3 Towing
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Statutory Documents - IMO Publications and Documents - Circulars - Maritime Safety Committee - MSC.1/Circular.1175/Rev.1 – Revised Guidance on Shipboard Towing and Mooring Equipment – (9 December 2020) - Annex - Shipboard Equipment, Fittings and Supporting Hull Structures Associated with Towing and Mooring - 3 Towing

3 Towing

3.1 Strength

The strength of shipboard fittings used for normal towing operations and their supporting hull structures should comply with the provisions of 3.2 to 3.6. 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 should also comply with these provisions. The strength of shipboard fittings intended to be used for both towing and mooring and of their supporting hull structures should also comply with the provisions of section 4.

 3.2 Arrangements

Shipboard fittings for towing should 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 equivalent arrangements may be accepted (for chocks in bulwarks, etc.), provided the strength is confirmed as adequate for the intended service.

3.3 Load considerations

3.3.1 The minimum design load applied to supporting hull structures for shipboard fittings should be:

  • .1 for normal towing operations, 1.25 times the intended maximum towing load (e.g. static bollard pull), as indicated on the towing and mooring arrangements plan;

  • .2 for other towing services, the ship design minimum breaking load of the tow line defined in appendix A; and

  • .3 for fittings intended to be used for both normal and other towing operations, the greater of the design loads according to .1 and .2.

3.3.2 The design load should be applied to fittings in all directions that may 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. However, in no case does the design load applied to the fitting need to be more than twice the design load on the line as specified in 3.3.1 (see figure below).

 3.4 Shipboard fittings

3.4.1 Shipboard fittings may be selected from an industry standard accepted by the Administration and at least based on the following loads:

  • .1 for normal towing operations, the intended maximum towing load (e.g. static bollard pull) as indicated on the towing and mooring arrangements plan;

  • .2 for other towing services, the ship design minimum breaking load of the tow line according to appendix A; and

  • .3 for fittings intended to be used for both normal and other towing operations, the greater of the loads according to .1 and .2.

3.4.2 When the shipboard fitting is not selected from an accepted industry standard, the strength of the fitting and of its attachment to the supporting hull structure should be in accordance with 3.3 and 3.5.

 3.5 Supporting hull structure

3.5.1 The reinforcing members beneath shipboard fittings should be effectively arranged for any variation of direction (horizontally and vertically) of the towing forces acting upon the shipboard fittings. Proper alignment of fitting and supporting hull structures should be ensured.

3.5.2 The acting point of the towing force on shipboard fittings should 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 should be taken not less than 4/5 of the tube height above the base (see figure below).

3.5.3 Under the design load conditions as specified in 3.3 the allowable normal stress should be taken as 100% and the allowable shearing stress as 60% of the specified yield point for the material used. Normal stress is the sum of bending stress and axial stress with the corresponding shearing stress acting perpendicular to the normal stress. No stress concentration factors being taken into account.

 3.6 Safe towing load (TOW)

3.6.1 TOW used for normal towing operations should not exceed 80% of the design load as given in 3.3.1.1 and TOW used for other towing operations should not exceed 80% of the design load as given in 3.3.1.2. For fittings used for both, normal and other towing operations, the greater of the safe towing loads should be used.

3.6.2 TOW, in tonnes, of each shipboard fitting should be marked (by weld bead or equivalent) on the fittings intended for towing. For fittings intended to be used for both, towing and mooring, SWL, in tonnes, according to 4.6, should be marked in addition to TOW.

3.6.3 The above provisions on TOW apply for the use of no more than one towing line.

3.6.4 The towing and mooring arrangements plan described in section 5 should define the method of use of towing lines.

Parent topic: Annex - Shipboard Equipment, Fittings and Supporting Hull Structures Associated with Towing and Mooring
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