Offshore wind turbine design - Wind Power Generation and Wind Turbine Design - page 374

Table 3: Comparing oil & gas industry to wind industry requirements [3].

Oil and gas Offshore
Structures Category Wind energy structures

Water depth 20-120 m and more 10-30 m

Vertical loading 5000-30,000 ton 100-300 ton

Horizontal loading 10-20% of vertical load 70-150% of vertical loads

Over-turning moment Water depth × horizontal (Water depth + 75 m) ×

load horizontal load

Number of installations 1 20-200

Structure width O(10 m): >20 m O(1 m): ∼ 5 m

Power generated O(100 MW) Max of <5 MW

per each installation

Above-mentioned challenges are certainly not new if we consider the signifi -
cant oil and gas installations around the world. However, it is erroneous to make
the assumption that overcoming these challenges amounts to direct translation of
the oil and gas experience to the wind industry. Table 3 illustrates key differences
between the two industries in terms of technical requirements.
Table 3 shows that magnitudes of loads affecting oil and gas structures are much
higher than those affecting wind energy structures. However, the ratio between the
over-turning moment and the vertical load, i.e. “eccentricity” which governs the
foundation geometry and shape, is several orders of magnitude higher in wind
energy structures than it is in the oil and gas structures. This problem is com-
pounded by (1) the fact that non-dimensional ratio between eccentricity and the
structure's width in the wind turbine structures will be even higher than in the oil
and gas structures, and (2) the heavily dynamic nature of the loads. Therefore, a
straightforward translation of foundation shapes and designs from offshore oil and
gas structures to offshore wind structures is simply not feasible.
In addition to the design limitations, there are other factors that differentiate the
oil and gas experience from the wind energy structures needs:
In contrary to one giant oil and gas offshore platform installation, a wind power

1.

plant normally consists of tens to hundreds of identical turbine installations.
Therefore, experience with assembly-line techniques and the associated com-
partmentalization, modularization, and quality control with six-sigma methods
become more relevant.
It is certainly economical to have $100 million worth of platform foundations

2.

to produce crude oil equivalent to hundreds of megawatts worth of electricity
from one oil and gas platform. If the same foundations are translated to the
wind industry, its cost must be in the neighborhood of one million dollar for the
overall economics to work.
On the other hand, ruling out direct translation of foundation designs from the
oil and gas industry to the wind industry does not mean that wind turbine founda-
tion designs should or need to start from a clean sheet of paper. The new research