If shipowners want a zero-emissions ship, they must look at not only eliminating greenhouse gas emissions, but also other pollution streams that could harm marine life
When thinking about the zero-emissions ship, shipowners should not just focus on what’s coming out of the stack. “When we think about emissions, we usually think air emissions,” noted Thordon Bearings vice president of business development Craig Carter. “But the shipowner also has to think about the risk of emissions below the waterline. One of the United Nations Sustainable Development Goals is SDG 14, ‘Life below Water’. One of its objectives is to prevent marine pollution by 2025, and now is the time to look at a zero-emissions ship,” said Mr Carter.
Speaking as part of a panel at Riviera Maritime Media’s Seals and bearings: condition monitoring, managing wearing and leakage webinar held in February, which was sponsored by Thordon Bearings, Mr Carter discussed the broad use of sealed, oil-lubricated propeller shaft bearing systems by the global fleet. “Unfortunately, this brings about a risk of oil leaking from the propulsion system into the marine environment if the shaft seal becomes worn down or damaged by foreign materials,” he said.
Citing a research paper from Sweden’s Linnaeus University, Mr Carter said seals in general cannot be leak free, noting panellists at a Riviera webinar were quoted as saying that they do hundreds of emergency seal repairs annually.
“Leakage is considered normal operating consumption for ships with a sealed system,” he said.
Among the most prevalent reasons for oil leakage is that the seals have become worn. He said rubber seals can become hard and less elastic. He also cited misalignment, vibrations, and actual and radial movements of the prop shaft could also affect the integrity of the seal and cause an increase in the amount of oil that leaks.
“An air seal will allow zero emissions as well”
He noted open seawater-lubricated shaftline systems are compliant with US VGP and Polar Code regulations and are technically equivalent to oil-sealed lubricated systems, with class providing extended shaft withdrawal notations.
“Many large commercial ships equipped with Thordon seawater-lubricated propeller shaft bearings have never had a shaft withdrawal in over 20 years and those have had the extended shaft with all notations very similar to an oil system,” said Mr Carter.
The other key benefit to using seawater is the reduced operating costs, because there is no aft seal to maintain or repair, no use of mineral oils or more expensive biodegradable oils. “When you use seawater as a lubricate, it’s free,” said Mr Carter.
There are about 700 commercial ships operating globally that use seawater-lubricated shaftline systems – still a small percentage of the overall fleet. Thordon, Wärtsilä, KEMEL, Duramax, Lagersmit and Maprom are the leading suppliers of the systems.
Joining the discussion, Lagersmit technical account manager Vincent van der Heiden noted a shipowner could still use mineral oil in a VGP-approved solution if the ship was converted to use an air seal. “An air seal will allow zero emission as well,” said Mr van der Heiden. Additionally, shipowners can also comply by switching to using more expensive environmentally acceptable lubricants (EALs) and installing seals for use with these lubricants.
In the case of Lagersmit’s Ventus Air Seal system, there is an extra air barrier between the oil and water-repellant lip seals. Any leakage is captured and transported to an onboard drain tank. A periodically-generated vacuum ensures that the drain channel remains clean. The air control unit (ACU) monitors the condition of the aft seal and alerts an operator if there is anything abnormal to its condition.
“So even if you have severe damage in your seal, you will still be able to have zero-emissions and to take action just to make sure you will not leak any oil into the sea,” said Mr van der Heiden.
On Site Alignment Group co-founder Martin van Leest made the point that all systems can fail and most times the cause of those failures is incorrect installation. “What is the reason they fail? Most of the time it is the wrong installation and the wrong alignment of the bearing. Also, the change from minerals to EALs resulted in failures of the bearings,” noted Mr van Leest.
“Among the most prevalent reasons for oil leakage is that the seals have become worn”
To determine the correct shaft alignment, Mr van Leest said several important measures and calculations must be undertaken that take into consideration the bearing loads, distribution of the loads, deflection of the vessel, and loading conditions.
Polls
Answers to several polls taken during the webinar were very revealing. Note that some of the polling numbers add to more than 100% because of rounding. When asked, “What do you believe is the best solution to eliminate ship propulsion oil pollution below the waterline?” 65% of respondents said, “improved shaft sealing” and only 35% said “use of biodegradable lubricants”.
Responses to another poll question, “Does an open seawater-lubricated propeller shaft system provide shipowners with extended shaft withdrawals similar to a sealed oil-lubricated shaft?” 41% of respondents said “not sure”, 12% “no”, 18% “Yes, but shaft withdrawal extensions are shorter with an open water system” and 29% “Yes – they are technically equivalent and offer similar extensions”.
The poll “If an open seawater-lubricated propeller shaft system offered similar wear life as a sealed oil-lubricated propeller shaft would you recommend it for newbuilds?” showed 67% of respondents “definitely would recommend”, 21% “Maybe”, 5% “I am happy with the current technology” and 7% “Not sure”.
Half of poll respondents said they favour water-lubricated bearings to comply with VGP, 18% “EALs with EAL seals”, 29% “Air Systems” and 3% “Other”.
When asked what the biggest advantage of an air seal was in a poll, 20% responded “Condition monitoring” , 10% “Less wear on the spare parts”, 48% “Zero emission” and 23% “Low total cost of ownership”.
There is also some willingness to spend more on seawater-lubricated shafting technology, with 33% responding “Yes”, 14% “No” and 53% “Unsure” to “Would you specify an open seawater-lubricated shaftline for your newbuild if the difference in capex was 20% higher compared to a sealed oil-lubricated system?”
© 2023 Riviera Maritime Media Ltd.