When in shallow offshore environments, the formation in which the conductor is set is normally weak, which means it struggles to contain the pressure that occurs during a gas kick. To avoid an underground blowout, the well should be diverted when a kick is detected in these circumstances. This should also prevent gas reaching the conductor shoe.
Whenever a situation demands a riser for drilling, when drilling for the surface casing, Company Policy will dictate that subsea dump valves and an annular preventer are installed at the mudline. Additionally, the surface should have a normal diverter system.
Thanks to extensive experience, we know that shallow gas blowouts cannot be controlled with current diverter systems alone. Instead, the annular preventer and subsea dump valves can be used at the mudline to control the shallow gas flows at the seabed. Once these are installed, the next step should be to unlatch the LMRP or pin connector before then winching off location (no downwind, only up current).
Prior to spud, a contingency plan should be considered with the Drilling Contractor so three main procedures have been covered;
- Winching the rig from location
- Shallow gas flow
- Any failure in major components of the riser, diverter, or BOP system
All issues and considerations for the contingency plan can be discussed at the pre-spud meeting. If ever the subsea system fails, the surface diverter system will act as a back-up. Furthermore, the surface diverter system can also be a useful feature for diverting gas in the riser (above the stack).
While the surface hole is open, certain precautions should be taken and these are listed below;
- Mud should always be kept on site; enough to fill the hole volume twice over.
- Moorings should allow, after the rig is moored, the rig to be winched some distance away from the plume (around 400 feet is recommended). Only if practical, and after the surface casting is set, the chain stoppers can be applied. Also, the windlasses should remain on their brakes.
- If sudden losses occur, facilities need to be available to allow the annulus to be filled rapidly from the surface.
- To prevent the invasion of voids, hatches should be secured and this should prevent inflammable gas and even downflooding when a loss of heel or buoyancy causes a reduction in the freeboard.
- In the drillstring, a float valve should always be run.
- The annulus shouldn’t become overloaded with cuttings so care must be taken to prevent this, because this can cause liberated gas and losses and therefore the possibility of unloading the annulus.
- While tripping, the hole should remain full and so care must also be taken to monitor this.
What if the well starts to flow?
If this occurs, the following steps may be useful as a guide;
- Start by opening the subsea dump valves and then close the annular preventer – this will allow the gas to vent at the seabed.
- Pump seawater or mud at the maximum rate as an attempt to control the well, assuming there’s no danger to the rig or any personnel nearby. If there is danger to either, consider shearing the pipe or dropping the drillstring. Additionally, winch the rig to a safe position after unlatching the pin connector or LMRP.
- If the subsea diverter system happens to fail, there’s still the option of unlatching the pin connector/LMRP or to divert at the surface; therefore, venting the gas at the wellhead. Although diverting at the surface isn’t recommended, it can become necessary at times and the process starts by maintaining maximum pump rate. Then, space out ensuring that the lower kelly cock is above the rotary table before then closing the shaker valve, opening the diverter lines, and closing the diverter element; the returns should then be diverted overboard. The upwind diverter line should also be closed. From here, all non-essential machinery and equipment should be shut down and this reduces the risk of ignition; as a precaution, deploy firehoses beneath the rig floor. Finally, get ready to unlatch the LMRP or pin connector and winch safely.
- If the situation is steadily getting worse and a loss of control is looking likely, consider shearing the pipe or dropping the drillstring. Once again, winch the rig to safety after releasing the LMRP or pin connector.
References
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.
