1 Introduction & Background

From the starting years of the 20th century, A-Frame drilling machines have been in use. The earliest drilling rigs in the US were diamond drilling rigs which recover cores from shallow coal deposits, and the shallow dry boring rigs for geotechnical exploration. Some were steam-powered, while gas-powered engines were also used for those early A-frame rigs. Those A-Frame rigs were easy to erect with a small crew using hand tools. With time, A-Frame rigs were further developed for use in SPT (Standard Penetration Test) sampling by the late 1920s. Those SPT sampling rigs used dropping-of-weight to drive the sampler into the ground and counting the number of blows for the penetration achieved. Those were followed by hollow flight augers where drive samplers were inserted inside hollow cylinders. In 1933, Hughes Tool Company developed tricone bit. Tricone bit used three exclusive rotating cutting heads coupled with internal flushing of cuttings by drilling mud. From that point, masts of rotary rigs started extending, and the drilling depth kept on increasing. Numerous other developments have been made to design the modern rigs that we use today for various applications, including oil and gas exploration activities. The maximum drilling depth, True Vertical Depth (TVD), of 12,262 meters (40,230 ft), was achieved to date in Kola Super-deep Borehole in the Russian Kola peninsula along Norway’s border. This was also the maximum bore length of drilled bore until surpassed by Al Shaheen Oil Well in Qatar in 2008, with Total Measured Depth (TMD) equal to 12,289 meters (40,318 ft).

2 Modern Drilling Rigs

Modern-day drilling rigs are operational in various types and applications. The most significant classification is based on their application on land (land rigs) or sea (offshore rigs). Both land rigs and offshore rigs have some basic components as described below:

  1. Power System
  2. Rotating System
  3. Hoisting System
  4. Circulating System
  5. Control and measurement System

Power system provides the necessary energy required for drilling hole, circulation of mud pumps, operations of draw-works, control system, utilities and support equipment present at the drilling rig site. In mechanical rigs, power is transmitted to various rig systems through belts, chains or driveshafts, while electric rigs get their power through DC cabling running from DC power generator.

The rotating system of the drill rig comprises a rotary table, drill stem, drill string and drilling bit. The drill string is the main component responsible for drilling progress and consists of drill pipes (screwed one after the other), collars and other bottom hole assembly components. The drill bit is attached to the end of drill collars and is the actual grinding or cutting component. Various types of drill bits used today are diamond bits, PDC ( Poly-crystalline Diamond Cut), insert, drag and tri-cone bits.

The hoisting system comprises draw-works, crown block, travelling block and other such parts used to raise drill stem. The top of the rig substructure is the derrick floor, which supports the crown block and provides for joining new pipe lengths during a drilling operation. The height of the rig substructure is often dictated by the length of blowout preventers installed.

The circulation system comprises mud pumps, standpipe, drill string intake, return annulus, flowline, surge tank, shale shakers and various tanks or pits. Drilling fluid circulation serves to cool the drill bit, stabilise the borehole, and recover formation samples for evaluation.

The control and measurement system comprises blowout preventers (BOPs), driller’s console, and dog house/service shed. BOPs are the last line of defence against a blowout. They are composed of hard rubber slips that fit around any sized pipe, metal doughnut-shaped pipe rams that fit a specifically sized pipe, blind or shear rams that can close the hole by cutting through pipe and crossover spools for attachment of choke and kill lines. The driller’s console is where the driller monitors equipment performance, distribution of power, changing of gears and oversee drilling crew activities. The dog house is where a geolograph is placed for the time-based recording of various rig functions like Kelly height, depth, pump pressure, circulation rate, string weight, stroke rate, rotary speed, rotary torque, etc.

  • What differentiates Offshore rigs from Land drilling rigs

Offshore rigs are similar to land rigs with additional features like helipad availability, living quarters, cranes, escape boats, etc. The most crucial difference is floating and stabilising the rig platform in a harsh sea environment. As such, based on floating and/or stabilisation mechanism, there are the following common types of offshore rigs:

  • Bottom Support rigs:
    • Barge mounted rigs
    • Submersible rigs
    • Jack-up rigs
    • Platform rigs
  • Floating rigs
    • Semisubmersible rigs
    • Drillships

3 Drilling Rigs for Land Applications

Broadly there are two types of land rigs, i.e. Conventional rigs and Mobile rigs. Mobile rigs being increasingly finding their utility in the field are further discussed in detail in this article. Mobile land rigs are classified as:

  • CATEGORIES
    • Jackknife or cantilever derrick rigs are assembled on the ground at a location where they are transported and raised to a vertical position with the help of draw-works.
    • Portable mast rigs are mounted on low-bed trucks as a single assembly and transported to a location. They are raised to a vertical position with the help of truck hydraulic pistons.

TYPES OF LAND RIGS

Reverse Circulation Drilling Rigs (RC). During exploration activities where special grade control is required, RC drilling rigs are recommended because of their dual wall drill rods with hollow inner tubes used for continuous and steady transfer of drill cuttings to the surface. Drill cuttings are blown through the annulus from the down-hole to the deflector box at the top of the rig. Drill cuttings then travel through a hose to the cyclone chamber, where they fall into sample bags – each bag is marked with the location and depth of the place where the sample was collected. They most commonly use a pneumatic hammer that is driving a tungsten-steel drill bit for crushing hard rock.

Down-the-Hole Drilling Rigs (DTH). These rigs consist of a jackhammer that is screwed on the bottom of the drill string, and the fast hammer action breaks the hard rock of the jackhammer. Drill cuttings and dust are removed by a fluid such as water or drilling mud.

