OFFSHORE DRILLING RIGS
The energy needed on offshore drilling rigs is usually supplied by diesel engines. Typically, these engines use 20–30 m3 diesel fuel per day, depending on the operations performed. Several measures can be used to reduce energy consumption, the amount of diesel burned, and emissions to the air. Measures to reduce energy consumption can be divided into two categories:
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1. Reducing the amount of energy needed on the rig
2. Enhancing power management system
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To reduce the energy demand on the rig, it is important to plan the drilling operations well. An efficient drilling process gives lower fuel consumption per drilled foot, and therefore fewer emissions. Automatic mud-mixing systems reduce costly mixing mistakes, exposure to hazardous material, and excessive emissions. Careful planning by drilling engineers and logistic personnel can reduce downtime and result in a more efficient drilling process. Integration of a remote-controlled rotating and hoisting cement head with top-drive casing-running operations reduces equipment rig-up time, leading to less transition time between casing-running and cementing operations. Finally, a reliability-centered maintenance (RCM) programme can also reduce rig downtime, improve safety, and provide a better return on investment.
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The design of the drilling rig is also important. Well-designed working areas and living quarters reduce the need for heating and cooling and are especially important in harsh, cold environments, where the need for heating is typically large. Hull shape and topside design of the drilling rig create wind drag. If this wind drag can be reduced, the energy consumption can be reduced.
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An important factor influencing the energy consumption on an offshore drilling rig is the means by which the drilling rig is positioned. Moored vessels have far lower energy consumption compared to dynamically positioned (DP) vessels, because the engines on DP vessels are using energy to position the rig.
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The choice of heave compensation system also has an impact on energy consumption. The use of active heave draw works (AHD), a fully electric solution, has different energy needs compared to the cylinder rig solution or traditional crown mounted compensator (CMC) because these compensation systems rely on different combinations of hydraulic and electrical equipment. The main advantages of hydraulic equipment are the power-to-size ratio of the actuators and their energy-storing capability; hydraulic equipment is smaller and lighter than its electrical equivalent, whilst the gas accumulators used in hydraulic systems store temporary energy fluctuations in a cost-efficient manner, and will continue to work in the event of a power failure. The disadvantages of hydraulic equipment are the need for a large and heavy hydraulic power unit (HPU) required to power the equipment, and the temperature dependency of the system. The placement of the HPU on the rig can be problematic, especially for floaters. The properties of hydraulic fluid vary with temperature and can have an impact on the overall performance of the system. On the other hand, the overall efficiency of electrical systems is 85–90% compared with approximately 70% for a hydraulic system. This increased efficiency makes electric power the preferred option for high-powered equipment. Electrical systems also allow accurate control of both torque and speed and eliminate the environmental hazard of hydraulic fluid leaks. The main limitation for the electrical system is energy storage, which is typically in the form of large and heavy batteries.
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The CMC system uses a standard derrick and standard draw works with a hydraulically compensated system installed on top of the derrick. This system inflicts the least amount of load on the derrick structure but has limited heave compensation ability. Its top-heavy weight distribution can affect vessel stability and reduce deck load capacity. The CMC will have far lower energy consumption while operating in harsh areas compared with other heave compensation systems. A diagram of a CMC system is shown below.
Figure 1: The Shaffer crown mounted compensator
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The cylinder rig solution replaces the derrick with a mast, and the draw works with hydraulic cylinders. This configuration lowers the centre of gravity of the rig and reduces the weight of the tower. The heave compensation ability is limited by the design of the compensating cylinder. Although the system requires a heavy HPU to operate, the typical placement of the HPU beneath the rig floor improves rig stability by lowering the centre of gravity. The use of multiple cylinders and wires provides redundancy in case of failure. The replacement of draw works with cylinders eliminates much of the noise on the drilling floor.
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The AHD system also uses a standard derrick but with fully electronic control of the draw works for heave compensation. AC motors provide accurate control of the draw works with a typical compensation accuracy of less than 2%. The regenerative power created by braking can be fed back into the rig for consumption by other equipment. Like the cylinder rig solution, the AHD design has a lower centre of gravity than CMC systems but has a lower weight than both the cylinder rig and CMC systems. Heave compensation is not limited as in the other systems. The main disadvantage of AHD systems is the use of AC-powered draw works, which can be noisy in a confined work environment.
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Enhanced flexibility in the energy production on the rig can be achieved by using power management systems and applying a power load philosophy. The intention here would be to run the generators at the correct load rather than run all generators on idle. To enable this, a mix of different power output (sizes) of generators can be used; alternatively, operating most generators on optimum load and one or two generators on variable load can be a solution. Simple electrical power distribution systems can reduce the frequency of blackouts by reducing the number of assignment systems and crossover connections. Where system components are fewer and more efficient, production and maintenance costs will be reduced, and the equipment room will have a smaller footprint on the rig.
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Heat recovery systems used to recover heat from exhaust gases can be used instead of heat production from steam boilers, thermal oil boilers or electrical heaters. This will also serve to reduce energy consumption.