ECA Methodology for Fatigue Design of Risers and Flowlines

Engineering Criticality Assessment (ECA) is a procedure based on fracture mechanics that may be used to supplement the traditional S-N approach and determine the flaw acceptance and inspection criteria in fatigue and fracture design of risers and flowlines. A number of design codes provide guidance for this procedure, e.g. BS-7910:2005 [1]. However, more investigations and example studies are still needed to address the design implications for riser and flowline applications. This paper provides a review of the existing ECA methodology, presents a fracture mechanics design method for a wide range of riser and flowline fatigue problems, and shows flaw size results from steel catenary riser (SCR) and flowline (FL) examples. The first example is a deepwater SCR subjected to fatigue loads due to vessel motion and riser VIV. The second example is a subsea flowline subjected to thermal fatigue loads. The effects of crack re-characterization and material plasticity on the Level-2 and Level-3 ECA results of the SCR and flowline examples are illustrated.

Materials Selection for Sandwich Pipes Under the Combined Effect of Pressure, Bending and Temperature

Recent researches point to the great potential of the sandwich pipe conception for ultra deepwater exploitation and production of oil and natural gas. Its configuration is very simple and comprises two concentric metallic pipes with a core material, polymeric or ceramic, in the annulus. The main functions of the annular layer are: to provide satisfactory thermal insulation so as to avoid the formation of wax and hydrates along the pipeline during production shutdown; to improve the overall structural strength of the system. Polypropylene and cement have been recently proposed for these applications. The reason for the choice of these materials was the low cost and the extensive availability in industry. Here a systematic material selection approach is employed in order to assess the applicability of other polymeric materials. The attributes of materials needed to meet the design specification are thoroughly studied. The list of possible materials was enlarged and the modified digital logic approach is used with the purpose to define a top group of materials for further numerical comparative study. Finite element analyses are carried out to assess the structural strength of the sandwich pipe under pure external pressure or longitudinal bending and combined external pressure and bending. Additionally, the effect of thermal gradient is included to the numerical analyses to evaluate each pre-selected material of the top group. Results indicate that other potential materials such as PEEK and polycarbonate can improve the structural performance of the sandwich pipe conception and yet meet other several design criteria.

Vortex Suppression Efficiency of Discontinuous Helicoidal Fins

Vortex-Induced Vibration (VIV) is one of the most demanding areas in the offshore industry, and detailed investigation of the fluid-structure interaction is becoming fundamental for designing new structures able to reduce VIV phenomenon. To carry on such analysis, and get reliable results in term of global coefficients, the correct modelling of turbulence, boundary layer, and separated flows is required. Nonetheless, the more accurate is the simulation, the more costly is the computation. Unsteady RANS simulations provide a good trade-off between numerical accuracy and computational time. This paper presents the analysis of the flow past a cylinder with several three-dimensional helical fins at high Reynolds number. Flow field, vortical structures, and response frequency patterns are analysed. Spectral analysis of data is performed to identify carrier frequencies, deemed to be critical due to the induced vibration of the whole structure. Finally, helical strakes efficiency in reducing the riser vibrations is also addressed, through direct consideration on the carrier shedding frequency.

Continuum FE Modeling of Lateral Buckling: Study of Soil Effects

2D and 3D Arbitrary Lagrangian Eulerian (ALE) Finite Element (FE) models are developed to study soil-pipe interaction and lateral buckling problems. A new in-situ testing method to characterize seabed soils and to obtain soil constitutive parameters is proposed. The influence of the soil properties on the soil-pipe interaction and lateral buckling processes are presented. Implications and observations for the design of high temperature and high pressure pipelines are discussed.

Numerical Study of the Wave Impact on a Square Column Using Large Eddy Simulation

This work concerns the numerical investigation of the impact of a wave on a square column. The wave is generated by a dam break in a wave tank. Two turbulence models were used: Large Eddy Simulations (LES) and Unsteady Reynolds Averaged Navier-Stokes (URANS). The numerical simulations were carried out using a finite volume approximation and the SIMPLE algorithm for the solution of the governing equations. Turbulence was modeled with the standard Smagorinsky-Lilly subgrid-model for the LES and the standard κ-ε model for the URANS. The results are validated against experimental data for the wave impact on a square column facing the flow. The results, especially for LES, show very good agreement between the predictions and experimental results. The overall accuracy of the LES, as expected, is superior to the URANS. However, if computational resources are limited, URANS can still provide satisfactory results for structural design.

