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Design Package

On-Bottom Stability (DNV-RP-F109 2010)

The lateral stability criteria for a pipeline lying on the seabed or in a trench under hydrodynamic forces have to be satisfied. This is achieved by calculating the steel wall thickness or concrete weight coating required to keep the pipe lateral movement below a code-specified limit. DNV-RP-F109 forms the basis for the development of module B109.

The B109 module is used to calculate the pipe wall thickness or concrete weight coating required for lateral stability (i.e. to limit the extent of lateral movement to a specified limit) in accordance with DNV-RP-F109 (2010). Alternatively it can be used to calculate the Safety Factor for a specified pipe wall thickness with (or without) weight coating. If pipe wall thickness is the calculated result, the nearest API 5L pipe size is selected.

The B109 module uses 3 analysis methods depending on the allowable lateral movement; The Absolute (zero movement), L_stable (0.5D) and L_10 (10D) stability methods. For a trenched pipeline, only the Absolute method may be used. The module is capable of analysing the stability characteristics of a pipeline length with varying wall thickness, water depth and environmental conditions (i.e. current and wave profiles).

Wall Thickness Calculation (DNV OS-F101 2010)

The Wall Thickness Calculation module (D2KM) calculates the minimum wall thickness that meets the requirements of the latest design code (DnV-OS-F101 2010), for a given design basis. The offshore standard F101 adopts the Load and Resistance Design Format (LRDF), a limit state method which uses partial safety factors, as opposed to the Allowable Stress Design Format, used by other PlusOne modules such as WALL.

The wall thickness calculated by the D2KM module satisfies the following design criteria specified in the Limit States chapter (Section 5 Part D) of the offshore standard, DNV-OS-F101 (2010), for submarine pipelines:

  • Pressure Containment (Bursting);
  • Local Buckling (General); which comprise of:

– Local Buckling External Overpressure only (System Collapse)

– Propagation Buckling

– Local Buckling (Combined Loading Criteria). This may be Load or Displacement Controlled.

Lateral Buckling Calculation

The Lateral Buckling (LATB) module predicts the susceptibility of pipeline to lateral buckling and checks whether the generated stresses in the pipeline are within the acceptance limit of the commonly used design codes.

Submarine pipelines often carry products which are hotter than surrounding water. The resulting thermal expansion is resisted by the friction between the pipeline and seabed causing large compressive forces in the line. The magnitude of these forces depend on many factor such as initial tension at the seabed, pressure difference across the pipe wall and temperature variations due to hot fluid passing through the line. This can result in lateral buckling or upheaval buckling.

For normal coefficients of friction, studies have concluded that for pipeline on seabed the propensity to buckle laterally is more than upheaval. This is because lateral buckling occurs at a lower axial load than the vertical mode and is dominant in pipelines unless the line is trenched or buried. Most of the work in this field has been done by R. E. Hobbs who did in-depth study and presented several modes of lateral buckling.

The design codes used for stress check in LATB module are as follow:

  • DNV-OS-F101 (2001) Submarine Pipeline Systems;
  • ASME B31.4 (2001) American Society of Mechanical Engineers, Gas Transmission and Distribution Piping Systems;
  • ASME B31.8 (2007) American Society of Mechanical Engineers, Liquid Transportation Systems for Hydrocarbons, Liquid Petroleum Gas, Anhydrous;
  • Ammonia, and Alcohols.

BS PD 8010 (2004) British Standard Practice Document 8010, Code of practice for pipelines; Part 2; Subsea Pipelines.

Free Spanning Analysis (DNV-RP-F105)

The Free Spanning Analysis (S105) module provides preliminary fatigue screening of observed submarine pipeline span and calculates the allowable spans due to Vortex Induced Vibration (VIV), Fatigue and Ultimate Limit State (ULS), in accordance with DNV-RP-F105 (2006). Following installation or during annual survey of pipeline, spans may be detected between high points on the seabed. The high spots may be natural seabed irregularities, as a result of hard soils encountered during trenching or as a result of scour or sand waves. Pipeline spans are prone to overstress due to self-weight and hydrodynamic loading, and vibrations induced by vortex shedding from the span.

Static loading on the span is produced by a combination of the self weight of the pipe and horizontal hydrodynamic loads. Although hydrodynamic loads with an oscillatory wave component are strictly dynamic loads, they are usually treated as static loads because the natural frequency of typical pipeline spans is often greater than the frequency of the wave.

Vibrations of the span are caused by periodic vortex shedding. Each vortex shed induces a reaction impulse and consequently a deflection in the pipeline. Where vortices are shed coincidentally, the impulse components perpendicular to the direction of the current cancel and the resultant oscillations are in line with the current. Where the vortices alternate, perpendicular components do not cancel and the resultant oscillations are predominantly perpendicular to the flow direction (‘cross flow’) but still have some in line component. If the frequency of oscillation matches the natural frequency of the pipeline span, resonance or “lock-on” occurs, resulting in high amplitude oscillations and possible damage to the pipeline or its coating.

