DNV has introduced new computerised buckling models in its rules for assessing buckling and ultimate strength limits of ship structures.

Founded on advanced mathematical models, DNV has developed a fast and user friendly Windows program interface called PULS (Panel Ultimate Limit State). In addition to assessing ship hull strength with a higher precision than more empirically based standards, the method provides important graphical design information.

According to Tor Svensen, DNVs technical director, the areas in a ship most critical to buckling and hull girder strength are the deck and bottom, which experience the highest longitudinal hull girder stresses. On a double-hull tanker the deck area within 0.4L midship will experience the highest stresses and hence be the most critical. The amount of steel in the bottom is considerably larger since it consists of both the bottom and the inner bottom.

Post-Marpol tankers are more sensitive to buckling since ships built according to Marpol requirements have a lower L/D ratio than Pre-Marpol tankers. Scantlings of deck plating and stiffeners could be reduced compared to Pre-Marpol tankers, with the result that buckling of plates and stiffeners became the decisive strength criteria.

Says Svensen, The PULS program features give an improved understanding of the actual buckling and failure mechanism in compressed stiffened hull panels. Hence PULS will be an important tool for the design of new ships, but equally important for assessing the strength of ships in service.