Establishment of Operational Criteria for Safe Launch of Free-Fall Lifeboats From a Turret-Moored FPSO

Abstract

The evacuation of personnel from floating production, storage and offloading (FPSO) vessels in case of accidental scenarios is generally ensured by free-fall lifeboats launched from skids. Safe lifeboat launches occur for a specific range of pitch angles at water entry: flat water impacts may cause dangerous acconal criteria for safely launching the lifeboats. The importance of the wind velocity and direction with respect to the lifeboats is investigated. The study mainly focuses on the acceleration loads experienced by the passengers during water-entry and it is based on numerical simulations and model tests for the validation of the simulation tools and models. Introduction Free-fall lifeboats launched from skids constitute the primary means for evacuating FPSOs in sea states that do not permit helicopter landing, i.e. typically when the significant wave height Hs is larger than 5m. These lifeboats are generally installed at the bow of the FPSO at ca. 20m above the calm water level. The lifeboat launch is then influenced by environmental parameters such as the significant wave height Hs, the spectral peak period Tp and the 1-hour mean wind velocity Uw. In addition to environmental parameters, other factors like the heading strategy of the FPSO with respect to waves and wind, the loading conditions of the FPSO and the lifeboat, possible damage scenarios for the FPSO including loss of thrust or additional trim and list are also of significance. The present paper is concerned with the establishment of operational criteria for safe launch of free-fall lifeboats from a turret-moored FPSO in the Norwegian Sea. The final goal is to estimate the probability of successful launch as a function of a selected set of the parameters listed above. So far, the main failure mode considered in this study has been possible human injury during water impact due to large accelerations. An example of quantity one aims to estimate is the conditional probability of being injured when seating in the aft seat row of a fully loaded lifeboat, if Hs is between 8 and 10 meters and the mean wind velocity Uw is larger than 20m/s. Provided a weather forecast, the obtained probabilistic model can then be used to decide if the lifeboats can be used in this sea state or if some seating restrictions have to be applied.

Experimental validation of CFD simulations of free-fall lifeboat launches in regular waves

“…In recent years significant efforts have been devoted to the assessment of the performance of free-fall lifeboats on the Norwegian continental shelf. The performance assessment focuses on the loads and structural integrity (Tregde and Nestegård 2013), the accelerations and human responses Luxcey et al 2010;Fouques et al 2013Fouques et al , 2014 and the lifeboat's ability to escape from the installation in large waves and wind without collision (Tregde and Nestegård 2014;Jin et al 2014). Numerical simulations, and sometimes model tests, are used in these studies.…”