Flexible Riser
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Flexible risers are an enabling technology for
floating production in harsh environments. These pipes encompass a layered
structure in which the various materials have functions such as: withstanding
external and internal pressure, coping with tensile forces, preventing
hydrocarbon leaks and protecting against seawater.
Norwegian statistics for 2010-13 show at least
1.5% probability of failure per riser per operational year, according to the
PSA. There are multiple causes, but the underlying factor is several years of
inadequate appreciation of the complexity of flexible pipes and possible
failure mechanisms.
Earlier this year, the PSA commissioned a
study to update knowledge of un-bonded flexible risers in Norway. DNV GL is
also calling for more comprehensive studies in this area through industry
collaboration, across commercial boundaries and barriers. We believe that this
could increase confidence in the sector and open the market to innovation and
standardisation in riser technology.
Flexible risers are being usurped by metallic
systems in some of the newer deepwater developments. Shell chose steel catenary
risers for its three latest TLPs in the Gulf of Mexico, and Petrobras has
followed suit for gas export from the Marlim semi. In Norway, titanium is
making its mark in the riser for the Heidrun TLP.
The downside of metallic systems is the extra
expense perceived for many applications. For this reason, flexible risers will
remain popular for all water depths. However, there are serious design
challenges to surmount as development heads deeper.
In 2013, there were about 300 flexible risers
offshore Norway. Many were subject to high pressures and temperatures, large
fluctuations in operating parameters, and high dynamic loadings. This is a
significant reliability challenge. According to an independent engineering
company, more than 25% of NCS risers have been replaced historically and few
have met their originally predicted service life.
Sumber :www.offshore-mag.com
www.dnvgl.com
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