A New High Strength Carbon Steel for Marine Applications
Authors: Smith R.C. , Phillips D.D. 
Published: October 2008

Eleven LASH (lighter aboard- ship) vessels currently under construction in a Gulf Coast shipyard are outstanding examples of the new type vessels designed for mechanized freight handling. These vessels achieve significant dead load weight reduction, high operating efficiency and minimum cost through the use of a new high strength quenched and tempered carbon steel.

After consultations with steel producers, the designers and shipyard selected a Modification of Armco QTC Steel to meet the stringent requirements of strength, notch toughness, formability and weld ability. QTG Steel was a logical candidate in light of the success of its application for North Sea offshore platforms. The resulting steel, a modification of Armco QTC, was tailored to meet the specific requirements of critically stressed hull members and to provide ease of fabrication.

In cooperation with the shipyard, manual and automatic submerged-arc welding procedures were evaluated to establish qualification under the standards of the cognizant inspection and regulatory agencies. This extensive test program included explosion bulge tests conducted in accordance with U. S. Navy procedures.

The results of the explosion bulge and weld qualification tests are discussed and analyzed to illustrate the favorable weld ability exhibited by this quenched and tempered steel for the intended service. The successful use of this steel in shipyard fabrication to date is reviewed in light of the production welding procedures. The mechanical properties of this new grade of high strength steel are described in detail to illustrate its potential for other marine applications such as offshore platforms.

INTRODUCTION

Friede and Goldman, Inc.; Marine Architects, have specified the use of a high strength steel for construction of the main decks of an innovative new cargo vessel designed to carry lighters. This paper details the results of an extensive testing program to evaluate Armco Modified QTC for this application.

The GOLDEN BEAR and LASH ITALIA are names of two of these new cargo vessels now under construction on the Gulf Coast. The names of these vessels seem to capture the ruggedness and efficiency being designed and built into these ships.

Each single screw vessel of 32,761 tons displacement al1.d 77 2 feet overall length is designed around a LASH cargo handling system. LASH, meaning lighter, or barge, aboard ship. Each lighter is to be 30 feet × 60 feet; the entire cargo of eighty capable of being stowed in the hold and on the main deck itself.

LASH Design.

The LASH design, while providing attractive cargo handling efficiency and ship utilization, results in large stresses in the main deck itself due to the many large unobstructed hatchways required to accommodate barge loading. The main deck and stiffening members, therefore, require the use of a high strength steel to keep deck section thicknesses within reason. In addition, the system of carrying some of the lighters above the main deck itself requires minimizing weight in the above-the-waterline structural members to eliminate excessive roll and stability problems.