DYNAMIC ANALYSIS OF WORKOVER RISER UNDER UNEXPECTED CONDITIONS

Abstract

The oil and gas business will meet new economical and technical challenges in the future. Increasing water depths and a decreasing number of new findings lead to a demand for higher recovery rates. Workover systems are used in operations where the aim is to open wells for production or conduct maintenance and service on already excisting wells. Optimisation of excisting oil fields are performed by well intervention through a workover riser, from the support vessel to equipment on the seabed. A workover riser connected to a support vessel is exposed to forces from waves and vessel motions. Dynamic positioning is applied for station keeping in horizontal direction, while a heave compensator system is implemented to minimise transfer of heave vessel motions to the riser. During normal operation the riser will have a curved shape due to the current profile and the weight and buoyancy distribution of the riser.Definition of a critical scenario will be when both drift off due to failure of the dynamic positioning system and lock up of the compensator system occurs when the support vessel has maximum vertical velocity. This will lead to a significant force increase as the riser is exposed to motions from the vessel and stretches out. The newly developed computer program Sima-Riflex was applied to perform nonlinear dynamic analyses. One of Aker Solutions' workover systems was modeled and critical scenarios with lock up for various support vessel offsets were executed as regular analyses. Wave height and period were calculated based on the compensation limit of the compensator system, the scatter diagram for the operational area and the RAO related to the support vessel Deepsea Atlantic.Analyes were first performed for a water depth of 1300.00 metres. Secondly, the workover system was extended and shortened to water depths of 3006.88 and 309.40 metres. The results showed an instantaneous tension increase, in the top end of the riser stack-up after lock up, for all offsets and water depths. The rapid tension increase created a subsequent load impulse that traveled through the riser with time. Zero support vessel offset gave largest values of effective tension along the whole riser stack-up for all water depths.The impulse velocity and time for impulse to travel through the riser stack-up are dependent upon the total length and steel type.Zero offset gives the riser larger geometrical stiffness compared to cases where offset is present. This leads to smaller elasticity and less tension is used to straighten out the riser when lock up occurs. With background in the results, there is no indication of a greater influence of significance to effective tension from drag forces or added mass for larger offset. A workover system that copes with the rapid tension increase and the load transfer through the riser with zero offset, will therefore also withstand the loads related to larger offsets.