ssc - 419 supplemental commercial design guidance for fatigue ship ...

7. Lifetime Secondary Loads PredictionTechnology Base for Commercial Ships7.1 PurposeThe purpose of this chapter is to identify and list the technology base that supportscommercial lifetime secondary load predictions. It covers external hydrodynamic pressure andinternal tank loads.7.2 BackgroundThe computation of secondary loads, such as hydrostatic and hydrodynamic loads onshell plating, has not received the same degree of emphasis in the literature as has primary hullgirder bending. However, estimates of this loading are essential for structural design. In manycases, standard hydrostatic heads are retained in classification society rules. Likewise, for cargoholds, design is based on standard design loads that have not been treated in a stochastic manner.McAffe and Nappi (1990) pointed out the importance of secondary loads on the design ofship structure. The cost to the U.S. Navy to repair damage from wave loads on superstructure,deck-mounted equipment, hull, and appendages was more than $10M in the decade from 1980 to1990. Because of the manner in which costs were characterized, not all of this damage was toship structure, but the results are nevertheless significant, especially as the costs of secondaryeffects, such as the loss of mission capability, were not included. The effect of secondary loadson ship design was shown, on a weight comparison basis, to be one-half to one-third as importantas primary hull girder loads in typical combatant ships, although for larger ships, secondary loadshave a greater effect on ship weight. The secondary loads used by the U.S. Navy for design arebased on historical empirical methods, and could be improved if methods such as ship motionprograms were used to predict them.This report describes only those secondary loads that are important for fatigue analysis:external hydrodynamic pressure, hydrodynamic impact loads, and tank sloshing loads. There aremany other secondary loads that are important for structural design that are not addressed. Theseloads include tire loads from vehicles or helicopters, hydrostatic loads on bulkheads fromflooding, typical deck live and dead loads, bow bulb or sonar dome slamming loads, anddynamic loads such as air blast, gun blast, or missile blast. In cases where these loadspredominate, local scantlings will be designed to accommodate them, and the cyclic fatigueloads will become of less importance in such areas of the structure.7.3 External Hydrodynamic PressureAccurate predictions of pressure distribution on the hull have received attention for thecomputation of fatigue loads on longitudinal stiffeners and transverse framing. Two differentinvestigators used linear strip theory to predict loads in the midship region in oblique seas7-1