Abstract. The main conclusions from analysis for the data collected are:

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1 Field experiments with subsurface releases of oil and dyed water Henrik Rye, Per J. Brandvik & Tove Strom SINTEF Applied Chemistry, N-7034 Trondheim, Norway Henrik. sintef.no Per.J. Abstract A field experiment with a subsurface release of oil and air was carried out in June 1996 close to the Frigg Field in the North Sea area. One of the purposes of this sea trial was to increase the knowledge concerning the behaviour of the oil and gas during a subsurface blowout. This was done by releasing oil and air at 106 meters depth with a realistic gas oil ratio (GOR=67) and release velocity of the oil. In addition to the oil release, several releases with dyed water and gas (GOR=7-65) were performed. Important and unique data were collected during these subsurface releases. In particular, the experiments with the dyed water releases combined with air turned out to be an efficient way of obtaining field data for the behaviour of subsurface plumes. The main conclusions from analysis for the data collected are: The field methodology used to study blowout releases in the field appears to be appropriate. The use of dyed water to determine the performance of the subsurface plume proved out to be an efficient way to obtain reliable and useful data. The behaviour of the subsurface plume is very sensitive to gas flow rates. For low gas flow rates, the plume did not reach the sea surface at all due to the presence of stratification in the ambient water. Some discrepancies were found between a numerical model for subsurface releases and field results. These discrepancies are pointed out, and recommendations for possible model improvements are given.

2 278 Oil & Hydrocarbon Spills, Modelling, Analysis & Control 1 Introduction As the oil industry develops and applies new technology for producing oil resources from the sea bottom, the probability for a blowout event to occur at the sea floor rather than at the sea surface increases. One of the reasons why is the use of subsurface solutions combined with floating production and storage units. These solutions are cheaper and easier to remove after the end of the production phase, but they enhance the probability of a blowout to occur at the sea floor rather than at the sea surface. The industry is therefore interested in understanding the behaviour of subsurface blowouts as well as computational tools to describe them. During the 1996 field trial at the Frigg field, 43 nf of stabilised Troll oil was released at 106 m depth over a time span of 40 minutes (approximately 17 1/s) jointly with air. The release conditions were arranged so that the release velocity and GOR (gas oil ratio, or, in this particular case, the "air oil ratio") is within a realistic range to what is expected during an early phase of a blowout event. In addition to the oil and gas release, a number of other subsurface releases were carried out at the same field trial for varying GOR (or GWR = gas water ratio), using dyed water (Rhodamine) instead of oil in the release. For some of these cases, concentrations of Rhodamine were recorded as well. As a part of the field trial, several supplementary activities were also undertaken to collect additional background information. These activities included ROY ( = remote operating vehicle) monitoring of the subsurface plume with video and sonar, video of the release at the outlet opening, sampling of oil in the slick, remote sensing of the surface slick, recordings of ambient conditions of currents, temperature and salinity and measurements of dye concentrations. 2 Description of the SINTEF blow-out model The model that was used for comparison with the field results is the same as the one used for comparison in a similar field trial carried out in 1995 (Brandvik et al.'). The results from the 1995 sea trial are also presented in Rye et. ala The model used is a combination of two other subsurface release models published in the literature (Koh and Fan\ Fannel0p and Sj0en^). These two models are complementary in the sense that they both model subsurface releases, but they cover different aspects of the problem. Both models simulate the mixing of a subsurface jet based on the principle of conservation of mass, momentum and buoyancy. The

