2.8 Ships engaged in towing and escort operations
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Statutory Documents - IMO Publications and Documents - International Codes - 2008 IS Code – International Code on Intact Stability, 2008 - Part B – Recommendations for ships engaged in certain types of operations, certain types of ships and additional guidelines - Chapter 2 – Recommended design criteria for ships engaged in certain types of operations and certain types of ships - 2.8 Ships engaged in towing and escort operations

2.8 Ships engaged in towing and escort operations

2.8.1 Application

The provisions given hereunder apply to ships the keel of which is laid or which is at a similar stage of constructionfootnote on or after 1 January 2020 engaged in harbour towing, coastal or ocean-going towing and escort operations and to ships converted to carry out towing operations after this date.

2.8.2 Heeling lever for towing operations

2.8.2.1 The self-tripping heeling lever is calculated as provided below:

  • .1 A transverse heeling moment is generated by the maximum transverse thrust exerted by the ship's propulsion and steering systems and the corresponding opposing towline pull.

  • .2 The heeling lever HLφ, in (m), as a function of the heeling angle φ, should be calculated according to the following formula:

  • where

    • BP = bollard pull, in (kN), which is the documented maximum continuous pull obtained from a static bollard pull test performed in accordance with relevant IMO guidelinesfootnote or a standard acceptable to the Administration;

    • CT = • 0.5,

      for ships with conventional, non-azimuth propulsion units;

      • 0.90/(1 + l/LLL),

      for ships with azimuth propulsion units installed at a single point along the length. However, CT should not be less than 0.7 for ships with azimuth stern drive towing over the stern or tractor tugs towing over the bow, and not less than 0.5 for ships with azimuth stern drive towing over the bow or tractor tugs towing over the stern;

  • For tugs with other propulsion and/or towing arrangements, the value of CT is to be established on a case by case basis to the satisfaction of the Administration.

    • Δ = displacement, in (t);

    • l = longitudinal distance, in (m), between the towing point and the vertical centreline of the propulsion unit(s) relevant to the towing situation considered;

    • h = vertical distance, in (m), between the towing point and the horizontal centreline of the propulsion unit(s) as relevant for the towing situation considered;

    • g = gravitational acceleration, in (m/s2), to be taken as 9.81;

    • r = the transverse distance, in (m), between the centre line and the towing point, to be taken as zero when the towing point is at the centre line.

    • LLL = length (L) as defined in the International Convention on Load Lines in force.

  • The towing point is the location where the towline force is applied to the ship. The towing point may be a towing hook, staple, fairlead or equivalent fitting serving that purpose.

2.8.2.2 The tow-tripping heeling lever HLφ, in (m), is calculated according to the following formula:

  • HLφ = C1 × C2 × ɣ × V2 × Ap ×(h × cosφ - r × sinφ + C3 × d)/(2 × g × Δ)

  • where

  • C1 = lateral traction coefficient = 0.10 ≤ C1 ≤ 1.00

  • C2 = correction of C1 for angle of heel = C2 ≥ 1.00

    • Angle to deck edge

  • C3 = distance from the centre of AP to the waterline as fraction of the draught related to the heeling angle

  • C3 = × 0.26 + 0.30 0.50≤ C3 ≤ 0.83

  • γ = specific gravity of water, in (t/m3);

  • V = lateral velocity, in (m/s), to be taken as 2.57 (5 knots);

  • AP = lateral projected area, in (m2), of the underwater hull;

  • r = the transverse distance, in (m), between the centre line and the towing point, to be taken as zero when the towing point is at the centre line;

  • LS = the longitudinal distance, in (m), from the aft perpendicular to the towing point;

  • LPP= length between perpendiculars, in (m);

  • φ = the angle of heel;

  • f = freeboard amidship, in (m);

  • B = the moulded breadth (m);

  • h = vertical distance, in (m), from the waterline to the towing point;

  • d = actual mean draught, in (m).

  • The towing point is the location where the towline force is applied to the ship. The towing point may be a towing hook, staple, fairlead or equivalent fitting serving that purpose.

2.8.3 Heeling lever for escort operations

2.8.3.1 For the evaluation of the stability particulars during escort operations the ship is considered to be in an equilibrium position determined by the combined action of the hydrodynamic forces acting on hull and appendages, the thrust force and the towline force as shown in figure 2.8-1.

2.8.3.2 For each equilibrium position the corresponding steering force, braking force, heel angle and heeling lever are to be obtained from the results of full scale trials, model tests, or numerical simulations in accordance with a methodology acceptable to the Administration.

