Section 1 Fractures and requirements for repair
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Clasifications Register Guidance Information - Naval Survey Guidance for Steel Ships, January 2001 - Chapter 13 Fractures - Section 1 Fractures and requirements for repair

Section 1 Fractures and requirements for repair

Parent topic: Chapter 13 Fractures

1.1 General

1.1.1 Fractures occur as a result of stresses imposed on a steel which is incapable of withstanding it. The stresses are imposed in a number of ways which will determine the action to be taken to rectify the defect.

1.1.2 Fractures caused by impact, being the result of abnormal loads, are normally straightforward to repair and need no further investigation. However, fractures occurring in service which are not the result of impact damage require to be fully investigated, and the reason for fracture ascertained.

1.1.3 Fractures are often difficult to detect and they can be located by indications such as leaks, corrosion or coating breakdown. If the presence or extent of a fracture cannot accurately be determined, suitable NDE methods are to be employed. One or more of the following fracture detection means may be used:

  1. Radiographic equipment.

  2. Ultrasonic equipment.

  3. Magnetic particle equipment.

  4. Dye penetrant.

Where it is not possible to determine the presence of a fracture and a preventative repair is not possible due to operational, economic or practical reasons, the suspect area should be raised as a condition of class.

1.1.4 In general, all fractures are to be removed. Any request to not repair a fracture is to be accompanied by a justification. Such a document should include:

  1. Likely cause for the fracture.

  2. Likely recurrence of the cause.

  3. Qualification of the crack propagation rate.

  4. Qualification of the critical crack length.

  5. Proposal for monitoring and future repair.

If the justification is accepted by the surveyor the location and nature of the fracture is to be recorded and a suitable condition of class raised.

1.1.5 Fractures in minor or secondary structure may be repaired by gouging and rewelding, see Ch 15, 7.2 Fractures. Arc-air gouging may be used, but care is to be taken to ensure that all carbon deposits are removed before welding.

1.1.6 Fractures in primary structure are usually to be repaired by cutting out the fractured material and fitting an insert, see Ch 2,2, Table 3.2.1 Part replacement of plates and Ch 15 Repairs. In exceptional cases, fractures in primary structure may be repaired by gouging and rewelding, see Ch 15, 7.2 Fractures. In such cases, care is to be taken to ensure that the crack tip is removed and that the root run is free from defects by using suitable NDE methods.

1.1.7 Insert plates, and supporting members, are to be of sufficient size to ensure that the strength of the structure is maintained or enhanced, and to ensure that there will be no recurrence, see Ch 2, Table 3.2.1 Part replacement of plates. Butts of inserts and supporting members must be staggered. The structure may be enhanced by increasing the thickness above that of the plate using a 1:3 taper, or by using a higher material grade. Differing material grade or thickness must be approved by LR.

1.1.8 Fractures found to have started due to misalignment are to be reported.

1.1.9 For fractures in association with wastage, inserts are to be fitted in way of damaged structure.

1.1.10 All accesses in the structure for the purpose of shipping items of equipment etc. are to be cut in approved positions utilizing existing butts wherever possible. Where this is not possible the radius of the opening corners is not to be less than 12,5% of the width of opening or 75 mm whichever is the greater.

1.1.11 Where a fracture has been found in the main hull structure on one side of a ship and the cause is other than contact damage, an examination of the corresponding structure on the opposite side of the ship should be made to ensure that a similar failure has not taken place. This precaution is especially important in cases where corrosion is associated with the failure and may have been a contributory factor.

1.1.12 Localized fatigue fractures, perhaps in association with localized corrosion, may be difficult to detect and it is stressed that any areas in question should receive close attention at Periodic Surveys. It is due to the interdependence of structural components that it is important that all fatigue fractures, however localized, are repaired or made the subject of a Condition of Class.

1.1.13 Where fractures have occurred previously, careful consideration of design detail is necessary. It has been found that re-welding of such fractures may not be a permanent solution and recommendations should be made to prevent a recurrence. Ch 13, 2 Examples of fractures gives details of improvements to detail design and all modifications to structure are to be in accordance with the tolerance and alignment requirements of Ch 3, 4 Survey during construction and repair requirements, Ch 15 Repairs.

1.1.14 Detailed information of all fractures is valuable in investigations into the strength of ships, and is to be reported. The type of information which may be relevant (if known) is as follows:

  1. Temperature of the sea and air, and of any refrigerated or other compartments relevant to the fracture.

  2. Weather conditions at the time of the damage.

  3. Account of the occurrence, if possible from witnesses.

  4. Particulars of voyage: position, course and speed of ship at time of casualty, dates of sailing and arrival, etc.

  5. Details of damage, showing position and extent, with origins, and ends of any fractures.

  6. Details of loading, including stores, fuel oil, water ballast, etc. at the time of damage.

  7. Draughts forward and aft.

1.1.15 In addition, it is sometimes desirable for samples of the material to be obtained for examination. Such samples should be representative of the plates involved in the fracture, and should be about 450 mm square encompassing the actual fracture. The positions of greatest interest are the beginnings and ends of the fractures which can usually be located by the ‘chevron markings’ on the fractured surfaces. When such samples are obtained, a sketch should be made to indicate clearly their position and orientation with regard to the fracture and ship’s structure. The samples obtained should be retained at the port of origin pending instructions from Headquarters after consideration of the report and sketches. They should be marked by paint. Photographs are also of great assistance.

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