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Full Package

Cathodic Protection Design (DNV-RP-B401 1983)

The Cathodic Protection Design (ANOD) module is used to design a bracelet anode system by helping the engineer to select an appropriate anode size and spacing along the pipeline. Alternatively, the module may be used to quickly check the validity of an existing or proposed design.

The ANOD module calculation method, default values and help screen information, are based primarily on Det Norske Veritas’ recommended practice for cathodic protection design (DNV RP B401, 1993).

Submarine pipelines are protected from external corrosion by a combination of coatings and cathodic protection (CP). The CP system generally consists of sacrificial anode bracelets attached to the pipeline at regular intervals.

The help menus also give additional advice to the engineer on the recommendations from other codes/standards, technical literature, and Penspen in-house data.

For a given set of design criteria, there are generally many feasible designs which satisfy the code requirements. The ANOD module provides graphical outputs to help the engineer to see quickly the benefits of one design over another, and hence to optimise the design by choosing a solution which minimises the total mass of anode material. A further design optimisation feature is the option to calculate an effective utilisation factor, which in certain cases can result in significant savings by increasing the required spacing or reducing the required anode dimensions.

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).

Bending Buckling

The Bending Buckling module (BUCK) determines the hydrostatic collapse pressure for a pipe having a defined eccentricity using Timoshenko’s elastic stability formula. The module also calculates the critical buckling curvature and moment taking full account of the elastic-plastic behaviour of the pipe material (the material model is defined using a bi-linear description of the elastic and plastic regions).

To operate this module, the user inputs the pipe dimensions and material properties and the water depth/density. BUCK then calculates the following:

  • the critical collapse pressure;
  • the critical buckling curvature and associate bending moment;
  • the hydrostatic pressure acting on the pipeline

Catenary Evaluation

The Catenary Evaluation (CLAY) is intended as a quick design aid, based on catenary analysis, allowing the engineer to assess the suitability of a particular laybarge/stinger configuration for the installation of a pipe whose physical properties are known and to calculate the tension required.

Submarine pipelines are usually laid in position by welding short sections of pipe together on a laybarge to make a continuous pipe which passes over a stinger connected to the barge. As the barge moves forward the pipe is lowered to the seabed. A tensioning device on the laybarge applies a horizontal force to the pipe; its purpose is to reduce the curvature of the pipe in the suspended span between the lift-off point at which it loses contact with the stinger and the touchdown point at which it reaches the seabed.

The horizontal tension is often large enough for the length of pipe in the suspended span to be several times greater than the depth of water. The design of such a system brings with it many mechanical problems. If the pipe is severely bent as it passes over the stinger or in the suspended span, it is likely to kink or buckle plastically under the combined action of bending moment, external water pressure, and axial tension.

Design of a system to avoid the possibility of this kind of buckling requires a means of analysing the configuration taken up by such a suspended pipe. The catenary configuration is determined by the submerged weight, the tension and the assumption that the pipe makes contact with the stinger and the seabed.

To operate this module, the user defines the pipe dimensions and properties, the environmental conditions, the lay barge stinger configuration and the amount of tension to applied to the pipe. (If the tension is not specified the module will calculate the minimum tension required).

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.

Single Phase Gas Flow

The Gas Flow (GASF) module predicts the pressure and temperature profiles and the flow rate of a single phase gas along a pipeline under steady flow conditions.

The pressure and temperature drop along the pipeline is solved using a numerical integration procedure. The integration is a function of the fluid properties which are in turn a function of the line temperature and pressure. Thus the integration is an iterative procedure, continuing until a balance between fluid property, pressure and temperature drop has been established.

The pipeline is defined as a series of segments. Each segment has its own orientation, geometry, roughness and thermal properties independently defined. The integration procedure divides each segment into elements and integrates down the line from the known starting conditions at the inlet. At each node the properties of gas density and gas viscosity are calculated as these are dependent on the pressure and temperature conditions.

The GASF module provides full tabulated and graphical output of the pressure, temperature and gas velocity profiles and elevation along the pipeline.

The accuracy of the profiles is dependent on the number of segments defined by the user. The more segments defined, the higher the resolution of the profile.

Heat Transfer Calculation

The Heat Transfer (HEAT) module calculates the overall heat transfer coefficient based on the internal diameter of either a fully exposed or fully buried pipe section or the coating thickness required to achieve a specified overall heat transfer coefficient.
To operate the HEAT module, the user defines the pipe dimensions, properties and environmental data.

Below is a summary of the HEAT module operations:

  • Calculate an overall heat transfer based on the internal area of a pipe if all the coatings have a specified thickness.
  • Calculate the required thickness of a coating to produce a given heat transfer coefficient.

The overall heat transfer coefficient is based on the pipe wall thickness and thermal conductivity. No account is made of the internal boundary layer between pipe wall and the transported fluid.

All exterior coatings are included in the calculation by specifying thickness and thermal property of each coating. For the unburied line, account is taken of either free or forced convection, and the temperature is assumed to be uniform about the external circumference.

The thermal resistance of the surrounding soil is only effective for the fully buried pipeline. Again the temperature is assumed to be constant around the buried pipe circumference.

