Strength assessment of rig foundations for a sailing car carrier

Abstract: The wPCC project (wind powered car carrier) is carried out by Wallenius Marine, KTH Royal Institute of Technology and SSPA Sweden. By developing a car carrier mainly propelled by the wind, the emissions are estimated to be reduced by up to 90 %. This thesis deals with the structural aspects of the design. Specifically, the interface between the sail rigs and hull is considered. Different rig foundation concepts should be developed and compared to each other. The objective is to focus on characteristics such as stress levels, relevant failure mechanisms and cargo capacity. Scantling of merchant vessels is to a large extent governed by rules issued by classification societies such as DNV-GL and Lloyd’s Register. While traditional car carriers are covered to a great extent in terms of scantling loads and structural requirements, there exists a regulatory gap for vessels powered by the wind. Four different concepts for the rig foundations are developed. They are all using transverse and longitudinal bulkheads for transferring loads from the rigs to adjacent hull structure. The foundation concepts are then evaluated by creating a finite element model representing the midship section of the ship. The model includes three rig foundations and generally follows the preliminary scantlings of the wPCC. For reference, a model without rigs is also created. The bow and stern of the vessel are excluded in the model and some other model simplifications are also made. The accuracy of stress levels is thus not sufficient for final sizing of the conceptual solutions but the qualitative differences should be valid. The focus of the study is thus how the four concepts compare with respect to each other. The main strength of the chosen approach is that it allows for comparison of multiple concepts. Had a more advanced approach been chosen, the results would likely prove to be of a higher accuracy but then at the expense of modeling effort and time. The transverse strength is deemed being critical for multiple deck car carriers such as the wPCC. Racking, the transverse shearing of the ship, is a common problem which is covered in detail by class guidelines. These loads, based on rule calculations, are therefore included in the analysis. The rig loads are in turn based on assumptions regarding parameters such as the wing geometry and rig weight. The results of the finite element models are evaluated at several critical areas, chosen as to represent different aspects of the structural response. Results from buckling analyses as well as characteristics such as cargo capacity are also recorded as to broaden the comparison. The different results represent criteria which the evaluation process is based on. Each criterion is given a weight to account for its importance. The Pugh method is then applied, yielding a score for each of the evaluated concepts. The total score consequently depends on the chosen weights. Regardless, the study clearly presents which concepts that are interesting and which should be discarded. Interestingly, the best performing concept turns out to be highly unconventional compared to supporting structures used in traditional shipbuilding. For the future, the rig loads as well as additional load cases should be addressed. Failure mechanisms such as fatigue will likely pose a problem, and at some stage a thorough, full scale finite element model should be created in order to accurately predict the response of the entire hull structure. However, the author believes that the presented study serves as a draft in identifying key aspects to consider when introducing wind propulsion in commercial shipping.