Guest Post from Mr. Jeffery Orner, Deputy Assistant Commandant for Engineering and Logistics (CG-4)
The Navy's Surface Warfare Center, Carderock Division (NSWC-CD) recently completed its review of planned fatigue design enhancements for the Legend Class National Security Cutter (NSC). Fatigue is the accumulation, across the structure of a ship, of cyclic loads (stress) due to waves that may manifest over time as small cracks. NSC fatigue design enhancements were developed by the dedicated professionals at the Engineering Logistics Center (ELC) and afterwards, the Coast Guard paid with the Naval Sea Systems Command’s (NAVSEA's) NSWC-CD to provide an independent assessment of those designs. A report outlining NSWC-CD's findings was provided to the Coast Guard on January 30, 2009 by NAVSEA. See the NSWC-CD letter here: /NSCAssessment.pdf
To conduct this analysis, NSWC-CD used advanced computer fatigue modeling software which led to two main conclusions. First, NSWC-CD determined that the Coast Guard's planned modifications to the hulls of NSCs three through eight will be adequate to achieve the desired 30-year fatigue life. NSWC-CD also identified the need for improvement in several localized areas, which the Coast Guard will review. Second, the report identified that major fatigue life improvements will result from modifications planned to be retrofitted on BERTHOLF and WAESCHE (NSC #1 & #2, already constructed), however there remain two areas that are still modeling a less than 30-year fatigue life. Consequently, NSWC-CD recommends more data be gathered for those areas to better validate the proposed modifications.
The Coast Guard is reviewing the findings identified by NSWC-CD, comparing them with the planned operational profile of the NSC, and will address those issues prior to implementing any related design solution. We plan to continue collaboration with NAVSEA in the future to conduct further analysis and may employ them to re-validate any potential design changes that may result from this report.
Because fatigue life assessment is an evolving science, the Coast Guard is partnering with an international consortium of shipbuilding industry experts and certification agencies, such as the American Bureau of Shipping and the Netherlands Maritime Research Institute, through an innovative program to advance technology in this complex field. This project will combine sophisticated data-gathering techniques employed on CGC BERTHOLF with industry-pioneering model testing to improve the ability of shipbuilders worldwide to analyze and estimate fatigue life. CGC BERTHOLF has already been outfitted with strain gauges and other sensors to measure actual fatigue stresses experienced while in an operational capacity and this data will be used to improve the methods for fatigue modeling. With what we've learned so far and will learn in the near future, I look forward to our results making a difference in the field of fatigue life assessment and in shipbuilding.
Once final design updates for BERTHOLF and WAESCHE are complete, upgrades will be made within the first three to five years of operations.<
