On the Creation of a Safe Operating Envelope for Ships

A Safe Operating Envelope (SOE) for sea going vessels seems like a good idea at face value. The process appears straightforward. The ship is modeled on a computer for a range of operating conditions. The responses are compared to a set of limits and anything that is above or beyond the limits is declared outside the SOE. These are tabulated into a format, typically Polar Plots, which the ships officers can use to safely operate the ship. There are serious practical challenges at each step of the process that can make this difficult to accomplish. When these challenges are not adequately and realistically addressed, an SOE of dubious validity can result. Some parameters like roll or pitch are straightforward to develop. A second group including accelerations, vertical velocities, cargo lashing limitations, Motion Sickness Index (MSI), or Motion Induced Interruptions (MII), are more involved but not difficult. A last group including, bow slamming, bow diving, catamaran or trimaran cross deck slamming, parametric roll, hull girder shear and bending in hogging and sagging, or green water on deck, can be problematic for a variety of reasons. This paper discusses these challenges and different ways that they have been addressed in practice at CACI (Formerly CSRA, Formerly CSC Advanced Marine Center). To the extent possible the discussion presented herein is independent of the software or model testing used to create the input data. The focus is on getting the requirements and process right to maximize the utility and validity of the results. A secondary goal is to discuss the typical methods in light of the variations found in real world ocean waves and how they differ from textbook approximations. This paper is intended to stimulate discussions within the SNAME community with the goal of eventually creating a SNAME T&R bulletin on this subject. This paper is about requirements and methodologies. The prior art where SOE's were developed well or poorly for specific vessels is deliberately not addressed. The SOE's the authors have developed or have reviewed are proprietary and cannot be discussed in this forum.