Section 2 Construction tolerances

2.1.1 Construction tolerances are to comply with Pt 3, Ch 1 General Regulations of the Rules for Special Service Craft. The additional guidance given in this Chapter should also be complied with where practicable.

2.2.1 Where defects are found in materials after delivery to the Builder, any rectification should be agreed with the LR Surveyor and should generally be in accordance with Ch 3, 1.12 Identification of materials or Ch 8, 1.10 Pressure weld tests of the Rules for Materials and aluminium respectively.

2.2.2 If lamination is found during plate preparation the extent of the lamination should be ascertained by ultrasonic examination. Renewal of the affected material is normally recommended.

2.3.1 Construction standards (including preparation of material, joint alignment, type of welded joint, fit-up prior to welding, steps to be taken for rectification of defects, etc.) are to be agreed between LR and the Builder. Such standards will be recorded and kept on file by LR. In a number of countries national standards exist in respect of the acceptable shipbuilding standards.

2.3.2 In shipyards certified under the LR Quality Assurance Scheme, the standards will have received general approval as part of the certification procedures and their application to particular craft will be included in the quality plan submitted to LR for approval.

2.3.3 In considering critical locations and their construction standards it also has to be borne in mind that craft construction is a traditional process with alignment standards based on heel lines. In addition, therefore, to establishing the more critical joints in terms of fatigue life, consideration of the thicknesses to be employed in these joints should be a fundamental factor. If the heel line principle is maintained at the toes of, for example, primary member end brackets, where increased thicknesses are employed, the arrangements will in reality, be out of line, even though perfect alignment is attained to the standard, see Figure 3.2.1 Recommended alignment of primary members. It is therefore recommended, particularly for aluminium, that a median line principle is employed at this local area so that an improved alignment can be more easily attained. From LR's point of view the various thicknesses of plating at structural joints, particularly the higher stress joints, is an important consideration. In this respect the gradient of load through the through thickness loaded plate should be controlled to a maximum of one in three, see Figure 3.2.1 Recommended alignment of primary members.

2.3.4 In addition to cruciform joint misalignment, the alignment of secondary stiffening and associated brackets is also important. Recommendations are made in Figure 3.2.1 Recommended alignment of primary members and Figure 3.2.2 Recommended alignment of secondary stiffening members. Where centreline alignment is recommended the tolerance guideline in Figure 3.2.2 Recommended alignment of secondary stiffening members should be followed.

2.3.5 Whenever possible the plate thicknesses, t ₁ and t _2, (see Figure 3.2.1 Recommended alignment of primary members) should be kept as close as possible in order to minimise the potential difficulties associated with a median line alignment.

2.4.1 Throughout the preparation of material and assembly of prefabrication units, the workmanship is to be inspected in order to ensure that correct procedures are being followed. By attention in the early stages of construction, undesirable procedures and faulty workmanship can be avoided, or their consequences minimised. When the existence of such defects is noticed prompt and suitable measures are to be taken for rectification.

2.4.2 Examination of structure will normally be carried out during the prefabrication of units, and liaison between the LR Surveyor and the Builder's drawing offices and quality control departments will ensure that attention is also given to details which may not have been included on approved plans (air and drainage holes, etc.) during early material preparation stages.

Figure 3.2.1 Recommended alignment of primary members

2.4.3 It should be borne in mind that visual examination of welds and plating of a finished structure does not necessarily ensure a complete and satisfactory survey. Procedures are to be such as to ensure that adequate inspection is made of joint preparation before welding. Attention is drawn to the guidance on welding and structural details in Pt 6, Ch 2 Construction Procedures and Pt 7, Ch 2 Construction Procedures of the Rules for Special Service Craft for steel and aluminium alloy craft respectively. Regular examination by the LR Surveyor, in conjunction with the Builder, of non-destructive examination and other Quality records provides a check on the quality of welding operations and any decline in standards should be investigated, including additional tests as considered desirable.

2.4.4 It is essential that a good standard of cleaning be achieved for these inspections. Special attention, as indicated in Ch 13, 1.7 Assembly and preparation for welding 1.7.8 of the Rules for Materials, are to be applied for aluminium. For steel structures welding slag should be removed and rusting of weld deposits should be removed by wire brushing.

2.5.1 The Builder and the LR Surveyors must ensure by regular and systematic examination that the control exercised up to the stage of block assembly is maintained by the efficient erection of blocks at the berth. It is particularly necessary at this point to ensure that fit-up, alignment, adjustment and welding of blocks is in accordance with the approved plans and building standards. Attention is to be given to the sequence of erection and of welding. Particular attention should be given to length allowance for unwelded stiffening member alignment.

2.5.2 With the assembly of large blocks careful attention should be paid to the areas in way of lifting lugs. It is not unusual to find small cracks in the vicinity of the weld area after removal of lugs. Where lugs are removed the dressing of the plate must be thorough and examination by means of crack detection of the finished surface is good practice. Repair of any cracks found must be carried out by skilled welders under strict control.

Figure 3.2.2 Recommended alignment of secondary stiffening members

2.5.3 Any unusual incidents during construction, such as fracturing of plates, should be noted and brought to the attention of the LR Surveyor. It will be necessary to have full information on the circumstances affecting such cases, such as position and extent of the fracture relative to adjacent structure and welds, atmospheric temperature, details of joints, precise stage and sequence of welding, type of electrode filler wire used, whether or not pre-heating was used, grades of steel or aluminium alloy involved and any other factors considered to have had a possible influence. Test details of affected plates and proposals for remedial measures should also be made available. Where doubt exists in establishing the source of such incidents, the assistance of LR's staff at Headquarters, should be sought.