Small-Anchor Tests To Predict Full-Scale Holding Power
Authors: Cole M.W. , Beck R.W. 
Published: December 2007

This paper presents a practical, low-cost method for predicting the holding power of conventional drilling vessel anchors from results of tests conducted with smaller anchors prior to moving the rig on location. The procedure, equipment, method for extrapolating small anchor data to drilling vessel-size anchors, and results of recent tests are discussed. Although the method was developed from tests conducted in mud, clay and sand using Lightweight-Type (LWT) anchors, it should be applicable for use with any type of anchor.

Introduction

Advance knowledge of anchor holding power at a drilling location may reduce lost rig time by permitting the selection of appropriately sized permitting the selection of appropriately sized anchors prior to moving the rig on location. In many instances, the time required to anchor a floating drilling vessel is excessive, because the anchors on board are inadequate and additional series or tandem anchors must ultimately be used. As a result, the time to properly set an eight-anchor pattern may vary from 12 to 50 hours or longer, pattern may vary from 12 to 50 hours or longer, depending on how many times the anchors must be handled and how many tandem anchors are necessary. Interpretation of small-anchor test results can provide necessary advance information to aid in provide necessary advance information to aid in the selection of proper size primary anchors, thereby allowing an increase in the efficiency of the anchoring operation and a reduction in hazards to personnel. Small-anchor tests may also be used to indicate whether conventional drag-type anchors are suitable or pile anchors are required. On occasion, considerable time and money have been wasted by trying to anchor with conventional drag-type anchors in areas where the ocean floor is hard and pile anchors are ultimately required to provide a suitable mooring. Conversely, both time and money have been spent on the installation of pile anchors where conventional anchors would have provided the necessary holding power. These costly delays can be reduced if the holding power of conventional drag-type anchors is determined in advance. Results of a few small-anchor tests have been published; however, these tests were limited to establishing whether conventional anchors would develop holding power or pile anchors were required.

PREDICTION OF FULL-SCALE HOLDING PREDICTION OF FULL-SCALE HOLDING POWER FROM SMALL ANCHOR TESTS POWER FROM SMALL ANCHOR TESTSAnchor pull tests have indicated that the holding power of conventional drag-type anchors is related power of conventional drag-type anchors is related to anchor weight and bottom soil by the following expression.

whereA.H.P. = anchor holding power, lb

C, b = soil constants for the type ofanchor, dimensionless

Wa = anchor weight in air, lb

The holding power of the anchor calculated from Eq. 1 corresponds to the maximum sustained load the anchor will resist before beginning to lose holding power, provided the mooring line does not exert any vertical uplift. Eq. 1 indicates that the maximum holding power developed in a given area depends on both the bottom soil and the weight or size of the particular type anchor. Values of soil constants, C and b, for test locations in the Gulf of Mexico and Santa Barbara Channel (Calif.) are tabulated in Table 1 for LWT anchors with 30 degrees) fluke angles. Soil constants used by the U.S. Navy to estimate holding power in "compacted sand" are also given. The data indicate that the constants C and b vary from one soil to another, so that a 20,000-lb LWT anchor, for example, should not be expected to develop the same holding power in both sand and mud. Also, soil characteristics vary from area to area as demonstrated by results in Timbalier clay and Santa Barbara Channel clay.

SPEJ

P. 127