The S105 module assesses spans for VIV fatigue onset (VIV fatigue screening criteria), direct wave loading (simplified direct wave fatigue) and ultimate limit states (ULS).

The module S105 is intended for preliminary allowable span evaluation of submarine pipeline span. The module performs the analysis by two distinct calculations as follows:

  • Provide data for screening of observed span length;
  • Evaluate allowable span length.

The loading on the span includes the self weight, buoyancy and maximum steady state hydrodynamic loading.

Analysis with S105 allows for variation in water depth, current and wave velocity profile data along the pipeline length.

Dynamic Span Evaluation (DNV 1976 & 1981)

The Span Assessment Evaluation (SPAN) module assesses spans for overstress and the risk of vortex-induced vibrations according to DNV 1976 & 1981. Following installation or during annual survey of pipeline, spans may be detected between high points on the seabed. The high spots may be natural seabed irregularities, as a result of hard soils encountered during trenching or as a result of scour or sand waves. Pipeline spans are prone to overstress due to self-weight and hydrodynamic loading, and vibrations induced by vortex shedding from the span.

Static loading on the span is produced by a combination of the self weight of the pipe and horizontal hydrodynamic loads. Although hydrodynamic loads with an oscillatory wave component are strictly dynamic loads, they are usually treated as static loads because the natural frequency of typical pipeline spans is often greater than the frequency of the wave.

Vibrations of the span are caused by periodic vortex shedding. Each vortex shed induces a reaction impulse and consequently a deflection in the pipeline. Where vortices are shed coincidentally, the impulse components perpendicular to the direction of the current cancel and the resultant oscillations are in line with the current. Where the vortices alternate, perpendicular components do not cancel and the resultant oscillations are predominantly perpendicular to the flow direction (‘cross flow’) but still have some in-line component. If the frequency of oscillation matches the natural frequency of the pipeline span, resonance or “lock-on” occurs, resulting in high amplitude oscillations and possible damage to the pipeline or its coating.

The SPAN module performs the analysis of the static behaviour of submarine pipeline spans by two distinct calculations as follow:

  • Static stresses due to bending under lateral loads in the extreme storm conditions. The loading on the span includes self weight, buoyancy and maximum steady state hydrodynamic loading;
  • Assessment of the risk of vortex-induced vibrations by the use of simple “reduced velocity” method.

The analysis is based on an idealised representation of a pipeline span, consisting of a fixed-fixed beam under lateral and axial loading. The span is assumed to have an effective length which is 1.1 times the observed length of the span. The span is assumed to be symmetrical, and rests on a rigid seabed foundation.

On-Bottom Stability (Classical Theory)

A pipeline resting on the seabed is acted upon by environmental loads comprising wave and current forces. In order to maintain integrity, the pipeline must resist lateral excursion. This can be achieved by designing the pipeline so that it has a sufficiently high submerged weight to resist these forces, or by shielding the pipe from the environment by trenching or burying. Additional weight is normally achieved by adding material in the form of a concrete coating or by increasing the wall thickness of the pipe.

The Pipeline Stability (1) (STAB) module is used to evaluate the stability of a pipeline using the hydrodynamic coefficients specified in DNV codes or by the user.

This is effected by:

  • Determining concrete thickness for a pipe specification;
  • Determining wall thickness to achieve a specified safety factor;
  • Determining a safety factor for a specific pipe input;
  • Batch processing several analyses for different water-depths, currents and wave profiles.

If a pipe wall thickness is the result of the calculation, then it is compared with the nearest API 5L pipe size. The user may also analyse the stability characteristics of a pipeline along a specified route. The user can vary water depth, and change both the current and wave profiles. The STAB module carries out a simplified analysis which calculates the worst combination of hydrodynamic loads and then assess pipeline stability. A set of 2 dimensional the forces acting on the pipe.

The accuracy of the results are dependent upon the suitability of the current and wave models chosen. Wave induced velocities can be derived from either Airy, Stokes, Cnoidal or Solitary wave theories and the current velocities from either an explicit value, the 1/7th power law or a logarithmic relationship. The STAB module also recommends the applicable wave theory for the conditions being analysed.

The method used by the STAB module to calculate the pipe’s lateral friction resistance is based upon Coulomb theory. The accuracy in this method is dependent upon the estimation of the lateral frictional coefficient, guidance for which is given in the manual.

On-Bottom Stability (DNV-RP-E305)

Environmental loads from waves and currents act upon a pipeline resting on the seabed. In order to maintain integrity, the pipeline must resist lateral excursion. This can be achieved by designing the pipeline so that it has a sufficiently high submerged weight to resist these forces, or by shielding the pipe from the environment by trenching or burying. Additional weight is normally achieved by adding material in the form of a concrete coating or by increasing the wall thickness of the pipe.

The Pipeline Stability (STB2) module evaluates the stability of a pipeline using the veritec recommended code of practice RP E305 (1988). This was intended to supersede earlier stability guidelines by using a modified 3D Morrison equation including wake effects downstream of the pipe. By using an interpolation methodology for the data presented in RP E305, the STB2 module has the ability to:

  • Determine concrete thickness for a pipe specification;
  • Determine a required wall thickness to ensure stability;
  • Calculate the safety factor between submerged weight as specified and that required for stability.

