Section 4 Structural design loads

4.1 General

4.1.1 The requirements in this Section define the loads and load combinations to be considered in the overall strength analysis of the unit and the design pressure heads to be used in the Rules for local scantlings.

4.1.2 A unit’s modes of operation are to be investigated using realistic loading conditions, including buoyancy, gravity and functional loadings together with relevant environmental loadings. Due account is to be taken of the effects of wind, waves, currents, motions (inertia), moorings, ice, and, where necessary, the effects of earthquake, sea bed-supporting capabilities, temperature, fouling, etc. Where applicable, the design loadings indicated herein are to be adhered to for all types of offshore units.

4.1.3 The Owner/designer is to specify the modes of operation and the environmental conditions for which the unit is to be approved, see also Pt 1, Ch 2, 2 Definitions, character of classification and class notations .

4.1.4 The design environmental criteria determining the loads on the unit and its individual elements are to be based upon appropriate statistical information and have a return period (period of recurrence) for the most severe anticipated environment of at least:

50 years for Mobile Offshore Units.
100 years for Floating Offshore Installations at a Fixed Location.

For sites susceptible to cyclones, the design of units which remain on-station during cyclones is to be based upon the most onerous of the cyclonic and non-cyclonic environments determined at the return periods in Pt 4, Ch 3, 4.1 General 4.1.4.(a) and Pt 4, Ch 3, 4.1 General 4.1.4.(b) for the respective unit type.

If a unit is restricted to seasonal operations in order to avoid extremes of wind and wave, such seasonal limitations must also be specified.

4.1.5 Model tests are to be carried out as necessary and the tests are to include means of establishing the effects of green water loading and/or slamming on the structure through video recordings of the model testing and by measurement of the following:

Relative motions.
Forces on local panels mounted at various locations on exposed areas including bow areas of ship units and other surface type units and accommodation areas, see also Pt 10 Ship Units for ship units and Pt 4, Ch 4 Structural Unit Types and Pt 3, Ch 10, 5 Design analysis for other unit types.

4.1.6 When carrying out model tests, account is to be taken of the following:

The test programme and the model test facilities are to be to LR’s satisfaction.
The relative directions of wind, wave and current are to be varied as required to ensure that the most critical loadings and motions are determined.
The tests are to be of sufficient duration to establish low frequency motion behaviour.

4.1.7 The unit’s limiting design criteria are to be included in the Operations Manual, see Pt 3, Ch 1, 3 Operations manual.

4.2 Definitions

4.2.1 Still water condition is defined as an ideal condition when no environmental loads are imposed on the structure, e.g., no wind, wave or current, etc.

4.2.2 Gravity and functional loads are loads which exist due to the unit’s weight, use and treatment in still water conditions for each design case. All external forces which are responses to functional loads are to be regarded as functional loads, e.g., support reactions and still water buoyancy forces.

4.2.3 Environmental loads are loads which are due directly or indirectly to environmental actions. All external forces which are responses to environmental loads are to be regarded as environmental loads, e.g., mooring forces and inertia forces.

4.2.4 Accidental loads are loads which occur as a direct result of an accident or exceptional circumstances, e.g., loads due to collisions, dropped objects and explosions, etc. See also Pt 4, Ch 3, 4.16 Accidental loads.

4.3 Load combinations

4.3.1 The structure is to be designed for the most unfavourable of the following combined loading conditions (as relevant to the unit):

Maximum gravity and functional loads.
Design environmental loads and associated gravity and functional loads.
Accidental loads and associated gravity and functional loads.
Environmental loads and associated gravity and functional loads after credible failures or accidents, see Pt 4, Ch 4, 1.3 Structural design 1.3.5 for redundancy assessment of column-stabilised units and Pt 10, Ch 2, 3.4 Return periods and probability factor, fprob for assessment of ship units in the flooded condition.

NOTE

Pt 4, Ch 3, 4.3 Load combinations 4.3.1 relates to the loading and condition of the unit at the time of the accidental event. Pt 4, Ch 3, 4.3 Load combinations 4.3.1 relates to the loading and condition of the unit following the accidental event and allowing for agreed documented mitigation measures to be put in place. See also Pt 4, Ch 3, 4.16 Accidental loads, Pt 4, Ch 4 Structural Unit Types and Pt 10 Ship Units for applicability to unit types.

4.3.2 Special requirements applicable to column-stabilised and self-elevating units are also defined in Pt 4, Ch 4 Structural Unit Types.

