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Accumulators for Surface Well Control System and Requirements

The accumulator unit is one of the critical well control equipment and its main aim is to supply the pumps with atmospheric fluid while also storing high pressure operating fluid for operating BOP stack. In this article, we will learn about requirements of critical components of an accumulator unit including  accumulators, reservoir, pneumatic pump, electric motor driven pumps and hydraulic control manifold/valve & fitting.

Surface BOP Control Systems Equipment

Accumulator Bottles

For storing high pressure fluid, accumulators are pressure vessels (ASME coded). Depending on requirements, the accumulators can be found in all sorts of types, sizes, pressure ratings, and capacities. Most commonly, ‘float’ and ‘bladder’ accumulators are used which come in ball or cylindrical shapes. Furthermore, they can be top or bottom loading.

Figure 1 - Accumulator Bottles

Figure 1 – Accumulator Bottles

If bottom loading, servicing will require them to be removed from the accumulator unit. If top loading, both float and bladder can be removed while mounted on the accumulator unit. Without destroying their stamp of approval, both types of accumulators can actually be repaired in the field whenever necessary. Continue reading

Wireline Tool Recovery and Junk Removal Equipment for Drilling and Workover

In order to fish for wireline tools out of the hole, we must use different equipment than would be used for pipe recovery. Common wireline tool issues center around the cable being tangled or wadded in the hole, as well as the fact that attempts at fishing can pull the wireline out of the rope socket or part, further complicating tool retrieval.

Which Part is Stuck? Cable or Tools

As soon as a wireline assembly becomes stuck, the operator will need to determine whether the problem is in the cable or the tool. Usually, one would apply normal logging tension on the cable and allow it to sit for a few minutes. During this time, four things should be recorded:

  • Current depth of the tool
  • Type and size of the cable
  • Surface tension of the cable just prior to becoming stuck
  • Cable-head’s weakpoint rating.

The cable will be marked at the rotary table, and a T-bar clamp will also be securely fitted to the cable just above the table. Should the cable break, then the clamp holds on to the cable end at the surface, so that the whole cable does not fall down the hole and cause additional blockage. The operator will then need to apply 1000 lbf of tension on the cable, and make a note of the distance that the cable mark moves at the rotary table. This figure shows the stretch produced in the elastic cable. It is then possible to estimate the length of free cable, using a stretch chart or from prior knowledge of the cable’s stretch coefficient. Should the length of free cable be the same as the current logging depth, then the problem does not lie with the cable; rather, the tool is stuck, and not the cable. If the length of free cable is less than current logging depth, then the cable is stuck at some higher point in the hole.

If it is the tool which is stuck, and not the cable, then pulling on the cable will cause one of three results. The tool may come free, the weakpoint can break and the tool will remain in the hole but the cable can be removed, or the cable will break at the point of maximum tension. Continue reading

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

Volume Gain from Slug

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

Figure 1 – Diagram Shows Volume Gain from Slug

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