Simulating dispersion of oils from a subsea release comparing mechanical and chemically enhanced dispersion - An experimental study of the influence of oil properties.

The main objective with subsea mechanical dispersion (SSMD) is to influence the fate of an oil spill in the marine environment by significantly reducing oil droplet sizes from subsea release of oil. Earlier studies have indicated that the capability of SSMD to reduce oil droplet sizes is comparable to subsea dispersant injection (SSDI). Earlier testing of SSMD has mainly used a low viscus paraffinic oil. Focus for this study was to study SSMD and SSDI effectiveness using five oil types spanning out a wide variation of relevant oil properties. Effectiveness was quantified as the reduction in oil droplet sizes measured by a Silhouette camera. Testing of the two technologies were completed in the same experiment on a simulated subsea release. The results show a variation in effectiveness for both technologies as a function of oil properties. SSMD and SSDI showed comparable effectiveness for all oils tested.

Reducing oil droplet sizes from a subsea oil and gas release by water jetting a laboratory study performed at different scales.

The main objective of subsea mechanical dispersion (SSMD) is to reduce the oil droplet sizes from a subsea oil release, thereby influencing the fate and behaviour of the released oil in the marine environment. Subsea water jetting was identified as a promising method for SSMD and imply that a water jet is used to reduce the particle size of the oil droplets initially formed from the subsea release. This paper presents the main findings from a study including small-scale testing in a pressurised tank, via laboratory basin testing, to large-scale outdoor basin testing. The effectiveness of SSMD increases with the scale of the experiments. From a five-fold reduction in droplet sizes for small-scale experiments to more than ten-fold for large-scale experiments. The technology is ready for full-scale prototyping and field testing. Large-scale experiments performed at Ohmsett indicate that SSMD could be comparable to subsea dispersant injection (SSDI) in reducing oil droplet sizes.

Large-scale basin testing to simulate realistic oil droplet distributions from subsea release of oil and the effect of subsea dispersant injection.

Small-scale experiments performed at SINTEF, Norway in 2011-12 led to the development of a modified Weber scaling algorithm. The algorithm predicts initial oil droplet sizes (d50) from a subsea oil and gas blowout. It was quickly implemented in a high number of operational oil spill models used to predict fate and effect of subsea oil releases both in academia and in the oil industry. This paper presents experimental data from large-scale experiments generating oil droplet data in a more realistic multi-millimeter size range for a subsea blow-out. This new data shows a very high correlation with predictions from the modified Weber scaling algorithm both for untreated oil and oil treated by dispersant injection. This finding is opposed to earlier studies predicting significantly smaller droplets, using a similar approach for estimating droplet sizes, but with calibration coefficients that we mean are not representative of the turbulence present in such releases.

Interfacial tension between oil and seawater as a function of dispersant dosage.

This paper presents a compilation of data describing interfacial tension between oil and seawater (IFT(oil-water)) as a function of dispersant dosage. The data are from several earlier laboratory studies simulating subsea oil blowouts to evaluate subsea injection of dispersant (SSDI). Three dispersants were tested with four oil types to give a large variation in oil properties (paraffinic, light, waxy and asphaltenic). A general expression for IFT(oil-water) as a function of dispersant dosage is proposed based on the compiled data. IFT(oil-water) versus dosage is needed by algorithms to predict oil droplet sizes from subsea releases. However, such a relationship based on averaged data should be used with care and IFT measurements on the actual oil-dispersant combination should always be preferred.

Shedding from chemically-treated oil droplets rising in seawater.

The degree to which droplet shedding (tip-streaming) can modify the size of rising oil droplets has been a topic of growing interest in relation to subsea dispersant injection. We present an experimental and numerical approach predicting oil droplet shedding, covering a wide range of viscosities and interfacial tensions. Shedding was observed within a specific range of droplet sizes when the oil viscosity is sufficiently high and the IFT is sufficiently low. The affected droplets are observed to reduce in size, as smaller satellite droplets are shed, until the parent droplet reaches a stable size. Shedding of smaller droplets is related to the viscosity-dominated modified capillary number (Ca'), especially for low dispersant dosages recommended for subsea dispersant injection. This, in combination with the IFT-dominated Weber number (We), characterise droplets into three possible states: 1) stable (Ca' < 0.21 &We<12); 2) tip-streaming (Ca' > 0.21 &We<12); 3) unstable and subject to total breakup (We>12).

Combined releases of oil and gas under pressure; the influence of live oil and natural gas on initial oil droplet formation.