4.3.3 Permissible stresses relevant to the combined loading conditions are given in Pt 4, Ch 5 Primary Hull Strength.

4.4 Gravity and functional loads

4.4.1 All gravity loads, including static loads such as weight, outfit, stores, machinery, ballast, etc., and live functional loads from operating derricks, cranes, winches and other equipment are to be considered. All practical combinations of gravity and functional loads are to be included in the design cases.

4.5 Buoyancy loads

4.5.1 Buoyancy loads on all underwater parts of the structure, taking account of heel and trim when appropriate, are to be considered.

4.6 Wind loads

4.6.1 Account is to be taken of the wind forces acting on that part of the unit which is above the still water level in all operating conditions and of the following:

Consideration is to be given to wind gust velocities which are of brief duration and sustained wind velocities which act over intervals of time equal to or greater than one minute, including squalls where relevant. Different wind velocity averaging time intervals applicable to different structural categories to be used in design calculations are shown in Pt 4, Ch 3, 4.6 Wind loads 4.6.1.
Wind velocities are to be specified relative to a standard reference height of 10 m above still water level for each operating condition.

The variation of wind velocity with height and averaging time interval for each applicable condition may be determined from the following expression:

V_HT

where

C	=	0,0573(1+0,148V_R)^0,5
l_U	=

and

V_R	=	wind velocity at specified reference height H_R and reference time period T₀, in m/s
H	=	specified height above sea level, in meters
H_R	=	reference height, in metres
T	=	wind speed averaging time interval, in seconds
T₀	=	reference time period, 1 hour, in seconds

Table 3.4.1 Structural parts to be considered for wind loading

Wind speed averaging time interval	Structural category
3 second gust	Individual members and equipment secured to them. Wind loads need not be combined with current and wave loads.
5 second mean (sustained)	Part or whole of a structure whose greatest horizontal or vertical dimension does not exceed 50 m. Wind loads need not be combined with current and wave loads.
15 second mean (sustained)	Part or whole of a structure whose greatest horizontal or vertical dimension exceeds 50 m. Wind loads need not be combined with current and wave loads.
1 minute mean (sustained) see Note	Part or whole structure of the unit regardless of dimension for use with the maximum wave and current loads.
NOTE
In no case is the one minute mean value to be taken as less than 25,8 m/s (50 knots). However, for unrestricted service the following wind criteria are also to be applicable for structural design considerations: for all modes of operation, whether afloat or supported by the sea bed, a sustained one minute mean wind velocity of not less than 36 m/s (70 knots) for the survival condition, a sustained one minute mean wind velocity of not less than 51,5 m/s (100 knots) The factors of safety are to comply with Table 5.2.1 Factors of safety for the combined load cases loadcase (b).

4.6.2 The wind force is to be calculated for each part of the structure and is not to be taken less than:

where

net force acting on any member or part of the unit. This includes the effect of any suction on back surfaces

0,613 (0,0625)

projected area of all exposed surfaces in upright or heeled position, in m²

wind velocity, in m/s, see Table 3.4.

shape coefficient as given in Table 3.4.2 Values of coefficient.

Table 3.4.2 Values of coefficient

Shape
Spherical	0,40
Cylindrical	0,50
Large flat surface (hull, deckhouse, smooth underdeck areas)	1,00
Drilling derrick	1,25
Wires	1,20
Exposed beams and girders under deck	1,30
Small parts	1,40
Isolated shapes (cranes, booms, etc.)	1,50
Clustered deckhouses or similar structures	1,10
NOTE
Shapes or combinations of shapes which do not readily fall into the specified categories will be subject to special consideration.

4.6.3 When calculating wind forces the following procedures should be considered:

Shielding may be taken into account when a member or structure lies closely enough behind another to have a significant effect. Procedures for determining the shielding effect and loading are to be acceptable to LR.
Areas exposed due to heel, such as underdecks, etc., are to be included using the appropriate shape coefficients.
If several deckhouses or structural members, etc., are located close together in a plane normal to the wind direction, the solidification effect is to be taken into account. The shape coefficient may be assumed to be 1,1.
Isolated houses, structural shapes, cranes, etc., are to be calculated individually, using the appropriate shape coefficient.
Open truss work commonly used for derrick towers, booms and certain types of masts may be approximated by taking 30 per cent of the projected block area of each side, e.g., 60 per cent of the projected block area of one side for double-sided truss work. An appropriate shape coefficient is to be taken fromPt 4, Ch 3, 4.6 Wind loads 4.6.2.

