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1 Introduction

The International Association of Classification Societies (IACS) is introducing a series of manuals with the intention of giving guidelines to assist the Surveyors of IACS Member Societies, and other interested parties involved in the survey, assessment and repair of hull st ructures for cert ain ship types.

This manual gives guidelines for a general cargo ship which is designed with one or more decks specifically for the carriage of diverse forms of dry cargo.

Figure 1 shows a typical general arrangement of a general cargo ship with single tween deck.

The guidelines focus on the IACS Member Societies’ survey procedures but may also be useful in connection with inspection/examination schemes of other regulatory bodies, owners and operators.

The manual includes a review of survey preparation guidelines which cover the safety aspects related to the performance of the survey, the necessary access facilities, and the preparation necessary before the surveys can be carried out.

The survey guidelines encompass the different main structural areas of the hull where damages have been recorded, focusing on the main features of the structural items of each area.

An important feature of the manual is the inclusion of the section which illustrates examples of structural deterioration and damages related to each structural area and gives what to look for, possible cause, and recommended repair methods, when considered appropriate.

The “IACS Early Warning Scheme (EWS)”, with the emphasis on the proper reporting of significant hull damages by the respective classification societies, will enable the analysis of problems as they arise, including revisions of these Guidelines.

This manual has been developed using the best information currently available. It is intended only as guidance in support of the sound judgment of surveyors, and is to be used at the surveyors' discretion. It is recognized that alternative and satisfactory methods are already applied by surveyors. Should there be any doubt with regard to interpretation or validity in connection with particular applications, clarification should be obtained from the Classification Society concerned.

Figures 2 (a) and (b)show cargo hold structural configurations for general cargo ships. As many different cargoes are carried by general cargo ships, hull structures

2 Class survey requirements

2.1    Periodical classification surveys

2.1.1    General

For Class the programme of periodical hull surveys is of prime importance as far as structural assessment of the cargo holds, and the adjacent tanks is concerned. The programme of periodical hull surveys consists of Annual. Intermediate, and Special Surveys. The Purpose of the Annual and Intermediate Surveys is to confirm that the general condition of the vessel is maintained at a satisfactory level. The Special Surveys of the hull structure are carried out at five year intervals with the purpose of establishing the condition of the structure to confirm that the structural integrity is satisfactory in accordance with the Classification Requirements, and will remain fit for its intended purpose until the next Special Survey. subject to proper maintenance and operation. The Special Surveys are also aimed at detecting possible damage and to establish the extent of any deterioration.

The Annual, Intermediate, and Special Sui*veys are briefly introduced in the following 2.1.2- 2.1.4. The surveys are carried out in accordance with the requirements specified in the Rules and Regulations of each IACS Member Society.

2.1.2    Special Survey

The Special Survey concentrates on examination in association with thickness determination. The report of the thickness measurement is recommended to be retained on board. Protective coating condition will be recorded for particular attention during the survey cycle. From 1991 it is a requirement for new ships to apply a protective coating to the structure in water ballast tanhs which form part of the hull boundary.

2.1.3    Annual Survey

At Annual Surveys overall survey is required. For saltwater ballast tanks, examinations may be required as a consequence of the Intermediate or Special Surveys.

2.1.4    Intermediate Survey

At Intermediate Surveys, in addition to the surveys required for Annual Surveys, examination of cargo holds and ballast tanks is required depending on the ship’s age.

2.1.5    Drydock Survey

Drydoch Surveys are requested twice during the Special Survey interval. In some cases it may be possible to replace one Drydoch Survey with an In-Water Survey. This will depend on the survey requirements of the relevant Classification Society.

2.2    Damage and repair surveys

Damage surveys are occasional surveys which are. in general, outside the programme of Periodical hull surveys and are requested as a result of hull

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damage or other defects. It is the responsibility of the owner or his representative to inform the Classification Society concerned when such damage or defect could impair the structural capability or watertight integrity of the hull. The damages should be inspected and assessed by the Society’s surveyors and the relevant repairs, if needed, are to be performed. In certain cases, depending on the extent, type and location of the damage, permanent repairs may be deferred to coincide with the planned periodical survey.

In cases of repairs intended to be carried out by riding crew during voyage, complete procedure including all necessary surveys is to be submitted to and agreed upon by the Classification Society reasonably in advance.

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3 Technical background for surveys

3.1    General

3.1.1    The purpose of carrying out the periodical hull surveys is to detect possible structural defects and damages and to establish the extent of any deterioration. To help achieve this and to identify key locations on the hull structure that might warrant special attention, knowledge of any historical problems of the particular ship or other ships of a similar class is to be considered if available. In addition to the periodical surveys, occasional surveys of damages and repairs are carried out. Records of typical occurrences and chosen solutions should be available in the ship's history file.

3.2    Definitions

3.2.1    For clarity of definit ion and reporting of survey data, it is recommended that standard nomenclature for structural elements be adopted. Typical sections in way of cargo holds are illustrated in Figures 3 (a) and (b). These figures show the generally accepted nomenclature.

The terms used in these guidelines are defined as follows:

(a)    Ballast Tank is a tank which is being used primarily for salt water ballast.

(b)    Spaces are separate compartments including holds and tanks.

(c)    Overall Inspection is an inspection intended to report on the overall condition of the hull structure and determine the extent of additional close-up inspections.

(d)    Close-up Inspection is an inspection where the details of structural components are within the close visual inspection range of the surveyors, i.e. normally within reach of hand.

(e)    Transverse Section includes all longitudinal members such as plating, longitudinals and girders at the deck, side, bottom and inner bottom.

(f)    Representative Spaces are those which are expected to reflect the condition of other spaces of similar type and service and with similar corrosion protection systems. When selecting representative spaces, account should be taken of the service and repair history on board.

(g)    Transition Region is a region where discontinuity in longitudinal structure occurs, e.g. at forward bulkhead of engine room, collision bulkhead and bulkheads of deep cargo tanks in cargo hold region.

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