Section 5 Helicopter landing areas

5.1 General

5.1.1 This Section gives the requirements for decks intended for helicopter operations on all unit types.

5.1.2 Attention is drawn to the requirements of National and other Authorities concerning the construction of helicopter landing platforms and the operation of helicopters as they affect the unit. These include the 2009 MODU Code - Code for the Construction and Equipment of Mobile Offshore Drilling Units, 2009 – Resolution A.1023(26) and Chapter II-2 - Construction - Fire protection, fire detection and fire extinction, CAP 437 7th edition, NMA/NMD 2013 and ISO 19901-3:2011, as applicable. Guidance on the provision and operation of helicopter landing or winching facilities may be drawn from international Standards such as the International Chamber of Shipping (ICS) Guide to Helicopter/Ship Operations and the International Aeronautical Search and Rescue Manual (IAMSAR).

5.1.3 Where helicopter decks are positioned so that they may be subjected to wave impacts, the scantlings are to be considered in a realistic manner and increased to the satisfaction of LR. Calculations are to be submitted for consideration.

5.1.4 Where the landing area forms part of a weather or erection deck, the scantlings are to be not less than those required for decks in the same position.

5.2 Plans and data

5.2.1 Plans and data are to be submitted giving the arrangements, scantlings and details of the helicopter deck. The type, size, weight and footprint of helicopters to be used are also to be indicated.

5.2.2 Relevant details of the largest helicopters, for which the deck is designed, are to be stated in the Operations Manual.

5.3 Arrangements

5.3.1 The landing area is to comply with applicable Regulations, International Standards or to the satisfaction of the National Authority, with respect to size, landing and take-off sectors of the helicopter, freedom from height obstructions, deck markings, safety nets and lighting, etc.

5.3.2 The landing area is to have an overall coating of non-slip material or other arrangements are to be provided to minimise the risk of personnel or helicopters sliding off the landing area.

5.3.3 A drainage system is to be provided in association with a perimeter guttering system or slightly raised kerb to prevent spilled fuel falling on to other parts of the unit. The drains are to be led to a safe area.

5.3.4 A sufficient number of tie-down points are to be provided to secure the helicopter.

5.3.5 Engine and boiler uptake arrangements are to be sited such that exhaust gases cannot be drawn into helicopter engine intakes during helicopter take-off or landing operations.

5.4 Landing area plating

5.4.1 Helideck support structures should be designed to carry all the loads imposed on the helideck through to the primary structure of the unit. Helideck loads derive from the parameters of the helicopter for which the helideck is intended (landing impact forces and wheel spacing), the deck weight, plus environmental loads (wind, snow and ice), and inertial loads due to unit movement, as applicable. Additionally, the effects of live loads and loads arising from parked helicopters (tied down) should be evaluated.

5.4.2 The designer of the support structure should ensure that all appropriate load cases have been applied to the helideck, and that the resulting maximum load cases are used in the design of the support structure. Similarly, it is important that the load cases are accurately transposed to the design conditions for the primary structure to which the support structure will be connected.

5.5 Load combination

5.5.1 The helicopter landing area is to be considered with respect to design loads resulting from the following conditions:

Emergency landing
Normal operation and
Helicopter at rest

5.5.2 Emergency landing The following loads are to be considered in helicopter emergency landing condition.

Helicopter landing dynamic loads: For an emergency landing, an impact load of 2,5 x the maximum take-off weight (MTOW) of the helicopter should be applied in any position on the landing area together with the combined effects of Pt 4, Ch 6, 5.5 Load combination 5.5.2 to Pt 4, Ch 6, 5.5 Load combination 5.5.2 inclusive.
Structural response factor for supporting structure: The helicopter landing dynamic loads shall be increased by a structural response factor to account for the sympathetic response of the helideck structure. The factor to be applied for the design of the helideck framing depends on the natural frequency of the deck structure. Unless values based upon particular undercarriage behaviour and deck frequency are available, a minimum structural response factor of 1,3 shall be used.
Area loads: A general area-distributed load of 0,5 kN/m² shall be applied to allow for minor equipment left on the helideck and for any snow and ice loads.
Horizontal loads as a proportion of MTOW: Concentrated horizontal imposed loads equivalent in total to half the maximum take-off weight of the helicopter shall be applied at the locations of the main undercarriages and distributed in proportion to the vertical loads at each point. These shall be applied at deck level in the horizontal direction that will produce the most severe load case for the structural component being considered.
Self weight of structure and fixed appurtenances: The self weight of the helideck structure and fixed appurtenances supported by each structural component concerned shall be evaluated.
Wind loads: Wind loads on the helideck structure shall be applied in the direction which, together with the horizontal imposed loads, produces the most severe load case for the structural component considered. The wind speed to be considered shall be that restricting normal (non-emergency) helicopter operations at the platform. Any vertical action on the helideck structure due to the passage of wind over and under the helideck shall be considered.
Inertial loads: The effect of accelerations and dynamic amplification arising from the predicted motions of the fixed or floating platform in a storm condition with a 10 year return period shall be considered.

