Snare Performance for Sunken and Submerged Oil Detection and Monitoring

ABSTRACT - 687127 Most oil spill response strategies, tactics, and equipment are designed to address floating oil. Previous research and historic events have shown that spilled oil can suspend (i.e., submerged oil) or sink (i.e., sunken oil) as a function of the oil's density relative to that of the receiving waters. Processes such as wave action or current velocity, sediment entrainment, and oil weathering (e.g., evaporation) may change the buoyancy of floating oils causing them to submerge or sink. Non-floating oil is more difficult and expensive to detect and poses significant challenges for containment and cleanup. Many existing detection techniques for non-floating oils rely on oleophilic sorbents, such as snare, which are weighted depending upon the oil's location in the water column and then towed behind a vessel in designated transects. Currently, there is no quantitative method to relate the amount of oil collected by snare to the amount of oil encountered during towing. In addition, the dynamics and interactions of towed snare and oil remain largely unknown. To address these knowledge gaps, various components of snare performance have been evaluated since 2016 by the Coastal Response Research Center (CRRC) at the University of New Hampshire (UNH). The research has evaluated: (1) the impacts of temperature, salinity, oil type, and tow velocity on adsorption and desorption of oil to snare, (2) snare dynamics and position in the water column as a function of tow velocity, (3) the impacts of material type and potential alternatives to snare (e.g., mosquito and fishing nets, plastic debris) for lesser developed countries (LDCs), and (4) the interaction of snare with sunken and submerged oil. The results determined: (1) adsorption of oil to snare was best for less viscous oils (No. 6 Fuel Oil) and lower water temperatures (5°C) and desorption was greatest at low temperatures (6°C) and low current velocities (< 1 knot), while salinity had no significant effect. (2) Tow depth for snare arrays decreased with increased velocity unless a vane was used. (3) Optimal spacing of snare on a chain is a function of tow and current velocity, and drag forces on the tow chain. (4) Snare alternatives with greatest potential for sunken oil detection in LDCs were nylon mosquito netting and plastic bags. The findings from this research improves understanding of the behavior of snare and how it interacts with sunken and submerged oil and can improve towing techniques used by oil spill responders, leading to more effective detection.

Flume Experiments to Assess the Effect of Bottom Roughness, Temperature, Water Velocity and Oil Condition on Critical Shear Stress Estimates of Heavy Fuel Oil

Non-floating oil is challenging to detect, track, and recover due to limited visibility inhibiting verification of the oil's location and subsurface movement. Oil that sinks to the bottom (i.e., sunken oil) can form large mats or small agglomerates on the bottom, mix into sediments, or remobilize into the water column and move with currents potentially impacting shorelines, benthic and pelagic organisms, intakes for drinking water, and power plants. Trajectory models exist that predict movement of floating and submerged oil; however, many models cannot accurately address sunken oil movement because the bed shear stress (BSS) necessary to mobilize oil (i.e., critical shear stress (CSS)), neglects the effects of bottom roughness and assumes an immobile bed. The goal of this research is to provide responders and modelers with more precise CSS estimates that include the effect of bottom roughness and incorporate results into a response tool to predict sunken oil movement. The transport of oil depends upon in-situ environmental conditions and oil properties. This research used the Coastal Response Research Center's (CRRC) 2180-liter straight flume to test the effects of water velocity, water temperature, oil mass, and bottom friction on fresh and weathered No. 6 Heavy Fuel Oil (HFO) on an immobile boundary. The flume's test section provided a uniform, one-dimensional flow field measured in 3D by an acoustic Doppler velocimeter (ADV), a Nortek AS (Norway) Vectrino II Profiling Velocimeter. The fresh or weathered (%Ev=5) HFO was mixed with kaolinite clay as a sinking agent, and 100 grams of the mixture was injected into static water via subsurface injection. The water velocity was incrementally increased in a stepwise manner by 0.07 m/s intervals and held for 15 minutes at each velocity. This occurred until: (1) oil had stopped eroding or was completely eroded from the substrate, or (2) the maximum velocity of 1.04 m/s was reached. Bottom roughness was evaluated using the velocity profile and bed shear stress (BSS) was calculated using multiple methods applicable to lab and field conditions. The oil's behavior was documented by downward- and side-facing GoPro cameras and reviewed to estimate mass loss per velocity interval, the distance the oil migrated along the bottom, and the corresponding CSS. In the case of an oil spill, responders can compare CSS estimates, determined through this research, with in-situ BSS estimates predicting under what conditions the sunken oil will become mobile.

