Assessment of Existing Structures and Repairs - Offshore Structures: Design, Construction and Maintenance - page 523

7.11 SCOUR PROBLEM
There are several methods for scour prevention and repair. Angus and Moore
(1982) detail the methods used in the southern North Sea. Gravel grout bags
and sand bags have been used effectively to fill in the scoured area and raise
the seabed profile up to the original level. Car tires have also been used success-
fully. They are tied together to suit the geometry of the local scour holes, are
dropped to the seabed and then are moved into position by the divers. Plastic
seaweed is another useful method. The seaweed reduces the water velocity
and thus mediates deposition of sediments. The seaweed system usually com-
prises continuous lines of overlapping buoyant polypropylene fronds that,
when activated, create a viscous drag barrier that significantly reduces current
velocity around the piles. The frond lines are secured to a polyester webbing
mesh base that is itself secured to the seabed by anchors pre-attached to the
mesh base by polyester webbing lines.
The action of reducing current velocity immediately prevents seabed sedi-
ment in the immediate area of the fronds from being transported (i.e., scoured
out) and causes sediment transported across the fronded area to fall into, and
collect within, the fronds.
At the design stage, jacket structures can be made less sensitive to scour by
providing stronger piles and jacket legs. According to API, this is done by con-
sidering scour as occurring by about 1.5 times the pile diameter (and this value
can be 2 times the pile diameter in special situations, depending on the soil type
and previous experience).
7.12 OFFSHORE PLATFORM REPAIR
7.12.1 Deck Repair
The topsides incorporate robustness through consideration of the effects of all
hazards and their probabilities of occurrence, to ensure that consequent damage
is not disproportionate to the cause. Damage from an event with a reasonable
likelihood of occurrence should not lead to complete loss of integrity of the
structure. The structural integrity in the damaged state should be sufficient to
allow a process system closedown and/or a safe evacuation. Framing patterns
that provide load paths are preferred.
The maintenance plan should include the risk-based inspection technique.
The first step of the inspection is visual inspection, which should be performed
periodically to monitor for degradation of the structure. Figures 7.29 and 7.30
present examples of what can be found during inspection (such as corrosion of
the main steel for the stairs and the main supports on the helideck truss support).
Repair of the deck structure is usually easy to perform: the stair can be replaced
by a new one and the helideck can also usually be replaced by a new one and
then repaired and transferred to another platform if there is a big fleet. If there is
no fleet, the helideck will be repaired.