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Oilfield Technology
August
2015
loss of riser margin and subsequent blowout
resulting from failure of a single cased riser
system. However, there are no recorded
events withwhich to draw lessons or
frequency of occurrence.
The following sections provide guidance
on determining appropriate risks and the
associated failure probabilities relevant to
dry tree top tensioned riser systems. The
identified risks can be used to develop Riskex
associatedwith both single and dual cased
systems, and provide a basis for decision‑making
in regards themost appropriate riser system.
Methodology
To determine the risk cost associatedwith a
TTR system, two primary pieces of knowledge are
required:
Ì
Ì
A list of all risks relevant to each riser
system.
Ì
Ì
The probability and consequence
associated with each risk.
Tomake a true comparison of risk costs, each
failuremodemust be evaluated based on its
individual probability of occurrence and potential
consequences. For instance, many operators elect
not to sidetrack drill through the production risers
in later life, and hence, there is reduced likelihood
of failure fromwear. Therefore, an operator’s
selected approachmust be taken into account to
correctly represent potential risks.
Identifyingriskyevents
The number of barriers to the reservoir varies
through the course of drilling and completion
activities frommultiple barriers (‘isolated’), to
two (‘exposed’), to only the hydrostatic head of
fluid contained in the riser (‘live’). Examples of
the number of barriers for various operations
are shown in Figures 1 and 2. When awell is live,
the integrity of the riser andwellbore casings
provide the only barrier to loss of well control
by supporting the vertical column of mud. This
overpressure created by themud is known as ‘riser
margin,’ and defined as the differential between
the reservoir pressure and themud hydrostatic
head. In a single cased riser, failure near the base
may lead to an uncontrolled flow (blowout) as the
hydrostatic pressure at the riser reduces to that of
the seawater, thus reducing the riser margin. This
scenario is the driving factor behind the choice
between single and dual cased risers. Dual cased
risers provide a redundant barrier against the loss
of riser margin, unlike a single cased riser system.
With thewell stages and applicable barriers
outlined, risk events can be identified for each
operation. A typical deepwater well may take
60 to 120 days to complete, duringwhich the
drilling the riser is subject tomultiple failuremodes
including overstress, fatigue, wear, corrosion,
operational error and combinations thereof. The
failures can occur in different locations as shown
Figure 1.
Dual casing riser: barriers and failures during operations.
Figure 2.
Single casing riser: barriers and failures during operations.
Table 1. Individual event probabilities
4,5
Well condition
(live, exposed,
isolated)
Riser casing
failure above
TBC
Well exposed
mechanical
barrier falls
Well bridges
<1 day
Well bridges
>1 day,
<5 days
Well bridges
on >5 days
<30 days
Live
0.1
Yes
0.004 Yes
0.00303 Yes
0.36 Yes
0.8
Yes
0.98
Exposed 0.5
No
0.996 No
0.99697 N0
0.64 No
0.2
No
0.02
Isolated 0.4
