Whole Body Vibration on Offshore Structures: An Evaluation of Existing Guidelines for Assessing Low-Frequency Motions

ABSTRACT

An extensive literature research has been conducted to create an insight into the existing norms and standards regulating the assessment of human exposure to motions in offshore environments. A summary of current threshold values and their specific fields of application is included. The presented literature is analysed with respect to their applicability for assessing low-frequency oscillatory motions of floating offshore wind turbines to which technicians are exposed during maintenance tasks. The review identifies the need for a consistent assessment method in combination with threshold values for floating structures.

INTRODUCTION

Whole body vibration (WBV) is a comprehensive term for vibrations transmitted to the whole body, not locally to specific extremities (e.g. hand-arm-vibrations), often induced through a seating surface, the backrest or the floor. The frequency, amplitude and duration of the vibrations influence the effects they have on the exposed person. Higher frequencies are associated with more severe health problems, which can persist or occur long after a person was exposed to the vibration, e.g. back pain, (Mansfield, 2005). A distinction is made for WBV in the frequency range below 1 Hz. They are classified as vibrations causing motion sickness and constitute their own class because of the different nature of symptoms they provoke.

The condition of motion sickness is a well-known phenomenon and yet the background of its origins has not been fully unveiled. Familiar under the name of seasickness it is mostly associated with high periodic motions on ships and vessels but can equally well occur on aircraft, train, car or roller coaster rides (Griffin, 1990). The symptoms appear in the form of dizziness, nausea and vomiting and are provoked through a conflicting stimulation of the vestibular system and the received visual information. Symptoms cease or disappear when the person is removed from the vibration source, (Mansfield, 2005).

Gresty and Golding (2008, 2009), have investigated the impact of vertigo and spatial disorientation on the performance of different tasks. They found that in the condition of motion sickness, cognitive performance on manual tasks is impaired but that habituation and familiarity to a specific test helps to increase the performance.