Life Extension of Deepwater Risers Used for a Spar Application in Gulf of Mexico

Deepwater development in Gulf of Mexico (GoM) started about two decades ago. It is the time to evaluate the global integrity of the offshore production unit and riser system and explore the possibility of life extension. This paper investigates life extension of deepwater risers used for a spar application in GoM. A spar in GoM has been put into service for the past 17 years since it was installed in 2000. It was expected to extend the riser design life of 20 years by 10 more years. This paper first introduces the configuration of Steel Catenary Risers (SCRs) and Top Tensioned Risers (TTRs) used for the Spar platform. It then discusses the philosophy for life extension of deepwater risers by assessing the integrity of the riser system. Latest technology and monitored data are used for the assessment. The TTRs and SCRs are modelled using latest analysis programs that were not readily available when the risers were designed. The interaction between the TTR and buoyancy can tensioning system is modelled as a pipe-in-pipe system that considers the function of centralizers with a gap size. The contact interaction between buoyancy can / stem and supporting guides of the hull platform is modelled as non-linear spring elements. The spring elements connect normally to the hull centerline and allow vertical sliding movement of the buoyancy can tension / stem system with friction. The base line inspections of the vessel, SCRs, and TTRs was carried out in the same inspection campaign with the spar hull structure. As-built information, production and operational data was also used for a continuous service assessment. Environmental condition data to the state of the art, including measured spar VIM data, was used in the assessment of VIM fatigue damage to the risers. The investigation was based on the up-to-date analytical tools including latest FEA program ABAQUS and VIV prediction Shear7. It further computes the riser global performance including dynamic strength and fatigue damage with the contributions from wave fatigue, VIV and VIM. This paper finally draws a conclusion for the life extension of the risers for the application in GoM.

Prediction of Combined Inline and Crossflow Vortex-Induced Vibrations Response of Deepwater Risers

Deepwater risers are susceptible to vortex-induced vibrations (VIV) when subjected to currents. When responding at high modes, fatigue damage in the inline (IL) direction may become equally important as the crossflow (CF) components. Accurate calculation of both IL and CF responses is therefore needed. Empirical VIV prediction programs, such as VIVANA “Passano et al. (2016, “VIVANA—Theory Manual Version 4.8,” Trondheim, Norway),” SHEAR7 “(Vandiver, J. K., and Li, L., 2007, “Shear7 v4.5 Program Theoretical Manual,” Department of Ocean Engineering, Massachusetts Institute of Technology, Cambridge, MA),” and VIVA “Triantafyllou et al. (1999, “Pragmatic Riser VIV Analysis,” Offshore Technology Conference, Houston, TX, May 3–6, Paper No. OTC-10931-MS.)” are the most common tools used by the offshore industry. Progress has been seen in the prediction of CF responses. Efforts have also been made to include an IL load model in VIVANA. A set of excitation coefficient parameters were obtained from rigid cylinder test and adjusted using measured responses of one of the flexible cylinder VIV tests. This set of excitation coefficient parameters is still considered preliminary and further validation is required. Without an accurate IL response prediction, a conservative approach in VIV analysis has to be followed, i.e., all current profiles have to be assumed to be unidirectional or acting in the same direction. The purpose of this paper is to provide a reliable combined IL and CF load model for the empirical VIV prediction programs. VIV prediction using the existing combined IL and CF load model in VIVANA is validated against selected flexible cylinder test data. A case study of a deepwater top tension riser (TTR) has been carried out. The results indicate that VIV fatigue damage using two-dimensional directional current profiles is less conservative compared to the traditional way of using unidirectional current profiles.

Prediction of Combined IL and CF VIV Response of Deepwater Risers

Deepwater risers are susceptible to Vortex Induced Vibrations (VIV) when subjected to currents. When responding at high modes, fatigue damage the in in-line (IL) direction may become equally important as the cross-flow (CF) components. If a riser experiences directional currents, fatigue damage must be evaluated at several locations on the cross-section’s circumference. Accurate calculation of both IL and CF responses are therefore needed. Empirical VIV prediction programs, such as VIVANA, SHEAR7 and VIVA, are the most common tools used by the offshore industry to design against VIV loads. Progress has been seen in the prediction of CF responses. Efforts have also been made to include an IL load model in VIVANA. A set of excitation coefficient parameters were obtained from rigid cylinder test and adjusted using measured responses of one of the flexible cylinder VIV tests. This set of excitation coefficient parameters is still considered preliminary and further validation is required. Without an accurate IL response prediction, a conservative approach in VIV analysis has to be followed, i.e. all current profiles have to assumed to be uni-directional or acting in the same direction. The purpose of the present paper is to provide a reliable combined IL and CF load model for the empirical VIV prediction programs. VIV prediction using the existing combined IL and CF load model in VIVANA is validated against selected flexible cylinder test data. A case study of a deepwater top tension riser (TTR) has been carried out. The results indicate VIV fatigue damage 1 using 2D directional current profiles is less conservative compared to the traditional way of using unidirectional current profiles.

Nonlinear Dynamic Analysis of Deepwater Risers With the Irregular Seabed

The stress of a steel catenary riser (SCR) at touch-down zone (TDZ) is substantially affected by its interaction with the seabed. Based on the slender rod theory, the paper adopts a realistic nonlinear load-deflection (P-y) curve to simulate soil deformation and resistance force. This seabed model can better reflect the change of soil stiffness along the vertical direction. According to the P-y curve, the equivalent stiffness at different points are obtained by an iterative method. In order to add the soil resistance force to the equation of motion, the seabed force is multiplied by the shape function a(s) and integrated from 0 to L of an element length. This paper use Gauss integral method to get the seabed force. In fact, the element may be not entirely in contact with the seabed. Therefore, the first step is to get the contact point between element and soil, and then to determine the real integral range. Meanwhile, existing riser-soil models mostly take the support of the seabed as a horizontal plane. However, many SCRs are unavoidable to be laid on the irregular seabed. It makes the strength characteristics of riser flow-line more complicated. This paper has studied the effect of irregular seabed on riser stress. The results indicate that there is a stress peak value at the irregular seabed. This value is almost equal to the value at TDP.