Tie-in spools - a verification study
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Within the Oil industry, subsea pipelines are used to transport hydrocarbons from one location to another. After the installation of a subsea pipeline, the final connection between the pipeline ant the interconnecting facilities are done by using tie-in spools. In principle, tie-in spools serve two purposes. First, it needs to provide an interface that bridges the inaccuracies associated with offshore pipeline installation. Inaccuracies related to pipeline installation are numerous, but can be related to the existing seabed infrastructure, orientation/position of the tie-in facilities with respect to pipeline installation vessel, bathymetry & soil accuracy of data among others. These factors cause the tie-in spool to be measured, fabricated and installed after the pipeline has been laid in order to make up the connection. Secondly, the tie-in spool needs to be a flexible element as pipelines expands during operational conditions because of heat and pressure differences between installation and operational stages. By the tie-in spool being flexible, the forces in connectors are reduced in order to ensure safe transportation of hydrocarbons. These key requirements can have significant impact of the overall cost of a project as they will affect all necessary operations related to tie-in spools. This report assesses key requirements related to tie-in spools by a detailed review about issues related to the design, fabrication, installation and operation of tie-in spools. By presenting details from the design of an actual installed tie-in spool in the southern North Sea this is sought achieved. By presenting a tie-in spool and its important design parameters, load steps that it is subjected to, and the results from loading analysis it is wanted to educate about the importance of tie-in spools. A modification of the tie-in spool where done to develop a simple technique to quickly assess the preliminary design/configuration of a tie-in spool based on bending moment capacity of the associated connector. The design parameters, such as pipeline expansion, where extracted from the presented tie-in analysis. Four different analysis methods where used in order to give recommendations on which method are most suited for spool piece analysis. By comparing results with the actual installed spool, calculated results showed that three methods can be suited for simplified spool piece analysis. In order to qualify one of the analysis methods, a downscaled tie-in spool was manufactured based on the modified spool. By innovative use of simple mechanical equipment, the tie-in spool was applied pipeline expansion via a winch. The bending moment in the connector where measured using an adjustable torque wrench. Measured bending moment was compared to the analysis methods and by comparison it was evident that numbers did not correlate. Due to this, no further recommendation on suitable analysis method could be given. A search for possible error sources contributing to no correlation was conducted. It is also proposed further development of experiment.
Master's thesis in Offshore technology