Section 7 Masts and derrick posts

7.1.1 The scantlings of masts and derrick posts intended to support derrick booms and similar lifting appliances are to be determined from the highest combination of stresses expected to arise when the gear is used in its most severe operating condition. Materials are to comply with Ch 2, 1.5 Materials 1.5.2.

7.1.2 The requirements of this Section apply to stayed or unstayed single masts of conventional design. The term ‘mast’ is used to include derrick post, king post or similar structure.

7.1.3 Calculations are to be made with the derrick booms at the operating position which results in the maximum stresses on the mast. For masts supporting derricks, angles of heel and trim of the ship in this condition of less than 5° and 2° respectively may be ignored. Where these angles are exceeded, and in all cases where the mast supports derrick cranes or derricks of special design, the actual angles are to be taken into account in calculating the stresses in the mast.

7.1.4 The angle of heel, ψ, of the ship is to be calculated for the specified condition of loading using the applicable stability data or may be approximated from:

7.1.5 Direct calculation procedures may be accepted as an alternative to the methods indicated in this Section.

7.1.6 Special consideration on the general basis of these requirements will be given to the scantlings and arrangements where:

1. The mast is of portal, bipod, lattice or other less common design, or is supported by rigid stays capable of being loaded in compression.

2. Significant forces other than those resulting from cargo gear loads will be acting on the mast.

7.1.7 In such cases, fully detailed stress calculations are to be submitted and these calculations are to take account of:

1. All horizontal, vertical and torsional forces.

2. Deflections of the structure.

3. Variations in the moment of inertia of the parts of the structure.

4. The effects of outriggers and similar structures.

5. Elasticity and sag in stays, where fitted.

7.1.8 A stayed mast is one that is supported wholly or partly by one or more stays. The term ‘stay’ includes shrouds, forestays, backstays and similar supports. Where a stayed mast is so designed that the stays are only required to be set up when loads exceeding a specified value are to be lifted, this fact is to be clearly indicated on:

• The plans submitted for approval.
• The lifting appliance certificates.
• The cargo gear particulars book.
• The mast itself.

7.1.9 The length of the mast, l, is to be measured from the uppermost deck or supporting deckhouse through which it passes. Arrangements where a deckhouse is specifically designed to give no effective support to the mast in either the transverse or the longitudinal directions will be specially considered.

7.1.10 The minimum outside dimensions of the mast at the level of the supporting deck are to be not less than . This dimension is to be maintained up to the level of the gooseneck fitting where this is entirely supported by the mast.

7.1.11 Where the mast is fitted with stays, the minimum outside dimension of the mast at a point midway between the supporting deck and the lowermost stay is to be not less than , but consideration will be given to reduce dimensions where it can be shown that no danger of crippling exists under service conditions of combined thrust, bending moment and torque.

7.1.12 The wall thickness of the mast is to be not less than the greatest of the applicable values determined from Table 2.7.1 Minimum thickness of mast plating.

Table 2.7.1 Minimum thickness of mast plating

Item	Minimum thickness, in mm
Curved plates
Flat plates
Note 1. where SWL = the safe working load of the largest derrick operating on the mast, in tonnes d = maximum outside diameter of the mast at the position under consideration, in mm. Where the mast is not circular, d is to be taken as the maximum diameter of the circle of which the plate forms a part b = width of flat plate, in mm, but is to be taken as not less than 60% of the width of the mast at that point measured parallel to the flat plate α = the ratio Note 2. Where stiffeners are fitted, b may be taken as the mean spacing of stiffeners. The required scantlings of the stiffeners to resist instability under end loading will be considered.

7.2.1 The symbols used throughout this Section are defined in Ch 2, 1.6 Symbols and definitions 1.6.2.

7.3.1 Calculations are to be made for the least favourable combinations of loading which may be imposed by the derrick systems. The following combinations are generally to be considered:

1. Swinging derrick systems and derrick cranes:

   1. For mast with one or two derricks:

      One or both derricks plumbing one hatch;

      One or both derricks slewed outboard on one side of the ship.

   2. For mast with three or more derricks:

      Two derricks plumbing one hatch;

      Two derricks slewed outboard on one side of the ship.

   3. For mast with a heavy derrick fitted:

      The heavy derrick plumbing the hatch;

      The heavy derrick slewed outboard.

