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MARINE & OFFSHORE ENGINEERING
Heave and VIM suppressed ‘HVS’ semi-submersible
design for Southeast Asian environments
by Qi Xu and Jim Ermon, Technip
The new design for oil production platforms is said to offer significant benefits.
Glossary of acronyms
CFD - Computational Fluid Dynamics
CG - Centre of Gravity
DTA - Dry Tree Application
DVA - Direct Vertical Access
GM - Distance between CG and metacentre
GoM - Gulf of Mexico
RAO - Response Amplitude Operator
SCR - Steel Catenary Riser
TLP - Tension Leg Platform
TTR - Top Tensioned Riser
VIM - Vortex Induced Motion
BACKGROUND
In offshore oil & gas production, with the trend towards
implementing deeper, larger, and more complex offshore
development projects, and moving into regions requiring more
flexibility, there is a greater need to meet both the technical and
cost challenges associated with this new frontier.
Although, over the last several years, deep-draft semisubmersible
concepts have been developed in the Gulf of
Mexico as wet-tree floating production systems, it is envisioned
that the production semi solution will be used in other parts of
the world, as well (most notably in Southeast Asia), to address
these challenges.
The wet-tree floater market has been growing, largely due to:
• Reservoirs favouring non-directional wells which lead to
subsea production
• Increased water depth (making SCR design easier and TTR
design more difficult)
• Choices of mature floater concepts
Current wet-tree floater concepts include semi-submersibles,
Spars, TLPs and ship-shaped vessels with and without turrets.
Of these choices, the semisubmersible has become a favourite,
primarily because of the key advantage of quayside integration
of the ever increasing size and weight of topsides, a large deck
area, ability to support a large number of risers, and overall costeffectiveness.
However, conventional semi-submersibles continue to face
challenges in supporting SCRs, particularly large ones, and the
associated strength and fatigue issues as a result of relatively
high heave motion. This is particularly true for semis operating in
harsh environments and relatively shallow water. Heave motion
increases the loading on the SCR near the riser touchdown
zone. In an effort to improve the heave motion, the draft of the
semi-submersibles has been increased.
Unfortunately, as a result of increasing the draft, another challenge
has been introduced - high VIM as a result of wind, loop, and tidal
currents. The high VIM of a deep draft semi-submersible is mainly
due to the increased VIM excitation from the longer columns
and reduced damping from the smaller pontoons, compared to
conventional semis. An SCR’s fatigue life at the touchdown point
can be greatly compromised by the high VIM. Other factors also
come into play as a result of VIM. For instance, VIM can make it
difficult to meet mooring fatigue design criteria.
INTRODUCTION
Though VIM has been experienced by shallow draft semisubmersibles
in the presence of strong surface current, it never
became a driving design issue. With increasing draft, however, it
has now become a fatigue design issue for risers and mooring,
with no simple solution.
Technip has long been recognised for its quality and reliable
hull designs, such as the Spar hull. Specifically developed as a
riser-friendly floater, suitable for both dry-tree and wet-tree
applications, the Spar is a proven concept with excellent motion
performance. Earlier Spars were primarily used as dry-tree units,
while the recent few Spars are wet-tree applications.
However, in the coming years, in order to meet evolving technical
challenges of deeper and more complex field developments
and increasing market needs, Technip is expanding its existing
floater product portfolio by introducing to the market a new
semi-submersible design - the heave and VIM suppressed ‘HVS’
semi-submersible.
The HVS semi-submersible was developed at Technip as a wettree
floater which achieves significantly improved heave motion
and VIM performance through hull form optimisation while
maintaining the simplicity of a conventional semi-submersible
design. Future enhancements of this design will also include
direct vertical access and dry tree applications.
This article explores the design of the new HVS semisubmersible,
points out its many features, quantifies its
performance, explains why it responds accordingly, and
highlights its many advantages and benefits.
October 2012 THE SINGAPORE ENGINEER
19