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29．Design for 1MW OTEC riser with HDPE pipe
DongHo Jung, YoungJu Kwon, HyeonJu Kim, BoWoo Nam, ○SaYoung Hong(Korea Research Institute of Ship and Ocean Engineering (KRISO), Daejeon, Korea)
is 20 years and the design requirements for thestructure are based on the return period of 100years sea environmental conditions.

The KRISO (Korea Research Institute of Shipand Ocean Engineering) has a plan to develop thefloating 1MW OTEC system at equator sea near
Kiribati in South Pacific to supply the electricity
to Kiribati by submarine cable. The size,configuration and construction method of an
OTEC platform will depend on many factors:
power output, site, type of installation (i.e. shorebased, bottom-mounted, floating but moored orcruising), method adopted for minimizing
interaction between platform motions associatedwith sea conditions and riser motions, produced
electricity transportation, and optimal systemcharacteristics that will maximize efficiency andminimize cost.

The selected HDPE pipe with lower density andweak strength is reinforced with an additional
lumped weight attached to the end of a riser
and connected to a floating platform with four
wire ropes along the riser. This additional
weight at the bottom end of a riser is tominimize the motion of the riser.
The two design requirements for riser system isdefined: the first is the bottom end of riser
should not be lifted over 10 % of water-depth
at operating condition in order to take the deep
seawater continuously. The second is the riser
should be safe in the harsh ocean
environmental condition. The results ofdynamic riser stress analysis are performed.
The maximum stress along the riser
corresponds to 9.45 MPa near sea surface dueto the extreme wave and floating structure
motion. The bending stress near top of the riser
dominates, but the axial stress near bottom of
the riser does. The effect of the bending
moment at the riser rapidly diminishes with thewater depth by the structural andhydrodynamic damping. Safety ratio of themaximum stress on the riser is 2.75 thatsatisfies the allowable safety factor. It is
verified that the designed riser is sufficiently
safe even for the harsh ocean environmentalcondition.

The OTEC system designed by KRISO consistsof the offshore floating structure and the plant
producing electricity of 1MW from seawatertemperature difference between surface and deep
seawaters. The new concept for the OTEC riser is
proposed in that a long and large diameter HDPE
pipe in 0.95 density is reinforced with wire ropes
along the riser and a lumped weight attached at
the riser bottom. The main components of 1MW
OTEC structure are shown as following; Floating
body, Riser : Long HDPE(High Density PolyEthylene) pipe reinforced with wire ropes andlumped weight, Mooring lines : Semi-tautmooring system in 4 points spread type
The platform specifications, materials, mooring
system, internal equipment, etc. have been
designed to be satisfied with the adequate
safety requirements of the sea conditions. Theriser pipe standing vertically hung on theplatform intakes deep sea water from the depthof 1100m. The material is HDPE (high densitypolyethylene) in order to have a strong
advantage for insulation and against oxidationand corrosion. The service life of the platform[ACKNOWLEDGEMENTS] This work was
financially supported by the R&D project of the"Development of Energy utilization technology with
Deep Ocean Water"(41)