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• Consider reducing the trip setpoint
• Relax allowed pressure rise (e.g. design pressure + 15%)
• Optimise process modelling of pressure rise (e.g. including hold-up time of full
inventory of piping instead of just vessel)
• Model shutdown valve movement (when the SIF operates, as the valve closes, it will
start to restrict the flow)
3.3
Redesign the process to make the hazard development slower
There are numerous possible ways in which the speed of hazard development could be
slowed. This could include installing a “surge dampening vessel” or in the case of the riser
de-packing hazard, this could be a speed limitation on the choke valve (the too-quick
opening of which was the initiating event). This could be the gearing on the handle for a
manual valve, or the speed of operation possible for an actuated choke. The maximum
opening speed needs to be reduced to such a level that the calculated Process Safety Time
can be achieved by the reactive SIF designed.
3.4
Redesign the process to eliminate the hazard
A process re-design could be to implement an inherently safe design by removing the HP/LP
interface by fully rating downstream equipment. Less expensive options could include the
provision of additional Independent Protection Layers which are able to reduce risk down to
the tolerable target level. These could include:
• Fit a restriction orifice to limit flow to within relief system capacity (and thus take
credit for the relief system as an IPL)
• Fit check valves to prevent backflow (e.g. 2 dissimilar check valves)
• Change valve mechanical locking philosophy to prevent hazard
3.5
Prevent the hazard (reduce the risk) by mechanical locking of valves
If the operation of a manual on/off valve is the initiating event, prevention may only be
possible via a mechanical means such as locking of valves in open/closed positions or key
interlocking solutions. An inhibit SIF cannot be designed to prevent the operation of a
manually operated valve.
In other cases, even when the Initiating Event is a remote operation through the control
system, and therefore an Inhibit SIF is under consideration, a change to the locking
procedures for manual valves could prevent the hazard. This solution was applied to one of
the Cargo Pump scenarios (see 1.3.2), via the lock open of cross-over valves on the suction
side to remove the overpressure scenario.
HAZOPs may consider the mal-operation of locked valves as not being credible. Even
IEC61511 refers to only considering hazards “under all reasonably foreseeable
circumstances…and reasonably foreseeable misuse”. But no protection layer is either 100%
reliable or completely resistant to abuse; a LO/LC valve is not a fool-proof solution. CCPS
[7] and other sources suggest finite values for risk reduction possible from LO/LC valves plus
procedure. Key interlocking systems can are often seen as having a higher reliability.
Applying a mechanical solution to eliminate the need for an Instrumented Function takes us