UNDERWATER INSPECTION 
The aim of any underwater inspection programme is basically the same as 
that for any structure above the waterline. Firstly, it must be capable 
of detecting areas of possible damage and fatigue and, secondly, it must 
provide sufficiently detailed and reliable information for an assessment 
to be made. Any repair must then be carried out to a satisfactory standard. 
A set of standards for underwater structural inspection has been agreed 
between the designers, constructors and operators of offshore 
installations. These take the form of guidance notes and specify: 
(i) visual examination of the whole structure above and below the 
waterline 
(ii) detailed visual inspection of selected welds and previous 
repairs 
(iii) additional close inspection as the surveyor may judge necessary 
(iv) provision of further information to verify suspected damage 
and defects 
(v) sea bed level inspection, particularly of scour 
(vi) survey and inspection of cathodic protection potential 
These notes relate to a survey performed immediately upon completion of 
the installation and again at intervals not exceeding five years. Since 
1975, no platform or installation can be operated in the North Sea unless 
it has a valid certificate of fitness granted by an appointed certifying 
authority. 
Any underwater inspection programme requires suitable vehicles or diving 
systems to perform the specified tasks. Whilst there are advantages in 
having a diver perform some of the tasks, the range of tasks that can be 
performed by the diver is limited owing to the quantity of equipment 
required, decompression restrictions and the diver's umbilical. The use 
of manned submersibles (Figure 1) and remote controlled vehicles for 
underwater inspection is, therefore, a logical step forward. Such vehicles 
are particularly suited for the survey of large structures and large areas 
of the sea bed and they can perform a larger range of tasks. 
One of the first operational, commercial, survey submersibleswas a Vickers 
Pisces submersible, equipped with side scan sonar, bathymetric profiler, 
doppler sonar, 35mm cameras and a long baseline acoustic positioning 
system (Westwood, 1979). The concept of a survey submersible has remained 
largely unchanged and development has concentrated on increased 
sophistication of position fixing and sensory equipment. These include 
developments of inertial navigation systems, short base line acoustic 
positioning systems and additional sensors in the form of echo sounders, 
trench profilers, cathodic protection sensors, buried pipeline sensors and 
video and still camera systems. Today the manned submersible is a fully 
operational survey vehicle capable of being positioned to an accuracy 
of 3m. 
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