Following to the great accidents the interest in preventing environmental pollution and the safety of people and means of maritime transport have increased. The idea of developing new, unconventional structures, imposed by the new requirements of Shipbuilding Rules related to safety and pollution is analyzed in the paper. The behavior of the unconventional double hull of a ship structure concept, named ARC, loaded to transversal impact is treated. In the paper the finite-element analysis is used because is important to provide the structural designer that no stress concentrations exist in a certain design. The results obtained for energy absorption and general ship hull behavior encouraged the authors to introduce in shipbuilding the unconventional double hull of the ship structure concept.
Double hull of a ship is a ship hull with double layers of watertight hull surface. The inner and outer layers of the hull are on the bottom as well as the sides of the ship. The double layer construction is designated to reducing the risks of marine pollution during collision, grounding, and any other form of ship’s hull damage. After the great disasters performed by the Exxon Valdez oil spill disaster and sinking of Erika off the coast of France in December 1999, The International Maritime Organisation (IMO) introduced the regulation 13 F of Annex 1 of MARPOL [
Structures of ballast spaces in double hull tankers are more susceptible to hull fractures and minor failures as a result of stress concentration, fatigue, or construction defects.
In the paper, extensive finite-element stress analyses in the case of transversal impact have been done on unconventional double hull of a ship structure concept. The analyses are important to provide the structural designer that no stress concentrations exist in a certain design.
In [
In [
Unconventional double hull structures have been developed as a more efficient response to the requirements of collision strength, in case of a ship collision or grounding.
PNTL (Pacific Nuclear Transport Limited) [
In the European research project MoVe IT! [
Additionally, in [
The analysed ship is a tanker for inland waterways having the following characteristics:
Length overall: 99.90 m
Breadth: 9.45 m
Depth: 4.75 m
Scantling draught: 3.20 m
Block coeff. CB: 0.9
Frame space: 625 mm
Web frane space: 1875 mm
Only a piece of ship hull from the midship, has been analysed, extending on one cargo hold length and including a corrugated transverse bulkhead.
Conventional analyzed structure is provided with double side of 0,8m width and double bottom of 0.7 m height in center line and 0.9 m at double side, ensuring practically for cargo area an ratio volume of cargo tanks/total volume of 70%.
Deck, bottom and double bottom, side and double side structures are in longitudinal framing system, otherwise are provided simple frames on the side at every 625 mm and web frames on all structures at every 1875 mm. In
In order to estimate the capacity of impact energy absorption, in collision case, a finite element analysis was performed in plastic deformation with a dynamic loading.
The unconventional solution of double hull is based on the idea of strengthen the inner side with transversal structural elements of arc shape. The name of this solution is “ARC”. In the same time, light elements have been used to compensate the weight of the supplementary elements added to the conventional double hull structure. In
The “ARC” structure can be a solution to modify existing structures, but can also be used to re-design new structures.
Finite element calculations of ship structures have to meet requirements imposed by the rules of the ship classification societies [
All structural elements have to be modeled with net thickness (without corrosion addition etc.), in consequence the strength and rigidity will be reproduced according to this thickness.
The model extension on longitudinal way has to take into account that the results from the analyzed area are not influenced by the boundary conditions. In the case of the center line symmetry the structure model can be done only on half of the ship.
Mechanical characteristics of the material used for the structure are according to steel S235, from Det Norske Veritas RP-C208 [
− Young’s modulus, E = 2.1 × 105 MPa
− Yield stress RY = 236.2 MPa
− tangent modulus 1105 MPa
− Poisson’s ratio 0.3
Additionally, the ultimate criteria according to strain εk = 0.171, was used. This criteria is obtained from [
ε k = ε g + ε e ∗ t l e (1)
where
εg = 0.08
εe = 0.65
t = 7 mm is the average thickness of the elements
le = 50 mm is the average length of the elements.
According TO BV Rules, 2016 [
− all three displacements fixed at fore end of the structure model,
− symmetry conditions in transversal plane,
− symmetry conditions in center line.
