Page 377 - Pipelines and Risers

348 Chapter 18

calculated for the both static and dynamic conditions. Accordingly, a “worst case” envelope
can be developed, which may be used as a basis for designating the allowable span criteria.

Any spans which exist may be detected by subsequent regular inspection program. The span
assessment and method of support should also take into account any proposed changes in the
submerged weight of the pipeline.

Span rectification measures will have to be employed if the pipeline span exceeds the
allowable span criteria. Generally span rectification measures will take the form of installing
discrete supports within the length of the unacceptable pipeline span, thus reducing the actual
freespan length. The installation of an engineered backfill may also be necessary to fill in the
voids between the supports and to ensure a smooth contour over the pipeline system.

Before the surface support vessel is mobilized, the repair contractor should, in consultation
with the Company, propose a design for span supports and the method of installing them.
Design calculations should be undertaken in order that the supports conform to the following
requirements:

The supports are positioned such that all relevant spanning conditions of the pipeline are
satisfied;
Realistic installation tolerance is to be included for the horizontal positioning of the
calculated spacing of the supports;
The supports are stable and fully support the pipeline over its remaining design life
period;
The support system is not susceptible to scouring action;
Lateral movement of the pipeline is prevented by the support installation.

Supports may be developed by placing numerous individual sand or grout bags under the
pipeline. An alternative to this is to install an empty fabric form-work under the pipeline and
subsequently fill it with grout. This technique is considered to provide a more reliable and
complete structural support than by using sand or grout bags and for larger supports may be
comparatively faster to install. Refer to Figure 18.4. The grouted fabric form-work may be
shaped to match the contours of the pipe and may be provided with straps to ensure a
permanent connection with the pipeline. Additionally, these units may be designed such that
during the grouting operation the injection pressure may provide an upward lifting mechanism
to the pipeline. This feature may provide a useful facility for stress relief in the pipeline span
if they are out with acceptable limits. Alternatively, if required, other equipment may be
installed to temporarily lift the pipeline during the support installation.