Verification of an Analysis Methodology for Riser Interference
Authors: Lambrakos Kostas F. , Ghosh Rupak , Moros Thanos 
Published: October 2008

Abstract
Riser interference has become a critical issue in riser design with the progression of oil production into deep water. This is particularly the case in the Gulf of Mexico due to strong loop and submerged currents. The generally accepted philosophy for riser interference or clashing is to prevent occurrence in extreme current conditions. In special cases limited clashing under these environmental conditions could be allowed provided no damage on the risers can be demonstrated. Regardless of the design philosophy, the need exists for accurate prediction of the threshold current above which clashing could occur.
A comprehensive methodology has been developed to estimate the potential for interference between two risers, one in the wake of the other. The computational procedure involves the use of ABAQUS as a core time domain solver in conjunction with SHEAR7 for VIV and drag amplification, and a wake model for the drag and lift coefficients of the downstream riser. The methodology has been benchmarked with blind comparison of predictions and measurements of the threshold velocity from long-riser tests at Marintek and NDP. The wake model has been benchmarked with BP data on the drag and lift coefficients of the downstream cylinder in two-cylinder tests.
The paper presents a discussion of the methodology, and the results from the benchmarking.

Introduction
Riser interference governs the riser layout, and is of greater importance for field development in deep water. Thus, for the design of riser layouts, it is critical to have an analytical capability which captures all the important physics of riser interference and has been validated with test data. The interference between two risers is governed by drag and lift forces which include wake and shielding from upstream risers, and vortex-induced-vibration (VIV).
This paper discusses an interference analysis tool, and the verification of aspects of this tool with model test data from the Norwegian Deepwater Program (NDP). The verification is carried out on (1) drag and lift coefficients for the downstream riser of two risers in tandem, and (2) threshold current (minimum current) that can cause clashing of the two risers.

Interference Methodology
The methodology for riser interference is based on a time domain analysis with the code ABAQUS. The methodology is applicable to both top tensioned risers (TTR), and steel catenary risers (SCR). The analysis iterates on the two riser configurations as indicated in the flow chart in Figure 1. The steps in the analysis are:Specification in ABAQUS of the properties of the two risers, riser coordinates, appropriate hydrodynamic parameters, current profile, and possible vessel offset.ABAQUS calls ABAFER for the hydrodynamic loads on the risers for each time step. ABAFER calculates the loads on the upstream riser on the basis of (1) the free stream current at each elevation,the free stream drag coefficient given in the input, (3) the amplification in the drag coefficient due to VIV, and (4) the adjustment in the drag coefficient because of the proximity of the downstream riser.