Product(s):	OpenFlows FLOOD
Version(s):	10.03.XX.XX
Area:	Modeling

Overview

This article describes the main necessary steps to set up an oil spill simulation using OpenFlows FLOOD's MOHID Water engine. 

Solution

To setup an oil spill, first create a workspace and then a new domain, selecting MOHID Water as the numerical engine. Then create a new simulation and add the Lagrangian module to the list of modules in the simulation. Then, in the Model input data options, set the option "Enable Lagrangian Module" to "True". Setup the Lagrangian module, defining an "origin" (i.e. a source or emission of particles representing the oil spill). 

The Lagrangian module is organized by “origins”. An “origin” represents an emission source of Lagrangian tracers. An emission can be instantaneous or continuous and a simulation can have one or more “sources”. An example of an origin (emission source) is an oil spill, where various Lagrangian particles/tracers are emitted and transported by currents and winds.

The configuration file of the Lagrangian module contains a specific structure. In the configuration file there are global keywords, which define options applicable to all defined “sources”, and specific keywords, referring only to the characteristics of each individual “source”. The configuration of these options is done by aggregating “keywords” in a block (or more) blocks by a “tag” at the beginning <BeginOrigin> and another at the end <EndOrigin>, as shown below.

<BeginOrigin>
ORIGIN_NAME : Oil spill
[...]
<EndOrigin>

Each <BeginOrigin>/<EndOrigin> block corresponds to an origin, the number of origins being unlimited.
Next, the “keywords” that can be changed to create a new scenario are indicated:

• DT_PARTIC        : 60       -> sets the Lagrangian transport integration time step
• OUTPUT_TIME  : 0 600   -> sets the time step (in seconds) for writing results in HDF5 format. The first value must be zero (indicating the instant of the first output), and the second value indicates the frequency of writing the following outputs until the end of the simulation (600 seconds = 10 minutes)

Another set of global options is related to how the Lagrangian tracers behave when they get close to the shoreline. In this case, MOHID allows defining a probability of a Lagrangian particle being retained on the ground (“beaching”) when it is at a distance lower than a predefined limit.

• ASSOCIATE_BEACH_PROB    : 1    -> Boolean (0 or 1) value to activate the beaching probability method
• DEFAULT_BEACHING_PROB : 0.7 -> Fraction (value between 0 and 1) of beaching probability. 0.7 means that there is a 70% probability that the particle will be retained on the shore when it is at a distance lower than a predefined threshold
• BEACHING_LIMIT                    : 10   -> Limit distance (in meters) from which a particle can be retained on the shore is almost at a distance from the shore less than this limit
• COASTLINE_FILE                    : ..\General Data\Digital Terrain\CoastLine.xy -> Path to a coastline file that contains the vertices of a polygon that defines the area contained by the coastline (land zone). When using this polygon, particles can be retained on the shoreline. If the polygon is not used, the coastline is defined based on the face of computational mesh cells that correspond to land cells. The polygons file format is described here. 

Once the global options have been configured, the specific options for each origin (or emission source) must be changed. These options are defined within a <BeginOrigin>/<EndOrigin> block where each set of options is similar for each origin but with different values (e.g. 1 origin with crude oil and another origin with diesel fuel, each in different positions).

For the specific case of an oil spill, the instantaneous “emission” type is normally used, which can be the special type Accident (“accident”). These options are defined by the “keywords”:

• EMISSION_SPATIAL       : Accident
• EMISSION_TEMPORAL : Instantaneous -> In this case (“Accident”), the initial spreading area is calculated based on the spilled volume and the physical properties of the oil, with the initial position of the particles randomly distributed within a circle with the calculated area and center at the position defined by the user. It is also possible to define continuous emissions at fixed points (e.g. a stranded ship with a continuous oil leak). In this case, define: 
  EMISSION_SPATIAL       : Point
  EMISSION_TEMPORAL : Continuous

Other options specific to each source are the position of the spill source, the spilled volume and the number of particles to use the simulation, described below.

• POSITION_COORDINATES   : -9.2733 38.6926 ->  coordinates indicating the longitude (X) and latitude (Y) of the position where the spill occurred (should be the same coordinate system as the one used in the computational grid) 
• POINT_VOLUME                    : 100 -> Volume (in m3) of spilled hydrocarbon 
• NBR_PARTIC                         : 500  -> Number of Lagrangian particles (tracers) to be used in the simulation.

Activate the initial spread method:  

ACCIDENT_METHOD : 1

Set the wind drag coefficient (as oil stays in the surface, its transport must account for wind)

WINDCOEF                  : 0.03

Then you will need to activate 3 properties (temperature, salinity and oil) inside the "origin" block as described below. 

<<BeginProperty>>
NAME                             : temperature
UNITS                             : ºC
CONCENTRATION         : 11
EQUAL_TO_AMBIENT  : 1
<<EndProperty>>

<<BeginProperty>>
NAME                             : salinity
UNITS                             : psu
CONCENTRATION         : 36
EQUAL_TO_AMBIENT   : 1
<<EndProperty>>

<<BeginProperty>>
NAME                               : oil
UNITS                               : m3
CONCENTRATION          : 1
EQUAL_TO_AMBIENT   : 0
AMBIENT_CONC : 0
<<EndProperty>>

Finally, it is necessary to configure the calculation options corresponding to the oil weathering processes. To make this configuration it is necessary to know the type of hydrocarbon and its properties. 
To do so, it is necessary to define, within the origin block  <BeginOrigin>/<EndOrigin> , a sub-block <<BeginOil>>/<<EndOil>> as shown below: 

<<BeginOil>>

OILTYPE                          : Crude

API                                   : 21

POURPOINT                    : -30

MAXVWATERCONTENT : 70

<<EndOil>>

Below is a list of the main keywords that parameterize the oil weathering processes:

• NAME                                     : Heavy Oil -> Spilled hydrocarbon name
• OILTYPE                                 : Crude ->Type of hydrocarbon: Crude or Refined
• API                                          :13 -> API Grade Value - American Petroleum Institute
• POURPOINT                           : -30 -> Temperature (ºC) from which the hydrocarbon loses its fluid properties
• MAXVWATERCONTENT       : 80 -> Maximum water content (value in %)
• VISCREF                                : 3603 -> Reference dynamic viscosity (in cP)
• TEMPVISCREF                      : 15 -> Temperature (ºC) at which the reference dynamic viscosity was determined
• OWINTERFACIALTENSION  : 20 -> Water-oil interfacial tension (Dyne/cm)