Abstract
Pre-installation Stability analyses of pipelines are required to prevent lateral and upheaval buckling in service. In this study, the hydrodynamic forces associated with an offshore pipeline is analyzed, thereby determining limiting steel wall-thickness and submerged weight necessary to prevent collapse and propagation buckling, contain pressure and ensure on-bottom stability. Relevant design equations, Codes and Procedures were integrated to create a comprehensive platform for analyzing lift, drag and inertia forces acting on submerged pipelines. Hence, a user friendly template with multiple design settings has been developed with MathCAD® for on-bottom stability analyses. The analysis tool is based on the absolute lateral stability method in DNV RP F109. A case study of Φ762 mm x 34 km pipeline to be installed Offshore Escravos, Gulf of Guinea is simulated and analyzed using the design tool developed. Pipeline behavior under different environmental and pipeline conditions such as water depth, wave height, steel and concrete thickness were investigated. The results showed that concrete and steel wall thicknesses are the most critical parameters in the on-bottom stability of offshore pipelines. With a determined optimal wall thickness of 20.6mm, concrete thicknesses of 78.796 mm, 61.386 mm, 53.043 mm and 42.58 mm corresponding to 5 m, 10 m, 15 m and 20 m water depths, respectively were obtained. Also, the results showed that for pipes OD > Φ32.5 in (Φ825.5 mm) alternative stability methods may be required as the necessary concrete thickness may exceed allowable limits.
Highlights
All submerged offshore pipelines and sections of onshore pipelines in swamps, floodable areas, high water table areas, river crossings etc., should be stable under the action of hydrostatic and hydrodynamic forces [1]
This load-resistance relationship has formed the basis for various design codes governing the stability of submarine pipelines such as the API RP 1111 [3], DNV RP E305 [4], DNV RP F109 [5], and DNV OS F101 [6]
The primary objective of pipeline design it to optimize the relationship between pipe diameter, pipe material, pipe wall thickness, appurtenances, economics, constructability and operability of the pipeline
Summary
All submerged offshore pipelines and sections of onshore pipelines in swamps, floodable areas, high water table areas, river crossings etc., should be stable under the action of hydrostatic and hydrodynamic forces [1]. The hydrodynamic forces on the pipeline on the seabed are the combined action of wave and current. On-bottom stability of pipelines is governed by the fundamental balance between loads and resistances. In offshore pipelines, these loads are hydrodynamic loads induced by waves and currents. These loads are hydrodynamic loads induced by waves and currents This load-resistance relationship has formed the basis for various design codes governing the stability of submarine pipelines such as the API RP 1111 [3], DNV RP E305 [4], DNV RP F109 [5], and DNV OS F101 [6]
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