101 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
— 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.
102 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.
103 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.
104 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.
105 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.
Table B2 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
106 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.
107 Ship impact loads are used for the design of primary support structures and foundations and for design
of some secondary structures.
108 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.
109 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.
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201 Variable functional loads on platform areas of the support structure shall be based on Table D1 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 D1. The following notions are used:
D 300 Ship impacts and collisions
301 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.
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 305.
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302 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.
303 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.
304 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. Broadside collision with appropriate fendering shall be assumed. 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 ⋅ Δ
Local design: For example design of plates, stiffeners, beams and brackets
Primary design: For example design of girders and columns
where F is the impact force in units of kN and Δ 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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305 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.
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D 400 Tank pressures
401 Requirements to hydrostatic pressures in tanks are given in DNV-OS-C101. D 500 Miscellaneous loads
501 Railing shall be designed for a horizontal line load equal to 1.5 kN/m, applied to the top of the railing. 502 Ladders shall be designed for a concentrated load of 2.5 kN.
Table D1 Variable functional loads on platform areas
Local design Primary design Global design
Distributed load p (kN/m2) Point load,P(kN) Apply factor to distributed load Apply factor to primary design load Storage areas q 1.5 q 1.0 1.0
Lay down areas q 1.5 q f f
Area between equipment 5.0 5.0 f may be
ignored Walkways, staircases and
external
platforms 4.0 4.0 f
may be ignored Walkways and staircases for
inspection only 3.0 3.0 f ignoredmay be
Internal
platforms, e.g. in towers 3.0 1.5 f ignoredmay be
Areas not exposed to
other functional loads 2.5 2.5 1.0 –
Notes:
— Point loads are to be applied on an area 100 mm × 100 mm, and at the most severe position, but not added to wheel loads or distributed loads.
— For internal platforms, point loads are to be applied on an area 200 mm × 200 mm
— q to be evaluated for each case. Lay down areas should not be designed for less than 15 kN/m2.
— f = min{1.0 ; (0.5 + 3/ )}, where A is the loaded area in m2.
— Global load cases shall be established based upon “worst case”, characteristic load combinations, complying with the limiting global criteria to the structure. For buoyant structures these criteria are established by requirements for the floating position in still water, and intact and damage stability requirements, as documented in the operational manual, considering variable load on the deck and in tanks.
503 Requirements given in EN50308 should be met when railing, ladders and other structures for use by
personnel are designed.