Summary
The riser is one of the key elements for deep offshore drilling. However,
the total weight of the system increases rapidly with the water depth in the
same way as the required tensioning capacity. Static loads and fatigue may
become critical and threaten riser integrity. Furthermore, the natural period
of the riser in the disconnected mode may also reach the wave-energy window and
may seem able to put the riser into a dangerous resonant state.
A new technology developed by the L'Institut Français du Pétrole (IFP) for
integrating riser joints, presented in the paper, reduces the weight of the
system by up to 30% and lowers the natural period significantly. It consists of
joining together all the pipes constituting the riser (main pipe and peripheral
lines) in such a manner as to share the axial tension between all of them. This
so-called hyperstatic-working mode, which provides an axial-load sharing
between all riser lines, may be used jointly with hybrids (steel/composite)
previously developed for kill and choke lines (Guesnon et al. 2002).
After the first step of riser analysis and technical studies through
extensive finite-element analyses (FEAs) and computer-assisted design (CAD), a
field test was decided upon to obtain information on the capacity of the
technology to work properly in the operational context of a drilling rig and
its capacity to maintain the hyperstatic-working mode of the riser joint in
that context. The Pride Angola drillship was chosen for the test.
This paper presents the basis of the technology, describes the three phases
of the field test, and comments on the first results obtained from the rig. To
conclude, the technology is then put in the context of deep-water drilling, and
its benefits are discussed for conventional, slender, and dual-gradient
risers.
Introduction
Drilling Deep Offshore
As the water depth increases, the cost of drilling operations remains a big
concern for operators and drilling contractors. One way to reduce this cost is
to reduce the weight of the marine riser, which is one of the key components in
the offshore-drilling system, especially for deep and ultradeep waters. Weight
reduction of the structures also becomes necessary to improve safety and
operational performances and to allow progressively deeper exploration and
production of hydrocarbon fields.
Assessment of Potential Need
Many current projects aim to drill deeper than 7,500 ft the maximum depth today
being at approximately 12,000 ft for the most ambitious ones. It is realistic
to think that within the next 10 or 15 years, a large number of these vessels
will be able to operate in the 10,000-ft water-depth range, or greater.
These new rigs, carrying the longest risers, justify the research work
concerned with technologies aiming at the reduction of riser weight, such as
the hyperstatic integration (HSI) of the peripheral lines described in this
paper. The challenge seemed strong enough to lead a research-and-development
(R&D) project aimed at the qualification of the technology to fulfill the
identified needs.
This paper presents the progress of the project. In the first part, we will
deal with drilling risers and describe the working conditions and the problems
resulting from use in the deepest waters. The second part will describe the
principle of HSI of the peripheral lines and the R&D works carried out to
qualify it, in the laboratory and in the field. The first results from field
testing are then presented. Finally, the use of the HSI principle is discussed
for emergent drilling-riser technologies, such as slender or dual-gradient
risers.
© 2008. Society of Petroleum Engineers
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History
- Original manuscript received:
20 December 2005
- Meeting paper published:
21 February 2006
- Revised manuscript received:
21 April 2008
- Manuscript approved:
28 April 2008
- Version of record:
10 December 2008
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