Southern LNG (SLNG), a subsidiary of El Paso Corporation, needed to reopen its liquefied natural gas (LNG) terminal and ship off-loading facility at Elba Island in Savannah Harbor, Georgia. To improve navigation safety and maneuverability in the harbor, they had a 1500-ft by 2300-ft ship turning basin constructed opposite the mooring dolphins and off-loading pier. The new turning basin required the relocation of approximately 2,500 linear feet of a confined disposal facility (CDF) dike on the Georgia side of the Savannah River. SLNG subsequently constructed a ship slip loading facility for much larger delivery tanks. To maintain continuous operations of the facility, SLNG is responsible for dredging the turning basin portion of the Savannah River as well as the newly constructed ship slip. Therefore, long-term management of the dredged sediments from these areas is paramount to the continued operation of the LNG facility. Materials dredged to maintain the ship turning basin and slip are deposited into two, 120-acre on-site CDFs specifically designed and constructed for the facility's operations. These CDFs cannot be laterally expanded due to existing physical site restrictions. To address these long-term management problems, SLNG retained Geosyntec to develop a more comprehensive approach for maximizing CDF storage capacity that considers not only vertical expansion of the existing CDF dikes, but also includes management of the dredged sediments within the CDFs to promote their desiccation and consolidation.
Geosyntec's Scope of Services
For the ship turning basin, Geosyntec designed and prepared construction drawings and technical specifications for approximately 1 million cubic yards (cy) of maintenance dredging and 3 million cy of new works materials. Our design included over 4,500 linear feet of shore protection to control erosion for the extreme tidal fluctuations and 15-knot container ship traffic. The location of the turning basin required the relocation of a CDF dike more than 200 ft. into an adjacent CDF. Geosyntec performed a geotechnical investigation consisting of cone penetration testing (CPT) and select material sampling. Stability analyses indicated ground improvement was necessary in the very soft marsh and dredged spoils to ensure continued performance of the relocated shoreline. Geosyntec also developed an innovative long-term dredged material management plan (DMMP) that defined and established working parameters and operational strategies to optimize CDF storage capacity and design life. The DMMP incorporated dredging quantities based on river sedimentation studies and the implementation of maintenance ditching for the dredged sediments within the CDFs to promote dewatering. It also required modeling the desiccation and consolidation of the dredged sediment using a US Army Corps of Engineers software program, and a schedule of periodic dike raisings to accommodate the annual dredging operations along the river and ship slip. The plan also includes use of the in-place dredged sediments as fill material for the periodic dike raising construction activities.
Geosyntec also provides construction quality assurance (CQA) and construction management (CM) services during dredging operations and dike raising construction, as well as routine monitoring of the established dikes, including periodic surveying of the CDFs mud line, assessment of available storage capacity, and compliance sampling and laboratory testing of the quality of water being discharged to the Savannah River.
Geosyntec's design and development of the long-term DMMP have provided SLNG with a plan for continuous operation of the LNG facility on Elba Island for at least the next 30 years. In addition, the use of in-place dredged sediments as fill material for the periodic dike raising construction activities provides significant cost savings to SLNG compared to importing off-site materials for construction. Geosyntec's approach can be (partially or fully) applied to any entity or facility that performs regular dredging operations, deposits the dredged material into a CDF, and manages and maintains the CDF.