Spill behaviour of Maui B Crude oil (offshore Taranaki, New Zealand) under simulated wind & wave conditions

Abstract

The physical crude oil properties currently known for New Zealand's Offshore Oil Spill Contingency Plan are limited to those routinely measured by oil producers and refiners. These are not the properties that an On-Scene-Commander (OSC) needs to know immediately after a spill has occurred. What needs to be known is the extent of evaporation, the extent of natural dispersion either naturally or with dispersants, whether water-in-oil emulsions form, if the oil is likely to sink or submerge and the viscosity of the oil at ambient temperature and as it evaporates. Further, all levels of contingency planning utilise spill surveillance or tracking programes that rely on the rule of thumb that oil travels at 3% of the wind speed. New Zealand's oil spill response capabilities could be improved significantly by ascertaining the various behaviours of indigenous crude oil when spilled in seawater, including its weathering properties and the accuracy of the standard 3% of wind speed rule, currently assumed for New Zealand's indigenous crude oil. All the above information may allow for better management of environmental risk, especially with potential loss of integrity of large storage facilities. In order to assess environmental risk and plan emergency response, flume tank tests were conducted to determine the behaviour of Maui B crude oil under simulated diverse Taranaki environmental conditions, including variations of wind, wave, combined wind and wave, and temperature. Also, oil behaviour testing was conducted to determine 2 of the most important weathering properties of the oil: emulsion formation and, rate of evaporation. New Zealand's Offshore Oil Spill Contingency Plan simply states that the Maui B crude oil cannot be dispersed chemically so an additional test objective was to determine the capability of 2 chemical dispersants (Tergo R40 and Corexit 9527), an organic cleaner (PES-51i) and other oil cleanup aids (Matasorb and oil snares). All experiments were replicated using Light Arabian crude oil, as a typical oil for comparison. Specifically, the overall objectives of this research were to develop and conduct a laboratory test programme to investigate, using suitably engineered and constructed equipment, the following: 1. Effect of wind on Maui B crude oil in warm and cold ocean conditions; 2. Effect of waves on Maui B crude oil in warm and cold ocean conditions; 3. Effect of combined wind and waves on Maui B crude oil in warm and cold ocean conditions; 4. Confirmation that the Maui B crude oil does not form an emulsion; 5. The rates of evaporation; 6. Maui B crude oil downwind velocity; 7. Effectiveness of the chemical dispersants Tergo R40 and Corexit 9527 on both weathered and unweathered Maui oil; 8. Effectiveness of the organic cleaner PES-51i on Maui B crude oil; and 9. Effectiveness of cleanup aids such as Matasorb (absorbent) and oil snares on weathered and unweathered Maui B crude oil. Maui B crude oil was chosen as the test medium, because of interest in its behaviours if released to the environment. It is specifically of interest because it is the only crude oil currently being produced and stored (in reasonable large quantities) offshore New Zealand, and the environmental risk associated with the potential spillage of large quantities of crude oil to water from the Maui B storage facility, during loadout or storage or from a leaking underwater pipeline is relatively high. Further, the Maui B crude oil has a moderate wax content (14%), and this oil is 'assumed' to travel at 3% of me wind speed. It is important to note that the samples of Maui B crude oil used in the laboratory flume tank experiments had undergone an initial 2-phase separation (separating gas from liquids), followed by processing in a heater treater to remove produced water from the crude oil. Also, the Maui B crude oil has had de-emulsifiers and asphaltene inhibitors injected at the well head as part of the production process. Therefore, the crude oil samples and their associated test results are representative of the behaviours of the treated Maui B crude oil that poses the highest risk to the environment, if released from the storage facility. Local seasonal temperature has a significant effect on the Maui B crude oil, as the oil's pour point (18-21°C) is at the summer ocean temperature. This in turn is due to its moderately high wax content, which begins to crystalise at temperatures below the pour point. The wax forms a stabilising element by offering rigidity to the compounds and when in the ocean environment, this wax imparts visco-elastic behaviours. Under local Taranaki summer conditions (warm 18°C) the Maui B crude oil displays strong visco-elastic behaviour, travels at velocities close to the standard 3% of wind speed and forms very few oil-in-water emulsions. The slower speed in warm conditions (18°C) is considered to be caused by the visco-elastic properties, and with the oil being at its pour point temperature. Thus, the oil simply stretches as the wave rolls through rather than being moved by the wave. Under local winter conditions (cold 13°C), Maui oil travels above the 'assumed' standard 3% of the wind speed at 3.7%. The increase in speed under cold conditions is a combination of the oil being below its pour point, the