Postma_E&P_quark6_Outsourcing_book_temp.qxd 12/11/2009 13:17 Page 77
Marine Operations During Transport and Discharge of SPAR Hulls
SPAR Free-floating Condition After Discharge to keep the barge deck out of the water, the HTV needs to be trimmed
Truss SPARs tend to have a free-floating draft that exceeds the to pass the critical phase.
allowable level of water above the deck of the submerged ship.
Discharge must be in the longitudinal direction because of cradle The optimal stability aid is tailor-made for each SPAR design and
height. The SPAR’s hull does not contribute much to the waterplane requires detailed investigation of the subsequent solutions or
area and therefore initial stability in the first period after the HTV’s main
deck is submerged.
Deck loads induced by
Restrictions of the Vessel Ballast System
From classification and layout of the ballast systems onboard, every
hogging and sagging conditions
HTV has restrictions on allowable trim. Typically the HTV can reach
or loads on the heave plate
some 6–8m trim aft. Furthermore, due to the free surface effect of
slack water ballast tanks, the stability of the vessel is reduced. These
supports resulting from torsion
factors will affect the design of the stability device or require an
in stern quartering seas must
adjustment of the HTV ballast system.
be assessed.
Vessel Stability During the Submerging Operation
For a large SPAR, calculations show that discharge without any
additional stability aid results in insufficient stability during at least part combination of solutions, which include stability element(s) on the
of the operation, and additional stability devices are required. SPAR, adjustment of the HTV ballast system and stability element(s)
A commonly used stability aid is a buoyancy barge with casings on the HTV. n
supporting the soft tank (see Figure 5).
Written permission has been granted by OTC to re-publish this
During the critical phase of the discharge operation, i.e. the phase in abridged and edited version of the original OTC Paper-20232-MS
which the SPAR and vessel bow have not generated sufficient written by Postma RL in 2009, titled ‘SS: Spar Technology – Marine
buoyancy and water plane area for vessel stability, the deck of the operations during transport and discharge of SPAR hulls’, presented at
barge stays at a fixed position above the waterline such that maximum the Offshore Technology Conference, Houston, Texas, 4–7 May. DOI:
waterline area is created and no barge ballasting is required. In order 10.4043/20232-MS.
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