4.6.4 For slender structures and components, the effects of wind-induced cross-flow vortex vibrations are to be included in the design loading.

4.6.5 For slender structures sensitive to dynamic loads, the static gust wind force is to be multiplied by an appropriate dynamic amplification factor.

4.7 Current loads

4.7.1 In storm conditions, the current has two main components: the tidal and wind driven components. Submitted information on currents is to include tidal and wind induced components and the variation of their profiles with water depth, see Pt 4, Ch 3, 4.9 Wave loads 4.9.6 and Pt 4, Ch 3, 4.9 Wave loads 4.9.7. In addition, the effects of general circulation and loop currents are to be included where appropriate.

4.8 Orientation and wave direction

4.8.1 Loadings are to be assessed using sufficient wave headings and crest positions to determine the most severe loading on the unit. In addition to the design wave height and period, the unit is to be designed to withstand shorter period waves of less height when these can induce more severe loading on parts or the whole unit due to dynamic effects, etc.

4.8.2 Where a unit is required to operate at locations exposed to wind waves and swell waves acting simultaneously then this is to be taken into account when determining the wave loads.

4.9 Wave loads

4.9.1 Design wave criteria specified by the Owner/designer may be described either by means of design wave energy spectra or deterministic design waves having appropriate shape, size and period. The following should be taken into account:

The maximum design wave heights specified for each operating condition should be used to determine the maximum loads on the structure and principal elements. Consideration is to be given to waves of less than maximum height, where due to their period, the effects on various structural elements may be greater.
Wave lengths are to be selected as the most critical ones for the response of the structure or element to be investigated.
An estimate is to be made of the probable wave encounters that the unit is likely to experience during its service life in order to assess fatigue effects on its structural elements.
When units are to operate in intermediate or shallow water, the effect of the water depth on wave heights and periods and of refraction due to sea bed topography is to be taken into account.

4.9.2 The forces produced by the action of waves on the unit are to be taken into account in the structural design, with regard to forces produced directly on the immersed elements of the unit and forces resulting from heeled positions or accelerations due to its motion. Theories used for the calculation of wave forces and selection of relevant coefficients are to be acceptable to LR.

4.9.3 The wave forces may be assessed from tests on a representative model of the unit by a recognised laboratory, see Pt 4, Ch 3, 4.1 General 4.1.5 andPt 4, Ch 3, 4.1 General 4.1.6.

4.9.4 Wave theories used for the calculation of water particle motions are to be acceptable to LR and when using acceptable wave theories for wave force determination, reliable values of and which have been obtained experimentally for use in conjunction with the specific wave theory are to be used. Otherwise published data are to be used.

4.9.5 Consideration is to be given to the possibility of wave impact and wave induced vibration in the structure, including superstructures.

4.9.6 Where sea current acts simultaneously with waves, the effect of the current is to be included in the load estimation. In those cases this superposition is deemed necessary, the current velocity should be added vectorially to the wave particle velocity. The resultant velocity is to be used to compute the total force.

4.9.7 The following methods may be used for load estimation:

The forces on structural elements with dimensions less than 0,2 of the wave length subject to drag/inertia loading due to wave and current motions can be calculated from the Morison’s equation:

where

force per unit length of member

drag coefficient

density of water

projected area of member per unit length

component of the water particle velocity at the axis of the member and normal to it (calculated as if the member were not there)

inertia coefficient

volume of water per unit length

component of the water particle acceleration at the axis of the member and normal to it (calculated as if the member were not there)

Overall loading on an offshore structure is determined from the summation of loads on individual members at a particular time. The proper values of and for individual members to use with Morison’s equation will depend on a number of variables, for example: Reynolds number, Keulegan-Carpenter number, inclination of the member to local flow and effective roughness of marine growth. Therefore, fixed values for all conditions cannot be given. Typical values for circular cylindrical members, will range from 0,6 to 1,4 for and 1,3 to 2,0 for . The values selected are not to be smaller than the lower limits of these ranges. For inclined members, the drag forces in Morison’s equation are to be calculated using the normal component of the resultant velocity vector.
General values of hydrodynamic coefficients may be used in the Morison’s equation for the calculation of overall loading on the structure, namely:
- For circular cylinders covered by hard marine growth, is to be not less than 0,7.
- For circular cylinders not covered by hard marine growth, is to be not less than 0,6.
- For circular cylinders, is to be not less than 1,7.
- The recommended C_D and C_M for the members of a lattice leg are given in Pt 4, Ch 4, 3.9 Unit in the elevated position.
Diffraction theory is normally appropriate to determine wave loads where the member is large enough to modify the flow field.

