Hull Girder’s Ultimate Capacity of the Inland Vessel Under Corrosion Effect

Abstract

In service, ships are likely to be subjected to various types of loads which are foreseen in the design phase. In addition, ships can experience rare events and consequently, extreme or oncein- a-lifetime loads (Paik, 2018). The cause of these loads can be attributed to failed attempts to evade storms, ship overloading, incidents of grounding and collisions. Such scenarios might produce a hull girder bending moment reaching the hull’s collapse, i.e., hull girder ultimate capacity (UC). Thus, the main idea behind the ultimate strength assessments of ships is to predict the structural capacity of the hull girder until its collapse. Rules and regulations for sea-going ships have acknowledged the issue (IACS, 2022). Nevertheless, for inland vessels (or even river-sea ships), no fully developed technical standards and studies involving state-of-the-art methods for UC assessments exist in practice. Moreover, studies using sophisticated progressive collapse analysis (PCA) are rare, see one in (Ilić & Momčilović, 2023). Compared to sea-going ships, inland vessels have a larger length to height ratio and low hull modulus, and thus, they are prone to longitudinal strength issues. Coupled with the prolonged service life of inland vessels, UC can be vastly affected. This can significantly reduce UC and decrease the margin between the elastic and the ultimate strength response. Therefore, in order to benchmark the phenomena, the effects of corrosion-induced degradation on UC of a typical inland waterway vessel are explored by using incremental-iterative progressive collapse analysis (PCA), a procedure thoroughly defined in (IACS, 2022). The selected inland vessel is a typical Danube mild steel made barge with the following dimensions: Loa = 72.9 m, B = 11.40 m, H = 3.7 m, T = 3.55 m. The cross-section of the vessel is divided into segments according to (IACS, 2022), see Figure 1. The vessel is already assessed for UC (Ilić & Momčilović, 2023), but just for intact condition. Firstly, UC is determined using as-built scantlings (new ship). Secondly, ageing effects are considered by modelling scenarios which include both pitting and uniform corrosion (ageing ship). Pitting corrosion is defined by pitting intensity degree (DOP) and corrosion intensity degree (DOC) which define the actual total loss of the volume of the element (ΔV), using the approach given by (Piscopo & Scamardella, 2021). On the other hand, uniform corrosion is defined by corrosion wastage (thickness reduction). Namely, uniform corrosion is considered by 10% of corrosion wastage applied to all structural elements, which is a corrosion level often found in such structures. In addition, pitting corrosion incidence is presumed in several locations (segments: 4*, 7*, 8*, and 9*.) and throughout three cases in which ΔV has occurred (Figure 1): 4%, 10%, and 20%. Pit affected segments were chosen due to their service life. Although corrosion models were not severe, the vessel’s UC is significantly reduced (up to 20%) when various real-service degradation scenarios were considered. Thus, the corrosion effect significantly influences the UC of the vessel and should not be neglected in the design phase for the estimations of the strength of ageing ships.