2.7 Ships engaged in anchor handling 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 - 2.7 Ships engaged in anchor handling operations

2.7 Ships engaged in anchor handling operations

2.7.1 Application

2.7.1.1 The provisions given hereunder apply to ships engaged in anchor handling operations.

2.7.1.2 A wire means a dedicated line (wire rope, synthetic rope or chain cable) used for the handling of anchors by means of an anchor handling winch.

2.7.2 Heeling levers

2.7.2.1 A heeling lever, HLφ, generated by the action of a heeling moment caused by the vertical and horizontal components of the tension applied to the wire should be calculated as:

  • HLφ = (MAH / Δ2) cos φ

where:

  • MAH = Fp × (h sin α × cos β + y × sin β);

  • Δ2 = displacement of a loading condition, including action of the vertical loads added (Fv), at the centreline in the stern of ship;

  • Fv = Fp × sin β;

  • α = the horizontal angle between the centreline and the vector at which the wire tension is applied to the ship in the upright position, positive outboard;

  • β = the vertical angle between the waterplane and the vector at which the wire tension is applied to the ship, positive downwards, should be taken at the maximum heeling moment angle as tan-1(y / (h × sin α)), but not less than cos-1(1.5 BP / (FP cos α)), using consistent units;

Figure 2.7.2 – Diagrams showing the intended meaning of parameters α, β, x, y and h. Ft shows the vector of the applied wire tension.

  • BP = the Bollard pull that is the documented maximum continuous pull obtained from a static pull test on sea trial, carried out in accordance with annex A of MSC/Circ.884 or an equivalent standard acceptable to the Administration;

  • Fp = (Permissible tension) the wire tension which can be applied to the ship as loaded while working through a specified tow pin set, at each α, for which all stability criteria can be met. Fp should in no circumstance be taken as greater than Fd;

  • Fd = (Design maximum wire tension) the maximum winch wire pull or maximum static winch brake holding force, whichever is greater;

  • h = the vertical distance (m) from the centre the propulsive force acts on the ship to either:

    • the uppermost part at the towing pin, or

    • a point on a line defined between the highest point of the winch pay-out and the top of the stern or any physical restriction of the transverse wire movement;

  • y = the transverse distance (m) from the centreline to the outboard point at which the wire tension is applied to the ship given by:

    • y0 + x tan α; but not greater than B/2;

  • B = the moulded breadth (m);

  • y0 = the transverse distance (m) between the ship centreline to the inner part of the towing pin or any physical restriction of the transverse wire movement;

  • x = the longitudinal distance (m) between the stern and the towing pin or any physical restriction of the transverse wire movement.

2.7.3 Permissible tension

2.7.3.1 The permissible tension as function of α, defined in paragraph 2.7.2, should not be greater than the tension given by paragraph 2.7.3.2,

2.7.3.2 Permissible tension as function of α can be calculated by direct stability calculations, provided that the following are met:

  • .1 the heeling lever should be taken as defined in paragraph 2.7.2 for each α;

  • .2 the stability criteria in paragraph 2.7.4, should be met;

  • .3 α should not be taken less than 5 degrees, except as permitted by paragraph 2.7.3.3; and

  • .4 Intervals of α should not be more than 5 degrees, except that larger intervals may be accepted, provided that the permissible tension is limited to the higher α by forming working sectors.

2.7.3.3 For the case of a planned operation to retrieve a stuck anchor in which the ship is on station above the anchor and the ship has low or no speed, α may be taken as less than 5 degrees.

2.7.4 Stability criteria

2.7.4.1 For the loading conditions intended for anchor handling, but before commencing the operation, the stability criteria given in paragraph 2.2 of part A, or where a ship's characteristics render compliance with paragraph 2.2 of part A impracticable, the equivalent stability criteria given in paragraph 2.4 of part B, should apply. During operation, under the action of the heeling moment, the criteria under paragraphs 2.7.4.2 to 2.7.4.4 should apply.

2.7.4.2 The residual area between the righting lever curve and the heeling lever curve calculated in accordance with paragraph 2.7.2 should not be less than 0.070 metre-radians. The area is determined from the first intersection of the two curves, φe, to the angle of the second intersection, φc, or the angle of down-flooding, φf, whichever is less.

2.7.4.3 The maximum residual righting lever GZ between the righting lever curve and the heeling lever curve calculated in accordance with paragraph 2.7.2 should be at least 0.2 m.

2.7.4.4 The static angle at the first intersection, φe, between the righting lever curve and the heeling lever curve calculated in accordance with paragraph 2.7.2 should not be greater than:

  • .1 the angle at which the righting lever equals 50% of the maximum righting lever;

  • .2 the deck edge immersion angle; or

  • .3 15°,

  • whichever is less.

2.7.4.5 A minimum freeboard at stern, on centreline, of at least 0.005L should be maintained in all operating conditions, with a displacement given by Δ2, as defined in paragraph 2.7.2. In the case of the anchor retrieval operation covered by paragraph 2.7.3.3, a lower minimum freeboard may be accepted provided that due consideration has been given to this in the operation plan.

2.7.5 Constructional precautions against capsizing

2.7.5.1 A stability instrument may be used for determining the permissible tension and checking compliance with relevant stability criteria.

Two types of stability instrument may be used on board:

  • either a software checking the intended or actual tension on the basis of the permissible tension curves; or

  • a software performing direct stability calculations to check compliance with the relevant criteria, for a given loading condition (before application of the tension force), a given tension and a given wire position (defined by angles α and β).

2.7.5.2 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. Access to spaces below the exposed cargo deck should preferably be from a position within or above the superstructure deck.

2.7.5.3 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 in 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.

2.7.5.4 The winch systems should be provided with means of emergency release.

2.7.5.5 For ships engaged in anchor handling operations the following recommendations for the anchor handling arrangements should be considered:

  • .1 stop pins or other design features meant to impede the movement of the wire further outboard should be installed; and

  • .2 the working deck should be marked with contrasting colours or other identifiers such as guide pins, stop pins or similar easily identifiable points that identify operational zones for the line to aid operator observation.

2.7.6 Operational procedures against capsizing

2.7.6.1 A comprehensive operational plan should be defined for each anchor handling operation, according to the guidelines given in paragraph 3.8, where at least, but not only, the following procedures and emergency measures should be identified:

  • .1 environmental conditions for the operation;

  • .2 winch operations and movements of weights;

  • .3 compliance with the stability criteria, for the different expected loading conditions;

  • .4 permissible tensions on the winches as function of α; in accordance with paragraph 3.8;

  • .5 stop work and corrective procedures; and

  • .6 confirmation of the master's duty to take corrective action when necessary.

2.7.6.2 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.

2.7.6.3 Counter-ballasting to correct the list of the ship during anchor handling operations should be avoided.

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