By Tom Coe, Senior Engineer, ManTech Advanced Systems International
I am in the final stages of completing a technology assessment project for the US Coast
Guard on the control of oil spills in one to six knot currents. Controlling oil on fast
moving water is like raising teenagers; you develop a plan, implement it that best you can
to try to keep them headed in the desired direction. You very often have to change
strategies as they seem to head in the wrong direction most of the time. It’s a real
challenge and a constant battle to do both successfully. I’m not an expert in either
but I have learned a few things about oil management in fast currents that I’d like
to share with you and hopefully you can share your knowledge with me via the internet.
The project goals are to quantify the oil spill threat in fast US waters, then evaluate
technologies and methods, and identify promising equipment and strategies for those threat
scenarios. Recommendations will be made to pursue methods, equipment, and training that
show the most promise to improve response capabilities for the U.S. Coast Guard and
industry in currents from one to six knots. Recommendations will include technology
development, testing in controlled conditions and field demonstrations.
Containment and removal of oil spilled in rivers and coastal tidal regions where
currents exceed one knot is very difficult because many skimmers and conventional booming
methods are not effective in fast currents. Boat handling, anchoring and boom deployment
are more dangerous in fast currents were forces are high and things happen quickly. Some
spill processes are accelerated in fast currents such as drift, oil dispersion,
emulsification and sedimentation (oil sinking). This necessitates, prior planning,
pre-staged equipment, fast transportation and deployment techniques, and effective
training to protect sensitive shorelines when spills occur. The oil will generally entrain
and follow the water path under the boom or skimmer when currents exceed one knot unless
it is tricked by the deflection, containment or recovery device. This can be accomplished
using specialized equipment and strategies; however, properly trained response personnel
are essential. The oil must be either skimmed as it goes by the recovery device or the
surface current with the oil must be slowed down without causing entrainment within the
skimmer or boom containment system.
Shallow draft, high speed current boom can be used to deflect the oil to shore at steep
angles where the currents are slowed by the rising shoreline and conventional skimmers can
be effectively used. For example a 4 knot current requires the deflection boom to be at a
10o angle to the current. Cascading the boom as shown in Figure 1, is a common
strategy; but, the key is to keep the boom straight without forming a J-shape which will
cause oil entrainment. In this case the 4" by 6" boom is kept in the proper
shape by using only 50-foot sections each attached to shore under high tension using two
upstream lines (one near shore and one far shore). Beach boom, a water ballasted boom that
sits upright on shore, (not shown here) is often preferred for the first shoreline boom
section of a diversion or containment system because it seals nicely to the shoreline with
changes of water level. The Canadians very often use longer continuous deflection booms
and keep them in shape by attaching shoreline ropes about every 30-feet which pull
downstream on the near shore collection side to help keep the J-shape out of the boom.
This however requires higher mooring line tension necessitating the use of wire rope for
the upstream mooring line. Other booming strategies such as chevron are used to deflect to
each side of a channel. Oil can also be contained by encircling it and slowly diverting
the boom to a lower current area. A Swedish company has recently perfected the boom
current rudder that once it is deployed allows one person to retrieve the deflection boom
to shore using the hydrodynamic forces on the current rudder to allow vessel passage in a
river. The boom can then be re-deployed in the opposite manor. This device also shows
promise to allow vessel of opportunity skimming with side sweeps without the use of
outriggers.
Figure (1). Cascading 4" X 6" Boom in 50-foot Sections
One Canadian company has recently improved upon the concept of boom deflectors
originally tested in the mid-1970’s. These devices connect between 50-foot sections
of boom and use deflection vanes on the back side of the deflector to push the boom into
the current using hydrodynamic forces. Only one upstream anchor is required. This is
applicable where boom deflection in a wide river or coastal area has to be done quickly
and using multiple anchors along the boom is too difficult or time consuming. The vanes
are adjustable depending upon the desired boom angle and current velocity.
Paravanes show promise in deflection scenarios with very high currents, shallow water
or difficult to anchor locations where boom cannot be easily deployed. The paravanes alter
the surface current and cause the oil to deflect to the side the paravanes are moored.
Similar the current rudder, but with many more in vanes in parallel, these can be
retrieved and deployed from shore by controlling the angle of attack to the current. They
can be cascaded down stream to herd the oil away from a sensitive area or into shore where
a conventional collection boom can be used to contain oil in slower currents.
Specialized boom systems have also shown to be successful in fast water. V-shaped boom,
held in place with a net across the foot of the boom, has been effective in 1.6 knots with
conventional weir skimmers in the apex and at speeds of 3 knots with several different
types of in-line skimmers during tests at OHMSETT. The University of New Hampshire is
developing a rapid current boom that uses incline plane technology to trap and contain oil
in currents up to 3 knots.
There are several high speed skimmers currently on the market and others that tested
well but never made it commercially that can recover oil at speeds ranging from 2 to 6
knots. Most; however, start to lose throughput efficiency and at speeds above 3 knots and
in waves. The skimmers that use zero relative velocity and incline plane technology tend
to be more effective in higher currents and waves than surface slicing type skimmers.
Quiescent zone skimmers fool the oil into thinking it has found a nice quiet lagoon while
it is skimmed in currents. As skimming speeds increase above 3 knots most skimmers cannot
use deflection systems due high drag forces, wave amplification and turbulence. Oil
encounter rate, recovery efficiency, throughput efficiency and cost are all important
considerations in selection of a high speed skimmer.
Other deflection strategies have also been tried. Alternate containment and diversion
techniques include: pneumatic boom, horizontal air and water jets, plunging water jets,
diversion paravanes, and floating paddle wheels. Most are limited to 2 knots but some can
work at higher velocities at steep angles to the current. Some of these techniques lend
themselves to permanent installations around transfer terminals, water intakes or piers.
If you know of any other strategies or equipment that are effective in high speed
currents please let me know. If you need additional information on any of these
technologies I’ll be glad to assist.
Tom Coe
Senior Engineer
ManTech Advanced Systems International
6400 Goldsboro Rd., Suite 200
Bethesda, MD 20817
USA
phone +1 540 659 7062
e-mail tjcoe@erols.com
