Page 491 - Pipelines and Risers

458 Chapter 24

Design Procedure and Acceptance Criteria
The design of bundle system shall ensure that the system satisfies the functional requirements
and adequate structural integrity is maintained against all the failure modes. In principle, the
design procedure and acceptance criteria for conventional single pipeline could be applied for
the bundle system. Some special design considerations are needed, which are presented in this
section.

Design Procedure for Bundle System
The first requirement of the bundle design is to determine the carrier pipe size. Having fixed
the carrier size the bundle is considered with regard to its on-bottom stability, tow stresses,
mechanical protection etc. The recommended design procedure is shown in Figure 24.2.

The carrier pipe is generally regarded as an installation aid. After installation the carrier pipe
provides the flowlines with protection from impact. Consideration of this policy is required
when carrier pipes contain flowing fluids to provide either a cool down or heat up process, as
they may be considered as a pipeline.
Flowlines are usually sized according to processing data. The wall thickness of the flowlines
depends on the internal pressure containment. However, in high temperature applications
especially with CRA material thermal loadings must be considered with regard to flowline
sizing and more likely material properties that will apply at elevated temperatures.

The insulation "U" value will be determined from thermal and processing analysis. The
weight and volume of the insulation are needed for bundle design. Thick coatings of polymer
insulation can result in carrier pipes of large cross section enclosing relatively small weights
producing in excess buoyancy. In such cases consideration may have to be given to flooding a
flowline to provide additional weight.

Pipe-in-pipe type insulation presents a good balance between volume and weight. Post
installation annulus fill insulation does not in itself affect the bundle weight but this type of
insulation may require pipelines to accommodate expansion or filling requirements.

The weight of all the bundle component parts must be determined. Generally only carrier pipe
displacement is considered in the buoyancy calculation. The displacement of externa1 anodes,
clamps and valves is accounted for by using a submerged weight for these items in the weight
calculations. The objective of this weight and displacement determination is to arrive at a
carrier pipe diameter that will provide a resultant buoyancy of 200 N/m +3% of steel pipe
weight. The 3% figure stems form the weight tolerance and the 200 N/m figure is suited to
carrier pipe diameters of 32-inch or greater. If diameters are less than 32-inch then 100 N/m
should be used. The optimum camer could be found through a reiterative process to calculate
the weights and buoyancy.

The installed submerged weight, expansion analysis and flowline equivalent stresses are
considered to ensure that the bundle design will be stable once installed and has no adverse
effect on the permanent flowlines.