5.5.3 Normal operations The following loads are to be considered in helicopter normal operation condition

Helicopter landing dynamic loads: For a normal operation, an impact load of 1,5 x the maximum take-off weight (MTOW) of the helicopter should be applied in any position on the landing area together with the combined effects of Pt 4, Ch 6, 5.5 Load combination 5.5.3 to Pt 4, Ch 6, 5.5 Load combination 5.5.3 inclusive.
Structural response factor for supporting structure: The helicopter landing dynamic loads shall be increased by a structural response factor to account for the sympathetic response of the helideck structure. The factor to be applied for the design of the helideck framing depends on the natural frequency of the deck structure. Unless values based upon particular undercarriage behaviour and deck frequency are available, a minimum structural response factor of 1,3 shall be used.
Area loads: To allow for personnel, freight, refuelling equipment and other traffic, snow and ice, rotor downwash, etc., a general area load of 0,5 kN/m² shall be included.
Horizontal loads as proportion of MTOW: Concentrated horizontal imposed loads equivalent in total to half the maximum take-off weight of the helicopter shall be applied at the locations of the main undercarriages and distributed in proportion to the vertical loads at each point. These shall be applied at deck level in the horizontal direction that will produce the most severe load case for the structural component being considered.
Self weight of structure and fixed appurtenances.
Wind loads: The 100 year return period wind loads on the helideck structure shall be applied in the direction which produces the most severe load case for the structural component considered.
Inertial loads: The effect of accelerations and dynamic amplification arising from the predicted motions of the fixed or floating platform in a storm condition with a 10 year return period shall be considered.

5.5.4 Helicopter at rest The following loads are to be considered in helicopter at rest condition

Helicopter static loads (local patch loads on landing gear): All parts of the helideck accessible to helicopters shall be designed to support a load equal to the MTOW of the helicopter at any location. This shall be distributed at the undercarriage locations in proportion to the position of the centre of gravity of the helicopter, taking account of possible different positions and orientations of the helicopter.
Area loads: To allow for personnel, freight, refuelling equipment and other traffic, snow and ice, rotor downwash, etc., a general area load of 2,0 kN/m² shall be included.
Horizontal loads from tie down helicopter, including wind loads from a secured helicopter: Each tie-down shall be designed to resist the calculated proportion of the total wind action on the helicopter imposed by a storm wind with a minimum one year return period.
Self weight of structure and fixed appurtenances.
Wind loads: The 100 year return period wind loads on the helideck structure shall be applied in the direction which produces the most severe load case for the structural component considered.
Inertial loads: The effect of accelerations and dynamic amplification arising from the predicted motions of the fixed or floating platform in a storm condition with a 10 year return period shall be considered.

5.5.5 Deck plate and stiffeners shall be designed to limit the permanent deflection (deformation) under helicopter emergency landing conditions to no more than 2,5 % of the clear width of the plates between supports.

5.6 Landing area plating

5.6.1 The deck gross plate thickness, t, within the landing area is to be not less than:

t = + 1,5 mm

where

thickness coefficient obtained from Pt 4, Ch 6, 5.6 Landing area plating 5.6.1

β	=	tyre print coefficient used in Pt 4, Ch 6, 5.6 Landing area plating 5.6.1
β	=	log₁₀

Figure 6.5.1 Tyre print chart

The plating is to be designed for the emergency landing case taking:

tonnes

where

are to be determined from Pt 4, Ch 6, 5.7 Deck stiffening and supporting structure 5.7.2

f	=	1,15 for landing decks over manned spaces, e.g. deckhouses, bridges, control rooms, etc.
f	=	1,0 elsewhere

the maximum all up weight of the helicopter, in tonnes

landing load on the tyre print, in tonnes;

For helicopters with a single main rotor, , is to be taken as divided equally between the two main undercarriage wheels.
For helicopters with tandem main rotors, , is to be taken as distributed between all main undercarriage wheels in proportion to the static loads they carry.
For helicopters fitted with landing gear consisting of skids, P_w is to be taken as distributed in accordance with the actual load distribution given by the airframe manufacturer. If this is unknown, is to be taken as 1/6 for each of the two forward contact points and 1/3 for each of the two aft contact points. The load may be assumed to act as a 300 mm x 10 mm line load at each end of each skid when applying Pt 4, Ch 6, 5.6 Landing area plating 5.6.1.

0,6 generally. Factor to be specially considered where the helicopter deck contributed to the overall strength of the unit

Other symbols used in this Section are defined in Section 6 and in the appropriate sub-Section.

For wheeled undercarriages, the tyre print dimensions specified by the manufacturer are to be used for the calculation. Where these are unknown, it may be assumed that the print area is 300 x 300 mm and this assumption is to be indicated on the submitted plans.
For skids and tyres with an asymmetric print, the print is to be considered oriented both parallel and perpendicular to the longest edge of the plate panel and the greatest corresponding value of α taken from Pt 4, Ch 6, 5.6 Landing area plating 5.6.1.

5.6.2 The plate thickness for aluminium decks is to be not less than:

t = 1,4+ 1,5 mm

where

is the mild steel thickness as determined from Pt 4, Ch 6, 5.6 Landing area plating 5.6.1.

