52 API RECOMMENDED PRACTICE 2T
7.2 System Modeling
7.2.1 TLP response to an extreme environment is complex, with forcing that is generally time-varying and
resulting from a number of sources. Since the total response includes contributions from a number of different
components, the modeling procedures should account for the statistics of joint occurrence of forces, and the
TLP system response to this combined forcing.
7.2.2 There are various ways of calculating both the forces acting on the TLP in an extreme environment,
and the system response to these forces. Both time and frequency domain response models have been
validated against field or model test data. Properly applied, either modeling approach is acceptable for use in
design. However, it is noted that all practical design methodologies include various approximations. None of
these methodologies are exact, and appropriate effort is needed to calculate the response with reasonably
accuracy commensurate with current experience. It is stressed that both validation and proper application are
prerequisites for using any calculation methodology for detailed design checking.
7.2.3 Existing TLP designs have been developed primarily using frequency domain tools enhanced with
statistical extremal distributions developed from physical model tests. Therefore, there is a base of experience
associated with these methods. On the other hand, recent developments in time domain simulation
procedures indicate that these may be capable of predicting the observed nonlinear response characteristics,
including independently predicting the distribution of extreme statistics for the global responses. Time domain
calculations may thereby provide a numerical procedure that can use model tests for confirmation, rather than
using the model test results as a direct part of the calculation.
7.2.4 The design procedures defined in the following sections may be based on either frequency or time
domain methods, or a combination thereof. However, time domain methods may be required for some
detailed response analyses, particularly for transient or highly nonlinear conditions that may be an integral
part of some designs. For instance, the detailed analysis of local tendon behavior, especially near slack
conditions, requires time domain simulations. Other load cases, such as transients during installation and
lock-off, may also require time domain simulations.
7.2.5 The use of model tests for global analysis confirmation is encouraged. Model tests and numerical
analysis are not to replace one another, but rather to complement each other. Model tests provide an
independent check of the system response in waves (also wind and currents, if included), and are generally a
more complete model of the physics than most computational models. Model testing is sensitive to the skill of
the practitioner and, as in the case of numerical analysis, good engineering judgment and practice shall be
applied. See 7.7 for a discussion of model testing practice.
7.3 Static and Mean Response Analysis
7.3.1 Introduction
Static and mean response analysis consists of determining the static equilibrium with no wind, wave, or
current present, and then determining a mean position due to steady environmental loads acting on the
platform. The determination of a mean or equilibrium position is necessary to proceed with a dynamic analysis
in the frequency domain or time domain.
7.3.2 Static Equilibrium in Still Water Condition
7.3.2.1 The determination of the static equilibrium (or “weight balance”) with the “still water” condition is
fundamental to sizing of the TLP and is the starting point for further analysis. A static equilibrium analysis
should be performed for each loading condition to be analyzed.
7.3.2.2 Determination of the static equilibrium should include the following:
a) the total platform weight associated with each loading condition to be analyzed;
b) the total platform displacement (the total platform buoyancy) for each draft to be analyzed;
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