Abstract

Reel-lay method is a fast, cost effective method of pipelaying for pipelines with an ideal diameter between 4" to 16". Reel-lay pipelines are plastically deformed to conform to the radius of the reel drum that is fixed on the vessel. The reeling operation requires a high level of engineering to ensure the pipe does not buckle nor have a high lift off on the reel or aligner during spooling and installation. This comes at the design level where the selection of wall thickness is driven by the requirements to avoid local buckling. The requirements imposed are taken from DNVGL-ST-F101 [1], a submarine pipeline systems’ standard that is widely used in the oil and gas industry in designing pipelines. However, the requirements are based on displacement control check equations with some fixed safety factors. The safety factor is designed to take into account the presence of mismatches in bending moment between pipeline joints and its system effect.

A refined assessment procedure for pipeline reeling have been developed within the industry and has been published due to the discrepancy in the minimum reelable wall thickness [[2]]. Finite element models are developed and analyzed to create a failure boundary between safe and unsafe regions using different combinations of mismatches. The reliability index is then determined using the failure boundary, which is then used to calculate the probability of failure. This method normally requires multiple FEA to create the failure boundary which can be very time consuming.

This paper outlines the probabilistic method which is an improvement to the past approach, where it firstly defines the probability of failure in calculating the mismatch. Finite element model is then developed and analyzed using finite element software Abaqus FEA [3] to verify that the level of safety associated with the method is met. The reeling studies carried out in this paper has shown that the probabilistic method requires far less analyses to be done and is able to analyze pipeline with features while still meeting DNVGL's requirement. This method also allows pipelines with different geometrical and mechanical properties such as transition design and bulkhead to be analyzed. This method normally requires only a single FEA.

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