Debris Management Strategy Engineered To Reduce Drilling NPT
On average, 30% drilling nonproductive time (NPT) in deviated offshore and onshore wells can be attributed to downhole tool failures related to ferrous and non-ferrous debris generated while drilling.
Although not as critical in vertical wellbores, debris control and containment have reached critical mass with the escalation of deeper wells with complex geometries, typically employing electronic-imbued intelligent BHAs under aggressive drilling parameters. Debris trapped in the circulating fluid can severely damage rotary steerable systems (RSS) and other debris-susceptible BHA components, while metal fragments entrained in the reused drilling fluid compromise the reliability of MWD and LWD measurements. With poorly designed surface ditch magnets and downhole filtering devices, the only recourse seems to be a trip to replace inoperative components.
The increasing costs associated with drilling debris prompted M-I SWACO, a Schlumberger company, to transfer lessons and technologies developed over 25 years of experience in pre-completion wellbore cleaning to the drilling environment. The emerging while-drilling debris management strategy capitalizes on proven technologies, employed individually or together to extract debris from the drilling path. Preventing debris ingress into BHA components, therefore, reduces trips and keeps the bit on bottom longer, while simplifying the mud-to-completion fluid displacement.
Field-proven technologies
A key element of the distinctively seamless approach, the MAGNOGARD* openhole magnet is ideally suited for hard and abrasive rock applications with aggressive drilling parameters. The tool is engineered to rotate and reciprocate in the open hole where it captures broken bit teeth and other metallic debris.
To illustrate, in a Texas horizontal drilling program, iron particles generated while drilling the abrasive formation had compromised the reliability of the RSS and MWD tools. In a subsequent well, the operator incorporated the MAGNOGARD magnet in the BHA where it recovered 33 lbm of ferrous material before it could damage the critical downhole tools.
The complementary MUDGARD* workstring filter is installed directly above the BHA to capture all fluid-borne debris before it can irreparably damage complex BHA components. In comparative analysis, the productive life of the internal filter is shown to be no less than 10 times greater than that of a conventional downhole filter.
The efficiency of the MUDGARD filter was demonstrated in multiple slimhole horizontal wells onshore Argentina where the drilling fluid had become contaminated with various debris, including O-ring and drive belt fragments, drill solids and fibrous material. After the workstring filter was installed, circulation testing showed no impairment of the MWD signal while pumping through the tool, which when pulled recovered 0.965 lbm of debris, enabling the operator to drill to programmed depth in a single run. On subsequent runs, the filter captured 0.21—3.30 lbm of the BHA-damaging debris. After more than 1,500 hours of circulating 13.0 lbm/gal through an individual MUDGARD tool, there was no apparent erosion of the filter screen.
Oftentimes, drilling deviated wells requires operators to boost annular velocities to better promote debris removal. For those applications, the drillstring can be augmented with the newest generation WELL COMMANDER* ball-activated drilling circulation valve. Likewise positioned above the BHA components, the valve when opened creates an alternate flow path to prevent hazards, such as the buildup of cuttings beds during the drilling operation.
The capacity of the technology to evacuate cuttings in complex wellbores was exemplified in its application in the 80-degree inclination of a 12-1/4in. interval of an offshore Sakhalin Island well, where the operator required a valve that would enable the fluid stream to bypass equipment downhole. Several weighted, high-viscosity sweep pills were pumped through the ports of the drilling circulating valve at high rates to facilitate hole cleaning and debris removal. Once the valve was in service, the operator observed a more than 150% increase in the cuttings volume at surface.
UPS Systems Safeguard Power Supplies On Offshore Rigs
When it comes to power on an offshore oil rig, there is no on/off switch for crew shift changes. For safety as well as operational and productivity reasons, the flow of energy must be 24/7 and completely protected from possible blackouts and malfunctions in the main power supply caused by frequency fluctuations or lightning strike over-voltages.
Functioning signal lights, which are used to ensure smooth operation on the platforms, run day and night and are essential to the safety of nearby ships, aircraft and the platform itself. That is why every rig is designed to accommodate customized and ruggedized uninterruptible power supply (UPS) systems.
Particularly critical to the offshore site is the ability of the UPS system to withstand the most severe environments—storms, salty air, intense vibration, high heat or extreme cold. All this plus corrosion, dust, atmospheric gas, electrical failures and other unforeseen problems can contribute to glitches in the power grid.
Offshore UPS systems must meet strict national and international industrial standards, including those required by the International Association of Classification Societies, American Bureau of Shipping, Lloyd’s Register and others.
UPS system engineers first determine the installation needs. How much space is available for the UPS system? Platform real estate is valued at about $50,000/sq ft. What amount of battery charger capacity is needed? How much power load can the UPS system support during backup? How much will the system weigh? Where will the platform be located?
Other factors include the ability of the equipment to operate when the platform is moving. It must be able to withstand pitch and yaw. The cabinet containing the UPS system must be more robust with particular attention paid to preventing corrosion and overheating. To ensure a continuous power supply for the lighting of the platform, each UPS system uses a distribution panel with a safe parallel feed from the standard power supply and the emergency grid.
Maintaining the UPS system
Maintaining an offshore UPS system is a delicate balancing act. When a platform is abandoned for safety reasons, the navigation beacons must stay on even though the batteries could be drained completely after the four-hour backup capacity has been used up and power for the generators has been depleted. That means the UPS system must increase its charging power.
The critical UPS component most affected by high ambient temperatures is the UPS battery, but other internal UPS components, such as DC bus filtering capacitors, might have their service life shortened by high-ambient temperatures unless special high-temperature components are selected.
Hybrid static switches
Another key component of all UPS systems is a hybrid static switch. Only the bypass pole of the static switch has the inverse-parallel silicon-controlled rectifier pair for electronic power switching. The inverter side of the hybrid static switch uses a power relay with normally open contacts to disconnect the inverter from the bypass during the normal static switch critical load transfer operation.
If the UPS hybrid static switch contactor contacts fail to open, the UPS output will be connected continuously to the bypass source. The mean time between failure of the UPS system is directly linked to the reliability of the static switch—the power path between the inverter and critical load.
UPS battery
The UPS systems employed on the oil-drilling sites use a much lower DC link voltage (60-cell, 125 voltage per cell), which has the advantages of using a UPS battery with fewer inter-cell connections and increased reliability. Typically, the charger capacity has to be robust because the battery support times can range from 60 minutes to eight hours or more. Also, it is important that each UPS system has enough battery recharge capacity built into the charger.
Unlike their commercial counterparts, offshore UPS installations have longer battery support times and use a higher end voltage of 1.75 voltage per cell.
Overall, the life expectancy of an offshore UPS system can be anywhere from 10 to 15 years. The equipment, including the cooling fans and DC filter capacitors, are designed to have more than 100,000 hours of mean time between failure under normal operation.
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