Where the deck is fabricated using extruded sections with closely spaced stiffeners the plate thickness may be determined by direct calculations but the minimum deck thickness is to include 1,5 mm wear allowance. If the deck is protected by closely spaced grip/wear treads the wear allowance may be omitted.

5.7 Deck stiffening and supporting structure

5.7.1 The helicopter deck stiffening and the supporting structure for helicopter decks are to be designed for the load cases given in Pt 4, Ch 6, 5.7 Deck stiffening and supporting structure 5.7.2 in association with the permissible stresses given in Pt 4, Ch 6, 5.7 Deck stiffening and supporting structure 5.7.2. The helicopter is to be positioned so as to produce the most severe loading condition for each structural member under consideration.

5.7.2 In addition to the requirements of Pt 4, Ch 6, 5.5 Load combination 5.5.1, the structure supporting helicopter decks is to be designed to withstand the loads imposed on the structure due to the motions of the unit. For self-elevating units, the motions are not to be less than those defined for transit conditions in Pt 4, Ch 4, 3.10 Legs in field transit conditions and Pt 4, Ch 4, 3.11 Legs in ocean transit conditions. The stress levels are to comply with load case 3 in Pt 4, Ch 6, 5.7 Deck stiffening and supporting structure 5.7.2, see also Pt 4, Ch 6, 5.1 General 5.1.3.

Table 6.5.1 Design load cases for deck stiffening and supporting structure

Load cases	Load
	Landing area		Supporting structure See Note 1
	Area load, in kN/m²	Helicopter patch load See Note 2	Self-weight	Wind load, return period in years	Inertia load, return period in years
(1) Helicopter emergency landing	0,5	2,5		See Pt 4, Ch 6, 5.5 Load combination 5.5.2	10
(2) Normal operation	0,5	1,5		100	10
(3) Helicopter at rest	2,0			100	10
Symbols
, and f as defined in Pt 4, Ch 6, 5.6 Landing area plating 5.6.1
= structural self-weight of helicopter platform
NOTES
1. For the design of the supporting structure for helicopter platforms applicable horizontal load, self-weight, wind load and inertia load are to be added to the landing area loads.
2. The helicopter is to be so positioned as to produce the most severe loading condition for each structural member under consideration.
3. For the emergency landing and normal operation, helicopter patch load shall be increased by a suitable structural response factor depending upon the natural frequency of the helideck structure. It is recommended that a structural response factor of 1,3 should be used unless further information allows a lower factor to be calculated.

Table 6.5.2 Permissible stresses for deck stiffening and supporting structure

Load case See Pt 4, Ch 6, 5.7 Deck stiffening and supporting structure 5.7.2	Permissible stresses, in N/mm²
	Deck secondary structure (beams, longitudinals, deck plating See Notes 1 and 2)	Primary structure (transverses, girders, pillars, trusses)		All structure
	Bending		Combined bending and axial	Shear
(1) Helicopter emergency landing	245/k	220,5/k	0,9
(2) Normal operation	176/k	147/k	0,6
(3) Helicopter at rest	176/k	147/k	0,6
Symbols
k = a material factor:
= as defined in Pt 4, Ch 2, 1.2 Steel for steel members
= as defined in Pt 4, Ch 2, 1.3 Aluminium for aluminium alloy members
= yield stress, 0,2% proof stress or critical compressive buckling stress, in N/mm², whichever is the lesser
NOTES
1. Lower permissible stress levels may be required where helideck girders and stiffening contribute to the overall strength of the unit. Special consideration will be given to such cases.
2. When determining bending stresses in secondary structure, for compliance with the above permissible stresses, 100% end fixity may be assumed.

Table 6.5.3 Deck plate thickness calculation

Symbols	Expression
a, s, u and v as defined in Pt 4, Ch 6, 5.6 Landing area plating 5.6.1
= load, in tonnes, on the tyre print. For closely spaced wheels the shaded area shown in Pt 4, Ch 6, 5.6 Landing area plating 5.6.1 may be taken as the combined print
φ1 = patch aspect ratio correction factor	= 1,0	for u ≤ (a – s)
φ2 = panel aspect ratio correction factor	=	for a ≥ u > (a – s)
φ3 = wide patch load factor	= 0,77	for u > a
	= 1,0	for v < s
	= 0,6 + 0,4	for 1,5 > > 1,0
	= 1,2	for ≥ 1,5

5.7.3 For load cases (1) and (2) in Pt 4, Ch 6, 5.7 Deck stiffening and supporting structure 5.7.2 the minimum moment of inertia, Ι , of aluminium alloy secondary structure stiffening is to be not less than:

cm⁴

where

Z is the required section modulus of the aluminium alloy stiffener and attached plating and as defined in Pt 4, Ch 2, 1.3 Aluminium.

5.7.4 When the deck is constructed of extruded aluminium alloy sections, the scantlings will be specially considered on the basis of this Section.

5.7.5 Where a grillage arrangement is adopted for the platform stiffening, it is recommended that direct calculation procedures be used.

5.8 Bimetallic connections

5.8.1 Where aluminium alloy platforms are connected to steel structures, details of the arrangements in way of the bimetallic connections are to be submitted.