On November 11, 2005, the double-hulled tank barge (TIB) DBL 152 struck the submerged remains of a drilling platform that collapsed during Hurricane Rita in the northwestern Gulf of Mexico. The allision and eventual capsize of the barge caused the release of approximately 1.9 million gallons of low-API gravity slurry oil, which sank to the seafloor. Throughout the response, information on the concentration of submerged oil (volume per unit area) was sought to prioritize cleanup of multiple sunken oil patches. Numerous methods were employed to detect and assess submerged oil including, among others, divers to measure oil thickness and chain-weighted sorbent drags to delineate patch size. However, none of the techniques used were able to quantify the percent cover of oil, which emerged as a critical parameter for estimating oil concentrations. This paper presents the methodology for quantifying percent cover of submerged oil used during the long-term monitoring phase of this incident. Orthographic (downward-looking) images of the seafloor were obtained using an underwater drop camera. Full-motion video was digitally recorded using a laptop computer and video capture hardware. Imagery was georeferenced by overlaying GPS coordinates onto the streaming video using a video annotator. Still images were extracted from the video using frame-grabbing software during post-processing. Percent cover of oil was then manually determined for each image using two different methods: ocular estimation and point counts. Seafloor imagery was acquired at discrete sample points along drift transects, and when visibility allowed, continuously between sample points. Percent cover data obtained using these techniques, combined with oil thickness measurements and patch size information obtained using other methods, was used to estimate the volume of oil present within a discrete patch of submerged oil discovered in September 2006. Successful application of this technique demonstrated its viability as a cost-effective means of obtaining quantitative information about percent cover of submerged oil. Moreover, this relatively low-tech system was comprised primarily of off-the-shelf components that can be readily sourced, assembled and mobilized to a submerged oil spill response area.

A variety of methods and equipment were employed during the response and long-term monitoring phases of the DBL 152 incident to locate, track and quantify the nearly 2 million gallons of low-API gravity oil that sank in the western Gulf of Mexico approximately 30 nautical miles off the coast of Cameron, Louisiana. Methods and equipment used to survey submerged oil included: divers; stationary snare sentinels; chain-weighted snare drags using devices known as vessel-submerged oil recovery systems or V-SORS; remotely operated vehicles (ROVs), underwater video drop camera, sled-mounted towed video, side-scan sonar and Rox-Ann sonar. This paper will describe each method and associated equipment and its specific application to submerged oil detection and assessment for this incident. It will also explore the evolution and refinement of approaches used throughout the course of the response and the underlying rationale for these changes. Guidelines and relevant considerations for selecting among these methods will be suggested. Finally, strengths and limitations of each approach will be discussed with the goal of capturing and communicating the lessons learned so that future submerged oil response efforts may benefit from the practical experience gained during the DBL 152 response.

In November 2005, approximately 1.9 million gallons of Group V slurry oil was released in the western Gulf of Mexico following the allision of the double-hulled tank barge DBL 152 with the submerged remains of a pipeline service platform that collapsed during Hurricane Rita. The released oil was denser than seawater and sank to the bottom. After approximately six weeks of intermittent cleanup using diver-direct pumping, submerged oil recovery operations were suspended by the Unified Command based on the high percentage (50%) of weather-related downtime, as well as indications that recoverable accumulations of oil were dispersing naturally, which further reduced the feasibility of cleanup. However, the responsible party was required to develop and implement a long-term monitoring program (LTMP) to track the fate and transport of the sunken oil and determine the potential need for resuming oil recovery operations once more favorable weather patterns returned in the spring. This paper will present an overview of the approach, methods and results of the long-term monitoring efforts performed over a 14-month period following the incident. Major objectives of the LTMP included tracking the movement and fate of non-recovered submerged oil to assess its extent and continued dispersion; providing advance warning of potential impacts to Gulf Coast shorelines and other sensitive areas; and documenting changes in the oil'S chemical composition and physical properties through time due to weathering processes. Major findings of the LTMP include the dissipation of the main submerged oil field over the course of several months and the discovery, differential behavior and eventual dissipation of a discrete high-concentration oil patch found several miles from the incident location. The importance of long-term monitoring data in the decision-making process to determine both the need for and feasibility of resuming submerged oil recovery operations will be emphasized. Information on the fate and transport characteristics of submerged oil and the adaptation of monitoring techniques to address evolving needs will also be addressed. Both the incident-specific information and the practical lessons-learned are intended to benefit those who may be faced with monitoring submerged oil spills in the future.