Reeling Strain Analysis of a Dynamic Pipe in Pipe Riser
Authors: Daly Roland , Bell Mike 
Published: October 2008

This paper describes a procedure for determining the full component behavior of a reeled PIP SCR system during deepwater pipelay from the CSO Deep Blue reeling vessel. The contribution of plastic strains resulting from reeling on the relative pipe tensions is presented. The technique is applied to a dynamic installation analysis and used to quantify the influence of wave motions, temperature and pressure cycles on strains, inner pipe shape and shake down of residual stresses. Technip-Coflexip have developed a suite of user defined FORTRAN subroutines, included within ABAQUS?, to increase functionality and simulate the complex cyclic elastoplastic material effects, resulting from the reeling process. These subroutines are further utilized to superimpose a residual inner pipe shape and a non-symmetric residual stress/strain state on a dynamic PIP SCR riser. Accurate assessment of maximum pipe tensions during deepwater installation is shown to be critical, to the detailed design of reeled components, comprising the pipeline system.

Introduction

The differences in construction and components within reeled and J-Laid pipes can lead to important differences in their installed behavior. The carrier and flowline pipes within reeled PIP systems, see Fig 1, are relatively unrestrained in comparison with those installed by J-lay. In a reeled system, bulkheads providing axial restraint, are normally only included at the pipeline extremes. Transverse pipeline interaction along the length of the pipeline occurs at the centralisers, which are typically spaced several meters apart. The centralisers serve to maintain the flowline and carrier concentric, and transfer load between the pipes during reeling, fabrication and operation. They are sized smaller than the ID of the carrier, to facilitate insertion of up to 1km flowline stalks into the carrier. Thus, the tolerance on the OD allows some lateral movement between the pipes. In comparison J-lay systems are usually more compliant structures which may incorporate annular shear stops at regular intervals (approx. 4 joints) along the PIP to restrict axial movement. In addition the use of foam insulation systems transfer lateral load continuously along the pipeline length. Depending on the design less compliant systems allow the use of efficient insulation materials which is advantageous in terms of thermal performance. In both systems the loading behavior during installation and operation is critical to the detailed design of components. Current modeling techniques are limited in that they often fail to accurately determine tension and contact forces between the two pipes and further do not account for the post installed condition during operation. These limitations can lead to excessive design conservatism compromising both weight and thermal performance. The paper describes a method to accurately predict the global behavior of a reeled PIP SCR during installation and operation, facilitating optimized component design. The analysis has also been extended to the installation pipe tension analysis of mid-line reelable bulkheads in deepwater.