Risk-Based Inspection Technique - Offshore Structures: Design, Construction and Maintenance - page 631

In some instances, structural members in the jacket may be removed where it
can be shown that the removal results in an increase in the overall system reliability.
Examples of structural members that can be removed include launch truss members
and redundant structural members. In an older platform fleet, change of the mode of
operation with time usually has resulted in redundant risers, and their removal can
reduce load significantly. However, removal of these risers requires performing a
comprehensive study, because removing risers is not easy and will be expensive,
although it can be the best solution to maintain the structural integrity.
Another solution is to decrease the hydrodynamic load by using marine
growth removal, as described in Chapter 7 .
In some cases, painting the surface of legs and bracing with a coating
prevents marine growth from accumulating.
Other actions include marine growth removal on a regular basis by means of
diver-held water-jetting equipment. A new trend is to put a piece of plastic
around the steel member so that the plastic rotates with wave movement and
provides a clean surface all the time. Inspections of actual growth levels in com-
parison to design values may sometimes show design values to be incorrect,
sometimes positively and sometimes negatively. Marine growth control also
has the added benefit of virtual mass reduction on platforms subjected to
dynamic excitation by waves or earthquakes.

Raising the Deck
If, after comparing the platform

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s elevation with the recent metocean data it is
found that the wave crest is expected to hit the deck, raising the deck out of the
wave crest will significantly reduce the structure

s global loading. In raising the
deck, the effects of increased unbraced deck leg lengths must be evaluated. Due
to the high cost and operational impact of raising the deck, cost-benefit analysis
should be done on a case-by-case basis.
An alternative to raising the deck is to remove or to relocate equipment and
nonessential structures from the lower deck elevations; this results in lower
hydrodynamic forces and will reduce equipment damage from direct wave loads.
The other suitable solution is to install deck grating, instead of plating; this
can be beneficial in reducing vertical loads on the underside of the deck by
allowing encroaching water and trapped air to dissipate more easily.
In some locations, field subsidence has caused a general settling of the sea
floor. Mitigation alternatives for this case often rely on reservoir pressure tech-
niques, such as water or gas injection. This approach does not recover lost
height, but it can be used to slow future subsidence.

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Strengthening
There are many strengthening and repair techniques available, as is illustrated in
Chapter 7 . Platform assessment will determine whether platform strengthening
or repair is required to meet the assessment acceptance criteria. If strengthening
and repair are to be considered, the assessment model should be used to develop