Proposal for making the cofferdam effective

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The recent attempts at setting a cofferdam over the riser leak on the Deepwater Horizon spill have failed and it appears through media reports that they are concerned with the hydrate (methane chlathrate) field surrounding the leak interfering with the setting of the cofferdam. The hollow cofferdam accumulates icing inside and around the apparatus causing buoyancy factors to make it unstable as is it lowered into position over the riser leak. I propose that CaCl brine in a liquid and then gel form may be applied to the site and the cofferdam to alleviate the icing. CaCl is a non-toxic material that is readily available. It is commonly used in de-icing operations on roadways and runways and it has a mildly exothermic reaction when it comes into contact with water. Methane chlarate (hydrates) has a density of 0.98gm/cm3 and CaCl has a density of 1.98gm/cm3. This density differential should allow for displacement of the hydrates away from the leaking riser.

There is HEC gel/polymer technology out there familiar to our Drilling units to gel up higher weight brines like CaCl. CaCl brine (12lb/gl) can be gelled up in to a solid (ultra stiff) gel pill that could be pumped into the cofferdam from the top that would allow the hydrates around the wellhead to be displaced. This would create added weight to the box that should allow them to manipulate it and set it over the wellhead. So first they should flood the leaking riser with liquid CaCl to remove as much of the methane chlarate (hydrate) away from the area. Then they pump the gelled CaCl into the top of the box, displacing the remaining hydrates and adding mass to the cofferdam. As the gelled CaCl is pumped through the cofferdam it would pass through the cofferdam to the riser leak and displace the hydrates away from the leaking riser. This would create a “channel” for the cofferdam to follow down to the sea floor. As the cofferdam got closer to the seafloor the gelled CaCl would “back up" into the cofferdam and displace the hydrates accumulating in the head space of the cofferdam. This would give the cofferdam the much-needed mass to combat the turbulence of the leak and make it stable for the ROV units to maneuver it.


It looks like that box is about 10x15x40, which would put it at 6000ft3 give or take. That means they could add potentially +/-500,000 pounds of gelled brine inside the cofferdam


1ft3 is 7.48gl, 6000ft3 is 44800gl, CaCl is 12#/gl, the cofferdam could theoretically hold 537,600 pounds of gel. The cofferdam itself weighs 100 tons (200,000 pounds roughly) and has proven to be too buoyant to hold against the hydrates around the wellhead. By using a weighted gel pill they could theoretically weight the box to 700,000 pounds (350 tons) give or take.


If necessary they could then run line through the cofferdam like a stinger pumping methanol (or the hydrate inhibitor/glycol product suggested) directly around the riser leak inhibiting the new hydrates pouring out of the well. Then they could run the pump intake line directly into the flow of the wellhead and crank up the pumps on the surface. The wellhead flowing pressure would push the gel away from the rupture creating a working area around the leak. This intake stinger would pull the oil/hydrate/methanol mix up the line into the tanker. The methanol or hydrate inhibitor mixed in with the leaking hydrates and oil should prevent the intake line to the tanker developing a hydrate plug.

The gel pill would be impervious to methanol or our hydrate inhibitor/glycol product but it would gradually degrade and dissipate. If the engineers felt it was necessary they could just replenish the gel weight with more gel and maintain the mass of the box to continue to displace the hydrates. Since the majority of the hydrates would be displaced and/or depleted by pumping and inhibiting during the initial process to set the cofferdam, it would not take nearly as much mass to maintain the operation as it would to get the box set and kick off the initial pumping operations.

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