Abstract

In 1999 onshore model tests on Position Installed Plate Anchors (PIPAs) were performed at Onsöy in Norway. The tests were extensively instrumented and performed in situ in typical normally consolidated clay. This paper describes tests, instrumentation selected test results and back calculations. Three different test anchors were installed, tested and retrieved. The anchors were installed vertically. Anchor line tension was applied until soil failure occured. Finally, the anchors were retrieved vertically to surface. Instrumentation includes load cells, inclinometers and depth measurements. Data were logged in real time. Manual measurements of anchor line geometry were also performed. Selected test results are presented herein. The results indicate significant repeatability. In addition, the different anchor types with their different shapes and functionality provide a valuable spread in the test data that can give improved understanding of anchor behaviour and be used for calibration of design methodology. Back calculations of selected results are also presented.

Introduction

Position Installed Plate Anchors (PIPAs) have several advantages that in many cases can favour their use. They are cheap to produce, easy to install and the position after installation is well defined. Accordingly, such anchors may often be preferred to drag-in anchors.

Several different anchor models are on the market, and new models are being developed. On this background, field tests with different types of model anchors could give useful information on their behaviour, as well as background to procedures for use of such anchors, design criteria etc.

In August 1999, Noregian Geotechnical Institute (NGI) was contracted by Saga Petroleum ASA (from 2000-01-01 merged into Norsk Hydro ASA) to plan and perform field tests of model anchors.

Possible anchor manufacturers in Norway were invited to take part in the project. Three manufacturers agreed to fabricate model anchors for the tests, according to given specifications. A model scale of ~1:3 was estimated to be suitable based on expected loads and practical considerations.

Test site
Onsöy test field.

For several decades NGI has utilized a test site at Onsöy, outside the city of Fredrikstad. This test field has been host for test fills, test piling, block sampling. Soil data. Due to previous geotechnical research activities, soil conditions at the test site are well documented1. The sediments consist of normally consolidated soft clay to large depth. Soil parameters for the natural clay deposits at Onsöy are summarised in Table 1.

Generally, the soil parameters at the test site are comparable with values for typical North Sea soft clay deposits, which makes the test site especially suitable for offshore applications. In Table 1, also values found at the Troll field in the North Sea2 are shown.

Instrumentation and data logging

The following instrumentation was used during the tests:

  • Two load cells: One for measuring pull force and retrieval resistance, and one for measuring penetration force during anchor installation.

  • Three inclinometers: One for measuring anchor pitch, one for measuring anchor roll and one for measuring angle of the pull wire at the pad eye.

  • Pressure sensor for measuring anchor depth.

  • Wires connected to the anchor line to measure anchor line geometry manually.

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