NSafe®-Hull & the Promising Bulwark of Energy Absorbing Steels

By April 5, 2016 Article, Technology No Comments

^ The Sinking of the RMS Titanic: A Tragedy of Epic Proportions

Image Courtesy of Willy Stower at https://en.wikipedia.org/wiki/File:St%C3%B6wer_Titanic.jpg#/media/File:St%C3%B6wer_Titanic_(colourized).jpg

Of Disasters & Techno-Administrative Evolutions

Some disasters leave behind vast and permanent imprints on human conscience. The sinking of RMS Titanic in April 1912 for example spurred the convening of the first International Convention for the Safety of Life at Sea (SOLAS). Another such epoch maker was the Exxon Valdez oil spill.

Four minutes past midnight on the fateful, 24th day of March 1989, an oil tanker named Exxon Valdez struck the Bligh Reef of Prince William Sound in the Gulf of Alaska. What followed was the second most disastrous oil spill in U.S. waters in terms of volume of spilled oil.

Statistics recount only part of the story. The real devil is in the additional detail. Spilling 11 million gallons oil is bad enough. The Deepwater Horizon Spill of 2010 has discharged more, 210 million gallons to be precise.

But spill this at Prince William Sound – a remote area reachable only via ships, airplanes, and helicopters; booming with rich and diverse wildlife amidst hundreds of miles of jagged shoreline – and you get the most environmentally damaging oil spill, to this very day.

Three Days After the Exxon Valdez Oil Tanker Ran Aground  Image Courtesy of the Office of Response and Restoration, National Ocean Service, National Oceanic and Atmospheric Administration at https://en.wikipedia.org/wiki/File:Exval.jpeg

Three Days After the Exxon Valdez Oil Tanker Ran Aground
Image Courtesy of the Office of Response and Restoration, National Ocean Service, National Oceanic and Atmospheric Administration at https://en.wikipedia.org/wiki/File:Exval.jpeg

The spill affected a monumental area of 11,000 square miles and 1,300 miles of coastline. For four summers, Exxon pressed 100 airplanes, 1,000 boats, and 10,000 workers into the cleanup effort. Not to mention the great assistance that the navy and the air force provided.

Usually it is the IMO that initiates the setting of standards for ship safety and environmental protection. It first came up IMCO 1973 for damage stability regulations for passenger ships. Later it introduced the same for cargo ships via Gilbert & Card 1990.

Spurred by the disaster, the U.S. government thrust into service the U.S. Oil Pollution Act 1990 (OPA 90) that required all oil tankers entering U.S. waters to be double hulled. This time, the IMO had to follow suit by prescribing equivalent requirements.

Double hulled tankers are not fool proof though. On a stormy night in March 2001, the cargo vessel Tern smashed into the double hulled tanker Baltic Carrier in the Baltic Sea at an angle of 50 degrees. The result: a 2,700 ton oil spill that was the worst ever on Denmark’s coastline.

What then, is the solution to this rather vexatious issue? Please note, the casualty rate in the maritime sector is fairly high. Although collisions, allisions, groundings, fires, and explosions are on the decline, they still grab headlines with frightening regularity.

For example, the maritime accident casualty rate in the U.K. is down from 358 per thousand in 1919 to 11 per 1000 for 1996-2005. Even this rate is 12 times as much of the general workforce, 8.5 times that in manufacturing, and 2.5 times the rate in the construction industry.

Plan of a Double Hulled Tanker  Image Courtesy of BoH at https://en.wikipedia.org/wiki/File:Double_Hull_Tanker.svg

Plan of a Double Hulled Tanker
Image Courtesy of BoH at https://en.wikipedia.org/wiki/File:Double_Hull_Tanker.svg

Then there is the financial aspect. Towage, repairs, medical costs, possible fines, shipment delays, and loss of reputation join forces to make maritime accidents a wastefully expensive proposition.

According to the U.S. Towing Industry Safety Statistics Report, a low severity maritime accident causes damages up to $50,000. The figure shoots up to $50,000-$250,000 for medium intensity incidents and over $250,000 for grave mishaps.

