Offshore crew and cargo transfer limits: efficient utilization of safe time

Anderson, Mitchell (2024) Offshore crew and cargo transfer limits: efficient utilization of safe time. Masters thesis, Memorial University of Newfoundland.

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Abstract

There have recently been significant petroleum discoveries in the Flemish Pass region of offshore Newfoundland, and their development is an active topic of conversation and research. The Flemish Pass region is further offshore than the currently producing oil installations in the province, and it is in a region characterized by higher winds, higher waves, deeper water, pack ice, and reduced visibility. Typical crew transfer operations in the province are conducted with helicopters, but the Flemish Pass poses some unique challenges in terms of visibility and range for the current helicopter fleet. Marine crew transfer with the current FROG-6 capsule Hs limitation of 4.0 m could make operability drop as low as 10% in the winter months based on public-domain environmental data. As such, improved efficiency in marine crew transfer is sorely needed. This thesis poses the question: can the limits for crane-based transfer methods be improved, if the problem is studied in more detail? This thesis has considered the specific ship and the specific environmental conditions, in terms wind speed and wave height, heading, and peak period. A secondary question is: even if the limits for routine crew transfer remain unchanged, is it possible to safely and practically complete crew transfer operations in seastates above the typical limits for emergency situations? A potential flow analysis is conducted on 6 representative OSV hull forms using ShipMo3D. Time domain analyses are conducted to compare anticipated deck velocities with the operating limits of a commonly-used crew transfer capsule, the FROG-6. It is determined that there is significant variation between the wave height that causes a limit exceedance, depending on the size of the ship, its heading relative to the waves, and its loading condition (GM). Through analysis of the time between limit exceedances, it is discovered that there is significant “safe time” in between limit exceedances, even in extremely rough sea-states. Filtering out the “safe times” with duration less than 10 minutes, there is still significant “useable time” in all the simulations, i.e. there are significant windows of time with benign vessel motions during which crew transfers can be conducted – even in extreme seas. This thesis concludes that: firstly, that operating limits for crew transfer could be reconfigured to account for differences in ship size/loading condition, and wave period/heading. Secondly, that the “safe time” in between limit exceedances could well be utilized for non-routine crew transfer operations, when supported by the appropriate procedures and motion monitoring/prediction systems.

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