Ultrasonics is a safe and reliable method for inspection and monitoring. The most common application of ultrasound in NDT is probably to measure the wall thickness, where the method gives a direct and quantitative result.
If ultrasonic sensors are installed permanently, more accurate monitoring is possible, because the error and uncertainty associated with operator and exact measurement position is removed. The current paper describes a system where ultrasonic monitoring is carried out by using an instrumented clamp. Contactless immersion ultrasound is used, which allows for this system to be retrofitted on existing subsea pipelines.
The paper describes lab experiments which indicate that this instrumentation can be used to estimate annual corrosion rate with an accuracy of 0.1mm per year within a short period (days).
The monitoring concept presented in this paper is based on the use of a large array of individual ultrasonic sensors which are installed on the pipeline in the form of an instrumented clamp1,2. A main challenge in this activity has been to develop this concept so that the instrumentation can be conveniently installed using standard subsea installation tools and procedures.
The monitoring clamp is designed to be installed onto the pipe from above. The clamp consists of two hinged halves, with hydraulic actuators for opening and closing. Everything is mounted on an installation frame with guiding structure for simple landing onto the pipe, and buoyancy on the top to ensure correct orientation and neutral weight in water. The ultrasound instrumentation is based on a modular component (ultrasound 'casette'), which contains all functions needed to carry out ultrasound measurements. Each of these casettes has been designed to survive in seawater and subsea pressure, so the clamp's only purpose is to provide mechanical protection and correct position for the instrumentation. Data communication between casette units and the outside world is digital, and a large number of units can be combined to implement large arrays. The electronics is made waterproof by polymer mould. The drawings in Figure 1 illustrates how each of these ultrasound casettes are placed relative to the pipe, and how they are mounted in the clamp for protection. A larger part of the circumference of the pipe can be covered by placing several ultrasound units side by side in the clamp. (available in full paper) The drawings in Figure 1 illustrates how each of these ultrasound casettes are placed relative to the pipe, and how they are mounted in the clamp for protection. A larger part of the circumference of the pipe can be covered by placing several ultrasound units side by side in the clamp.
If the pipeline is buried, some excavation is needed.
Pipeline coating is removed by a stripping tool (high pressure waterjetting)
The clamp is brought in place by the ROV, and moved to its final position
The unique property with this monitoring concept is that it is possible to install the equipment on an existing subsea pipeline assisted only by an ROV. Installation procedure of the subsea retrofit monitoring clamp will be as follows:
