ENGINEERING, CONSTRUCTION, & INSTALLATION
DeepStar study assesses
cost-effective dry tree solutions
Research examines applications for deepwater marginal felds
The development of deepwater felds (< 5,000 ft) in the Gulf of Mexico (GoM) with dry tree (DT) and direct vertical access (DVA) operations have been mostly performed using TLPs and
spars. Ultra-deepwater felds (> 5,000 ft)
have been developed with spars.
Due to a signifcant increase in tendon
cost with water depth, the use of TLP design, with advantages from low tensioner
stroke and quay-side integration of topsides,
has remained in the 3,000 ft to 5,200 ft range.
The spar design requires long stroke RAM
style tensioners and offshore integration of
topsides by heavy-lift vessel (HLV). The current qualifed limit of top tension riser (TTR)
tensioner stroke is 28 ft (design level) for
the spar design, and it requires larger hull
and mooring sizes to keep the riser tensioner stroke within this limit.
The alternative semisub shaped hull has
been used as a production platform in the
GoM with tieback of subsea wells at distance. These wells are drilled by a mobile
offshore drilling unit (MODU), and tied to
the semisub by fexible risers in lesser water
depths and by steel catenary risers (SCRs)
or steel lazy wave risers (SLWRs) in deeper
water depths. One large semisub, the
Thunder Horse platform, also includes a drilling
The feasibility of platforms in GoM has
been further impacted by signifcant increase in the metocean criteria after Katrina
and other hurricanes. The platforms designed to post-2005 (post-Katrina) specifcations would require higher deck elevations,
greater topsides payload, longer tensioner
stroke, more robust station-keeping systems, and larger hull displacements.
The new metocean criteria also led to the
division of GoM into three zones (central,
transition, western) with varying metocean
design data. The new maximum wave height
is about 30% higher for the Central GoM and
13% higher for the Western GoM than the
pre-2005 criteria. The impact on designs and
estimates of TLP, spar, and semisub from
new criteria varies. The impact on semisub
designs is generally less than on the TLP
and spar designs.
The DT or DVA capability is sought to
reduce the associated opex when a MODU
is mobilized to perform operations such as
ESP replacement. The DT capability is obtained with the drilling rig supported on the
foating production platform. Alternatively,
pre-drilling is performed from a MODU and
the foating production platform supports
a workover rig in the case of wells below
the platform. In the case of subsea wells
at distance from the platform, a MODU is
required for workover operations. These
approaches have provided economic solutions for the development of large felds. In
the case of deepwater marginal felds at remote locations with low feld life and lower
number of production risers, it is desirable
that drilling, production, and export functions are undertaken from a single foating
platform. Thus lower cost hull and mooring
system designs are required for economic
viability, which could be obtained by use of
semisub hull designs with polyester mooring system. In general, the semisub hull-shaped designs require long stroke RAM
tensioners as with a spar platform.
A large number of semisub or hybrid
shape hull designs with dry tree capability
have recently been developed by various
concept owners. These have been evaluated
for specifc feld applications with unique
requirements and drivers, or are in various technology development stages. Some
of these have been evaluated as dry tree
solutions for development of large felds
with 12 production risers and a large drilling rig for deeper reservoirs. For two novel
hull designs, efforts have been undertaken
in RPSEA-funded projects to increase their
technology readiness levels (TRL).
The feasibility of several of these novel solutions to provide an economic solution for
development of deepwater marginal felds
is unknown. Their technical performance in
multiple regions and water depths, scalability,
constructability, and associated economic factors are important to determine their competitive advantage against proven TLP and spar
designs. The evaluation of new designs as enabling solutions, beyond the feasibility of TLP
and spar, is required for a combination of water depths, regional parameters, and payloads.
Thus DeepStar funded this project to evaluate a large number of novel concepts for a
common design basis, focusing on identifcation and characterization of key technical performance parameters, constructability, and
economic factors. The economic factors are
most important for these designs to be considered attractive for development of marginal
felds with short feld life and fewer wells.
The goal here is to present the alternative development themes for obtaining dry
tree and direct vertical access capability, key
design basis and design constraints, alternative hull and riser designs considered in various groups for comparative assessment, and
important variations among design groups.
The fve development themes evaluated
with DT or DVA capability are illustrated in
on page 73.
Theme C0 with benchmark hull.
Conventional development using 4-column TLP or
truss spar with dry trees and TTRs with tensioners for drilling and production and SCRs
for water injection (WI) and export. TLP requires hydro-pneumatic tensioners (pull or
push type) with low stroke while truss spar
requires RAM tensioners with long stroke.
DVA to wells is obtained through TTRs.
Conventional TLP hull with 4-column design
and 2 level deck enables quay-side integra-
tion and pre-commissioning of topsides,
whereas truss spar design with slender hull
and 3 level deck requires longer duration
offshore integration and commissioning of
topsides by use of HLV and other vessels.
Theme C1 with novel hull. This development
theme considers a novel semisub shaped hull
Granherne, a KBR Company