As production systems extend to water depths beyond 3,000 feet, the effects of mooring and riser become increasingly significant when predicting a Spar's response. For these water depths, the viscous damping, inertial mass, current loading and restoring effects from both the mooring and riser system should be included to accurately solve the system's motion response. For some systems, these effects can magnify the extreme response, but for Spar platforms, coupling the mooring and riser with the vessel motion typically results in a reduction in extreme motion response.The ability to predict a significant reduction in the extreme motions can directly result in a smaller and less expensive mooring and riser system and indirectly in a lighter Spar platform through a reduction in payload requirements. Given the limitations of current model basins for deepwater Spar systems, quantification of the coupling effect of the mooring and riser system with the Spar hydrodynamic response can best be done through analytical software. This paper will describe recent efforts to predict and quantify these effects.
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.
A joint probabilistic model of the metocean environment is assembled, taking account of wind, wave and current and their respective heading angles. Mooring line tensions are computed in the time domain, for a large set of short-term stationary conditions, intended to span the domain of metocean conditions that contribute significantly to the probabilities of high tensions. Weibull probability distributions are fitted to local tension maxima extracted from each time series. Long time series of 30 hours duration are used to reduce statistical uncertainty. Short-term, Gumbel extreme value distributions of line tension are derived from the maxima distributions. A response surface is fitted to the distribution parameters for line tension, to allow interpolation between the metocean conditions that have been explicitly analysed. A second order reliability method is applied to integrate the short-term tension distributions over the probability of the metocean conditions and obtain the annual extreme value distribution of line tension. Results are given for the most heavily loaded mooring line in two mooring systems: a mobile drilling unit and a production platform. The effects of different assumptions concerning the distribution of wave heading angles in simplified analysis for mooring line design are quantified by comparison with the detailed calculations.
Structural reliability analysis (SRA) has been used to calculate the probability of two adjacent mooring line failures. The initial failure is caused by some exceptional causes which most likely is related to substandard strength, but could also be exceptionally high tension caused by mal operation. Empirical failure data are used to assess the probability of initial failure. The ALS in the context here should control the probability a second mooring line failure with ordinary strength, adjacent to the initial failure of a weak substandard line. This check is also called the ULS redundancy check in ISO 19901-7. 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. Considerations for when the initial failure occurs have been made, and three different time intervals are considered: i) Failure of the second line during the transient motion after first failure, ii) failure of both lines in the same storm and iii) failure of the second line during stationary conditions after the initial failure. Time interval ii) is identified as most critical, when there is practically no time to implement mitigating actions. Detailed SRA analyses have been carried out, and include the concept of having a weak line in the system that represents the first failure. It was found that weak lines, with strength distributions that are consistent with the empirical probability of line failure, are too weak to contribute significantly to reduce the probability of the 2nd line failure of an ordinary line. The probability of the combined event of 2 line failure can therefore be simplified and set equal to the product of the probability of the presence of a weak line and the probability of 2nd line failure of an ordinary line in a system with one line missing. Time domain analysis is applied to obtain the short-term, extreme value distribution of line tension in the most loaded line after one line is removed from the mooring system. A large number of different metocean conditions are considered. A response surface is used to interpolate on the distribution parameters 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. 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. The probability of failure, conditional on the initial failure, is calculated using SRA. Calibration of safety factors are provided for different reliability levels, and for different assumptions for the probability of the presence of a weak line. It is demonstrated how the ALS criterion can be relaxed if the frequency of initial line failures due to exceptional causes is reduced. The final recommendations on target reliability level and on the probability of having a weak line in the mooring system are given in a companion paper at OMAE 2017, which comprises 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 the Gulf of Mexico to achieve acceptable risk, and options are discussed. This paper is the second one in a series of three at OMAE 2017, where the first one deals with structural reliability analysis of the ULS and the third one provides summary and recommendations for safe mooring design in ULS and ALS.
DNV GL is currently running a Joint Industry Project, “NorMoor JIP”, with participants from oil companies, engineering companies, rig-owners, manufacturers and marine authorities. It is a global study covering Gulf of Mexico, Northern Europe and Brazil waters. Our motivation for initiating a study on mooring line reliability was that all the global standards (API, ISO, DNV GL, others) are mostly based on work from late 1990s, when frequency domain analysis was prevalent. The reliability level implied by these regulations is not known, and we also see that the mooring standards are interpreted and applied differently. Thus, there is a need for a mooring design code with a consistent analysis methodology and with safety factors that are in line with this methodology and calibrated at an appropriate target reliability level. This is achieved through reliability-based calibration for a range of different units, mooring systems, water depths and geographical locations. The focus in the present paper is the calibration of safety factors and selection of target reliability level. The underlying probabilistic analysis results used for the calibration are reported in two accompanying papers at OMAE 2017, [1] and [2], dealing with structural reliability analyses for the ULS and ALS respectively. For mobile units frequency domain analyses are common, and although the main attention in the JIP is towards time domain analyses, it is part of the JIP to calibrate safety factors for frequency domain analyses as well. The annual extreme value distribution of line tension for all cases is calculated in time domain and is applied both in the calibration of safety factors for time domain and frequency domain analyses. It is seen that characteristic tensions from time domain analyses are likely to be higher than those from frequency domain analyses. The dilemma of not being penalized when using more refined time domain analyses is discussed, and different safety factors have been suggested for use with time domain and frequency domain analyses. A discussion about target reliability level is included, and the target levels are proposed with basis in the existing mooring design practice for mobile units, where frequency domain analysis is prevalent. Different targets are proposed depending on consequences of failure. Calibration for different design formats are carried out. The current format using a single safety factor is challenged with a format with two safety factors. The objective is to arrive as close as possible to the target reliability for all cases analyzed. A different design philosophy is needed in the Gulf of Mexico in order to achieve acceptable risk, and options are discussed. The present work provides a unique and comprehensive set of results, where advanced reliability methods are used to calibrate a mooring design code where the mooring line tensions are calculated in the time domain. The results provide a basis for regulators, such as ISO, to update their rules. ULS and ALS are covered here, and a potential phase 3 of the JIP will cover the fatigue limit state. When the NorMoor JIP is completed the plan is to implement the results into DNVGL-OS-E301, [5].
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