Tbis paper presents a practical, for predicting the boiding power M. W. COLE, JR. R. W. BECK M13413ER AIME low-cost method oi converitiord drilling vessel anchors~om results of tests conducted with smaller anchors prior to moving the rig on location. The procedure, equipment, method for extrapolating small anchor data to drilIing vessel-size anchors, and reszdts of recent tests are discussed Altbougb the method was developed from tests conducted in mud, clay and sand usirig Ligbtweigbt-Type (L WT) anchors, it should be applicable for use with any type of anchor.
Holding-power tests on several types of anchors in a variety of mud and sand bottom revealed varying degrees of reliability. Balling up and failure to dig in were the principle problems encountered. A new streamlined anchor, the BOSS, generally outperformed the others by digging in quickly, holding well, and avoiding ball-up. Introduction The anchors used for mooring drilling vessels are usually of a lightweight design similar to the U. S. Navy Lightweight or the Danforth. Commonly used sizes range from 20,000 to 30,000 lb. Holding power in favorable sea floors may exceed 10 times anchor weight, but in some types of mud the holding power may be only one to two times anchor weight.Minimum holding-power requirements for drilling vessels range from about 100 kips per mooring line on a small vessel to as much as 300 kips on some of the larger units. These requirements are usually verified by applying a test load on the anchors before spudding the well. If the anchors fail to hold the test load, tandem anchors or pile anchors may be required to achieve the necessary holding power.A dependable mooring system is important in the safety of floating drilling operations. It is therefore desirable that drilling vessel anchors perform well and be reliable. That is, they should achieve holding powers of at least 10 times anchor weight, and should powers of at least 10 times anchor weight, and should be able to do so in all types of penetrable bottoms ranging from sand to very soft mud. This paper discusses the results of tests carried out by Esso Production Research Co. (EPR) on the performance of Production Research Co. (EPR) on the performance of anchors in a variety of bottom soils. A significant result of these tests was the development of a new type of anchor that showed improved performance and reliability. Reported Tests of Lightweight Anchors Various branches of the U. S. Navy have conducted anchor holding-power tests with lightweight anchors in sand and mud bottoms. Some of the test data are briefly summarized below, with the holding power shown as "holding-power ratio" (holding power/ anchor weight). The Naval Civil Engineering Laboratory tested two types of lightweight anchors in a mud bottom of San Francisco Bay and in a sand sea Boor near Port Hueneme, Calif. Average holding-power ratios for one type of lightweight anchor ranged from 3.8 to 13.3 in mud and from 12.4 to 23.6 in sand. Tests with the other type of lightweight anchor showed average holding-power ratios ranging from 2.1 to 5.1 in mud. The Naval Amphibious Test and Evaluation Unit tested lightweight anchors in both mud and sand at Little Creek, Va.:, Average holding-power ratios ranged from 5.5 to 7.5 in mud, and from 16.8 to 26.0 in sand. These U. S. Navy tests indicated that the lightweight anchors generally perform better in sand than in mud.In 1958, the Naval Civil Engineering Laboratory developed a new type of lightweight anchor that included large tripping palms. The anchor design aimed at good performance in both mud and sand. Four sizes of this anchor, with weight ranging from 3,000 to 12,000 lb, were tested in San Francisco Bay mud and Port Hueneme sand . 4 After 50 ft of drag in mud, the average holding-power ratios ranged from 13.3 to 19.2; and after 50 ft of drag in sand, the holding-power ratios ranged from 19.6 to 23.2. JPT P. 337
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
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