_MNL_34-1998.pdf

TPerformance Review of Sl2(6)large waves,foam crests,and some spray Special wave conditions axe often defined in skimmer tests,such as Harbor Chop and Regular Waves.These test waves have very specific heights and periods,so these defi-nitions are retained in the text.Further,waves generated in test basins are small,usu-4 OIL SPILL RESPONSE PERFORMANCE REVIEW OF SKIMMERS ally 1 ft(0.3 m)or less,so nearly all test waves fall in the ASTM range of Calm Water.In a few cases,waves may reach about 1.6 ft(0.5 m)which is in the Protected Water range.A note has been added to tables indicating where the test waves fall according to ASTM definitions.Skimmers with a large inertial mass generally have problems following the oil-water interface.To solve this problem,some are designed so that the mass of the skimmer in the water is quite low and heavy equipment,such as pumps and tanks,are stored on a host ship.Some skimmers have collection elements with a low mass per unit length to provide good conformance with wave patterns.Rope mops and boom-skimmers are ex-amples of these kinds of devices.Nearly all skimmers are able to follow long period wave patterns quite well.In this case their performance would be the same as in calm water because the skimming head maintains its position relative to the surface of the water.Sorbent lifting belt,sorbent submersion belt,and chain brush skimmers can operate in a range of wave patterns in which the waves are not higher than the vertical dimen-sion of their belts.Similarly,fixed submersion plane and submersion moving plane skimmers can operate in waves that are not higher than the vertical dimension of their submersion planes.Currents affect the performance of skimmers because fast currents generally cause oil to escape under collection booms.Also,high currents may swamp intakes or cause sur-face oil to move past the collection element so fast that it is not effectively recovered.Skimmers effective in high currents often have a collection element that moves with a zero relative velocity to the current(ZRV).These skimmers generally have a rope mop or sorbent belt collection element that moves aft in a well or between a catamaran hull at the same speed as the vessel is moving ahead,or at zero velocity relative to the oiled sur-face.Some of these devices can effectively recover oil in currents up to 6 knots.In the Review,current is represented by Tow Speed.Advancing skimmers or skimmers in cur-rents have the same problems.Slick Thickness Slick thickness is most important in determining the effectiveness of skimming systems.Nearly any device is effective if the oil is thick enough.As the accumulation of oil de-creases,performance in terms of recovery rate and recovery efficiency(percent oil)also decreases.This is particularly true of simple devices such as suction and weir skimmers.Some skimmers can improve their performance in thin slicks by changing the operating parameters.For example,the performance of some oleophilic skimmers,such as disc skimmers,can be improved by reducing the speed of the oleophilic surface.A disc skim-mer can be operated at a very low speed in a thin slick to increase recovery efficiency.This,of course,is done at the expense of recovery rate,which becomes very low.Operation in Debris The presence of debris can cause a substantial obstacle to skimmer performance.Some skimmers,such as oleophilic skimmers,are relatively insensitive to the presence of de-bris.Suction and air conveyor devices are generally tolerant of debris up to the size of the transfer hoses or the size the pump can handle.Weir devices are vulnerable to de-bris;however,some weir devices using integral archimedean screw pumps can process most debris that enters the system.In selecting skimmer types,sensitivity to debris is an important consideration.Skimmer Performance Parameters Significant performance parameters are listed for each skimmer type.Certain perfor-mance parameters are basically the same for all skimmer types and therefore they are not mentioned separately for each skimmer.These common performance parameters include the following:Slick thickness Oil type and viscosity Wave height and period Sweep width Sweep speed INTRODUCTION 5 In some test reports,the way in which skimmer performance parameters were mea-sured is described in great detail;in some reports it is not.The paragraph titled Test Pro-cedures contains a brief statement describing how test parameters were measured.Test Procedures This paragraph head appears for every test report.It briefly describes how the test was conducted,how test parameters were measured,and how much oil was distributed dur-ing the test.SKIMMER MEASURES OF EFFECTIVENESS The expected performance of skimmers is described in terms of standard measures of effectiveness,which are:Recovery Efficiency(RE)The percent oil in the recovered mixture.Throughput Efficiency(TE)The ratio of oil recovered to oil encountered,ex-pressed as a percent.Oil Recovery Rate(ORR)The