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Calvez  -  Decision  making  process

corresponding  window  of  opportunity,  according  to  metoceanic  conditions  (temperature,
wind).
During  a  spill  emergency,  the  most  straightforward  way  to  assess  dispersibility  of  the  spilled
oil  would  be  to  perform  dispersibility  tests  on  in  situ  oil  samples.
Nevertheless,  such  field  sampling  may  not  be  possible,  and  spilled  oil  viscosity  should  then
be  assessed  according  to  the  following  steps:
•  does  the  viscosity  of  fresh  oil,  given  sea  temperature,  makes  it  dispersible  a  priori
(>  or  <  10,000  cSt)?
•  according  to  the  oil  pour  point  vs.  sea  temperature,  is  there  a  chance  for  the  oil  to
stay  fluid?
•  after  having  assessed  the  actual  oil  viscosity  at  sea,  generally  through  oil  weathering ­
  models,  is  the  oil  dispersible  at  the  time  of  dispersant  application?  Data  from
weathering  pre-studies  (dispersibility  as  a  function  of  viscosity),  if  available,  may
be  useful  to  answer  this  question.  Otherwise  simplified  tables  (expected  dispersibility ­
  based  on  oil  viscosity  vs.  sea  state)  may  be  used.

Figure  9.2:  Detailed  flowchart  for  Q1  “Oil  dispersible  from  a  physico-chemical  viewpoint?”  {Note:  Yellow
boxes  indicate  the  information  which  should  be,  as  possible,  made  available  in  NOSCP)  (adapt,  from  Merlin, ­
  2015).
Chemical  dispersion:  an  environmentally  “beneficial”  trade-off?  (Figure  9.3)
Toxicity  of  dispersed  oil  (i.e.  bioavailable  small-sized  droplets  -typically  tenths  of  micrometers) ­
  may  adversely  impact  marine  fauna  and  flora.  For  this  reason,  chemical  dispersion  is
not  applicable  everywhere,  and  is  generally  not  recommended  :
(i)  in,  or  in  the  immediate  vicinity  of,  ecologically  vulnerable/sensitive  areas,  or
(ii)  in  areas  where  renewal  and  mixing  of  water  do  not  guarantee  rapid  dilution  of  the
droplets.
Also,  if  implemented  in  sheltered  and/or  shallow  waters,  dispersion  process  may  lead  to  adsorption ­
  of  droplets  on  suspended  matter  and  eventually  its  incorporation  into  sediments.