The present study is an attempt to re-assess the level of reliability of the mooring system of an existing Floating Production Storage and Offloading (FPSO) unit in West Africa. The study made use of field data for the environment including wind, waves and current together with simultaneous measurements of the FPSO offset and of the mooring line tensions. Three different approaches to predict the extreme response are compared. More specifically, the traditional design method is compared with Response Based Design (RBD) and First Order Reliability Method (FORM) analysis associated with Response Surface Models (RSM) of the moored FPSO. The results of this case study allow assessing the level of conservatism that is currently embedded in classical design rules. Highlights► A reliability analysis of an FPSO mooring system is proposed. ► Field data including wind, waves and current are used. ► Models for FPSO offset and the mooring line tensions were benchmarked. ► The traditional design method is compared with Response Based Design (RBD) and First Order Reliability Method (FORM) analysis associated with Response Surface Models (RSM). ► The level of conservatism currently embedded in design rules for moored structures is assessed.

Squalls are one of the main issues for the design of West Africa floating units mooring systems. At the present time and due to the lack of more relevant information and models, squalls are represented by on site time series of time varying wind speed and relative heading. The first FPSO units were designed on the basis of a reduced Squall database. Nowadays, the number of squall records has been significantly increased and a response based analysis can be carried out. The present paper is focused on the Gulf of Guinea environment. The area has been divided into two zones: North (Nigeria…) and South (Congo, Angola…). This approach enabled us to deal with 90 Squall events for North zone and 115 Squall events for South zone. Two different mooring systems, with quite different natural periods, have been investigated in order to cover the range of already installed spread moored FPSO’s. For every Squall of the database, time domain and modal simulations have been carried out in order to obtain the maximum values of the axial tension in mooring lines and of the offset of a standard spread moored unit. Then a statistical procedure is applied a) to estimate 100-year return period values for these parameters and b) to assess overall trends besides the differences between results from both zones and both mooring systems. A comparative study has also been carried out to relate the 100-year return period extrapolations with the values derived from classical design procedures in order to evaluate the potential design margins for extreme responses. Finally, areas needing further investigation are identified.

SUMMARYThe hydrodynamic mass matrix for a vibrating ship is evaluated using a three-dimensional finite element discretization of the surrounding water. For the important case of vertical vibrations of the ship, values of the added masses obtained in this way are compared with those given by the conventional semi-empirical formulas currently in use in the shipbuilding industry. A numerical example is presented.It is concluded that conventional two-dimensional methods, the usefulness of which is confirmed when the first few modes of the hull girder only are sought, do not apply to higher modes, for which significant deformation of the cross-section may occur. On the other hand, a finite element approach has the advantage of its generality as well as flexibility. Suggestions for future research are given.