Guttorm Alendal

Two‐phase, near‐field modeling of purposefully released CO2 in the ocean

Injection of carbon dioxide (CO2) in the ocean has been proposed as an option for accelerating the natural net flux of CO2 from the atmosphere into the ocean. Liquid CO2 released as droplets at depths <3000 m will create an ascending plume of droplets and entrained water. As the CO2 droplets dissolve, carbon is transferred into the plume water, yielding increased density and a lowered pH value of the plume water. As ambient water entrains the CO2-enriched water by mixing, the density difference disappears and the injected CO2 follows the ocean dynamics as a dynamically passive tracer. Here we report on numerical experiments performed with a two-phase Navier Stokes solver. The effects of different droplets sizes, background currents, and injection rates are examined. The numerical experiments show that the droplet size and the background current are key parameter for predicting the vertical distribution of the plume water, the associated reduction in the pH field, and the increase in the plume water density. If rapid dilution of the CO2-enriched water is the objective (leading to modest reduction in the pH value), large initial droplets and high background currents are preferable. On the other hand, if the objective is to increase the density of the plume water in order to generate a sinking plume (yielding enhanced residence time of the released CO2), CO2 injection with small droplets in a stagnant water column is optimal.

Ocean release of fossil fuel CO2: A case study

The natural ocean uptakeof the greenhouse gas CO2can be accelerated by collecting and liquefying the gas from point sources, and bypumpingit intothe ocean at ap- propriate locations and at sucient depths. Results from a numerical modelling system indicate that injection sites lo- cated at about 1,000 m depth in the eastern Norwegian Sea lead to ecient and long term sequestration in the abyss Atlantic. For a release rate corresponding to the CO2emis- sions from a 220 MW gas power plant, it is found that the volume of the near-source water with a pH-reduction0:1 is0.5km 3 . Thesendings,togetherwithavailabletechnol- ogyandfeasibleeconomics, indicatethattheNorwegianSea represents a possible location for large scale demonstration of operational ocean release of CO2. Steadilyincreasingdemandsforenergycausedbyagrow-

Dissolution of CO2 in the ocean

Abstract A numerical model for plumes of dissolving CO 2 droplets has been developed to investigate the efficiency and environmental impact of disposal at 500–2000m depth. The model results indicate that the fate of disposed CO 2 is strongly dependent on droplet size. If small droplets can be created and maintained, open ocean outlets may become a practical disposal technique. However, the pH of water peeling off from the plume is so low that marine life outside the plume would be affected.

A bottom gravity current model for CO2-enriched seawater

Abstract A bottom gravity current model, including chemistry, has been used to study the behaviour of heavy, CO 2 -enriched seawater released on a sloping seabed. The model has been fitted to the emission of CO 2 from a 2 GW gas power plant, and the CO 2 -enriched seawater is released at a depth of 300 m. A plume is modelled for three different areas; the Norwegian Sea, the north Atlantic and the north Pacific. Temperature and salinity profiles have been used to simulate the vertical density gradient for each of the areas. The numerical values of the different model parameters are discussed, and special attention is paid to the bottom friction parameter. It is argued that natural bottom gravity currents need a bottom friction parameter between 0.01 and 0.1, which is one to two orders of magnitude higher than the drag coefficient commonly used in tidal modelling. It is shown that a friction parameter near 0.01 leads to quick dilution of the plume, but that the plume may reach the deep ocean if the friction parameter is near 0.1. Furthermore, it is shown that the density profiles in the Atlantic and Pacific tend to reduce the excess plume density, whereas a plume released at 300 m depth in the Norwegian Sea may transport CO 2 to great depths.

Low shear turbulence structures beneath stress-driven interface with neutral and stable stratification

This study deals with turbulent flow below a stress-driven interface with stable density stratification, as a simplified configuration of a turbulent flow below a calm gas-liquid interface with wind forcing. A simple shear stress, which is constant in time and uniform at the interface, is enforced, hence only turbulence below the interface is studied. A flat interface is assumed to simplify transport phenomena beneath the interface by limiting the strength of the enforced wind stress and avoiding violent wave motions. A direct numerical simulation technique is employed to obtain three-dimensional turbulence structures below the interface. Results from the present study indicate two major changes of turbulence structures below the sheared interface in the presence of stable density stratification; one is a reduction of the Reynolds stress at the near-interface region, which leads to a coincidental increase of mean velocity gradient. The other effect of stable stratification is the quantitative variation of...