Hydrodynamic Coefficients for Riser In-Line VIV in Sheared Currents

A methodology for analyzing risers for in-line VIV fatigue damage has been developed that is based on the code SHEAR7, and laboratory in-line VIV coefficients. The in-line VIV fatigue in many instances governs the design of the riser since in-line VIV starts at a reduced velocity of about 1 whereas the threshold reduced velocity for cross flow VIV is about 4. The methodology can treat sheared currents on the basis of the cross flow VIV modeling in SHEAR7. Through the SHEAR7 modeling, the methodology removes conservatism implicit in the present ad hoc procedures for calculating riser in-line VIV response on the basis of the DNV-RP-F105 code. The reduction in conservatism is due to accounting properly for the power-in region in the VIV excitation, the hydrodynamic damping, and competing modal excitation (multiple mode response). The inline VIV coefficients have been derived from laboratory tests at the Norwegian University of Science and Technology (NTNU). The paper presents the in-line VIV coefficients, and examples to demonstrate the methodology for riser in sheared currents. The coefficients derived from the NTNU tests are functions of both the in-line VIV response amplitude and the reduced velocity. The coefficients presented in the paper are scaled test coefficients. The scaling of the NTNU coefficients assures that the methodology calculates in-line VIV amplitudes that are consistent with the response amplitudes in DNV-RP-F105 for the case of a simply supported riser in uniform current. This DNV code, although written for pipelines, has been extended to risers in sheared currents on the basis of conservative approaches.

Clashing of Risers and Umbilicals With Tension Leg Platform Tendons

Clashing of risers and umbilicals with other risers, umbilicals, mooring lines, tendons, the hull, or other objects is a major design issue. Preventing clashing is the preferred course of action; however, clashing between catenary risers or catenary umbilicals and tension leg platform (TLP) tendons is typically more challenging to prevent due to the difference in configuration/stiffness and the light weight in the case of umbilicals. In this article, the effect of parameters such as the hang-off angle, hang-off location, azimuth heading, Drag coefficient and weight on the clashing between TLP tendons and umbilicals is considered. Alternatives to preventing such clashing are also discussed.

Numerical Simulation of Offshore Pipeline Installation by Lateral Deflection Procedure

Conventional offshore pipeline installation operations in Brazil have been performed in an S-Lay procedure employing the BGL-1 barge, owned by Petrobras. However, this procedure has some limits, and may not be feasible in some particular scenarios. Therefore, the objective of this work is to present the numerical simulation of an alternative pipeline installation procedure. This procedure basically consists of performing the pipeline assembly on shore, and deflecting it to the sea using a tug boat. The numerical simulations employ the SITUA-Prosim computational tool, which is able to incorporate the correct definition of the seabed and shore from bathymetric curves. An actual pipeline installation by this lateral deflection procedure is analyzed and discussed. The characterization of the procedure passes through the determination of the better velocity and direction of the tug boat in order to minimize the efforts on the pipeline (especially due to the curvatures).

Hybrid Riser Tower for Deepwater Drill Centres

Hybrid Riser Towers, whether single bore or bundles minimise the riser load on Floating Processing Units and are field proven. Recent work has emphasised the effect of wake instability of parallel risers, and provided guidance for minimum distance between the risers. The paper describes the general architecture of a minimum Hybrid Riser Tower riser production system to be used for a drill centre, and the results of wake interference analysis for single bore HRTs and for a drill centre HRT’s in deepwater West Africa and GOM for turret-moored FPSO’s.

Numerical Simulation of the Flow Around an Array of Free-Surface Piercing Cylinders in Waves

This work deals with the viscous flow around an array of cylinders impinged by an incoming wave. Different configurations are considered in order to evaluate the effects of both wave heading and wave height on the loads applied to the bodies and on the run-ups. The results are also compared to previous calculations obtained with the assumption of inviscid flow with the aim of evaluating the contribution of viscosity.