Proceedings of the ICCGS 2016 15-18 June, 2016 University of Ulsan, Ulsan, Korea 131 Influence of compressive ice force in bow-to-aft ship collision Kristjan Tabri 1) , Floris Goerlandt 2) , Pentti Kujala 2) 1) Faculty of Mechanical Engineering, Tallinn University of Technology, Estonia 2) Dept. of Applied Mechanics / Marine Technology, Aalto University, Espoo, Finland Abstract Ice interaction studied in Nelis et al (2015) for symmet- ric ship collisions revealed that ice crushing force had only modest influence on the collisions, where the struck ship was supported by level ice at the side oppo- site to the collision side. To extend the understanding of ice interaction in ship collision, this paper quantifies the ice interaction in a scenario where the collision takes place in the ice channel and where the striking ship collides to the aft of the ship stuck in the channel. The aim of the paper is to study whether the ice forces are to be considered when evaluating collision damage in abovementioned scenario. First, the structural resistance of the aft structure is stud- ied using displacement controlled finite element analy- sis. This provides the force-penetration curves that are used in the external dynamic analysis. As a result of the external dynamics analysis, ships’ motions, penetration depth and the force histories are obtained providing detailed insight into the phenomena. The importance of the ice interaction is evaluated by comparing the magni- tude of the contact force to the ice force. The same is done for the energies absorbed via different mechanisms. It is revealed that the ice interaction has only minor influence and accounts for about 6% of energy in max- imum. Keywords Ship-ice interaction, bow-to-aft collision, numerical analysis, compressive ice force Introduction Maritime transportation is an important economic ac- tivity, and navigation in ice conditions is a common operation in several northern sea areas. As in open water conditions, navigational accidents have occurred and may happen also in the future. A recent risk analysis of winter navigation in Finnish sea areas indicates that ship-ship collisions are the most important risk event, due to a relatively high occurrence probability and the potential for severe consequences, including oil spills (Valdez Banda et al., 2015a). It is therefore important to develop ship-ship collision models for assessing colli- sion energy and related hull damage, for instance to assess the volumes of spilled oil. Such knowledge is of crucial importance for maritime oil spill risk analysis (Goerlandt and Montewka, 2015; Van Dorp and Mer- rick, 2011), response preparedness assessment (IMO, 2010; Lee and Jung, 2013; Lehikoinen et al., 2013) and ultimately for assessing the biological impacts of such spills (Lecklin et al., 2011). In existing maritime water- way risk analyses models, the scope is either limited to non-ice covered waters (Li et al., 2012), or methods for open water collisions are applied also for ice conditions (COWI, 2012), which may be questionable. While for collisions in open water conditions, several models for assessing collision energy and hull damages have been proposed (Ehlers, 2011; Terndrup Pedersen, 2010), the extension to ship collisions in ice conditions is not yet much elaborated upon and involves relatively high uncertainty. Nelis et al. (2015) studied symmetric ship collisions where the struck ship was supported by level ice at the side opposite to the location of impact, finding an increase in energy absorption by the struck vessel of 15-20% in rather thick ice fields. However, this impact scenario is not very realistic in actual navi- gational practice, whereas it has been found that the proper definition of impact scenarios is very important in maritime transportation risk analysis (Goerlandt et al., 2012) and in ship design (Ståhlberg et al., 2013). To increase the understanding of the ship-ship collision phenomenon in ice conditions, it is therefore important to assess the contribution of ice forces to collision ener- gy in more realistic scenarios. Results from a risk analy- sis (Valdez Banda et al., 2015a) and human error studies (Valdez Banda et al., 2015b) indicate that especially convoy operations are relevant. In such operations, one or several ships follow an icebreaker which breaks an ice channel, thus facilitating the movement of the assist- ed vessels through the ice field. Collisions can occur if the icebreaker or one of the assisted vessels suddenly becomes stuck in the ice channel, e.g. due to compres- sive ice forces, and when the following vessel cannot stop in time or maneuver out of the channel. Such sce- nario occurred on 5 th of March 2006 in the Gulf of Fin- land, where Dominican-registered cargo ship Runner 4 sank after being collided to the aft by the Malta-