Advanced Analysis and Design

Z-Subsea personnel have distinguished track record in advanced finite element and have vast experience in applications to subsea and offshore engineering valuable to clients. Main industry widely used software packages (ABAQUS & ANSYS) have been used by the personnel of Z-subsea as part of their experience.

This is particularly important for pipeline advanced analysis, HPHT pipelines, and pipe-in-pipe systems where special design considerations, methodologies, and materials are required.

Some of Z-Subsea experiences and capabilities in advanced FE modelling and design for offshore industry are summarised.

Design by analysis (DBA)

Subsea pipelines usually require the design of mechanical components for tie-in to other parts of the subsea system.

These components are traditionally designed based on linear finite element analysis and stress linearization and categorisation. However, with increased computational power of computers, economical and robust design of components is possible using nonlinear finite element and Limit State design methods and Design By Analysis.

Pipeline Wye Piece

Ultra-Deep Water Pipeline Inline Tee

Pipeline Reducer

Pipe-in-Pipe End Bulkhead

End Bulkhead Contour Plot

Inline Bulkhead

Nonlinear Finite Element and Design By Analysis (click here) method is widely used for the design of subsea and onshore components such as:

  • Bulkheads (Pipe-in-Pipes, Bundles)
  • Flanges
  • Pipeline Tee forging
  • Wye Piece forging
  • Pipeline Reducer

The components can be designed to:

  • ASME VIII Div. 2
  • BS EN-13445
  • PD5500

using Design-By-Analysis (DBA) and advanced nonlinear finite element. This method is applicable to components with sufficiently ductile materials, providing rules for the design of any component under any loading.

 

Pipeline Flange Analysis & Design

Advanced flange design is part of Z-Subsea expertises. Flange design is based on ASME VIII Div. 2. Similarly, it can be performed to BS EN-13445 or PD5500. This requires advanced nonlinear finite element and conducting design checks. Below is one of the examples of a flange analysis.

This involves many kinds of nonlinearities including contacts and plasticity and bolts pretension, prior to design.

 

Cracked body modelling

Fracture mechanics based Engineering Critical Assessment (ECA) of non-standard welded and non-welded components require FEA modelling of crack and determination of crack-tip parameters including stress intensity factor (SIF) and plastic collapse (reference stress) solution. The former is a measure of likelihood of brittle fracture and the latter is a measure of plastic collapse. For components with standard geometry however, codes and standards such as BS 7910 and API 579 provide analytical solutions for such crack-tip parameters. These parameters combine with material mechanical properties and information about flaw dimensions, are used in the ECA procedures in order to decide whether detected flaws can be acceptable (tolerated) or immediate repair or mitigation is required.

 

 

Pipeline Reeling Analysis

To find out the effect of reeling on a pipeline, it can be modelled using detailed FEA. Based on the reeling hub diameter, it is accurate way to quantify:
  • Ovality during reeling process
  • Wrinkling and local buckling
  • Assessing weld fracure due to large strains
  • plastic strain cycles
  • residual stresses & strains due to reeling, unreeling, and straightening
  • Optimising wall thickness for reeling
  • Assessing reeling effect on CRA (corrossion resistant alloy) lining

A detailed mesh shown below is used to capture the required outputs and features from the analysis.

 

Pipe-in-Pipe Riser and pipeline and Bulkhead Design

With advanced capabilities, pipe-in-pipe risers/spools/pipelines are modelled for high pressure-High Temperature (HPHT) systems. The global model of riser and spools will provide the design forces and moments for pipeline and bulkheads.

In this approach, LRFD method is applied using load and resistance factors. Nonlinear FEA is employed to check the acceptability of design based on the above codes. The code check is usually based on analysis convergence, maximum strain, non-accumulative plasticity, buckling, or fatigue.

 

Global Buckling of Subsea Pipeline

Nonlinear finite element models are developed to assess and design pipelines for lateral buckling. The design parameters such as:

  • Pipeline cross sectional dimensions
  • Pipeline configuration and seabed features
  • Pipe-soil monotonic and cyclic interaction
  • Cyclic start-up/shut-down
  • Pressure/temperature transient profiles
  • Material and geometric nonlinearities
  • Pipeline sleepers or buoyancy modules

are the parameters which will be used for an optimized and acceptable lateral buckle. The sensitivity of target design function (local buckling, stress/strain level, low cycle fatigue, walking, etc.) will be established through running many FE cases.

The results of sensitivity analyses will be used for Structural Reliability Analysis (SRA) and Monte Carlo simulation to ensure acceptability of design and reliability of intended system for an acceptable buckle. A two-stage SRA can be suggested for a lateral buckle assessment.

On-Bottom Roughness & Spanning Analysis (VIV)

On-Bottom-Roughness and Spanning analysis of single pipes or pipe-in-pipes is required to find out the stress due to seabed features as well as fatguie life of the pipeline due to spans VIV.

 

Boat-Jacket Impact Analysis

Finite element modelling is frequently used for boat impact analysis.

Explicit and Implicit dynamic analysis methods are employed by Z-Subsea to model the impact analysis and interpret the results to design/protect offshore installations against boat impact (click here).

 

This type of ship collision analysis requires proper and accurate choice of analysis parameters and material models to have realistic and reliable results from the analysis. The analysis results are used for design purposes and assess jacket resistance against boat impact with various impact energy levels and configurations

 

FE Model for fatigue & fracture of Structural components

 

For jackets, structural members, or floating structures FE models are required for local design or stress concentration/fatigue calculation and strength/life estimation.

These models are usually required to prevent local failures due to strength or weld fatigue as basis for structural modifications.

Also, based on analysis results, engineering criticality assessment (ECA) is performed to ensure there is sufficient safety factor against brittle fracture and adequate life for fatigue fracture.

 

Structural Reliability Analysis (SRA) and FEA

Among Z-Subsea experience is using FE modelling and Structural Reliability Analysis (SRA) to address the design conditions not fully covered in design codes. The method is applied to low probability events such as Well Head Shut-in Pressure (WHSIP) on risers and pipelines in fortified zones, as well as pipelines and risers under seismic conditions.
To check a pipeline/riser loss of pressure containment:

  • A global FE model of the pipeline/riser is developed under an event which is usually a low probability condition;
  • The maximum stress or plastic strain in the global model is identified;
  • The bending moments and forces corresponding to this location is captured and applied to a local FE model representative of pipeline/riser section;
  • Using FE analysis, the equivalent plastic strain/stress in this local model is calculated for various combinations of design variables (wall thickness, yield strength, ovality, hi-lo, weld defect size, etc.);
  • Having the plastic strain for appropriate various combinations of design variables, interpolation is used to estimate the plastic strain for any other given combination of design variables;
  • Having the probability distribution function of the design variables, they are chosen randomly using Monte Carlo simulation, and the corresponding plastic strain/stress is obtained by interpolation from FE results for each random combination of design variables;
  • Assuming that the rupture of the pipeline/riser occurs at a given strain/stress (due to excessive strain, brittle fracture, etc.) the Probability of Failure (PoF) is calculated and compared with the acceptable PoF.