Both oil droplets and gas bubbles have simultaneously been quantified in laboratory experiments that simulate deep-water subsea releases of both live oil (saturated with gas) and additional natural gas under high pressure. These data have been used to calculate particle size distributions (50-5000 µm) for both oil and gas. The experiments showed no significant difference in oil droplet sizes versus pressure (from 5 m to 1750 m) for experiments with live oil. For combined releases of live oil and natural gas, oil droplet sizes showed a clear reduction as a function of increased gas void fraction (increased release velocity) and a weak reduction with increased depth (increased gas density/momentum). Oil droplets were reduced by a factor of 3 to 4 during simulated subsea dispersant injection (SSDI) and no significant effect of pressure was observed. This indicates that SSDI effectiveness is not dependent on water depth or pressure.

Quantification of oil droplets under high pressure laboratory experiments simulating deep water oil releases and subsea dispersants injection (SSDI).

Limited experimental and field data are available describing oil droplet formation from subsea releases at high pressure. There are also analytical challenges quantifying oil droplets over a wide size and concentrations range at high pressure. This study quantified oil droplets released from an orifice in seawater at low and high pressure (5 m and 1750 m depth). Oil droplet sizes were quantified using a newly developed sensor (Silhouette camera or SilCam). The droplet sizes measured during experiments at low and high pressure, using the same release conditions, showed no significant difference as a function of pressure. This lack of a pressure effect on oil droplet sizes was observed for both untreated oil and for droplet formation during subsea dispersant injection or SSDI. This strongly indicates that the effectiveness of SSDI is not influenced by water depth or pressure, at least for simulated subsea releases of oil alone (no gas).

The sensitivity of the surface oil signature to subsurface dispersant injection and weather conditions.

Subsea blowouts have the potential to spread oil across large geographical areas, and subsea dispersant injection (SSDI) is a response option targeted at reducing the impact of a blowout, especially reducing persistent surface oil slicks. Modified Weber scaling was used to predict oil droplet sizes with the OSCAR oil spill model, and to evaluate the surface oil volume and area when using SSDI under different conditions. Generally, SSDI reduces the amount of oil on the surface, and creates wider and thinner surface oil slicks. It was found that the reduction of surface oil area and volume with SSDI was enhanced for higher wind speeds. Overall, given the effect of SSDI on oil volume and weathering, it may be suggested that tar ball formation, requiring thick and weathered oil, could possibly be reduced when SSDI is used.

Surface weathering and dispersibility of MC252 crude oil.

Results from a comprehensive oil weathering and dispersant effectiveness study of the MC252 crude oil have been used to predict changes in oil properties due to weathering on the sea surface and to estimate the effective "time window" for dispersant application under various sea conditions. MC252 oil is a light paraffinic crude oil, for which approximately 55 wt.% will evaporate within 3-5 days when drifting on the sea. An unstable and low-viscosity water-in-oil (w/o) emulsion are formed during the first few days at the sea surface. This allows a high degree of natural dispersion when exposed to breaking wave conditions. Under calm sea conditions, a more stable and light-brown/orange colored water-in-oil (w/o) emulsion may start to form after several days, and viscosities of 10,000-15,000 mPa s can be achieved after 1-2 weeks. The "time window" for effective use of dispersants was estimated to be more than 1 week weathering at sea.

Droplet breakup in subsurface oil releases--part 1: experimental study of droplet breakup and effectiveness of dispersant injection.

Size distribution of oil droplets formed in deep water oil and gas blowouts have strong impact on the fate of the oil in the environment. However, very limited data on droplet distributions from subsurface releases exist. The objective of this study has been to establish a laboratory facility to study droplet size versus release conditions (rates and nozzle diameters), oil properties and injection of dispersants (injection techniques and dispersant types). This paper presents this facility (6 m high, 3 m wide, containing 40 m(3) of sea water) and introductory data. Injection of dispersant lowers the interfacial tension between oil and water and cause a significant reduction in droplet size. Most of this data show a good fit to existing Weber scaling equations. Some interesting deviations due to dispersant treatment are further analyzed and used to develop modified algorithms for predicting droplet sizes in a second paper (Johansen et al., 2013).

Droplet breakup in subsea oil releases--part 2: predictions of droplet size distributions with and without injection of chemical dispersants.

A new method for prediction of droplet size distributions from subsea oil and gas releases is presented in this paper. The method is based on experimental data obtained from oil droplet breakup experiments conducted in a new test facility at SINTEF. The facility is described in a companion paper, while this paper deals with the theoretical basis for the model and the empirical correlations used to derive the model parameters from the available data from the test facility. A major issue dealt with in this paper is the basis for extrapolation of the data to full scale (blowout) conditions. Possible contribution from factors such as buoyancy flux and gas void fraction are discussed and evaluated based on results from the DeepSpill field experiment.

Composition of in situ burn residue as a function of weathering conditions.