3 Oil & Hydrocarbon Spills, Modelling, Analysis & Control 279 Koh and Fan model includes arbitrary stratification combined with an arbitrary orientation of the outlet opening. The Fannel0p and Sj0er/ model includes the features of an expanding gas present in the plume as well as a description of the resulting surface flow and oil slick thickness generated by the subsurface plume. The present version of the model assumes a constant GOR (gas oil ratio, Sm^/Srn^) throughout the water column, although some evaporation of the most volatile components must be expected during the rising of the subsurface plume. In the model, adiabatic expansion of the gaseous components is assumed until the water and gas temperatures are equal. An illustration of the subsurface plume is given in Figure 2.1 (from Fannel0p and Sj0en*). The model is formulated by means of integral versions of the conservation principles (mass, momentum and buoyancy). The equations are formulated in such a way that the changes of these quantities are calculated along the plume path. The initial conditions of the mass flux, the momentum, the angle with the horizontal plane and the buoyancy (density) have to be specified. The equations are then solved numerically along the plume path by means of a numerical integration method (Runge-Kutta of the fourth order). Note that at the surface, the subsurface plume will be converted to a surface plume that will spread radially outwards from the point that the subsurface plume reaches the sea surface. The propagation of the radial outflow away from the subsurface plume area may be limited if the ambient water mass has a density different from that in the plume. In the summer, the water mass in the surface plume is generally colder than the surface water. When the plume velocities become sufficiently low, the plume will tend to sink. Fannel0p and Sj0en* suggested in their work that this will happen when where v is the velocity of the radial outflow at the sea surface and hw is the thickness of the surface plume. The symbol g' is the "reduced gravity" which is the gravity g times the ratio between 1) the density difference between plume water and ambient water and 2) the density of the ambient water. This criterion can also be formulated in terms of a densimetric Froude number smaller than one, where the densimetric Froude number is defined as the (square root of the) ratio between the left and the right hand sides of the inequality (2.1).

4 280 Oil & Hydrocarbon Spills, Modelling, Analysis & Control Surface flow Interaction zone Buoyancy profile, p (r,z) Sea bed Virtual origin Figure 2.1. An illustration of a subsurface plume as presented by Fannel0p and Sj0en\ Equation (2.1) expresses an imbalance between buoyancy forces and momentum forces. The criterion for plume sinking (buoyancy dominates over momentum) becomes then the fulfilment of the inequality (2.1). The level of plume sinking below the sea surface may be approximated by the depth where the density of the surface plume flow equals the density in the recipient. Further details of the model used for comparison can be found in Rye et.al.*\ 3 Ambient conditions The experiment took place in June 1996 in the North Sea area, under moderate weather conditions. Weather: Cloudy, low cloud base, no rain

5 Oil & Hydrocarbon Spills, Modelling, Analysis & Control 281 Wind strength: 9-10 m/s at 3 m height Wind direction: From dir (from W-NNW) Wave height: Sign, wave height m One interesting feature of the ambient conditions is the water stratification, that is, the vertical variation of temperature and salinity, This stratification was recorded by a CTD sensor (Conductivity, Temperature and Depth recording unit) from a nearby ship. The stratification in the water mass remained relatively stable during the experiment. The profile recorded at 0800 GMT at 12. June 1996 (one hour prior to the oil release) is shown in Figure 3.1. The profiles show the presence of a salinity gradient at m depth, increasing by about 0.45 ppt over this range. The temperature shows a more gradual variation, with homogeneous water mass in the upper 20 meters of the water column. 4 Subsurface blow-out simulations with dyed water and air Some of the experiments were carried out with dyed water (Rhodamine) and air. The purpose of this effort was to observe the presence of the plume at the sea surface qualitatively and to record concentrations in the plume. Dilution factors could then be calculated and compared to model results. Trials were made with four different GWR's (gas water ratios): 7.25, 18, 46 and 67 SnrVSm\ For all the cases, the release rate of sea water (or oil) was 1 mvminute. For the two lowest release rates of gas (GWR = 7.25 and 18), no plume was observed at the sea surface at all. For the GWR = 7.25 case, the plume was observed by the ROV to diminish at about m depth. Also, the numerical model gave similar results, with a maximum plume rise level at about 30 m depth. The reason why was found to be an effect of the water stratification. The buoyancy induced by the presence of the air bubbles is counterbalanced by the negative buoyancy caused by the entrained water in the plume from below (which is heavier than the ambient sea water). When the subsurface plume approaches the sea surface, the buoyancy of the plume changes to be negative. For the GWR = 46 case, the plume did appear at the sea surface. This gave an opportunity to record concentrations in the plume that could form a basis for estimation of dilution factors for the subsurface (and surface) plume.

6 282 Oil & Hydrocarbon Spills, Modelling, Analysis & Control Temperature, *C Salinity, ppt , ,935 35,1 E. Density In sigmat unit 26, ,7 26,8 26, , ,4 Figure 3.1. Temperature, salinity and density recorded on the site at 12. June 1996 at 0800 GMT. Temperature in deg. C. Salinity is given in ppt. Density is given in sigmat units = density in kg/nf minus 1000 kg/nf. Data from Johansen*.

