Using Model Basin Tests to Predict Motion Characteristics of Floating Rigs

Abstract

A scale model of a floating rig was subjected to motion tests in a wave basin under a variety of wave conditions representing actual wave heights and periods experienced in Lake Maracaibo in Venezuela. The motion of the rotary table relative to a fixed wellhead was recorded on motion picture film. The observed vertical, lateral and fore and aft motion was then related graphically to wave height and period. These data permitted predictions to be made that the actual floating rig would be able to work effectively more than 98 per cent of the time, and that the maximum motion to be expected under severe storm conditions would not endanger the floating rig, or the wellhead and foundation. The actual floating rig went into operation in mid-1963. It is equipped with a three-dimensional continuous motion recorder. During a prolonged test period, the floating rig operated effectively 99.44 per cent of the time, and the maximum motion experienced to date is 83 per cent of the theoretical maximum predicted from the model basin test data. Introduction Lake Maracaibo in Western Venezuela is a brackish body of water 110 miles long by 70 miles wide and a little over 100 ft deep. The Bolivar coastal field, one of the largest oilfields in the world, is in the northern half of the lake. Creole Petroleum Corp. is one of the major operators in Lake Maracaibo. A Creole drilling rig operating in the lake is shown in Fig. 1. The derrick and drawworks are mounted on a reinforced concrete pile foundation which has a working load capacity of 400 tons. The rest of the drilling equipment is carried on a standard drilling tender which is 180 ft long by 70 ft wide and has a normal operating displacement of about 2,700 tons. The heavy-duty foundation used on Creole's Lake Maracaibo wells represents about 30 per cent of the total well investment. If the foundations had to support only the wellhead and working platform and not the heavy drilling loads, a simplified tripod design could be used and the cost would be substantially reduced. With this in mind, Creole decided to examine the possibilities for converting a standard drilling tender to a floating rig by supporting the rig outboard of the front end of the barge. This system would permit the use of a tripod wellhead support platform costing only about one-half as much as a conventional foundation. Although it appeared probable that a floating rig of the type contemplated could be used successfully, it was deemed advisable to perform sufficient model basin tests to obtain quantitative answers regarding barge motion under the conditions to be expected in the lake. Test Methods The Model Creole already had on hand an accurate 1:48 scale model of a standard drilling tender and drilling rig. A rig-support structure was prepared for the model so the rig could be mounted either over the bow or over the side. Photographs of the model rig are shown in Fig. 2. JPT P. 1361ˆ