API BULL 4719-2017 pdf download.Industry Guidelines on Requesting Regulatory Concurrence for Subsea Dispersant Use.
5 Evaluating the Use of Subsea Dispersant Injection The primary goal of dispersant use is to increase the amount of oil that dissipates into the water column and is subject to microbial degradation, thereby reducing the amount of oil remaining on the surface. The use of SSDI offers an available and efficient method of achieving a high encounter rate directly at the source, thereby reducing the potential for floating oil to threaten worker health and safety, and to reach ecologically and economically sensitive shoreline environments. Research and experience has shown that hydrocarbon exposures decline rapidly away from the subsea source and are further mitigated by microbial degradation  . This enhanced dispersal of oil is an important factor when using SIMA as a part of the response decision–making process. Past government and industry experience with responding to open-water oil spills has shown that mechanical recovery alone has often yielded limited rates of recovery  because of low encounter rates due to oil spreading into a thin film, and reduced efficiency due to higher wave conditions offshore. As industry operates in deeper waters farther offshore, there are additional limitations posed by greater transit distances for boats supporting the response, and adverse weather conditions that can hamper safe operations and transits to and from port. For these reasons, the use of SSDI can provide an effective means of minimizing significant quantities of oil from the surface quickly, and reduce potential threats to sensitive near-shore, shallow-water environments. Several factors should be considered in making a decision about subsea dispersant injection in any given scenario, and the decision process should be documented for potential presentation to the RRT. Table A.2 in Annex A may be used to help assess the feasibility of SSDI as a response method in the context of a given spill scenario.
6 Use of Modeling to Support Response Decision-making The forecast skill of oil spill trajectory models is dependent upon the accuracy and availability of the data requested when the model is developed, accuracy of input information, the judgment of the modeler, and the formulation of the oil spill model itself. Important inputs include wind and current data from meteorological and hydrodynamic models. For spill responses, models are recommended for 24- to 72-hour forecasts due to the decreased accuracy of input information for longer future projections; however, longer (>72 hours) projections provide valuable conceptual and predictive data for planning purposes, especially for potential resources at risk. Subsea 3-D models consider both vertical and horizontal transport, which depend upon many of the same factors as a surface spill but also include model predictions of droplet size, gas content, depth and stratification, and the oil constituents themselves. Current subsea deep ocean hydrodynamic models have not been verified to any degree of accuracy, nor are they configured to resolve local current velocities important to near-term trajectories, which should be considered during a subsea release. For modeling dispersed oil at depth, full water column measurement of current velocities near the spill site should be a priority, along with oil characteristics and droplet size, as well as a baseline conductivity, temperature, and depth (CTD cast) to provide inputs for the modeling. Predictions using this information should have a relatively high forecast accuracy within a 24- to 72-hour trajectory forecast. Oil spill modeling should be conducted well in advance of an event, using a (credible) worst-case discharge scenario, allowing prediction of oiling extent and character for use in response exercises, training, or planning. It can be modified during an event.