Design of subsea spools: Investigating the effect of spool size

Abstract

Rigid horizontal spools provide the connection between flowlines and subsea structures. A typical subsea development may consist of a number of wells and subsea structures, which each need to be tie-in with the help of subsea spools. The spool design consequently need to be highly reliable as they also serve an important function of accommodating displacements caused by pipeline expansion to avoid damage to the connecting structures, in addition to forming the connection between pipelines and subsea structures such as manifolds and templates. Spools also have to accommodate tolerances for metrology, fabrication and installation. Loads imposed on the spool connecting hubs due to misalignments during tie-in as well as the pipe expansion set the limitations for the spool design. Different spool shapes, provide different levels of flexibility. The main objective of this thesis is to design horizontal subsea spools at a water depth of more than 1000m that is able to accommodate a 1m pipeline expansion whilst complying with the limitations set by the hub capacities. An analysis was carried out for different spool shapes in order to judge their ability to accommodate the imposed loads. The minimum spool size for the three spool configurations was determined by the use of the finite element program ANSYS 15.0. The limiting design criteria were found to be the hub capacities and the spools were optimized based on this limitation. The wall thickness of the spools complies with the limit states described in DNV-OS-F101. The spools were analysed through a series of six load steps. In the first load step, the spool self-weight was applied, followed by a tie-in sequence in the second load step. An evaluation was made to investigate which combination of metrology and fabrication tolerances were governing. Operating and design conditions were subsequently applied. The workings of DNV standard for pipeline design for wall thickness design of spools along with other design considerations such as installation, fabrication and operational issues is presented.