Page 321 - Pipelines and Risers

294 Chapter I6

by a release. The wind speeds and relative frequencies used to determine the hazard ranges
associated with various releases are summarized in Table 16.3.

Table 16.3 Relative Frequency of Representative Wind Speeds.
Wind Speed Representative Wind Relative
Range (ds) Speed (ds) Frequency
0 to 5 2 0.26
5 to 11 I 8 0.49
11 to17 I 14 0.2 1

Hazard Ranges
Hazard ranges are calculated in terms of the extent of the lower flammability limit (LFL) for
different release rates, wind speeds and water depths. A concentration of 5% by volume has
been used to represent the LFL.

A total of eighteen gas dispersion analyses have been undertaken. These results are combined,
using the data for relative frequency of Pasquill Class and wind speed, to provide an estimate
of the hazard area associated with each of the three hole sizes. These are shown in Table 16.4.

Table 16.4 Average Hazard Areas for Different Hole Sizes.
Hole Size Hazard Area (mz)

200 mm 18650

16.8.3 Individual Risk
Acceptance Criteria
The risks to which workers will be exposed are compared with the maximum operational
FAR of 10 fatalities per 10' hours worked.

Cause Analysis
Statistics of dropped object frequencies have been obtained from the 1992 Offshore
Reliability Data Book, OREDA-92. This data source records a total of 7 dropped objects
against a total calendar time of 648,200 hours or an operational time of 22,800 hours.
Assuming an average lift duration of 5 minutes this is equivalent to 0.42 lifts per hour with a
probability of a dropped object of 2.56 x per lift.

Two lifting operations have been assumed at each work location, corresponding to one lift for
installation of structures and one lift for pigging operations.