A Field Study of Flow Induced Vibrations on a Deepwater Drilling Riser
Authors: Furnes G.K. , Hassanein T. , Halse K.H. , Eriksen M. 
Published: October 2008

A data acquisition program has been performed to examine accelerations and displacements of a full scale drilling riser in 1300 m water depth off the coast of mid Norway. The data was collected during BP's Nykhøgda (Nyk High) drilling programme that was carried out from the 7th May to the 20th July 1997. The present experiment is the first of three planned under the "Norwegian Deepwater Programme". The riser instrumentation consisted of five accelerometer units with two linear accelerometers in each unit. Two of the units were in addition equipped with two rotational rate sensors. An accelerometer unit consisting of five sensors was installed on the platform itself to monitor the platform motion. In addition, a set of instruments was moored to record the time and depth dependent currents at the location. A successful data retrieval was achieved for all the accelerometers covering a period of 74 days. The data analysis has revealed flow induced riser oscillations with several possible mode shapes. The present paper deals with the field experiment design and some features of the observed accelerations.

Introduction

A deepwater drilling riser is a very flexible structure. The dynamic properties of a flexible structural system are determined by the mass, stiffness, and damping of the system. The dynamic mass is given by the mass of the riser joints including buoyancy and the associated hydrodynamic added mass. The stiffness is given mainly by the effective tension in the riser, but with an additional contribution from the bending stiffness of the riser joints. The bending stiffness contribution has increased importance for vibration of high modes. The major contribution to the damping is the hydrodynamic damping caused by the relative velocity between the surrounding fluid and the riser as it vibrates. In addition there will be some damping contribution from the drilling riser itself, e.g. material damping, friction in couplings etc. In addition to wave loading and rig motion a riser is subjected to severe current loading. For a deepwater riser the current loading becomes increasingly important relative to the other environmental loads. Shedding of vortices from the riser surface may set up an oscillating lift force which excites riser motions. If the shedding frequency is close to one of the natural frequencies of the riser, significant Vortex Induced Vibrations (VIV) of the riser may occur. VN may contribute significantly to the fatigue damage of a deepwater riser, and it is therefore of vital importance to be able to predict the VIV response with sufficient accuracy. In connection with the 15th round licence awards on the Norwegian continental shelf it was decided to establish the Norwegian Deepwater Programme (NDP) as an umbrella organization for all the partners of the 6 licences. Members of the NDP are BP, Conoco, Exxon, Mobile, Norsk Hydro, Saga, Shell, and Statoil. The intention with NDP is to focus on technological development in fields of mutual interest for the licences. NDP is organized in different projects of which the Riser & Mooring Project is one. Within this projects workscope is the response analysis of risers for deepwater applications, and one of the critical tasks is to be able to predict the VlV response of a riser.