Part A | 3.4
58 Part A Fundamentals
cerns about the accuracy and consistency of SODAR
measurements, especially in storm conditions or near
obstructions.
Regardless of the variable being measured, interference from nearby man-made or natural features must
always be considered. Cooper et al. [3.75] discuss some
of the challenges of taking metocean measurements
off of oil-industry facilities like jackets. Similar issues crop up with wind measurements from stations on
the coast, and these have been well studied and codified into numerous recommended practices, e.g., ASCE
(American Society of Civil Engineers) 7-05 [3.76] and
EUROCODE [3.77].
Another issue that frequently arises is the averaging interval. As explained in Sect. 3.2, it is especially
important for wind velocity because there is considerable wind energy at higher frequencies. Another
important factor affecting winds is the elevation of
the measurement above the sea or land, which is also
explained further in Sect. 3.2. In short, specification
of wind velocity should always include a minimum
of four variables: speed, direction, averaging interval,
and elevation. Similar issues arise with ocean currents,
although they tend to be less pronounced because of
the inherent difference in the turbulence spectra of
winds and currents. In the case of waves, the issue of
elevation is irrelevant. However, the temporal scale of
the sampling is important when calculating statistical
values like significant wave height. This issue will be
discussed in more detail in Sect. 3.7.8. A good general
rule is that the minimum sampling period for wave
spectra should never be less than 20 min.
As indicated above, a great deal of in situ measurements, including wind velocity, are reported in real time
and archived at the NDBC. Another good source for
measurements from limited duration deployment is the
National Oceanographic Data Center (NODC).
3.3.4 Mobile Measurements
Instrumented wind measurements have been taken
from vessels for decades. In the 1980s, oceanographers began mounting ADCPs on ships [3.78]. In
both cases, correcting for ship motion presents challenges but GPS (global positioning system) has largely
resolved these. Vessel-based wind measurements remain susceptible to flow interference from the ship
superstructure.
Starting in the 1980s there was a rapid increase
in the use of semi-automated or fully automated mobile platforms, starting with Lagrangian drifting buoys
whose paths are tracked by satellite, e.g., [3.79]. With
the advent of GPS, the drifter position could be precisely tracked and accurate velocities estimated. Coholan et al. [3.80] describe the use of drifters to measure
the strong currents associated with the Loop Current in
the Gulf of Mexico.
Autonomous underwater vehicles (AUV) have not
been used much for current measurements because of
their cost and limited range. However, as mentioned in
Sect. 3.2.3, Dhanak and Holappa [3.35] made good use
of an AUV to measure turbulence.
In the late 1990s gliders became increasingly
common thanks to their light weight (50100 kg),
small size (2 m), relatively low cost ($100 k), and
lengthy deployment capability (several months). Rudnick et al. [3.81] describe the technology in some detail.
Though gliders can only progress horizontally at about
1 knot, their long endurance and the ability to remotely pilot them make gliders highly cost effective
and adaptable. The present crop of sophisticated gliders can reach 1000 m depth, though this will likely be
extended in the near future. Gliders have limited payload and power capacities. They are typically equipped
with CTD (conductivity-temperature-depth) sensors,
although other sensors have been deployed, including
fluorometers, dissolved oxygen, and pH. A time-mean,
depth-averaged water velocity can be derived from the
surfacing coordinates of a glider. Efforts are underway to incorporate ADCPs into a glider, though power
consumption and obtaining an absolute velocity measurement in deeper water remain challenges.
The Global Drifter Program [3.82] began deploying
large numbers of drifting buoys in 1999 as a fundamental component of the global ocean observing system
(GOOS) and as of August 2011, nearly 11 000 buoys
had been deployed worldwide and are available from
the GDP website. Unfortunately, there is not yet an
equivalent to NDBC for obtaining real-time or archived
measurements from other mobile instruments.
3.4 Modeling
Since the advent of relatively cheap computing power in
the past 30 years, numerical modeling has started to replace measurements as the primary feedstock for metocean criteria. There are many reasons for this change.
