Page 260 - Petroleum and Gas Field Processing

Therefore, the treatment could be achieved using two 108-in.-
diameter vertical skimmers with SP packs.
The selection of any of the above two options will depend on cost and
other considerations such as the availability of gas for flotation units.

9.6.9 Sizing of Offshore Disposal Devices
Disposal Piles
As mentioned earlier, disposal piles are not designed for produced water
treatment; they are simply used to dispose of the treated produced water.
However, disposal piles are also used to dispose of the deck drains, normally
rainwater or washdown water. Such water is usually contaminated by oil
and, therefore, the disposal pile will be expected to provide the means to
settle oil droplets of certain size. In most cases, the disposal pile will be
designed to separate oil droplets 150 mm and larger. This, however, depends
on local regulations.
Equation (16) developed for vertical skimmers is used to determine
the diameter of the disposal pile; the equation is rewritten as follows:

2
D ¼ 6691 Q w w in: 2 ð33Þ
ð
w
o Þ
where Q w is given by
Q w ¼ Q pw þ Q rw þ Q wd BPD ð34Þ

Normally, the larger of the rainwater and washdown water flow rates is
used in Eq. (34), as it is unlikely to have both at the same time. The water
viscosity, w , is taken as 1 cP.
The length of the pile submerged below the normal sea level is
determined such that the high-level and shutdown alarms are sensed before
the oil reaches 10 ft from the bottom. This could be determined from a
hydrostatic pressure balance at the bottom of the pile. The normal tide
and expected storms should be considered.
Let L w be the normal water level, L st be the design storm range, L t
be the average tide range, and assigning 4 ft for the alarm and shutdown
levels, the length of the pile submerged, L, will be given by

ð4 þ L st þ L t þ L w Þ
o
L ¼ 10 þ ft ð35Þ

w
o
In deep waters, however, the minimum length should be 50 ft. In shallow
water, the length of the pile should extend as deep as possible.

Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved.