Page 171 - Offshore Electrical Engineering Manual

158 CHAPTER 14 Offshore Lighting

luminaires. A simple point-by-point lighting programme is shown in Appendix 3.
Point-by-point calculations are required particularly for floodlighting schemes in
order to examine the uniformity of illuminance. Eq. (2.14.8) may be applied for the
illuminance on a surface due to a source of intensity I cd at distance r metres away
when the light falls at an angle 0 to the normal to the surface, as follows:
Icos θ
E =
r 2 (2.14.8)
A programme is available for such calculations from Chalmit Ltd, called
ChalmLite.

FLOODLIGHTING

General information on floodlighting can be obtained from the CIBSE Technical
Report TR13, Industrial Area Floodlighting.
In floodlighting, the effective source intensity is the photometric value multiplied
by the maintenance factor and by the atmospheric light loss factor. The most useful
form of luminaire intensity data is one using the vertical/horizontal (V–H) coordinate
system, such as the isocandela contours shown in Fig. 2.14.6. This accompanies a
zonal flux diagram deliberately, for ease of point-by-point calculation after a lumen
calculation.
Eq. (2.14.1) can be rewritten as:
(I cos 3θ)
E H =
H 2
where E is the horizontal illuminance due to a source mounted H metres above the
H
horizontal.
When an external area such as a drillpipe laydown area is to be lit, the contribu-
tion of each floodlight installed can be summated at the points of minimum intensity
(usually the corners) and the minimum value of illuminance found. The change in
illuminance over the area to be lit is a measure of uniformity and is quoted in the
CIBSE Memorandum 5 in terms of the following:

Criteria Value for Working Areas
1. Maximum to minimum 20:1
2. Average to minimum 10:1
3. Minimum distance over which a 20% change in 2 m
illuminance occurs