4.4.1 General
4.4.1.1 Variable functional loads are loads which may vary in magnitude, position and direction during the period under consideration, and which are related to operations and normal use of the installation. Examples are: — personnel
Table 4-2 Basis for selection of characteristic loads and load effects for operational design conditions Limit states – operational design conditions
Load category ULS FLS ALS SLS
Intact structure Damaged structure
Permanent (G) Expected value
Variable (Q) Specified value
Environmental (E)
98% quantile in distribution of annual maximum load or load effect (Load or load effect with return period 50
years) Expected load history or expected load effect history
Not applicable Load or load effect with return period not less than 1 year
Specified value
Accidental (A) Specified value
Abnormal wind
turbine loads Specified value
Deformation (D) Expected extreme value
Table 4-3 Statistical terms used for specification of characteristic loads and load effects Term Return period (years) Quantile in distribution of annual maximum
Probability of exceedance in distribution of annual maximum
100-year value 100 99% quantile 0.01
50-year value 50 98% quantile 0.02
10-year value 10 90% quantile 0.10
5-year value 5 80% quantile 0.20
— crane operational loads — ship impacts
— loads from fendering
— loads associated with installation operations — loads from variable ballast and equipment
— stored materials, equipment, gas, fluids and fluid pressure — lifeboats.
4.4.1.2 For an offshore wind turbine structure, the variable functional loads usually consist of: — actuation loads
— loads on access platforms and internal structures such as ladders and platforms — ship impacts from service vessels
— crane operational loads.
4.4.1.3 Actuation loads result from the operation and control of the wind turbine. They are in several categories including torque control from a generator or inverter, yaw and pitch actuator loads and mechanical braking loads. In each case, it is important in the calculation of loading and response to consider the range of actuator forces available. In particular, for mechanical brakes, the range of friction, spring force or pressure as influenced by temperature and ageing shall be taken into account in checking the response and the loading during any braking event.
4.4.1.4 Actuation loads are usually represented as an integrated element in the wind turbine loads that result from an analysis of the wind turbine subjected to wind loading. They are therefore in this standard treated as environmental wind turbine loads and do therefore not appear as separate functional loads in load combinations.
4.4.1.5 Loads on access platforms and internal structures are used only for local design of these structures and do therefore usually not appear in any load combination for design of primary support structures and foundations.
4.4.1.6 Loads and dynamic factors from maintenance and service cranes on structures are to be determined in accordance with requirements given in DNV Standard for Certification No. 2.22 Lifting Appliances, latest edition.
4.4.1.7 Ship impact loads are used for the design of primary support structures and foundations and for design of some secondary structures.
4.4.1.8 The characteristic value of a variable functional load is the maximum (or minimum) specified value, which produces the most unfavourable load effects in the structure under consideration.
4.4.1.9 Variable loads can contribute to fatigue. In this case characteristic load histories shall be developed based on specified conditions for operation.
Guidance note:
For a specified condition for operation, the characteristic load history is often taken as the expected load history. ---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
4.4.2 Variable functional loads on platform areas
Variable functional loads on platform areas of the support structure shall be based on Table 4-4 unless specified otherwise in the design basis or the design brief. For offshore wind turbine structures, the platform area of most interest is the external platform, which shall be designed for ice loads, wave loads and ship impacts. The external platform area consists of lay down area and other platform areas. The intensity of the distributed loads depends on local or global aspects as given in Table 4-4. The following notions are used:
4.4.3 Ship impacts and collisions
4.4.3.1 Boat landings, ladders and other secondary structures in and near the water line shall be designed against operational ship impacts in the ULS. The primary structure in and near the water line shall be designed against accidental ship impacts in the ALS. Furthermore, if an accidental ship impact against a secondary structure results in larger damage of the primary structure than an accidental impact directly against the primary structure, then this load case shall also be considered in the ALS.
Local design: For example design of plates, stiffeners, beams and brackets Primary design: For example design of girders and columns
Guidance note:
The requirements for design against accidental ship impacts in the ALS are merely robustness requirements, which are practical to handle as ALS design. They are not really requirements for full ALS design, since designs against rare large accidental loads from impacts by larger vessels than maximum authorised service vessels are not considered, see
[4.4.3.5].
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4.4.3.2 Impacts from approaching ships in the ULS shall be considered as variable functional loads. Impacts from drifting ships in the ALS shall be considered as accidental loads.
4.4.3.3 When primary structural parts such as the support structure and its foundation are exposed to ship impacts, these structural parts shall not suffer such damage that their capacities to withstand the loads they are going to be exposed to become compromised. In the ULS, secondary structural parts, such as fenders, boat landings and ladders, shall not suffer damage to such an extent that they loose their respective functions as access structures. In the ALS, secondary structural parts are allowed to become torn off, e.g. by including weak points or by local strengthening of supporting structural parts, thereby to avoid excessive damage to these supporting primary structural parts.
4.4.3.4 For design against operational ship impacts, the characteristic impact load shall be taken as the expected impact load caused by the maximum authorised service vessel approaching by bow and stern in the most severe sea state to be considered for operation of the service vessel. A vessel-specific speed shall be assumed. The speed shall not be assumed less than 0.5 m/s. Effects of wind, wave and current shall be included as well as effects of added mass, which contributes to the kinetic energy of the vessel.
Guidance note:
Data for the maximum authorised service vessel, including service vessel layout and service vessel impact velocities, are usually given in the design basis for structural design of the wind turbine structure. Data for wave, wind and current in the most severe sea state to be considered for operation of the service vessel are also usually given in the design basis. A risk analysis forms the backbone of a ship impact analysis. The expected impact load is part of the results from the risk analysis.
When specific loads are not given, the contact area can be designed by assuming an impact force F = 2.5 ⋅∆
where F is the impact force in units of kN and ∆ is the fully loaded displacement of the supply vessel in units of tons. This is based on an assumption of ship impact against a hard structure. When a damper or spring device such as a fender is provided in the area subject to the impact, a lower impact force can be used. For further background and guidance, reference is made to the DNV High Speed Light Craft Rules.
IEC61400-3 states that if no information about the service vessel is known, the impact force can normally be accounted for by applying 5 MN as a horizontal line load over the width of the support structure. This load is meant to include dynamic amplification.
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4.4.3.5 For design against accidental ship impacts, the characteristic impact load shall be taken as the impact load caused by unintended collision by the maximum authorised service vessel in daily operation. For this purpose, the service vessel shall be assumed to be drifting laterally and the speed of the drifting vessel shall be assessed. The speed shall not be assumed less than 2.0 m/s. Effects of added mass shall be included. Effects of fendering on the maximum authorised service vessel shall be considered.
Guidance note:
The maximum authorised service vessel is the largest expected vessel used in daily operation. Data for the maximum authorised service vessel, including impact velocities of a laterally drifting vessel, are usually given in the design basis for structural design of the wind turbine structure. Note that supply vessels may grow in size over the years and the accidental load may become substantial. Larger special purpose vessels used for replacement of larger components etc. should be handled by specific case-by-case safety assessments.
4.4.4 Tank pressures
Requirements to hydrostatic pressures in tanks are given in DNV-OS-C101.
4.4.5 Miscellaneous loads
4.4.5.1 Railing shall be designed for a concentrated load of 1.0 kN as well as for horizontal line load equal to
0.3 kN/m, applied to the top of the railing.
4.4.5.2 Requirements given in EN 50308 should be met when railing, ladders and other structures for use by
personnel are designed.