2. Union purchase systems:

   1. One pair of derricks plumbing one hatch;

   2. One pair (or two pairs if fitted) of derricks with the load outboard on one side of the ship.

7.3.2 Where any other combination of operating derricks is proposed or where it is possible for the greatest stresses to arise at other positions of the derricks, the resultant loads are to be considered.

7.3.3 The effects of wind, ice and the normal motion of a ship in a seaway may generally be ignored in the calculations.

7.3.4 Where it is intended to operate the derrick system in a specified service category, see Ch 1, 2.3 Service category 2.3.2, the resulting additional forces imposed on the system will be specially considered.

7.3.5 The maximum allowable combined bending and direct stress is not to exceed the value given in Table 2.7.2 Allowable stresses in masts. The maximum allowable shear stress is not to exceed 0,58 times the value given in Table 2.7.2 Allowable stresses in masts.

7.3.6 For masts of controlled design, where it is proposed to adopt the maximum stress value of 0,83σ_y permitted by item (5) of Table 2.7.2 Allowable stresses in masts, the following requirements are to be met:

1. A detailed stress calculation is to be made.

2. All scantlings are to be based on the guaranteed minimum thickness of the materials used.

3. Full account is taken in the calculations of heel and trim of the self-weight of the gear, including guys.

4. The effect of any guy tension which could occur in operation is to be included.

5. Means are to be provided for controlling the tension in the stays, if fitted.

6. The mast, fittings and loose gear are to be manufactured to high engineering standards.

7.4.1 The forces imposed on the mast by the cargo runner, span tackle and gooseneck are to be determined from the force diagrams or calculations prepared in accordance with Ch 2, 2 Design criteria. The resulting stresses in the mast are to be calculated taking into account the effect of any offsets in the lines of action of the forces.

7.4.2 The total stress (σ_t) at any particular location is to be taken as:

7.4.3 The total stress is to be determined at each change of plate thickness or other change of section along the mast. It is recommended that a plot or table of stress to a base of mast length be prepared. At no point is σ_t to be greater than the allowable stress determined from Ch 2, 7.3 Loading and allowable stresses 7.3.5.

7.5.1 Calculations are to be prepared for the conditions with the derrick operating parallel to the centreline of the ship and when slewed to the most outboard operating position. Other positions are to be examined where the arrangement of stays is such that higher stresses can be expected in the system.

7.5.2 The forces acting on the mast resulting from the cargo runner, span tackle and gooseneck are to be determined from the force diagrams or calculations prepared in accordance with Ch 2, 2 Design criteria. Where cross trees are fitted or where the vertical separation of the highest and lowest points of attachment of the mast head span cargo lead blocks and the stays exceed 0,1H m, the calculations of forces will be specially considered. A fully detailed direct calculation may be required.

7.5.3 In the absence of stays, the mast will deflect under the influence of the imposed forces. Where stays are fitted, they will extend under tension, with the amount of elongation being related to the deflection of the mast at the point of attachment of the stays.

7.5.4 The distribution of forces in the mast and stays may therefore be obtained by consideration of:

1. The equilibrium between the deflection of the mast and the corresponding elongations of the stays.

2. The equilibrium between the imposed loads on the mast and the reactions in the mast and the stays.

7.5.5 The bending moment (BM) in a single-stayed mast and the tension (T) in the stay are to be determined as follows:

where

φ	=	the angle of the stay to the horizontal in degrees
A	=	the cross-sectional area of the stay, in m2
x	=	the distance below the mast head about which bending moments are to be calculated, in metres
l	=	the height of the mast/stay attachment above the deck, in metres
l1	=	the length of the stay, in metres
E p	=	the Young’s modulus of steel for the mast, in N/m2
E s	=	the Young’s modulus of steel for the stay, in N/m2
	=	second moment of area for the mast section, in m2
P	=	the component of force acting on the mast head, in Newtons.

7.5.6 These calculations are to be made using appropriately defined co-ordinate axes. Attention is drawn to the importance of assigning the correct sign to the angles and dimensions used. Any stay which would be required to work in compression is to be ignored.

7.5.7 Elongation of the stays is to be calculated on the basis of the area enclosed by a circle of diameter equal to the nominal diameter of the rope in association with an effective modulus of elasticity of 61300 N/mm². Consideration will, however, be given to the use of a higher modulus of elasticity where this is demonstrated by suitable tests to be applicable.

