Analysis And Model Testing Of Flexible Riser Tie-Ins

Abstract

Abstract As part of a study conducted by Esso Australia investigating means of reducing the high cost of construction of submarine pipelines in Bass Strait, flexible risers connecting bottom-towed steel pipelines and the topsides of freed platforms were identified as a potential means of providing significant cost savings. To assess the concept, an extensive program of dynamic analyses and state model tests in a large wave-current flume were undertaken. This paper presents the findings of these analytical and test studies. Additional dynamic analyses were performed to assess the sensitivity of flexible riser performance to various changes in hydromechanics and other design parameters. A number of recommendations were developed for design criteria and analytical parameters for flexible riser systems, not addressed in current codes of practice for such systems. Introduction The relative remoteness of Australia from major centres of offshore oil and gas activity presents a number of problems to the viability of potential offshore field developments, notably the cost of mobilization of specialist construction vessels, such as derrick and pipelaying barges, pipe reeling vessels and diving support vessels (DSVS). In recent years Esso Australia Ltd. (EAL) has used innovative engineering and new construction techniques, minimizing or avoiding the use of such vessels altogether, to make substantial reductions in the cost of development of offshore structures and facilities in Bass Strait. Most future field developments in Bass Strait involve much smaller reservoirs than in the past, such that the high cost of conventionally installed pipelines, which has References, tables and figures at end of paper been identified as a major component of the total cost of such projects, would either preclude development or would make it a marginal proposition. In view of these considerations, EAL initiated the "Pipeline Cost Reduction Study" (PCRS), investigating novel pipelaying procedures to identify cost effective means of installing pipeline links to new marginal field developments. One of the innovations investigated in the PCRS was the use of single-catenary flexible risers as tie-ins to fixed platforms from on-bottom steel pipelines. This paper describes wave tank testing and dynamic numerical analysis of this concept, and discusses design criteria and analytical parameters for use in the design of such flexible riser systems. The following section introduces the PCRS. Section 3 develops the criteria used for the design of flexible riser systems. Section 4 describes the dynamic numerical analysis and the use of FLEXCOM-3D, while Section 5 describes the physical model studies including the effect of spectral versus regular waves and the influence of the structure. Section 6 discusses the design parameters obtained and compares the capabilities of numerical and physical models this discussion is summarised in Section 7 Conclusions. The Pipeline Cost Reduction Study and Its Application The PCRS identified a number of alternative methods of construction and new pipeline configurations which might offer substantial savings in pipeline cost. The most promising concepts were reviewed and the technical feasibility of specific aspects which offered the possibility of significant reductions in the cost of installation of the pipelines for a number of future projects were established.