Page 135 - Offshore Electrical Engineering Manual

122 CHAPTER 12 Process Drives and Starting Requirements

weight to the drive package, and therefore direct-on-line, low-starting-current
machines are preferred, unless technical considerations for selection are overridden
by those of cost and delivery. Motors with starting currents ranging from 3.5 to 4.5,
instead of the normal 6–10 times full load, are usually available from the larger
manufacturers.

SPEED
Higher pump speeds at 60 Hz is marginal, and with motors having ratings in the order
of several megawatts, weight and power savings can be substantial.
With smaller installations, however, operating at 60 Hz may be a disadvantage
where it is decided to select a reciprocating engine rather than a gas turbine main
generator prime mover. The problem is that the optimum engine speed of around
1500 rpm is better suited to generation at 50 Hz. Reciprocating engines running at
1200 rpm tend to have too low power-to-weight ratio, and operating at 1800 rpm
leads to short cylinder life or even piston speeds which would be beyond the design
limitations of the engine.
The higher synchronous speeds obtainable at 60 Hz also lead to higher inherent
noise levels, although this can be deadened with better module insulation.
From a machinery standpoint, a major disadvantage in adopting 60 Hz for a
European offshore installation is related to testing the equipment before installa-
tion on the platform, as full-load tests cannot be carried out using the British and
European national supply networks. Until recently, tests were mainly carried out
at 50 Hz and the results extrapolated to give projected machine characteristics at
the design conditions. However, currently, test facilities are available in the United
Kingdom for motors of up to 6 MW at 60 Hz. For larger machines, where capital
investment is high and full-load tests are considered essential, it is usually possible
to arrange full-load tests in conjunction with testing of the main generators to be
installed on the offshore installation. Although this procedure is usually expen-
sive, the costs should be more than offset by the benefits of adopting the higher
frequency.
Once the system frequency has been established, it becomes increasingly expen-
sive to change and hence may no longer be considered a variable after that point
in the system design. It is therefore important to consider the number, rating and
purpose of the larger drives on the installation at an early stage in the power system
design, before the frequency is selected. However, in many cases the shaft speeds
available from a motor, even from a 60 Hz two-pole machine (i.e., 3600 rpm) is lower
than the required shaft speed, and it will still be necessary to install a gearbox in the
drive string. Once the requirement for a gearbox has been established, changes in
the drive ratio have only minor effects and the motor speed may be chosen to give
the optimum motor design in terms of dimensions, weight, reliability, noise emis-
sion, etc.