The initiation of oscillatory ripple marks and the development of plane-bed at high shear stresses under waves,
The gathering momentum for the use of polyester ropes for long term production system moorings in deep water has instigated a thorough review of their fatigue performance. The traditional view of designers has been to use spiral strand steel wire rope T-N curves to assess the fatigue performance of polyester ropes on the basis that available evidence indicates that the latter's fatigue performance is superior to spiral strand wire rope. The authors have examined all the available fatigue testing data and the proposals for polyester specific T-N curves and have identified a number of issues pertinent to the derivation of fatigue design data for fibre ropes and propose an improved design curve. This paper systematically identifies the various issues and proposes a rational way of including the available data (including runouts) for the development of a T-N curve for polyester ropes. The new data was used to re-visit the fatigue life and reliability for an example West of Shetland FPSO moored using polyester ropes (Ref.1).
The integrated design of deepwater risers and moorings has the potential to bring substantial benefits in terms of overall system response, cost and safety to a offshore development. Existing design methods, with their origin in shallower water design, have typically not considered design integration. The associated design assumptions need to be reassessed in deeper water, where substantial benetits can accrue from integrated design. The justification for integrated design methods lies in the importance of hydrodynamic loads, stiffness, damping, added mass and potential compliant effects of risers as partof a moored system and also in the need to recognise the precise station keeping requirements of riser systems. This paper presents the work of a JIP managed by MCS International and Noble Denton Europe and supported by 20 participant operators, contractors, manufacturers and regulators, which has investigated methods of integrated mooring and riser design over 2 1/2 years between 1997 and 1999. Several altenative innovative riser and mooring analysis methodologies, corresponding to varying levels of design integration, have been developed and investigated for five selected FPS/FPSO systems to evaluate alternative design approaches. The vessels were selected to represent recent state-of-the-art mooring and riser designs in water depths up to 2000m. Introduction A comparison of alternative methodologies for the analysis of integrated mooring and riser system designs, representing several different levels of design integration, are described, together with their design assumptions. Their results and relative merits are evafuated against the results of mooring model tests and original design calculations. Finafly, observations are made on appropriate integrated design methodologies, particularly for deeper waters. Each of the design methods under review represent increasing levels of complexity and integration of the mooring and riser design. The objective of their evaluation was to investigate the analysis methodology which is robust in representing actual conditions while still being a practical design tool. The five vessels considered throughout the investigation include a Central North Sea (CNS) FPSO in 150m water depth, a West of Shetland (WOS) FPSO in 400m water depth, a Northern North Sea (NNS) semi-submersible located in 350m water depth, a Brazilian FPSO in 1000m water depth and a generic Gulf of Mexico (GOM) floating production unit (FPU) located in 1700m water depth. The vessels and their associated mooring and riser systems, which are briefly described in Table 1, incorporate a variety of water depths, mooring system types, riser system designs and environmental conditions.
The paper deals with the reliability of offshore mooring systems. Reliability techniques have been applied to assess mooring systems designed by the quasi-static method. The study identifies that the variability of mooring line strength due to uncertainties in link strength and dynamic tension response are the two crucial parameters which affect the system safety, and further study is carried out to investigate their effects. It has been concluded that due to the simplistic and deterministic approach adopted, the quasi-static method is unable to address the variability of design parameters, and therefore mooring systems so designed often have inconsistent safety levels. 1. INTRODUCTION Mooring system failure frequently occurred in the past (about average once every 2 or 3 years per rig) and questions have been raised as to whether the presently used quasi-static method is adequate to cope with mooring system designs in extreme conditions. In this paper, mooring systems designed using this method have been assessed using reliability techniques. The main objectives of the study are:To perform a reliability analysis of a typical semisubmersible mooring system in order to identify the general acceptance level of mooring system safetyTo assess the reliability of mooring systems designed according to the present quasi-static methodTo identify the critical design parameters and assess their effects on mooring system safety. To perform the reliability analysis, typical North Sea environmental conditions are modelled and the mooring system static and dynamic responses are predicted. Probabilistic descriptions of tensions and line strength are then defined to compute the probability of failure. The technique is first applied to the mooring system of a semisubmersible unit, Sedco 700 series in 150m water depth designed according to the quasi-static method. The reliability analysis has shown that the variability of chain link strength and the line dynamic response are the two most important parameters which affect the probability of failure. The study has concluded that mooring systems considered to be at the limit using the quasi-static method have inconsistent safety levels. The study has also identified a number of areas which warrant future studies. 2. PROBLEM DESCRIPTION As most engineering design problems, the design of offshore mooring systems can be broadly categorised into two main approaches:Allowable working stress approachProbabilistic or reliability approach In design approach (a), the system resistance capacity required can be determined by:(mathematical equation) (available in full paper) where Q is the resistance, L is the load and SF is the safety factor. This approach implies that both the load and resistance can be perfectly predicted as shown in Fig.1, and when successfully designed, the probability of system failure is zero. However, in reality, due to the randomness of environment and our ability to predict the system response precisely, uncertainties are inevitable. Therefore every system has a finite probability of failure. This is why the design method (b), reliability based approach can be utilised for a better description of system safety.
Europe eUPrIW IQ*,~-T~..@Y C= feren-l%k papu wm. pmp.rmd for prese.ialio. atti. IS% C%hore Tedmolow Conference held (n Hmx.tam, T.x% 3-S thy !SSS llk paw was selectsd far Pwentanon by the OTC Program Cmmiifee follutdng Maw of in#m?nalkm c=xralnsd k!.. .Mmct s.bmii+d by the author(s) Can fe.ts of tie paper. as presen~cd, hw not been revwwad bY the Gffshcfe Technolcqy Conference and are s.tyua to cormcbon by the author(s). The material, as presented, does not !wessanly Meet any w.ilf..~~. Offahorw Twhndow Cm ferem or its ob%ma. Electr.mc mpreduchen, dmtnbuhon, or swags M any Pan of this paw for twnm.rci.l p.rpo=s withour t+. Wriwn cu.-t &tie Oifshore Technology CMcwnc. B pmhibiW. Permtw.n to reprtiuc. in pwnt IS WAIictec to an abstract of nti mom than s07 word% i[lustmtions may not b. copied. TIIe abstract must ca.ti. cmwpkuo.s acknowledgment of where and by horn the paper W% pres%nted. AbstractThe trend to deep water oilficlds has accelemterl the development of mooring systems. Fornrc monring requirements represent a considerable step out on current practice, Polyestermonrings offer considemble economic and operational advantages.The authors have previously used experience gained orr an FPS in combination with reliability analyses tn predict the failure probability of two harsh enviromnent FPS(I Riseo%fooring systems.Reeent work has built on this exqserience to prediet the failure probability of polyester mooring systems and compmc it with steel wire mooring performance.The paper outlines the previous work which provides the commorr basis for comparative reliability prcdictimra of polyester moorings. The new polyester mooring reliability predidians are presented which make a significant contribution to demonstrating the accept~bili[y of polyester moori lgs for production systems.
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