Offshore wind turbine design - Wind Power Generation and Wind Turbine Design - page 368

with a thickness extending to 90 mm at 25 m water depth from around 5 m and
50 mm accordingly at 10-12 m depth. The resulted weight could be 250 metric tons.
This level of dimensions and weights stretch the maximum capability of the fabrica-
tion factories and installation vessels to their limits. In fact, only a few vessels in the
world can even handle the hammering requirements of such a large pile.
Design limits: •

At 25 m depth, and for a hub-height of approximately 75 m above sea level, a
monopile foundation's natural frequency becomes dangerously close to the tur-
bine's natural frequency. The engineering solution is to increase either pile diam-
eter or thickness, which is either at the manufacturing limit or at the construction
limit already.

4.3 Overcoming the 25-m barrier

Due to these manufacturing and design limitations, the 25-m barrier is quite real,
especially as coastal communities demand wind power plants to be placed even
farther offshore. However, overcoming the 25-m depth is not without precedent
and this gives strong hope for the wind industry to cross the barrier.
Before 1996, the gravity foundation was the industry's standard offshore foun-
dation. However, in 1996, the industry reached the gravity foundation potential,
approximately 7.5 m deep. Danish research performed in 1996-1997 recom-
mended Danish wind industry to use monopile foundation for the deeper sites.
Since then, the monopile has become the primary foundation choice for the off-
shore wind industry.
As an additional benefi t from the 1997 monopile development, overcoming the
gravity foundation depth limit gave Danish companies a signifi cant lead in the
offshore wind development race. While the offshore challenges are more signifi -
cant for the United States as compared with European sites for reasons such as
higher waves, susceptibility to hurricanes, and increased wave breaking energy,
the 25-m barrier presents the United States with a signifi cant opportunity for tech-
nology break through and leadership in the offshore wind industry.
The opportunity exists for the United States to spearhead a development effort
similar to the European effort to break the 7.5-m barrier. Because of the coastal
community demands for farther offshore deployment location, overcoming the
25-m barrier will be the enabler for the growth of the offshore wind power plants
market in the United States.
The offshore wind challenges are not without parallel in other U.S. industries,
particularly oil and gas. Given its offshore oil and gas experience, the United States
is ideally suited to assume the lead on attacking the 25-m barrier challenge. How-
ever, it would be a mistake to assume that the solution amounts to the direct trans-
lation of the oil and gas experience to the wind industry.
The magnitudes of loads affecting oil and gas structures are much higher than
those affecting wind energy structures. However, force magnitudes are neither the
sole factor nor the most important one affecting foundation design. The ratio
between over-turning moment and vertical load is known as “eccentricity” and is