The accuracy of the results is dependent upon the suitability of the current and wave loads applied to the pipeline. Wave induced velocities are calculated in accordance with RP E305 which can be made physically realistic by use of a calibration factor. Wave states are characterised by energy density functions specified by wave height, spectral wave period and the Jonswap peakedness parameter. Current velocities may be specified as an explicit value from the 1/7th power law or a logarithmic relationship.

The method used by the STB2 module to calculate the pipe-seabed interaction is as specified in RP E305 for sand soil and for clay. This involves determining the generalised weight parameter by interpolating between data-figures presented in the code.

The STB2 module is supplied with comprehensive theory, user and validation manuals describing worked examples in detail.

Upheaval Buckling Analysis

Pipelines in service are often subject to high compressive axial forces. These forces are due to temperature and pressure induced axial expansion, which is resisted by the friction force generated between the pipe and the seabed. For upheaval buckling to occur, the pipeline must first have an initial imperfection. Imperfections are typically due to the pipeline being laid over a boulder or due to irregularities in the seabed profile.

The object of the Upheaval Buckling (UPBK) module is to carry out a simplified upheaval buckling analysis based on idealised pipeline imperfections. The UPBK module is suited to the conceptual design stage in order to determine whether upheaval buckling problems exist for a given pipeline.

The UPBK module is used to:

  • Determine whether the pipeline is susceptible to upheaval buckling;
  • Evaluate the required height of backfill to prevent upheaval buckling.

Pipeline buckling is very sensitive to the size and shape of the initial imperfection. Small diameter pipelines are particularly susceptible to upheaval buckling problems for the following reasons:

  • The Pipeline is trenched for protection, therefore preventing lateral movement;
  • Submerged weights are low since concrete is often necessary for stability;
  • The contents temperatures are high and Pipeline flexibility is high;
  • The UPBK module enables the engineer to quickly and easily carry out upheaval buckling calculation. This in turn allows flexibility in conceptual design and assessment of the pipeline for a range of operational and environmental conditions.

Wall Thickness Calculation – ASME, BS, DNV & API

As part of the initial design of a pipeline the wall thickness has to be selected. The wall thickness selected must achieve all the requirements set out and specified by the design code being used.

The Wall Thickness (WALL) module calculates the minimum wall thickness that will meet the requirements of the selected design code.
The design codes implemented by the WALL module are:

  • API 1111 (2009, Errata 2011) – American Petroleum Institute (Design of Offshore Hydrocarbon Pipelines);
  • ASME (B31.4: Liquid, B31.8: Gas) – American Society of Mechanical Engineers, Pipeline Transportation Systems for Liquid Hydrocarbons & Gas
  • Transmission and Distribution Piping Systems;
  • DNV 1981 – Det Norske Veritas (Rules for Submarine Pipeline Systems);
  • BS PD8010 (2004) – Code of Practice for Pipelines (Part 2: Subsea Pipelines);
  • BS8010 (1993) – British Standard, Code of Practice for Pipelines Part 3 (Pipelines Subsea).

The WALL module follows the following procedure:

  • Calculate the minimum wall thickness for each of the selected design code requirements (hoop stress, hydrostatic collapse, buckle propagation, diameter to thickness ratio);
  • Select the minimum wall thickness that will comply with all the selected design code requirements;
  • Recommend the nearest API pipe size that will comply with all the selected design code requirements.

The WALL module enables the engineer to quickly and easily calculate the required wall thickness of a pipeline for any of the codes selected. The module can also be used to optimise the pipeline steel grade.

Pipeline Expansion Analysis

The Pipeline Expansion (XPAN) module enables the engineer to quickly calculate the expansion of a pipeline and the pipeline conditions along its length. This information is typically required for sizing spoolpiece offsets.

When a subsea pipeline is installed its ambient temperature is the same as its surrounding environment. The product that the pipeline is designed to transport is usually much hotter than the installation temperature of the pipeline. For the product to flow an internal pressure difference will also be applied. Once operational the temperature and pressure of the pipeline will rise and consequently the pipeline expands. The amount of pipeline expansion will be dependent upon the seabed frictional resistance.

The objective of the XPAN module is to calculate the amount of expansion that will occur at either end of the pipeline after it becomes operational for a specified temperature and pressure profile.

The Calculations can be carried out for both thick and thin wall theories. The XPAN module is used to:

  • Calculate the thermal, pressure, frictional and total strain at each node along the pipeline;
  • Calculate the displacement of both the hot and cold ends of the pipeline (this takes into account the variation of temperature and pressure profile along the length);
  • Calculate the maximum and minimum stresses at the nodal points.

The output from this package is in tabular and graphical form. The tabular output summarises the expansion, stresses and strains at each nodal point. The graphical output shows the temperature profile, displacement and stress along the length of the pipeline.

The XPAN module is supplied with comprehensive theory, user and validation documentation describing worked examples in detail.