As the costs of DP technology fall it is becoming an affordable option for smaller vessels and unmanned commercial surface vessels
Dynamic positioning (DP) technology is playing a key role in the offshore sector’s development of autonomous survey vessels and coastal renewables. Vessels are now being built with DP systems for marine construction, offshore wave and tidal energy projects. The technology is also being fitted on remote control towage tugs * and has been proposed for unmanned subsea survey and inspection work.
The falling costs of DP technology mean vessel designers and builders can incorporate it on smaller vessels. For example, Damen Shipyards designed, and is building, a Shoalbuster vessel for Herman Senior that has a DP2 system and engines that meet IMO Tier III requirements. Once built, Brutus will be used for multiple operations including offshore project support, towage assistance, oceangoing towage and anchor handling, plus remote vehicle surveys, mattress installations, mooring, pushing and dredging support.
Damen is constructing a Shoalbuster with DP2 for Herman Senior
This Shoalbuster 3514 shallow draught design vessel is due to be delivered in 2020 from Damen’s Hardinxveld facilities in the Netherlands. Its DP system will enable Brutus to hold position around structures, such as platforms and offshore turbine foundations. DP could also be used in open-sea operations, such as cable-laying.
Herman Senior managing director Jack van Dodewaard thinks more of these types of DP-driven vessel will be built in the near future. “The demand for DP2 vessels is increasing and this will fill a gap in the market,” he says. Brutus will have four Caterpillar C32 ACERT engines that will deliver a total of 3,876 kW to four 1,900 mm-nozzle waterjets, giving this Shoalbuster 60 tonnes of bollard pull.
Other examples of DP installation on marine construction vessels include Reygar’s StemTide DP control systems on Keynvor Morlift (KML)’s multipurpose installation and survey support vessel, Severn Seas and 60-m multipurpose barge Mormaen 15. These vessels were retrofitted in 2018 for a development programme that included testing technology for construction and maintenance of offshore renewables in shallow waters and in tidal environments.
On Seven Seas, StemTide DP manages a Schottel azimuthing thruster, a Denison bow thruster and a Torkmaster stern thruster. On Mormaen 15, StemTide controls two modular deck-mounted azimuthing 430 kW thrusters.
“DP would be useful for manoeuvring vessels at slow speeds for pipe and cable survey and light marine construction in tidal areas where there are tight windows of opportunity”
Reygar managing director Chris Huxley-Reynard says that StemTide DP was designed to manoeuvre and position marine and offshore construction units in these environments. He says there is an increasing need for vessel positioning technology as more wave and tidal energy generation devices are installed worldwide.
StemTide DP controlled two modular deck-mounted azimuthing 430 kW thrusters on Mormaen 15
“DP would be useful for manoeuvring vessels at slow speeds for pipe and cable survey and light marine construction in tidal areas, where there would be tight windows of opportunity,” he explains.
“DP controls and computer hardware costs have come down, making it more affordable for smaller vessels, such as multicats and Shoalbusters,” says Mr Huxley-Reynard. “Multicats can lay anchors, install subsea equipment, conduct surveys – there are plenty of applications for DP on multicats.”
Following on from UK deployments of StemTide, Reygar plans to test it in East Asia and North America. It is in discussions with Chinese manufacturers to integrate thrusters with its DP controls and has a project planned in the US.
Mr Huxley-Reynard says this DP system can be adapted for use on autonomous and remotely controlled vessels. It is being used to develop the first generation of autonomous survey vessels for renewables and pipeline inspection.
“We are in an R&D project for expanding the capabilities for coastal and near-shore autonomous vessels for survey work,” he explains. This project is in association with the UK’s Offshore Renewable Energy Catapult, marine survey firm Safeguard Nautica and communications system developer Core Blue.
An unmanned surface vessel (USV) will be built and tested for environmental monitoring and surveys in inland waterways. A larger vessel will be tested in more extreme tidal and near-shore environments, including on a tidal energy project off the Isle of Wight, UK.
StemTide DP will be used for navigation, positioning, tracking and following waypoints. “It features intelligent navigation and information from charted data, radar and AIS targets to deduce safety navigation strategies,” says Mr Huxley-Reynard.
“Autonomous DP vessels can reduce costs of surveys and visual inspections of subsea cables and pipelines,” he continues. “We are looking at operating autonomous vessels alongside a manned vessel to reduce vessel charter costs for surveying areas and turbines.”
One manned vessel could command multiple USVs during surveys using a radio mesh network for communications. “As more vessels are added, the mesh network increases range and its resilience,” says Mr Huxley-Reynard.
The value of DP technology in the offshore space was clearly demonstrated when Heerema Marine Contractors (HMC) completed the first Quad lift using two semi-submersible crane vessels in the Gulf of Mexico in October 2018. Thialf and Balder were both operating in DP mode to lift a structure with four cranes.
HMC crane vessels Thialf and Balder completed a successful Quad lift in Q4 2018
To push the boundaries of offshore lifting operations, Kongsberg Maritime developed a specific heavy-lift function for its K-Pos DP system, as Quad lifting requires a synchronous move by both vessels operating on DP.
However, it was not all plain sailing: prior to the successful operation HMC identified an issue that Kongsberg needed to solve to enable the Quad lift. During heavy lifting, instabilities can occur in the DP control system, when the lift vessel is connected to another fixed or floating object through its hoist wires. Kongsberg already had a heavy-lift function in place to mitigate these instabilities for a single crane lift, but when two vessels are lifting a single object together, the existing solution was insufficient.
Studies by HMC into the behaviour of the DP control system during Quad lift operations revealed that modifications to the K-Pos DP Kalman filter (see below) were required. These studies were informed by simulators at the Maritime Research Institute Netherlands (MARIN) and Kongsberg’s K-SIM simulator, coupled to the K-Pos DP system.
A team, led by Kongsberg Maritime DP product department and cybernetics principal engineer Petter Stuberg, derived the modifications to the K-Pos DP system that were required to conduct the Quad lift operation.
A modified K-Pos DP system with high Kalman settings was tested during simulations to confirm its ability to deal with a wide range of specific heavy-lift conditions. Kongsberg optimised the Kalman filter parameters and retested it during a dedicated offshore DP-trial programme on Thialf in 2018. These high Kalman filter functions were installed on all HMC heavy-lift vessels.
*DP systems are used on tugs by Svitzer and Kotug International and on OSVs by Wärtsilä for testing remote control technology
Technology explained - Kalman filtering
Kalman filtering, or linear quadratic estimation, is an algorithm that produces a joint probability distribution of variables over a timeframe using a series of measurements to filter out statistical noise and other inaccuracies.
The algorithm produces estimates of the current state variables, including their uncertainties. This is updated after the next measurement using a weighted average, with more weight being given to estimates with higher certainty. This process is recursive, producing more accurate estimates each run.
Shapshot CV: Chris Huxley-Reynard
Chris Huxley-Reynard, managing director Reygar