A leading owner of walk-to-work vessels believes technicians could safely be transferred to turbines in higher sea states than current practice suggests, that operating practices could be improved and costs reduced
Contracts for walk-to-work vessels in the offshore wind industry are usually defined in terms of operating conditions, based on a significant wave height (Hs), within which transfers must be capable of being undertaken.
But a leading vessel owner believes focusing purely on Hs could place unnecessary limits on the ability of a vessel to undertake transfers.
It believes if safe transfers are possible in more challenging conditions, the industry should not arbitrarily limit a vessel’s ability to safely transfer personnel and cargo. It also argues that a new approach might mean more transfers are possible, in worse weather, thus reducing project timelines and costs, and operating procedures for transfers of all types could be enhanced and costs reduced.
Bibby Marine Services has been operating walk-to-work vessels for a couple of years and has two Damen-designed vessels, Bibby WaveMaster 1 and Bibby WaveMaster Horizon. WaveMaster 1 was built on speculation and has maintained >90% utilisation in multiple sectors and charters for 28 months. It utilises an Uptime 23.4-m gangway and Palfinger active heave compensated crane. Bibby WaveMaster Horizon is a dedicated windfarm vessel with an SMST gangway and 3D crane.
Bibby Marine Services chief executive Stephen Bolton told the 2020 Offshore Wind Journal conference in London in February that an analysis the company conducted based on using a digital twin and data collected over the last two years suggests that the industry needs to “move the conversation from significant wave height.” Mr Bolton and his colleagues believe the percentage of operating time that a vessel can achieve a transfer is a better measure of effectiveness. His presentation at the conference provided evidence to support this change, a move Mr Bolton said would enable charterers “to plan around actual vessel access, not hard limits.”
The company used a digital twin and data collected from actual transfers over 24 months to determine the probability that a gangway on a vessel could safely connect in specific conditions. “This enabled us to demonstrate that if the probability is above a certain threshold, a transfer can be made, allowing us to shift from Hs limits to percentage access,” he explained.
“The hardware in the loop simulator we used predicted that the vessel should actually be capable of providing access for windfarm technicians in sea states up to 3.6 m Hs, which translates into an access certainty of circa 94%.”
In addition to demonstrating safe transfers are possible in higher sea states than the 2.5 m Hs usually specified by the industry, Bibby Marine Services’ analysis suggested there may also be times when Hs is lower than 2.5 m when the crew of the vessel might be too cautious about connecting, or when the vessel is not set as efficiently as it might be.
Sub-optimal practice, including using sub-optimal headings for the vessel, could have an impact on fuel consumption and emissions, Mr Bolton said. Effective walk-to-work operations is not just about maximum wave height, he explained. “We need to make sure the vessel is optimally set up when transfers are easy, not just when it is challenging.” Equally, there might be times, he said, when Hs was higher when connections could have been made but were not attempted.
The company used technology from BMO Offshore and Next Ocean to track gangway use, DP performance and weather conditions, including sea state and wind and found that information about wave period, not just wave height, was crucial to decisions about how to set up a vessel to undertake a transfer.
Sometimes, said Mr Bolton, energy associated with longer wave periods was not identified visually by the crew or conventional wave measuring devices. “Better information about wave period has changed the way decisions are taken on the bridge,” Mr Bolton explained.
“Predicting access using a digital twin has been validated and can be relied upon, but the human factor should not be overlooked,” Mr Bolton concluded. “Also, what we see with our eyes and even what we measure may not be the full picture. The industry still has a lot to learn about walk-to-work and about the way conditions affect operations.
“At the moment tenders do not place any value on this kind of detailed assessment of what a vessel can actually do, but by using it we can achieve better access and reduce costs.”
Manufacturers of motion-compensated gangways continue to refine their products with new technology and are modifying them to handle cargo as well as personnel. A new walk-to-work gangway from one of them could provide a boost for operators by using AI technology.
The 30-m gangway from Uptime in Norway benefits from an automatic slip-off detection system and an autonomous landing system. The new access system also has an integrated crane capability. In crane mode, the gangway can lift up to 4 tonnes in static conditions, or 2 tonnes in dynamic offshore conditions.
“Removing the possibility of human error by using technology, that is really what this is all about,” says Uptime. “You still have an operator there to provide commands and input, but the system will take care of critical operations itself.”
The autonomous landing system uses stereoscopic cameras and other sensors to identify acceptable landing spots on a structure, based on distance from the gangway-end to the landing point, and the vessel position relative to the landing point. These are then passed on to an operator for approval. Once this is given, the control system starts up the gangway’s hydraulic power unit, extends the gangway and connects it, all fully autonomously with no further input from the operator.
Artificial intelligence (AI) means the gangway ‘learns’ from each landing operation, improving its ability not only to identify preferred landing spots but also the best method of approach. To start off, the operator will manually select the landing spot; as more landing operations are carried out, the gangway will pick up on the operator’s preference in terms of landing spots and approaches.
If the gangway unexpectedly slips off its landing point the automatic slip-off detection system detects it and the system withdraws to a safe position. The company notes that an electronic system is able to detect such events and react much faster than a human operator.
Another well-known manufacturer, Ampelmann, recently unveiled a version of its E1000 motion-compensated gangway system, the E5000, that can transfer personnel and up to 5 tonnes of cargo. It is expected to be available in Q3 2020.
The switch between people and cargo mode takes less than a minute and is accomplished with the push of a button. The E5000 could, for instance, be used to lift fully fuelled generators on to wind turbines or support blade repair projects.