Mooring System Calibration of the Intact Condition, Ultimate Limit State (ULS)

Abstract

Structural reliability analysis (SRA) has been used to calculate the probability of mooring line failure in an intact mooring system as a function of the magnitude of the safety factor applied in design. A range of different units have been considered, comprising ship shaped units and semisubmersibles at different water depths from 100 m to 2200 m. Environmental conditions representative for the Norwegian continental shelf and the Gulf of Mexico are used in the analyses, and the characteristics of the results in the different environments are compared and discussed. Analyses for Brazilian environment are currently ongoing, but not included here. Time domain analysis is applied to obtain the short-term, extreme value distribution of line tension, conditional on stationary metocean conditions. A large number of different conditions are considered. A response surface is used to interpolate on the distribution parameters in order to describe the tension response in varying conditions. Joint probabilistic models of the metocean environment corresponding to the different geographical locations have been applied, taking account of wind, wave and current and their respective heading angles. A continuous model is used for the metocean conditions at the Norwegian continental shelf, whereas a hurricane model is used in the Gulf of Mexico. The effects of uncertainties in the response calculation are included. The mooring line component strength is based on strength data from break load tests. Conventional catenary chain-wire chain systems as well as polyester moorings are considered. With the probability of failure as a function of the safety factor, it is shown that present regulations result in a significant scatter in reliability level between the cases. Safety factors have been calibrated considering all cases. Alternative design formats are proposed and tested including a format with 2 safety factors. Calibration results are provided as a function of the target reliability level. The final recommendation on target reliability level is given in an accompanying paper at OMAE 2017, comprising both the ULS and the ALS. It is demonstrated that alternative design formats can provide a more consistent safety level across the cases. A different design philosophy is needed for the Gulf of Mexico in order to achieve acceptable risk. Options for design are discussed. The present work provides a unique and comprehensive set of results, where advanced reliability methods are used in combination with detailed response calculations in the time domain. The results provide a basis for calibration of mooring design for ULS and subsequently for regulators to update their rules. The work has been carried out as part of the NorMoor Joint Industry Project, with participants from oil companies, engineering companies, rig-owners, manufacturers and marine authorities. This paper is the first one in a series of three at OMAE 2017, where the second deals with structural reliability analysis of the ALS and the third one provides summary and recommendations for safe mooring design in ULS and ALS.

Mooring system reliability in tropical cyclone and North Sea winter storm environments

“…The location for the FLNG in the NS is chosen to be Haltenbanken on the Norwegian Continental Shelf. This location and the metocean conditions were deliberately selected to be identical to those used in the reliability study performed by NorMoor JIP ( [8], [9], [10]) so as to facilitate a direct comparison of the results with those obtained in the NorMoor Project. The metocean criteria are presented in Table 4 and the contour line for HS vs TP is presented in Figure 1.…”

“…For more details on the derivation of the environmental model refer to [8]. Due to the nature of the NS environment, the maximum angle between wave and wind was considered to be 30°, while the maximum angle between wave and current was taken to be 45°.…”

“…Such a demonstration is difficult if the FoS in design standards is not related to a reliability target. Recently under the NorMoor Project ( [8], [9], [10]) reliability targets were stated. For permanently connected and permanently manned floating systems, the target probability of failure of a mooring system was stated to be 10 -5 /annum.…”

“…the NS Norwegian continental shelf and the GoM [8], [9], [10]. The study was performed for both the intact condition and the damaged condition.…”

“…This paper extends the previous study and draws conclusions from the relative performance of the mooring system in the two regions. The details of the NS environment are selected to be those of the Norwegian continental shelf that were utilised in the NorMoor Project [8] so as to establish a basis for direct comparison with the NorMoor JIP. A probability of failure is calculated for the mooring chain and the pile, using FoS as currently recommended in ISO [5] and API [7].…”