Energy-Absorbing Steel Offers Higher Rupture Limits
While retaining the same yield and tensile strength and weldability as conventional steels, highly ductile steel (HDS) is distinguished by greater ductility. The consequence is that HDS plate has been shown to be able to absorb significantly more energy than its conventional counterpart without rupturing, should it be involved in a 90 degree collision.
This is clearly significant for ship structures, and HDS is already available in this application from Nippon Steel & Sumitomo Metal Corporation under the NSafe-Hull brand and has been used in the construction of a Mitsui O.S.K. Lines bulk carrier built by Imabari Shipbuilding Co Ltd.
The characteristics of HDS would appear to be especially significant in the case of tankers, however, where the regulatory response to historical oil spills has been the enforcement of double-hulled vessels where conventional steel plate is used.
A collision or incident at sea involving an impact at right angles to the hull’s steel plate would be the most probable occurrence indeed and is supposed to cause the most severe damage to the struck ship. In reality however, the crashworthiness of hull structures needs to be considered in relation to oblique collisions as well.
The infamous 2001 Baltic Sea collision between the stem of the vessel Tern and starboard tank No.6 of Baltic Carrier, which contained 2,700 tons of fuel oil, involved a hull built of conventional steel struck at an angle of incidence of around 50 degrees. Although a double hull vessel, Baltic Carrier was holed, with the main part of the 2,700 tons of oil discharged into the sea.
Currents carried the oil spill towards the Danish islands Moen and Falster, and into the narrow Groensund waters between the two. The pollution along the coastline was the most severe ever experienced off the coast of Denmark.
While such consequences are mercifully rare, oblique collisions are not. For this reason ClassNK has supported work as part of its Joint R&D for Industry Program on a new series of three-dimensional, non-linear ship-to-ship collision simulations. The society is working together with the National Maritime Research Institute, Nippon Steel & Sumitomo Metal Corporation, and Imabari Shipbuilding to quantify the extent to which ships featuring HDS would offer better protection against shell rupture.
A series of finite element simulations was carried out between two 333m (1,000 feet) LOA very large crude carriers (VLCCs) where a comparison was made covering the same incident scenarios between ships constructed using conventional steel plate and HDS. Both the striking ship’s speed and the angle of collision were varied.
The collision scenario assumes that the striking ship collides with the midship section of the struck ship where damage is the most severe for the struck ship. The output offers a comparison of energy absorbed at a variety of angles of incidence and the way the ‘critical striking velocity’ changes. The critical striking velocity is defined as the maximum impact that could be sustained without the ship’s inner shell being ruptured.
In order to encompass a higher level of real world factors into the investigation, consideration has been given to the use of three types of conventional steel (one mild steel for the bow and side stringer, HT32 (High Tensile Strength Steel 32kn/mm2) for the outer and inner shell, and HT36 for the upper deck and bottom shell). The simulation makes a direct comparison across different collision scenarios between these steels and their corresponding HDS (HDS, HDS32, and HDS36).
An example of the normalized nominal stress-strain relationships between the two steel types has been offered for HT32 – as noted the main material for the outer and inner shells – and HDS32. In this case, HDS is shown to offer elongation one and a half times that offered by high tensile strength steel. This elasticity has a significant bearing on energy absorption.
To investigate critical striking velocity in the case of an oblique collision, a series of finite element simulations establish the rupture limit curve for both the outer and inner shells of the VLCC that has been struck.
In the case of HDS, although the precise value of the critical striking velocity will be the subject for a later study, the model shows that rupture of the outer shell and the inner shell does not take place if the impact speed is up to and including 12 knots, even if the collision occurs at a 90 degree angle.
In the case of conventional steels, the rupture limit curve modelled suggests that a 90 degree collision at as little as three knots would penetrate the outer shell, with the inner shell being penetrated if the impact velocity was just under six knots. If the angle of incidence were 60 degree, a striking velocity of 7.5 knots would still represent an impact large enough to breach both skins.
The study’s findings are that if the collision angle is 30 degrees or 150 degrees, the striking ship will slip against the struck ship and the two ships float apart from each other, consequently avoiding severe penetration, but if the angle of incidence is 45 degrees or above no such ‘slip condition’ occurs in the present study. For the ship built using high tensile strength steel, then, the critical collision angle is therefore somewhere between the two. For the ship built using HDS struck by another with a velocity of 12 knots, there would be no critical collision angle at all.
With the practical application of HDS in vessel designs, expectations are high for helping shape a safer future.
Source: ClassNK Customer Magazine
The opinions expressed herein are the author's and not necessarily those of The Maritime Executive.