NEW METHOD TO ASSESS STRUCTURE DURABILITY OF AGED SHIPS ANNOUNCED BY SHIP STRUCTURE COMMITTEE
The Ship Structure Committee is pleased to announce the publication of a new report, SSC-474, titled Structural Assessment of Aged Ships, authored by Mr. Gregory Walker, Mr. Brendan Connell and Mr. Sean Kery of CSRA.
The report describes the development of an assessment process to accurately predict the survivability of a corrosion-degraded ship in specific wave conditions. The method developed utilizes a ship specific 3-D hydrodynamic model to simulate the ship’s rigid body dynamic response to wave conditions, measuring the resulting ship motions and pressure distribution on the hull. Pressure and acceleration data from the hydrodynamic model is then input into a 3-D finite element model of the degraded ship structure where the resulting stresses in stiffeners and plating are assessed against various failure modes, including buckling modes, which are calculated according to IACS Common Structural Rules. The results form the basis of a degraded ship strength assessment which can be provided to a ship owner and operator to make operational and repair decisions.
The report describes the development of an assessment process to accurately predict the survivability of a corrosion-degraded ship in specific wave conditions. The method uses a ship-specific 3-D hydrodynamic model to simulate the ship’s rigid body dynamic response to wave conditions, measuring the resulting ship motions and pressure distribution on the hull.
After conducting a limited study of the Expeditionary Sea Base (ESB), a US Navy Auxiliary ship, with uniform corrosion beyond the typical 25% limit, hull bottom plate buckling coefficients rise dramatically, so that even in the lower sea states buckling of the hull plating is possible.
A more thorough investigation may reveal other failure locations with less than 25% wastage. Future research on this topic could consider and evaluate the residual strength of a ship structure with localized corrosion damage. Localized and nonuniform structural corrosion and pitting are probably more common than uniform corrosion across the entire hull structure, but is very case-specific.
The approach that the report uses to assess a degraded ship structure can be expanded and used to develop a safe operating envelope for a ship’s hull structure with various degrees of corrosion. While many simplifications of scope and assumptions were made for this project, a more thorough assessment of a degraded ship structure can be accomplished, but would require modeling the ship’s actual corrosion levels, more seaway conditions, more headings and more ship loading conditions, amplifying loadings to account for expected exposure times, and investigating more structural components such as internal tank bulkheads and their internal fluid loadings.
Download the 75 page report at
https://bit.ly/2uvju5I
The Report • September 2018 • Issue 85 | 7
Marine News
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