Abstract

Pipeline free span mitigation is an integral part of field developments and their design for shallow water conditions poses a number of challenges which require special considerations. This paper discusses the main aspects of pipeline free span mitigation in shallow water conditions as is the case for the Arabian Gulf Region in which the water depths could be close to the limits of applicability of the conventional design codes based on experiences for other regions.

Free span mitigation design according to conventional codes will induce tremendous quantities of intervention works. An optimization design procedure is developed to minimize the number of IWs without compromising pipeline's integrity. Part of the procedure is the implementation of a finite element model which can capture the main features of on-bottom roughness analysis, e.g. pipe-soil interaction and residual lay tension etc. The whole process from pipeline laying on the seabed to operating are simulated. Post-process tools are developed for prompt visualization of the span profiles at different stages to provide efficient judgement for design of mitigation scheme.

The predicted spans from the FE model are compared with as-laid survey spans and good agreement is found, w.r.t locations and span lengths. Sensitivity studies have been performed to investigate the effect of various parameters on pipeline span length. The optimization design procedure constitute mainly three aspects. Firstly, allowable span length (ASL) can be increased by eliminating over-conservatism. The second aspect is reducing span lengths by various approaches, e.g. increasing concrete coating thickness, optimizing the residual lay tension and micro re-routing etc. Lastly, new optimization philosophy is also proposed, which is based on fatigue analysis for the entire design life of pipeline instead of screening by ASL for as-laid condition. Total fatigue damage during as-laid and operating conditions are calculated and compared with allowable value. For locations with total fatigue damage less than allowable value, IWs are not required. The material and installation costs are thus saved without compromising pipeline integrity.

To date, few studies have fully addressed the free span mitigation in shallow water regions. The optimization design procedure and extensive results in this paper will be a good reference for free span mitigation in shallow water regions. This will have direct cost and schedule impact on the execution of brown field developments where crossings and free span mitigation constitute a major element of the project.

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