Improved methods for flexible riser fatigue reassessment

Transcription

1 MARINTEK do Brasil Multiclient Project Proposal Improved methods for flexible riser fatigue reassessment 01 October

2 Presentation outline 1. Introduction / Objectives 2. Work description 3. Schedule / Cost estimation 4. Organization

3 Introduction Background Flexible risers operating for more than 20 years Is it safe to continue operation or should they be replaced? Conservative assumptions made during the design phase may overestimate the accumulated damage at the end of the riser life Motivation/Objectives Improved numerical methods and analysis procedures may help reducing lifetime assessment conservatism - more accurate prediction may lead to cost reduction - may allow life extension and better reassessment due to operational changes or for re-allocation purposes

4 Work description (preliminary) PHASE 1 IMPROVED CALCULATION METHODS FOR FATIGUE ASSESSMENT WP1 State-of-practice and art review WP2 - Environmental loading and load case matrix definition for bi-modal/bi-directional sea states WP3 Coupled irregular wave dynamic analysis methodologies for fatigue assessment WP4 Flexible pipe / bend stiffener intermediate modelling WP5 Methodologies for fatigue assessment of the cross sectional components in a flexible pipe PHASE 2 RE-ASSESSMENT AND LIFETIME EXTENSION (Optional) Client specific scenario for: Option 1 Lifetime extension of a riser in operation Option 2 Re-assessment of a riser due to operational changes

5 PHASE I Improved calculation methods for fatigue assessment Main topics to be assessed: i. Environmental loading revisited and load case matrix definition for bi-modal / bi-directional sea states; ii. Simultaneous irregular wave dynamic analysis (coupled) of the floating unit, mooring lines and risers considering hysteretic bending behavior; iii. Bend stiffener rate dependent response (non-linear viscoelasticity); iv. Definition of a stress transfer function Transformation of curvature and tension time series into stress time series of many points around the riser cross section and along length. Environmental loading definition Global / intermediate loads Local stress analysis Fatigue assessment

6 WP2 Environmental loading and load case definition for bi-modal / bi-directional sea states Objective To develop and apply methods for wind-sea and swell partitioning considering different directional spreading To propose a procedure for fatigue loading matrix definition Work description TASK 2.1 Environmental loading definition for bi-modal/bi-directional sea states TASK Statistical uncertainty of simulation time and blocking of scatter diagram

7 combining wind-sea and swell with different spreading functions Wind sea JONSWAP Hs=4m, Tp=12s s = 2 Swell Hs=4m, Tp=16s s = 150 Total wave spectrum Teixeira and Guedes (2009) What s the ompact of comboned sea states on the fatigue assessment?

8 WP3 Coupled irregular wave dynamic analysis methodologies for fatigue assessment Objective To propose procedures for an efficient global dynamic analyses that incorporates the multi-directional environmental definition, the system coupling effects and the riser bending moment curvature hysteresis Work description TASK Simultaneous dynamic analysis of floater motions, mooring lines and risers efficient methodology for fatigue assessment TASK Hysteretic bending behavior of flexible risers

9 a irregular wave time domain coupled dynamic analysis is feasible for fatigue assessment? In a coupled analysis, both the offset and vessel heading varies according to the environmental condition at each time step Assumptions removed : Simulation time 6h time domain irregular wave 20 mooring lines ~ 3200 bar elements 15 risers 900 beam elements bar elements Total computational time 2,5 hours 2 x Quad Core Intel Xeon X Gb RAM i. Offset estimation ii. Heading distribution (considering joint probabilistic description of wave and current) iii. Riser and mooring system damping and inertia Fatigue contribution from WF as well as LF can be intrinsically considered SIMO/RIFLEX coupled analysis

10 structural damping occurs due to bending hysteresis -Large pre-slip bending stiffness -Loss of energy due to friction (structural damping) It will be directly modelled in the coupled dynamic analysis without converting it to an equivalent form of viscous damping May reduce the bending motions in a pressurized riser and assist in demonstrating a safe fatigue life