Application of the SOSim v2 Model to Spills of Sunken Oil in Rivers

Sunken oil transport processes in rivers differ from those in oceans, and currently available models may not be generally applicable to sunken oil in river settings. The open-source Subsurface Oil Simulator (SOSim) model has been expanded to handle spills of sunken oil in navigable rivers, utilizing Bayesian inference to integrate field concentration data with bathymetric data to predict the location and movement of sunken oil. A novel prior likelihood function incorporates bathymetric input, with sampling grid and default parameters adapted appropriately for rivers. SOSim v2 was demonstrated versus field observations taken following the M/T (Motor Tanker) Athos I oil spill. The model was also modified to operate in 1-D, to assess the longitudinal distribution of sunken oil in a non-navigable river using available poling data collected following the Enbridge Kalamazoo River oil spill in 2010. Results of both case studies were consistent with observed data and local bathymetry in 2-D and 1-D, and the model is suggested as a complement to deterministic models for oil spill emergency response in rivers.

T/B APEX 3508: Best Practices for Detection and Recovery of Sunken Oil

ABSTRACT On September 2, 2015, two towing vessels collided on the Lower Mississippi River at Mile 937, near Columbus, Kentucky, resulting in the complete breach of the #3 starboard cargo tank on the T/B APEX 3508 and the release of 120,588 gallons of clarified slurry oil (CSO; Group V oil; Specific Gravity: 1.14) into the navigable waterway. The incident was classified as a Major Inland Spill in accordance with the National Oil and Hazardous Substance Contingency Plan and a Major Marine Casualty that was jointly investigated by the United States Coast Guard and the National Transportation Safety Board. Over flights conducted as far as 20 miles downriver indicated only light, sporadic sheening for 1–2 days. On-water and shoreline assessments conducted up to six miles downriver revealed no visible signs of surface oiling. Based on its properties, the vast majority of the CSO was presumed to have sunk, but its precise disposition and location was not confirmed. Using side scan sonar (SSS) technology, two distinct subsurface anomalies with an approximate combined area of 9,200 m2 were identified on the river bed in the vicinity of the incident. The anomalies were confirmed as oil by divers and direct sampling, and were then divided into 25 m grids for identification and tracking. The Unified Command evaluated best available technologies and determined that GPS guided environmental dredging would be the safest, most effective and efficient of the recovery options. The established cleanup endpoint was a maximum of 10% sporadic oil distribution in each grid. Two endangered mussel species were identified as potentially inhabiting the affected area. A diver survey was conducted in the area and concluded that bottom habitat was not likely to support the listed species. Further consultations with the resource manager indicated that proposed recovery operations posed low risk to the species. Recovery operations commenced on September 15, 2015 and concluded on September 25, 2015. Endpoint verification was conducted via SSS. In total, response operations lasted 23 days (eight operational periods), involved over 120 responders and 75 specialized response assets, and cost approximately $5 million. Approximately 2,524 m3 of dredged material (liquid and solids) were removed. After decanting, approximately 1,730 m3of oiled solids representing approximately 50 to 75% of the spilled product was recovered. This case serves as a benchmark for sunken oil detection and recovery operations, and identified many best practices that should be considered on future cases with similar spill conditions.

OIL BEHAVIOUR AND THE RESPONSE TO A SUNKEN OIL SPILL OF SLURRY IN QUINTERO BAY, CHILE

ABSTRACT Slurry oils are one of the few groups of oils that have a density greater than full salinity seawater. Slurry spills at sea are rare, but when they do occur, the response can be difficult, with low final recovery rates. Oil spill response planning and recovery techniques have been refined over many years and the research and development of most methods has been underpinned by the principle that oil floats on water. This recent case study provides an example of the behavior of a slurry spill and the subsequent response operations. A leak of an API gravity 4° slurry occurred from a submarine pipeline in Quintero Bay, Chile, on 14th May 2016. The oil sank to the adjacent sea bed and covered an area of approximately 10,000 m2 in water depths of 18–22 m. The initial response involved over 20 dry-suit divers with surface air to recover the oil with vacuum hoses. These were then supplemented by four diver-operated vacuum dredges. Fortunately weak bottom currents did not redistribute the oil; nevertheless “sea bed booms” were deployed around the perimeter to prevent migration. These “booms” were initially a screen mesh that was later supplemented by sturdier metal frame screens deployed around the zone of the sunken oil in a zigzag configuration. Baseline benthic sediment samples were collected from the adjacent unaffected area and subsequently from the treatment area after completion of operations. A benthic SCUBA SCAT survey using a track-line positioning system was conducted to systematically document the distribution of the oil and post-treatment systematic ROV survey ensured that the end-point oiling conditions had been achieved.