August 2015
Oilfield Technology
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by red dots in Figures 1 and 2. Furthermore, the failure can occur at different
timeswhen thewell is exposed, isolated or live.
For example, one can consider the failure of a single cased riser due
to fatigue. The example well has a reservoir depth of 10 000 ft and bore
pressure of 6000 psi in 5000 ftwater depth, giving a total depth of 15 000 ft.
With 12 ppgmud in the well, there is a 210 psi margin to the reservoir
pressure tomaintainwell integrity. Based on the assumptions provided,
well integrity would bemaintained for fluid loss down to approximately
1000 ft belowmean still water level (MSL), or the upper 20%of the riser.
For loss of well control, the fatigue failure would need to occur in the lower
portion of the riser when the well is live. However, in later life, reservoir
pressuresmay have declined such that the riser marginmay not be critical.
This example demonstrates that each scenario should be considered
fully and independently to develop appropriate failure probability and
consequence costs to give an overall risk cost.
Each failure scenario can be developed for both
dual and single cased systems and describe the
following:
Ì
Ì
Failed component.
Ì
Ì
Cause of failure resulting in full break or
small leak.
Ì
Ì
Consequence of failure in terms of time
shut in, barrels leaked, emergency
response and other mitigating factors.
Ì
Ì
Assumptions such as well pressure and
riser configuration. These assumptions will
provide justification so that a reviewing
engineer could plausibly re‑create the
scenario.
Ì
Ì
Recommendations of actions that would
prevent or better control the defined event.
Avoidingbias
For each of the failure events, a qualitative risk
assessment is initially conducted to evaluate
the probability and consequence costs. For low
Table 2. Total event outcome probability
Event
Frequency
Normal operation 0.988
Ignited
Non‑ignited
Delay due to
event/failure
0.002
20%
80%
Well flows for
<1 day
1.462E‑04
2.924E‑05
1.169E‑04
Well flows for >1
day <5 days
2.079E‑05
4.158E‑05
1.663E‑04
Well flows for >5
days <30 days
5.094E‑05
1.019E‑05
4.075E‑05
Well flows for
>30 days
1.040E‑06
2.079E‑07
8.316E‑07
Table 3. Example of a Riskex calculation
Event
Single casing riser
Dual casing riser
Likely Cons.
Riskex
Likely Cons.
Riskex
Accelerated fatigue leading to
outer casing failure
1.0E‑3 US$15
million US$15 000
1.0E‑3 US$5
million
US$5000
Slow leak failure at riser base leading to leak of mud/annulus contents to
environment. For single casing riser, the mud is able to be replaced faster
than leaked. Consequence cost driven by downtime for inspection/repair and
environmental impact.
Impact from critical lift dropped
object
1.0E‑2 US$3
million US$30 000
1.0E‑2 US$3
million
US$30 000
Impact at top of riser or ‘glancing blow,’ therefore no benefit from second riser
casing. Consequence cost driven by downtime for inspection/repair.
Lower riser failure leading to
ignited surface blowout
5.0E‑4 US$4.5
billion US$2 250 000 5.0E‑5 US$4.5
billion
US$ 225 000
Consequence of a blowout similar in both cases but order of magnitude
reduction in likelihood due to second casing.
Note: The numbers provided here are for demonstration purposes only and not to be used as a reference in future
assessments.
Figure 3.
Event tree for determiningprobability.
