Propulsion, power and computer issues are reasons why dynamic positioning (DP) control can be lost on offshore support vessels, risking a collision, grounding and danger to divers
International Marine Contractors’ Association (IMCA) produces a report of all DP incidents, observations and events reported by vessel operators each year.
IMCA technical adviser for marine Captain Andy Goldsmith summarised the key findings from the 2020 report at Riviera Maritime Media’s European Dynamic Positioning virtual conference, on 24 March.
IMCA received 144 reports in 2020 from 104 DP vessels and around 40 vessel operator companies. Capt Goldsmith was pleased with the response, but warned, “if we were to consider the worldwide DP fleet, there are still many events that go unreported to IMCA”.
He cautioned a DP event could result in a vessel’s losing redundancy or position-keeping ability.
The largest percentage of cases reported for 2020 were due to thruster or propulsion issues; 47 reports representing 33% of the total.
“Of concern is that 11 of the 47 reports last year (23%) resulted in the vessel not maintaining automatic DP control,” said Capt Goldsmith. “This is an increase on 2019 when only 9% resulted in a loss of automatic DP control.”
Issues with power and computers were the other two most reported causes in recent years. “During 2020, 18% of these reported events culminated in a loss of automatic DP control,” said Capt Goldsmith.
IMCA also looks at secondary causes for DP events, the most frequent of these being human factors and issues with electrical and mechanical systems. In 2020, a secondary cause was identified in 129 of the 144 reports.
“In 2020, an electrical issue was identified as the secondary cause in 52 of the received reports and a human factor in 29 cases,” said Capt Goldsmith. “These causes together with the newly introduced cause of mechanical represent more than 75% of all secondary causes.”
He added the category of human factors is broad in nature and all of 2020 reported causes could be categorised as unintentional behaviour.
“We need to ensure our key DP personnel have the skills and tools,” said Capt Goldsmith, “and are not put in the position where they need to cut corners to get the job done.”
IMCA provides a quarterly DP bulletin outlining issues to feed back lessons learned to industry.
It categorises a DP incident as a major system failure, environmental or human factor which has resulted in loss of DP capability; while a DP undesired event is a system failure causing a loss of redundancy and/or compromised DP capability; and a DP observation is an event that has not resulted in a loss of redundancy or compromised DP operational capability, but is still deemed worthy of sharing.
During the virtual conference, Wärtsilä product performance manager for thrusters and propulsion control systems Petra Stoltenkamp explained how vessel builders could optimise station keeping through better DP configurations. She explained through design studies how optimising thruster positioning and pitching can deliver high-performance DP.
This is important for vessels built to install offshore wind turbines as they lift higher, larger and heavier nacelles and blade units. These next-generation windfarm projects will be “further from shore in more challenging sea conditions and increased environmental forces,” said Ms Stoltenkamp, “meaning larger thrusters with more power will be needed for larger vessels.”
When thruster forces counteract the environmental forces, the vessel will remain in position. These are needed for safe DP even in failure modes when one of more thrusters are out of action in an overall system configuration. This system could include underwater mountable, steerable, retractable and transverse thrusters.
“We use computational fluid dynamics to optimise thruster pitch,” said Ms Stoltenkamp. Through this study, Wärtsilä discovered by pitching a thruster gearbox at 8 degrees there was reduced interaction with the vessel hull.
“With the wash of the thruster directed downwards, it is 20% more effective, depending on its placement,” said Ms Stoltenkamp.
She also demonstrated how thruster-thruster interaction losses can be reduced through vessel design. “This prevents one thruster blowing into another by accurate modelling of thruster performance,” she explained.
Wärtsilä also developed Thrust Allocation Logic (TAL) with algorithms to determine thruster loading as a percentage, their azimuth angle and optimal configuration.
“So resultant forces and moment of all units together balance the environmental forces and moments,” said Ms Stoltenkamp. “It uses hydrodynamic modelling for optimal thruster and system configuration for efficient and safe DP operations.”
Capt Goldsmith and Ms Stoltenkamp were joined in the first session panel by AqualisBraemar LOC senior electrical engineer Julius Partridge, Louis Dreyfus Armateurs newbuilding and projects engineer Pierre-Arthur Fortin and DEME Offshore marine co-ordinator and DP authority Captain Matthijs van der Moer.
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