Development
Introduction
The
Deep Penetrating Anchor™ is a novel concept in anchor technology that
can now be offered to the oil and gas industry as a vialble alternative
to present day solutions. Since its conception as a mere idea in
the mid 1990s, it has undergone the scrutiny from a broad range of
disciplines through feasibility studies comprising fluid dynamic
analyses of the anchor and chain configuration during the drop or
free-fall phase, geotechnical analyses including anchor penetration and
FEM calculations for estimation of anchor capacity, soil consolidation
after installation and deformation during short and long-term loading,
evaluation of suitible fabrication, transportation and installation
techniques and cost estimates. In addtion much effort has been applied
through small scale (1:25) and large scale (1:3) testing in both
laboratory facilities and deep water (300m) fjord and offsshore
testing.
Model Testing - Scale 1:25Model
scale (1:25) drop tests were performed in a water-filled five meter
deep tank at the MARINTEK facilities in Trondheim, mainly in order to
determine the behavior of the anchor line chain and its influence on
the anchor after release. The tests were carried out with varing
vertical length of length of chain (a) and chain loop width (c) as
shown in the figure below.
Tests disclosed the following main results:
- The
chain does not apparently alter the behavior of the anchor durig its
descent other than gently tugging vertically at the connection point
due to hydrodynamic drag on the chain.
- Less drag from the chain is realized with increasing chain length suspended above the anchor.
- Anchor velocity is not noticeably altered due to chain drag.
- During
the drop, the portion of the chain, which hangs down adjacent to the
anchor contracts to a degree depending on the horizontal distance (c)
of the mooring line loop. This effect decreases with increasing
distance (c).
Model Testing - Scale 1:3Large
scale model testing was carried
out in both Trondheimsfjord and at the Troll field in the North Sea
using the anchor as illustrated in the figures right. Two anchor models
were
fabricated i.e. one instrumented while the other a so called
dummy i.e. no instrumentation installed. Instrumentation consisted of a
digiquarts depth sensor, accelerometer, pitch and yaw inclinometers,
and pore pressure sensors to monitor excess pore pressure over a
period of time after installation.
Probably
the most important finding from the 1:3 scale tests was
that the hydrodynamic stability was verified. The bottom
right figure shows the anchor just before release during one of the
Trondheimsfjord tests. (Click the figure to start the anaimation). In
Trondheimsfjord the current can be extreme in relation to normal sea
tidal currents. Therefore, due to the current, the anchor starts off
with a rotation but quickly recovers due to the hydrodymanic design. The final anchor tilt is approximately one degree.
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