LES study of flow around a CO2-droplet plume in the ocean

Abstract Transport of liquid CO 2 to depths of 500–2000 m, and subsequent solution of CO 2 in the sea water, is one of several CO 2 mitigation options which have the potential to become both technically and economically feasible, and environmentally acceptable. A three dimensional Large Eddy Simulation (LES) code with conservation equations for salt and total carbond concentration is used to simulate spreading from a CO 2 dropet plume, in an ocean linearly stratified through a salinity gradient of 0.025 psu km −1 . Results from an integrated model for release of CO 2 droplets rising in the water column are used to prescribe the carbon source for the LES model. For a plume with release of 25 kg-CO 2 s −1 the CO 2 enriched water spreads out at a neutral depth in a layer with center at a depth about 300 meter below the release port ranging from an upper bound at the center of the plume down to 1600 meter. The pH value in this layer becomes low and will affect the marine life.

Simulating spatial and temporal varying CO2 signals from sources at the seafloor to help designing risk‐based monitoring programs

Risk-based monitoring requires quantification of the probability of the design to detect the potentially adverse events. A component in designing the monitoring program will be to predict the varying signal caused by an event, here detection of a gas seep through the seafloor from an unknown location. The Bergen Ocean Model (BOM) is used to simulate dispersion of CO2 leaking from different locations in the North Sea, focusing on temporal and spatial variability of the CO2 concentration. It is shown that the statistical footprint depends on seep location and that this will have to be accounted for in designing a network of sensors with highest probability of detecting a seep. As a consequence, heterogeneous probabilistic predictions of CO2 footprints should be available to subsea geological CO2 storage projects in order to meet regulations.

Gas exploration beyond the shelf break: An oceanographic challenge

Norways second largest gas field, Ormen Lange, is located 140km west off Kristiansund at an unprecedented depth when it comes to exploration. It will be the first Norwegian project beyond the shelf break. Exploration and development of the field is thus a challenge. An important issue during the planning stage is to understand the current conditions and hydrography of the site. This is especially important regarding pipeline design, deployment and operations. A complicating factor for estimating design currents is the extreme roughness of the local topography. Submarine slides have produced escarpments and sea mounts with height variations of up to 100m. The hydrography seems to be equally complex; in situ moorings have revealed strong variations in current speed and temperature close to the seabed. A variety of numerical experiments have been and are being set up in order to recapture and if possible forecast the observed variability. The results show that the flow is influenced by the inflow of Atlantic Water, tides, atmospheric forcing and by flow of water masses inside the Norwegian Sea basin. The variability near the seabed at Ormen Lange is strongly influenced by the local topography and the stratification. Realistic model studies therefore require high resolution models for the Ormen Lange topography connected to basin scale models. The models must be non-hydrostatic and the stratification realistic to enable realistic estimates of extreme events.

Survey strategies to quantify and optimize detecting probability of a CO2 seep in a varying marine environment

Designing a marine monitoring program that detects CO2 leaks from subsea geological storage projects is challenging. The high variability of the environment may camouflage the anticipated anisotropic signal from a leak and there are a number of leak scenarios. Marine operations are also costly constraining the availability of measurements. A method based on heterogeneous leak scenarios and anisotropic predictions of chemical footprint under varying current conditions is presented. Through a cost function optimal placement of sensors can be given both for fixed installations and series of measurements during surveys. Ten fixed installations with an optimal layout is better than twenty placed successively at the locations with highest leakage probability. Hence, optimal localizations of installations offers cost reduction without compromising precision of a monitoring program, e.g. quantifying and reduce probabilities of false alarm under control. An optimal cruise plan for surveys, minimizing transit time and operational costs, can be achieved. Seafloor monitoring design to detect leaks from geological CO2 storage.Based on site characterization, local environmental statistics and a detection threshold.Quantifies the probability of detecting a leak and finds the optimal layout.Likelihood of adverse measurements being false alarm is quantified.Can be extended to include other constraints, e.g. cost or vulnerable areas.

Solutions near the hydraulic control point in a gravity current model

A steady‐state gravity current model that incorporates entrainment and friction is used to describe large‐scale gravity currents and channel flows. When the model includes pressure effects from varying current thickness, critical points occur when the current velocity is equal to the phase velocity of waves on the interface. Some solutions have the possibility to pass from super to subcritical flow, or vice versa. These solutions pass through a hydraulic control point and the objective is to analyze the behavior of the solutions in the vicinity of such points. Using a phase space in which the hydraulic control points occur as equilibrium points, and performing Taylor expansion to the first order, the result is a system of autonomous differential equations with constant coefficients that can describe the behavior of the solutions for different parameter regimes near a hydraulic control point. If an equilibrium point in phase space represents a saddle point, it is distinguished between three different solut...