Log in to your subscription Username. Thusyanthan Atkins, UK S. Advanced search Show search help. Rock Mechanics Symposium and 5th U. With ever increasing demand for energy, the need for new pipelines in harsher seabed conditions is on the rise and hence accurate on-bottom stability assessment of pipelines c a vital role in cost effective pipeline design and ensures the long-term integrity pipelines.
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General 1. The premises of the document are based on technical development and experience. The basic principles applied in this document are in agreement with most recognised rules and reflect state-of-the-art industry practice and latest research.
Vertical hydrodynamic lift load. Passive soil resistance, Ref. Vertical contact force between pipe and soil, Ref. Significant wave height during a sea state. Maximum wave height during a sea state. Ratio between steady velocity component applied with single design oscillation and with design spectrum. In case of conflict between requirements of this RP and a referenced DNV Offshore Code, the requirements of the code with the latest revision date shall prevail.
In case of conflict between requirements of this code and a non DNV referenced document, the requirements of this code shall prevail. Guidance note: Any conflict is intended to be removed in next revision of that document. For the absolute stability criterion, the set of safety factors is calibrated to acceptable failure probabilities using reliabilitybased methods.
For other design criteria, the recommended safety level is based on engineering judgement in order to obtain a safety level equivalent to modern industry practice. Spectral moment of order n. Water depth. Mean grain size. Pipe outer diameter including all coating. Acceleration of gravity.
Should be taken as 9. Transfer function. D s Load reduction factor. Load reduction factor due to penetration. Load reduction factor due to trench.
Load reduction factor due to a permeable seabed. Reduction factor due to spectral directionality and spreading. Spectral spreading exponent. Un-drained clay shear strength. Relative grain density. Peak period for design spectrum. Period associated with single design oscillation. Wave induced water particle velocity. Spectrally derived oscillatory velocity significant amplitude for design spectrum, perpendicular to pipeline.
Oscillatory velocity amplitude for single design oscillation, perpendicular to pipeline. Steady current velocity associated with design spectrum, perpendicular to pipeline. Steady current velocity associated with design oscillation, perpendicular to pipeline. Pipe submerged weight per unit length.
Elevation above sea bed. Reference measurement height over sea bed. Bottom roughness parameter. Penetration depth. Trench depth. For a temporary phase with duration less than 12 months but in excess of three days, a year return period for the actual seasonal environmental condition applies. An approximation to this condition is to use the most severe condition among the following two combinations: 1 The seasonal year return condition for waves combined with the seasonal 1-year return condition for seasonal current.
One must make sure that the season covered by the environmental data is sufficient to cover uncertainties in the beginning and ending of the temporary condition, e. For a temporary phase less than three days an extreme load condition may be specified based on reliable weather forecasts. Guidance note: The term load condition refers to flow velocity close to the seabed. The highest wave induced water particle velocity does normally not correspond to the highest wave and its associated period, but for a slightly smaller wave with a longer period.
This effect is more pronounced in deeper waters. Coefficient of friction. Shields parameter. Angle between current direction and pipe. Angle between wave heading and pipe. Safety factor. Can be taken as 18 clay. Submerged unit soil weight. Pipe content can be included with its minimum nominal mass density in the relevant condition. If other limit states, e. When considering the displacement criterion, one should keep in mind that instability in this sense is an accumulated damage that may also get contributions for storms that are less severe than the design storm that is normally analysed.
For larger displacements one should perform a full dynamic analysis with adequate analysis tools, or e. Special considerations with respect to bending and fatigue should be made. The design curves given in Section 3. It should be noted that these analyses are one dimen- 2. Design 2. If this displacement leads to significant strains and stresses in the pipe itself, these load effects should be dealt with in accordance with e. For permanent operational conditions and temporary phases with duration in excess of 12 months, a year return period applies, i.
Design Methods 3. A design equation is presented for vertical stability, i. Design in order to ensure vertical stability of pipelines resting on the seabed or buried in soil is presented in general terms. The dynamic lateral stability analysis gives general requirements to a time domain simulation of pipe response, including hydrodynamic loads from an irregular sea-state and soil resistance forces.
The generalised lateral stability method and the absolute lateral static stability method give detailed specific design results for two approaches to stability design. The generalised lateral stability method is based on an allowable displacement in a design spectrum of oscillatory waveinduced velocities perpendicular to the pipeline at the pipeline level.
The design spectrum is characterised by spectrally derived characteristics Us oscillatory velocity , Tu period and the associated steady current velocity V.
As a special case a virtually stable case is considered whereby the displacement is limited to about one half pipe diameter and is such that it does not reduce the soil resistance and the displacements do not increase no matter how long the sea-state is applied for.
The absolute lateral static stability method is a design wave approach, i. If the soil is, or is likely to be, liquefied, the depth of sinking should be limited to a satisfactory value, by consideration of the depth of liquefaction or the build up of resistance during sinking.
If the specific gravity of the pipe is less than that of the soil, the shear strength of the soil should be documented as being sufficient to prevent floatation.
Consequently, in soils which are or may be liquefied, the specific weight of the pipe should not be less than that of the soil if burial is required. Exposed lines resting directly on the seabed should be checked for possible sinking in the same manner as explained above for buried pipes.
The surface wave spectrum must be transformed to a time series for the wave induced particle velocity at the pipe position at the sea-bed. Normally a constant current velocity is added to the wave induced velocity and the hydrodynamic loads are based on the relative velocity and acceleration between the pipe and the total particle velocity.
The resisting force from the soil consists normally of two parts, a pure friction term and a passive resistance term depending on the pipes depth of penetration into the soil.
The dynamic simulation should be performed for a complete sea state. If no information is available on the duration of sea states, a sea state of three hours is recommended. This is particularly important to keep in mind for large values of current to wave ratios and large wave periods, and more so for stiff clay and rock than for soft clay and sand where the build up of penetration and passive resistance is more pronounced.
Storm build up may be modelled by applying a linear ramp function on wave induced particle velocity and acceleration so that the load increases from zero to full load during approximately the first 20 per cent of the analysis. This will subject the pipe to moderate waves with small displacement that leads to increased penetration and increased passive resistance. Very small time increments may be required to accurately capture the highly non-linear stick slip behaviour of a stability problem.
The application of different phase shift between the harmonic wave components give rise to different time series realisations with varying maximum wave height and sequence of waves that both are important factors for the calculated maximum displacement. Hence, at least seven analyses with randomly, or onerously, chosen seeds to the random number generator should be performed.
When the standard deviation in the resulting displacement has stabilised, the mean value plus one standard deviation should be used as design value. The pipe may be modelled by finite beam elements extending over a part of, or the whole pipeline length. In this case, end conditions may be accounted for. If end effects are negligible, e.
A very heavy pipe will resist the hydrodynamic loads from the largest wave in the design sea state and the criterion for achieving this absolute stability requirement is given in Section 3. Sinking should be considered with maximum content density, e. If the specific weight of the pipe is less than that of the soil including water contents , no further analysis is required to document the safety against sinking.
This displacement will typically be less than half the pipe diameter, and the corresponding weight parameter is here denoted Lstable. An even lighter pipe will regularly be moved out of its depression and can assume that the displacement is proportional with time, i.
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