Page 112 - 4274

In general, the DBD is a living document that goes through
several revisions during the course of a project. However, after the
front end engineering design (FEED) phase of a project, the DBD
must define the majority of the project requirements. After FEED,
a successful project will incorporate change only by an established
“management of change” process, which provides evaluation of
the proposed change, and its implication with regard to safety, cost
and schedule.
5.2 Route Selection and Marine Survey
In the FEED phase of a project, typically the seafloor
bathymetry data is available to the so-called “regional survey”
level. This means that coarse surface tow and swath bathymetry
survey data are available for preliminary route selection, but not to
a level of detail required for a finalisation. At this point, the
pipeline lead engineer should select the base case route based on
the regional survey data.
If a challenging bathymetry is present, alternative routes
should also be defined; environmental sensitivity zones should be
avoided, as well as excessive span areas. During the detailed
marine survey, a pipeline engineer should be on-board to perform
a real-time bottom roughness analysis. Frequent communications
should take place between the on-board pipeline engineer and the
design office to assure a successful marine survey, which will
suffice for purposes of supporting a final route selection as well as
the required geohazard survey report.
5.3 Construction Feasibility
Pipelines are installed on the seafloor by one of the four
typical installation methods: J-lay, S-lay, Reel-lay and Tow. The J-
lay and the S-lay method are shown schematically in figs. 5.1 and
5.2 (the shape each pipe assumes justifies the corresponding
name). The reel-lay method includes one or more pipe spools on
board the vessel, and the pipeline is un-spooled during offshore
works. It departs the vessel in a J-lay or S-lay configuration,
depending on the vessel method employed. By J-lay mode it is
meant a large departure angle, thus the J-lay tower can assume a
large departure angle to the horizontal, leading the pipe to a single
curvature, or J-shape. Conversely, the S-lay mode has a smaller
departure angle and the pipe has a double curvature, or S-shape.

112