John Morton

Project Title: Dynamic anchors: a cost effective anchoring solution for the offshore energy industry

Name: John Morton

Supervisor: Dr. Conleth O Loughlin

Abstract:

The proposed project aims to investigate the geotechnical performance of dynamic anchors in normally consolidated clay as this is the dominant deep water seabed deposit. Considering the scarcity of experimental data from field and model tests, the first objective of the proposed project is to develop an experimental database by conducting an extensive suite of reduced scale centrifuge tests on model anchors. Such a database would have considerable merit in the development of design charts and verification of numerical/analytical techniques that predict the embedment and eventual capacity of dynamic anchors under working and ultimate loading conditions. The second major objective therefore, is to develop a design tool that can be readily employed by industry for the prediction of anchor embedment and subsequent capacity.

Three distinct phases have been identified in the proposed project, the completion of which will meet the aforementioned objectives. Phase 1 involves an extensive suite of centrifuge tests that will allow for an appreciation of the expected penetrations and anchoring capacities for geometry specific anchors. The particular aims related to Phase 1 are:

  1. Investigate the parameters that govern anchor embedment: impact velocity, anchor shaft length/diameter ratio, anchor fluke width/height ratio, anchor tip geometry, anchor mass and soil shear strength profile.
  2. Examine the effect of anchor embedment on anchor capacity under monotonic and cyclic loading conditions.
  3. Quantify the contribution of anchor soak period (duration between anchor installation and loading) to anchor capacity.

Phase 2 focuses on the development of an analytical approach for the prediction of anchor penetration and anchor capacity under monotonic and cyclic loading conditions. The particular aims of Phase 2 are:

  1. Development of an analytical approach for anchor penetration. It is anticipated that the anchor penetration model will be based upon conventional bearing and frictional capacity theory but with provision for viscous enhanced shearing resistance and fluid mechanics drag resistance.
  2. Development of an analytical approach for anchor capacity. For the case of purely vertical monotonic loading this approach will be based upon conventional end bearing resistance and shaft friction. However for inclined and cyclic loading, models will be developed which, a) employ load interaction diagrams to assess the contribution of horizontal capacity during inclined loading, and b) account for the reduction in soil shear strength during cyclic loading by introduction of appropriate degradation factors.

These approaches will be validated using experimental data acquired during Phase 1.

Phase 3 involves more sophisticated model tests on instrumented model anchors. The particular aims of Phase 3 are:

  1. Measurement of the deceleration of the anchor during penetration using miniature accelerometers.
  2. Measurement of near and far field soil displacements in the zone of anchor installation as these will yield information about the level of excess pore pressure generation.
  3. Identification of rate-dependent soil failure mechanisms at the interface of the advancing anchor.