The Azipod propulsion system enables a vessel to break ice using the revolutionary Double Acting (DA) principle,  Heideman et al (1996). The DA principle means that the vessel can be designed with the stern optimized for icebreaking and the bow optimized for another condition such as a bulbous bow for open water or a heavily ice-strengthened bow for multi-year icebreaking by repeated ramming. It is well known that when going astern, the ice resistance of a ship will decrease as a result of the propeller flow around the aft part of the hull, which, among other factors, reduces friction. However, ships equipped with conventional rudders are difficult to steer when going astern. This problem does not affect ships equipped with an Azipod system, as the propeller thrust can be steered in any direction (see Figure 1).
The Azipod system greatly improves the maneuverability of ice-going vessels. The turning unit allows the propeller thrust and wake to be directed against the ice, meaning it can be effectively used in ice management tasks such as : This paper will explain how the correct operation of an azimuth thruster can further enhance each of these ice management tasks. Differences in ice management tasks in various ice-covered seas will be discussed.
– Breaking the vessel through ice ridges
– Vessel operation in ice rubble
– Clearing a wide channel behind the vessel
– Clearing ice around the hull of the vessel or from a structure or platform
– Breaking level ice or pack ice to smaller pieces
– Clearing ice between the pier and the ship
Full-scale ice management tests and measurements recently carried out onboard an icebreaker will also be summarized.
What do we mean by ice management? How can we define it? Players in the industry have different perceptions of what it is depending on their own experience. We will briefly go through IM in different areas in the world and highlight what is required from the vessels that operate in those areas.
In the Sakhalin area, IM used to be closely connected to the Sakhalin II project and the Molikpaq platform in the 1990s. Offloading from Molikpaq was via a SALM (single anchor leg mooring) buoy and a floating storage and offloading unit (FSO) named Okha. Both the SALM and the FSO were moderately ice strengthened and the operating window was very dependent on effective IM. The area consists of drifting sea ice of varying thickness with occasional stamukhas or parts of them. A stamukha is an ice feature that has been grounded and grown in size due to rafting. Storms and high tides can push it back into the sea, where it starts to drift with the pack ice. In this area ice concentration is normally well below 1 (10/10).
The supply vessels expanded the operating window for the SALM, Okha and the offloading tanker by positioning themselves upstream in the ice flow and creating an ice-free path by turning the thrusters more or less transversely and then pushing the ice to the sides using the propeller wash. In this kind of operation the force and the direction of the propeller wash stream are important and this should be reflected in the design of the propulsion arrangement.
Nowadays, this mode of operation is no longer needed since offloading from the Molikpaq platform is by pipeline to the shore. IM tasks are now mostly to clear ice away from loading and manifold areas and to keep the rescue area, where the emergency escape capsules descend, clear of ice. These tasks are better performed by milling the ice with the propellers. It is important to understand that the gravity-based platforms do not need IM to survive because they are designed to withstand all ice loads.
IM is quite different in the Baltic and other sub-Arctic areas. Here we generally speak about assisting and convoying merchant vessels. When an icebreaker frees a vessel in compressed ice, the ice milling effect of the azimuthing thrusters can be fully utilized. On the other hand, during vessel convoying, the flushing effect from the transverse thrusterwash can be used to clear the channel and reduce the required power outtake of the vessels that follow, thereby saving fuel and reducing emissions. These are design points for assisting icebreakers.
The ice conditions in the Caspian Sea are highly dynamic. Outside of the landfast ice, the ice is constantly moving, rafting and ridging. This, in combination with very shallow water in the northeastern part of the Caspian, produces grounded ridges that reach high above the water level. In these conditions, the only way to operate is to mill the ridges with propellers and remove the milled ice with the propeller wash. This is how the supply vessels manage the ice at the Kashagan site when they clear the loading and escape areas.
IM performed in North America is, again, completely different from that described above. On the east coast, in the Labrador Sea between Newfoundland and Greenland, IM means mainly iceberg towing in more or less open water during the summer season. This kind of operation requires different capabilities from vessels. Azimuthing thrusters offer excellent maneuverability and are able to manage growlers and bergy bits using the directed propeller wash effect. If the thrusters are equipped with nozzles, they protect the propellers against towing wires in the sea to a certain extent. It is a common misperception that iceberg towing requires very high bollard pull force. As a matter of fact, iceberg towing is normally performed using low speeds and pull forces – normally less than 100 tons – to prevent the iceberg from tumbling over.
IM on the North Slope in the Beaufort Sea is more violent than anywhere else in the world. Here the focus is on protecting exploration vessels from drifting pack ice that contains multi-year (MY) ice. There are several IM techniques, some involving several vessels with different roles. However, all the vessels used for IM have a high ice class in common and they are usually equipped with very strong bows intended for breaking the MY floes by repeated ramming. When operating in MY ice conditions, great care should be taken when rudders and propellers or azimuthing thrusters come into contact with the ice. With MY ice, the main mode of operation is bow first.
There are also different kinds of IM in the Russian Arctic. In the waters to the west of Kara Gate there is an established shuttle tanker service system and another ready to start up any day. Here the IM vessels are supposed to prepare the offloading site before the tankers arrive and help them maintain the correct position with respect to the ice drift direction during offloading. During operations, both flushing, milling and breaking bow-first have been used. In the high Russian Arctic to the east of Kara Gates, IM mostly involves convoying vessels through the ice. Nuclear icebreakers normally break a lead through the ice ahead of the convoy. The reason for the large distance between the icebreaker and the convoy is safety related. The ships following the icebreaker need a proper stopping distance in case the icebreaker is stopped by severe ice. If the vessel(s) cannot follow, the icebreaker returns to free the vessel by maneuvering close to the stuck vessel, thereby releasing the ice pressure on the hull.
IM is thus not a clearly defined function but various tasks involving milling or flushing with the propellers or managing the ice by breaking it with the bow. Different IM tasks require different capabilities from the IM vessels. Elegant vessel designs can produce compromises that work reasonably well for several IM tasks.