Page 486 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

breaks or internal flaws, the acoustic energy is dissipated, allowing easy measurement at a receiver and thus revealing the flaw.
18.2 Metal object detection
Metal object detection is necessary for various purposes including salvage, pipeline/cable tracking, subsea construction, mine countermeasures, and the like. There are many methods for sensing submerged metal objects through the use of electromagnetism. In this section, the three most common methods used for subsea metal object detection and their application to subsea operations will be examined—namely:
1. Active pulse inductance
2. Passive inductance sensing of toned lines
3. Sensing of anomalies in the Earth’s magnetic field
18.2.1 Active versus passive
Metal detection techniques typically use some form of magnetic field detection technology. The methods vary based upon whether the magnetic field is being (i) induced or (ii) sensed passively (Figure 18.5).
Magnetic fields may be induced through active stand-off pulse inductance or through remote alternating current tone generation using a conducting line (such as a pipeline or subsea cable). Passive sensing involves varying techniques of sensing anomalies in the Earth’s magnetic field gen- erated by a magnetic field surrounding ferrous magnetic metals.
18.2.2 Active pulse inductance
Active pulse inductance (termed simply “pulse inductance”) is widely used in the shore-based con- struction industry as a “cover meter” for determining the status and cover depth of structural rebar embedded within concrete. In the subsea industry, this technique is used to track and determine the depth of cover for buried cables or pipelines.
The basic principle of induction is quite commonly used throughout the power generation indus- try with applications such as transformers (Figure 18.6). A magnetic field is induced around any conductive metal coil once a current is passed through that coil. The magnitude of the field sur- rounding the coil is proportional to the instantaneous magnitude of the current.
With pulse inductance, an instantaneous current is generated through a coil that produces a known magnetic field surrounding the subject coil. This field decays at a constant rate over time in the absence of any other magnetic field, that is, a field induced through a nearby conducting metal. However, if other conductive materials are present in the near vicinity, a sympathetic current (termed an “eddy current”) is induced within the target.
As depicted in Figure 18.7(a), a sample voltage pulse of 215V amplitude with pulse width 1500 μs drives a current into the sample search coil. At the conclusion of the drive period, the magnetic field collapses with the current falling toward a nominal zero value. The current induced into the search coil results in a magnetic field that decays at a known rate. In Figure 18.7(b), the magnetic field then induces a sympathetic current in a target (Figure 18.7(c)). The current induced into the target then generates and decays further inducing a current back into the search coil—that
18.2 Metal object detection 481