Page 381 - Marine Structural Design

Chapter I8 Fatigue Loading and Stresses 357

significant wave height H, and spectral wave period T,. Swell may in some instance come
from a single direction without much variation of the direction. However, in general, the
directionality should be explicitly considered in defining the scatter diagrams. The selection of
sea states for combined sea, swell, current and wind is a complex subject, and requires certain
engineering judgement based on the understanding of the environmental data and structural
dynamic response.
Another critical issue to be taken into account is load cases and the loading conditions. To
estimate fatigue damage during operating conditions, the vessel motions and FL40 data should
be generated for the normal operating condition. Similar statement may be valid for estimation
of fatigue damage during transportation and installation phases. The total accumulated damage
is then obtained by adding the damage for each phase of the design fatigue life and the
periodprobability of the respective phase. For fatigue analysis of TLP tethers, mooring lines
and risers, it is necessary to define the vessel motions and the RAO at the point where the
tethers, mooring lines and risers are attached to the vessel.
Francois et a1 (2000) compared fatigue analysis results from classification societies nd full-
scale field data.

An example analysis was conducted by Nordstrom et a1 (2002) to demonstrate the heading
methodology and assess its efficiency for project use for an FPSO. Their proposed heading
and fatigue analysis procedure may lead to more effective fatigue design for FPSOs in non-
collinear environment.

18.6 Examples
Example 18.1: Long-term Stress Range Distribution - Deterministic Approach
Problem:
Determine the long-term stress range distribution of the spanned riser clamped to a jacket
platform, as shown in Figure 18.2 below. This example is chosen to illustrate the deterministic
approach in sub-section 18.3.3 (Almar-Nms, 1985). It may be assumed that the riser span
length is 1=1Om, outer diameter OD=0.27m, wall thickness WT=0.0015m, moment of inertia
I=9.8*10-’m4 and water depth is 100m. All waves are assumed to approach from the same
direction.
Solution:
The first natural period of the span, f, , can be calculated from:

f --.
N-
27t
where,
E1 = Bending stiffness
1 = Span length
m = Mass per unit length
a, =Numerical constant, for a beam fixed at both ends, a, =22 for the first mode.