As the need for drilling and production platforms in deep waters becomes more and more pressing, several alternative structural concepts are being evaluated for potential application. One concept that has been studied for some time. and has been brought to a high level of development is the Guyed Tower Platform (GTP). This paper discusses some basic variations of the original GTP concept which might enhance its feasibility and range of application as well as its economic advantages over other deep water solutions. In particular,
The spud can, foundation is replaced by a system of piles.
An integrated deck and predrilling template have been included.
Details in the mooring system design have been changed.
The paper contains a description of the main components of the structural system and a discussion of the principal design criteria. The results of some parametric studies and a brief description of fabrication and installation procedures are also included.
Over the past thirty years fixed jacket-type platforms have represented the most common structural solution for drilling and production facilities. As the need to move into deeper waters arose technological advancements, ever-growing expertise. more sophisticated analytical techniques and faster and larger computers pushed the state-of-the-art further wld further. Today the tallest jacket stands in 1050 ft. of water in the Gulf of Mexico. However, there are various indications that with the present technology, water depths beyond 1000 ft. may require an altogether different approach.
One of the main problems to be faced by the designer of deep water platforms is the dynamic interaction between waves and structure. Figure I, taken from Ref. 11, shows a set of design conditions typical of the Gulf of Mexico. The main diagram shows three wave spectra, labeled as operating weather, winter storm and design storm. The curve in the frame describes how the dynamic amplification factor (D.A.F.) varies with the ratio of the structural period to the dominant wave period. It should be noted that the curve shown is valid for systems with one degree of freedom. while an offshore structure is obviously a multi-degree of freedom system. For a qualitative discussion however, the simplification is acceptable.
The shallow water (300 ft.) jacket has a natural period of about 2 sec; this period is much smaller than the peak periods of the various sea states. Accordingly, the ratio of the periods is less than one. and the representative point on the dynamic amplification factor curve is on the left side of the resonance peak. As long as the periods ratio is small enough, the D.A.F. is very close to 1 and the structural response is essentially static.
As the water depth increases, the structural period increases and approaches the spectral peaks while the D.A.F. becomes larger than 1 and moves closer to the resonance peak.
