Page 380 - Marine Structural Design

356 Part 111 Fatigue and Fracture

will typically dominate. For instance, the vertical bending moment related stress fluctuation at
ship deck is predominant, while the stress range on the side shell near waterline is nearly
entirely due to local (intemaVexterna1) pressure. Structural details in the ship bottom is under a
combination of bending and local pressure effects.
Pressure variations near the waterline are the main cause of fatigue damages on side shell
(Friis-Hansen and Winterstein, 1995).
For spectral fatigue analysis of ships for unrestricted service, the nominal North Atlantic wave
environment is usually used. For a site-specific assessment (of FPSO) or for a trade route
known to be more severe than the North Atlantic, the more stringent wave scatter diagram
should be applied. When motion and loads are highly frequency dependent, it is necessary to
include wave-period variation.
The fatigue loading conditions for ships is fully laden and ballast. According to classification
Rules (e.g. BV, 1998), for each relevant loading condition, two basic sea states should be
considered: head sea conditions and oblique sea condition. The total cumulative damage may
be estimated as:

D=a D,+pD; (18.26)
where the coefficients CY and are given in Table 18.1. Do and 0: are cumulative damage due
to full laden load conditions and ballast load conditions respectively.
Do = (01 + 02) (18.27)

Db = (D; + Di) (18.28)
where.
Di = max(Di,, Di2), i = 1,2 for full laden load condition (18.29)
Di = max(D,;:, , DiJ, i = 1,2 for ballast load condition (18.30)

where Dll, D12 or Dl’, , D;, are cumulative damage for static sea pressure associated to maximum
and minimum inertia cargo or blast loads, respectively. D,,, D,, or D;,, D;, are cumulative
damage for maximum (ship on crest of wave) and minimum (ship on tough of wave) wave-
induced sea pressure associated to static internal cargo or ballast loads, respectively.
18.5.3 Fatigue Load Combinations for Offshore Structures
In defining the environmental conditions for offshore structural design, it is necessary to
derive combinations of directional sea, swell, wind and current that the offshore structure will
encounter during its life. The fatigue of hull structures, mooring lines and risers will largely
dependent on the sea and swell conditions, while the current may cause vortex-induced
vibrations of risers, mooring lines and TLP tethers. It is therefore required to define a
directional scatter diagrams for sea states, swells and sometimes for currents. Swells will only
be considered properly (typically by adding a separate swell spectrum into the analysis and so
obtaining a multi peaked sea plus swell spectrum) if it is of particular importance as, for
instance, offshore west Africa and Australia (Baltrop, 1998). An alternative approach to
properly account for swells is to use two separated scatter diagrams for directional sea and
swell respectively. In this case, the probability of individual bins (sea-states, cells) should be
properly defined, and each bin (cell) is represented by a single peak spectrum defined by