Congenital vallecular cysts are rare. In this report, four infants having vallecular cysts encountered over a six-year period from 1992 to 1997 were reviewed. All of them presented with upper aerodigestive tract symptoms. Marsupialization was performed in three of them and CO2 laser excision was performed in the fourth patient. There was no recurrence of the cyst in any patient. One of them also had co-existing laryngomalacia. The degree of airway collapse caused by laryngomalacia improved after cyst removal. The laryngomalacia resolved spontaneously. Cyst fluid culture was performed in one of the patients and yielded Staphylococcus aureus but there was no other definite indicator of infection. Staphylococcus aureus could also be isolated in the respiratory tract from two of the other patients.

This paper outlines the essential steps taken in performing structural reliability calculations during the process of laying out a risk-based inspection program. The structural reliability analysis described in this paper essentially takes the deterministic finite element method (FEM) stress/fatigue analysis results, coupled with uncertain degradation mechanisms (e.g. corrosion rate, crack propagating parameters, etc.), and tracks the time-varying structural reliability index of the structural components under consideration. This can then be used to determine the timing for inspection of structural components. For the assessment of structural strength, an efficient and straightforward method is proposed to calculate the time-variant reliability index. This method is verified by an example problem and compared to the random process first-passage reliability solutions. Load combination issue is briefly discussed, in which an approach stems from the ABS Dynamic Loading Approach (DLA) coupled with concepts from Turkstra’s rule. This proposed simplistic load combination approach is verified through an example problem in which the result is compared to the solution calculated from a more sophisticated approach. Establishment of target reliability levels is also briefly discussed. For the assessment of fatigue behavior for welded connections, both S-N curve based and fracture mechanics based reliability methods are discussed. Their usefulness will be discussed in terms of both inspection interval as well as selecting the proper sampling percentage of connections to inspect. Statistical correlation among a group of similar connections is discussed to assist the selection of appropriate locations in the population of the aforementioned sampling. The usefulness of fatigue reliability analysis is also demonstrated by an example problem.

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.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractStructural system inspection practices for floating offshore facilities, such as FPSO's and FSO's, have for the most part followed the practice employed and developed via trading ships/maritime practice. Often, the influence of a particular repeated service/cargo loading regime, cargo corrosivity, or site environmental influence can play a major role for a particular asset. More specifically, there has been only cursory attempts made to first understand and then, where possible, take advantage of site-specific conditions and loading in the development of in-service inspection plans.In recent years there has been significant interest by the marine and offshore industry to apply structural reliability techniques to risk based inspection planning for marine vessels and floating production installations. In this regard, structural reliability based methods can assist in providing a framework for quantifying site-specific loading and degradation mechanisms (such as fatigue and corrosion), through a systematic consideration of the probabilistic uncertainty in each degradation mechanism. By applying structural reliability analysis and risk assessment techniques to inspection planning, the operator is given a tool by which he can justify the allocation of resources to those structural components with a higher risk profile, and at the same time potentially relax inspection activities for low risk components to optimize and target inspection efforts.In a companion paper [1], available structural reliability methods developed to date were summarized, and then applied to determine the inspection intervals based on site specific loading as applied to strength considerations of the hull girder as well as to stiffened and un-stiffened plate panels. By tracing the time-varying reliability index for these structural components, the risk-based inspection intervals can be determined.This methodology has recently been implemented to provide the foundation in a risk-based inspection (RBI) plan for a floating production unit offshore West Africa. The current paper will further consider the sensitivities resulting from the following two conditions for the strength reliability: (1) environmental load for different regions of the world, and (2) corrosion rate corresponds to offshore environment as well as storage conditions. Another two conditions are considered for fatigue reliability: (3) assumed initial crack size for a connection, and (4) crack growth parameter for either the air environment or marine environment that a connection may be subjected to. The sensitivity studies presented in this paper provide a quick reference to understand the RBI plans benefits via work scope optimization and cost reductions for floating production units operating in various regions of the world.

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