Section 9 Structural requirements associated with towing and mooring

9.1.1 This Section applies to the design and construction of shipboard fittings and supporting structures used for the normal towing and mooring operations. Normal towing means towing operations necessary for manoeuvring in ports and sheltered waters associated with the normal operations of the ship.

9.1.2 The arrangements, equipment and fittings of sufficient safe working load are to be provided to enable the safe conduct of all towing and mooring operations associated with the normal operations of the ship.

9.1.3 For ships, not subject to SOLAS Regulation II-1/3-4 Paragraph 1, but intended to be fitted with equipment for towing by another ship or a tug, e.g. such as to assist the ship in case of emergency as given in SOLAS Regulation II-1/3-4 Paragraph 2, the requirements designated as ‘other towing’ in this Section are to be applied to the design and construction of those shipboard fittings and supporting hull structures.

9.1.4 This Section is not applicable to the design and construction of shipboard fittings and supporting hull structures used for special towing services such as escort towing, canal transit towing, emergency towing for tankers etc. These requirements are also not applicable to special purpose ships.

9.1.5 Shipboard fittings means bollards and bitts, fairleads, stand rollers, chocks used for the normal mooring of the ship, and the similar components used for normal or other towing of the ship. Any weld or bolt or equivalent device connecting the shipboard fitting to the supporting structure is part of the shipboard fitting. Other components such as capstans, winches, etc. are not covered by this Section.

9.1.6 Supporting hull structures means that part of the ship structure on/in which the shipboard fitting is placed and which is directly submitted to the forces exerted on the shipboard fitting. The supporting hull structure of capstans, winches, etc. used for normal or other towing and mooring operations mentioned above is also to comply with the requirements specified in this Section.

9.1.7 The nominal capacity condition is defined as the theoretical condition where the maximum possible deck cargoes are included in the ship arrangement in their respective positions. For container ships, the nominal capacity condition represents the theoretical condition where the maximum possible number of containers is included in the ship arrangement in their respective positions.

9.1.8 Ship Design Minimum Breaking Load (MBL_SD) is the minimum breaking load of new, dry mooring lines or tow line for which shipboard fittings and supporting hull structures are designed in order to meet mooring restraint requirements or the towing requirements of other towing service.

9.1.9 Line Design Break Force (LDBF) is the minimum force at which a new, dry, spliced, mooring line will break at. This is applicable to all synthetic cordage materials.

9.2.1 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. For fittings intended to be used for both towing and mooring, Pt 3, Ch 13, 9.3 Mooring is also to be applied.

9.2.2 Where a ship is equipped with shipboard fittings intended to be used for other towing services, the strength of these fittings and their supporting hull structures are also to comply with the requirements specified.

9.2.3 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 (for chocks in bulwarks, etc.), provided that the strength is confirmed adequate for the intended service.

9.2.4 The design load applied to shipboard fittings and supporting hull structure is not to be less than that given in Table 13.9.1 Minimum design load for deck fittings and supporting structure - Towing.

9.2.5 The design load is to be applied to fittings in all directions that could occur by taking into account the arrangement shown on the towing and mooring arrangements plan. Where the towing line takes a turn at a fitting, the total design load applied to the fitting is equal to the resultant of the design loads acting on the line, see Figure 13.9.1 Design load applied to fittings. However, in no case does the design load applied to the fitting need to be greater than twice the design load on the line.

Figure 13.9.1 Design load applied to fittings

9.2.7 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.

9.2.8 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 by the Table 13.9.1 Minimum design load for deck fittings and supporting structure - Towing based on the acceptance criteria given in Pt 3, Ch 13, 9.2 Towing 9.2.10 or Pt 3, Ch 13, 9.2 Towing 9.2.11 as appropriate. The capability of the structure to withstand buckling is also to be assessed. 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 this Section.

9.2.9 The net scantlings of the supporting hull structure for the fittings are to be adequate for the loads specified by the Table 13.9.1 Minimum design load for deck fittings and supporting structure - Towing based on the acceptance criteria given in by Pt 3, Ch 13, 9.2 Towing 9.2.10 or Pt 3, Ch 13, 9.2 Towing 9.2.11 as appropriate. The capability of the structure to withstand buckling is also to be assessed. 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 13.9.2 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 13.9.2 Supporting hull structure. Corrosion additions are to be added to the net scantlings as defined in this Section.

Figure 13.9.2 Supporting hull structure

9.2.10 In the case of strength assessment using beam theory or grillage analysis, the stress within the supporting structure of fittings with net scantlings is not to exceed that given in Table 13.9.2 Allowable stress within the supporting structure of shipboard fittings.

