Page 103 - Handbook Of Multiphase Flow Assurance

Frequency of blockage remediations 99

Days for subsea remediation
160
Hydrate remediation
140
Other flowline
remediation
120
Linear (hydrate
remediation)
Time (days) 80 Linear (other flowline
100
remediation)
60

40
20

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10,000
Water depth (ft)
FIG. 5.2 Days for remediation of subsea flow assurance blockages.

this duration showed an increasing trend with water depth. Remediation project cost may
include, besides renting specialized remediation equipment, hiring a support vessel to de-
liver the equipment to the flowline which may range from a service boat at approx. 50–100
thousand of US dollars per day to a workover rig or a drilling rig which may cost 300–600
thousand USD per day. Additional cost for staff, engineering support, chemicals and mobili-
zation/demobilization should also be considered when a flow assurance strategy is selected.
Typical cost limit for a subsea remediation project has been around 20 million USD, but valu-
able high productivity assets may increase this limit. Costs are approximate and shown in
2018 US dollars. While costs tended historically to approximately double every 20 years due
to inflation, future costs in USD may vary.

Hydrate versus other flowline remediation time
With time the flowline remediation methods become more effective and take less time.
However, the time for hydrate remediation subsea keeps increasing from year to year as
shown in Fig. 5.3.
The actual frequency of blockages occurrence is an important input into risk models.
Recent changes in the price of the produced hydrocarbon commodity made operators op-
timize cost of new projects to ensure these are still profitable. One of such optimizations is
risk-based flow assurance, discussed further, when the production system is estimated to get
plugged once per a time period. A common flow assurance strategy is to build a dual multi-
phase pipeline tieback to a new field which can be scraped, and live produced fluids can be
displaced with stock tank crude to mitigate most flow assurance risks.
An example of a risk-based flow assurance strategy approach is to build a single multi-
phase flowline subsea tieback to a new field if the probable cost savings of not installing the
second line outweigh, by a pre-set margin, the probable operating cost losses from the unmit-
igated flow assurance risks.
It would be useful for such risk-based approach to know the relative frequency for
remediation of certain types of flow assurance blockages. A summary Table 5.2 for hydrate