NUMERICAL MODELLING OF CURRENT ON WAVE DRIFT LOAD OF A SHIP SHAPED OFFSHORE STRUCTURES (CASE STUDY – EXCRAVOS IN WEST AFRICAN OFFSHORE)
Abstract
On account of the piecemeal reduction of oil and gas resources onshore and shallow waters,
oil exploration is gradually moving into deeper waters. One of the major means of exploration of these
resources is by ship-shaped offshore structures. Owing to marginal field and the increment of depths of
exploration, there comes the need for the reusable, re-locatable and field-independent ship-shaped
offshore structures. They include FPSO, FLNG, FSO, drill ships etc. A usual phenomenon these
structures face at their location is drifting i.e. losing its course. The research presents the numerical
analysis of sea loads on these structures. It focuses more on waves and current effects. This work uses a
theory developed by Faltenses 1990 and modified by Said in 2010 and extended the theory to verify the
effects of current on second-order wave force on an operational FSO LPG ESCRAVOS in West Africa,
Nigeria. This FSO is co-owned by the Nigerian government and Chevron Nigeria limited. Data used for
computation of the current forces were obtained from the Oil Companies International Marine Forum of
2014. Computations were carried out in irregular wave’s conditions data prevalent in Escravos, Offshore
Nigeria. Based on it, steady drift motion responses and effect of the current forces are examined.
Environmental conditions, such as effect of current velocity, current forces and current angle of attack
on wave drift load are analyses, which significantly affected the FSO motion in surge, sway and yaw
moments. It is found that the effect of current forces is quite significant when the current velocity is
increased. In this numerical analysis, while the current velocity is increased to 2.0 meter/seconds (3.90
knots), the impact on FSO motion is quite significant, which should be taken into consideration from the
point of view of design, safety, failure of mooring systems, operating responses, production shut down
and the positioning of the FPSO. The results of the discussion conform to facts about sea loads and its
effects.
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