Structural Analysis of Oil-Spill Booms - Mathematical Modelling and Numerical Simulation of Oil Pollution Problems - page 160

7.4.4 Interpretation
Toward the oil-spill response tactics, we begin by the interpretation of the boom
tension in term of the current direction. The Elorn boom plan is oriented approxima-
tively fromWest to East. During flooding tide the current at the buoy comes from the
West and slightly South. For ebbing tide the current goes to the South East. During
November 5, 2009, and flooding tide, the Elorn boom plan will have the deviation
function for a pollution coming from the Bay of Brest. Under these conditions the
boom is weakly stressed. For ebbing tide, the Elorn plan will have the containment
function for an oil pollution coming upstream from the Elorn river. In that case the
boom stress is high.
The boomgeometrymeasured byGPS permits an interpretation in termof oil-spill
preparedness, during a typical period of flooding tide. We consider an oil pollution
drifting according to the tide current direction. At that time of November 4, 2009,
the section AB tends to have a flag-like geometry. During a pollution deviation by
the curtain, this section should contain a reduce volume of oil. The sections BC and
CD take the J-form geometry. This favours a large oil accumulation in the vicinity
of the section bottom. There is suggested a valuable position to install a skimmer or
a sorbent material for oil recovery.
Several considerations must be underlined when using ADCP sensor in our situa-
tion. We enumerate several of them. We underline that the ADCP uses the magnetic
north while the GPS uses the geographic north. The sensor gives the current profile
when both the small boat and the sea bottom are immobile. When the ship moves,
the sensor can give its velocity if this one is moderate. By time integration we can
obtain the ship displacement. If the sea bottom is composed of tidal mud flat and has
a small drifting velocity, from sea current entertainment, then we can observe that
the small boat position drifts slightly.
The measured current profiles around the boom are homogeneous along the ver-
tical. It is observed from the first cell at 0.5m depth. It suggests homogeneity of the
current in the first meters of the water depth. The elaboration of the Elorn estuary
contingency plan depends principally on the temporal and horizontal current varia-
tions. The implementation of 3D oil-spill or hydrodynamic models must give such
current velocity field with a vertical homogeneity along the Elorn plan.
The skirt angle evaluation uses light stick attached to boom section connection.
We underline that this measurement delivers the angle of the connection which is
a rigid beam having low flexibility. The instrument must be improve to measure
the angle of the fabric part of the skirt which is a flexible tensile structure. A subsea
photography systemwithin low turbidity water may be envisaged to capture the angle
and the vertical curvature of the boom skirt.
The stress map in the boom structure permits to define the suitable materials. The
finite-element mesh of the boom float is based on a single cylinder. The float can
buckle depending of the loadings on the structure. To avoid that effect, the boom
sections are built with a sequel of elementary tubes having a reduced length. Each
boom element used in the Elorn experiment has a length of 5m. An improvement of