Deck drains, also known as "slop water" on offshore drill ships and platforms, can often contain drilling and completion fluids, solids and various hydrocarbons. These sources of contaminated water are mixed together and can be stored in a settling tank (also known as a "slop tank"). Other contaminants can enter the settling tank from the machinery space, from a non-machinery space or from other processing systems that contribute hydraulic oils, lubricants, solvents, grease and various solids. Oil separating and filtering equipment normally used onsite to process the slop water and reduce the oil content to less than 29 ppm or 15 ppm, depending on the regulations at the operation area. Hydrocarbons in slop water can differ from the hydrocarbons contemplated by Marine Environment Protection Committee MEPC 107(49). Anything subject to International Maritime Organization (IMO) MEPC 107(49) must meet a 15 ppm or 5 ppm requirement.
A spill of solids can enter into the drains and then into the slop water storage tanks. Often, the tanks for slop water are mingled with bilge water storage tanks that are aligned with the space governed by MEPC 107(49), making anything that gets into the machinery space water subject to MEPC 107(49) testing and treatment requirements. Most IMO-certified oily water separators (OWSs) are not equipped to handle a full range of hydrocarbons or solids that are in the deck drain water and that are often difficult to define. The solids and free oils separate in the holding tanks, resulting in high concentrations of solids settled by gravity at the bottom of the holding tanks, high concentrations of oil floating on the top of the tank, and emulsified oil and water in the middle. When these high concentrations of oil or solids are fed to the OWSs; the systems are overloaded and the OWSs plug up.
In some cases, offshore drilling operations are required to have independent offline testing of the water, even if there is an in-line oil content monitor with the system. In most cases, the recommended lab test by United States Environmental Protection Agency (EPA) uses either an EPA 1664A test (three different options) or an EPA 5520 test (six different options: B-G). This creates a problem because the IMO oil content monitors (OCMs) are certified based on the International Organization for Standardization (ISO) 9377-2:2000 procedure, which does not cover the same range of hydrocarbons. IMO-certified OCMs are not designed to measure hydrocarbons in the C3 to C9 range or above C38 because they are not required to be measured by IMO. Frequently, the independent lab tests produce a different result, usually higher than the OCM on the OWS, and the offshore drilling operation is not allowed to discharge overboard, resulting in significant cost.
This paper describes a separation system specifically designed to handle the full range of contaminants that can be found in deck drain and machinery space effluent water. The system will separate oils, grease and other hydrocarbons from water in accordance with IMO standards as defined in MEPC 107(49). This separation system also offers greater removal of other hydrocarbons that are not included in the IMO standards. In addition, the system will remove high concentrations of drilling contents and other solids that generally plug up other water treatment systems.
The system is adjustable so that each challenge can be addressed separately in an effective manner. The system provides a 0.02-micron positive barrier against most contaminants by using a back washable and low-pressure ultra-filter media. This system offers a processing rate of 45 gpm (higher amounts during rainstorms), requires a smaller floor space, enhances operational safety with fewer movable parts, and minimizes any environmental impact while enabling continuous drilling.