Marine Drilling and Well Completions from Floating Platforms
Authors: Shields C.M. 
Published: December 2007

Floating drilling vessels and barges have been used extensively in California coastal waters for core drilling since the early 1950's. The techniques developed in these operations have recently been used for full- scale drilling operation, and some wells have been completed on the ocean floor as successful producers. The floating drilling vessel technique, although still in an early state of development, provides a method for economically exploring deep-water areas; developing marine reserves when additional platforms cannot be justified; and performing exploratory and developing drilling in remote arias. The limitation on the method are in deep diving, vessel mooring systems and high drill pipe and casing stresses caused by vessel movement.

Introduction

The purpose of this paper is to review the recent developments and accomplishments of the floating drilling vessel techniques, compare the relative economics of developing marine oilfields from floating and fixed platforms, and to examine the risks and intangible advantages in drilling from floating vessels. Drilling operations from floating barges may be divided into three phases. The first phase, starting shortly after World War II, was concerned with obtaining shallow cores in water depths to approximately 100 ft. For the most part cores were obtained using punch or jetting methods, although some work was done with small rotary rigs operated "over-the-side" of small vessels. During this phase over 500,000 ft. of 4 to 6 in. diameter core holes were drilled to depths of 500 ft. below the water surface. The second phase of the floating drilling operations was started about 1954. This phase was initiated with the construction of medium size, 150- to 200-ft vessels employing regular rotary drilling equipment mounted on the center line of the vessel. Ten- to 12-ft holes were cut in the vessel hulls, through which the drilling operation was performed. Operating through the center of the vessel, rather than overside, minimized the effect of vessel roll and pitch and substantially improved the over-all drilling operation. During this second phase, as many as six floating drilling vessels were in service off the California coast. Operations were conducted in water depths to 400 ft and holes drilled to 10,000 ft below the water surface. Drilling operations were also conducted in other areas with shallower water. The third phase of the floating drilling operations was started about 1958 and continues to the present time. This phase is characterized by a further refinement in the drilling technique and development of ocean-bottom drilling equipment, by which wells have been completed on the ocean floor as successful producers. During this phase, new, large drilling vessels have been developed for operations in water depths beyond 300 ft and for drilling to 15,000 ft as efficiently as land or fixed-platform equipment.

Description Of Drilling Method

The procedure in drilling from a floating vessel is in many respects similar to drilling from a fixed platform or land location. The principal difference, however, is in accounting for the relative motion between the vessel and the ocean floor, caused by currents, wave motion and the inherent springing characteristics of the mooring system. Drilling methods and equipment used with floating vessels have been improved in recent years, although the basic method is essentially unchanged. As shown in Fig. 1 the ocean bottom equipment usually consists of a base plate or template, blowout preventer stack, disconnect coupling, flexible joint, riser pipe and telescopic joint. Guide lines attached to the template provide a means of re-entering the hole during the initial drilling operation. During subsequent drilling operations the riser pipe is used for guiding pipe and tools into the hole as well as providing a conduit for drilling fluid returns. The flexible member and telescopic joint are required to accommodate the lateral and vertical motion of the moored vessel. Disconnect couplings as shown in Fig. 1 are installed above and below the blowout preventers. The upper disconnect joint will permit the riser and associated equipment to be removed for repairs and replacement without disturbing the blowout- preventer assembly. This connection may also be disengaged when operations are secured because of storms and sea conditions.

JPT

P. 605ˆ