Hydrostatic testing of a manned underwater vehicle using fiber optic sensors

Underwater vehicles are capable of diving thousands of feet below the water's surface. At these depths, extreme pressures are exerted over every square inch of the hull. As with most current sophisticated structural designs, modern submarines are designed with the use of finite element methods (FEM). Despite the demonstrated accuracy of FEM, the safety of the vessel and crew can only be ensured through extensive structural testing prior to certification and deployment of the submarine for use. However, structural strain measurements under these high hydrostatic pressures are difficult to obtain because of the need for waterproofing, potentially long data transmission distances through water, and the large number of desired sensors. Systems Planning and Analysis, Inc. (SPA) recently planned, installed, and tested a fiber Bragg grating (FBG) strain sensor system with a total of 97 sensors to validate FEM predictions of a new submarine design undergoing American Bureau of Shipping (ABS) certification testing. Fiber optic triaxial, biaxial, and uniaxial gages were selected based on the FEM analysis. FBGs were placed on six optical fibers with two fibers (33 sensors) mounted internally to the hull and four fibers mounted externally. Testing was performed by lowering the submarine to the design depth and recording strain measurements. The optical sensor signals were transmitted directly to the surface and interrogated by top-side electro-optical instrumentation through over 2000 feet of armored fiber optic cable. The measured temperature-compensated strain values were compared to the FEM predicted strain values with a high degree of correlation. To the author's knowledge, this successful test represents the first time that FBG sensors have been used during the certification testing of a submarine design and to validate FEM analysis on a large-scale, marine structure