Models are typically much less expensive than measurements and can provide results at a specific site and
for durations of many years. In contrast, one rarely
has the luxury of having more than a year or two of
58 Part A Fundamentals
cerns about the accuracy and consistency of SODAR
measurements, especially in storm conditions or near
obstructions.
Regardless of the variable being measured, interference from nearby man-made or natural features must
always be considered. Cooper et al. [3.75] discuss some
of the challenges of taking metocean measurements
off of oil-industry facilities like jackets. Similar issues crop up with wind measurements from stations on
the coast, and these have been well studied and codified into numerous recommended practices, e.g., ASCE
(American Society of Civil Engineers) 7-05 [3.76] and
EUROCODE [3.77].
Another issue that frequently arises is the averaging interval. As explained in Sect. 3.2, it is especially
important for wind velocity because there is considerable wind energy at higher frequencies. Another
important factor affecting winds is the elevation of
the measurement above the sea or land, which is also
explained further in Sect. 3.2. In short, specification
of wind velocity should always include a minimum
of four variables: speed, direction, averaging interval,
and elevation. Similar issues arise with ocean currents,
although they tend to be less pronounced because of
the inherent difference in the turbulence spectra of
winds and currents. In the case of waves, the issue of
elevation is irrelevant. However, the temporal scale of
the sampling is important when calculating statistical
values like significant wave height. This issue will be
discussed in more detail in Sect. 3.7.8. A good general
rule is that the minimum sampling period for wave
spectra should never be less than 20 min.
As indicated above, a great deal of in situ measurements, including wind velocity, are reported in real time
and archived at the NDBC. Another good source for
measurements from limited duration deployment is the
National Oceanographic Data Center (NODC).
3.3.4 Mobile Measurements
Instrumented wind measurements have been taken
from vessels for decades. In the 1980s, oceanographers began mounting ADCPs on ships [3.78]. In
both cases, correcting for ship motion presents challenges but GPS (global positioning system) has largely
resolved these. Vessel-based wind measurements remain susceptible to flow interference from the ship
superstructure.
Starting in the 1980s there was a rapid increase
in the use of semi-automated or fully automated mobile platforms, starting with Lagrangian drifting buoys
whose paths are tracked by satellite, e.g., [3.79]. With
the advent of GPS, the drifter position could be precisely tracked and accurate velocities estimated. Coholan et al. [3.80] describe the use of drifters to measure
the strong currents associated with the Loop Current in
the Gulf of Mexico.
Autonomous underwater vehicles (AUV) have not
been used much for current measurements because of
their cost and limited range. However, as mentioned in
Sect. 3.2.3, Dhanak and Holappa [3.35] made good use
of an AUV to measure turbulence.
In the late 1990s gliders became increasingly
common thanks to their light weight (50100 kg),
small size (2 m), relatively low cost ($100 k), and
lengthy deployment capability (several months). Rudnick et al. [3.81] describe the technology in some detail.
Though gliders can only progress horizontally at about
1 knot, their long endurance and the ability to remotely pilot them make gliders highly cost effective
and adaptable. The present crop of sophisticated gliders can reach 1000 m depth, though this will likely be
extended in the near future. Gliders have limited payload and power capacities. They are typically equipped
with CTD (conductivity-temperature-depth) sensors,
although other sensors have been deployed, including
fluorometers, dissolved oxygen, and pH. A time-mean,
depth-averaged water velocity can be derived from the
surfacing coordinates of a glider. Efforts are underway to incorporate ADCPs into a glider, though power
consumption and obtaining an absolute velocity measurement in deeper water remain challenges.
The Global Drifter Program [3.82] began deploying
large numbers of drifting buoys in 1999 as a fundamental component of the global ocean observing system
(GOOS) and as of August 2011, nearly 11 000 buoys
had been deployed worldwide and are available from
the GDP website. Unfortunately, there is not yet an
equivalent to NDBC for obtaining real-time or archived
measurements from other mobile instruments.
3.4 Modeling
Since the advent of relatively cheap computing power in
the past 30 years, numerical modeling has started to replace measurements as the primary feedstock for metocean criteria. There are many reasons for this change.
Models are typically much less expensive than measurements and can provide results at a specific site and
for durations of many years. In contrast, one rarely
has the luxury of having more than a year or two of