Page 320 - Pipelines and Risers

Risk Analysis applied to Subsea Pipeline Engineering 293

The duration’s given above do not take into account emergency response actions initiated
following the detection of a loss of containment. Hazard durations have therefore been
assumed based on the time that it is expected to take for the existence of a release to be
detected. These duration’s have been assumed to be 168, 48 and 6 hours respectively. It
should be noted that these times represent hazard duration’s rather than leak duration’s, i.e.
they are estimates of the time required for the detection and location of a leak and for the
imposition of measures to exclude shipping traffic from the affected locality. It should also be
noted that the risk analysis results are not sensitive to the value assumed for the hazard
duration for 20mm holes, since these do not result in flammable releases.

Subsea Plume
The effect of a subsea gas release may be modeled as an inverted conical plume with a half
cone angle of between 11 and 14 degrees in a zero current velocity situation. Assuming the
most conservative case, this results in a 150m diameter release zone at the sea surface for the
assumed 300m water depth.

Airborne Dispersion
Airborne dispersion will be modeled using the program HEGADAS-S, part of the HGSystem
suite. This program assumes that the gas evolves as a momentumless release from a
rectangular pool. The pool has been taken to be 15Om by 150m, so as to reflect the release
into the atmosphere of the subsea plume.

Effect of water depth
Releases from greater depths will result in somewhat reduced mass flow rates. This is due to
the increased seawater pressure at the site of loss of containment. Subsea dispersion over a
greater depth will result in a larger gas evolution zone at the surface. These effects mean that
the surface concentrations, and hence the dispersion distances and hazard zone dimensions
will reduce with increasing release depth. The assumption of a 300m release depth for all loss
of containment incidents is therefore conservative.

Stability
Pasquill stability classes define meteorological conditions from very unstable, A, to
moderately stable conditions, F. These parameters are used in the Modeling of airborne
dispersion.

Two values of the Pasquill Stability Class have been used, these are Class D (Neutral
Stability) and Class F (Moderately Stable Conditions). Class D is appropriate for night time
and overcast day time, and has therefore been assumed to be representative of 75% of the
time, with Class F being representative of the remaining 25%.

Wind Speeds
Since there are no fixed installations at hazard as a result of subsea releases from the pipeline,
wind direction is not required as an input to the risk assessments. Wind speeds are however
required, since they determine the extent of the flammable gas clouds that may be generated