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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 6.10 - 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. 6.11). 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 anticipated loading conditions, spaced to support topsides modules, and tuned

for a natural period of at least 20 seconds. These columns are supported by two

parallel pontoons or a ring pontoon. Pontoons are sized to provide adequate

buoyancy to support all weights and vertical loads, and proportioned to maximize

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