Metocean Extreme and Operating Conditions 3.2 Overview of WWC Processes 49
Part A | 3.2
that make little difference to the design or operation of
the facility. The first and best way to eliminate variables
from investigation is to understand the basic responses
of the particular facility. In other words to answer the
question: which metocean variables impact this facility
most and which have little or no impact? For example, squalls and their dynamic effects can be especially
important in designing the moorings for floating production vessels near the equator, such as off Indonesia.
Carefully quantifying squall intensity and its change
over time scales of a few minutes and length scales of
the order of 50 m is of highest importance. In contrast,
quantifying water, storm surge, and air temperatures is
not critical.
Section 3.2 is an overview of key processes, including a discussion of WWC spectra, wind and current
profiles, and important though arguably tangential discussions of wave growth and wave breaking.
Sections 3.3 and 3.4 briefly address some of the
more important issues that can arise regarding measurements and models. Since all metocean criteria are
founded on one or both of these inputs, it is important
to understand the various sources and databases, and
their advantages and limitations. Otherwise one may
well suffer the consequences of garbage in, garbage
out.
Section 3.5 examines ways to calculate the marginal
probability of WWC processes. One of the more interesting cases is when two variables are statistically
independent (or nearly so) in time or space and yet there
is often a non-negligible probability that the two can occur simultaneously and generate loads that exceed the
load from an individual process at the same probability
level.
Section 3.6 describes some of the analysis products
that are typically used to quantify operating conditions.
The discussion begins with the simplest approaches
such as univariate probability density functions and
then moves on to address more sophisticated products
to characterize storm and calm persistence, directional
dependence, and vertical space variations.
Finally, the last section covers the topic of extreme criteria. It is important because the economic
and safety consequences of getting it right are so high.
It is also important because there is no general and
all encompassing methodology to estimate extremes so
the topic is rich in subtleties, complexity, and potential
traps.
3.2 Overview of WWC Processes
3.2.1 Winds
Most winds that are important for offshore design and
operations come from extra-tropical or tropical storms.
Extra-tropical storms are large-scale systems that are
well represented on standard meteorological charts. The
measurements used to produce these charts are discussed in Sect. 3.3. Tropical storms are relatively small
features on common weather charts. Observations in
them are scarce. Detailed wind fields in tropical storms
are produced using dynamic or kinematic numerical
models, which are discussed in Sect. 3.4.
Wind specification requires especially careful attention to definitions. Richardson [3.3] made an eloquent
statement of the problem many years ago:
Does the wind possess a velocity? This question, at
first sight foolish, improves on acquaintance . . . let
us not think of velocity, but only of various hyphenated velocities.
Richardson was concerned that x=t might not
have a limit in a turbulent fluid. Examples of hyphenated velocities which do have a clearly defined meaning
are the one-hour or three-second wind. The one-hour
wind is the wind velocity averaged over an hour. The
three-second wind is the maximum three second average velocity in an hour interval unless another interval
is stated. The three-second wind gust is about 30%
higher than the one-hour average.
Wind speeds also vary with altitude. Friction at the
water surface reduces the wind speed near the boundary.
Wind speeds increase with height through the atmospheric boundary layer. The speed at 30 m height is
about 15% higher than that at the common anemometer
height of 10 m. Unless the averaging time and height of
a wind measurement are given, that measurement is not
very useful.
The standard offshore engineering method for converting wind speeds from one averaging period and
height to another is given by Standards Norway (NORSOK) [3.4] and serves as the basis for the ANSI
(American National Standards Institute)/API [3.1] recommended practices. These guidelines give the wind
speed u.z; t/ o at height z above mean sea level for averaging period t o as
u .z; t o / D U .z/
Ä
1 0:41I u .z/ ln
 t
t o
ÃÃ
;
(3.1)
where the one-hour mean wind U.z/ is given by a modified logarithmic profile that depends on the one-hour
Part A | 3.2
that make little difference to the design or operation of
the facility. The first and best way to eliminate variables
from investigation is to understand the basic responses
of the particular facility. In other words to answer the
question: which metocean variables impact this facility
most and which have little or no impact? For example, squalls and their dynamic effects can be especially
important in designing the moorings for floating production vessels near the equator, such as off Indonesia.
Carefully quantifying squall intensity and its change
over time scales of a few minutes and length scales of
the order of 50 m is of highest importance. In contrast,
quantifying water, storm surge, and air temperatures is
not critical.
Section 3.2 is an overview of key processes, including a discussion of WWC spectra, wind and current
profiles, and important though arguably tangential discussions of wave growth and wave breaking.
Sections 3.3 and 3.4 briefly address some of the
more important issues that can arise regarding measurements and models. Since all metocean criteria are
founded on one or both of these inputs, it is important
to understand the various sources and databases, and
their advantages and limitations. Otherwise one may
well suffer the consequences of garbage in, garbage
out.
Section 3.5 examines ways to calculate the marginal
probability of WWC processes. One of the more interesting cases is when two variables are statistically
independent (or nearly so) in time or space and yet there
is often a non-negligible probability that the two can occur simultaneously and generate loads that exceed the
load from an individual process at the same probability
level.
Section 3.6 describes some of the analysis products
that are typically used to quantify operating conditions.
The discussion begins with the simplest approaches
such as univariate probability density functions and
then moves on to address more sophisticated products
to characterize storm and calm persistence, directional
dependence, and vertical space variations.
Finally, the last section covers the topic of extreme criteria. It is important because the economic
and safety consequences of getting it right are so high.
It is also important because there is no general and
all encompassing methodology to estimate extremes so
the topic is rich in subtleties, complexity, and potential
traps.
3.2 Overview of WWC Processes
3.2.1 Winds
Most winds that are important for offshore design and
operations come from extra-tropical or tropical storms.
Extra-tropical storms are large-scale systems that are
well represented on standard meteorological charts. The
measurements used to produce these charts are discussed in Sect. 3.3. Tropical storms are relatively small
features on common weather charts. Observations in
them are scarce. Detailed wind fields in tropical storms
are produced using dynamic or kinematic numerical
models, which are discussed in Sect. 3.4.
Wind specification requires especially careful attention to definitions. Richardson [3.3] made an eloquent
statement of the problem many years ago:
Does the wind possess a velocity? This question, at
first sight foolish, improves on acquaintance . . . let
us not think of velocity, but only of various hyphenated velocities.
Richardson was concerned that x=t might not
have a limit in a turbulent fluid. Examples of hyphenated velocities which do have a clearly defined meaning
are the one-hour or three-second wind. The one-hour
wind is the wind velocity averaged over an hour. The
three-second wind is the maximum three second average velocity in an hour interval unless another interval
is stated. The three-second wind gust is about 30%
higher than the one-hour average.
Wind speeds also vary with altitude. Friction at the
water surface reduces the wind speed near the boundary.
Wind speeds increase with height through the atmospheric boundary layer. The speed at 30 m height is
about 15% higher than that at the common anemometer
height of 10 m. Unless the averaging time and height of
a wind measurement are given, that measurement is not
very useful.
The standard offshore engineering method for converting wind speeds from one averaging period and
height to another is given by Standards Norway (NORSOK) [3.4] and serves as the basis for the ANSI
(American National Standards Institute)/API [3.1] recommended practices. These guidelines give the wind
speed u.z; t/ o at height z above mean sea level for averaging period t o as
u .z; t o / D U .z/
Ä
1 0:41I u .z/ ln
 t
t o
ÃÃ
;
(3.1)
where the one-hour mean wind U.z/ is given by a modified logarithmic profile that depends on the one-hour