PLANNING, DESIGNING, AND CONSTRUCTING TENSION LEG PLATFORMS 19
Different types of risers between the platform and seafloor may be utilized, including integral and non-integral
risers, and risers integral to the tendons. Drilling blowout preventers (BOPs) and well completion systems
may be located either at the platform deck level or subsea. Anticipated workover frequency and wellhead
maintenance will influence the decision as to surface or subsea completions. Anticipated changes in future
operation (e.g. gas lift or water injection) might require the need for flexibility within components selected.
Well component design and selection should be primarily based on reliability and safety of the system. Field
proven technology and equipment should be used where possible. Design reliability should include
redundancy, backup procedures, and fail-safe designs whenever practical. Component and well system
reliability studies could be useful in determining the consequences of failure, and identifying those
components needing a higher degree of reliability. Identification of those components that cannot be retrieved
to the surface, the consequences of such components being damaged, and how to mitigate the
consequences should be considered. In all cases, consideration should be given to an acceptable means of
stopping the well flow near the seafloor in the event of an accident.
4.7.5 Facilities
The planning and selection of facilities involve many problems that are unique to compliant structures. The
selection and design of the facilities should consider the platform motions. Facilities will have interfaces
between individual systems and the overall structure, including dynamic load input from drilling rigs, sharing of
utilities between drilling/production systems and hull systems, and escape means for various damage states.
Such loads and interfaces should be identified and considered.
TLP facilities design should recognize the highly interactive nature of the design process, and the importance
of proper coordination and integration of drilling rig, production, hull systems, and structural needs. Specific
definition of all facilities criteria and requirements early in the design process should prevent changes in the
platform resulting from changes in facilities. There should be close coordination between the facilities and
structural designers throughout the design project to ensure that routine interactions, changes, and interfaces
are properly addressed.
Facilities and drilling layouts should be considered in the initial stages of design when the development of the
overall configuration is being made. Layouts should initially be guided by the overall function of the platform
and should include the influences of well location(s), production systems needs, accommodation
requirements, and area classification considerations. Facilities construction, whether fully integrated, semi-
integrated, or modular, will affect the layout and weight as well. Damage control, personnel safety and
evacuation, and spillage/containment requirements also influence the facilities layout. It may be beneficial to
examine a variety of facilities layouts.
Weight, center of gravity (CG), and space requirements should be managed to develop a facility efficient in
cost and operation. Weight management is the key to controlling parameters that affect the stability and
global performance of a floating structure.
The design process should consider the use of “growth allowances” in the form of weight and space factors,
which can help in two respects. First, platform facilities have a tendency to grow during the design process
with potentially detrimental implications. Thus, realistic allowances for weight and space growth during the
design process should help to prevent major design recycling at late stages. Second, experience has shown
that the originally intended operational parameters for offshore facilities frequently are no longer adequate
once the facility has been in operation for several years. Accordingly, it is appropriate to utilize space and
weight growth allowances as a means of allowing flexibility in future operations. Operational growth scenarios
should also include examination of the weight or space flexibility that may be gained by the removal of certain
facilities at later stages in the operation.
Benefits may result from keeping the design growth and operational growth allowances separate during
design. Operational growth allowances can easily be preempted by unexpected design problems, but the
implications to future facility operation should be considered.
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