2.8.3.3 For each relevant loading condition the evaluation of the equilibrium positions is to be performed over the applicable escort speed range, whereby the speed of the assisted ship through the water is to be considered.footnote

2.8.3.4 For each relevant combination of loading condition and escort speed, the maximum heeling lever is to be used for the evaluation of the stability particulars.

2.8.3.5 For the purpose of stability calculations the heeling lever is to be taken as constant.

Figure 2.8-1: Escort tug equilibrium position

2.8.4 Stability criteria

2.8.4.1 In addition to the stability criteria given in part A, section 2.2, or the equivalent stability criteria given in chapter 4 of the explanatory notes to the 2008 IS Code where the ship's characteristics render compliance with part A, section 2.2 impracticable, the following stability criteria should be complied with.

2.8.4.2 For ships engaged in harbour, coastal or ocean-going towing operations the area A contained between the righting lever curve and the heeling lever curve calculated in accordance with paragraph 2.8.2.1 (self-tripping), measured from the heel angle, φe, to the angle of the second intersection, φc, or the angle of down-flooding, φf, whichever is less, should be greater than the area B contained between the heeling lever curve and the righting lever curve, measured from the heel angle φ = 0 to the heel angle, φe.

where:

  • φe = Angle of first intersection between the heeling lever and righting lever curves;

  • φf = Angle of down-flooding as defined in part A, paragraph 2.3.1.4 of this Code. Openings required to be fitted with weathertight closing devices under the ICLL but, for operational reasons, are required to be kept open should be considered as down-flooding points in stability calculation;

  • φc = Angle of second intersection between the heeling lever and righting lever curves.

2.8.4.3 For ships engaged in harbour, coastal or ocean-going towing operations the first intersection between the righting lever curve and the heeling lever curve calculated in accordance with paragraph 2.8.2.2 (tow-tripping) should occur at an angle of heel less than the angle of down-flooding, φf.

2.8.4.4 For ships engaged in escort operations the maximum heeling lever determined in accordance with paragraph 2.8.3 should comply with the following criteria:

  • .1 Area A ≥ 1.25 × Area B;

  • .2 Area C ≥ 1.40 × Area D; and

  • .3 φe ≤ 15 degrees.

where:

  • Area A = Righting lever curve area measured from the heel angle φe to a heel angle of 20 degrees (see figure 2.8-2);

  • Area B = Heeling lever curve area measured from the heeling angle φe to a heel angle of 20 degrees (see figure 2.8-2);

  • Area C = Righting lever curve area measured from the zero heel (φ = 0) to φd (see figure 2.8-3);

  • Area D = Heeling lever curve area measured from zero heel (φ = 0) to the heeling angle φd (see figure 2.8-3);

  • φe = Equilibrium heel angle corresponding to the first intersection between heeling lever curve and the righting lever curve;

  • φd = the heel angle corresponding to the second intersection between heeling lever curve and the righting lever curve or the angle of down-flooding or 40 degrees, whichever is less.

Figure 2.8-2: Areas A and B Figure 2.8-3: Areas C and D

2.8.5 Constructional precautions against capsizing

2.8.5.1 Access to the machinery space, excluding emergency access and removal hatches, should, if possible, be arranged within the forecastle. Any access to the machinery space from the exposed cargo deck should be provided with two weathertight closures, if practicable. Access to spaces below the exposed cargo deck should preferably be from a position within or above the superstructure deck.

2.8.5.2 The area of freeing ports in the side bulwarks of the cargo deck should at least meet the requirements of regulation 24 of the International Convention on Load Lines, 1966 or the Protocol of 1988 relating thereto, as amended, as applicable. The disposition of the freeing ports should be carefully considered to ensure the most effective drainage of water trapped on the working deck and in recesses at the after end of the forecastle. In ships operating in areas where icing is likely to occur, no shutters should be fitted in the freeing ports.

2.8.5.3 A ship engaged in towing operations should be provided with means for quick release of the towline.footnote

2.8.6 Operational procedures against capsizing

2.8.6.1 The arrangement of cargo stowed on deck should be such as to avoid any obstruction of the freeing ports or sudden shift of cargo on deck. Cargo on deck, if any, should not interfere with the movement of the towline.

2.8.6.2 A minimum freeboard at stern of at least 0.005×LLL should be maintained in all operating conditions.

Parent topic: Chapter 2 – Recommended design criteria for ships engaged in certain types of operations and certain types of ships
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