Performance Tests of Drilling-Vessel Anchors
Authors: Beck R.W. 
Published: December 2007

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