11 WP4 Flexible pipe / bend stiffener intermediate modelling Objective To develop a new mathematical model considering the polyurethane non-linear viscoelastic behaviour To propose a force/curvature transposition methodology Work description TASK 4.1 Bend stiffener viscoelastic modelling TASK 4.2 Global to intermediate to local transposition methodology A novel approach for the top connection fatigue assessment is being proposed and will be developed and assessed considering the inherent loading rate dependent response of the bend stiffener (non-linear viscoelasticity).

12 why a viscoelastic model?

13 Large deflections, beam theory what we have developed up to now is a 2D nonlinear viscoelastic beam model in: i. Time domain - arbitrary loading conditions -time consuming (20h) ii. Steady-state harmonic loading - harmonic loading conditions - perturbation method - fits engineering purposes (3min) and what need to be further developed is: iv. Steady-state solution for arbitrary loading conditions - irregular wave loading v. 3 dimensional modelling

14 Related external activity BEND STIFFENER POLYURETHANE EXPERIMENTAL CHARACTERIZATION Research cooperation project between MARINTEK do Brasil and Federal University of Rio de Janeiro (COPPE / UFRJ) Financed by the National Innovation Agency FINEP (Cooperação ICTs-Empresas Pré-Sal 03/2010) Relevant activities Creep and/or relaxation tests for polyurethane non-linear viscoelastic characterization Data fitting

15 WP5 Methodologies for fatigue assessment of the cross sectional components in a flexible pipe Objective To propose a procedure to generate a stress transfer function from the local finite element model that convert forces and moments obtained in the global coupled time domain analyses into stresses in the tensile armours Work description TASK 5.1 Stress transfer function for irregular wave loading TASK Fatigue damage calculation

16 ...from the global time series of forces and moments to the stresses in the tensile armour -4 corners -8 wires in each layer DOYNOV et al. (2007) SOUSA et al. (2012) Axisymmetric loads Bending loads f T P, f M,, P,, P ax int ext, P b int ext Coefficients calculated to adjust the function using BFLEX Stress time series are generated using the transfer function for 32 points in a selected critical region along riser length (e.g. bend stiffener area)

17 Phase II Reassessment and lifetime extension (optional) The client who has participated in Phase I may choose one of the following scenarios for further analysis: Reassessment scenarios i. Extension of service life ii. Deviations of operational and/or environmental conditions when compared to the design basis Reanalysis requirements (e.g.) a. Modifications made to the floater which can possibly influence the riser displacements b. Updated operational data such as riser content density, pressure and temperature during the service life of the riser c. Latest/updated environmental data if available

18 Schedule (preliminary) PHASE I 18 months PHASE II 6 months

19 Cost estimation (preliminary) Assuming 3 participants for the Phase I:

20 Pre-requisites and limitations The data to be used in the case study shall be defined by the Steering Committee and should contain, but is not limited to the following: a. Environmental conditions Wind and current data Wind sea and swell wave data c. Mooring system System configuration Stiffness properties b. Floating unit Hydrodynamic coefficients Mass matrix, damping matrix d. Flexible riser system data Cross section geometry and material properties S x N curves

21 Personnel

22 Thanks!!! For any questions, please contact: Marcelo Caire (21) For the companies interested, individual meetings will be scheduled for clarification purposes

23 What if no environmental data is available for Phase I? Global Reanalysis of Ocean Waves Fine South America (GROW-FINE SAM) -Oceanweather product sold on a per-grid-point -Continuous operational winds and waves from (20 years) -The data is provided subject to an end-user license agreement applicable to a single project or application

24 in case no specific riser cross section is available J.A. Witz (1996) A case study in the cross-section analysis of flexible risers

25 in case no specific SxN curves are available DNV RP-C203 Fatigue design of offshore steel structures