9.2.11 For strength assessment by means of finite element analysis the mesh is to be fine enough to represent the geometry as realistically as possible. The aspect ratios of elements are not to exceed 3. Girders are to be modelled using shell or plane stress elements. Symmetric girder flanges may be modelled by beam or truss elements. The element height of girder webs must not exceed one-third of the web height. In way of small openings in girder webs the web thickness is to be reduced to an appropriate mean thickness over the web height. Large openings are to be modelled. Stiffeners may be modelled using shell or plane stress elements. The mesh size of stiffeners is to be fine enough to obtain proper bending stress. If flat bars are modelled using shell or plane stress elements, then dummy rod elements are to be modelled at the free edge of the flat bars and the stresses of the dummy elements are to be evaluated. 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 Von Mises stress within the supporting structure of fittings, calculated with net scantlings, is not to exceed the specified minimum yield strength of the material.

Table 13.9.2 Allowable stress within the supporting structure of shipboard fittings

	Normal stress, in N/mm2	Shear stress, in N/mm2
Allowable stress
where σ0 = 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 accounted for and as such may need to be considered separately.

9.2.12 The safe towing load (TOW) is the safe load limit of shipboard fittings used for towing purposes. TOW used is not to exceed 80 per cent of the design loads specified by Table 13.9.1 Minimum design load for deck fittings and supporting structure - Towing.

9.2.13 TOW, in tonnes, of each shipboard fitting is to be marked (by weld bead or equivalent) on the deck fittings used for towing. For fittings intended to be used for both, towing and mooring, SWL, in tonnes, according to Pt 3, Ch 13, 9.3 Mooring is to be marked in addition to TOW.

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

9.2.15 The towing and mooring arrangements plan mentioned in Pt 3, Ch 13, 9.4 Towing and mooring arrangements plan is to define the method of use of towing lines.

9.3.1 The strength of shipboard fittings used for mooring operations and their supporting hull structures as well as the strength of supporting hull structures of winches and capstans are to comply with the requirements specified in this sub-Section. For fittings intended to be used for both mooring and towing, Pt 3, Ch 13, 9.2 Towing is also to be applied.

9.3.2 Shipboard fittings, winches and capstans for mooring are to be located on stiffeners and/or girders which are part of the deck construction so as to facilitate efficient distribution of the mooring load. Other arrangements are acceptable (for chocks in bulwarks, etc.) provided that the strength is confirmed adequate for the service.

9.3.3 The design load applied to shipboard fittings and supporting hull structure is not to be less than that given in Table 13.9.3 Minimum design load for deck fittings and supporting structure - Mooring.

9.3.4 The design load is to be applied to fittings in all directions that could occur by taking into account the arrangement shown on the towing and mooring arrangements plan. Where the mooring line takes a turn at a fitting, the total design load applied to the fitting is equal to the resultant of the design loads acting on the line, see Figure 13.9.1 Design load applied to fittings. However, in no case does the design load applied to the fitting need to be greater than twice the design load on the line.

9.3.5 Shipboard fittings are to be selected from an acceptable National or International standard and to be based on the ship design minimum breaking load as given in Pt 3, Ch 13, 7.5 Mooring lines (Equipment Number ≤ 2000) or Pt 3, Ch 13, 7.6 Mooring lines (Equipment Number > 2000), corresponding to the ship’s equipment number (see Notes 2 and 3, Table 13.9.3 Minimum design load for deck fittings and supporting structure - Mooring).

9.3.6 Mooring bitts (double bollards) are to be chosen for the mooring line attached in figure-of-eight fashion if the industry standard distinguishes between different methods to attach the line, i.e. figure-of-eight or eye-splice attachment. With the line attached to a mooring bitt in the usual way (figure-of-eight fashion), either of the two posts of the mooring bitt can be subjected to a force twice as large as that acting on the mooring line. Disregarding this effect, depending on the applied industry standard and fitting size, overload may occur.

9.3.7 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 Table 13.9.3 Minimum design load for deck fittings and supporting structure - Mooring based on the acceptance criteria given in Pt 3, Ch 13, 9.3 Mooring 9.3.10 or Pt 3, Ch 13, 9.3 Mooring 9.3.11 as appropriate. The capability of the structure to withstand buckling is also to be assessed. Mooring bitts (double bollards) are required to resist the loads caused by the mooring line attached in figure-of-eight fashion. 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 as defined in this Section.

