Environmental Engineering Reference
In-Depth Information
marine mammal activity in parallel with sound level monitoring during construction
and operation. Baseline sound surveys would be needed against which to measure
the added effects of energy generation. It will be important to measure the acoustic
characteristics produced by both single units and multiple units in an array, due to
the possibility of synchronous or asynchronous, additive noise produced by the array
(Boehlert et al., 2008). Minimally, the operational monitoring would quantify the
sound pressure levels across the entire range of sound frequencies for a variety of
ocean and river conditions in order to assess how meteorological, current strength,
and wave height conditions affect sound generation and sound masking. The moni-
toring effort should consider the effects of marine fouling on noise production, par-
ticularly as it relates to mooring cables.
i
MpaCTs
oF
e
leCTroMagneTiC
F
ields
Underwater cables will be used to transmit electricity between turbines in an array
(inter-turbine cables), between the array and a submerged step-up transformer (if
part of the design), and from the transformer or array to the shore (CMACS, 2003).
Ohman et al. (2007) categorized submarine electric cables into the following types:
telecommunications cables; high-voltage, direct-current (HVDC) cables; alternat-
ing-current, three-phase power cables; and low-voltage cables. All types of cable will
emit electromagnetic fields (EMFs) in the surrounding water. The electric current
traveling through the cables will induce magnetic fields in the immediate vicinity,
which can in turn induce a secondary electrical field when animals move through the
magnetic fields (CMACS, 2003).
Nature of the Underwater Electromagnetic Field
In 1819, Hans Christian Oersted, a Danish scientist, discovered that a field of mag-
netic force exists around a single wire conductor carrying an electric current. The
electromagnetic field created by electric current passing through a cable is composed
of both an electric field (E field) and an induced magnetic field (B field). Although E
fields can be contained within undamaged insulation surrounding the cable, B fields
are unavoidable and will in turn induce a secondary electric field (iE field). Thus, it
is important to distinguish between the two constituents of the EMF (E and B) and
the induced field (iE) (
Fig u r e 7.10
)
. Because the electric field is a measure of how the
voltage changes when a measurement point is moved in a given direction, E and iE
are expressed as volts/meter (V/m).
The intensity of a magnetic field can be expressed as magnetic field strength or
magnetic flux density (CMACS, 2003). The magnetic field can be visualized as field
lines, and the field strength (measured in amperes/meter [A/m]) corresponds to the
density of the field lines. Magnetic flux density is a measure of the density of mag-
netic lines or force, or magnetic flux lines, passing through an area. Magnetic flux
density (measured in teslas [T]) diminishes with increasing distance from a straight
current-carrying wire. At a given location in the vicinity of a current-carrying wire,
the magnetic flux density is directly proportional to the current in amperes. Thus,
the magnetic field B is directly linked to the magnetic flux density that is flowing in
a given direction.
