5 Bollards and Bitts, Fairleads, Stand Rollers, Chocks and Capstans

5.1 General

5.1.1

Article 5 provides requirements applicable to the design and construction of shipboard fittings and supporting structures used for the normal towing at bow, side and stern and mooring operations as well as the strength of supporting structures of winches and capstans.

Normal towing means towing operations necessary for manoeuvring in ports and sheltered waters associated with the normal operations of the ship.

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

5.1.2

Article 5 is not applicable to design and construction of shipboard fittings and supporting structures used for special towing services defined as:

• a) Escort towing: Towing service, in particular, for laden oil tankers required in specific estuaries. Its main purpose is to control the ship in case of failures of the propulsion or steering system. It should be refered to local escort requirements and guidance given by,e.g., the Oil Companies International Marine Forum (OCIMF)

• b) Canal transit towing: Towing service for ships transiting, e.g. the Panama Canal. It should be refereed to local canal transit requirements.

• c) Emergency towing for oil tankers: Towing service to assist tankers in case of emergency. For the emergency towing arrangements, ships subject to SOLAS regulation II-1/3-4 Paragraph 1 are to comply with that regulation and resolution MSC.35(63) as may be amended.

5.1.3

Where fairleads are fitted in bulwarks, the thickness of bulwarks may need to be increased. See Ch 11, Sec 2, [2.2].

5.1.4

The supporting structure is to be dimensioned to ensure that for the loads specified in [5.2.1] and [5.3.1], the stresses do not exceed the permissible values given in [5.5]. The capability of the structure to resist buckling failure is to be assured according to Ch 8.

5.1.5

These requirements are to be assessed based on net scantlings.

5.2 Towing

5.2.1 Towing design loads

When a safe towing load, TOW, greater than the value determined according to [5.2.4] is provided by the designer, the design load is to be increased in accordance with the appropriate TOW/design load relationship given in [5.2.1] and [5.2.4].

The design load is to 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 (see Figure 4). However, the design load applied to the fitting needs not to be greater than twice the design load of the line.

Figure 4: Design load on fitting

5.2.2 Shipboard fittings

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

When the shipboard fitting is not selected from an accepted industry standard, the strength of the fitting and of its attachment to the ship is to be in accordance with the requirements of this Article.

Towing bitts (double bollards) are required to resist the loads caused by the towing line attached with eye splice.

5.2.3 Towing force acting point

The acting point of the towing force on shipboard fittings 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 (see Figure 5).

Figure 5: Attachment point of the towing line

5.2.4 Safe Towing Load (TOW)

The Safe Towing Load (TOW), in t, is the load limit for towing purpose.

TOW 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 t, according to [5.3.4] is to be marked in addition to TOW.

5.3 Mooring

5.3.1 Mooring design loads

The minimum design load applied to supporting structures for shipboard fittings is not to be less than 115% of the minimum breaking strength of the mooring line provided by the designer (refer to Sec 3, [3.9])

The minimum design load applied to supporting structures for winches is not to be less than 125% of the intended maximum brake holding load, where the maximum brake holding load is to be assumed not less than 80% of the minimum breaking strength of the mooring line provided by the designer (refer to Sec 3, [3.9]).

The minimum design load for the supporting structure for capstans is to taken as 125% of the maximum hauling in force.

When a safe working load SWL greater than the value determined according to [5.3.4] is provided by the designer, the design load is to be multiplied by the ratio SWL/design load, where the design load is given above as appropriate.

The design load is to 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 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 4). However, the design load applied to the fitting needs not to be greater than twice the design load on the line.

5.3.2 Shipboard fittings

Shipboard fittings may be selected from an industry standard accepted by the Society and at least based on the minimum breaking strength of the mooring line provided by the designer (refer to Sec 3, [3.9]).

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.

When the shipboard fitting is not selected from an accepted industry standard, the strength of the fitting and of its attachment to the ship is to be in accordance with this Article.

