Risks and Equipment Considerations for Surface Diverting (Well Control)

Of all diverts, many studies show a failure rate of between 50% and 70%. According to the same studies, when it comes to well control issues, shallow gas blowouts is the leading cause of offshore rig damage and loss. On the US Outer Continental Shelf, the MMS agrees with these findings and has suggested a 46% failure rate between 1971 and 1991. Even though mandatory well control training was introduced during this period, the MMS has noted that a reduction in blowout frequency wasn’t experienced during this time.

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Introduction to Diverters in Well Control

Considering the danger of shallow steam or gas zones requires unique well control considerations. Whenever the necessary casing shoe integrity cannot be obtained due to the shallowness of the zones (before encountering pressure), a kick will need to be diverted because it cannot be shut-in. For this situation, a diverter shown in Figure 1 is a mandatory equipment to divert the undesirable flow to allow personal to have proceed the next plan; i.e., evacuation and/or dynamically kill a well.

Figure 1 - Diverter Package in Well Control (Courtesy of Cansco Dubai LLC)

Figure 1 – Diverter Package in Well Control (Courtesy of Cansco Dubai LLC)

By directing the flow from an unloading well, diverting allows physical damage to be limited to all equipment and rig personnel. With specialized procedures and equipment, the idea is to impose limited back pressure on the weak downhole formations. Although not strictly a well control procedure, diverting successfully will allow the well to be dynamically killed, to bridge over, or be depleted (without losing equipment or life). Continue reading

Basic Understanding about Cameron U BOP – Rams Blow Out Preventer

Suited towards surface or subsea applications, the Cameron Type ‘U’ preventer is one of well known wellbore pressure assisted ram preventers . It can come with a single ram (Figure 1) or double rams unit (Figure 2). When it comes to see whether the rams is in closed or opened position, this isn’t possible through observation alone and this is due to the operating rod’s tail end being enclosed inside the preventer itself. Since 1979, all Type ‘U’ preventers have required H2S service capabilities. One of key features of this BOP is a capability to pump open the bonnet doors. Once the four bonnet bolts have been removed, top-load ram changing is made easy by  applying closing pressure to push the bonnet out.

Figure 1 – Single Rams Unit – Camron U BOP

Figure 2 – Double Rams Unit – Camron U BOP

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Drilling (from a bottom supported rig) for the Surface Casing and Encountering Gas

When penetrated from a platform or a jack-up, shallow gas reservoirs have the potential of being more hazardous. Since the conductor almost reaches the floor of the rig, any kick products discharge into the hazardous zone directly.

To direct the flow overboard, the diverter will close automatically when a shallow gas flow occurs. During a period of stress, the diverter system’s reliability is questionable which is why failure should always be considered.

If a restriction forms in the diverter line, a hazardous situation quickly develops on a bottom supported rig. Around the seabed’s casing, gas can actually broach as a result of the pressure build-up. Whenever this occurs, the risk of the seabed becoming fluidized increases and therefore so does the risk of a rapid reduction in spudcan resistance.

Shallow gas encountered on a jack up rig (Ref – officerofthewatch.com)

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Drilling with a Riser from a Floating Rig for the Surface Casing and Encountering Gas

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.