Page 275 - Hydrocarbon

262 Subsea vs. Platform Trees

not the only issue. Other issues include well interventions, reserves and production
profiles, operating costs, flexibility and decommissioning.
Well interventions might be for value protection (e.g. repairing or preventing
corrosion or scale, maintaining artificial lift systems) or value creation (e.g. shutting
off water or adding artificial lift). If a platform has been properly designed, well
intervention from a fixed platform should be relatively straightforward and cheap.
If these simple well interventions can be done independently of the rig, costs are
reduced further and interruptions to the drilling schedules are avoided. Subsea well
interventions on the other hand are rarely easy or cheap. They require mobilisation
of a rig or a multipurpose service vessel (MSV); neither of which are low cost.
These interventions will also require a significant lead time for equipment and
planning. As a result, there tend to be fewer well interventions on a subsea well than
its equivalent platform well. This has three main consequences
1. It promotes the use of smart wells on subsea wells as we found in Section 10.7.
Smart wells are more expensive.
2. Partly because well interventions are less common subsea, data on the state of
the well or the reservoir can be missing. This can reduce the probability of
success or increase the cost for the intervention. This loop can be self-
reinforcing.
3. Well interventions are a primary means of protecting or increasing reserves and
the production profile. This can lead to a reduction in reserves for a subsea development
compared to its equivalent platform development.
The last two effects are hard to quantify. Some pointers however

Try to estimate the frequency (and cost) of well interventions from analogue
fields.
Use reservoir simulators to estimate the value of techniques like water shut-off.
Reservoir simulators can automate the decision-making process in a plan and
scripts can be used to decide whether these interventions can be done economically.
The effect on production profiles and reserves can therefore be quantified.
Subsea developments do however have a few advantages over platform wells
when it comes to phasing and sequencing of a field development

1. A platform, by virtue of its fixed size and large cost requires a relative low level
of uncertainty in reserves and productivity before the project is sanctioned.
A subsea development is much more flexible; the subsea wells and architecture
are typically phased in over many years, often with multiple drill sites. The cost
of a floating production vessel will typically be lower than its equivalent
platform. As a result, a subsea development can be sanctioned with a relative high
level of reserves uncertainty.
2. The ability to place the seabed locations of subsea wells remote from the
host platform or floating production facility, gives huge flexibility for drilling
and construction activities. For example, multiple drilling rigs can be used,
or drilling on one location can progress whilst the subsea infrastructure
is installed.