Use of Float Modules To Supplement Mechanical Tensioning of Marine Risers
Authors: N-Dietz Daniel 
Published: December 2007

Introduction
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 counterweights linked to the riser by a system of wire lines and sheaves. These have largely been replaced by hydropneumatic rams that have the advantages of smaller physical size, greater flexibility, increased capacity, and better response to vessel heave.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 provide lift and thus reduce tension requirements in water depths up to 1,500 ft.

Float Modules
In the Santa Barbara Channel, different types of float modules were used in two different riser systems. The first system was used with a barge-form vessel and employed 2-in.-diameter hollow plastic spheres embedded in syntactic foam to make a composite material with a density of approximately 24 lb/cu ft. This was cast into 30-in. OD x 16 1/2-in. ID float modules that are 18 ft long and approximately semicircular in cross-section. Thus, four float modules cover 36 ft of a 40 ft-long, 16-in. OD riser joint, leaving room for handling the joint during makeup. The float modules are provided with an outer fiber glass sheath and end plates. Each module is attached to the riser with the same type of clamps as are used on standard riser joints to restrain the integral choke and kill lines. Since this riser is equipped with a third external conduit similar to a choke or kill line, a hole through the modules on one side of each riser joint is provided to allow its passage. The clamps are molded into the inner surface of each module and are equipped with cleats that extend several inches into the float material to increase pull-out resistance. Fig. 1 is an internal view of two float modules showing the clamp geometry. Additional longitudinal restraint for the floats is provided with simple circular friction clamps bolted around the riser pipe at the ends of the modules. These floats provide a net lift of 3,700 lb/riser joint, thus reducing the in-water weight of each joint by 90 percent. The only problem encountered with floats of this type was problem encountered with floats of this type was occasional separation of the fiber glass shell from the float material. This was easily rectified with adhesive. Installation of these large modules required the use of a crane, and alignment and makeup of clamps was somewhat time consuming. However, no maintenance of the clamp attachment system was necessary after installation. The second riser system was used with a semisubmersible drilling vessel, with modules having a molded 34-in. OD x 16 1/2-in. ID high-density-polymer outer shell. The shells are 4 ft long, semicircular in cross-section, and are filled with syntactic foam. No hollow plastic spheres are provided to reduce float density. Sixteen modules are required to cover 32 ft of each 40-ft-long riser joint. They are attached to the riser with three steel bands for each pair of modules.
JPT
P. 445