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with interconnecting truss-like structural members below the water
surface. These structures often comprise of two submerged
horizontal pontoons which provide the main buoyancy for the
platform but act as catamaran hulls when moving location at low
draft. Typically, four to eight vertical surface piercing columns are
connected to these pontoons. The columns themselves may have
cross and horizontal bracing to provide structural strength and
triangulated rigidity for the platform. The deck of the platform is
located at the top of the columns.

Figure 2.3 - IRAN-ALBORZ Semi-submersible, GVA-
4000 Class

Semi units offer a number of benefits, including large
payload capacity, limited sensitivity to water depth, quayside
integration and the ability to relocate after field abandonment. A
typical Semi design has four columns connected at the bottom by
pontoon with a nominally rectangular cross-section. A truss
structure connects the column tops and supports topsides modules
(fig. 2.4). This arrangement provides a high degree of flexibility in
fabrication methodology.
The Semi is designated as a “column stabilized units”
(USCG, ABS, DnV, etc.). The columns are “stability columns”
and primarily provide floatation stability. Important design
variables are column dimensions and spacing, pontoon size and the
ratio of pontoon width to pontoon height, draft of the hull, etc. In
order to satisfy the stability and motion requirements, ranges for
the variables and critical parameters such as GM value, free board
value, heave natural period, etc are set as the constraints. Columns
are sized to provide adequate water plane area to support all

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