Page 37 - Pipelines and Risers

In trodiiction 11

Interference with human activities (fishing).

Due consideration to these requirements will result in the evaluation of an allowable freespan
length. Should actual span lengths exceed the allowable length then correction is necessary to
reduce the span for some idealized situations. This can be a very expensive exercise and,
consequently, it is important that span evaluation is as accurate as possible. In many cases, a
multiple span analysis has to be conducted accounting for, real seabed and in-situ structural
behavior.

The flow of wave and current around a pipeline span, or any cylindrical shape, will result in
the generation of sheet vortices in the wake (for turbulent flow). These vortices are shed
alternately from the top and bottom of the pipe resulting in an oscillatory force being exerted
on the span (see Figure 1.4).

If the frequency of shedding approaches the natural frequency of the pipeline span then severe
resonance can occur. This resonance can induce fatigue failure of the pipe and cause the
concrete coating to crack and possibly be lost.

The evaluation of the potential of a span to undergo resonance is based on the comparison of
the shedding frequency and the natural frequency of the span. The calculation of shedding
frequency is achieved using traditional mechanics although some consideration must be given
to the effect of the closeness of the seabed. Simple models have, traditionally, been used to
calculate the natural frequency of the span, but recent theories have shown these to be over-
simplified and multiple span analysis needs to be conducted.

Another main consideration with regard to spanning is the possible interference with fishing.
This is a wide subject in itself and is discussed in Chapter 11.

1.4.4 On-bottom Stability Analysis
Pipelines resting on the seabed are subject to fluid loading from both waves and steady
currents. For regions of the seabed where damage may result from vertical or lateral
movement of the pipeline it is a design requirement that the pipe weight is sufficient to ensure
stability under the worst possible environmental conditions. In most cases this weight is
provided by a concrete weight coating on the pipeline. In some circumstances the pipeline
may be allowed to move laterally provided stress (or strain) limits are not exceeded. The first
case is discussed briefly in this section since it is applied in the large majority of design
situations. Limit-state based stability design will be discussed in Chapter 8.

Thc analysis of on-bottom stability is based on the simple force balance or detailed finite
element analysis. The loads acting on the pipeline due to wave and current action are; the
fluctuating drag, lift and inertia forces. The friction resulting from effective weight of the
pipeline on the seabed to ensure stability must resist these forces. If the weight of the pipe
steel and contents alone or the use of rock-berms is insufficient, then the design for stability
must establish the amount of concrete coating required. In a design situation a factor of safety
is required by most pipeline codes, see Figure 1.5 for component forces.