P-scan goes deeper

Exploration in deeper waters is driving technology development, not least in subsea inspection. Meg Chesshyre reports.

Diver-based pipeline scanning. Images from FORCE Technology.

Denmark’s FORCE Technology, with its P-scan system, is a key player in the specialized NDT nondestructive testing technology (NDT) subsea inspection market.

The company was formed in 1939, as the Danish Welding Institute, to perform X-rays on pressure vessels. Now, FORCE Technology is looking to take technology first developed in the 1980s into ever deeper waters.

FORCE’s proprietary Subsea P-scan enables inspection of subsea structures, such as tension legs, jacket structures, monopiles and pipelines, for fatigue cracks, welding defects and corrosion mapping. Based on a magnetic wheel scanner, the P-scan can be fitted with a wide selection of ultrasonic probes and/or eddy current probes, depending on the type of application, such as crack detection, corrosion mapping or weld inspection. The ultrasonic probes detect cracks in the full volume of the objects, and the eddy current probes detect surface-breaking cracks.

The main purpose of the inspection is to verify that no service-induced indications are present in the welds. Welding defects can also be detected, but it is normally assumed that their size is below the original acceptance criteria and therefore will not be taken into consideration during development of an inspection procedure.

FORCE Technology developed the first diver-operated subsea ultrasonic inspection system in the mid-1980s. In the late-1990s, requirements for performing inspections in deeper waters led to the development of the first remotely operated vehicle system to enable subsea inspection depths of 1000m.

An ROV performing a Subsea P-scan.

“Presently, we operate down to 1000m, but we are working on getting down to 3000m,” says Ole Nørrekær Mortensen, who has served as a project manager for the Advanced NDT Global division of Force Technology for the past 14 years. The next generation of the technology is comprised of phased array ultrasound, which will provide more information about the condition of the subsea structures and allow for faster inspection. FORCE Technology is developing 3D results presentations—an upgrade that is due to be available by the end of 2015—which would replace 2D.

Tether string weld inspection specialists

Tether string weld inspection is also an area of expertise for FORCE, demand for which is increasing, Mortensen says. “There are a lot more platforms of this type, and they are all coming of age. The timing depends on the design life of the tension-leg platforms (TLPs) and the requirements from authorities and classification companies.”

FORCE Technology has already carried out an inspection of Statoil’s Heidrun and Snorre A platform tethers in the Norwegian sector of the North Sea and of MC Offshore Petroleum’ Jolliet TLP in the Gulf of Mexico. The first inspection for Heidrun was in 2003, and FORCE Technology will revisit the platform this year as part of an on-going inspection program. The Jolliet inspection in 2012 was the first in the region for the US Coast Guard and the client before any legislation was in place.

Ole Nørrekær Mortensen

During the past decade, FORCE Technology’s subsea inspection system has proven its ability to perform valuable inspection on various tether string welds and has at least the same capability as the inspection system used to perform inspection during production. The inspection system is fitted with 16 ultrasonic channels that can include any type of ultrasonic probe, shear wave, compression wave, creep wave or time-of-flight diffraction. The probes can be combined arbitrarily as required by any inspection procedure. The inspection system also allows for the addition of up to eight eddy current channels.

Based in Brøndby, near Copenhagen, FORCE Technology has grown during the past 75 years through mergers and acquisitions to employ about 1400 people throughout its various business divisions. About 40-50% of its income is derived from its NDT business, about half of which is focused on offshore activities. The NDT department has about 250 employees, and the advanced NDT department, which focuses on subsea, employs 30. The firm also operates subsidiaries in Sweden, Norway, China and Singapore.