7 Oil & Hydrocarbon Spills, Modelling, Analysis & Control 283 The 40% Rhodamine dye was added to the release through the sea water with a rate equal to 375 ml/min (±5%). With a sea water release rate equal to 1 mvmin, the concentration in of active Rhodamine in the release was 150 ppm. The concentrations in the water mass were then recorded by towing the UV Fluorescence recording units through the resulting plume at the sea surface. The measurements indicate concentrations with maximum values close to 25 ppb in m depth and maximum values close to 15 ppb in 1-10 m depth. These concentrations indicate a dilution factor of order 1:6000-1: With a release rate of 1 nrvmin, this dilution then amounts to a sea water flux in the surface plume equal to order nrvs. The modelled value for the Rhodamine concentrations are given in figure 4.1 and are in the range of 1-8 ppb. The modelled volume flux of sea water is given in figure 4.2 and is approximately 300 nf/s at the front of the surface plume. The transects of the Rhodamine concentration measurements show significant changes with time, indicating that the plume is very patchy with local high concentration in the patches and very low concentrations in between. Also, the concentrations appear with high numbers in all the 5 depths recorded that were 1,5, 10, 15 and 20 m depth. Figures 4.1 and 4.2 show the results from calculation of concentration in the surface plume and the sea water flux in the surface plume as formulated by Fannel0p and Sj0en\ Concentrations of Rhodamine in surface plume Radial distance in m Figure 4.1.Calculation of the average concentration of Rhodamine in the surface plume by means of the numerical model. GWR = 46.

8 284 Oil & Hydrocarbon Spills, Modelling, Analysis & Control Transactions on Ecology and the Environment vol 20, 1998 WIT Press, ISSN J» 1500 c x 1000 ul 500 Sea water flux in surface plume Radial distance In m Figure 4.2. Calculation of the volume flux of sea water in the surface plume by means of the numerical model. GWR = 46. By comparison between measurements and calculations, we obtain a larger surface plume water flux in the model than estimated from the measured concentrations. The concentrations are therefore smaller to a similar extent, calculated to be 1-8 ppb, dependent on the distance from the start of the surface plume. The patchiness of the observed concentrations may explain some of the discrepancy between the measurements and the calculations, because the model only calculates the average concentrations and not local variations in the vertical and in time. However, the averaged level of the concentrations measured appears still to be somewhat larger for the measurements, compared to the calculations. One interesting feature of the Rhodamine recordings was that the concentrations in the surface plume appear to be larger at m depth than at 1-10 m depth. This is surprising, because the surface plume is not expected to penetrate down to m depth in the vicinity of the subsurface plume. Therefore, the vertical extent of the surface plume appears to be deeper than the calculations indicate. One possible explanation may be effects from subsidence of the surface plume, which is not accounted for by the theory used in the modelling. An interesting additional feature of the recordings carried out was the registration of the salinity and the temperature of the surface plume by means of buoys and IR images from air planes. These data gave valuable supplementary information which proved to be useful in interpreting the results from the field trial. The IR images indicated a horizontal extent of

9 Oil & Hydrocarbon Spills, Modelling, Analysis & Control 285 the surface plume with diameters of order 200 metres before subsidence, which is somewhat smaller than what the model calculations predict. The measurements thus gave a somewhat different picture of the circulation patterns close to the upstream area. An interpretation of the circulation pattern anticipated is shown in figure 4.3. Surface plume /tegner/hyie pkjrnel Figure 4.3. Illustration of the current pattern interpreted from the field trial in 1996 with GWR = 46. The establishment of the cell brings the surface plume down below the sea surface. After the subsidence, the plume is expected to spread out at a depth where the density of the plume equals the density of the ambient water mass. The practical implication of the subsidence of the surface plume is that this would stop or limit the surface spreading of oil at the sea surface. Thus, the spreading of the oil (and the corresponding size of the slick) becomes therefore reduced. This finding makes it questionable to use the Fannel0p and Sj0en^ theory for the size of the surface slick in stratified waters. 5 Oil and air release for the GOR = 67 case For the oil and air release, the size of the surface slick was recorded my remote sensing. In addition, the thickness of the oil slick was determined by means of pads absorbing the oil on the sea surface. Comparison between measurements and calculations showed that the calculated oil slick dimensions in the horizontal are larger than the observed ones by a factor of order 2-3 (upstream penetration, width of the slick). The reason for this is attributed to the behaviour of the surface plume below the slick. The surface plume in particular appears to behave differently in stratified water mass, compared to the behaviour in homogeneous (non-stratified) waters. Significant factors in stratified water mass are influences from entrainment into the radiant surface flow from the water mass below, and re-entrainment and submergence of the

