Cortis K. Cooper

DeepSpill––Field Study of a Simulated Oil and Gas Blowout in Deep Water

Abstract With the world’s increasing demand for oil and gas and dwindling onshore reserves, the need to exploit oil and gas has moved into deep water. This move brings with it the potential of accidental releases from well blowouts and pipeline or riser ruptures. While there is a low risk of such accident thanks to today’s technology, the oil industry has to be prepared. To better understand how oil and gas would behave during a deep water release, the DeepSpill experiment was conducted in the Norwegian Sea at the Helland Hansen site (65°00′N, 04°50′E) in 844 m of water roughly 125 km off the coast of central Norway. Four controlled discharges of oil and gas were made during late June 2000 amounting to a total of 120 m3 of oil and 10,000 standard m3 of natural gas. The main objectives of the experiments were to calibrate numerical models and to test methods of subsurface surveillance. Extensive observations were made of wind, currents, water density, surface and subsurface oil concentrations, and chemical and biologic samples in the water column. Results showed that the oil started reaching the surface about an hour after the release began and within a few hundred meters of the release site. Oil continued to surface for several hours after the release stopped. No gas hydrates were formed even though thermodynamic equilibrium suggested they should have. No gas bubbles reached the surface indicating that gas dissolution was complete but not as quickly as predicted by standard algorithms. The echo sounders on-board the research vessels were able to track the oil/gas plume as it rose through the water column. In general the surface slick was much thinner than a slick initially released at the surface would have been. Emulsified oil was observed at the surface after the crude oil discharge, with water content increasing with time after the oil came to the surface. An integral plume model [Spill Science and Technology Bulletin 6 (2000) 103] did a reasonable job of predicting the time to surface and the location of the slick though some tuning of the bubble/droplet sizes, gas dissolution rate, and hydrate formation were needed. Finally, the results showed that all gas was dissolved well beneath the surface suggesting that today’s safety restrictions governing surface vessel activity could possibly be revised.

Evolution and kinematics of a loop current eddy in the Gulf of Mexico during 1985

The large eddy that broke off from the Loop Current in July 1985 was the most extensively studied eddy ever to appear in the Gulf of Mexico. Other investigators have described its early evolution based on Lagrangian drifters and its later evolution using moored current meters in the western gulf. This paper provides additional insight on the early evolution of the eddy using results from air dropped expendable bathythermographs and air dropped expendable current profilers in early May, a hydrographic ship survey in mid July, and a detailed ship survey in August using expendable bathythermographs and a current profiler. The May survey established a center of circulation at about 26°N but showed that the eddy had not separated from the Loop Current. A maximum velocity of 171 cm/s was observed near the northern edge of the feature. The evidence suggests that a large elongated eddy then separated from the Loop Current and later split into two smaller eddies. The July hydrographic cruise showed a clear separation of the large eddy from the Loop Current to the southeast. Two weeks later, the August survey showed an asymmetric eddy, with the maximum surface current 178 cm/s south of the center of circulation and 132 cm/s to the north. A western eddy named Ghost Eddy then separated from an eastern eddy named Fast Eddy. Using the current profiles and data from the drifters, we constructed a simple kinematic feature model for eddies in the Gulf of Mexico.

Intercomparison of oil spill prediction models for accidental blowout scenarios with and without subsea chemical dispersant injection.

We compare oil spill model predictions for a prototype subsea blowout with and without subsea injection of chemical dispersants in deep and shallow water, for high and low gas-oil ratio, and in weak to strong crossflows. Model results are compared for initial oil droplet size distribution, the nearfield plume, and the farfield Lagrangian particle tracking stage of hydrocarbon transport. For the conditions tested (a blowout with oil flow rate of 20,000 bbl/d, about 1/3 of the Deepwater Horizon), the models predict the volume median droplet diameter at the source to range from 0.3 to 6mm without dispersant and 0.01 to 0.8 mm with dispersant. This reduced droplet size owing to reduced interfacial tension results in a one to two order of magnitude increase in the downstream displacement of the initial oil surfacing zone and may lead to a significant fraction of the spilled oil not reaching the sea surface.

A Regional Data‐Assimilative Model for Operational Use in the Gulf of Mexico

In this chapter, we describe the results from a multi-year study to develop and test a regional data-assimilative forecast/hindcast model of the Loop Current and associated eddies in the Gulf of Mexico. We start by describing the forward model and its skill in reproducing the broad statistical features of the Loop Current and its variability. The model is then applied to hindcast the 1993-2001 period. The model skill in replicating the Loop Current eddy shedding events, and associated warm- and cold- eddy movement is examined. Finally, the model is extended to include part of the Caribbean Sea, and run in both hindcast and nowcast/forecast modes to assess its skill in reproducing the Loop Current and Loop Current eddy fronts using measures that quantify error in the distance of the front to a fixed site (i.e. an oil platform). While the results can no doubt be improved further by higher model resolution and more altimetric data, the model has useful skill for operational applications, and is now being run daily for operational use by the offshore industry.