Direction Drilling Rigs. Direction drilling rigs are capable of controlling the directional deviation of the drill bore towards the target location. These are different from horizontal drilling rigs as they start the drilling in the vertical direction and can change the bore direction as desired by the driller – even horizontally. Modern direction drilling rigs are capable of achieving a TMD of 7000 – 8,000 meters.

Diamond Core Rigs. These are core drilling machines using a rotary drill fitted with a diamond drill bit and can create precisely measured boreholes. These drilling rigs are equipped with lubrication methods to keep the drill bit from overheating.

Sonic Drill Rigs. These drilling rigs are the most advanced drilling machined that use high-frequency, resonant energy generated by a sonic head. Simultaneously with the rotation of the drill string, the sonic energy is used to push the core casing into subsurface formations.

Borehole Rigs. These rigs are used for drilling boreholes using high-pressure water jets for the extraction of minerals. High impact water jet helps in advancing the rotating drill into hard rock. Drill cuttings are carried back to the surface due to the turning action of the drill bit. These machines can be used on land surface as well as in underground mines (limited space).

Pole Hole Rigs. These are special application drilling machines used in civil construction activities like concrete piling, pole erection foundations, conductor casing installations for oil and gas facilities, etc.

Blast Rigs. These small machines help in blasting operations. These machines drill “blast holes” into the rock’s surface that are later packed with explosive material and detonated.

Dewatering Rigs. These are commonly used rigs for various infrastructure projects where a critical aspect of work is dewatering systems. The applications include dewatering systems like deep wells, vacuum well-points, eductor wells, recharge wells in different subsurface conditions like sand, gravel, cobbles, boulders and intact rocks. If not used properly, the dewatering rigs can hamper the success and schedule of an overall project.

Underground Drill Rigs. These rigs have applications in underground metal ore mines and tunnelling and are also referred to as master rigs or face rigs. They are commonly available in single drill booms and double drill booms.

4 Land Drilling Rig Performance Evaluation

  • Key Terms Used
    • ADPM = Automatic Drilling Performance Measurement
    • BHA = Bottom Hole Assembly
    • ILT = Invisible Lost Time (defined herein as the difference between a predefined KPI target and the actual KPI performance shown by a rig. ILT due to rig crew or other components of the entire rig fleet are not considered here)
    • KPI = Key Performance Indicator
    • NPT = Non-Productive Time

Performance evaluation of drilling rig is measured with two broad criteria. The first one is the time required to move the rig to the desired location and erect it to start drilling operations. The second one is the performance of the drilling rig once it is erected and is ready for operation. Based on the first criterion, there is no doubt that truck-mounted mobile rigs perform better than conventional drilling rigs. The second criterion needs more thorough evaluation and is usually better to evaluate rig performance based on an automated detection and judgement process that is not prone to any bias. For this purpose, some auto-detection and evaluation software like ADPM is suitable. ADPM is a tool that is commonly used for real-time analysis of drilling crews and equipment performance. In this article, the scope of such analysis is limited to rig performance KPI, and formation-dependent KPIs will not be discussed.

Automatic measurement and recording of ILT and NPT events during rig operations are almost impossible without a tool like ADPM. Such a tool provides an analysis and calculation of KPIs, which are discussed here. Analysis and quantification of NPT result in determining the root causes of NPT, while KPIs help avoids ILT. A combination of both these findings helps in the determination of saving potential percentage for a drilling operation. This results in learning and improvement through corrective actions for subsequent similar operations.

Possible root causes of NPT linked to drilling rig are rig equipment, downhole tool failure, casing/wellhead issues, lost circulation, hole conditioning problems, miscellaneous and third-party issues. Precious rig time is lost due to any of these route causes, and recorded reading help in their identification. ILT of a rig is, however, calculated through KPIs, especially those that are the highest time consuming among others and are routinely performed while drilling, such as:

  • Tripping indicators
    • Casing running – slip-to-slip connection time
    • Casing – pipe movement time
    • Tubing running – slip-to-slip connection time
    • Pipe movement time
  • BHA Slip-to-Slip connection time
  • Drilling Weight to Weight Time
  • Wellbore conditioning and treatment

Through this method, it is easily possible to create a quantified drilling performance assessment ratio. Such ratios are generated by first establishing technically possible quickest time limit standard for every sequential step in a multi-step drilling operation. Actual time performance comes from the time recorded for each sequential step in the actual drilling operation. A comparison of actual with standard time limits initially will give the drilling performance ratio for a particular drilling rig.

For all companies deploying drilling rigs should be conscious about minimising precious drilling rig time. This is particularly true for oil and gas companies where cost-cutting is currently the name of the game. Increasing drilling efficiency by reducing drilling cost provides the best recipe for cost reduction in today’s low-price environment. Barring other costs like casing, personnel, drilling fluids, etc., rig movement and deployment costs are the high costs in oil and gas exploration and development activities. Explorationists agree that such costs are a direct function of time – the cost of a well depends mainly on time it takes to drill a well and complete it successfully. The quicker a well is drilled and completed, the lesser is the overall cost of the exploration and development activity. Thus, it is imperative for the companies that are engaging drilling rigs to minimise NPT and identify ILT, which often occur due to avoidable actions such as inexperienced crew and sub-optimal drilling equipment. Therefore, knowledge of ILT and NPT events is the key to saving a considerable sum of valuable money spent on drilling projects.

It can be said without doubt that monitoring the performance of a drilling rig and taking actions to improve it has become essential for petroleum asset operators for improving a company’s bottom line.

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