Troll B crude oil was weathered under Arctic conditions with different ice coverage: open water, 50% ice and 90% ice. Samples (100 mL) were taken during the experiment and tested for ignitability in a burning cell. From each burning a residue sample was taken for analysis. The burning process removed the light compounds eluting before C13. No effect from the prior weathering time or the different ice coverage was seen in the burn residue composition. The content of selected Poly Aromatic Hydrocarbons (PAHs) was determined and it was noted that the concentration of PAHs with more than 4 rings were increased. The source origin of the PAHs was investigated by use of relative ratios of PAH isomers and indicated that some formation of PAHs was additionally taking place during burning.

Chemical composition and acute toxicity in the water after in situ burning--a laboratory experiment.

The chemical composition and toxicity of a water soluble fraction (WSF) of oil versus the underlying water after in situ burning (ISB), has been studied in a laboratory experiment. A system for allowing water sampling after ISB was developed. Seawater samples and oil were collected prior to and immediately after ISB, and chemical analysis was conducted. The chemical characterization of the water showed that the disappearance of water soluble oil components during ISB was insignificant. Acute toxicity tests with the marine copepod Calanus finmarchicus and Microtox® bioassay was performed to establish LC(50)/EC(50) values of the water. The results were compared with regular WAF systems with unburned weathered oil, and indicated no increase in toxicity in the underlying water after ISB.

Measuring ignitability for in situ burning of oil spills weathered under Arctic conditions: from laboratory studies to large-scale field experiments.

This paper compares the ignitability of Troll B crude oil weathered under simulated Arctic conditions (0%, 50% and 90% ice cover). The experiments were performed in different scales at SINTEF's laboratories in Trondheim, field research station on Svalbard and in broken ice (70-90% ice cover) in the Barents Sea. Samples from the weathering experiments were tested for ignitability using the same laboratory burning cell. The measured ignitability from the experiments in these different scales showed a good agreement for samples with similar weathering. The ice conditions clearly affected the weathering process, and 70% ice or more reduces the weathering and allows a longer time window for in situ burning. The results from the Barents Sea revealed that weathering and ignitability can vary within an oil slick. This field use of the burning cell demonstrated that it can be used as an operational tool to monitor the ignitability of oil spills.

Large-scale oil-in-ice experiment in the Barents Sea: monitoring of oil in water and MetOcean interactions.

A large-scale field experiment took place in the marginal ice zone in the Barents Sea in May 2009. Fresh oil (7000 L) was released uncontained between the ice floes to study oil weathering and spreading in ice and surface water. A detailed monitoring of oil-in-water and ice interactions was performed throughout the six-day experiment. In addition, meteorological and oceanographic data were recorded for monitoring of the wind speed and direction, air temperature, currents and ice floe movements. The monitoring showed low concentrations of dissolved hydrocarbons and the predicted acute toxicity indicated that the acute toxicity was low. The ice field drifted nearly 80 km during the experimental period, and although the oil drifted with the ice, it remained contained between the ice floes.

Responses of microbial communities in Arctic sea ice after contamination by crude petroleum oil.

Microbial communities associated with Arctic fjord ice polluted with petroleum oils were investigated in this study. A winter field experiment was conducted in the Van Mijen Fjord (Svalbard) from February to June 2004, in which the ice was contaminated with a North Sea paraffinic oil. Holes were drilled in the ice and oil samples frozen into the ice at the start of the experiment. Samples, including cores of both oil-contaminated and clean ice, were collected from the field site 33, 74, and 112 days after oil application. The sampled cores were separated into three sections and processed for microbiological and chemical analyses. In the oil-contaminated cores, enumerations of total prokaryotic cells by fluorescence microscopy and colony-forming units (CFU) counts of heterotrophic prokaryotes both showed stimulation of microbial growth, while concentrations of oil-degrading prokaryotes remained at similar levels in contaminated and clean ice. Analysis of polymerase chain reaction (PCR)-amplified bacterial 16S rRNA gene fragments by denaturing gradient gel electrophoresis (DGGE) revealed that bacterial communities in oil-contaminated ice generated fewer bands than communities in clean ice, although banding patterns changed both in contaminated and clean ice during the experimental period. Microbial communities in unpolluted ice and in cores contaminated with the paraffinic oil were examined by cloning and sequence analysis. In the contaminated cores, the communities became predominated by Gammaproteobacteria related to the genera Colwellia, Marinomonas, and Glaciecola, while clean ice included more heterogeneous populations. Chemical analysis of the oil-contaminated ice cores with determinations of n-C17/Pristane and naphthalene/phenanthrene ratios indicated slow oil biodegradation in the ice, primarily in the deeper parts of the ice with low hydrocarbon concentrations.