Otso (icebreaker)

Otso in unbroken level ice outside Raahe in February 2011.

Although the 99-metre (325 ft) Otso is slightly shorter than her predecessors Urho and Sisu, she is slightly wider and was, at the time of her delivery, the widest subarctic icebreaker in the world with a beam of 24.2 metres (79 ft).[4] However, with a maximum draft of 8 metres (26 ft) and a minimum operating draft of only 7.3 metres (24 ft), Otso is able to escort ships to shallower ports than her predecessors.[10] The displacement of the icebreaker is 9,222 tonnes.[2]

In many ways, Otso was a significant departure from the traditional Finnish icebreaker design. The extended bridge wings on top of the stylish superstructure feature full-depth all-round windows that provide almost 360 degree visibility from the main steering position, which has been moved to the starboard bridge wing from the centerline.[3][13] Furthermore, the traditional colours dating back to the Imperial Russian era, yellow and black, were replaced with the colors of the flag of Finland, blue and white. Due to the extensive use of advanced automation, which allowed everything ranging from starting and stopping the main engines to raising and lowering the flag to be done remotely from the bridge, Otso had the smallest crew among Finnish state-owned icebreakers when she entered service in 1986.[5] Later her crew has been further reduced to 20.[2] Otso was also the first icebreaker in which the lifeboats, unusable in ice-covered waters, were replaced with inflatable life rafts that could be lowered on the ice.[13]

While Otso holds the highest Finnish-Swedish ice class, 1A Super,[2] it has hardly any meaning for icebreakers which are of considerably stronger build than merchant ships operating in their care. Coated with low-friction Inerta 160 epoxy paint, the hull plating has thickness of over 30 millimetres (1.2 in) in the bow and the waterline is protected by an explosion welded stainless steel compound plating that further reduces ice friction and resists abrasion.[3][11] Galvanic corrosion is prevented by active cathodic protection.[2]

Like all modern Finnish icebreakers, Otso has a diesel-electric propulsion system. It utilizes the so-called power plant principle in which the main generators provide electricity for all shipboard consumers through a common busbar. Her main generators, four 16-cylinder Wärtsilä Vasa 16V32 four-stroke medium-speed diesel engines producing 5,460 kW each and driving 7,540 kVA Kymi-Strömberg alternators, are located on the upper deck beneath the helicopter platform.[3][14] In addition, she has two smaller Wärtsilä 4R22/26 generators with an output of 350 kW for use in ports.[13] The uncommon location of the engine room was chosen to improve the seakeeping characteristics of the icebreaker by moving the center of gravity higher to calm the ship's motions in heavy weather. In addition, the maintenance of the main engines is easier as the heavy engine components can be handled with the ship's main crane and the heavy fuel oil tanks could be placed amidships to reduce the probability of an oil spill in case of grounding.[10] As of 2013, Otso and her sister ship Kontio are the only Finnish icebreakers running on heavy fuel oil which, being the cheapest fuel available, reduces operational costs of the icebreaker.[15]

Otso was the first icebreaker in the world to utilize cycloconverters and alternating current (AC) propulsion motors.[3] Although electrical propulsion had already been used in Finnish icebreakers for almost five decades due to its flexibility and good torque characteristics at low propeller speeds, the older vessels had direct current (DC) propulsion motors which were very heavy and bulky at such power levels. Although it would have been possible to use two smaller DC motors in tandem, recent advances in drive and control technology as well as excellent experiences from a prototype installation on board the Finnish research vessel Aranda were in favor of the new type of propulsion system. As a result, Otso was fitted with two Kymi-Strömberg synchronous AC motors rated at 7,500 kW. The maximum power is available at propeller speeds ranging from 115 to 220 rpm to both directions and at lower speeds the propulsion motors are capable of providing 130% of the rated torque to prevent the four-bladed stainless steel propellers from jamming in severe ice conditions.[3][14] The bollard pull of Otso is 160 tonnes.[2]

Prior to the construction of Otso, all Finnish state-owned icebreakers built after the Second World War had been fitted with two bow propellers to reduce the friction between ice and the hull of the vessel in difficult ice conditions. However, the quad-shaft propulsion system was expensive to build and maintain, and the forward propeller bossings considerably increased the resistance of the ship. Although Finnish icebreaker captains voiced their concerns about the maneuverability and icebreaking capability of a vessel without bow propellers especially in heavy ice ridges and at crawling speeds,[16] extensive model testing at Wärtsilä's new ice model test basin showed that the ice resistance could be reduced by replacing the bow propellers with a "clean" hull and adopting an air bubbling system to lubricate the hull.[3] The patented Wärtsilä Air Bubbling System (WABS) onboard Otso consists of three compressors with a combined output of 1,900 kW that pump air through 46 nozzles located below the waterline on both sides of the vessel. At low speeds, the system can also be used for manoeuvering. In addition, she has large ballast tanks and high-capacity pumps that can be used for rapid heeling and trimming to release the icebreaker if she is immobilized by compressive pack ice.[13]

In 1996, Otso was fitted with a bow thruster to assist manoeuvering while the icebreaker was drydocked at Vuosaari shipyard.[17]