|
|
"How well are fundamental physical laws incorporated in the
design of oil spill recovery equipment, and in specifications related to
RFQs (Requests For Quotations)?"
Sometimes, there is a lot of wishful thinking incorporated when
manufacturers write their technical specifications on their oil spill combat
equipment:
- Containment booms can concentrate and hold oil at speeds far above the
limits created by fundamental physical laws.
- Skimmers can recover and transfer high viscosity oils at high rates,
it is claimed, but the documentation may be based on the pump’s
performance on water, the only thing you wouldn’t want to recover and
transfer. And the rated capacity for the pump is against zero discharge
pressure, an information which can best be described as useless.
However, some purchasers of oil spill combat equipment seem to have
adapted the manufacturers "translation" of the word
"specifications":
- You may observe Requests for Quotations where containment booms are
specified with a minimum sweeping speed of 1.5 knots (even in seastate
4…). Without stating type of configuration (U, V, J+V, etc.). Without
stating how it is expected to get rid of the contained oil and how fast.
We are talking about budget prices of up to several hundred thousands of
US dollars.
- Whilst a simple 1000 USD centrifugal marine pump for a ship will be
specified in all possible details, including performance curves, annual
energy consumption, etc., you may see specifications for a 100.000 USD
skimmer system listed something like "recovery rate must be 45
m3/hour (270 bbl/h)". Sometimes a minimum viscosity capability will
be required, but this will not necessarily be related to the overall
capacity of the system….
The facts are:
- The maximum sweeping speed (perpendicular to the boom) with a
conventional inflateable or foam filled containment boom is 0.7 knots,
if you don’t want to loose oil, escaping under the boom. Maximum 0.7
knots, no matter how much draught the boom has. There are, however,
minimal variations, for instance depending on the viscosity and specific gravity of
the oil, but based on fundamental physical laws, the max. sweeping speed
can be computed to app. 0.7 knots.*) Understood: Calm water. Anything
apart from calm water will reduce the maximum speed.
- The capacity or recovery rate of a skimmer (system) is a complex size.
Besides the design, it depends on the available amount of oil for the
skimmer to work on: Oil layer thickness, and how much oil is constantly
entering the recovery area. The viscosity of the oil, and how far away
and up you want to transfer the recovered oil through the discharge
hose, and especially the size (diameter) of the hose, are important
factors. But the available power (size of power pack, kW or HP, less the
power losses in the hydraulic system, fittings, valves, and hydraulic
hoses) also matter a lot.
- Example: The above mentioned 45 m3/h capacity skimmer, at an oil
viscosity of 20,000 cSt, a discharge hose of 4" (100 mm) diameter
and 40 m (120 ft) long, will require a pump discharge pressure of about 40 bar
(600 psi) in order to deliver the 45 m3 per hour through the hose, EVEN
without any vertical lift! 40 bar is probably far beyond the bursting pressure
for both hose and skimmer pump….
But if you instead want to send the same quantity of 20,000 cSt oil
through 40 m of 6" (150mm) hose, the necessary pressure drops down
to 8 bar (120 psi). Suddenly things work, by just using the right hose
diameter!
Maybe you can imagine the limitations of a skimmer system with a claimed
capacity of 2-300 m3/hour, which, however, is equipped with only a
5" (125 mm) discharge hose, 60 or 80 meters long?
Canadian and US authorities are trying to create equipment evaluation
standards. And they do a good job. But till now we haven’t seen specific
demands or requirements for skimmer or pump performance curves at various
oil viscosities. In the US, a derating of skimmer pumps have been
implemented, but the factor is the same for a centrifugal pump, which cannot
pump high viscosity oil, as it is for a positive displacement pump, which
can.
Likewise, we haven’t seen demands for containment boom performance
curves at various speeds and seastates.
It really is expensive to make the necessary testing and to have them
certified by an agency, who is familiar with oil spill combatting
technology. But on the other hand: The - in general - low recovery
percentage of the total volume of an oil spill, may partly be due to
insufficient requirements and knowledge to equipment capabilities: The booms
may be operated at too high sweeping speed. They may not be suitable for
tough conditions. The skimmer can maybe recover the oil, but the pump cannot
transfer it. The pump can maybe transfer anything, but the skimmer cannot
recover anything but water. Debris interrupts the operation frequently. Etc.
Therefore, it is from this forum recommended that the international oil
spill industry positively involves itself in the preparation of joint and
common performance standards for the key recovery equipment: Booms,
skimmers, and pumps.
*) Certain designs, such as the NOFI Vee Sweep and Oil Stop, Inc.’s
Fast Sweep System, with a fine meshed net, forming a bag between the lower
parts of the skirts at the apex of the V shaped boom (thus creating a
strainer bag effect where the water escapes easier than the oil through the
net), may reach more than 1.5 knots before significant amounts of oil are lost
under the boom.
The patented LAMOR/LORI circulation system, where the collected oil via a side
sweep on a vessel is circulated away from the (critical) apex of the boom to
an internal recovery area inside the vessel’s hull, and (if not recovered
by the bristles) back to the collection area via side doors in the hull, may
likewise reach more than 1.5 knots without significant losses.
By Flemming Hvidbak, flemingCo environmental aps, March
1997
|
 |
