Loads, Responses, Failure Modes

1. EN358 Ship Stuctures Loads, Responses, Failure Modes Naval Architecture and Ocean Engineering Department U.S. Naval Academy

2. Classifying Loads On Ships Static loads Stillwater loads Hydrostatic, weight Other static Drydocking (grounding) loads Thermal loads Slowly-varying loads Wave-induced dynamic pressure Wave encounter + ship motions Sloshing liquids Shipping (green) water Wave slap (sides/foredeck) Launching, berthing Rapidly-varying loads Slamming Mechanical vibration Propeller, machinery Other dynamic loads Combat loads, collision, grounding, ice-breaking Loads to be combined Basic loads “Live” loads “Dead” loads Liquid loads Equipment loads Sea environment loads Hydrostatic loads Still water, waves (sagging, hogging) Other sea loads Heeling, transverse waves, shipping (green) water, slamming Ship motion loads (DAF) Individual loads Operational environment loads Flooding, aircraft landing, docking, ice loads, etc. Combat loads UNDEX, topside missile, airblast, gun-blast (self), etc. Hughes / PNA NAVSEA DDS / IMO

3. Static Loads • “Stillwater” loads • External pressures (hydrostatic/buoyancy – no waves) • Internal pressures (tanks) • All weights onboard • Fixed (“lightship”) weights • Structure (steel), machinery and piping (propulsion & non-propulsion including fluid in piping systems), fixed deck gear, outfitting/furnishing, fixed portions of weapons systems • Variable weights • Cargo (incl. “non-fixed” portion of weapons systems), fuel & lube, water (variable ballast & fresh water), holding & waste, provisions & stores, crew & effects, etc.

4. Static Loads • Special static loads • Grounding • Drydocking • Lifting, parbuckling, etc. • Thermal loads

5. Slowly-Varying Loads • Wave-induced “dynamic” pressure distribution • Due to both wave motion and ship motion • Wave-induced buoyancy distribution • Longitudinal → longitudinal bending • Transverse → transverse “racking” • Oblique → torsion/twisting, bending, racking

6. Slowly-Varying Loads • Wave-induced buoyancy distributions (longitudinal) • Hogging • Wave crest amidships • Main deck in tension • Keel & bottom plating in compression • Sagging • Wave trough amidships • Main deck in compression • Keel & bottom plating in tension • Most Navy combatants → worst case loads WHY?

7. Slowly-Varying Loads • Other slowly-varying loads • Wave slap on sides and on foredecks • Sloshing of liquids in tanks • Shipping (green) water on deck • Localized inertial loads (masts, elongated structures, other heavy objects) • Launching & berthing loads

8. Rapidly-Varying Loads • Slamming → whipping (2 or 3 node flexural vibration) → local buckling, shell plating damage • Springing • Flexural hull vibration due to increased frequency of encounter with waves in head seas • Mechanical Vibration • Propeller, machinery

9. Rapidly-Varying Loads • Other rapidly-varying dynamic loads • Combat loads • UNDEX • shock wave, bubble pulse → whipping • Above water weapons • nuclear air blast, missiles, etc. • Self-launched (missile blast, gun blast) • Impacts: collisions & groundings

10. Response Analyses • Basic types of response analyses • Static only vs. static & dynamic vs. “quasi-static” • Probabilistic vs. deterministic • Linear vs. non-linear • Naval architects usually deal with “quasi-static”, deterministic, linear analyses • Consider slowly-varying wave loads as “quasi-static” pressure/buoyancy distribution (neglect ship motions) • Sometimes deal with dynamic analyses via dynamic amplification factor (DAF) → “quasi-static” • Probabilistic nature of loads and strength capacities are addressed via factor of safety (FOS) • Suitable FOS keep material behavior in linear range • But, geometric nonlinearities may occur !