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Anchor Handling Simulator Course.pdf
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3

Practice

Introduction

Practice

 

Although theoretical knowledge of anchors is essen-

 

tial for good anchor design and selection, the practi-

 

cal issues are just as important. The handling of an

 

anchor and the selection and use of support equip-

 

ment is of equal importance.

 

Anchor handling is a critically important and often

 

complicated process. It is influenced by such factors as

 

the weight and shape of the anchor, the nature of the

 

soil, the depth of the water, the weather conditions,

 

the available handling equipment and the type and

 

weight of mooring line. It is for these reasons that

 

anchor handling is a subject which requires careful

 

consideration. Without proper anchor handling, opti-

 

mal performance of an anchor is not possible.

51

In the process of handling anchors, various types of support equipment are necessary or beneficial. An anchor manual would be incomplete without consideration of these auxiliary items, the reasons for their use, their operation and the advantages and drawbacks involved.

This chapter gives an overview of the recommended procedures that should be followed for anchor handling and the types and use of the support equipment during the handling operations.

The following handling procedures are by no means complete, but they do give some suggestions which can be applied to each anchor handling procedure and adapted for specific circumstances and locations.

Some of the topics covered in this chapter are: requirements for a soil survey, connection of the anchor to the mooring line, chasers, handling the Stevpris anchor, handling the Stevmanta anchor, the Stevtensioner, anchor handling/supply vessels.

Soil survey

For the dimensioning of drag embedment anchors, the availability of site-specific soil data is important. For advice on specifying drag embedment anchor type/size and calculating expected behaviour, the site-specific soil data should be compared with soil data of previous drag embedment anchor (test) sites.

The soil survey requirement for the design of drag embedment anchors usually consists of only shallow boreholes, while in anchor pile design deep boreholes are required. For suction anchor design therefore a more extensive soil investigation is generally required when compared to drag embedment anchors. When choosing between anchor pile, suction anchor and drag embedment anchor the financial implications of the soil survey should be taken into account.

52

A typical soil survey for drag embedment anchor design requires a survey depth of twice the length of the fluke in sand and 8 times the fluke length in very soft clay. In most cases a depth of 8 to 10 meters is sufficient, although in very soft clay a reconnaissance depth of 20 to 30 meters should be considered. For optimal drag embedment anchor dimensioning, each anchor location should ideally be surveyed. The soil investigation can consist of boreholes, vibrocores, cone penetration tests or a combination of these. Cone penetration tests including sleeve friction are preferred, but they should be accompanied by at least one vibrocore or sample borehole per site to obtain a description of the soil. Depending upon the type of survey performed and the soil conditions encountered, the survey report should present the test results obtained on site and in the laboratory including the points as shown in table K.

It is possible to dimension the drag embedment anchors based on limited soil information (for instance fewer boreholes). The ‘lack’ of soil data can be compensated by choosing a conservative (larger) anchor size.

Typical contents survey report

Cone penetration resistance.

Sleeve friction.

Pore pressure.

SPT values.

Granulometry and percentage fines.

Wet and dry densities.

Water content.

Drained and undrained triaxal tests.

Undrained shear strength, also remoulded.

Unconfined compression tests.

Plasticity limits.

Specific gravity.

CaCO3 content.

Shell grading.

Angularity and porosity.

Compressibility.

Cementation.

Normalised rock hardness test (point load test).

RQD index, rock quality designation.

table K

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