Hydrate Formation Prediction

The Hydrate Formation (HYDR) module determines hydrate formation temperatures in natural gas lines using the molar percentages and tabulated data of vapour-solid equilibrium constants for the seven hydrate forming hydrocarbons.

Hydrocarbons are known to produce hydrate deposits, in natural gas lines, below the hydrate formation temperature. Since 1969, gas engineers have been using published hydrocarbon equilibrium constant data to calculate the temperature at which these hydrates begin to form. In conjunction with the equilibrium constant data, the composition and working pressure of the gas is required to calculate this unique temperature.

The water dew point temperature, in a gas line, may also be calculated using a standard gas equation. This equation uses the pressure, temperature and water content of the gas.

The amount of water condensate that forms in gas lines, at the dew point pressure and temperature, can also be calculated by manipulating the standard gas equation.

The HYDR module calculates the following three pipeline conditions:

  • Hydrate formation temperatures for a defined pressure range;
  • Water dew point formation temperatures for a defined pressure range;
  • Water content for a given dew point temperature and pressure.

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.

Lay Rate Estimation

In the laybarge method of installation, the pipeline profile from stinger to seabed is determined by a delicate interaction between applied tensioned force, pipe weight and flexural rigidity. Unless carefully investigated, the combined action of bending moment, external water pressure and axial tension can lead to severely stressed sections of pipe, resulting in buckling, kinking and plastic deformation.

Using a relaxation method for the finite difference technique [1], the Lay Rate Estimation (LAYS) module permits the pipeline engineer to quickly assess the suitability of a particular laybarge-stinger configuration for installation. Once the user has specified the stinger configuration for a pipeline and other case specific parameters the LAYS module calculates:

  • Pipeline profile as a function of distance from the laybarge
  • Bending stress, tensile forces and shear forces at each position of the nodal geometry
  • Reaction loads at the stinger rollers
  • Seabed touchdown point
  • Hoop and equivalent stressThe LAYS module provides full tabulated outputs. The LAYS module is supplied with comprehensive theoretical and validation documentation.

[1] Palmer, A.C, Hutchinson, G. and Ells, J.W. “Configuration of submarine pipelines during laying operations”. ASME, paper no. 73-WA/OCT-4 (1973).

Lay Stress Analysis

In the laybarge method of installation, the pipeline profile from stinger to seabed is determined by a delicate interaction between applied tensioned force, pipe weight and flexural rigidity. Unless carefully investigated, the combined action of bending moment, external water pressure and axial tension can lead to severely stressed sections of pipe, resulting in buckling, kinking and plastic deformation.

Using a relaxation method for the finite difference technique [1], the Pipeline Analysis (LAYS) module permits the pipeline engineer to quickly assess the suitability of a particular laybarge-stinger configuration for installation. Once the user has specified the stinger configuration for a pipeline and other case specific parameters the LAYS module calculates:

  • Pipeline profile as a function of distance from the laybarge
  • Bending stress, tensile forces and shear forces at each position of the nodal geometry
  • Reaction loads at the stinger rollers
  • Seabed touchdown point
  • Hoop and equivalent stressThe LAYS module provides full tabulated outputs. The LAYS module is supplied with comprehensive theoretical and validation documentation.

[1] Palmer, A.C, Hutchinson, G. and Ells, J.W. “Configuration of submarine pipelines during laying operations”. ASME, paper no. 73-WA/OCT-4 (1973).

Single Phase Oil & Liquid Flow

The Oil & Liquid Flow (LIQF) module predicts the pressure and temperature profiles along a pipeline under steady flow conditions. This includes the calculation of the inlet or outlet conditions, or the flow rate.

The pressure and temperature drop along the pipeline is solved using a numerical integration procedure. The integration is a function of the fluid properties which are in turn a function of the line temperature and pressure. Thus, the integration is an iterative procedure, continuing until a balance between fluid property, pressure, and temperature drop has been established.

The pipeline is defined as a series of segments. Each segment having its own orientation, geometry, roughness and thermal properties independently defined. The integration procedure divides each segment into elements and integrates down the line from the known starting conditions at the inlet. At each node, the liquid viscosity and consequently the Reynolds number are calculated as these are dependent on the pressure and temperature conditions.

The LIQF module provides full tabulated and graphical outputs of the pressure and temperature profiles and elevation along the pipeline.

The accuracy of the profiles is dependent on the number of segments defined by the user. The more segments defined, the higher the resolution of the profile.

Material Selection & Corrosion Allowance

The Material Selection & Corrosion Allowance (MATS) module provides an aid to linepipe material selection, given a set of pipeline operating conditions and fluid compositional data. The decision is based on an internal corrosion estimation. The object is to design to mitigate the effects of corrosion by selection of either a corrosion allowance, the injection of inhibitors or the use of high alloy steels.

The corrosion module first estimates a corrosion rate for carbon steel from the data provided. It then uses this information to recommend a suitable linepipe material to enable the design life specification to be achieved. Criteria used in assessing the corrosive environment include:-

  • Oil / gas / water ratio;
  • Quantities of CO2, H2S and O2 present;
  • Pressure and temperature;
  • Salinity;
  • Acidity;
  • Flow velocity;
  • Bacteria;
  • Dissolved solids;
  • Undissolved solids.

