For the last few years, the main focus on shipping’s environmental impact has been in the areas of ballast water treatment and emissions of SOx and CO2. This is not surprising, given that these are both high profile aspects and come with a high price tag, well into the tens of billions of dollars for the industry.
There are, however, other sources of pollution from ships with leaking stern tube and thruster seals being the major one. This coupled with higher rates of leakage from damaged seals can mean that significant pollution occurs. Some estimates put the annual leakage of oil from stern tubes into the marine environment as double that of the Exxon Valdez oil spill.
Recognition of the problem by seal manufacturers has resulted in improved products, and the advent of environmentally acceptable lubricants (EALs) that are bio-degradable. Whether the issue of stern tube leakage can ever be eliminated entirely is debateable, but manufacturers such as Wärtsilä Seals & Bearings are constantly refining existing products and developing new ones to reduce leakages. The latest of these is the new Wärtsilä Dualguard seal for stern tubes and thrusters that is to launch this month (November).
Wärtsilä has developed the Wärtsilä Dualguard with the VGP requirement for vessels operating in US waters very much in mind. Wärtsilä also believes that something similar to the US VGP regulations will also be adopted in other areas in due course.
Seals, whether on conventional shaft lines or thrusters are first and foremost needed to protect the ship’s machinery from ingress of seawater and debris such as sand, grit, ropes and other material. The protection of the environment from leaking oil is a regulatory requirement in most parts of the world and one that has taken on increasing importance with developments such as the US vessel general permit (VGP) system aimed at preventing pollution from operational discharges.
The new Wärtsilä Dualguard is based on the face seal principal. The basis for a face seal is two lapped surfaces, one rotating surface and one static surface. When the two lapped surfaces rotate against each other, a thin stationary lubricant film develops in the sealing interface. The presence of a stationary lubricant film results in low friction and thus limited wear during the seal lifetime.
The Wärtsilä Dualguard has two sealing interfaces, one water lubricated and one oil lubricated. The airspace between the two sealing interfaces is the void space, connected to an inboard tank thereby preventing oil pollution to the sea water and preventing water ingress in the lubricant. The seal consists of two main sections, the static section and the rotating section.
The rotating section contains the rotor support and the dual-faced rotor. The aft side of the rotor is in contact with the seawater stator, creating the water sealing interface. The forward side of the rotor is in contact with the seal ring, creating the oil sealing interface. The seawater stator and the seal ring are being pushed towards the rotor by independent pressure springs ensuring continuous face pressure and enabling axial movements.
The seawater stator and the seal ring can be moved in an axial direction to compensate for axial travel caused by thermal expansion of the shaft line and possible gaps in the thrust bearing. During axial travel of the seawater stator and seal ring, the pressure springs ensure that the interface of the seal is pressurised to maintain sealing effect and a suitable stationary lubricant films in the sealing interfaces are maintained.
Since the seawater stator and the seal ring move in an axial direction, a sealing method is required between the seawater stator/seal ring and the remaining static components. The Wärtsilä Dualguard uses o-rings in a secondary sealing layout for this purpose, situated on the outside diameter of the seal ring and on the inside diameter of the carrier. These o-rings prevent water ingress and loss of lubricant under axial movements.
The design of the seal is aimed at reducing or eliminating the effect of debris such as sand, grit, fishing lines and rope on the sea side of the seal. This is aided by the use of composite materials for components and faces. Wärtsilä does also recommend that a bolted conventional rope guard, equipped preferably with cutting knives, to be fitted to the stern tube/thruster flange to further increase the protection against fishing lines and other debris.
The oil lubricated side of the seal can use either mineral oils or EALs depending upon the owner’s choice. With some seal types, EALs may be the only option for ships operating in US waters assuming the manufacturer of the shaft line endorses their use. Although the Wärtsilä Dualguard can be used with EALs, it has been designed for use with mineral oils as the seawater lubricated side and the drainage system will prevent any leakage to the environment. This means that the risk of bearing failure that is associated with some EALs can be avoided.
Problems on the oil side of the seal will be mainly due to common issues such as shaft misalignment, wear and vibration. The seal has been designed to handle high radial and axial movements and withstand vibration at high speed. The operating temperatures for the Wärtsilä Dualguard seal range from minus 5°C and plus 40°C on the water lubricated side and up to 80°C on the oil lubricated side somewhat higher than the more normal 60°C that some other seal types are designed for.
The drain system between the two seals makes use of air pressure to aid movement of any leakage to the system tank which has a capacity of around 30-40 litres and requires minimal piping. Installation of a seal requires either drydocking or the use of an underwater habitat. As the seal is of a split design type, withdrawal of the shaft is not required. Normal maintenance would be five years to coincide with drydocking schedules.
Wärtsilä sees the market as merchant marine and offshore vessels as the seal has been designed for use in thruster applications including azimuthing and tunnel types as well as conventional stern tubes. There is a large fleet of offshore vessels operating around the US in oil and gas and increasingly in offshore wind where the US could be generating more than 80GW by 2050.
Production of the Wärtsilä Dualguard seal is ready and Wärtsilä is currently looking for possible candidate vessels. The seal comes in various sizes from 100mm to 1176mm for thruster applications and from 300mm to 1176mm for conventional propulsion use.
