ACFM

Shipping Applications

Introduction

Merchant ships are generally constructed from large steel plates welded together either manually or on submerged arc lines. This offers many opportunities for ACFM inspection. The ability of ACFM to inspect through paint, even underwater, means that ship hulls can be inspected without having to enter dry-dock.

Using ACFM in conjunction with a rope-access technician or diver also avoids the need for scaffolding or overside staging.

Overview

As well as inspecting ships hulls, ACFM has been used to inspect storage and ballast tank welds on chemical and gas tanker ships, LPG spheres and propeller blades and thrusters. In most cases large areas require inspection, a process that can be shortened through the use of an ACFM array probe which can be marinised if required.


Propeller blades are usually inspected underwater by a diver with an underwater instrument. However this is one example where costs can be reduced through the use of a topside AMIGO instrument and a marinised probe with 50m cable in place of a subsea instrument.

 

Although most of a ship is made from ferritic steel, some components (such as the propeller) may be made from more exotic non-ferrous alloys, for example phosphor bronze or aluminium bronze. ACFM probes can be used to inspect virtually any type of metal, with a simple change in set-up.

Projects and Applications

Voith Schneider Propulsor Blades

Project Equipment

Propulsor blade inspection

 

U19 AMIGO system

Underwater 40mm flat array probe 5KHz

50m underwater probe extension

Standard underwater weld probe 5KHz

 

 

In collaboration with a leading underwater maintenance company in the UK, TSC Inspection Systems developed an ACFM system and underwater array probe to provide accurate and reliable detection of defects in aluminium bronze Voith Schneider propulsor blades and phosphor bronze ships propellers.

 

The array probe is capable of inspecting a width of 40mm at once and detecting defects in all orientations in one pass. It is also capable of differentiating between cracks and cavitational pitting, a common problem on all propulsor and propellor blades.

 

Early detection of defects allows instant decisions to be made on whether components need to be replaced, or condition monitoring carried out.

The ability of ACFM to save all scan data allows direct comparison of defects and assessment of any propagation over time.

High Strength Steel (HSS) Welds on Submarines

Project Equipment

Submarine high strength steel (HSS) weld inspection

 

U19 AMIGO system

Standard weld probe 5KHz

 

 

A recent trial for certifying underwater welds in high strength steel, in collaboration with one of the UK's leading underwater ship inspection companies realised a major breakthrough in the assurance that an underwater welding capability, that allows warships, in particular submarines, could be supported afloat.

This avoids, at least, part of the cost of similar work currently undertaken in dock on de-commissioned submarine ‘hulks'.

During 2007, tests were carried out using a nickel-based electrode to provide fillet welds for high strength steels. At the time, the austenitic properties of the electrode restricted the NDT of the fillet welds to visual inspection only. The recent trial of ACFM technology removed that limitation.

ACFM technology was developed to allow crack sizing underwater without the need for any electrical contact and is already being used successfully by numerous underwater contractors involved in defence applications, to find and size cracks and discontinuities in warship and submarine propeller blades.

After a review of the available technologies, TSC's client concluded that ACFM may also be suitable for testing these fillet welds.

The ACFM trial was carried out at the client's UK based offices utilising their own test tank. Divers were used to pass an ACFM probe over fillet welds, into which surface and sub-surface cracks had been induced.

Cargo and Chemical Tankers

Project Equipment

Tanker cargo tank inspection

 

U19 AMIGO system

Standard weld probe 5KHz

Straight mini pencil probe KHz

Right angled mini pencil probe 5KHz
 

 

ACFM inspection techniques have been used on a major international shipping company's chemical transport tankers; for the investigation of possible defects, located in cargo tank floors and top and bottom sloping wall welds, as well as leak detection between cargo and ballast tanks in chemical tankers.

In one case, inspection was carried out during the time the vessel was in transit between offload and load ports, thus no costly downtime or docking was required.

Thruster Inspection

Project Equipment

FPSO thruster blade maintenance

 

 

U19 AMIGO system

Underwater array probe

Micropencil probe

Mini pencil probe

50m array umbilical

Custom build stop hole probe

 

ACFM inspection was carried out on forward and aft stern thruster blades on an FPSO located in the North Sea. The blades on both stern azimuth thrusters had serious cavitation erosion damage. Due to the extent of the erosion the leading edges had become perforated, with a subsequent loss of blade area. Such a situation could accelerate a fatigue failure resulting in a greater loss of blade area, rendering the unit inoperable.

The leading edges were trimmed back to remove the edge damage and blended into the tip without changing the diameter. All five associated lifting padeyes on each thruster were also inspected. 

Diving operations were carried out from a 12m Rigid Inflatable Boat (RIB), launched from the RIB Mother Vessel (RMV).

All areas inspected were located subsea, using diver intervention from the RIB. Alternating Current Field Measurement (ACFM) was used to determine the presence or absence of surface breaking defects in all the specified areas of the propeller blades.

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