Minimum Structures: New Recommended Practices In The 20th Edition Of API RP2A
Authors: Isenhower W.M. 
Published: October 2008

The use of minimum structures, especially in the Guy of Mexico, has greatly increased over the past jive years. As the economic pressures on the oil and gas industry have increased, the incentive for more cost effective means of producing has resulted in various minimum structure concepts. API RP 2A, Recommended Practice for Planning, Designing and Constructing Fixed Offshore Platforms (RP2A) was Developed over the last twenty plus years primarily for so-called conventional structures such as the four or eight piled, template type platforms. Almost all of the currently used minimum structure concepts have attributes or characteristics not shared by the more conventional structures and thus are not adequately addressed by the current version of RPM therefore, the API Subcommittee for Fixed Systems charged a Work Group to formulate proposed additions and Revisions for the Subcommittee's consideration. The Work Group's proposals were subsequently approved by the API organization and will be included in the 20th Edition of RP2A as Section Sixteen. This paper describes the more significant portions of Section Sixteen and its associated Commentary some of the thoughts and reasoning of the Work Group, and assumes that the reader is somewhat familiar with the 20th Edition.

GENERAL DESCRIPTION

One of the more difficult tasks for the work group was the attempt to define "... what is a minimum structure?". The approach chosen was to describe physical attributes that are characteristic of minimum structures. This approach is reflected in RP2A Section 1.6.4., Many minimum structures are marketed under various trade names; many may be more descriptively defined as:Braced caissons, utilizingtubulars and pin connected skirt pilingguyline systemsTripodstemplate or modified template typeutilizing two legs, clamped or otherwise attached towel conductor thus forming tripodSkirt pile supported well conductors, utilizingthreaded connectionsclamped connectionsother non-welded connections

Section 16 attempts to directly address all of the above except for caissons braced by a guyline system. The Work Group was of the opinion that there was not enough usage of this configuration to warrant it's inclusion type under the scope of Section 16. Of course the original minimum structure, the free standing tubular caisson, is included.

In general, the Work Group concluded its task with the thought that Section 16 will need to be broadened, perhaps to include guylined systems, and otherwise refined in later years, much as the rest of API RP2A has evolved.

ISSUES SPECIFIC TO MINIMUM STRUCTURES

The Work Group decided that the current RP2A adequately addresses most issues and only needed the addition of recommendations, emphasis, or guidance specific to minimum structures. The issues of concern included:

Primary Issues

Minimum structures are generally more flexible than conventional structures. Therefore, dynamic effects could be of significance even in shallow water depths. Minimum structures are, in almost all configurations, less redundant. Sensitivity to design, fabrication and/or installation deviations is thereby increased.