4.9.8 Account is to be taken of the increase of overall size and roughness of submerged members due to marine growth when calculating loads due to wave and current, see Pt 4, Ch 3, 4.13 Marine growth 4.13.1.

4.10 Inertia loads

4.10.1 Dynamic loads imposed on the structure by accelerations due to the unit’s motion in a seaway are to be included in the structural design calculations. The dynamic loads may be obtained from model test results or by calculation. The methods of calculation are to be acceptable to LR.

4.11 Mooring loads

4.11.1 Mooring loads are to be considered for units operating afloat with positional mooring systems, see Pt 3, Ch 10 Positional Mooring Systems. The following are to be considered:

The overall strength of the structure.
The local strength where the mooring line forces are transmitted to the hull.

4.11.2 The support structure in way of mooring equipment is to be designed for the minimum design breaking load of the mooring line, determined in accordance with Pt 3, Ch 10 Positional Mooring Systems. See also Pt 4, Ch 6, 1 General requirements.

4.12 Snow and ice loads

4.12.1 Consideration is to be given to the extent to which snow and ice may accumulate on the exposed structure under any particular weather conditions. The wind resistance of exposed structural elements will be increased by the growth of ice. Details of the thickness and distribution of accumulation are to be established and taken into account in the design, see also Pt 3, Ch 6 Units for Transit and Operation in Ice.

4.12.2 The increased loading caused by the accumulation of snow and ice on any part of the structure is to be taken into account.

4.12.3 Values for the thickness, density and variation with height of accumulated snow and ice are to be derived from meteorological data acceptable to LR.

4.12.4 The overall distribution of snow and/or ice on topside structure is to be taken as a thickness on the upper and windward faces of the deck structures or members under consideration, where is the basic thickness obtained from the meteorological data. The distribution of ice on individual members may be assumed to be as shown inFigure 3.4.1 Assumed distribution of ice on individual members for calculation purposes.

4.12.5 It may be assumed that there is no increase of drag coefficient in the presence of ice.

4.12.6 The appropriate combinations of snow and ice loadings with other design environmental loads are to be specially considered and agreed with LR. In general, extreme snow and ice loads are to be combined with other environmental loads corresponding to the design five-year return criteria for the unit.

Figure 3.4.1 Assumed distribution of ice on individual members for calculation purposes

4.13 Marine growth

4.13.1 Marine growth will increase the weight and the overall dimensions of submerged members and alter their surface characteristics. These effects will increase the loads applied to the structure. The thickness of marine growth taken into account in the design is to be stated in the Operations Manual and the design limit is not to be exceeded in service. Unless more accurate data is available from the marine fouling study, the density of marine growth in air is to be taken as 1325 kg/m³.

4.14 Hydrostatic pressures

4.14.1 The hydrostatic pressure head to be used as the basis for the design of internal spaces is to be the greatest of the following:

For tanks, the maximum head during normal operation.
For shell boundaries, the hydrostatic head due to external sea pressure.
For watertight boundaries, the head measured to the worst damage waterline, see Pt 4, Ch 7 Watertight and Weathertight Integrity and Load Lines.

The minimum design pressure heads for local strength are to be in accordance with Chapter 6.

4.14.2 Where testing the tank involves pressure heads in excess of those derived in Pt 4, Ch 3, 4.14 Hydrostatic pressures 4.14.1, the excess may be taken into account by the use of a load factor applied to the design head. Where this is done, it is to be clearly stated in the calculations.

4.15 Deck loads

4.15.1 The maximum design uniform and concentrated deck loads for all areas of the unit in each mode of operation are to be taken into account in the design. The minimum design deck loads for local strength are to be in accordance with Pt 4, Ch 6 Local Strength.

4.16 Accidental loads

4.16.1 The following credible failures and accidents are to be considered in the design as applicable to the function of the unit:

Collision.
Dropped object.
Blast.
Accidental flooding.
Loss of primary bracing (column-stabilised unit).
Emergency helicopter landings.
Abnormal metocean.

