Hull: The Armor of Ships
Technically and economically, hulls are a paramount design consideration in shipbuilding. Hulls determine a ship’s structural integrity, safety, and stability while contributing about 20% of the ship’s total cost. Remember, ships transport over 90% of globally traded cargo through 90,000 cargo ships.
Erika Tanker Failed in 1999 due to a Weakened Structure
(Source: http://www.timesofmalta.com/articles/view/20080116/local/updated-total-found-guilty-in-1999-erika-oilspill-case.192110)    
With corrosion, fatigue cracks are the main cause for failure of ships, railway tank-cars, and road tankers. Fatigue cracks can grow in a branched or curved manner different from the shape in which they are formed. Repairs are not always viable in view of exorbitant maintenance expenses.
Shipbuilding is no longer ‘experience engineering’; advances are no longer based on past experience. The necessity to build larger, more eco-friendly, and more fuel-efficient ships means shipyards are making never-made-before ships. You cannot accurately predict all work conditions for such ships and this increases the possibility of fatigue cracking.
Operating Conditions & Stresses
Ships face the onslaught of waves, wind, cargo loading-unloading, engine thrust and vibrations, and irregular seaways. Hull loads are first transferred from panels to stiffeners, then to primary members viz. longitudinal girders and transverse rings, and finally to shell plates.
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Hull Structure
(Source:
1.     http://www.globalsecurity.org/military/systems/ship/hull.htm
2.     http://navyadvancement.tpub.com/14325/css/14325_196.htm)
Joints between longitudinal stiffeners/girders and transverse web frames are particularly prone to fatigue failure due to high stress levels. And it is precisely these areas that are critical for the ship’s total strength. 
Double hulled ships with larger cargo capacities have more joints making them more vulnerable to fatigue cracks. Corrosive marine environment further reduces the fatigue life of hull members.
Ship structures are designed to behave elastically during their design life of about 20 years. Three primary design requirements for hulls are high reliability, good performance, and easy maintenance. Ships can fail due to:
·      Yielding by excessive tensile loads
·      Buckling by excessive compressive loads
·      Fatigue by small loads applied repeatedly
Fatigue cracks can cause:
·      structural failure of the entire ship
·      cargo and ballast leakage
·      flooding or water intrusion
·      hull degradation
Predicting the growth of fatigue cracks requires precise knowledge of the sea environment the ship will face. Data on wave measurements is available from satellites and buoys.
However, fatigue cracks occur earlier than expected due to uncertainty in forecasting sea conditions. This happens because:
·      Lack of Data: ship classification societies provide stress-range-distribution data for fatigue assessment that is not applicable for larger ships and innovative hulls that are presently in fashion  
·      Limitations of Assumptions: the actual sea conditions may be different from the conventionally assumed ones
·      Redistribution of Residual Stresses: residual stresses persist even after the original cause for the stress is removed. Such original stress causers include heat gradients, machining operations, welding, deformation, corrosion etc.
Hogging & Sagging
(Source: http://sailor-ru.narod.ru/chapter2/2-4.htm)
Waves are the primary cause of fatigue failure in ships. Fatigue crack inducing loads on ships are:
·      Longitudinal Strength Loads affect the ship’s longitudinal members. These include shear force, longitudinal bending movement, and torsional moment and can be:
Ø  Static Longitudinal Loads due to local load asymmetries in stationary water
Ø  Dynamic Longitudinal Loads are induced by waves. These include the hogging and the sagging bending movement, shear force, and dynamic torsional moment
·      Transverse Strength Loads distort the transverse members of the ship and include:
Ø  Weight of the Ship Structure, Cargo, and Ballast Water
Ø  Hydrostatic and Hydrodynamic Loads induced respectively by the static pressure of the surrounding water and by the waves
Ø  Inertia of the Cargo and Ballast Water exerts stresses when the ship changes direction or speed along any of its three axes
Ø  Impact Loads of Slamming and Sloshing. Slamming is the severe impact of ships on the water surface. Sloshing is the impact of the free surface of the oil in the tank when the oil resonates with ship movement
·      Local Strength Loads affect local structural members such as stiffeners, shell panels, and stiffener connectors
Design-Level Safeguards
Currently, ships that resist fatigue completely may be technically feasible but economically unviable. Fatigue cracks are therefore inevitable. The following design techniques can however minimize fatigue cracks:
·      Better Load Calculation Techniques such as the long-term prediction method that forecasts a ship’s response to irregular waves over 20 years. Such methods are more practical and convenient than conventional methods
Fatigue Crack Prone Areas
(Source: http://www.twi-global.com/technical-knowledge/published-papers/integrity-of-fpso-floating-production-storage-and-offloading-hull-structures-march-2000/)
·      Better Design Techniques such as replacing the conventional slot structures that use a fatigue-crack-prone bracket to connect longitudinal stiffeners with transverse web members. Such connection transfers load from the side shell to the transverse members
·      Incorporating Whipping Stresses in Load Calculations for whipping stresses have multiplied in recent times with increase in the flare, the outward angle between the vertical and the ship’s hull plate at the bow and stern
Designers do not usually consider the whipping stress caused by vibrations due to collisions between the hull and the waves