Abstract

This study presents a review and new approach to VIV design of large diameter pipes hanging off from a collar and restrained laterally by guides. The specific application is for a subsea tie-back, which are common in deepwater field developments. The exported fluids are transported by a pipeline from a deepwater Floating Production Installation (FPI) to a valve assembly on the seabed, and then through a tie-back bottom spool to a fixed platform in shallow water. The incoming riser is attached to the structure of the receiving platform via clamps that allow no restriction in motion along the axial direction of the riser. VIV is often one of the controlling parameter that governs the design of the bottom spool and the riser.

Vortex-shedding design of subsea pipeline spans, and subsea spool tie-backs have been studied by other authors. This is a review study of the subject matter, but the main difference is that it considers the spool, bottom bend span and the upper riser as one single structure; thus the vibration and soil stiffness interaction of the bottom spool is accounted for in the VIV response of the upper riser spans; and viceversa, the vibration response of the riser is accounted for in the bottom spool.

Future bypass and pre-commissioning capabilities for large diameter subsea tie-back spools of incoming import risers that connect to a dual subsea isolation valve assembly; as well as pulltubes of deepwater Spars will benefit from the findings of using this approach. Both cases consist of large diameter pipes hanging-off from a collar restrained laterally by guides.

The main conclusion of this study is that VIV and specifically in-line VIV cannot be ignored because usually becomes one the governing design parameters for this type of applications.

Methodology

The riser and bottom spool are modeled as a continuous multi-span beam using Flexcom3D and Orcaflex. The riser pipe may exhibit VIV motions for both: in-line and cross-flow current directions. The programs Shear7 and RISVIC are used to simulate VIV in the frequency and 3D time domain respectively. A numerical example of a large diameter 20-in export riser is presented to demonstrate some of the challenging design issues in 750 ft. of water in the GOM environment. The fatigue damage resulting from both in-line and cross-flow VIV is estimated from the frequency and time domain responses. Among the key design issues are the required clamp distance, radial gap, appropriate boundary conditions at the clamp locations and their effect on the resulting modes and fatigue damage.

The riser clamp spacing is usually designed so that no cross-flow VIV occurs and in-line VIV is mitigated: This means that if it is not possible to eliminate in-line VIV by design, then a fatigue analysis shall be performed to demonstrate whether the riser presents an acceptable fatigue life.

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