Pending further trials and the results thereof, Highly Ductile Steel (HDS) might just be the answer to this dilemma. For, it promises three times better safety in case of impact. This is particularly useful during collisions, the maritime accident with the greatest frequency.

Highly Ductile Steel (HDS) & NSafe®-Hull

Experiments have demonstrated the ability of HDS to absorb more energy vis-à-vis traditional steels even when ships collide at 90 degree – this is particularly valuable for tankers that have to use double hulled vessels.

Despite being more ductile than other steels, HDS retains the high weld-ability, machinability, tensile strength, and yield strength of conventional steels. The ability of a metal to be drawn into wires, ductility is the reverse of brittleness i.e. the tendency to break without much elongation.

For more reasons than one, steel is the most widely used alloy. For one, it is made from iron – the second most abundant metal in the earth’s crust after aluminum. This brings down its cost to affordable levels.

Polar Endeavour: A Double Hulled Tanker  Image Courtesy of Walter Siegmund at https://en.wikipedia.org/wiki/File:Polar_Endeavour_31930.JPG

Polar Endeavour: A Double Hulled Tanker
Image Courtesy of Walter Siegmund at https://en.wikipedia.org/wiki/File:Polar_Endeavour_31930.JPG

Then, you can tailor its properties to your particular requirements by adding alloying elements. Steel can thus be corrosion resistant, hard, tough, and ductile – you name it.

Nippon Steel & Sumitomo Metal Corporation (NSSMC) developed HDS under the brand name NSafe®-Hull in collaboration with Imabari Shipbuilding Company Ltd. and the National Maritime Research Institute (NMRI).

These organizations worked to control the material’s chemical composition and microstructure at the crystalline scale. The first application of the NSafe®-Hull is a Mitsui O.S.K. Lines’ bulk carrier built by Imabari Shipbuilding and launched on August 2, 2014.

NSafe®-Hull could absorb three times the energy as conventional steels can. Ships with an NSafe®-Hull will therefore be at least three times safer than those with hulls made from conventional steels.

For oil tankers, such improved safety minimizes the possibility of disastrous spills. Neither can water enter the cargo holds and damage the cargo and while also escalating the haunting prospect of capsize.

The said bulk carrier of Mitsui O.S.K. Lines is 299.94 meters long while being 50 meters wide and 24.7 meters deep. It has a deadweight of 206,600 tons. Imabari Shipbuilding has used 3,000 tons of NSafe®-Hull for its hull.

Research Findings

To evaluate 3-dimensional, non-linear, ship-to-ship collisions, Japanese classification agency ClassNK is working on a collaborative research program with NSSMC, NMRI, and Imabari Shipbuilding. The aim is to study if and how much better protection do HDS hulls offer in collisions.

A collision between ships with paths at right angles is the most severe because it takes the heaviest toll. Not to say that we should ignore oblique collisions. Impact at the midsection of the struck ship also produces maximum damage.

Simulation trials involving two very large crude carriers (VLCCs) of 333 meter length found HDS hulls offering 1.5 times the elongation vis-à-vis that offered by high tensile strength steel hulls. Greater elasticity means better absorption of energy and lower probability of rupture.

The SS Andrea Doris: Her Sinking after a Collision is Among the Most Infamous Maritime Disasters Image Courtesy of https://en.wikipedia.org/wiki/File:Andreadoria02.jpg

The SS Andrea Doris: Her Sinking after a Collision is Among the Most Infamous Maritime Disasters
Image Courtesy of https://en.wikipedia.org/wiki/File:Andreadoria02.jpg

Studies also established the critical striking velocity for HDS at 90 degree collision at 12 knots – the struck ship’s outer and inner shells do not fracture. Critical striking velocity is the maximum impact velocity the struck ship can withstand without breaking its inner shell.

If we now use hulls of conventional steel for the same 90 degree collision, a speed of 3 knots would be higher than the critical striking velocity because the impact would penetrate the outer hull.

