MEASUREMENT OF MARINE STRUCTURES 
  
Fic. l. A completed steel jacket awaiting the 
flooding of the graving dock prior to tow-out into 
the sea. 
Longdin and Browning (Surveys) Ltd in con- 
junction with the Department of Surveying, 
University of Newcastle upon Tyne. 
In this study photogrammetry has been 
applied on three different types of unit: 
(1) Nodes. These consist of a main tube, 
which may be up to 15m in length, from 
which a series of circular stubs project at vary- 
ing angles (Figure 2). After fabrication, nodes 
must be checked to ensure that they lie 
within the specified dimensional tolerances. 
The information required to check this is the 
radii and roundness of the main tube and all 
circular stubs, the direction cosines and 
length of each stub. This information is de- 
rived from the calculation of the centre of 
each stub in the plane of its end. Such units 
are difficult to measure by conventional sur- 
veying techniques but are ideally suited to 
photogrammetric measurement. In order to 
cover the complete node it is sometimes 
necessary to take a series of stereopairs from 
various viewpoints. 
(2) Sub-assemblies. These are larger in 
size than the nodes and are formed from sev- 
eral small units (Figure 2). Each sub- 
assembly must be checked after fit-up for 
completeness, and to ensure that it lies 
within given tolerances for overall size and to 
allow line-up of pile and conductor guides. 
As well as overall dimensions, therefore, the 
coordinates of the centres of all pile and con- 
ductor guides are needed. Once these coor- 
dinates are obtained, the sub-assembly may 
be translated and rotated in a computer into 
its eventual position on the main structure so 
as tocheck that it will match up prior to lifting. 
In this way it should be possible to eliminate 
the need for adjustment once erection com- 
mences. 
(3) Deck stabbing guides. Once the jacket 
1503 
  
Fic.2. Twotypes of steel jacket unit which have 
been measured by photogrammetry. In the centre 
is conductor panel-136E complete with strings to 
define the centre of each conductor guide. Above 
is a typical node. 
is complete, it is necessary to determine the 
relationship between the centres ofthe tubes 
which form the deck stabbing guides. These 
may be seen protruding from the top end of 
the jacket (i.e., to the left-hand side) in Figure 
1. These measurements are needed so that 
the deck module will fit onto the stabbing 
guides when jacket and deck structure are 
united at sea. If specified tolerances are ex- 
ceeded it is necessary to modify the deck 
structure before floating it out to sea. Photo- 
grammetry is again ideally suited here be- 
cause the stabbing guides are relatively inac- 
cessible. 
The application of photogrammetric meas- 
urement to these three types of unit marks 
only the beginning of its usage in this indus- 
try. Undoubtedly with time the range will be 
extended. 
ExAMPLE—A CONDUCTOR PANEL 
A typical sub-assembly is the conductor 
panel shown in Figure 2. This panel was 
measured both by photogrammetry and by 
conventional surveying techniques and will 
serve as an example to show the accuracies 
which may be achieved by using photo- 
grammetry. 
The panel, which measures about 13m X 
6m, is composed of 15 conductor guides con- 
nected to a main tube whose axis is at right 
angles to the axes of the conductor guides. 
Preparation of the panel for measurement by 
conventional means had taken place by fixing 
strings across two diameters of each guide in 
order to define its centre. 
Prior to photography, the ends of the main 
tube and conductor guides were accentuated 
with white chalk to allow measurement of a 
series of points around the outside perimeter 
of each guide during the analysis phase. For