The Use of Satellite Altimeter Data to Estimate the Extreme Wave Climate

Abstract Since 1986, nine years of wave data derived from satellites have been accumulated, and this database will expand dramatically in the next two years as two more satellites are added. Several researchers have begun using this data to estimate extreme value statistics for waves. However, one potential problem with satellite data is space–time resolution, which is a poor match for the scales of storms. Satellites only revisit a site once every 10–35 days, and their tracks are separated by 100–200 km. With this coarse sampling, the satellite may miss storms since they have characteristic length and time scales as short as a few hours and tens of kilometers. The purpose of this paper is to explore the impact of this undersampling on the calculated 100-yr wave height. This is accomplished by running Monte Carlo simulations of simplified but realistic storms sampled by a simulated satellite and site. The authors study the sensitivity of the calculated 100-yr wave to variations in storm type, radius, and ...

Newly recognized turbidity current structure can explain prolonged flushing of submarine canyons

Runaway turbidity currents stretch into the deep ocean to form the largest sediment accumulations on Earth. Seabed-hugging flows called turbidity currents are the volumetrically most important process transporting sediment across our planet and form its largest sediment accumulations. We seek to understand the internal structure and behavior of turbidity currents by reanalyzing the most detailed direct measurements yet of velocities and densities within oceanic turbidity currents, obtained from weeklong flows in the Congo Canyon. We provide a new model for turbidity current structure that can explain why these are far more prolonged than all previously monitored oceanic turbidity currents, which lasted for only hours or minutes at other locations. The observed Congo Canyon flows consist of a short-lived zone of fast and dense fluid at their front, which outruns the slower moving body of the flow. We propose that the sustained duration of these turbidity currents results from flow stretching and that this stretching is characteristic of mud-rich turbidity current systems. The lack of stretching in previously monitored flows is attributed to coarser sediment that settles out from the body more rapidly. These prolonged seafloor flows rival the discharge of the Congo River and carry ~2% of the terrestrial organic carbon buried globally in the oceans each year through a single submarine canyon. Thus, this new structure explains sustained flushing of globally important amounts of sediment, organic carbon, nutrients, and fresh water into the deep ocean.

Metocean Extreme and Operating Conditions

Metocean stands for meteorology and oceanography, an acronym that is commonly used in the offshore oil industry to encompass almost all topics involving the quantitative description of the ocean and atmosphere needed to design and operate man-made structures, facilities, and vessels in the ocean or on the coast. The metocean environment controls many aspects of facility design and operation, so errors in quantifying metocean conditions can cascade through the design and operational decisions. Errors can result in damage and lost lives. Conversely, if the variables are overestimated, costs will be overestimated perhaps to the point that the project becomes uneconomic and is never built.

Real-time Ocean Surface current measurements in the Gulf of Mexico

This paper reports on a series of airborne data collections we made in the Gulf of Mexico using the Remote Ocean Current Imaging System (ROCIS) to measure surface currents in real-time. ROCIS provides high spatial resolution over large areas of the ocean in a way not previously available with existing technologies. Such current measurements can be employed for improved regional ocean forecasting and a better understanding of mesoscale processes.

Development of Gulf of Mexico Deepwater Currents for Reference by API Recommended Practices

The metocean conditions contained in the 21st edition of API RP2A, last updated in 1993, are in the process of being revised to account for: the effects of recent major hurricanes, the shift of production to deeper water, and improvements in our understanding of metocean conditions in US waters. As the Oil Industry has moved into the deeper Gulf of Mexico waters, it has become exposed to strong currents generated by the Loop Current, its associated eddies, and by topographic Rossby waves. This paper describes the basis for the draft extreme conditions we have developed for these strong deepwater ocean currents. Further work is underway to develop conditions for extreme near-bottom currents on the continental slope and for joint hurricane-Loop currents. Once accepted by API, the conditions will ultimately be published as part of a stand-alone API recommended practice (RP) which will in turn be referenced by other API recommended practices such as those addressing shallow-water fixed platforms, jack-ups, deepwater platforms, and floating MODUs. The Metocean RP will also include hurricane-generated conditions the development of which is documented in a separate paper (Berek, et al. 2007).Copyright

A general model for the helical structure of geophysical flows in channel bends

Meandering channels formed by geophysical flows (e.g. rivers and seafloor turbidity currents) include the most extensive sediment transport systems on Earth. Previous measurements from rivers show how helical flow at meander bends plays a key role in sediment transport and deposition. Turbidity currents differ from rivers in both density and velocity profiles. These differences, and the lack of field measurements from turbidity currents, have led to multiple models for their helical flow around bends. Here we present the first measurements of helical flow in submarine turbidity currents. These ten flows lasted for 1-to-10 days, were up to ~80-metres thick, and displayed a consistent helical structure. This structure comprised two vertically-stacked cells, with the bottom cell rotating with the opposite direction to helical flow in rivers. Furthermore, we propose a general model that predicts the range of helical flow structures observed in rivers, estuaries and turbidity currents based on their density stratification.