Offshore wind turbine design - Wind Power Generation and Wind Turbine Design - page 372

4.4.7 Unique U.S. challenges and oil and gas experiences
There are other challenges that are more signifi cant in the U.S. compared with
potential European sites:

1. Because of the longer fetch length, wave heights are signifi cantly higher in the

open Atlantic Ocean than in the most challenging European sites in North Sea.
The most utilized extreme wave power spectral density function is modifi ed
Pierson-Moskowitz (PM) spectrum [6]. It assumes a fully developed sea, i.e.
all the wind energy has been imparted to the sea over an infi nite fetch length.
However, in many coastal situations, that may not be the case and much smaller
fetch lengths may exist. The Joint North Sea Wave Project (JONSWAP) pro-
posed the JONSWAP spectrum [7, 8] that includes scale parameters for the
fetch length. The marked difference between PM and the JONSWAP spectrums
is that JONSWAP is signifi cantly more peaked. The shorter the fetch length, the
more peaked JONSWAP becomes. Over long fetch lengths, the sea receives more
energy from wind shear and has more opportunity to develop random waves
with more variable spectrum of wave heights and lengths. So the JONSWAP
spectrum experience may need amendments when it is applied to the U.S.
coastal wave evaluation.
For offshore structures, Germanisher Lloyd has certifi cation requirements for
applying both wind and wave loads on offshore wind turbine without being too
conservative. For fatigue analysis, load spectra are to be determined which
include the infl uences to be considered for the wind turbine plus those from
wave, currents, and sea-ice [9]. These load cases are also common with the oil
and gas industry and are largely borrowed from that industry. However, it has
to be carefully reconsidered and studied for the U.S. market. The fi rst reason is
that wave heights in the Atlantic are on the average higher than those in the
North Sea since North Sea and Baltic Sea coasts have shorter fetch lengths and
lower wind speeds compared with the U.S. North Atlantic coasts. Figure 5
shows the 95% confi dence intervals for the long-term Weibull distribution of
the probability of the extreme signifi cant wave heights. Figure 5 shows that the

Wave Height Wave Height

50 50

40 40

North Atlantic North Atlantic

30 30

20 20

North Sea North Sea

10 10

0 0

1
Years

1
Years

1
Day

1
Day

100
Years

100
Years

Return Period Return Period

Figure 5: Wave heights in the U.S. east coast and Europe [3].