Identification of the safety operating envelope of a novel subsea shuttle tanker

Abstract

A baseline Subsea Shuttle Tanker (SST) was proposed as a cost-efficient maritime transportation method. It is designed to be a 164 m length, 17 m beam autonomous underwater vessel with a cargo capacity of over 16,000 m3. One of the crucial topics for such underwater vehicles is recoverability during undesired malfunctions. A Safety Operating Envelope (SOE) must be identified for military submarines. It considers the submersibles' malfunctions, including partial flooding, jam-to-rise, and jam-to-dive. This paper aims to identify the SOE to enclose the safety operation zones of the SST. In this work, a planar SST manoeuvring simulation model considering the combined contributions from hydrodynamic loads, compensation tank blowing, propeller thrust, and control planes is derived based on semi-empirical formulas. Second, standard operating procedures of recovery actions are established to cope with each malfunction. After that, free-running simulations are conducted. Three cases are presented to discuss SST recovery responses during each incident. Finally, the SOE of the SST is identified. This established SOE determines the SST's feasible speed and depth excursion ranges from an operational safety perspective. The safety depth is sufficient for the SST to recover from a jam-to-rise failure. Moreover, the study found that the existing safety factor on the structural design suggested by the Norwegian classification society Det Norske Veritas (DNV) naval submarine code is exceedingly conservative and potentially leads to a heavy and complex SST structure. The SOE helps reduce the designed collapse depth from the operational safety perspective and contributes to reduced material cost and considerable payload capacity. Also, this work fills in the blanks of SOE analysis on commercial submersibles.