Analytical Estimation of Pretension Requirement to Inner Pipe of Pipe-in-Pipe Flowline in Ultra Deep Water Using J-Lay Installation

摘要

The pipe-in-pipe (PIP) offshore flowline is gaining popularities in ultra deep water developments due to its better characteristics in thermal insulation, thermal expansion, seabed stability, free span and maintenance. So far most of the larger size PIP flowline are installed using J-lay method. During the pipe welding and lowering process, the vessel tensioner grabs the outer pipe with the inner pipe free-standing inside the outer pipe. Therefore a large top lay tension is applied to the outer pipe while the inner pipe is under a sizable compression due to its own weight. A bulkhead is welded to the both inner and outer pipes with a predetermined spacing. As the PIP is being lowered toward the sea floor, the large lay tension on the outer pipe is gradually released, and the outer pipe tends to go back to its original length. This results in a push to the inner pipe from the outer pipe and develops a locked-in compression in the inner pipe and a locked-in tension in the outer pipe. In the ultra deep water PIP case, the locked-in loads and locked-in stresses could be critical if they are not well addressed. The solution to limit the locked-in effects to the inner and outer pipes is to apply a predetermined pretension to the inner pipe before both the pipes are welded together through the bulkhead. The pretension is a function of water depth, pipe length, lay angle, bottom lay tension, soil friction, pipe wall thickness, content density, content temperature, content pressure and bulkhead spacing, installation sea state and vessel motion. The accurate prediction of the pretension demands significant resources and time through utilizing the advanced nonlinear finite element analysis (NLFEA) tools such as ABAQUS. This paper proposes analytical formulations to estimate the pretension requirement to the inner pipe. The formulations are validated using ABAQUS modeling. The proposed formulations can be used as a cost effective alternative to the more time consuming and cost driven FEA method, especially when only a high level analysis is needed such as in the FEED stage or as a starting point for the detailed design stage.