Offshore wind turbine design - Wind Power Generation and Wind Turbine Design - page 373

North Atlantic has 30% higher wave heights on the average compared with
North Sea. It is worth noting that the 30% difference can be accounted for
almost exactly using the JONSWAP spectrum [8]. The higher signifi cant wave
heights in U.S. pose many challenges for the structural modeling and design.
Back to the hurricane problem at the turbine design level, the current wind

2.

turbines are typically designed for up to IEC class 1 wind load, which can be
roughly translated to a maximum wind speed between the Hurricane category
1 and category 2 conditions. However, many population centers along the U.S.
east coast are hit by higher than category 2 hurricanes in a frequency higher
than once per 50 years. So the safety of the offshore wind turbine erected near
these regions could be a challenge to the system and structure design.
Wave breaking occurs when the ratio between the wave height and the sea depth 3.

exceeds a certain limit. Furthermore, in certain circumstances wave breaks
as plunging wave instead of surging wave, which can cause signifi cantly more
fatigue load force on any structural element present at the breaking point. At a UK
experimental offshore wind power plant in Blyth Harbour, one of the towers was
erected at a wave breaking point and sustained signifi cantly increased fatigue load.
A study of wave breaking probability in U.S. sites vs. European sites shows that
the probability of plunging wave breaking is about the same. However, because
the U.S. sites have higher wave heights and are deeper in depth, the plunging wave
could break with as much as four times the energy of its European counterparts.

There are many breaking wave theories available in the literature. The simplest
and oldest model is:

H = 0.78 h

(1 )

b

where H b is the wave breaking height and h is the sea depth. A little more sophisti-
cated equation that considers the sloping seabed effect is termed the Goda expres-
sion:

(2 )

H = 0.071tanh( kh )

b

Many other theories exist in the literature with varying degree of complexity.
Anastasiou and Bokaris [10] evaluated 19 of the hypothetical expressions avail-
able in the literature against their own data and found the two simplest equations
above to be the most accurate ones. On the contrary, Kriebel [11] found that other
equations to be better using different sets of data. So there is a high degree of dis-
agreement that may never be resolved because of the complex and uncertain nature
of the wave breaking phenomenon. However, it is possible to devise a methodol-
ogy to generate a site-specifi c wave breaking prediction by fusing the site-specifi c
short-term observations, the 3D bathymetry measurements, and the known wave
breaking models. This task is not easy given the fact that the local current has
impact to the wave breaking condition because increased current velocity increases
the probability of wave breaking. The fi nal prediction should be based on site-
specifi c measurements to calibrate known physics and models. It should also be
included as part of the wind turbine certifi cation process for the site.