# Span & Stability Bundle

## 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.

## 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.