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SemiTEC™
Semisubmersible (Semi) Technology
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.
Typical Semi RAO’s
 The FloaTEC, LLC DeepDraft™ Semi
FloaTEC, LLC's new DeepDraft™ Semi has
been designed with particular
emphasis on reducing damage in SCRs. Compared to a typical
shallower
draft Semi, the heave response in environments with wave
periods in the
region of 8 seconds or lower is significantly reduced because
of the
deeper draft. The low heave motion reduces the fatigue damage
in the SCR
by reducing the stresses at the riser hang-off connection
and at the
touch down point on the sea bed. In severe environments,
both heave and
angular (pitch and roll) motion of DeepDraft(tm) Semi are
significantly lower thus reducing stresses in the SCR.
DeepDraft™ Semi Model
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