10 286 Oil & Hydrocarbon Spills, Modelling, Analysis & Control surface plume. This part of the model should thus be revised in order to improve on the comparison with the measurements. The results from the present field trial resulted in a slick thickness that was less than predicted by existing theory. The reason for this is that the bulk of oil did not reach the sea surface, but was contained within the surface water plume (below the slick). One possible explanation for this is that details in the release arrangement or the size of the release velocity contributed to create a large amount of small droplets with a small ability to rise through the water mass in the plume. This design of the release arrangement may not have been sufficiently realistic. During the IXTOC I blow-out case in the Gulf of Mexico, the release velocity was probably larger (may be of order 100 m/s) than in the 1996 field trial (of order m/s). The bulk of oil in the IXTOC I case was found at the sea surface and not contained within the water mass. The explanation why the oil did not reach the sea surface in the present field trial but did so in the IXTOC I case should therefore be looked into further. Further details of the oil and air release results and comparison with simulation results are also found in Rye et. al/. 6 Conclusions The recordings carried out in the field showed that the behaviour of the surface plume appeared to be different from what was expected from the Fannel0p and Sj0en^ theory. The reason why is attributed to effects from the stratification in the water masses. Based on the results from the field trials, the following behaviours of the subsurface and the surface plumes can be anticipated for stratified waters: 1. Low release flux of gas combined with ambient stratification. The subsurface plume does not reach the sea surface at all. No surface plume. 2. Higher release flux of gas combined with stratification. The subsurface plume reaches the sea surface. Provided that the criterion given by equation (2.1) is fulfilled, the surface plume will sink down immediately and form a closed cell at and below the sea surface (Densimetric Froude number is always smaller than one for the surface plume) 3. Higher release flux of gas combined with weaker stratification. The subsurface plume reaches the sea surface. Provided that the criterion given by equation (2.1) is not fulfilled, the surface plume will spread out on the sea surface. When the criterion given by the equation (2.1) is fulfilled, the surface plume will sink down below the sea surface (Densimetric Froude number larger than one in the closest vicinity of

11 Oil & Hydrocarbon Spills, Modelling, Analysis & Control 287 the area where the subsurface plume reaches the sea surface). This situation corresponds to what is illustrated in figure No stratification. The surface plume will not sink at all. This case corresponds to the Fannel0p and Sj0en* case. The inequality (2.1) is never fulfilled because g' is zero. This situation corresponds also to what is illustrated in figure Acknowledgements The field work could not have been carried out without the close cooperation and the financial support from a number of different bodies and organisations. The financial support from NOFO and Norsk Hydro is hereby acknowledged. Assistance from NOFO, Norsk Hydro, Oceanor, Maritex, SFT (aircraft) and the German Marine Pollution Control Unit (aircraft) are also greatly acknowledged. 8 References ' Brandvik, P.J., Str0m-Kristiansen, T.S., Lewis, A., Baling, P.S., Reed, M., Rye, H., Jensen, H. 1995: Summary report from the NOFO 1995 oil-on-water exercise. IKU report no /01/ p. * Rye: "Subsurface Oil Release Field Experiment - Observations and Modelling of Subsurface Plume Behaviour". Proceedings, 19th Arctic and Marine Oil Spill Program (AMOP), June 12-14, Calgary, Canada. * Koh and Fan: "Mathematical Models for the prediction of Temperature Distribution resulting from the Discharge of Heated Water into Large Bodies of Water". EPA Report, Water Quality Office, October Fannel0p and Sj0en: "Hydrodynamics of subsurface blowouts". Norwegian Maritime Research, No. 4, pp 17-33, * Johansen, 0., 1996: "NOFO olje pa vann 0velse juni Milj0datamalinger". (In Norwegian). Report prepared by Oceanor, Trondheim, Norway. ^ Rye: "Model for Calculation of Underwater Blow-out Plume". Proceedings, 17th Arctic and Marine Oil Spill Program (AMOP), June 8-10, Vancouver, Canada. ^ Rye et al., 1998: Subsurface Blowouts. Results from Field Experiments", Paper to be published in: Spill Science and Technology Bulletin, Elsevier, 1998.