Shallow Hazard while Drilling Without a Riser (from a floating rig) and Encountering Gas

According to many company policies, unless circumstances and conditions apply, as stated in the Drilling Policy and Guidelines Manual, the surface hole should be drilled riserless. By doing this, it’s possible to eliminate the most common cause of blowouts in a shallow and pressured gas reservoir (most importantly, the loss of hydrostatic head). Of course, there will still be a risk of penetrating an overpressured reservoir so there must always be a contingency plan in place. Prior to stud, the operator and the drilling contractor must also together on the plan and it needs to include;

  • Common procedure when winching the rig off location
  • Common procedure when a shallow gas flow occurs

Normally, the pre-spud meeting will be the ideal time and place to discuss contingency plans. A 10-degree cone of low-density water is normally produced after a gas blowout in open water and there will also be a discharge of highly-flammable gas. The current and water depth will decide the intensity of the blowout with greater water depth leading to more dispersed water from the plume. When a current is active, the result would be a plume away from the rig.

West Vanguard Blowout

West Vanguard Shallow Gas Blowout

A loss of buoyancy can occur for a floating vessel within a plume of expanding gas; this being said, when the water depth creates a negligible effect on a semi-submersible at operating draft, then this reduces somewhat. A vessel can be displaced after an eruption of gas, and a drillship can also keel towards the plume when constrained by its moorings; this would reduce its freeboard even further. When the conditions are calm, there’s always a risk of fire if the gas is trapped within a confined area. In these conditions, the gas disperses slowly.

All hazards can only be assessed, in terms of severity, at the time. In truth, the crew and vessel aren’t likely to be under excessive danger but there are still some precautions and considerations that must be considered. Before and during the surface hole being open, this includes;

  • Securing all hatches and therefore preventing inflammable gas in the voids, or even downflooding when a loss of heel or buoyancy causes a reduction in the freeboard; this is very important for a drillship.
  • Running a float valve in the drillstring.
  • Monitoring all weather conditions and the current. Signs of gas should also be considered in the sea surface.
  • Mooring the rig with enough moorings so the rig can be winched around 400ft from the plume itself. The chain stoppers should only be applied after setting the surface casting, and windlasses should remain on their brakes as long as it’s practical.
  • Keeping enough mud on site so the hole volume can be filled twice over.
  • Distributing the cuttings and drilled gas by limiting the ROP, drilling the pilot hole, and circulating at a high rate.
  • Keeping personnel and facilities available at all times to heave in up current (not downwind), and slack off whatever moorings lie closest to the plume. Before anything takes place, a contingency plan should be created so every individual on the site knows their roles and responsibilities when dealing with potential issues.

Shallow Gas Flow – Whenever a shallow gas flow is detected, the priority should be to control the well. As long as there’s no danger to the rig or to personnel nearby, this can be done by pumping either seawater or mud at the maximum rate possible.

Immediate Danger? – If the rig or individuals are under immediate danger, the priority should be to shear the pipe or drop the drillstring. From here, the rig can be winched to safety (away from the gas plume!).


Cormack, D. (2007). An introduction to well control calculations for drilling operations. 1st ed. Texas: Springer.

Crumpton, H. (2010). Well Control for Completions and Interventions. 1st ed. Texas: Gulf Publishing.

Grace, R. (2003). Blowout and well control handbook [recurso electrónico]. 1st ed. Paises Bajos: Gulf Professional Pub.

Grace, R. and Cudd, B. (1994). Advanced blowout & well control. 1st ed. Houston: Gulf Publishing Company.

Watson, D., Brittenham, T. and Moore, P. (2003). Advanced well control. 1st ed. Richardson, Tex.: Society of Petroleum Engineers. 2005. West Vanguard Blowout – Oil Rig Disasters – Offshore Drilling Accidents. [online] Available at: <> [Accessed 21 May 2020].

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