Governing Seismic Design Scenarios for Graving Docks

Graving docks are used throughout the world to dry dock large vessels and many of these facilities were built prior to the advent of modern seismic design. In addition, no seismic design code specific to graving docks exists. Consequently, many existing graving docks are seismically deficient. These structures are costly and time consuming to construct and the vessels which use them are often worth billions of dollars. As a result, earthquakes pose a large financial risk to the owners and operators of graving docks in seismically active regions. This paper presents the results of a study evaluating the operational scenarios most likely to govern the seismic design or evaluation of various types of graving docks.

The seismic performance of three fundamental types of graving docks will be evaluated. First, hydrostatic docks, which are designed to resist the full buoyant and hydrostatic forces from the surrounding groundwater. Second, continuously relieved docks in which the groundwater pressures on the dock are reduced at all times by a pumped drainage system. Third, intermittently relieved docks in which the groundwater pressures on the dock are reduced by a pumped drainage system only when the dock is unflooded. These dock types are represented by three generic graving dock designs, each sharing the same interior geometry, but with different structural thicknesses, soil conditions, and groundwater elevations.

The seismic performance of the fundamental graving dock types will be evaluated for three common operational scenarios, including unflooded and empty, unflooded with a docked vessel, and flooded. The most likely governing scenarios for both global displacements and local demands in the wall-to-floor joint are presented for each type of graving dock. Global displacements are of particular concern to owners in congested sites or when a graving dock must provide significant utility services to a dry-docked vessel, which is common for Navy graving docks. Local demands in the wall-to-floor joint are important because they are the best predictor of collapse.

The in-depth seismic analysis required to evaluate each of the three operational scenarios for a particular graving dock can be very complex and time consuming. Each analysis would include evaluation of the nonlinear response of soils, the nonlinear response of structural elements, soil-structure interaction, fluid-structure interaction, and vessel-structure interaction. Consequently, this paper concludes by using the results from the generic graving dock evaluations to provide guidance to aid design professionals in identifying the governing seismic design scenario for their facility. This can make their work more efficient and provide value to owners when they are allocating resources for seismic design or evaluation of a graving dock.