4.16.2 Collision loads imposed by attending vessels which may be approaching, mooring or lying alongside the unit are to be considered in the design. The unit is to be designed to withstand accidental impacts between attending vessels and the unit and be capable of absorbing the impact energy.

Recommended practice is given in LR's Guidance Notes for Collision Analysis to assist in identifying potential collision scenarios, establishing representative collision loads and assessing the impact of these loads on structural integrity.

4.16.3 The kinetic energy to be considered is normally not to be less than:

14 MJ for sideway collision;
11 MJ for bow or stern collision;

corresponding to an attending vessel of 5000 tonnes displacement with impact velocity 2 m/s.

4.16.4 A reduced impact energy may be accepted upon special consideration, taking into account the environmental design criteria.

4.16.5 The energy absorbed by the unit during a collision impact will be less than or equal to the total impact kinetic energy, depending on the relative stiffnesses of the relevant parts of the unit and the impacting ship/unit and also on the mode of collision and ship/unit operation. These factors may be taken into account when considering the energy absorbed by the unit, see also Pt 4, Ch 4, 1 Column-stabilised units and Pt 4, Ch 4, 3 Self-elevating units for column-stabilised and self-elevating units respectively.

4.16.6 Collision is to be considered for all elements of the unit which may be hit by sideway, bow or stern collision. The vertical extent of the collision zone is to be based on the depth and draught of attending ships/units and the relative motion between the attending ships/units and the unit.

4.16.7 The accidental impact loads caused by dropped objects from cranes are to be considered in the design of the unit when the arrangements of the unit are such that the failure of a vital structure member could result in the collapse of the structure.

4.16.8 Critical areas for dropped objects are to be determined on the basis of the actual movement of crane loads over the unit.

4.16.9 The structural bulkheads protecting accommodation areas, and other structures that may be subject to blast pressures, are to be designed for accidental blast loading, where applicable. The design blast pressures are to be defined by the Owners/designers, see Pt 7, Ch 3, 2.4 Fire and Explosion Evaluation (FEE) 2.4.2 and are to comply with National requirements. Blast loads are to be combined with the still water loads. Environmental loads need not be considered. Design calculations are to be submitted which may be based on elastic analysis or elastoplastic design methods, see also Pt 4, Ch 3, 4.16 Accidental loads 4.16.11.

4.16.10 Accidental flooding of a single hull compartment is to be considered in the design of the unit. As a minimum, the compartments to be addressed are to include those set out in Chapter 3 - Subdivision, Stability and Freeboard as applicable to the unit type. Special consideration will be given to unit types not addressed by the 2009 IMO MODU Code.

4.16.11 Units with slender members where the failure of a single member could result in the overall collapse of the unit’s structure are to be considered for credible failure of such members, see Pt 4, Ch 4 Structural Unit Types.

4.16.12 Requirements for helicopter landing areas are given in Pt 4, Ch 6, 5 Helicopter landing areas.

4.16.13 Abnormal metocean with a return period of 1000 ~ 10000 years are to be considered in the design of the floating structure. Unless the upper hull structure is designed for wave impact, the unit is to be designed to have a positive air gap in abnormal metocean.

4.16.14 Permissible stresses for accidental load conditions are given in Pt 4, Ch 5, 2 Permissible stresses.

4.16.15 When a National Administration in the country in which the unit is registered and/or in which it is to operate has additional requirements for accidental loads these are to be taken into account in the design loadings.

4.17 Fatigue design

4.17.1 Fatigue damage due to cyclic loading must be considered in the design of all unit types.

4.17.2 Fatigue design calculations are to be carried out in accordance with the analysis procedures and general principles given in Pt 4, Ch 5, 5 Fatigue design or other acceptable method.

4.17.3 The factors of safety on calculated fatigue life are to comply withPt 4, Ch 5, 5 Fatigue design . Additional factors of safety are given in Pt 10, Ch 1, 18 Fatigue for ship units.

4.18 Other loads

4.18.1 If attending ships/units are to be moored to the unit, the forces imposed by the moorings on the structure are to be taken into account in the design.

4.18.2 Other local loads imposed on the structure by equipment and mooring and towing systems are to be considered in the design of the structure.

4.18.3 When partial filling of tanks is contemplated in operating conditions, the risk of significant loads due to sloshing induced by any of the vessel motions is to be considered. An initial assessment is to be made to determine whether or not a higher level of sloshing investigation is required, using the procedure given in Pt 3, Ch 3, 5 Design loading of the Rules for Ships.