Full resolution (JPEG) - On this page / på denna sida - The Öresund Power Cable of 1958, by Anders Bergman, Wilhelm Carlshem and Gottschalk von Geijer
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errors due to the iron in the drum. A rough
estimation of the angles between the wire connecting
"Bien" and the drum, and the route has been made.
Most of the slack was laid during the first part of
the crossing. This is also confirmed by the track
plotter chart shown in fig. 6. This shows that the
greatest deviations from the straight course occured
during the first part of the crossing.
According to Decca measurements some 90 metres
of slack had been paid out when reaching a point
600 metres from the Danish shore. From this point
to a point 180 metres from the shore the drum was
towed by small boats. The slack laid during this
part is believed to be negligible. During the last 100
metres the cable floated on barrels and was pulled
ashore as before. Here the slack is possibly some
10 metres. According to Decca measurements,
together with the assumptions made regarding the
slack at the Swedish and Danish shores, the total
amount of slack laid is 100 metres. The true value
is around 90 metres, thus making an error of 10
metres. This is a somewhat better value than was
expected. The great number of readings made has
tended to reduce the random errors. In fig. 7 the
cable-slack as a function of cable paid out is shown.
On a test run made a week before the laying, the
line drawn on the track plotter chart was found to
agree fairly closely with the route at points near
the Swedish and Danish shores. Unfortunately no
attempt was made to determine the errors at
different points along the route. On the track plotter
chart the line connecting the terminal points is
slightly curved, but the computed curvature is much
less than the actual curvature drawn by the track
plotter pen. The possibility of a varying
fixed-error along the route cannot be excluded. More-
Fuj. 7. (’able slack as a function of cable paid out.
over, the position given by Decca may conceiveably
have been incorrect from the start. The error seems
to have decreased during the crossing, and when
arriving at the Danish shore it cannot have been
more than some ten metres. Fortunately the linearity
along the route has not been seriously affected,
otherwise a noticeable distorsion of the cable-slack
diagram would have been the result. A possible
explanation is that the Decca pattern was altered
between the two occasions (the test run and the
cable-laying) due to instability of the transmitting
equipment. This could have been checked by
locating a receiver at a fixed point and transmitting
corrections to the Decca operator on board the ship.
After-inspection
Diving equipment
In order to enable the position of the cable on the
bottom to be checked after laying an investigation
was carried out with the help of underwater
television and frogmen. Investigations of this kind have
been undertaken previously by divers of the
conventional type. The heavily equipped diver stirs up
a quantity of mud when walking on the sea bottom
however, and thus impairs visibility. A frogman, on
the other hand, swims above the cable, and the
water consequently remains clear. By directing a
television camera on to the cable, its position on the
bottom can be accurately observed from a following
boat. A group of workers attached to the State Power
Board are studying the problems associated with
diving investigations by frogmen in combination
with television transmission to a following boat, and
they have developed a technique which is also
adopted in the after-inspection of the cable. The
TV-equipment consists of a camera unit and a
monitor with a 10-inch picture tube connected to a
plas-tic-insulated multicore cable. The installation
operates with separate picture- and deflection signals,
resulting in an improved resolving power in
comparison with a combined signal. The line system is
625 with interlaced scanning, which is the same as
in commercial television. The resolving power lies
at about 500 lines. The camera tube is a resistron
which possesses a high photo-sensitivity. Acceptable
pictures are obtained with light intensities of less
than 10 lux. It has been found possible to obtain
entirely satisfactory pictures down to water-depths
of 35 metres. The resistron is affected by a certain
inertia, however, particularly at low light
intensities owing to the fact that the resistivity of the
tube’s light-sensitive layer does not change
instantaneously with the variations in lighting. This results
in a certain blurring of the picture when a moving
object is being observed and when the camera is
not held stationary, which may present a problem
in fast-flowing water. The lens employed has a
focal length of 10 mm and an aperture of f: 1.8. The
short focal length gives such great sharpness in
depth that focussing is rendered unnecessary. In
order to correct refraction between the water and
air a supplementary lens is used. By these means
a normal picture angle is obtained under water. The
camera casing consists of a cylinder with ends of
plexi-glass. The correcting lens is mounted in the
front end which is threaded into the cylinder. The
] 26 ELTEKN I K 1959
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