GNSS


Offshore application of EGNOS

Mar 2009 | No Comment
The GIANT project has been pushing ahead with the necessary activities to make EGNOS a viable navigation system in the hazardous North Sea environment

Oil was first commercially exploited in the North Sea in the 1960’s. Production is underpinned by regular and reliable helicopter operations enabling the movement of staff and equipment to and from the shore. However, the North Sea environment is challenging for rotorcraft operations from many perspectives, not least its remoteness from the shore, the exacting weather conditions and the changeable nature of rigs. There have been six fatal accidents since 1976, with the loss of 79 lives.

Today there are more than 300 helidecks in the UK sector alone being serviced by regular flights. Approach options in Instrument Meteorological Conditions (IMC) are limited to using the aircrafts’ weather radar to identify the rig. This is neither designed nor certified for the task and following a UK Civil Aviation Authority (CAA) review the need for an accurate and reliable instrument approach aid for conducting offshore approaches has been highlighted.

With support from the UK CAA, the GNSS Supervisory Authority’s (GSA) GIANT project has sought to develop a new approach procedure based upon SBAS guidance, in this case EGNOS.

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Table 1

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Figure 1

North sea operations

The North Sea – particularly at the latitudes of the oil fields – can be an inhospitable environment. A helicopter operating to the rigs is exposed to the full wrath of the weather. During winter lightning strikes to helicopters are common. There is hail and sleet to contend with along with icing conditions at typical operating flight levels. Furthermore the helipads can be located hundreds of feet from sea level. This means that a cloud base of down to 200’ may obscure the pad.

Even without the weather there are still many factors for the flight crew to contend with that are of particular significance once operating in IMC on an instrument approach. Whilst many oil platforms are fixed in their location a number of rigs are moveable and can be relocated at short notice. Some platforms are semi-submersible – tethered to the sea bed, but still free to heave on a heavy swell. Rigs often have moving cranes, gantries and chimneys that are used in ‘flaring’ – burning natural gas. Perhaps most hazardous for a crew on an instrument approach is the potential for moving obstacles. It is not unknown for supply vessels to arrive at the rig whilst the aircraft is on approach. As many of these ships are relatively large their superstructure can easily impinge on the planned flight path.

Despite all of these factors the vast majority of approaches are conducted successfully – even those in IMC at night. This is testimony to the skill and training of the current flight crews.

The current instrument approach procedures to North Sea oil rigs utilise the aircrafts’ weather radar. They are known as Airborne Radar Approaches (ARA). The helicopter initially navigates to the proximity of the rig, it then identifies.the rig using the weather radar display and flies toward it descending on the altimeter at the same time. When closing on the rig the crew will level off and adopt an offset heading to guide them abeam the rig whilst still maintaining radar contact. If by the closest point on this approach (typically 0.75NM) the crew have not achieved visual contact with the rig they will instigate a banking, climbing missed approach. In addition to being used for navigation the weather radar is used continually to look for other mobile obstacles in the final approach such as supply ships.

EGNOS in the North Sea environment

EGNOS provides accuracy and integrity sufficient to enable guided vertical descent procedures and its ability to contain the gross navigation system errors could also permit closer approaches to the rigs to remove the necessity for a climbing, turning missed approach procedure. EGNOS could also maintain cost effectiveness as it requires no ground infrastructure on rigs.

Removal of the traditional procedures using radar equipment not intended for the task, together with the ability to autopilot couple the EGNOS guidance to aid workload could all help to improve achieved safety levels and perhaps even operating minima in the future.

However, the majority of platforms are located in the latitude range of 58°N to 63°N. At these latitudes the EGNOS geostationary satellites are at a low elevation angle (approx 20-25°) and appear clustered in the sky to the south. This factor coupled with the often suboptimal GPS antenna installations common on rotorcraft result in a major challenge to the successful application of EGNOS in this environment. [Table 1]

A new approach

An ideal offshore approach procedure would have a number of new features. Ideally the approach would be straight in but offset from the rig. This would allow for an optimal straight ahead climb for the missed approach. The approach should bring the helicopter close enough to enable visual identification whilst also far enough away to allow for final speed and height adjustments.

One positive element of the current ARA approach procedure is that it provides the crew with the ability to choose their approach heading. This flexibility should not be lost as it allows the crew to optimise their approach taking into account wind direction whilst maintaining clearance from moving obstacles or superstructure. This would however, require the SBAS avionics to generate the approach procedure on-the-fly. This is a challenging requirement for current avionics as SBAS approaches usually consist of a set of waypoints that are typically hard coded in a database.

New approach procedures should be able to provide guidance to the aircraft autopilot. This will help to reduce overall cockpit workload during the critical approach phase of flight and allow the crew to focus on just monitoring the guidance whilst undertaking other tasks. Ideally the procedure should also provide guidance for the descent too that will encompass a stable descent (typically 4° to 6°). The potential for mobile obstacles will remain a reality of North Sea operations.

Therefore a new procedure will still need to provide the crew with the ability to ensure that the path ahead of them remains clear of obstacles. It is likely therefore that the need will remain for the use of the weather radar even on an SBAS approach.

SOAP trial procedure

Within the GIANT project a new offshore oilrig approach procedure was designed that used EGNOS to address the recognised shortcomings of the ARA. The result of this work is the SBAS Offshore Approach Procedure (SOAP). The approach is divided into four segments [Figure 1]. The arrival segment is used for the helicopter to descend to the minimum safe altitude of 1500ft. Once the helicopter reaches the Initial Approach Fix (IAF) it enters the initial approach segment, in which it aligns itself on the final approach heading and decelerates to the final approach speed between 60kts and 80kts – depending on the environment and the capabilities of the helicopter.

During this time the crew use the weather radar to check the system-generated final approach and missed approach areas and verify that they are clear of radar returns.

On reaching the Final Approach Fix (FAF), the helicopter enters the final approach segment and begins its descent to the Minimum Descent Height (MDH, defined as the height of the rig’s helideck plus 50ft, with a minimum value of 200ft in daylight and 300ft at night).

The descent angle can vary depending on the elected final approach speed. Once the helicopter reaches the MDH, it flies a level segment during which the pilot and co-pilot attempt to acquire visual contact with the rig. If visual contact is not made, the helicopter will reach the Missed Approach Point (MAP) and perform a missed approach procedure simply by climbing straight ahead at the steepest safe angle.

The various lengths and angles of the approach procedure can be seen in the figure [Figure 2]. Certain points, such as the FAF, are positioned depending on distances that will vary, such as the distance covered whilst the helicopter is descending (which depends on the descent angle and the MDH value). The procedure also dictates the sensitivity of the guidance to be used, which can be seen in the two figures (3 & 4), the first showing lateral sensitivity and the second showing vertical sensitivity. Throughout the procedure, lateral guidance is provided by EGNOS. Vertical guidance during the arrival and initial approach segments is provided by the helicopter’s baro-altimeter, and by EGNOS backed up by the radar altimeter during the final approach segment.

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