Cuttings re-injection on deepwater drilling operations was once an expensive luxury, but increasingly stringent legislation and a drive to cut costs has led to a resurgence of interest and recognition that a cuttings re-injection system is vital to any drilling operation.
The disposal of contaminated drill cuttings at sea is often not allowed, and transporting separated cuttings to land for further treatment can be expensive and weather dependent. Cuttings re-injection present a practical solution to dealing with contaminated cuttings while complying with environmental legislation. They have the added benefit of reducing transportation requirements, making the rig self-sufficient.
Cuttings re-injection systems are designed to inject the slurrified cuttings back down the borehole and out into a fracture zone. They are normally supplied as complete packages comprising a slurrification system and a re-injection system. A positive displacement pump unit, driven by an induction motor with speed control by an inverter is used to re-inject the slurry.
The desired injection rates and pressures will determine the pump selection and ultimately the power required and package size. Power requirement and unit weights are obviously critical considerations for offshore applications. The pumps are selected for continuous duty service, to ensure long term reliability and low maintenance requirements in the field.
A system designed by Procon Drilling Services of Norway for Statoil's Gulfaks A platform in the Norwegian sector of the North Sea used the following components:
- 300 kW high-pressure pump (Calder Ltd) and Eexde motor housed in an offshore container
- Centrifugal pump that acts as a boost pump for the system
- High pressure pipework, valves and operator control panel
- Drives and controls instrumentation (Siemens).
- All equipment was housed in a series of containers that could be transported from one drilling unit to another.
Injection control
Cuttings re-injection systems generally use pressure gauges and transducers that can then be used as a means of controlling the pump. As the fluid is a slurry, there is always the possibility that the slurry can damage the transducer. In response, Siemens and Calder worked closely to design a special control system for Procon which involved measuring the motor current drawn, interpreting this in the form of pressure, and using this to control the injection process.
Further reasons for choosing this system were the motor's suitability for offshore duty, the drive and motor's capability to accommodate variations in pressure and flow rate, and its flexibility to accommodate the system's simple yet safe method of control.
Drives and controls
At the heart of cuttings re-injection systems are the drives and controls. The drives and controls systems are usually manufactured separately and supplied as complete pre-assembled packages to the pump manufacturer.
The drive is through a pump-mounted gear box with the motor mounted horizontally or vertically above the pump to save footprint space. The electric motor in this case (Eexde) was rated for gas groups IIA and IIB. The control of the pump unit is by a variable speed controller that is designed to be mounted in a safe area and a Zone II mini-control center (Eexde) at the slurrification unit achieves local control of the pump unit.
The operator can start and stop the pump, adjust the speed and flow, and read the pressure at the local control center. Some systems incorporate a full suite of monitoring devices to protect the unit, such as pressure, temperature and level switches, thermostatically controlled heaters and micro-switches.
The variable speed controller and motor must provide an optimum speed-torque envelope and minimum current demand. The design of the frequency converter minimizes harmonic distortion and current (EMC) radiation. It achieves high frequency switching and recalculation against the motor model, thus providing rapid and direct control of the motor.
As the number of installed cuttings reinjection systems increases, there is greater tendency to move away from DC drives to those incorporating high performance AC drives. The AC drive system offers the benefits of superior speed consistency, high performance, accurate speed control capabilities, flexibility, smaller installation sizes, reduced power system harmonics, and lower capital costs and low lifetime costs.
As rig operators come to terms with the requirements for cuttings re-injection systems, it is important to consider the variable factors which affect not only the efficiency of the system, but also the flexibility and sophistication of the control systems. With infinite permutations in different areas and conditions, and the opportunities to move packaged cuttings re-injection solutions from one rig to another, an efficient control system is crucial.
