An innovative hammer-union pressure transmitter, which can be easily repaired or replaced on location, has improved the accuracy and response time for mud-pulse communications used in most directional-drilling operations.
This sensor, which measures the hydraulic and pressure characteristics of the drilling fluid, plays a pivotal role in communicating with downhole measurement-while-drilling (MWD) tools.
Since mid-1998, more than 300 of these devices have been installed in directional drilling applications worldwide, with only five reported failures. This reliability, in light of previous failure rates of 50% over a 6-month period, provides a testament to the integrity of the design.
These devices have been used by Conoco Inc., Shell Offshore, Coastal Oil & Gas Co., Union Pacific Resources, and others on a variety of directional drilling operations.
Pressure regimes
Control the flow of mud within the directional drilling process, typically used for bit cooling, lubrication, hole cleaning, and mud-motor hydraulics, it is necessary accurately to measure and control the static pressure, normally maintained within a range of 2,000-4,000 psi.
To use the mud as a communication's medium, however, it becomes necessary to measure accurately a dynamic pressure component, otherwise known as a pulse. This pulse, which is superimposed upon the static pressure, is normally maintained in the range of 10-80 psi.
Unfortunately, the pressure transmitters used to make these measurements have not always been reliable as a result of severe shocks and vibrations suffered throughout the drilling operation.
For years, the oil and gas industry has measured static drilling mud pressure using a bonded, strain-gauge type of pressure sensor that incorporates a diaphragm that is integral to the machined WECO front end. For safety reasons, a special fitting has been incorporated as part of the sensor. This fitting, also known as a WECO "wing union," consists of a large mushroom-shaped head held in place by a large wing nut (Fig. 1).
In order to achieve a high-integrity seal, the wing nut is hammered tight with a 20-lb sledgehammer, hence the nickname "hammer union." The advantage of using the hammer union is that it can be assembled quickly. And because no threads are relied on for containing the pressure, it can be easily removed, even if the drilling mud has dried to a cement-like hardness.
When the wing nut is hammered on, however, the compression causes a serious offset for a traditional bonded-strain gauge-pressure sensor, typically in the full-scale pressure (FSP) range of 5-20%. The tighter the wing nut is hammered, the higher the offset. Thus the electronic system must be zeroed out prior to start-up.
The problem
In 1997, Baker Hughes Inteq determined that hammer-union devices created one of its most serious maintenance problems, resulting in interrupted services that cost an average $3,000/occurrence.
This prompted a concentrated effort to design a hammer-union pressure sensor that would improve measurement accuracy while solving the reliability issue. Altogether, Baker Hughes invited seven companies to participate in this endeavor, with one company ultimately addressing the following nine design issues:
- Premature failure of the electronics.
- Ingress of water and other contaminants.
- Reparability.
- Large zero offsets during installation.
- Slow response time.
- Longer length body for easier handling.
- More comfortable handle.
- Fundamental improvements on static and thermal accuracy.
- Improved long-term stability.
The first two problems are related in that improper sealing of the electronics enclosure creates unavoidable repair issues. For example, the use of O-rings, nuts, bolts, and threaded assemblies provide leak paths for moisture ingress. Additionally, the lack of an encapsulant in the electronics module results in direct shocks and vibration damage to the sensor.
The new hammer-union pressure transmitter, developed to solve these problems, has been rigorously tested against six devices. Previous static accuracy, including non-linearity, hysteresis, and non-repeatability remained in the range of ± 0.25-1% FSP with temperature errors in the range of ± 2-3% FSP. The new hammer-union pressure transmitter's accuracy is improved to better than ± 0.1% FSP with temperature errors in the range of ± 1% FSP.
The environment on a drilling rig is extremely demanding and hard on equipment. For example, the hammering of the sensor can induce shock levels greater than 2,000 g. Additionally, the device remains exposed to constant vibrations and varying temperature extremes ranging from -40 to +130° F. And new drilling muds combined with other chemicals and salt water also present corrosive conditions that only selected materials can withstand.
One company reports more than a 50% failure rate over a 6-month period on some hammer union-type pressure sensors. In other cases, 20% of the devices failed while being hammered into the fixtures due to excessive zero-pressure offset-a condition in which the device shows pressure when no pressure is present.
The technology
The piezo-resistive silicon technology used to solve these problems has been developed and refined over the past 26 years into one of the most accurate and stable sensors of its kind. Extensive applications in the oil and gas, aircraft, and automotive industries have resulted in improvements in accuracy, stability, and reliability.
The heart of the sensor is a high-stability, pressure-measuring element that has been micro-machined from single-crystal silicon. The piezo-resistive silicon element is mounted within a high-integrity glass-to-metal seal. This element is fully isolated from the pressure medium by an Inconel isolation diaphragm that has been electron-beam welded to the front of the seal.
This proprietary, low oil-volume isolation feature allows for fast dynamic response times with minimal thermal errors. Additionally, the small, compact design, configured as a replaceable sensing element, provides mechanical isolation that eliminates the stress exerted on the sensor by the tightening of the wing-nut (hoop stress). As a result, no zero offset error occurs while hammering on the wing nut as encountered by traditional designs.
Surface-mount electronics also condition the output from the silicon sensing element, correct the data for thermally induced errors, and configure the output as required.
Design features built into the electronic circuitry enable minimum sensor size. The electronic systems incorporate power-supply regulation, reverse-polarity, over-voltage, and short-circuit protection into the design, coupled with electro-magnetic compatibility (EMC) protection components.
