Preliminary Optimal Design of Deepwater Steel Lazy-Wave Riser

Abstract

The importance of shale oil and shale gas has increased greatly for the global energy industry and especially in the United States. Nevertheless, the demand for liquid hydrocarbon has continued to increase, and growth in deepwater development is expected to continue. Meanwhile, as the deepwater development technologies are already mature, capital expenditure (CAPEX) and operating expenditure will decide continued developments in deepwater oil and gas. In reducing CAPEX and risk for deepwater development, especially in smaller marginal fields, use of tieback systems to existing platforms has been widely used. Such developments continue to increase despite recent lower oil prices. In development of tieback systems, limitations of host platform payload and interference with existing facilities are major design restrictions. For the limited payload, steel lazy-wave riser (SLWR) offers lower payload over steel Catenary riser (SCR). The interference issue can be resolved in preliminary design stage with precise data on host platform. In this study, SLWRs with buoyancy modules modeled as continuous sections of constant buoyancy to determine required buoyancy, without considering for possible variation in cross section. In selecting SLWR modes, the length of buoyancy catenaries have been varied by 2% increments form 10% through 20%, while lengths of hang-off catenaries have been selected using catenary equations. To determine optimal configuration among the selected models, strength and fatigue analyses have been performed on a preliminary design level. For strength analysis, two operating cases, three extreme cases, and two survival cases have been considered with fifty models. For fatigue analysis, long-term wave fatigue under wave scatter diagram, short-term vortex-induced vibration (VIV) fatigue and vortex-induced motion (VIM) fatigue under 10 year and 100 year return period loop currents have been considered. The differences in cost from material and installation time for rigid pipe, buoyancy module and strake have been compared for cost evaluation. To present the flexibility of SLWR, a normalized parameter has been proposed. Using this parameter, it is convenient to show the relations with the API utilization for strength analysis and the fatigue damage for fatigue analysis. Especially, for strength analysis, the interpolation function for each buoyancy catenary length has been derived which might be useful to engineers.