9.3.8 The net scantlings of the supporting hull structure for the fittings are to be adequate for the loads given in Table 13.9.3 Minimum design load for deck fittings and supporting structure - Mooring based on the acceptance criteria given in Pt 3, Ch 13, 9.3 Mooring 9.3.10 or Pt 3, Ch 13, 9.3 Mooring 9.3.11 as appropriate. The capability of the structure to withstand buckling is also to be assessed. The arrangement of reinforced members beneath shipboard fittings, winches and capstans is to consider any variation of direction (horizontally and vertically) of the mooring forces acting upon the shipboard fittings, see Figure 13.9.3 Supporting hull structure for a sample arrangement. Proper alignment of fitting and supporting hull structure is to be ensured. The acting point of the mooring force on shipboard fittings is to be taken at the attachment point of a mooring line or at a change in its direction. Corrosion additions are to be added to the net scantlings as defined in this Section.

9.3.9 For bollards and bitts the attachment point of the mooring line is to be taken not less than 4/5 of the tube height above the base, see Figure 13.9.3 Supporting hull structure. However, if fins are fitted to the bollard tubes to keep the mooring line as low as possible, then the attachment point of the mooring line is to be taken at the location of the fins, see Figure 13.9.3 Supporting hull structure.

Figure 13.9.3 Supporting hull structure

9.3.10 In the case of strength assessment using beam theory or grillage analysis, the stress within the supporting structure of fittings, with net scantlings, is not to exceed that given in Table 13.9.2 Allowable stress within the supporting structure of shipboard fittings.

9.3.11 For strength assessment by means of finite element analysis the mesh is to be fine enough to represent the geometry as realistically as possible. The aspect ratios of elements are not to exceed 3. Girders are to be modelled using shell or plane stress elements. Symmetric girder flanges may be modelled by beam or truss elements. The element height of girder webs must not exceed one-third of the web height. In way of small openings in girder webs the web thickness is to be reduced to an appropriate mean thickness over the web height. Large openings are to be modelled. Stiffeners may be modelled using shell or plane stress elements. The mesh size of stiffeners is to be fine enough to obtain proper bending stress. If flat bars are modelled using shell or plane stress elements, then dummy rod elements are to be modelled at the free edge of the flat bars and the stresses of the dummy elements are to be evaluated. 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 Von Mises stress within the supporting structure of fittings, calculated with net scantlings, is not to exceed the specified minimum yield strength of the material.

9.3.12 The Safe Working Load (SWL) is the safe load limit of shipboard fittings used for mooring purposes. Unless a greater SWL is requested, the SWL assigned shall be the ship design minimum breaking load given in Pt 3, Ch 13, 7.5 Mooring lines (Equipment Number ≤ 2000) and Pt 3, Ch 13, 7.6 Mooring lines (Equipment Number > 2000), corresponding to the ship’s equipment number (see Notes 2 and 3 of Table 13.9.3 Minimum design load for deck fittings and supporting structure - Mooring).

9.3.13 The SWL, in tonnes, of each shipboard fitting is to be marked (by weld bead or equivalent) on the deck fittings used for mooring. For fittings intended to be used for both, mooring and towing, the TOW, in tonnes, according to Pt 3, Ch 13, 9.2 Towing is to be marked in addition to the SWL.

9.3.14 The above requirements on SWL apply for the use with no more than one mooring line.

9.3.15 The towing and mooring arrangements plan mentioned in Pt 3, Ch 13, 9.4 Towing and mooring arrangements plan is to define the method of use of mooring lines.

9.4.1 The SWL and TOW for the intended use for each shipboard fitting is to be noted in the towing and mooring arrangements plan available on board for the guidance of the Master. It is to be noted that TOW is the load limit for towing purpose and SWL that for mooring purpose. If not otherwise chosen, for towing bitts it is to be noted that TOW is the load limit for a towing line attached with an eye splice.

Furthermore, information provided on the plan is to include:

9.4.3 The above information as given Pt 3, Ch 13, 9.4 Towing and mooring arrangements plan 9.4.2 is to be incorporated into the pilot card in order to provide the pilot with proper information on harbour and other towing operations.

9.5.2 For double hull oil tankers and bulk carriers with a CSR notation (see Pt 1, Ch 2, 2.3 Class notations (hull)), corrosion addition for the hull supporting structure is to be in accordance with IACS Common Structural Rules for Bulk Carriers and Oil Tankers.

9.6.1 In addition to the corrosion addition given in Pt 3, Ch 13, 9.5 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.