The legislation, adopted Jan. 1, regulates the design, equipment and operation of ocean liners, tankers, cargo ships and icebreakers in an effort to prevent pollution from oil discharges in arctic ecosystems, as the remoteness of many areas makes cleanup operations difficult and costly. Like the 2013 Vessel General Permit of the United States Environmental Protection Agency, the Polar Code recommends that vessel operators use EALs in equipment with oil-to-sea interfaces such as thrusters.

A Harsh Environment

Icebreakers are special-purpose ships with strengthened hulls to break through thick ice, clearing waterways for other vessels. Because of the operating conditions such ships face, lubricants for their propulsion systems must provide improved load carrying capacity and reduce wear caused by pushing through the frozen seas.

These ships are commonly equipped with azimuth thrusters, a configuration of propellers external to a submerged pod that can be rotated 360 degrees around a vertical axis. The setup provides maneuverability without a rudder, which has reduced performance in cold waters. The thrusters power transmission line contains mechanical bevel gears between the engine and propeller, requiring environmentally acceptable gear oils.

The thrusters body, propeller blades and propeller shafts can be impacted due to encounters with ice, causing structural stresses. [Ice loads] include the thruster body hitting a floating ice block, the thruster body pushing into an ice ridge, and propeller-ice interaction loads, said Jari Halme, senior scientist at the VTT Technical Research Center of Finland. At worst, these might cause high-peak and resonance-type loadings and torque, leading to a reduced lifetime of the thrusters critical components such as gears and bearings.

Phil Cumberlidge, business development manager for Panolin Internationals GreenMarine lubricant product range, pointed to additional challenges. Thrusters can be prone to suffering seawater ingress, through seal wear or damage. This can lead to corrosion issues that can be catastrophic to rolling element bearings if the water cannot effectively be removed from the oil, he explained. These failure modes mean that base oils that readily separate from water so it can be managed by draining or filtering out-such as synthetic esters-and those that are non-emulsifying, are best suited for propulsion systems.

Temperatures in arctic regions are not as favorable to an EALs bacterial biodegradation, Cumberlidge said. Polar waters are generally calmer than oceans such as the Atlantic or Pacific. They are also damped by ice floes, which means that oil slicks are not as quickly broken up and dispersed like in the open oceans, he continued.

The 2013 VGP requires EALs to have at least 60 percent biodegradation in 28 days. Faster rates of up to 70 percent can be achieved with base oils such as esters, according to Cumberlidge. The faster the EAL biodegradability time is, the better for the arctic environment.

Additive solubility is also of particular importance in arctic conditions, for example, to maintain a protective oil film on thruster rolling bearings. Pour point depressants, extreme pressure additives, friction modifiers and other additives are susceptible to crystallization or separation from base oils that do not exhibit good additive solubility, which can decrease their performance, Cumberlidge noted.

Even at only around -25 degrees Celsius, these lubricants additives can separate from the base oils and potentially be filtered out of the lubrication system, leading to wear and possible equipment failure, he warned.

Braving the Cold

The main choices for formulating EALs that can survive in extremely low temperature conditions are fully saturated and unsaturated synthetic esters, water soluble polyalkylene glycols, and polyalphaolefins; the latter, Cumberlidge noted, are only biodegradable in low viscosities unless blended with synthetic esters or viscosity improvers. Natural esters exhibit poor performance at low temperatures and are not ideal for applications in polar regions.

Fully saturated synthetic esters function at temperatures lower than -50 C, and at -60 C if they are below an ISO 32 viscosity grade. Unsaturated esters, by comparison, operate in temperatures down to -40 C. Depending on the blend, PAO based formulations with viscosity improvers have a cold temperature operating limit between -40 and -50 C, followed by PAGs at around -35 to -45 C, Cumberlidge said.

Though PAGs have good load carrying ability and low temperature performance without leaving an oil slick on ocean surfaces, he remarked, the VGP raises the question of their toxicity, and the oils tendency to attract and retain water raises concerns about corrosion.

In the case of PAOs, Cumberlidge said that they require viscosity improvers when used as gear oils. These additives can break down under high shear conditions, requiring frequent monitoring of the viscosity. Unsaturated esters are prone to thermal oxidation, which can adversely raise viscosity in higher operating temperatures and lead to oil thickening.

Fully saturated synthetic esters seem to gain the edge when it comes to formulating EALs to withstand arctic conditions, according to Cumberlidge, whose company produces such oils. Due to their high polarity, they provide increased corrosion protection to metal surfaces, in addition to shear stability, thermal oxidative resistance and endurance against oil breakdown.

The synthetics are all better in the cold than mineral oils, which have around -10 to -30 C operating temperature capability, he pointed out.

Testing the Oils

To determine how EALs can help manage loads and wear on thrusters in arctic conditions, the European Commissions Maritime Technologies II Research Programme established the Arctic Thruster Ecosystem project (Arteco), which VTT coordinates. Arteco is a consortium of manufacturers and academic institutions in Finland, Sweden and Germany conducting research to reduce ice loads, manage vibration in mechanical propulsion units and improve EAL formulations for these types of components.

The project has evaluated thruster conditions in Finnish-Swedish ice classes, which are requirements for vessels operating in ice, based on their structural design and engine output. Tests have been conducted on a full-size azimuth thruster at the Wartsila Propulsion Test Center in Tuusula, Finland, with bench studies done by Swedens Lulea University of Technology, Finlands Tampere University of Technology and the Technical University of Dresden in Germany.

Arteco has focused on the conditions that icebreakers face during start-up operations in Scandinavian waters, which have temperatures of -20 C in winter to 35 C during summer, said Ichiro Minami, professor at Lulea Universitys department of engineering sciences and mathematics.

The Swedish university measured the capacity of three oils to reduce wear when exposed to different temperatures through a ball-on-flat reciprocating motion tribotest (ASTM D6425), and Tampere University tested the oils load carrying ability using a twin-disc rotating test with unidirectional motion (ASTM G99).

The samples included two fully saturated, synthetic ester based EALs, a fresh mineral oil and a mineral oil aged at 1,700 hours, all ISO viscosity grade 150, Minami told attendees at the Society of Tribologists and Lubrication Engineers annual meeting in Atlanta. The step temperature test examined the oils by gradually increasing them from 25 to 100 C over a two-hour period, then cooling them back down to 25 C over 1.5 hours. These conditions reflect a thrusters heating and cooling cycle during operation.

The mineral oil samples performance remained relativity constant. The two EALs friction and wear properties at 25 C were similar to mineral oils but diminished once they were heated past 75 C; after cooling, the friction control properties did not recover like they did in the mineral oil sample, said Minami.

In Tamperes step load test, which analyzed the oils ability to prevent scuffing in increasing contact stress between 100 to 300 Newtons over an hour and 20 minutes, the results favored EALs over mineral oils. The scuffing resistance test for petroleum based oils showed a friction coefficient of 0.10; for the EALs, it was 0.11. Load carrying capacity for the EALs was better than petroleum, under these conditions, Minami said at the conference held in late May.

While they show potential for EALs to address wear and friction from ice loads and low temperatures, Minami later clarified for LubesnGreases that the results are not yet conclusive. Currently, we are collecting test results for publication. After STLE, our student found considerable advantages of EAL over mineral [oil] under elastohydrodynamic lubrication, to decrease friction and wear, he highlighted.

VTTs Halme added that Arteco is continuing to conduct tests until the project ends in December, when the results will be published in a final report and shared publicly in conferences and other forums.