On-Bottom Stability Design of Submarine Pipelines

On-Bottom Stability Design of Submarine Pipelines

2. Design

2.1 Target failure probability

Excessive lateral displacement due to the action of hydrodynamic loads is considered to be a serviceability limit state SLS with the target safety levels given in DNV-OS-F101., Ref. /1/. If this displacement leads to significant strains and stresses in the pipe itself, these load effects should be dealt

with in accordance with e.g. DNV-OS-F101.

2.2 Load combinations

The characteristic load condition shall reflect the most probable extreme response over a specified design time period. For permanent operational conditions and temporary phases with duration in excess of 12 months, a 100-year return period applies, i.e. the characteristic load condition is the load condition with 10-2 annual exceedance probability. When detailed information about the joint probability of waves and current is not available, this condition may be approximated by the most severe condition among the following two combinations:

1) The 100-year return condition for waves combined with the 10-year return condition for current.

2) The 10-year return condition for waves combined with the 100-year return condition for current.

For a temporary phase with duration less than 12 months but in excess of three days, a 10-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 10-year return condition for waves combined with the seasonal 1-year return condition for seasonal current.

2) The seasonal 1-year return condition for waves combined with the seasonal 10-year return condition for current.

One must make sure that the season covered by the environmental data is sufficient to cover uncertainties in he beginning and ending of the temporary condition, e.g. delays. 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.

2.3 Weight calculations

Pipe weight should be based on nominal thicknesses of steel wall and coating layers. If metal loss due to corrosion, erosion and/or wear is significant, the wall thickness shall be reduced to compensate for the expected average weight reduction. Pipe content can be included with its minimum nominal mass density in the relevant condition.

2.4 Resistance calculations

Resistance, both the Coulomb friction part and that from passive resistance should be calculated based on nominal pipe weight.

2.5 Design criterion

Away from end constraints, the design criterion for lateral stability may be written on a general form as:

where Yall owable is the allowed lateral displacement scaled to the pipe diameter. If other limit states, e.g.maximum bending and fatigue, is not investigated, it is recommended to limit the sum of the lateral

displacement in the temporary condition and during operation to 10 pipe diameters. 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.g. data bases established by such analyses. Special considerations with respect to bending and fatigue should be made.

The design curves given in Section 3.5 are based on maximum displacement from several dynamic analyses with varying seed value for the random phase shift and can thus be regarded as upper bound values. I.e. no additional safety factors are required. It should be noted that these analyses are one dimensional, neglecting pipe bending – and axial stiffness, and that close to constraints and/or if very large displacements are allowed, two (or three) dimensional analyses may be required.

RECOMMENDED PRACTICE

DNV-RP-F109

OCTOBER 2010