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. 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 columns dimension 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 waterplane 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 heave damping.