The working principle of hydraulic drilling equipment

Generally, a hydraulic drilling machine spread has a smaller footprint than a conventional drilling rig. This is, in part, because hydraulic drilling usually involves smaller hole sizes requiring smaller drill string diameters, smaller OD and lighter BHAs, and smaller mud systems and pumps that require only the circulation rates necessary to clean the hole. All of this means less areal space is required on the wellsite for the equipment spread.

 

Less equipment, lower pump rates, and smaller horsepower requirements also mean less fuel consumption and fewer air emissions. There is less equipment on the location, so exposure risk to leaks or spills is reduced. Smaller hole sizes also mean that a smaller cuttings volume must be handled, and the risk of a cuttings spill is also reduced.

 

The snub drilling spread may be required to drill the same size holes as the conventional drilling rig. It does not have a substructure and mast, however, and most of the other equipment is smaller. So, the snub drilling spread will usually have a smaller footprint than a conventional drilling rig.

 

Some items on the snub drilling site may be the same size as those on a conventional drilling rig such as the mud logger's shack, the MWD/LWD and computer facilities, and supervisors' offices. Pipe will still be staged, picked up, and run in the hole. On most rigs, each joint must be pulled and laid down on a trip out of the hole as well. Some racking systems allow the drill string to be stood back in singles or doubles. This further reduces the footprint of the snub drilling equipment.

 

The weights of most of the individual pieces of equipment are usually less than those on a conventional drilling rig. So, offloading and spotting them requires smaller lift equipment and is less complicated. For packaged rigs, such as an offshore jack-up rig, this does not apply. Onshore, much of the work in spotting equipment is done by tandem-axle gin pole trucks, so the crane comparison does not apply. Inshore equipment spreads still require a crane to load and position equipment on barges, however.

 

Most onshore conventional drilling rigs are designed to be self-contained and sized to manage the deepest, largest-diameter hole and casing string that the derrick and substructure are rated to handle. Because onshore rigs may not have to deal with compressed locations, there is little need to place the equipment items close together.

 

Often, the mast and substructure require extra lifts and truckloads. This is not the case with snub drilling equipment since there is no heavy substructure and mast involved in snub drilling. Many conventional rig substructures are self-elevating and most of the derricks are raised by the drawworks. However, a crane is sometimes required to erect both. A crane is usually required to assemble the unit and BOP stack in a hydraulic unit drilling spread.

 

Mud systems on conventional rigs are also sized for the largest and deepest hole capable of being drilled by the rig. On many conventional rigs, over 1,000 bbl total mud system capacity is required where snub drilling rigs may only require a quarter of that volume. Similarly, conventional rigs need pumps that can clean large-diameter holes. Some have two large pumps; others have three. Usually, only two smaller-capacity (and lower-horsepower) pumps are required for hydraulic unit drilling.

 

The result of the smaller equipment package for hydraulic unit drilling often means that there are fewer mobilization and rig-up costs associated with this type drilling. Mobilization cost for an offshore jack-up rig is quite expensive. The rig must first rig down, deballast, and free itself from the seafloor at the previous location before moving to the new well, either under its own power or towed by large tugs.

 

Once the jack-up is spotted, the spud cans and legs must be jacked down to lift the rig above the waterline while avoiding punch-through (penetrating the sea floor unexpectedly). The rig must be preloaded with seawater pumped into the ballast tanks to place weight on the spud cans. This is commonly done to ensure stability before the rig is raised to the proper height to drill. Then, the drilling package must be cantilevered out to the well centerline above the wellhead deck. Only then can the drilling package be extended and rigged for drilling. Several days or even weeks may be required to perform this work depending on weather and sea conditions. The hydraulic unit package is rigged up on the platform floor, and aside from hoisting individual lifts from the transports, rigging up and drilling is sea-state-independent. Costly delays are unnecessary.

 

Inshore locations normally use canals to reach existing wellsites in marshes or swamps. Shallow draft barges can navigate these canals with little difficulty. A large, bottom-supported conventional jack-up drilling rig might require widening and deepening an existing canal that has partially filled in over time. A dredging permit is often required. All dredging is expensive especially when one considers the cost of spoil disposal.

 

Environmental concerns may delay or prohibit securing a dredging permit from regulatory agencies especially when the routes to the well or the well itself are in sensitive areas. In some cases, these areas became sensitive after the original hole was drilled. Securing a dredging permit in these areas is improbable, at best, even though there is an existing well at the end of the canal. So, new drilling operations on old inshore wells may not be possible at all using a conventional rig. The smaller footprint of a hydraulic unit drilling spread mounted on barges is certainly attractive in these situations.