Page 312 - Offshore Electrical Engineering Manual

Structural Considerations 299

using bolted brackets rather than welding, as it may be necessary depending on the
sea state, etc. to temporarily remove the equipment and reinstall it offshore.

SHOCK AND VIBRATION
With platform structures installed in sea depths of 150 m or more, rotating machin-
ery rated at 5 MW or more may be operating near the top of a structure over 200 m
high. The structure may resonate with vibrations produced by the machinery and,
particularly if the machinery skid floor is cantilevered out beyond the main structure,
a catastrophic failure may occur. To ensure that this is not the case, vibration analy-
ses need to be carried out to establish structural resonance frequencies. If these are
close to those of the machinery, modifications will need to be carried out to change
the offending frequency to an acceptable value. At such heights, the swaying of the
platform and the shock from drilling activities, etc. may produce damaging torques
on the bearings and shafts of large motors and generators if these are not catered for
in the machine design.

LOCATION OF ENGINE INTAKES AND EXHAUSTS
An offshore production platform is a compact three-dimensional arrangement of
modules. Some modules contain prime movers and others, such as separator and gas
compression areas, require to exhaust ventilation air from hazardous areas; all must
take in fresh, uncontaminated air from the surrounding atmosphere. The prevention
of cross-contamination between exhausts and intakes in all wind and weather condi-
tions can be extremely difficult, and in some weather conditions, it may be necessary
to accept a small drop-off in gas turbine performance due to the thermal contamina-
tion of other exhausts. Water curtains around turbine exhaust ducts are sometimes
used to good effect in reducing contamination by taking exhaust gas downwards,
away from intakes. Prime movers for emergency equipment such as emergency gen-
erators and fire pump alternators are not so critical because testing can be carried out
in a favourable wind which blows the exhaust away from ventilation intakes.
With emergency equipment, it is more important that aspects of emergency sce-
narios such as gas cloud boundaries and likely flame passages are given consideration.

MECHANICAL PROTECTION
On any platform, routine production maintenance and drilling operations demand
the movement of equipment, containers, scaffolding poles, drillpipe, etc. from the
laydown area, at which it was received from the supply vessel, to its point of use,
and then possibly the reverse journey back to the laydown areas before transferring
back to the supply vessel. During these movements, there is a risk that the item will
be dropped on to, or swung into, some exposed piece of electrical equipment. Worse
still, an item may be dropped by a crane so that it pierces the roof or wall of an
operating module. To avoid such occurrences the following should be considered.