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.
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)
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
Slug mud is typically pumped into the drill string in order to push the mud inside the drill pipe down so the drill pipe will be clean and ready for pulling out of hole. Since slug is heavier, it will push the lighter mud out of the well. Figure 1 demonstrates how slug displaces current mud out of hole.
Figure 1 – Diagram Shows Volume Gain from Slug
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
For back-off operations to be successful on the first try, plans must be devised and then carefully followed; this should also keep the risk of injury low for rig floor personnel. Of course, the Contractor Driller/Tool pusher and the Fishing Tool Supervisor need to oversee the process since it’s considered a non-routine operation.
Steps for Safety
1) Firstly, a safety meeting, coordinated by the Drilling Supervisor, should take place before the procedure itself. With all rig personnel in attendance, the meeting will explain the no-go areas during torque application (and when torque is held on the drill string), the hazards of the operation, and the proper use of equipment in order to prevent injury.
For personnel not essential to the task at hand, they should stay well away from the rig floor until completion.
2) To hold right or left-hand torque in the string, sometimes rotary slips and rig tongs will be used. If this is the case, the slip insert dies need to be sharp (while also fitting into the slots themselves). If any dies have signs of wear during an inspection, they should be replaced. Continue reading