Oil Skimmers Overview

A skimmer is an essential tool in our days. Designed to recover oil from land or water, these oil collection devices come in a variety of sizes and forms.

SKIMMERS

The common name used for various oil recovery units is oil skimmer or simply skimmer. A skimmer is defined as any mechanical device specifically designed for the removal of oil (or oil/water mixture) from the surface of water without altering the water's physical and/or chemical characteristics. The principles for skimmers' operation are based on the fluidity properties of oil and oil/water mixture, density differences between oil or oil/water mixtures, and water or differences in adhesion to materials.

Skimmers are often deployed in combination with oil containment booms, which are temporary floating barriers used to contain the oil by concentrating it into thicker surface layers. These devices are constructed of weather and water-resistant materials such as stainless steel, rubber, aluminum, and polypropylene. Skimmers are directed to oily areas by pilots flying over designated areas.

Skimmer Components

Skimming head - the component separating oil from water
Transfer system - built-in pump or vacuum unit, discs, brushes, belts, tubes or ropes, hoses and couplings
Containment unit - Tank or container for recovered oil

Three factors normally used to describe skimmer performance

Recovery Rate
Recovery Efficiency
Throughput Efficiency

SKIMMER TYPES

Skimmers vary considerably in their working principles and construction. The method of collecting the oil is normally used to distinguish between skimmers and their respective uses.

1. Weir Skimmers

Weir skimmers include any device using gravity to drain oil from the water surface. Weirs are normally launched from a vessel, using a crane, and are guided by ropes. The edge of the weir is positioned just below the upper slick surface, allowing oil to flow over the weir edge into a collecting sump and then to be pumped to storage. It is probably the most commonly used skimmer type due to its simple construction. Weir systems can either be remote-controlled or self-adjusting. Remote adjusting systems are based on compressed air.

2. Vacuum Skimmers

Vacuum skimmers do not have a pump incorporated in the actual floating skimmer device. This device is normally called a skimmer head. The recovered oil/water mixture is sucked from the skimmer head by a suction or vacuum pump. The simplest type of vacuum skimmer is a hose directly connected to a vacuum truck, which can easily be employed in harbors or rivers. Vacuum skimmers are restricted to use in harbors and calm waters as they are very sensitive to waves.

3. Oleophilic Skimmers

Oleophilic skimmers recover oil based on specific materials that have a greater affinity for oil than for water. Such materials are known as oleophilic (mops, rope, brushes, tubes, or discs). The moving part of the skimmer (rope, belt, tube, drum, and disc) with the oleophilic surface is rotated or drawn through the oil slick. The oil is then scraped or squeezed off and guided into a sump to be pumped or sucked away.

Oleophilic skimmers are divided into subgroups: Disc Skimmers, Belt and Brush Skimmers, Tube Skimmers, Rope Mop Skimmers

Disc skimmers have recovery capacities from 40-100 m3/h and can be used for open sea operations. Disc skimmers work best with lighter types of oil (medium viscosity) and cannot handle emulsified oil. The volume and weight of these discs are quite large due to the size and number of rotating discs. Crane operators deploy, guide, and control the heavy disc skimmers.

Brush and Belt skimmers are mostly large and either mounted on a barge (self-floating unit) or on a specially constructed vessel. Deploying large skimmers can be complicated and requires heavy equipment and specially trained personnel. These skimmers have a high recovery efficiency and a good recovery rate but are specialized products. However, nowadays you can pick portable belt skimmers of average and even mini sizes.

Tube skimmers. Flexible and versatile, tube skimmers are capable of removing oils, grease, and floating sludge from a variety of containment systems, and can be adapted to any number of applications. These can range from small indoor tanks and sumps to large outdoor basins and ponds. Tube skimmers can extend as far as 16 feet, and when equipped with a balanced boom system, can move around as desired for maximum portability, eliminating the need for expensive bridging or foundations – OR – can be mounted directly over top of a sump or manway opening. Tube skimmers have the ability to snake over, under, and through debris to continuously pick up oil.

Vertical Rope Mop skimmers are large units launched from a vessel or shore using a crane during the entire operation. The vertical rope mop skimmer is normally used for single sweep operations only. This skimmer type is not sensitive to waves. Very little water is collected during recovery which makes this type of skimmer suitable for lighter oil types. Oil is wrung out of the mop ropes and stored in a collection sump on board the vessel; debris will not affect the pump operation.

DEPLOYMENT AND USE OF SKIMMERS FOR ON-SHORE OPERATIONS

Spills on Soil

For oil spills on soil, the best solution is digging a pit at a collection point and guiding the oil towards the pit. A vacuum skimmer can be used for the recovery of oil from the pit. It may be necessary to mechanically remove the topsoil for further treatment using an excavator or shovels.

Spills on Concrete, Asphalt, or other hard surfaces

To avoid further spreading of the oil, sorbent booms can be laid out to contain the spill. If the oil is fresh, it can be recovered using a vacuum skimmer. If the oil is not fresh(solidified), using high-pressure cleaners combined with chemical treatment is an option.

Shallow water areas (marsh, swamp, or lagoons)

Sorbent booms can be used to guide the oil towards the skimmer. Skimmers could be rope mop or disc skimmers, since there might be a high content of debris in the water. If it is not possible to concentrate the spilled oil, rope mop skimmers can be used to cover a larger area than a disc skimmer. Any type of mechanical treatment in sensitive areas (e.g. marsh or shallow water) should be avoided where possible; heavy equipment will damage the environment much more than smaller skimming recovery devices.