15.11.2012 Crash tests in the shipbuilding industry

In April 2011 we delivered forged round bar material that was used for collision experiments by the Institute for Ship Structural Design and Analysis of Hamburg University of Technology. In its 02/2012 edition, the quarterly journal Ingenieurspiegel has related in detail about the experiments.

In April 2011 we delivered forged round bar material that was used for collision experiments by the Institute for Ship Structural Design and Analysis of Hamburg University of Technology. After forging, the round bars with a diameter of 825 mm (round) and a length of 975 mm were sawed, drilled, then turned inward and outward and ground. Tests according to the US standard SEP 1921-84 3C/c as well as the inspection certificate DIN EN 10204/3.1B ensured that the workpiece fulfilled all the requirements of the experimental arrangement. Within the framework of the experiment, the steel round bar represents the bulbous bow of a ship (cf. fig. 2) colliding with another ship. Ingo Tautz of  the Institute for Ship Structural Design and Analysis of Hamburg University of Technology published an article about the collision experiments in the 02/2012 edition of the quarterly journalIngenieurspiegel:

The systematic experimental study of the safety of ship constructions in the event of a collision began as early as the 1960’s in parallel to the testing of nuclear propulsion technology for merchant naval vessels. In response to serious accidents involving oil tankers, further collision experiments were carried out in the late 1990’s to validate the calculation techniques that had significantly advanced since the first experimental studies. The focus of those experiments was the construction of the rammed ship and its energy absorption capacity. In the experiments of the most recent past, the collision opponent ramming the other vessel was considered as ideally rigid.

Actually there are no regulations yet in the shipbuilding industry concerning the collision-friendly design of forecastle constructions. This is why, in practice, very sharply formed bulbous bows are designed to be downright rigid, which in the event of a collision can cause particularly severe damage to the rammed ship. A number of simulation results published in the last few years, however, lead to the conclusion that a collision-friendly construction of those structures, similarly to the crumple zone of a passenger car, is likely to have a considerable influence on the energy absorption capacity of the rammed ship. Nevertheless, the simulation of a ship collision which also takes into consideration the deformation of the rammed ship is an extremely complex task. Within the framework of a research project scheduled for a period of several years a number of collision experiments are being carried out at the moment, which will be used as a basis for the validation and a reliable background for future calculations.

The experiments described in this publication are based on the scenario of a collision in an angle of 90°, which is the scenario widely established today for assessing the collision safety of sea vessels. The experiments are limited to the field of pure bulbous bow collisions and the analysis of the so-called inner collision mechanics, i.e. the purely structural-mechanical processes, without taking into consideration the ship movements. The extent of the investigation is shown in fig. 1, where the collision of two RoRo ferries is analysed as an example. The rammed ship has a conventional double hull construction in the area of investigation.

The collision opponents are shown on a model basis in the area of collision which is the object of this investigation. The basic construction characteristics of the reference constructions are shown at a scale of about 1:3. In principle, the energy input in the case of ship collisions is very much dominated by the exerted mass forces, i.e. the collisions take place at relatively low speeds. Dynamic effects, particularly the influence of the material’s strain rate, can therefore be neglected usually. This is the reason why the experiment can be limited to a quasi-static analysis of the collision mechanics.      Another advantage of the quasi static performance of the experiment is that the failure process can be observed and documented in every detail.

Fig. 2 shows the experimental set-up with the collision in vertical direction. The area of the model’s side shell (dark red) represents the typical ship structure design of a double hull construction. It is welded in a steel frame designed to be very rigid (grey) which represents the boundary conditions of the surrounding ship structure. The side shell is linked in the x-direction by four instrumented drawbeams with one abutment each, on both sides of the model. The abutments also record forces in the z-direction via a combination of four pressure gauges. Force is introduced by a total of four servo-hydraulic cylinders which are linked via a 10 m long cross beam. With this arrangement maximum collision forces of up to 4,000 kN can be exerted. The cross beam is linked to a rotationally symmetric collision body similar to a bulbous bow. This test arrangement is linked to two massive base carriers arranged in parallel in order to ensure a closed force flow loop within the test arrangement.

All in all, four experiments are planned to be carried out in this arrangement, two of which have already been completed. In both cases, the object of observation was a side shell model with a double hull conventional design against which a rigid bulbous bow was directed in the first experiment. In the second experiment the design of the bulbous bow was modified in such a way that deformation of the front part became easier. While in the first experiment the maximum collision forces before the failure of the side shell of the ship were still below 1,000 kN, this figure grew up to over 2,500 kN in the second experiment, due to the considerably blunter collision process of the deformable bow. In addition, the failure of the side shell happens only at considerably larger collision distances when deformable bulbous bows are used. In the full-scale version, this allows the forecastle constructions which are over the water surface to penetrate even further. Kinetic energy can thus dissipate over the whole ship into areas that are less at risk than the areas below the water line.

In the first phase of the project, in a test arrangement that was slightly simplified in comparison to fig. 2, a total of two bulbous bow structures were directed against a rigid opponent in order to study the deformation behaviour of the bulbous bow separately. Fig. 3 shows one of those experiments. Together with the results of the crash tests described above, the numeric simulation models are being validated at the moment. Once this work has been completed, a number of computer simulations are planned which are supposed to consider all relevant structural areas of both opponents. Those calculations will then form the work basis for a design proposal concerning a collision-friendly bulbous bow that is meant to meet the requirements of the operational demands, e.g. those arising from rough seas or navigation in ice, and at the same time constitutes a significant improvement in the event of a collision, when compared to conventional constructions.

Ingo Tautz, Institute for Ship Structural Design and Analysis, Hamburg University of Technology