As well as providing a linepipe material recommendation the module provides a list of warnings and prompts which indicate other factors which may affect corrosion, together with a method of preventing these factors from reducing the life of the pipeline.

Quick Lay Stress Analysis

The Quick Lay Stress (QLAY) module is intended as a quick design aid, allowing the engineer to assess the suitability of a particular laybarge and stinger configuration for the installation of a pipeline whose physical properties are known under static conditions.

The QLAY module can analyse a comprehensive range of configurations and incorporates the theory necessary to calculate stresses and shear forces on the overbend region due to the effect of stinger geometry and in the sagbend region along the span. If requested, the QLAY module will also calculate the necessary force at the tensioner to produce an acceptable pipeline configuration. The QLAY module also incorporates the facility to model a sloping seabed. An option exists in the module to either calculate hoop and equivalent stresses according to the latest pipeline design codes.

Submarine pipelines are usually laid in position by welding short sections of pipe together on a laybarge to make a continuous pipe which passes over a stinger connected to the barge. As the barge moves forward the pipe is lowered to the seabed. A tensioning device on the laybarge applies a horizontal force to the pipe; its purpose is to reduce the curvature of the pipe in the suspended span between the lift-off point at which it loses contact with the stinger and the touchdown point at which it reaches the seabed. The horizontal tension is often large enough for the length of pipe in the suspended span to be several times greater than the depth of water.

The design of such systems has many associated mechanical problems. If the pipe is severely bent as it passes over the stinger or in the suspended span, it is likely to kink or buckle plastically under the combined action of bending moment, external water pressure, and axial tension. The design of such a system to avoid the possibility of this kind of buckling requires a means of analysing the configuration taken up by such a suspended pipe. The configuration profile is determined by the interaction between the flexural stiffness of the pipe, the pipe weight, the forces applied to the pipeline by the stinger, tensioner, seabed and by the sea itself.

Laybarge configurations are stored in files and can be accessed individually for evaluation as required. From a given set of data supplied by the input screen for the QLAY module, the pipeline configuration and associated stresses are calculated.

The output from the QLAY module includes both graphics and text. The graphical output displays the pipeline configuration together with associated bending stresses and shear forces.

The theory is based on small deflection theory for a submarine pipeline and therefore is ideally suited to large stiff pipes which are laid in shallow to medium water depths. A semi-analytical approach, based on invariant embedding techniques, has been used to solve the resulting equations.

To operate the QLAY module, the user defines the pipe dimensions and properties, the environmental conditions, the laybarge stinger configuration and the amount of tension to be applied to the pipe. (If the tension is not specified the module will calculate the minimum tension required).

Below is a summary of the QLAY module operations:

  • determination of the configuration of the pipe from the freeboard to the seabed;
  • calculation of the forces and moments at each of the nodal points along the pipe;
  • calculation of the bending, longitudinal and equivalent stresses induced at the nodal points and highlighting where the maximum stresses occur;
  • calculation of the minimum tension required to lay the pipeline.

Currently only single fixed ramps or circular arcs can be used to describe the stinger geometry.

The QLAY module is limited by the mathematical model, which is theoretically limited to small deflections. Therefore the QLAY module is generally limited to shallow or medium depth water. However the QLAY module will produce results for other (deep water) configurations and in such cases the results should be treated with caution.

The QLAY module is intended as a quick design tool for use in shallow to medium depth water only.

Pipeline Reeling

The Pipeline Reeling (REEL) barge method of subsea pipeline installation is a fast and economical technique but poses specific problems to the design engineer. The plastic deformation of the pipeline when spooling-on and spooling-off the reel ship precludes the use of relatively large wall thickness to avoid ovalisation.

The REEL module addresses these difficulties through a consideration of the wall thickness ratio D/t and critical buckling curvature. Bending-buckling behaviour has been widely investigated through model experiments and full scale tests, leading to correlations of critical strain resulting from high bending curvatures. For the purposes of the REEL module, the correlation which is most conservative for the specified D/t ratio is selected, indicating that the pipeline curvature must not exceed this critical strain criteria.

Bending Moment-Curvature Relationships

The dependence of the bending moment with curvature has to be calculated taking the elasto-plastic material behaviour into account. The moment distribution is found by integrating the stress field across the linepipe cross-section for a given radius of curvature. Ovality effects are taken into consideration by using Brazier’s Formula.

Critical Buckling Curvature

At very high curvatures, a local buckle may develop on the compression side of the pipe. Kink formation of a short inward dent extending over a length of about two diameters is accompanied by a marked reduction in the bending moment the pipe can withstand.

The REEL module assesses bending-buckling behaviour with either the Shell, Exxon or Palmer empirical results published independently in the academic literature. Maximum curvature and the associated percentage of critical buckling curvature are calculated and the results reported.

Pipe Configuration on the Reel

The problem of determining pipe configurations as they are spooled onto the reel is evaluated using analytical geometry. Reel capacity can be determined either by weight or volume.

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.