Comparative Risk Analysis of Two FPSO Mooring Configurations

Abstract

A methodology is set out for comparing the risks and reliabil-ities associated with two FPSO configurations offshore Brazil, one turret-moored and the other spread-moored. Both qualita-tive risk assessment (HAZID) and detailed quantitative as-sessment (structural reliability analysis) methods were used. Risk comparisons are important to Operators when choos-ing between potential development options, and to Regulators when trying to accept new concepts. This paper shows how qualitative and quantitative risk analyses are best combined to generate a rigorous and objective risk comparison between potential concepts. It also highlights areas where effort is best focussed to mitigate the marine risks. The operational HAZID comparatively assessed the ma-rine risks, with emphasis on offloading operations. It was appreciated that the limited weather vaning of the spread moored FPSO would lead to different risks for shuttle tanker operations. The dominance of wind and current as the vessels changed draft during oil transfer needed to be carefully as-sessed. Other areas of particular interest were the differences between the use of risers through a conventional turret, and external risers hung off the side of the spread moored vessel. The quantitative assessments are concerned with the strength and fatigue of the mooring lines subject to uncertain environ-mental loads and mooring line strength. Both single-line and double-line mooring failures were analyzed. Special attention is paid to an original method, which attempts to synthesize the mooring failure probabilities contributed from all potential failing lines and environmental directions. Introduction Two new FPSOs production facilities for the Caratinga and Barracuda fields offshore Brazil will be designed to accom-modate up to 80 and 107 risers, respectively. The large num-ber of risers needed at the locations makes a standard turret moored system impractical. This is because all the risers that would have to come up through the turret system, and such a large diameter turret capable of accommodating all of the riser I-tubes has yet to be developed. As a result of this, and other factors, a spread-moored FPSO will be installed that will ac-commodate the large number of risers along the exterior of the side shell. Spread-moored FPSOs are not uncommon; there are many operating throughout the world, particularly in West Africa, and currently other new facilities are being designed. However, the proposed spread-moored system for the new FPSO will use a combination of relatively new approaches and materials, including combination chain/polyester mooring lines and suction piles anchors. None of these items are unique, and all have been used at some other locations in the past. Notwithstanding the past experience with other spread moored FPSOs, there was general interest by the Brazilian oil industry and regulatory bodies to assess the comparative risks between the spread-moored and a conventional turret moored designs, which has been the common FPSO mooring system used offshore Brazil to date. Based on this experience, there is confidence in the conven-tional turret moored units in this region, and there is a desire to ensure that there are no undue risks associated with a spread moored system.

Reliability-based comparative study for mooring lines design criteria

“…If the components of a mooring segment are correlated, the correct distribution of the segment resistance could be obtained by means of an "appropriate number" of segments N e to be employed in Eq. (8) instead of N . It is simple to observe that N e varies from 1 for fully correlated components to N for completely uncorrelated ones.…”

“…In order to consider the effect of the line length on its global resistance the characteristic resistance of each line segment has been defined in this work as the most probable value of the distribution given in Eq. (8). If F R (r ) is the lognormal distribution, the most probable value of Eq.…”

“…Some of these studies are concerned with the application of reliability techniques to mooring lines in order to assess quantitatively their failure probability and its associated sensitivity to some design parameters [4,5]. Others used the reliability analysis to asses the risk of using new mooring materials by the offshore industry [6] or to compare the overall mooring safety of different mooring systems, e.g., turret and spread-moored FPSO [8]. However, a great part of these works have been concerned with the calibration of safety factors for LRFD (Load and Resistance Safety Factor) mooring line design criteria [7,9,10], i.e., the changing of a single usage safety factor to a multiple safety factors design approach.…”

“…Recently, a lot of attention has been given to the reliability analysis of mooring lines [4][5][6][7][8][9][10]. Some of these studies are concerned with the application of reliability techniques to mooring lines in order to assess quantitatively their failure probability and its associated sensitivity to some design parameters [4,5].…”

Progressive reliability method and its application to offshore mooring systems

“…Stiff et al [1] calculated the probability of failure of mooring systems using a closed-form solution accounting for the ultimate-strength (overloading) failure only and based on a quadratic model for the failure of the individual line components. Shah et al [2] assessed the reliability of mooring systems by calculating the probability that incoming waves have a frequency near the natural frequency of the mooring system.…”