7.5.8 The total stress in the mast at any particular location is to be determined in accordance with Ch 2, 7.4 Stress calculations – Unstayed masts 7.4.2 and Ch 2, 7.4 Stress calculations – Unstayed masts 7.4.3. Attention is drawn to the fact that increased stiffness of the mast leads to a rapid increase in stress in the mast with a corresponding reduction in the effectiveness of the stays. It is desirable, therefore, to design the mast for the required section modulus in association with the least practicable moment of inertia.

7.6.1 Masts are to be supported by at least two decks and are to be effectively scarfed into the main hull structure. The hull structure is to be suitably reinforced. A deckhouse may be considered as a support provided it is of adequate strength.

7.6.2 Alternative means of achieving efficient support for the mast will be considered. Where brackets are fitted to the deck at the mast heel, they are to be of sufficient size to provide an adequate path for loads to be carried to the underdeck stiffening and surrounding structure.

7.6.3 Where the lower part of the mast is integral with the deckhouse, the plating is to be increased in thickness and additional stiffening fitted to ensure adequate strength and resistance to plate buckling. Openings are, in general, to be avoided in these areas, but where required are to be well rounded and suitable edge stiffening is to be fitted.

7.6.4 In general, mast scantlings are not to be reduced inside deckhouses.

7.6.5 Cross trees, outriggers, brackets on bridge fronts and similar structures are to be of such design that the stresses on them resulting from the cargo gear and any other significant forces do not exceed the values in Ch 2, 7.3 Loading and allowable stresses. The design is also to be such as to minimise the moments acting on the mast. Attachment to the mast is to be such as to avoid distortion of the mast under load. Local stiffening, doublers or diaphragm plates are to be fitted to the mast as necessary.

7.6.6 Special attention is to be paid to the structural continuity and abrupt changes of the section are to be avoided. Manholes, lightening holes and other cut-outs are to be avoided in way of concentrated loads and areas of high shear. Where required, openings are to be well rounded, suitably framed and stiffened.

7.6.7 Adequate reinforcement is to be fitted in way of concentrated loads. The toes of brackets and corners of fittings are not to land on unstiffened panels of plating. Suitable arrangements are to be made to avoid notch effects.

7.6.8 Care is to be taken in the design of masts and fittings to reduce the likelihood of water collecting in inaccessible parts of the structure. Drains or other means are to be provided to remove any water which might otherwise accumulate. All parts are to be accessible for inspection and painting except where closed box construction is adopted.

7.6.9 Welding and weld details are to comply with the requirements of Pt 3, Ch 10 Welding and Structural Details of the Rules for Ships.

7.6.10 Where a mast is intended to support a derrick with a SWL exceeding 25 t, all welded joints below a distance of 3,0 m above the uppermost supporting deck, or to the level of the derrick heel if more than 3,0 m, are to be examined by nondestructive crack or flaw detection methods.

7.6.11 Where higher tensile steel is used, preheating or other heat treatments may be required at the Surveyor’s discretion and will normally be required for all ring seams on masts supporting derricks with a SWL exceeding 60 t. Nondestructive methods of examination may be required in areas of high stress in way of fittings at the Surveyor’s discretion.

7.6.12 Lightning conductors are to be fitted to masts having wood, aluminium or plastic topmasts or where a break in electrical conductivity occurs in other arrangements.

7.7.1 Wire rope stays are to be in one length and their construction is to comply with the requirements of Ch 8 Fittings, Loose Gear and Ropes.

7.7.2 The scantlings of the stay are to be such as to provide the tensile force and elongation to meet the requirements of Ch 2, 7.5 Stress calculations – Stayed masts. The breaking load of wire rope stays is to be not less than 3,5 times the maximum calculated force on that stay. Man-made stays are to have a breaking load not less than 4,38 times the maximum calculated force.

7.7.3 Stays are to be arranged so that they do not foul running rigging or derrick booms when in service and are to be set up with an initial tension of about 30,0 N/mm².

7.7.4 The connection of the stay to a deck, bulwark, house or mast is to be such as to allow rotation at the point of attachment and is to be designed so that the stay cannot become disconnected while the derrick system is in use.

7.7.5 It is undesirable to connect stays to the ends of cross trees where deflection under load may significantly affect the load bearing efficiency of the stay.