Section 1 Thickness measurement philosophy

1.1.1 The objective of taking thickness measurements (TM) is twofold:

1. Assessment of global strength considerations, in order to recommend renewal or compensation for loss of topside or bottom area on the basis of TM readings.

2. Assessment of local strength considerations, in order to recommend renewal, or partial renewal of individual plates and stiffeners on the basis of thickness measurement readings.

1.2.1 To ensure that the global hull strength has not been eroded to an unacceptable level, thickness gauging is to be carried out as detailed in Figure 11.3.2 Thickness measurement of steel shipsSpecial Survey I - Ship 6 years old and Vol 1, Pt 1, Ch 3, 6 Special Survey – Thickness measurement requirements for steel ships of the Rules and Regulations for the Classification of Naval Ships (hereinafter referred to as the Rules for Naval Ships). The extent of TM depends upon the age of the ship, and typically is undertaken to the wind and waterline and to transverse sections around the ship.

1.2.2 In well-maintained naval ships, it is unlikely that there will be overall plate diminution without it being accompanied by a heavily pitted surface or breakdown of coating. Therefore the TM requirements for the hull in accordance with Vol 1, Pt 1, Ch 3 Periodical Survey Regulations of the Rules for Naval Ships, may be partially waived (at the Surveyor’s discretion) where it can be confirmed by random measurement and visual inspection of the coated surface that there is no sign of any corrosion having taken place. Care should be taken when performing a visual inspection that the surface of the plate is being inspected, and that only thin coatings are present. Lagging, lining, screed and cladding material may need to be removed.

1.2.3 Note that for existing ships being taken into Naval Class for the first time thickness measurements are mandatory. The minimum extent of hull thickness gauging is given in Vol 1, Pt 1, Ch 3 Periodical Survey Regulations of the Rules for Naval Ships (to establish the thickness of plate fitted at build). For ships built under special survey by a recognised Classification Society or other on-site Owner’s or independent quality control and inspection procedures, where the thickness of plate can be confidently established, the extent of gauging may be reduced.

Figure 11.1.1 Material thickness

Figure 11.1.2 Plan notation

1.2.4 Particular attention is drawn to the need for maintaining the buckling resistance of longitudinals. Longitudinals must be renewed if there is any evidence of buckling or if the diminution is below design values.

1.3.1 Where the size and extent of local repairs to corroded plate needs to be determined, additional thickness measurements may be required. The number and spacing of the measurements is at the discretion of the surveyor and the number should be adequate to map the affected area such that the repair will reinstate the material strength.