Design Methodology for Offshore Platform Conductors

Abstract

Summary The problem of analyzing and designing that segment of an offshore platform conductor located between the mudline and deck is examined. A new set of efficient and easy-to-use conductor design equations has been developed on the basis of the fundamental principle that internal casing string loads do not produce any buckling tendencies. An example conductor design has been included which illustrates that the method, when compared with the typical conductor design approach, is extremely efficient and cost effective, capable of producing several million dollars of cost savings for a major platform project. platform project. Introduction The offshore structural designer is occasionally confronted with a special problem for which an efficient and easy-to-use design solution is not ready problem for which an efficient and easy-to-use design solution is not ready available. The designer, often working under the pressures of time constraints, is usually hard-pressed to develop the special design solution needed. Consequently, when special design problems do arise, the resulting design solution may be extremely conservative. While such a design is undoubtedly safe from the standpoint of structural collapse, it can be wasteful of resources. Furthermore, the constraints imposed by an extremely conservative design approach may lead to decisions that complicate and hamper offshore drilling and producing operations. In the conductor design problem presented here, very large components of the axial load and bending moment acting on the conductor are produced by the casing and tubing strings located inside. The effect of these internal loads is quite different from that produced by loads resulting from sources external to the member. The familiar American Inst. of Steel Construction (AISC) specifications for steel construction, for example, are frequently used to check the structural adequacy of load-carrying members. Since the AISC interaction equations for strength and stability are intended for stress conditions produced by external loads, direct application of these equations to conductor design produces extremely conservative and unrealistic results, which not only require excessive amounts of steel but also can lead to increases in platform environmental loadings and attendant construction costs. Designers have at times concluded that reasonably sized conductors do not have sufficient strength and stability to allow hanging the casing and production tubing off at the top of the conductor, and that mudline suspension systems are therefore necessary. Use of mudline suspension systems, however, increases the cost of drilling and work over operations and adds to the complexity of a well. This paper develops a methodology for checking the strength and stability of conductors, using the well-known AISC criteria as a basis. The fundamental principle that axial loads and bending moments caused by internal casing strings do not produce any buckling tendencies is used to derive a modified design method that more accurately reflects the actual behavior of a conductor. Designs produced with the modified method should be more realistic and cost effective. An example problem is presented to demonstrate use of the modified design procedure and to estimate potential cost savings. JPT p. 1973