Unmanned Aerial Vehicles: Safe air inspection

Unmanned Aerial Vehicles (UAVs) are available in all sizes and are applied to various fields of services including aerial mapping, surveying, security, inspections, etc. When it comes to the wind energy market the UAVs or drones are already being used for blade inspections on land based turbines. The idea has been picked up by a few companies to apply the drones for inspections in the offshore environment. Offshore WIND spoke to Pieter Franken, Managing Director of one of such companies, Amsterdam based Skeye B.V.

Created in January of this year, Skeye B.V. is specialising in aerial surveying and inspections using unmanned aircrafts. The company combines the experience of two existing Dutch companies, Image2Info bv, specialised in aerial photography, and Deep BV, a survey company specialised in hydrography, geophysics and oceanography.

A multicopter, the Eagle-Eye, and an UAV, Bird-Eye, were acquired for aerial photography and measurements on land. In January of this year Skeye did their first project for the wind industry. The Eagle-Eye was used to carry out an inspection on the blades of a land based wind turbine after reports of gun shots fired in the vicinity of the turbine.

Mr Franken is confident that this can also be done offshore. “We believe that using unmanned vehicles adds to the safety aspect of the industry. There would be no need for helicopter transport of inspection personnel.” Beside the safety factor it would also be a quicker, and possibly a more economical method of making an inspection offshore.

These existing drones, though, would not be adequate for use in the harsh offshore environment. As a result, the company invested in a microdrone, a miniaturised VTOL-aircraft (Vertical Take Off and Landing), type md4-1000, from German manufacturer microdrones GmbH, which, according to Mr Franken is the ‘Mercedes of the drones’.

Technical specs

The md4-1000, renamed by Skeye into Falcon Eye, is a four-engine armed gearless drone with a diameter of 1.70m (with the blades configured for flying). With a total weight of around 6kg it fits well inside the regulation standard of the 7kg maximum weight for this class of small flying craft. Built using light-weight carbon fibre, most of its weight is in the batteries.

The drone manufacturers state a flight time maximum of 88 minutes, depending on external factors such as payload, weather conditions, etc.

However Mr Franken explains, “This might well apply for the use on land, offshore the conditions will reduce this flight time to what is expected to be 45 minutes. Even with this reduction in flight time we expect the Falcon Eye to still have around 30 minutes of effective operation time, enough to inspect the blades of one turbine with around 15 minutes taken for getting to and from the blades.” As an example, the smaller version which is used for land turbines can stay in the air for only 10 minutes.

In contrast to its smaller ‘sisters’ the Falcon Eye is expected to operate in winds up to force 6. Moreover, it can operate in rainy and dusty weather to a certain limit. This is a very important factor as it is much more difficult to act on sudden weather changes at sea than on shore.

Another important feature is the fact that it is not affected by the electromagnetic fields produced by the turbines. Several independent sensors like GPS, barometric altitude sensor, magnetometer and an inertia measurement unit make this possible. This was tested and proven at the TU Dresden in Germany.

The Falcon Eye is equipped with a 24MP camera, which can also take video films. Due to the use of software that calibrates the lens of the camera it provides images that are geometrically correct. With the use of a point of reference the camera can precisely locate and accurately determine the position of any possible damage to the blades such as cracks.

Requirements for operation

Equipped with AAHRS (Attitude, Altitude and Heading Reference System) the drone is not very difficult to operate. It can be used from any type of vessel with a minimal requirement of a deck space of 6-8m2 and can be operated by remote control or with the aid of a GPS navigation system.

Two persons are needed to do the inspection work. There is only one person needed to operate the drone but with both the eyes of one person fixed on the object on the screen there is no way to see what is happening around you. For that reason you always need a second pair of eyes. This is also a requirement with land based projects, Mr Franken explains.

An additional base station enables another person, for example the project leader, to follow the Falcon Eye’s proceedings in real time downlinked via radio transmission.

Mr Franken is very clear though, “The drone is not a replacement for inspections by rope access but can be used as a preliminary inspection in order to document the state of the blades and make further decisions based on the results.”

Proper documentation will reduce the human error factor and provide solid proof in case of possible insurance claims. With follow-up inspections being carried out by rope access one can also see whether, for example, a crack has deteriorated. It is then up to the company responsible for the maintenance of the blades to determine whether repair work is necessary.
At the moment Skeye is looking to test the Falcon Eye on location in an actual offshore environment. Offshore WIND will keep an eye on the developments.

Sabine Lankhorst