floatation aids may be required due to the
awkward shape of the jacket.
Apart from that, the end of the IC jacket legs
that are to be connected to the topside may
require the addition of Leg Mating Units (LMU) to
absorb the impact of the topside weight for the
safe landing of the topside legs onto their jacket
counterparts.
Fabrication and Lead-out
Construction Constraints
The construction of an MC deck requires additional
steel which is can be avoided with the IC structure.
Although a higher material cost is incurred for the
MC deck, the work of constructing the modules
can be distributed to different yards. This generally
results in quicker construction of the MC topsides.
Another advantage of the MC design is the
improved access to internal areas as a result of
reduced size and a greater work boundary area.
Also, smaller steel sections are required in the
MC module fabrication (i.e. tubular/ extruded vs.
fabricated sections), which then reduces material
lead time.
Against this, there are fewer work faces
available for the IC deck design throughout the
fabrication phase. As a result, the IC deck has to
be fabricated in sequence and tends to be more
labour intensive at one location towards the end
of fabrication and hook-up and commissioning
(HUC). Therefore, fabrication sequence has to be
thoroughly studied to account for any possibility
of late delivery of equipment and materials to
site. Also, heavier cranes are required as heavy
panel liftings can be anticipated for IC decks.
Apart from that, limited yards have the facilities
required to construct offshore mega structures
which can weigh more than 17,500 MT. Hence,
a comprehensive assessment of local yards’
capability such as maximum bulkhead capacity,
draft of wharf area, capacity of cranes and
available manpower is required.
A soil foundation analysis needs to be carried
out to determine the ability of the soil at site to
withstand the high load induced by the heavy a
IC structure and to undertake the improvements
to the soil foundation if required. In some cases,
the bearing stress induced on the soil may be
very high which necessitates the use of pile
foundation. Typically, fabricators prefer the use of
spun piles for the pile foundation due to its high
bending moment capacity. This also accounts for
the large horizontal force induced during skidding
operations and barge entrance during load-out.
The entrance and departure paths of float-over
barge also need to be checked. As the minimal
keel clearance required is one metre, dredging
might be required for some yards.
Load-out Method
For IC decks, skidding is a necessity. Other loadout methods which are available for MC decks
such as lifting and rolling may not be possible
due to the relative heavy nature of the IC deck.
Skid beams are used to transfer the structural
weight of the topside safely to the ground during
lead-out operation. Furthermore, the number of
skid beams required for IC structure should be a
minimum of three units to cater for its excessive
weight.
Also, an IC deck requires an additional LSF to
support its weight during lead-out, transportation
and installation. The weight of a LSF normally
ranges between 15-30% of the topsides weight
is normally disposed of or reformed for future
projects.
Installation
Modular Concept Lifting
A MC installation normally requires the use of a
dumb barge with tug boats to bring the structures
to its offshore site. At site, a Heavy Lift Vessel
(HLV) is required to lift the modules onto the
jacket. Therefore, the weight limit of the modules
will be determined by the barge derrick lift
capacity. The cost of mobilising and demobilising
a HLV can prove to be uneconomical and their
limited availability is also a risk to the project.
MC also requires lifting gear and in some cases,
a spreader frame which increases the structural
steel requirement. The most significant impact
of a MC is the long duration required to complete
an installation notwit