SPA™ – Subsea Phased Array
SPA™ – Subsea Phased Array
In recent years, Phased Array (PA) technology has become a standard in-service inspection discipline for high-resolution corrosion mapping and crack assessment for topside operations. TSC Subsea, working closely with its business partners, Eddyfi Technologies, has brought the many benefits of Phased Array into the subsea inspection world. The resulting Subsea Phased Array (SPA™) system incorporates conventional PAUT and ToFD but also some advanced applications using the latest technology developments of TFM (Total Focussing Method).
Phased Array has the ability to simultaneously collect A-scan data at a number of given angles. This unique feature produces a volumetric beam allowing operators to distinguish between geometric reflectors and defect signals and therefore increasing the likelihood of detection (see figure 1). In addition, this ability also improves flexibility on complex geometries as the beam can be steered to suit the requirements of the inspection. Conventional methods of ultrasound inspection are based on fixed angle probes which can be severely restrictive when inspecting parts with unfavourably orientated discontinuities.
Further advantages of Phased Array are that the data sets can be saved and with the advantage of visual aids makes this inspection fully auditable allowing clients to review data sets as they are collected.
Industrial volumetric inspections have predominately included either radiography and/or ultrasonic methods. Radiography has the obvious disadvantage of being an extreme safety hazard, and in addition has a poor record at detecting certain planar defects such as cracks and lack of fusion as shown in Figure 2. Manual ultrasonic testing is more sensitive than radiography to planar defects, but is slow, and the results are highly subjective.
These factors were the driving force behind the introduction of automated ultrasonic testing (AUT). Unfortunately, AUT typically involved large, expensive and inflexible systems that were impracticable for subsea ROV deployed advanced inspection. Phased Array has the ability of bridging the gap between radiography and UT by reducing unwanted safety hazards, improving flexibility, data storage imaging and repeatability.
In general terminology PAUT technology is separated into two inspection forms: linear and sectorial scanning. Linear scans are constructed by multiplexing chosen apertures along an array of elements. Typical arrays can have between 64- 128 elements, pulsed in groups of 8 to 16. Linear scanning permits large coverage with a tight focal spot, ideal for highly accurate wall thickness measurements when corrosion mapping. Phased array corrosion mapping allows high resolution imagining without sacrificing productivity.
Figure 3. C-Scan display showing thickness variations
Sectorial scans use a fixed set of elements but adjust the time delays to sweep the beam through a series of angles simultaneously. Because of the versatility and flexibility of sectorial phased array scans, there are currently a vast number of recognised applications.
Figure 4. Sectorial scan being using advanced backscatter for crack detection and sizing
TSC Subsea’s SPA system utilizes the Eddyfi M2M Gekko 64:128 TFMPA housed in a diver or ROV mountable bottle rated to 750 meters. Where applicable, TFMPA has many advantages over conventional PA. TFMPA consists of adapting the receiving delay laws to focus on many points which form a grid, after a single pulse which generates a large and/or divergent ultrasonic beam. This means focusing can be achieved in the entire inspection area.
Comparisons between PA and TFMPA demonstrates clearer imaging, offering higher accuracy, as show in the below Figure 5 & 6.
Figure 5. Near surface increased resolution using TFMPA
Figure 6. Far surface increased resolution using TFMPA