Non Intrusive Sensors - An Answer to Annulus Pressure Monitoring in Subsea Wellhead Equipment
Authors: Adamek F.C. , Jennings C. , Aarskog A. 
Published: October 2008

ABSTRACT
On offshore platform and jackup surface wellhead completions, there is the potential for leakage from the high pressure production tubing and casing strings into the low pressure outer casing string, or from poor cementing jobs. Historically, these completions maintain the capability of regularly monitoring wellhead annulus pressure so that appropriate action can be taken should a leak be detected.
In the past, subsea completions have been oil producers, however, gas production, extreme reservoir pressures, and deeper waters are becoming commonplace. Although subsea wellhead technology and reliability have significantly improved with the introduction of the metal-to-metal sealing system, the potential for annulus pressure buildup still exists. Up to the present, the ability to monitor pressure beyond the first casing string has been virtually non-existent. Such monitoring would require penetration of the wellhead housing, a very hazardous practice. Conversely, non intrusive sensortechnology will be a significant industry advancement. It provides a means of measuring pressure through several inches of steel housing wall thickness.
This paper describes the design, development, testing, and application of non intrusive sensor technology for pressure measurement in subsea wellheads and production trees. The data and test results define and describe the phenomenon of "inverse magnetostriction". This phenomenon allows agnetic sensors to non intrusively penetrate three to four inches of steel in a subsea wellhead housing and measure annulus pressure from less than 30 psi to more than 15,000psi. In addition, test data, charts, and graphs illustrate the sensor's capability of differentiating between pressure, tension, compression, and bending stress imposed on the wellhead. The electronic interface description details how the data is obtained from the sensors, stored, and Iatertransmitted to existing control systems or to the user interface at the surface via an ROV.

THE NEED FOR NON INTRUSIVE ANNULUS PRESSURE MONITORING

Surface Capability To Monitor:
Because hydrocarbons can gain entry into the casing by leaking past the downhole packers, seeping through threads on the pressurized production-tubing string, or leaking through the cement, it remains important to monitor wellhead annulus pressure. When pressure buildup is detected and determined to be unsafe, action can betaken relieve the pressure or kill the well if necessary.
Monitoring pressure in the wellhead varies with the type of hookup. On some offshore facilities (i.e., jackup rigs), the surface wellhead with all the casing strings is suspended from the deck. In this type of arrangement, the innermost casing annulus is accessible, and all the annuluses can be monitored with standard pressure gages fixed to the casing spools and casing heads. With subsea satellite trees, only the innermost annulus pressure maybe monitored. This can be donethrougha separateannuluslinerunningbackto the platform and paralleling the flow line on the ocean floor [1]. Alternately, the annulus pressure maybe monitored through the flow line itself by closing the master valve to interrupt the flow and opening the crossover valve at the tree.

Pressures To Expect:
In the Gulf of Mexico, well pressure can be low enough to require pumping, or it may exceed 15,000 psi.