The friction between the bow and the side was considered with constant friction coefficient μ = 0.3. The friction coefficient has been estimated according to European Agreement concerning the International Carriage of Dangerous goods by Inland Waterways [
μ c = FD + ( FS − FD ) e − DC | v rel | (2)
where:
FD = 0.1
FS = 0.3
DC = 0.01
| v rel | is relative friction speed.
The loading of the model was made by considering an initial kinetic energy to the bow model defined by through:
− initial speed 4 m/s on transverse direction, direction Oy,
− bow model weight is of 750 t.
The ship bow indenter is the bow of a classical inland ship, suggested by ADN, 2017 [
The following criteria have been taken into account for unconventional double hull structures analysis:
− the behavior until end of the impact (total internal energy, the length of total penetration),
− the behavior until the collapse of the tank bulkhead (total internal energy, the length of total penetration),
− structure weight (idea to obtain a light structure),
− the efficiency of the structure (rate total intenal energy/weight).
For the plastic-dynamic analysis by using FEM, the module ANSYS-Explicit Dynamics [
According to ADN requirements, [
In the following chapters the numerical simulation results are presented.
In
By analyzing the equivalent stress map at various intermediate moments, it is noted that the area of stress concentrators generated by the contact between the outer shell and the inner side was taken over by the added stiffener elements. At the same time, it is noticed that vertical failure of bulkhead tanks have not been
initiated, and the inner side failure has occurred in a much narrower area and only at the fore area of the deck (
In order to assess the level of participation of the stiffeners in the total deformation of the structure during the impact, the internal energy map was analyzed at the end of the calculation at time t = 0.73 s.
From
From the figures can be more accurate estimated the participation of each structural element to the deformation process during collision.
Analyzing the map of the deformations of the unconventional structure we found the following: a deeper deformation of the deck due to arched elements, as it is seen in
In
Following to the FEM analysis, the results obtained to the end of the impact (the moment of double side failure) are: total internal energy is 5.633 MJ, maximum displacement is equal to −1.820 m and ratio internal energy/structure mass equal to 0.446 MJ/t.
By comparison with the conventional structure it is observed that the unconventional structure shows a faster decrease of the kinetic energy throughout the phenomenon compared to the conventional structure. The unconventional structure consumes total kinetic energy at a final time t = 0.8 s, that is 20% earlier than the conventional structure.
On the other hand, the weight of the analyzed unconventional structure model is increasing by 6.2% regarding to the conventional structure, which means an increase of 18 t of the entire structure of the vessel, which translates into a 20 mm increase in the draft of the ship.
To build this structure it is possible to use accessible materials (customary naval steel) and to use usual assembly technology, that means welding,
The “ARC” structure complies with the 2017 requirement for the inspection of compartments adjacent to the cargo tanks, the added structural elements do not impede access and movement through the double and double bottom spaces.
Using of unconventional structure analyzed in the paper involves modernization in the following directions:
− the addition of the “arch” cross-sectional elements inside of the double-sided panel, of a thickness of 6.5 mm
− Relief of deck transverses structure, floors, shoulder and side girders.
Benefits of using ARC structure are obtained by increased internal energy (46.6% compared to the conventional structure), simple technology (basically the “arc” elements are made of common steel and the usual shipbuilding technology).
Of course for certain cases the using of ARC structure involves disadvantages: in the case of viscous cargo such as oil, asphalt etc., the “arc” elements used to the cargo tanks can create functional problems (complication of the heating system, accumulation of substances transported around structural elements, tanks washing process can be difficult). Also, ARC structure cannot be applied to bulk or container type vessels.
The authors would like to thank SHIP DESIGN GROUP Ltd. for their help in providing data and support to this research study. Some parts of this work are included as a chapter in the doctoral thesis “Stress States that Occur in Ship Unconventional Double Hull Structures”, made by Adrian Presura.
Presura, A. and Chirica, I. (2017) Collision Analysis of the Unconventional Double Hull of a Ship. World Journal of Engineering and Technology, 5, 601-612. https://doi.org/10.4236/wjet.2017.54051