Finally, the fully encapsulated solid-state design incorporates dual glass-to-metal headers that provide a secondary pressure containment backup system to the sensing diaphragm. This provides component integrity under high levels of shock and vibration.
The solution
The problem of premature failure of the electronics, a result of shocks and vibrations, is solved by using low-mass, surface-mount components, fully encapsulated in a potting compound.
The use of high-reliability components provides protection against electrical failures while the use of electro-magnetic interference (EMI) filters and other components protect against electrical noise.
Additionally, the fully encapsulated hermetic enclosure solves the problem of moisture ingress. This is made possible using a replaceable or throw-away pressure transmitter concept. All primary seals of the replaceable pressure transmitter are electron-beam welded, eliminating potential leak paths.
Engineers solved the problem of zero offset, caused during the hammering operation, by redesigning the way the replaceable pressure transmitter is mounted. Additionally, with the introduction of a metal-to-metal cone-type seal, located at the rear of the sensing element, the unit undergoes less stress.
Thus, substantially less zero offset is suffered than from traditional hammer-union devices that are machined from a solid piece of Inconel, including the sensing diaphragm. In extensive testing, the new device encountered no zero offset during hammering operations.
The device also incorporates several improvements that make handling easier. The handle is fitted with a plastic rotating tube that increases the diameter of the handhold, making it more comfortable to carry.
Additionally, the length of the device has also been increased, making handling easier once the handle has been detached. This also allows the connector and protective cap to be completely recessed, reducing the possibility of damage if dropped or from a misjudged sledge hammer blow.
Response time
During directional drilling applications, decisions are based on data received from the MWD tool. Communications are normally accomplished by transmitting mud pulses through the drilling fluid then encoding the data with a transponder. The decoded data are used in algorithms to present data that then assist the operator in making better drilling decisions.
In the current process, this can take from 2 to 5 min, depending on current TD. Considering the high cost of rig time, any time wasted increases the expense of the operation.
Fortunately, the device has a 2-khz response time that allows pulsing to be compressed three to five times faster than normal. It is conceivable that real-time decisions can be made, saving hundreds of thousands of dollars in rig-time.
Detecting pump problems
Fig. 2 shows a screen print of the raw mud-pulse signal (top) and the decoded or filtered signal of the mud pulse telemetry (bottom). As shown, mud flowed at a rate of 300 gpm accompanied by pulsations of ±0 psi from a base pressure of 380 psi.
The lower left corner of the figure shows the presence of harmonic or rhythmic pulsations that denote pump noise. Worn-out liners and other pump anomalies caused these harmonics.
Because the piezoresistive pressure element, manufactured by Druck Inc., operates with a 2-khz response, the operator noticed the pump noise immediately. With the former bonded strain gauge-type sensors, however, this phenomenon could not be detected.
Therefore, sensor responsiveness can also be used as a maintenance tool to help prevent premature pump failure. It is interesting to note that this improved response time, along with the excellent EMI and RFI (radio-frequency interference) protection, prevented the pump noise from interfering with the signal, resulting in excellent decoding.
With traditional hydraulic and electrical systems that are noisy, the raw signal can be garbled using conventional bonded-strain gauge sensors. Thus, because the signal amplitude is not much greater than the noise, appearing as a fuzzy curve, it is nearly impossible to decode this signal.
Data quality
In the example shown in Fig. 2, one can see by the bottom chart that the decoding resulted in a clean stream of data from the downhole tool. Without high-quality decoding, no tool information can be made available at the surface for interpretation by the mud-pulse telemetry software.
An additional advantage of the piezoresistive, silicon hammer-union sensor is its fast response time. As more data become required uphole and as the operator requires faster mud-pulse telemetry signal rates, the 2-khz response time is able to keep up.
This allows faster data collection with a higher degree of accuracy. Faster decisions mean less rig time and a reduction in operating costs.
The sensor also improves full-scale static accuracy to ±0.1% FSP with an overall temperature error band of 1%. This is an improvement of more than 2.5 times over traditional designs. The long-term, full-scale stability of the device is 0.1% FSP/year. These factors improve information reliability.
Replaceable transmitter
The replaceable, throw-away pressure transmitter module forms a compact, rugged design that is easily replaced if damaged (Fig. 3). One of the most common causes of damage occurs when attempting to clean out dried mud using a screwdriver or other pointed object.
The actual housing with the large head is reusable. And if the connector becomes damaged, its assembly is also replaceable.
Replacement can be accomplished aboard offshore platforms or other remote locations, eliminating the necessity for shipping damaged devices back to the manufacturer for analysis and repair. And by stocking the replacement elements, the operator maintains inventory control on a local level.
Thus, down time is all but eliminated, and excessive inventory of complete transmitter assemblies is reduced to onsite storage of spare transmitter elements, O-rings, and connectors.
The Authors
Norm Ditrich is a surface systems hardware team leader in Baker Hughes Inteq's drilling and evaluation engineering department. His duties include new equipment design, product reliability improvements, field failure analysis, and reduced life-cycle cost evaluation for equipment used in MWD applications.
Ditrich holds a BS in mechanical engineering from Penn State University (1969) and has held a variety of positions in engineering, manufacturing, and management over the past 25 years.
Jan Matthews is product manager of industrial sensors for Druck Inc. After a career in the US Navy as a helicopter pilot, he entered the field of engineering and gravitated into the sales and marketing field where he held various positions in sales and management. Matthews holds a BS in electrical engineering from the US Naval Academy (1966).