moderate wax content, and suppression of the visco-elastic properties. Under both warm and cold conditions with wave action, the wave energy causes the oil to form semi-solid lumps, considered 'tar balls'. The temperature variation that can be expected off the coast of Taranaki will have a considerable effect on the behaviour of Maui B crude oil in a spill situation. The Maui B crude oil does not form an emulsion. Maui B crude oil forms a water-oil mixture called "entrained water" when spilled into the ocean. This entrained water formation is a result of favourable energy inputs and oil properties. However, although the de-emulsifier "Aquanox" has been added to the crude oil, the necessary quantity of compounds needed to form an emulsified oil are missing naturally(as stated by Fingas and Fieldhouse, 1999) from the oil properties. Thus, should no de-emulsifier have been added, the Maui B crude oil is still not expected to form an emulsion but rather an entrained water mixture. It is the low quantity of asphaltenes and resins in the Maui B crude oil, which means that the stabilising class of compounds necessary for an emulsion to form is missing. The visco-elastic properties displayed in the Maui B crude oil means that this entrainment mixture will perpetuate, resulting in the oil remaining in this state over time and with increased energy inputs. Measurements of the absolute viscosity of Maui B crude oil at the end of the emulsion experiment (after 12 hours) showed that its viscosity on the water surface increased between 207 and 227 times, viscosity of the oil at the end of the tank on the wave energy dissipation board (location of highest energy) increased by 2,372 times and the viscosity of the oil on the sides of the tank increased by 791 times. Experiments conducted on the rate of evaporation of Maui B crude oil indicate that most of the evaporation occurred over the first 60.7 hours, at which time 25.2% of the oil was evaporated. In comparison the rate of evaporation of Light Arabian crude oil showed that most of the evaporation occurred over the first 48.6 hours with 15.1% of oil evaporated. Fingas (1995) predicted the evaporation of crude oils by the equation: Ev = Tin(t) Ev = Percentage evaporating per unit of time T = Temperature t = Time However, the Maui B crude oil did not fit this curve. An evaporation model was developed for both the Maui and Light Arabian crude oils, which had the equation: where C is a constant specific to each oil. C = 2.7 for Maui B crude oil C = 3.5 for Light Arabian oil The effects of 2 chemical dispersants were tested, as it is currently assumed that the Maui B crude oil is not dispersable via chemical dispersants applied to the oil on water. New research (NETCEN, Australia) has raised some uncertainties over this assumption. The dispersant Tergo R40 was effective at dispersing the Maui B crude oil under cold (13°C) water temperatures. However, with induced mixing only a few oil droplets were dispersed throughout the water column. Under higher energy inputs significant dispersion may occur. Testing showed that the dispersant Corexit 9527 did not effectively disperse the Maui B crude oil. The organic cleaner, PES-51I, was tested as interest has been raised over its capabilities of washing oil from rocky shorelines. PES-51i proved effective at lowering the viscosity of oil and removing oil from rocks and their crevices. However, Swirling Flask Tests and toxicity tests conducted by Environment Canada place PES-51i above the acceptable Canadian toxicity threshold values. Therefore, its use is not recommended. Matasorb, an absorbent, was tested as during the experiments, interest was raised over Maui B crude oil's absorbtion properties. Matasorb as a cleanup aid, worked via adsorption of entrained Maui B crude oil rather than absorption, for which it was designed. However, the Matasorb did manage to remove semi-solid lumps of oil from the water surface. The oil snares proved by far the most effective method for removing weathered and unweathered oil from the water surface, adsorbing up to 80% of the crude oil. Due to the lack of quality information available on the weathering and spill behaviour of the Maui B crude oil and Light Arabian crude oil, correlation with independent results could not be performed at this time. However, Environment Canada, Emergencies Sciences Division will be running a full suite of spill behaviour laboratory analysis on the Maui B crude oil. These results are not yet available. Based on the results from the flume tank laboratory experiments on the behaviours of Maui B crude oil under various environmental conditions (simulating leakage or spillage of Maui B oil to seawater and wind and wave action) it is recommended that an appropriate oil spill response might be to boom the oil regardless of sea conditions (this will at least help to contain a large quantity of the spill), and to mop-up using oil snares. These snares could later be burned via controlled incineration. Another option could be to use vacuum pumps to suck up the oil from the water, separate the oil and water, if possible, recycle the oil, and dispose of the water with the processed produced water. Under warm and cold conditions both the above methods would prove effective whereas it is doubted whether methods such as mechanical recovery would work on the Maui B crude oil under cold environmental conditions.