Mooring bitts (double bollards) are required to resist the loads caused by the mooring line attached in figure-of-eight fashion.

5.3.3 Mooring force acting point

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. 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 6 a). However, if fins are fitted to the bollard tubes to keep the mooring line as low as possible, the attachment point of the mooring line may be taken at the location of the fins (See Figure 6 b).

Figure 6: Attachment point of the mooring line

5.3.4 Safe Working Load (SWL)

The Safe Working Load (SWL), in t, is the load limit for mooring purpose.

The following requirements on Safe Working Load apply for the use with no more than one mooring line.

Unless a greater SWL is provided by the designer, the SWL is not to exceed the minimum breaking strength of the mooring line provided by the designer (refer to Sec 3, [3.9]).

The SWL of each deck 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, TOW, in t, according to [5.2.4] is to be marked in addition to SWL.

5.4 Supporting structure

5.4.1

Shipboard fittings for towing and mooring, 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 towing or mooring loads. Other arrangements may be accepted (for chocks in bulwarks, etc.) provided the strength is confirmed adequate for the intended service.

5.4.2

The reinforced structural members beneath shipboard fittings are to be effectively arranged for any variation of direction (horizontally and vertically) of the towing/mooring forces acting upon the shipboard fittings (see Figure 7).

Figure 7: Sample arrangement

5.4.3

Shipboard fittings (bollards and bitts, fairleads, stand rollers and chocks) and capstans used for mooring and/or towing operations are to be fitted to the deck or bulwark structures.

5.4.4

The structural arrangement is to provide continuity of strength. Proper alignment of fittings and supporting structure is to be ensured.

The structural arrangement of the ship’s structure in way of the shipboard fittings and their seats and in way of capstans is to be such that abrupt changes of shape or section are to be avoided in order to minimise stress concentrations. Sharp corners and notches are to be avoided, especially in highly stressed areas.

5.5 Acceptance criteria

5.5.1

For the design load specified in [5.2.1] and [5.3.1], the stresses induced in the shipboard fittings, the supporting structure and welds are not to exceed the following permissible values defined in [5.5.3] and [5.5.4], as applicable:

5.5.2

The strength assessment of the shipboard fittings can be performed by means of either beam theory or grillage analysis, or by finite element analysis.

At the discretion of the Society, load tests of the fittings may be accepted as alternative to strength assessment by above mentioned analysis.

5.5.3

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 are taken into account.

5.5.4

For strength assessment with finite element analysis, the von Mises equivalent stress to be considered is not to exceed ReH.

For strength calculations by means of finite elements, the geometry is to be modelled 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 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 a mean thickness over the web height. Large openings are to be modelled. Stiffeners may 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.

5.6 Corrosion addition of the fittings

5.6.1

The corrosion addition, tc, of the fittings is not be less than the following values:

a) For pedestals and foundations on deck which are not part of a fitting according to an accepted industry standard, 2.0 mm.

b) For shipboard fittings not selected from an accepted Industry standard, 2.0 mm.

5.6.2

In addition to the corrosion addition the wear allowance, tw, for shipboard fittings not selected from an accepted Industry standard is not to be less than 1.0 mm, added to surfaces which are intended to regularly contact the line.

5.7 Towing and mooring arrangements plan

5.7.1

The SWL and TOW for the intended use for each deck fitting is to be stated in the towing and mooring arrangements plan available onboard for the guidance of the Master.

Item c) with respect to items d) and e), is subject to approval by the Society. 5

5.7.2

The information provided on the plan is to include:

• f) The arrangement of mooring lines showing number of lines (N), g) The minimum breaking strength of each mooring line (MBL),

  h) The acceptable environmental conditions:

  • 30 second mean wind speed from any direction
  • Maximum current speed acting on bow or stern (±10°).

This information is to be incorporated into the pilot card in order to provide the pilot with proper information on harbor and other towing operations.

5.7.3

The towing and mooring arrangements plan is to define the method of use of towing lines and/or mooring lines.