Ocean Systems Engineering, Vol. 7, No. 4 (2017) 345-369 DOI: https://doi.org/10.12989/ose.2017.7.4.345 345 Copyright © 2017 Techno-Press, Ltd. http://www.techno-press.org/?journal=ose&subpage=7 ISSN: 2093-6702 (Print), 2093-677X (Online) Swell description for Bonga offshore Nigeria location Akinsanya Akinyemi Olugbenga 1 , Ove Tobias Gudmestad 1 and Jasper Agbakwuru 2 1 Department of Civil, Mechanical and Material Sciences, University of Stavanger, 3056 Stavanger, Norway 2 Federal University of Petroleum Resources, FUPRE, Warri, Nigeria (Received May 27, 2017, Revised October 12, 2017, Accepted November 7, 2017) Abstract. The ocean environment offshore West Africa is considered to be mild. However, the generated swell from distant North and South Atlantic during austral winter and summer can reach high wave amplitudes with relatively low wave periods or low wave amplitudes with long wave periods, the later can be a crucial scenario to consider when the assessment of vessel resonance is of importance. Most offshore operations, which include offshore drilling, and installation in West Africa, are carried out from floating systems. The response of these systems and performance are governed by characteristics, such as amplitude and frequency of the wave and swell seas. It is therefore important to fully understand the sea conditions offshore Nigeria. This study covers the description of the swell sea offshore Nigeria using Bonga offshore wave measurements collected from the directional wave-rider (DWR), positioned at the Bonga site off the coast of Nigeria. Keywords: West African swell waves; wave periods; wave heights; extreme value estimates; safe operations of floating vessels 1. Introduction As there is a growing increase in offshore activities off the West Africa coast, the need to be equipped with good knowledge of swell seas, being dominant, offshore West Africa becomes more important than before, for the planning of successful marine operations in this geographic region. This knowledge is needed to specify the sea states not only for delicate offshore operations (such as drilling) but also for the safe operations of weather sensitive floating vessels in the ocean in this region. It is worthy to note that the knowledge of swell sea is not limited to marine operations; it is also very well applicable for adequate design of offshore facilities specific to this region. It is known that the ocean environment offshore West Africa is mild, but it is dominated by swell waves, which are generated by high wind energies, far away from offshore West Africa. The waves originate in the South Atlantic and North Atlantic during the austral winter and austral summer, respectively (Prevosto et al. 2013). High energies transferred by the wind to the sea results in the propagation of fast moving swell waves, covering enormous distance to West Africa coast. These swells, being generated from sources far from the West African ocean environment, are characterized as low long waves, having long wavelength, with reduced amplitude/wave height, then move further away from the source. However, these swells possess enormous amounts of Corresponding author, Professor, E-mail: Ove.t.gudmestad@uis.no