The top suspension system comprises the kelly, the swivel the drilling hook, the travelling block, the drill lines and the derrick. This paper includes measured high-rate data of accelerations and forces and a discussion on simple and more complex models for the top suspension system. The simple model of lumped mass and spring is shown to be insufficient for describing the boundary conditions for longitudinal drillstring vibrations. The two models for explaining the observed dynamics included in this paper are i) resonant derrick vibrations and ii) non-linear coupling to transversal drill lines vibrations.
Drillstring vibrations have been discussed in many publications during the last decades. There are several reasons for studying such vibrations, fatigue problems, improving rate of penetration, directional control and data transmission can be mentioned as examples. Drillstring vibrations do mainly occur due to bit-formation infractions or drillstring-borehole interactions.
In the present work a discussion on the boundary conditions for longitudinal vibrations are given. With a few exceptions publications on drillstring vibrations do not seem to be very much concerned about boundary conditions. An early paper on longitudinal and angular vibrations modelled the downhole boundary condition as a sinusoidal excitation of the displacement with a constant amplitude and the top boundary condition as a spring from the elasticity of the cables and a lumped mass of the travelling equipment. For most situations these two boundary conditions are relevant to get an overall idea of the problem. This paper has also contributed considerably in the understanding of drillstring vibrations and several later publications have used the same type of boundary conditions when modelling longitudinal drillstring vibrations.
In a recent paper a thorough discussion on some phenomena concerning the boundary condition at the bit was given. The authors conclude that a non-linear coupling between longitudinal and torsional vibrations may exist at the bit. In this paper a discussion of the boundary condition at the rig-floor for longitudinal vibrations is given. It is shown that real drillstring vibration data cannot be explained by the commonly used boundary condition. Two different possible explanations to the phenomena seen are discussed, i) the effect of internal rig vibrations and ii) a coupling between longitudinal drillstring vibrations and transversal vibrations in the drill lines.
The theory presented will be supported by measurements from Ullrigg. Ullrigg is a full-scale research drilling rig with dimensions similar to the rigs on off-shore installations.