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compare them to the undeformed situation. The
points selected were the following:
- points near the joints, i.e. the intersection
points of the tubular roads. These are
important points both from a geometrical and
structural point ‘of view for the whole
structure;
- points near the quarters of the lenght of the
bracings; in this case it was kept in
consideration that beams that have fixed
boundaries undergoing deformations have the
maximum relative movements in these positions.
Luna B jacket was loaded at the Intermare Sarda
yard at Arbatax (Italy). In this case it was slid
on teflon coated guide lines. The pushing
mechanism was formed by a pair of oleodynamic
pull/push units applied on each guide.
The jacket then moved without continuity, with a
piston expulsion phase and, at the end of the
stroke, with a recovery phase of the cylinders and
control panel applied to them. The frequency was
about 90 seconds.
Given the type of operation it was not possible to
use a traditional topographic method to control
the structure because the time needed to take the
reading at the points between one phase and
another was insufficient. It was needed to use
stereophotogrammetry which, with particular
adjustments, allows shooting photograms in rapid
succession of a moving subject.
3. STEREOPHOTOGRAMMETRY
Stereophotogrammetry is a special application of
photogrammetry and it is used generally in a
close-range context.
It consists of photogrammetric takes of an object
using two metric cameras simultaneously. In this
way it is possible to measure moving objects as
long as the movements which occur during the
exposition time are negligible.
In the sector of offshore industrial applications
this technique is often indespensable. Below a
list of examples of the most frequency cases:
- measuring structures at open sea where there is
no fixed support on which to place the
equipment;
- measuring large metallic structures in the
shipyard where quick thermal variations can
greatly alter the distances to be measured;
- measuring geometrically complex structures in
sunny environments where the different positions
of the shade may compromise the operator’s
stereoscopic vision.
For these reasons Agip has developed the
stereophotogrammetric technique so that it may be
used onshore, offshore and also from helicopters
(G.Bozzolato, 1988; M.Rampolli, 1990).
In fact, in 1988 the S.E.R. (Supporto Eliportato
Ruotabile) was produced with financial aid from
the EEC. The system, derived from idea of G.
Bozzolato, is patented by Agip.
S.E.R. allows the application of two large sized
metric cameras on a 6.5 m beam fixed to a
helicopter. The results obtained were positive in
the industrial and geological/architectonic
fields. In February 1992 The Valley of Temples
(Agrigento) was surveyed using this method, with
good results.
4. MEASURING OPERATIONS
4.1 Instruments used
The following equipment was used:
- 3 AUS JENA 10/1318 cameras.
During the reference survey a single camera was
used in the four station positions (figure 1).
During loading 3 cameras were used: two of these
were connected to each other and synchronised
and the third one was isolated but with manual
synchronisation.
- WILD T2000 electronic theodolite with a WILD GRE
3 data recorder. This instrument was used to
measure the data points.
During loading the theodolite was equipped with
a WILD DI4 diastimeter which allowed the
immediate reading of the coordinates of some
reflector prisms installed on the structure.
4.2 Preparation
Every photogrammetric survey is divided into 3
main steps:
- marking of the points to be collimated during
plotting to have greater precision;
- triangulation of the data points for every pair
of photograms to allow subsequent orientation;
- photographic shooting.
The following operations were carried out in the
survey discussed in this report.
During a preliminary stage the area was inspected
and the survey's various stages were planned so
that the shipyard could cooperate in terms of
means and personnel.
The shooting bases were chosen as a function of
some determining factors such as the sunlight and
shipyard obstacles. The accurate definition and
materialisation of the station points were carried
out on the basis of the area covered by the
photograms and on the precision required (figure
1). There were 7 station points divided into 4
shooting bases. In particular, since the
structure must be measured in the initial
undeformed situation, bases 1 and 2 were chosen.
For shooting during loading, bases 3 and 4 were
chosen, near the quay.
The points on the structure to be measured when
plotting were identified using 200x200 mm self
adhesive targets with a Malta cross.
The data points, deermined topographically, and to
be used for orientation operations, were
distributed in order to have a good coverage of
the photographed area. They were placed on
suitable locations on the ground and on the
structure for measuring the underformed structure.
Analogously, 13 data points were identified on
land and on the runway for the dynamic phase
survey.
It was difficult however to put data points in the
high part in order to have a good orientation of
the ohotograms. In "fact, the large moving
structure excluded any suitable position.
It was therefore decided to place 3 data points
directly on the structure at a height of about 20
meters and to apply reflecting prisms at these
points.
In this way it would be possible to determine the
coordinates directely using a wave diastimeter
exploiting the time when the structure is
stationary during loading operations.
