Section 2 Towing

2.1.1 The strength of shipboard fittings used for normal towing operations at bow, sides and stern and their supporting hull structures are to comply with the requirements specified in this sub-Section.

2.2.1 Shipboard fittings for towing are to be located on stiffeners and/or girders which are part of the deck construction so as to facilitate efficient distribution of the towing load. Other arrangements are acceptable, provided that the strength is confirmed adequate for the intended service.

2.3.1 The design load applied to shipboard fittings and supporting hull structure is not to be less than 1,25 times the intended maximum towing load (e.g. static bollard pull) as indicated on the towing arrangements plan.

2.3.2 When a safe towing load (TOW) greater than that determined according to Pt 3, Ch 10, 2.8 Safe towing load (TOW) 2.8.1 is requested, then the design load is to be increased in accordance with the appropriate TOW/design load relationship given in this Section.

2.3.3 The side projected area is to be considered for selection of towing lines and the loads applied to shipboard fittings and supporting hull structure.

2.3.4 The increase of the minimum breaking strength for synthetic ropes need not to be considered for the loads applied to shipboard fittings and supporting hull structure.

2.4.1 Shipboard fittings are to be selected from an acceptable National or International Standard and to be based on the intended maximum towing load (e.g. static bollard pull) as indicated on the towing arrangements plan.

2.4.2 Towing bitts (double bollards) are to be chosen for the towing line attached with an eye splice if the industry standard distinguishes between different methods to attach the line, i.e. figure-of-eight or eye splice attachment.

2.4.3 When the shipboard fitting is not selected from an accepted industry standard, the strength of the fitting based on net scantlings and its attachment to the ship is to be adequate for the loads specified in Pt 3, Ch 10, 2.3 Design load 2.3.1 based on the acceptance criteria given in Pt 3, Ch 10, 2.5 Strength of supporting hull structures 2.5.2 or Pt 3, Ch 10, 2.5 Strength of supporting hull structures 2.5.3 as appropriate. Towing bitts (double bollards) are required to resist the loads caused by the towing line attached with an eye splice. For strength assessment, beam theory or finite element analysis using net scantlings is to be applied, as appropriate. Corrosion additions and wear down allowance is to be added to the net scantlings as defined in Pt 3, Ch 10, 2.6 Corrosion addition and Pt 3, Ch 10, 2.7 Wear allowance.

2.5.1 The net scantlings of the supporting hull structure for the fittings are to be adequate for the loads specified by the Pt 3, Ch 10, 2.3 Design load 2.3.1 based on the acceptance criteria given in by Pt 3, Ch 10, 2.5 Strength of supporting hull structures 2.5.2 or Pt 3, Ch 10, 2.5 Strength of supporting hull structures 2.5.3 as appropriate. The reinforced members beneath shipboard fittings are to be effectively arranged for any variation of direction (horizontally and vertically) of the towing forces acting upon the shipboard fittings, see Figure 10.2.2 Supporting hull structure for a sample arrangement. Proper alignment of the fitting and its supporting hull structure is to be ensured. The acting point of the towing force on a shipboard fitting is to be taken at the attachment point of a towing line or at a change in its direction. For bollards and bitts the attachment point of the towing line is to be taken not less than 4/5 of the tube height above the base as indicated in Figure 10.2.2 Supporting hull structure Corrosion additions and wear down allowance are to be added to the net scantlings as defined in Pt 3, Ch 10, 2.6 Corrosion addition and Pt 3, Ch 10, 2.7 Wear allowance.

2.5.2 In the case of strength assessment using beam theory or grillage analysis, the stress within the supporting structure of fittings is not to exceed that given in Table 10.2.1 Allowable stress within the supporting structure of shipboard fittings.

2.5.3 For strength calculations by means of finite element analysis, the geometry is to be idealised as realistically as possible. The ratio of element length to width is not to exceed 3. Girders are to be modelled using shell or plane stress elements. Symmetric girder flanges are generally to be modelled by beam or truss elements. At least three elements are to be used across the depth of the girder. In way of small openings in girder webs the web thickness is to be reduced to a mean thickness over the web height. Large openings are to be modelled. Stiffeners are generally to be modelled by using shell, plane stress, or beam elements. Stresses are to be read from the centre of the individual element. For shell elements the stresses are to be evaluated at the mid- plane of the element. The equivalent stress within the supporting structure of fittings is not to exceed the specified minimum yield strength of the material.

Figure 10.2.2 Supporting hull structure

Table 10.2.1 Allowable stress within the supporting structure of shipboard fittings

	Normal stress, in N/mm2	Shear stress, in N/mm2
Allowable stress
where σo = specified minimum yield strength of the material in N/mm2 Note Normal stress is defined as the sum of bending and axial stresses. No stress concentration factors are accounted for and as such may need to be considered separately.

2.6.1 An allowance for corrosion is to be added to the net thickness derived as indicated below:

• For the supporting hull structure, a corrosion addition of 2 mm is to be added to the net thickness derived.
• For pedestals and foundations on deck which are not part of a fitting according to an accepted industry standard, 2,0 mm.
• For shipboard fittings not selected from an accepted industry standard, 2,0 mm.

2.7.1  In addition to the corrosion addition given in Pt 3, Ch 10, 2.6 Corrosion addition, the wear allowance, t_w, for shipboard fittings that are not selected from an acceptable National or International standard, is not to be less than 1,0 mm, added to surfaces which are intended to regularly contact the line.

2.8.1 The safe towing load (TOW) is the load limit for towing purposes. The TOW used is not to exceed 80 per cent of the design load specified by Pt 3, Ch 10, 2.3 Design load 2.3.1.

2.8.2 The TOW, in tonnes, of each shipboard fitting is to be marked (by weld bead or equivalent) on the deck fittings used for towing.

2.8.3 The above requirements on the TOW apply for the use with no more than one towline line. If not otherwise chosen, for towing bitts (double bollards) the TOW is the load limit for a towing line attached with an eye-splice.

2.8.4 The towing and lifting arrangements plan mentioned in Pt 3, Ch 10, 4.1 Towing and lifting arrangements plan is to define the method of use of towing lines.