Section 3 Thickness measurement requirements

3.1.1 The extent of thickness measurement required is defined by the Rules for Naval Ships, Vol 1, Pt 1, Ch 3, 6 Special Survey – Thickness measurement requirements for steel ships. These requirements are summarised in Figure 11.3.2 Thickness measurement of steel shipsSpecial Survey I - Ship 6 years old.

3.1.2 As part of the preparation for Special Survey, the measurement may be dealt with in the year preceding the due date of the Special Survey.

3.1.3 If provisional recommendations for renewals have been made based on the thickness measurement results, repairs may be deferred to the Special Survey subject to the Surveyor’s agreement. Where repairs are deferred, they are to be recorded in the Survey Report and a suitable condition of class raised.

3.1.4 The Thickness Measurement Report should, where possible, be made on Lloyd’s Register’s forms: (see Ch 25 Appendix 6 Form TM1)

• Report on thickness measurement of all deck plating, all bottom shell plating or side shell plating
  Form TM1 (06/93) 1/2
• Report of thickness measurement of shell and deck plating (one, two or three transverse sections)
  Form TM2(a) (06/93) 1/2 and Form TM2(b) (06/93)
• Report on thickness measurement of miscellaneous structural members
  Form TM6 (06/93) 1/2
• Report on thickness measurement of topside and bottom area assessments
  Form TM8(06/93).
• Other forms from the same package may be used.

3.1.5 In selecting the position of transverse sections for measurement, a careful overall assessment is to be made utilizing shell expansion and deck plans in order to avoid local reinforcements, doublers or any other obstructions, both on deck and at the shell.

3.1.6 Transverse sections are to be chosen where the largest reductions in thickness are revealed from deck plating measurements or where suspected during survey. Transverse sections should be well clear of:

• The ends of superstructures where local increases have usually been made to the sheerstrake and stringer plate.
• Strengthening in the way of the corners of openings or deck-house which may not always be apparent when welded insert plates have been used.

3.1.7 Areas of structure where the corrosion is considered substantial are to be listed as memoranda items and subject to annual inspection and gauging to the satisfaction of the attending Surveyor.

3.2.1 Thickness measurement is normally to be by means of ultrasonic test equipment and carried out by a firm qualified as Grade 1 or 2 in accordance with LR’s Approval for Thickness Measurement of Hull Structure.

3.2.2 Surveyors are to co-ordinate the survey with the thickness measurement company to ensure that the full requirements are satisfactorily dealt with. Surveyors should collaborate with the Owner and forward a full verified report.

3.2.3 Thickness measurement companies are to be reminded that where significant diminution of the structure is found during the survey, then this is to be brought to the attention of the Owners and Surveyors as early as practicable.

3.2.4 Where diminution of the structure is found to be zero or minimal, this should also be brought to the Surveyor’s early attention in order that the extent of measurement can be given further consideration.

3.2.5 Owners’ representatives and Thickness Measurement Companies are to be informed that the thickness measurement report is to be submitted to the Surveyor for verification.

3.2.6 Thickness measurement should indicate the general condition of plates and not the thickness in way of isolated pits or localized corrosion which should be dealt with as necessary by the Surveyors, see Figure 11.3.1 Method of measuring (a) individual plates in deck and shell and (b) transverse sections.

3.2.7 Thickness Measurement firms are to be encouraged to use LR’s Thickness Measurement Software for reporting.

3.3.1 As an alternative to large scale renewal, doubling plates of the requisite cross-sectional area and extent may be considered. This will depend on the amount and extent of corrosion and the type of structure affected. The requirements of Ch 15, 7.4 Doubling plates are to be complied with.

3.3.2 The longitudinal extent of renewals or doubling is to be determined after measurement and assessment of sufficient structures within 0,5L, forward and aft of the section under consideration. In general terms, it should extend at least 500 mm beyond the corroded area.

3.3.3 Additional structures outside 0,5L may also be assessed to determine the full extent of renewals or doublings.

3.3.4 Doubling plates should be efficiently attached to plating using slot welds, in accordance with Ch 13, 2.7 Closing plates of the Rules for Materials, or by arranging in strips approximately 200 mm wide. The ends of the strips are to have a 1 in 3 taper to a 50 mm radius nose, the weld throat thickness is to be increased in way of the taper, so that an abrupt change in section is avoided.