Page 333 - Hydrocarbon Exploration and Production Second Edition

320 Maintenance Engineering Input to the FDP

early wear
failure failure

Failure rate

random
failure

Time
Figure 12.4 The bathtub curve for failure frequency.

Increasingly, maintenance engineers think in terms of the performance and
maintenance of equipment over the whole life of the field. This is often at the
centre of the decision on CAPEX–OPEX trade-offs; for example spending higher
CAPEX on a more reliable piece of equipment in anticipation of less maintenance
costs over the life of the equipment.
Statistical analysis of failures of equipment shows a characteristic trend with time,
often described as the ‘bathtub curve’ (Figure 12.4).
Early failures may occur almost immediately, and the failure rate is determined by
manufacturing faults or poor repairs. Random failures are due to mechanical or human
failure, whilst wear failure occurs mainly due to mechanical faults as the equipment
becomes old. One of the techniques used by maintenance engineers is to record the
mean time to failure (MTF) of equipment items to find out in which period a piece of
equipment is likely to fail. This provides some of the information required to
determine an appropriate maintenance strategy for each equipment item.
Equipment items will be maintained in different ways, depending upon their

criticality which is associated with the consequence of failure
failure mode.

Criticality refers to how important an equipment item is to the process. Consider
the role of the export pump in the situation given in Figure 12.5.
The choice of the size of the export pump will involve both maintenance and
production operations. If a single export pump with a capacity of 12 Mb/d is
selected, then this item becomes critical to the continuous export of oil, though
not to the production of oil, since the storage tank is sufficient to hold 4 days of
production. If continuous export is important, then the pump should be maintained
in a way which gives very high reliability. If, however, two 12 Mb/d were provided
for export as part of the production operations ‘sparing’ philosophy, then the pumps