An impact at a velocity of just below 6 knots would penetrate the inner hull as well. And, an impact at 60 degrees angle and 7.5 knot speed would be enough to rupture both hulls.

If two ships with hulls made from high tensile strength steels collide at an angle of 30 degrees or 150 degrees, the ships will slip on each other thereby avoiding destructive impact. But if the angle of incidence is 45 degrees or more, the ships will not slip.

This means, the critical angle of impact for hulls made from high tensile strength steel is somewhere between 30 and 45 degrees. But if the ships have HDS hulls and are moving at 12 knots or below, there is no such critical impact angle – they are safe even for a 90 degree collision.

Of course, these are provisional findings based on simulation trials. There are still miles of real world trials to go before we can confirm the utility of HDS and NSafe®-Hull.

The Curious Case of Bent Ramparts

A struck ship experiences maximum damage when the striking ship hits it at:

  • right angles
  • its midsection

A 2002 paper viz. Probabilistic Method for Predicting Ship Collision Damage by Alan Brown and Donghui Chen, Department of Aerospace and Ocean Engineering, Virginia Polytechnic Institute and State University, provides valuable insights on collision damage.

Sloped Armor of the Soviet T-54 Tank: Note How the Inclination Increases the Horizontal Distance of Penetration Image Courtesy of Balcer~commonswiki at https://en.wikipedia.org/wiki/File:T54_Training_Parola_Tank_Museum_3.jpg

Sloped Armor of the Soviet T-54 Tank: Note How the Inclination Increases the Horizontal Distance of Penetration
Image Courtesy of Balcer~commonswiki at https://en.wikipedia.org/wiki/File:T54_Training_Parola_Tank_Museum_3.jpg

According to the paper, the main factors that influence collision damage are:

  • collision angle
  • speeds of the two ships involved in collision
  • type and displacement of the striking ship

There is an interesting corollary here related to collision damage and collision angles. Tanks and other armored fighting vehicles use what is called as sloped armor i.e. armor that is neither vertical nor horizontal. Tanks first fitted completely with sloped armor were the SOMUA S35.

Sloped armor offers better protection from assault projectiles such as artillery shells and bullets. The same is true for ship collisions at angles other than right angles. Such armor is a superior safeguard on account of the following reasons:

  • A horizontally traveling projectile has to cover greater horizontal distance in order to penetrate the sloping armor vis-à-vis vertical armor of the same thickness
7  - How Inclination Lowers the Active Component

How Inclination Lowers the Active Component of a Projectile’s Force

For example, armor of thickness 50 mm sloped at 300 will double the horizontal distance (to 100 mm) that the projectile will need to cover if it has to pierce the armor

  • Related to the first reason is the second. Only a fractional component of the total force of the projectile will act along the width of the sloped armor
  • Next, designers build tanks and assault vehicles to a more rounded shape. Such forms have lesser surface area relative to their volume, something that provides enough internal space for men and gadgets while minimizing the area of exposure

Sloped armor is more similar to the contours of such shapes than that of vertical or horizontal armor

  • Finally, sloped armor is better at deforming, deflecting, and ricocheting a projectile
Deflection, Deformation, & Ricocheting of a Projectile on a Sloped Surface Image Courtesy of ProcopHapala(talk) at https://en.wikipedia.org/wiki/File:Projectil_deflection_effects.jpg

Deflection, Deformation, & Ricocheting of a Projectile on a Sloped Surface
Image Courtesy of ProcopHapala(talk) at https://en.wikipedia.org/wiki/File:Projectil_deflection_effects.jpg

If we regard the striking ship as the hostile projectile, an angle of impact of 90 degrees becomes equivalent to vertical or horizontal armor where the effects of the collision are most disastrous.

Finally

History stands testimony to the fact that progress in material technology has always promoted human civilization. Chances are, this old maxim will come true again. We only have to wait for the results of further trials on the efficacy of HDS and NSafe®-Hull.

Wish to know more of such state of the art technologies and materials? Visit our blog.

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