The number of stern tube-related damage claims has increased significantly over the past few years, and such damage is now one of the most common types of claims handled by Gard under the hull insurance policy. A recent analysis by Gard dissects this issue.

The nature and severity of damage to the stern tube/stern system can vary from simple rear seal leaks that can be repaired at sea in days to loss of propulsion failures involving expensive salvage operations and long dry dock repairs.

In the past, inspectors generally regarded stern tube damage as a relatively simple accident. The cause of the accident is usually obvious-for example, touching floating objects such as ice floes, fishing nets or ropes.

Of course, these reasons still exist, but it is indeed worrying that the frequency of accidents has increased significantly in the past few years without obvious cause.

Shipowners and insurers face new“Distress”

Over the past ten years, the number of stern/stern tube-related cases settled by Gard (whether as the main insurer or from the insurer) exceeded 700, with the following year-on-year distribution:

Total number of stern tube-related cases with Gard claims exceeding $5,000

It should be noted that the number of vessels covered by Gard also increased during the same period. So does the rise in claims simply reflect an increase in the number of ships insured. Even taking into account the increase in the total number of ships covered, we can still see that the share of stern tube damage cases is increasing (see figure below).

So, what happened?

Frequency of stern tube-related cases with Gard claims exceeding $5,000

Use of environmentally friendly lubricants

As shown in the first histogram, the significant increase in the number of claims cases appears to have occurred after the U.S. Environmental Protection Agency (EPA) introduced the revised Vessel General License (VGP) requirements in 2013. The revised regulations require all ships over 79 feet sailing in U.S. waters to use environmentally friendly lubricants (EAL) at all their oil-water interfaces.

A 2019 study conducted by Det Norske Veritas with the assistance of Scandinavian insurance companies (Gard, Skuld, Swedish P & I and Norwegian Hull Insurance Association) found that the following two main characteristics of environmentally friendly lubricants cause them to differ from the traditional mineral oils widely used before the VGP requirements came into effect:

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  Pressure/viscosity coefficient => Under high load operation (ie high speed sharp rotation), the minimum oil film safety margin formed by the environmentally friendly lubricant between the stern shaft and the bearing is low.

   

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  Viscosity index (eco-friendly lubricants are significantly higher than mineral oil) => eco-friendly lubricants have lower viscosity at lower temperatures (I. e., mooring test and cold start).

   

By using a higher viscosity environmentally friendly lubricant (up one level) or adjusting the design of the stern bearing (changing the single slope to the double slope design), the contact surface between the bearing and the stern shaft can be smoothed/enlarged, which can be used to some extent Prevent damage caused by the above characteristics.

In addition, another notable feature of environmentally friendly lubricants is their poor hydrolytic stability when interacting with seawater around the rear seal. When contaminated with sea water, the lubricant will decompose and produce carboxylic acids, which may cause damage to the sealing device, which in turn will cause more sea water to enter.

The rate of hydrolysis depends on a number of factors, such as the chemical nature of the oil, temperature and additives. However, the most important factor is the amount of seawater present in the lubricant. According to the International Association of Classification Societies (IACS), the recommended water content limit is 1%, whether it is traditional mineral oil or environmentally friendly lubricants. Nevertheless, experience has shown that this upper limit is quite "loose" and may be somewhat misleading in the case of environmentally friendly lubricants, as even small amounts of water may trigger hydrolysis reactions of the environmentally friendly lubricant and affect its properties (including stability).

In summary, environmentally friendly lubricants have proven to be unable to match the properties of traditional mineral oils-strong lubricant films, consistency when the oil is hydrated, and excellent seal compatibility. For this reason, shipowners tend to use a combination of "traditional" mineral oil plus air seals on new and existing ships.

New Root Cause Analysis

Gard has conducted an internal investigation of all stern tube damage cases encountered during the past 10 years (2013-2022) in an attempt to find a pattern of accident causality.

From the beginning of the investigation, it was found that the types of ships with the largest number of such damage cases were bulk carriers and container ships, accounting for 53% of the total number of cases. Together, these two ship types account for 44% of Gard's total hull risk (see pie chart below).

Gard's stern tube-related claims, classified by ship type

As shown above, a large number of accidents were attributed to fishing nets/ropes entering the stern tube sealing area, resulting in seawater contamination of the stern tube oil, accounting for 38% of the total number of accidents. In fact, according to the World Wide Fund for Nature (WWF), about 500000 to 1 million tons of fishing gear are lost or abandoned at sea each year. However, we believe that the "fishing net cause" may have been exaggerated as the main cause, so there is little further investigation into other possible factors.

In fact, many stern tube bearing failures since 2014 do not seem to have signs of external factors (such as touch marks on the propeller blades, fishing nets wrapped in the rear sealing system, bottoming, etc.).