Y-shaped support web for unsinkability

Earlier this year the Waterhuizen shipyard in Groningen, the Netherlands, delivered a gas tanker with a collision resistant hull to which the hull designer attaches the bold adjective unsinkable. The ship, for Dutch owner Chemgas, is the first of two vessels built to the concept. It is designed to carry ammonia and propane on inland waterways, the area of operation where the hull designer, Royal Schelde Shipyard, says, in theory, it is unsinkable.

The hull structure was developed and patented by Hans Ludolphy, R&D coordinator at Royal Schelde Naval Shipbuilding in the Netherlands. His design, called Libra Dynamics Collision Resistant Ship Structure, has been in development for a number of years in response to growing international concern of the serious consequences arising from ship collisions. Also, the expanding knowledge of the dynamic behaviour of structures under impact loads, makes it a challenge for researchers and naval architects to create ship structures that are able to withstand impact loads.

The design concept

The concept is based on the simple expedient of incorporating a Y-shaped support web structure on the inside of the hull. It resembles folded trapezoidal steel plates with a T-bar welded on top of it. The idea of the structure is based on causal interaction of the construction parts during impact where local failure of the web structure supports a membrane deformation of the hull plating. The structure is designed and constructed in such a way that its deformation absorbs the force of the colliding object to produce optimal resistance by means of a complex system of bending, buckling and stretching.

By improving the energy absorption of hull plating, the hull is better able to avoid the skin being pierced or torn open thus maintaining the hull?s integrity and, at the very least, prevent oil spills or gas leakage. This avoids putting lives at risk and prevents serious damage to the environment. The concept has shown during two full-scale tests to contribute substantially to ship safety during collisions, says the designer.

These trials have been coupled with the opportunity to perform extensive international verification tests involving the Netherlands Research Institute TNO, Mitsubishi HI, Technical University of Hamburg and Germanischer Lloyd in addition to Royal Schelde Naval Shipyard.

During the early test stages, three classification societies ? Germanischer Lloyd, Lloyd?s Register and Bureau Veritas ? were involved. The final design has received approval from GL, BV and ADNR-Rijnvaart for application for inland water gas tankers. Also, for the development and design of the Y-shape support web, knowledge and experience from defence-related expertise have been used, especially design experience with shock, impact and explosion loaded structures.

In constructing a hull structure with the Y-shaped support web, the design of the connections for bulkheads, decks, tank supports and other connected construction parts follows the same design considerations. In practice the structure can be fitted easily into a standard ship design. This applies equally to a single as well as a double-hulled ship.

Commercial viability

From a commercial point of view, because the Y-shaped support web is deemed to be safer and more acceptable to regulatory bodies, the concept offers an opportunity to fit fewer but larger tanks. For a conventional tanker, for example, the number of tanks could be reduced from six to four which means less pumping facilities and better use of available cargo space.

Another positive aspect is that the building costs for a new vessel are reportedly only slightly more than for a standard ship while offering a substantial improvement to the structural safety of the ship. It is therefore a cost-effective solution particularly when, during the fabrication of the first Chemgas tanker barge, it turned out that the welding process of the Y-shaped stiffening structure lends itself very well to automation. This could well lead to shorter construction time while the outer hull of the vessel would remain perfectly flat as a result of the controlled heat input during the welding process of the long welds.

Finally, the application of the collision resistant structure resulted in a lighter ship and therefore a larger deadweight and increased payload. In the design of the initial barge, only straight sections of the hull were equipped with the collision resistant structure but by bending the trapezoidal profiles, curved sections can also be equipped with the stiffening structure.

Future developments

Although it is now technically feasible to build a gas tanker for the inland waterways that can resist hull damage following a collision or grounding, fracture mechanics still has a long way to go until it can be commonly used for calculating the impact effects on a ship?s hull. In addition, finite element methods are still not accurate enough for predicting within reasonable limits the dynamics and actual damage of collisions between ships.

With the valuable experience gained in designing the two 76.5m long inland waterway tanker barges for Chemgas, Schelde R&D is currently finalising the design for a coastal vessel measuring 135m long with a beam of 21m to carry highly volatile gas oil. A spokesman for R&D Schelde confirmed that this coaster would be built by one of the yards within the Damen Group for delivery during 2003.

The next step in the development of the Libra Dynamics Collision Resistant Ship Structure will involve adapting the design concept for large ocean going gas and chemical carriers. This, however, is unlikely to materialise for another two to three years since further optimisation of the performance by ongoing research in the field of collision dynamics and fracture mechanics will be needed. All this has to be done within the constraints defined by classification societies and within the financial limitations of commercial exploitation for the shipowner.

A growing demand is also anticipated from navies around the world and already discussions are taking place with the US, the UK and the Netherlands since structures like these can be expected to give protection against terrorist attacks and other kinds of explosions. Further development towards application in naval shipbuilding can therefore be expected.

The Libra Dynamics collision resistant ship structure can be manufactured under licence from Schelde Naval Shipbuilding, now part of the Damen Shipyard Group, while assistance can also be given for the design and integration of the collision resistant structure in existing ship structures. Already, a number of licencing agreements have been issued to ship owners mainly in the inland waterway sector.

The collision tests

The new international patented Libra Dynamics Collision Resistant Ship Structure was tested in Holland in July 1998 under the auspices of TNO (Building and Construction Research Centre for Mechanical Engineering) in Delft and the Dutch shipyard Royal Schelde. Also participating were the Technical University of Hamburg-Harburg, Germanischer Lloyd and Mitsubishi Heavy Industries of Japan.

The test ships consisted of two inland waterway tankers, of some 1,200t each, which were specially prepared for the trials at a Dutch shipyard. The modifications involved modelling the test section of the hull to represent a tanker of some 40,000 dwt, scaled down 1:3. It comprised a section from the side hull which reached from the deck down to and including the double bottom, thus corresponding to the entire depth of a double-hulled tanker. The special ramming bow, also reduced to the scale 1:3, corresponded to the bulbous bow of a 40,000 dwt ship and hit the test vessel at right angles doing a speed of seven knots.

As intended, but contrary to what was expected using a finite element analysis, the bulbous bow of the colliding vessel did not penetrate the test section and, even after a second collision on the same spot, the section was only dented and not ruptured. After these successful tests the concept was further developed into an economically profitable and feasible structure that can be applied to protect the vulnerable parts of high-risk ships such as chemical- and gas-tankers.

However, although the tested structure showed no cracks, dynamics of the impact caused a whipping effect that was so heavy for the original structure of the collided ship, that one of the tanks in the fore part of the ship cracked and leaked after the second impact. This is a well known phenomenon in shock design.