Use of Float Modules to Supplement Mechanical Tensioning of Marine Risers

ABSTRACT USE OF FLOAT MODULES TO SUPPLEMENT MECHANICAL TENSIONING OF MARINE RISERS Tensioning systems for marine risers used for floating drilling units have passed through several phases of development as the water depths of interest to the industry have steadily increased. The first tensioning systems made use of large counter weights linked to the riser by a system of wire lines and sheaves. These have largely been replaced by hydro-pneumatic rams which have the advantages of smaller physical size, greater flexibility, increased capacity, and better response to vessel heave. More recently, syntactic foam floats attached to the riser have been used to provide lift and thus reduce tension requirements in water depths up to 1500 feet. This paper shows the effects of floats in reducing tension requirements for various combinations of environmental and operating conditions as determined by a mathematical model of a riser system. Factors involved in designing riser systems with optimum combinations of tension and flotation are explained. Actual behavior of a riser with attached floats designed on the basis of this mathematical model for deep water drilling operations in the Santa Barbara Channel is discussed. INTRODUCTION In late 1968 Exxon Co., U.S.A. equipped two rigs for drilling in the deep waters of the Santa Barbara Channel, California. The rigs were equipped with 16" OD marine riser systems and with pneumatic riser tensioning systems having 360 kip nominal tensioning 1 capacity. At that time, Harris and Lifrey concluded that these riser and tensioner systems would allow operation in 1300 ft of water with 12 1b/ga1 mud in the riser and in 1500 ft of water with 10 1b/ga1 mud in the riser. It was also concluded that extension of system capability would be possible on the basis that (1) experience might prove design parameters to be unduly conservative and (2) that the range of system adequacy would be improved through the development of new technology. ESTABLISHMENT OF OPERATING CONDITIONS, TENSIONING CRITERIA, AND TENSlONER PERFORMANCE With further experience in deep water drilling in the northwestern portion of the Santa Barbara Channel, it became apparent that the maximum wave height likely to be experienced in operations would probably not exceed 20 ft and that drilling vessel offsets would not exceed 3% of water depth during most of the time when work through the riser was necessary. Repeated current measurements encountered in this area could be idealized by a "standard" current pattern of .65 kt for the upper 25% of water depth and 0.2 kt for the lower 75% of water depth. It was also determined that mud weights in excess of 13 Ib/gal were not likely to be required. Meanwhile, thorough consideration of all factors involved in riser operation while well work was being done through the riser resulted in the following tensioning criteria.