Practical aspects and the application of lateral buckling mitigation for subsea pipelines with Residual Curvature Method (RCM) are discussed in this paper. The main purpose of this paper is to promote the RCM in the Asia Pacific region as a practical and a cost-effective alternative to the existing buckle initiation methods for subsea pipelines. The RCM is assessed and compared with two existing buckle initiation methods (i.e. sleeper and zero-bend radius) using finite element analysis. The build-up of the effective axial force is the key driving mechanism in inducing a buckle in the pipeline. The effective axial force builds up along the pipeline with buckle length and the critical buckling force for buckle mitigation methods mentioned above are presented for comparison. Some practical aspects and design considerations of the RCM are also discussed. The local residual curvature section, which so far has been applied with reel-lay vessels can be implemented with S-lay vessels as a buckle mitigation scheme. Discussion on a proposal to consider the use of RCM with S-lay vessel is also covered. Additionally, the advantages and disadvantages of RCM are compared with respect to the technical challenges, the construction cost and time, handling operations and installation. This paper shall provide some good exposure to practising engineers and local/international operators in the Asia Pacific region with a relatively new and efficient method for lateral buckling mitigation and has, to date, been utilized for shallow water pipelines installed by reel-lay vessels.
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.
In 2014 Asia Pacific, McDermott International, Inc (NYSE: MDR) completed installation of its first pipein-pipe (PIP) flowline system in water depths in the range of 1200m to 1500m by reel-lay using its reel Lay Vessel North Ocean LV105. During spooling onto the installation vessel and offshore installation, the pipeline is reeled, unreeled, straightened and deployed to the seabed. These operations subject the pipe to cyclic plastic deformation and induce significant residual ovality in the pipe after reeling and straightening. An accurate prediction of this residual ovality is a key parameter for deep water pipe collapse checking and is a dimensional requirement to determine expected welding hi-lo during offshore tie-in welding. An empirical design formula for ovality prediction has been provided in DNV-OS-F101 [1] (which makes reference to Murphey and Langner (1985) [2]. However the formula does not take into account the effect of pipeline contact pressure with the reel and therefore its predicted ovality is not in line with that of experienced during actual pipeline reeling. Alternative methods for ovality prediction are to conduct a full scale bending test (FSBT), or to perform a finite element analysis (FEA). However, a conventional bending rig test generally results in higher ovality because its boundary conditions such as back tension and shear force are different from those of actual reeling. On the other hand, FEA can predict the initial reeling-on ovality accurately, however it over-estimates the residual ovality. The consequences of over-conservative ovality estimation are that it increases the pipeline design wall thickness requirement and limits the number of re-reeling cycles that can be allowed during pipelay operations (reversing the pipe back onto the reel or back over the aligner). This paper tackles the prediction of ovality for reeling and straightening processes using the finite element simulation software ABAQUS [3] by addressing the over-estimation issue caused by the standard isotropy plasticity model with a simplified but effective and practical post-processing method. A new empirical formula for predicting ovality under pure bending has been formulated based on curve fitting results from FEA simulation of pipeline pure bending and a novel approach has been developed to estimate the on-reel ovality for actual reeling. It started with estimating the pure bending ovality using the newly developed formula as mentioned above, followed by another newly developed formula that accounts for the effect of contact load on reel. This contact load ovality empirical formula is developed considering the pipe stiffness definition used in buried pipe design for plastic pipe [4] and steel pipe [5]. Combining both the ovalities due to pure bending and contact load respectively ultimately gives the on-reel ovality of interest. Finally, a new simplified approach for estimating as-laid ovality has been developed. This new approach is based on the above mentioned newly created on-reel empirical formula together with reduction factors based on in-house data.
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