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Under construction
GRP Cover Design
General
GRP covers are made of glass fiber reinforced plastic and are commonly used in the North Sea
for protection of infrastructure on the seabed. Most commonly, the GRP covers are supported
by rock dump. Depending on the water depth, hydrodynamic loading will be the governing load case
or alternatively trawl net loads.
Geotechnical engineering of GRP covers
The limit equilibrium method is used for the geotechnical design of the GRP covers.
A series failure mechanisms are considered to find the correct capacity of the geotechnical problem.
The geotechnical design is particularly complicated when the GRP cover is placed on very soft clay.
To verify the geotechnical design, it is recommended to complete FEM analysis, i.e. with use of PLAXIS 2D.
Limit state analysis - a strong geotechnical tool
General
Limit state analysis have been a preferred design methodology in danish geotechnical engineering since Brinch Hansen.
There are two options in limite state analysis; kinematic admissible solution or a static stress distribution.
In my opinion, the kinetic admissible solution is suited for all geotechnical problems related to ultimate capacity assessment,
especially for educated estimates of foundation sizes.
Lately, an interesting development in limite state analysis was made by
LimitState:Geo. They have developed a software which
assesses all kinematic solutions to a given geotechnical problem. Subsea 7's geotechnical team has acquired
a license for the software.
Identification of stability envelope utilizing limit state analysis
Utilizing LimitState:Geo to accurately predict the ultimate capacity has proven to be excellent tool for geotechnical
design. The software can easily be used determine the stability envelope for all geotechnical problems. The figure below
illustrates the stability envelope for a foundation placed on a rock berm installed on top of very soft clay veneer.
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Pipe Soil Interaction
General
Pipelines, flexibles and umbilicals are installed by Subsea 7. As part of the installation analysis, a geotechnical
assessment of the pipe-soil interaction is performed.
The installed pipelines or flexibles will frequently need protection.
This protection comprise of rock dump or backfill from trenching activities. The pipelines and flexibles may
expand due to the pressure in the products leading forces interacting between soil and pipe. The geotechnical assessment of
these forces are called pipe-soil interaction.
Trenched products
Pipelines, flexibles and umbilicals are often trench by either jetting or ploughing. DNV-RP-F110 does provide geotechnical approaches
on how to determine uplift resistance and downward resistance for trenched pipes; however, no appropriate recommendations are given for
lateral resistance of trenched pipelines. Subsea 7's geotechnical team has developed approaches to assess lateral resistance.
Rock dumped products
On soil inappropriate for trenching, rock are typically installed for the protection of pipelines.
Rock dump consist of crushed rock from quarries and generally has a particle size between 1 inch and 5 inch.
No geotechnical design codes or papers seem to have appropriate methodologies described to assess pipe-soil interaction
in a rock dump. Subsea 7's geotechnical team has developed approaches for the geotechnical assessment of a rock dumped
pipeline.
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