OPINION: Targetless DP is the way forward offshore

Although they are essential to dynamic positioning, co-operative targets, or reference points, on each new turbine structure could be a prohibitive cost.

Sam Wood, head of Product Management IA and Autonomous Vehicles at Navtech Radar, says W-band radar will be increasingly important in aiding situational awareness that depends on accurate, reliable and continuous position information so essential for dynamic positioning and navigation.

Radar has been synonymous with the marine industry since its inception in the 1930s.

Less well known is relative newcomer W-band, with its much higher resolution and reliability at near-distance ranges where Lidar has conventionally been used with cameras and GNSS (global navigation satellite systems).

To aid situational awareness and dynamic positioning, W-band is becoming increasingly crucial to the many applications we see in the marine sector today, from offshore environments that are increasingly concentrated with man-made structures, to near-shore and inland waterways, where the cost of collisions and congestion can be of colossal proportions.

The necessity for dynamic positioning

Dynamic positioning (DP) originated in the 1960s and developed with the advent of offshore drilling. As drilling operations moved into ever deeper waters, conventional methods like jack-up barges became inadequate, while anchoring in deep waters were uneconomical.

DP technology has advanced. From its initial development with analogue controllers and limited redundancy, it has evolved into a highly sophisticated system with superior capabilities and is no longer restricted simply to maintaining a fixed position.

Modern DP systems offer the versatility of sailing an exact track, making them invaluable for tasks like cable and pipe-laying, surveying, and myriad other tasks required to service wind farms.

A modern DP-equipped vessel is reliant on its position reference sensors to enable the advanced functionality afforded by advanced computer control, which falls broadly into global and local references.

Global references are systems such as GNSS, which rely on a set of globally visible satellites or other known global reference to provide a position. Local reference systems provide a range and bearing to a local cooperative target, typically laser or Lidar position reference sensors.

Most of these platforms – oil and gas installations in the main - are equipped with laser targets that allow supply vessels to line up with the appropriate spot on the platform and then maintain position using DP. An ideal operation will allow the vessel to approach the platform using multiple local reference sensors, improving safety, thorough redundancy and speed of operation.

In the growing seascape of wind farms, however, there is a paucity of targets, which has meant new methods and systems being deployed to counter the requirement to fit every wind turbine with a target, each with their own power supply, electronics and inherent vulnerabilities, not least cost. The first on the scene has been Lidar, which measures the time difference light travels to and from the target or structure. Lidar has proved a highly accurate aid to DP and navigation: however it’s not without its disadvantages, which is why W-band radar is required for redundancy.

Why W-band?

As we explain in our recent report ‘Marine Report: Charting a Course for Marine Automation and the Role of Radar’, most GNSS systems cannot provide positional accuracy to less than 10 metres when they are close to large objects. This is due to signal blocking and multipath errors, the latter occurring when satellite navigation signals are reflected off nearby structures such as platforms, other ships and now, wind turbines.

With its higher frequency, Lidar has a part to play: however it has limited detection ranges and, perhaps more importantly, severe limitations in adverse weather conditions, requiring the sensors to be constantly attended to, which has its own dangers.

Radar however is more robust. Most marine radar systems on ships operate on S-band and X-band. Operating at 3 GHz, S-Band has a range of 74km or 40 nautical miles. However, it is W-band radar (76-77Hz) that can deliver the most spectacular imagery and accuracy, with a 500m range, as well as being independent of GNSS and infrastructure and highly resilient to rain, fog and glare.

Comprising a single 360° radar sensor and a small, low-power processing unit, the picture it can acquire of its local environment can be used to output accurate odometry, in which it can use a local map that it builds to locate the position of the sensor, much as driverless cars do.

Using a single high-resolution sensor, the system uses radar images alone to provide accurate motion estimation in three degrees of freedom, x, y, angle. Radar motion estimation can be used in tandem with a radar map of the area to provide absolute positioning. The system outputs NMEA style messages that can then be used by downstream systems. The system uses an innovative keypoint-based framework, ensuring centimetre-level accuracy, resilience and reliability across diverse conditions.

In this way, the radar data can be integrated with data from other sensors such as GPS, GNSS, inertial data, laser, camera and sonar.

Future opportunities

The importance of safe navigation through today’s more congested seaways cannot be understated. Equally, smarter solutions need to be deployed that require minimal setup and cost, which W-band radar immediately provides through enabling targetless positioning.

Developed in the UK, Navtech Radar’s Terran360 solution can enable autonomous navigation of boats in environments where GNSS cannot operate, in any weather conditions and at any time of day or night. This improves vessel productivity, operable 24 hours a day without worrying about unreliable GNSS positioning or sensor failures. Vessels taking the most efficient routes also produce fewer emissions.

With the burgeoning of remote, autonomous and unmanned vessels, sensory systems, from millimetre wave radar to satellite enabled propulsion controls, become ever more commonplace in maritime automation, as will the fusion of multiple sensors become the default, of which near-distance, W-band radar will be core.