Energy Efficiency

Energy saving potential in diesel electric drilling rig

Abstract

The life of oil stock in India with the present rate of consumption is not more than 21 years. The life of natural gas stock is predicted to be 38 years. India though a developing country, its consumption per dollar of GDP is as high as 0.77 kg of oil equivalent compare to developed country like Japan's 0.1 kg of oil equivalent. This indicates that there is a huge potential for conservation by efficient usage of all forms of energy including petroleum products. With this point in mind, a comprehensive energy audit of a diesel electric (ACSCR) drilling rig was undertaken. The paper presents a few of attractive energy options: optimum loading of DG sets, load management of transformers, replacement of over-sized motors by appropriately sized ones, replacement of V-belts by modern flat belts, plugging of compressed air leakage and use of time switches.

Introduction

Energy is a critical input for the growth of national economy. The energy need in India have grown exponentially due to explosion in population coupled with high rate of industrial growth. Even with huge investment, it does not appear that shortage of power & oil will be met by the planned increase in production. Hence, to meet the growing demand of energy in our country, it is essential to adopt low cost option of increasing the end use efficiency of energy resources for the same level of production. With this point in mind, a comprehensive energy audit of a diesel electric (ACSCR) drilling rig was undertaken with the following objectives.

* To carry out a detailed performance evaluation of various equipment with a view to identify thrust areas for energy conservation

* To quantify the extent of energy savings/performance improvement that can be achieved based on analysis of the measurements to be carried out/data to be collected

* To recommend various technically sound and economically viable measures to improve the end-use energy efficiency

The recommended improvements ranged from low-cost maintenance (housekeeping) items to long term measures. Table 1 shows that total savings of Rs.5 lacs can be achieved in drilling one well by investing Rs. 1.7 lacs in such measures. The investment would provide a total estimated energy savings of 39.6 MWh in electrical energy and 36.8 KL of High-speed diesel (HSD).

Table 1: Summary of potential energy savings

Sl Suggested measure Savings for 3000 hrs

of operation Total annual saving (Rs.) Required

Investment (Rs.) Simple Payback period (Years)

Electricity (kwh) HSD (L)

1. Optimum loading of DG sets - 36,800 3,68,000 - Immediate

2. Load management of transformers 1000 - 3500 - Immediate

3. Replacement of oversized motors 13821 - 48,300 - Immediate

(swapping of motors)

4. Replacement of V-belt by flat belts 16,065 56,200 45,500 1.24

5. Plugging the compressed air leakage 3,420 11,900 - Immediate

6. Use of timer switches 625 2200 3,000 0.5

7. Use of slim tube lights 864 3024 2880 1 (Year)

Total 35,795 36,800 4,93,124 51,380 1.3 months

Rig description

Drilling rig looks like a 40 to 50 meters high electric transmission tower consisting of rotary system, draw works, engines, mud pumps etc. It produces a vertical hole to create a communication channel for products like crude oil and associated gas to be made available on the surface. The hole is drilled by piercing the rock with the help of the cutting bit attached to hollow drill pipe driven by (electric motor) engine power. The process of drilling for hydrocarbons requires the circulation of mud slurry between the drilling rig and well which lubricate and cool the drill bit, 'plug' the well and return rock chips (cuttings) to the rig. These cuttings are then separated from the mud through de-sanders and de-silters.

Drilling has to be suspended at different depths to change bit if worn out, lower casing pipes or perform special operations like corning, electrologging, drill stem test etc. After the well is drilled to the target depth, cased and cemented, it is hermetically tested. In case hermetical test of the casing and well is ok, the well becomes ready for production of various prospective horizons, which starts from the lowermost horizon.

The main source of energy at drilling rig is HSD oil used for captive power generation by Diesel Generating (DG) sets. Loads at rig can be broadly divided into AC and DC type. The power is generated at 600 V, AC and goes to a common bus-bar from where it gets distributed to both AC and DC loads. For AC loads, the power is first stepped down from 600 V to 415 V using two transformers of 500 kVA rating each. The power at 415 V is then distributed to different AC motors and lighting loads. For DC loads, the power at 600 V, AC is first converted to DC power and then fed to different DC motors of 900 HP rating each.

The total installed load of the rig is 4719 kW. AC loads form only 14.6 % of the total installed load while 85.4% is constituted by DC loads.

Energy conservation opportunities

Energy conservation opportunities (ECOs) were explored in five major energy-using systems at the rig: DG set, electric motor, and transformer, air compressor and lighting systems.

The audit team spent one week at the rig for carrying out measurements and collection of data. Discussions were also held with officials and engineering staff regarding the operations and scheduling of the energy-using systems. Various instruments such as ultrasonic leak detector, power master, lux meter, digital stopwatch, etc were used for measurements.

The ECOs that were examined at the rig encompassed of the five major energy-using systems and are discussed below, following a description of existing systems.

DG sets

The captive power through DG sets is the only source of electrical power at the rig. During non-drilling