C HALLENGES

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The challenges in both systems vary from each other. Starting with the conventional BOP system, the first things come to one’s mind are its complexity, weight, size, and dimension. As the exploration operations are moving towards ultra-deep-water locations, the operator companies will need heavier, bigger, and more advanced equipment. This sentence has been already mentioned many times in different forms in the report, but now let us consider why the companies need to increase the properties of the BOP as going to deeper water depth.

Deeper water depth means higher hydrostatic head of water, for which the parts of the BOP is to be adapted by increasing wall thickness, strengthening seals. Additionally, the capacity of the hydraulic accumulators is also needed to be increased to overcome the hydrostatic pressure of water head. As a result, the number of the accumulators, their wall thickness is increasing, and the BOP support frame are designed to carry the increasing weight, and overall, the size and weight are going up. Just to note here, this increasing weight of the BOP causes well fatigue issues on the wellhead.

The above-mentioned challenge is the main driving force for the electric BOP control system.

Because the elimination of the hydraulic system results in 154 metric tons weight reduction, including the shrinkage of the BOP stack dimensions. On the contrary to the hydraulic accumulators, electric accumulators do not need to be increased in capacity as going deeper water depth locations. Adding to that the fact that the volume and weight of the electric accumulators are much smaller than the hydraulic ones, is also important. This difference is visualized in Figure 44 by showing the space gained due to using the electric system.

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2. BOP

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Another challenge will be the time spent on running BOP/marine riser or retrieving them back.

Traditionally, the BOP stack is lowered via the drilling riser and therefore, running BOP/marine riser joint by joint and retrieving them consume duration of weeks. And in case of its failure during pressure test after placement can increase NPT significantly. In comparison, all-electric BOP can be lowered with the help of a rig crane due to its much lower weight, and the whole process will take just some days. Just to compare, Table 4 shows the running duration of the electro-hydraulic BOP/marine riser and all-electric BOP without marine riser in the depth of 1,000 ft:

Figure 44: Comparison of BOP Stacks - Hydraulic (left) and All-electric (right) Control Systems (Dale, Rød, & Howes, 2017)

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Table 4: Comparison of BOP running & retrieving duration (Electrical Subsea & Drilling , 2020)

B. A

The actuating concept in electro-hydraulic BOP control system is based on the power of hydraulic fluid and has its own challenges. After pressing the close BOP command, it is not possible to follow-up the closing process, such as the ram position, speed, and closing force etc. The only option to be sure about the successful closing/opening BOP operation is following the surface well pressures. Addition to that fact, another challenge is the complexity of the hydraulic fluid power transmission from the rig to the actuator. This has negative effects on the reliability of the BOP control system. Fluid leakage is the most common problem associated with the conventional BOP. The leakage is often seen in flex joint, annular preventer, hydraulic connectors, and ram preventers. In flex joints worn joints can result in external leakage, in annular preventer internal leakage during closed position is a problem.

Hydraulic connectors can leak externally to the environment and can fail to be unlocked from the BOP stack. Ram preventers can leak internally/externally, can fail to close/open fully or to keep closed. Regarding subsea control pods and accumulators, again the leakage is common and losing one or all function can also be seen in the control pods.

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Earlier we mentioned three electro-mechanical actuators in all-electric BOP control system, ring piston, ram, valve actuators. Starting with the ring piston actuators, the main challenge for it is improved sealing systems. Since annular preventers are frequently used preventer type and they allow vertical movements of the drilling string in closed position, the sealing efficiency can be degraded after a while. In fact, all-electric BOP system offers longer period of usage of the seals than the hydraulic one. It is because the CPU can adjust the speed and force of the preventer in the electric BOP in such a way to minimize wearing. Additionally, the automation system also reduces the time of the closing operation. For example, the CPU will increase the speed of the shear rams until touching the drill sting and then will decrease to obtain higher cutting load. Higher cutting capacity is the main challenge for the ram preventers. Regarding valve actuators, it is aimed to reduce the design variations and to create a typical one which can be used for different purposes (choke, kill etc.)