The Effect Of Variations In Design Criteria On Steel Offshore Structures
Authors: McKinney William M. 
Published: December 2007

The effect of design parameters on offshore piled platforms is discussed for structures in piled platforms is discussed for structures in approximately 150 m of water. Variations in the following design criteria were investigated: marine growth, current, wave height, number and size of conductors, loading density and area, drag and inertial coefficients used in Morison's equation to calculate wave forces. Curves are presented that relate changes in the parameters to resultant weights. Two applications of the curves are presented:predicting platform weights for future structures from predicting platform weights for future structures from known weights of existing platforms andanalyzing the effect on calculated weights for a future platform due to changes in design criteria.

Results platform due to changes in design criteria. Results show significant changes of 5 to 15% of platform weights due to relatively small variations in environmental and configuration criteria.

Introduction

Many of the parameters used in defining the design criteria for offshore platforms are variables. They change because the physical conditions and functional requirements vary from site to site or because of insufficient data and/or inadequate technology. In most design operations, guidance in the selection of particular values for the parameters is obtained from certifying agencies, onsite measurements, and operational requirements. These values are not arbitrary, but the result of extensive study and investigation. But since they are not absolute, a range of designs and costs is possible. The object of this study was to determine the effect of variations in some of the more questionable parameters on platform weight as an indication of parameters on platform weight as an indication of platform cost. platform cost. PROCEDURE PROCEDURE A base case or reference structure to use as a datum line was required. To assure meaningful and acceptable results, the following requirements were set up for the reference design. (1) It must be based on a set of real design criteria. (2) The loadings must be severe enough to require design changes. (3) The criteria has to permit maximum latitude in choosing a range for the parameter being studied. (4) The actual design procedure has to have sufficient detail to account for the major portion of the structural weight required.

The design criteria established for a current in-house design study fulfilled these requirements. See Table 1 for the basic drilling and production requirements and Table 2 for the environmental criteria.

Other restraints imposed on the study were:no variations in the foundation water depth, or over-all platform configuration,an elastic design based on static loading,no allowance for dynamic loading and/or fatigue, andone parameter varied at a time.

BASE CASE

The basic space frame model is illustrated in Fig. 1. It is very similar in appearance to other jacket configurations for platforms in approximately the same water depth. Typical elevations of the math model are illustrated in Figs. 2 and 3.

The environmental load due to waves and current resulted in a total horizontal shear of 7.56 × 10(7) newtons and a total overturning moment about the base of 9.54 × 10(6) newton-meters. The member sizes in the jacket framing ranged from 508 mm in diameter for some of the horizontal bracing near the water line to 2743 mm in diameter for the corner legs at the mud line. Corrosion of all members in the splash zone was allowed for by reducing the thickness and radius of such members by 12 mm for the jacket analysis.

The optimized design for the base case resulted in the following weights excluding the deck: 12,565 tons of structural steel and 1,135 tons of nonstructural steel. The total tower weight was 13,700 tons. A further breakdown of weight is given in Table 3.

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