Analysis of Riser Loads during BOP sailing.

Abstract

Transferring a drilling rig and its subsea equipment from one location to another by conventional means tends to be time consuming. With low oil prices and increasing rig rates, operators are looking into making drilling rigs more efficient. This thesis will examine a method called BOP sailing, which may expedite the process of moving a drilling rig and its subsea equipment from one location to another. It will also make sure that the BOP sailing method is feasible from a safety and design point of view.

The BOP sailing operation is modelled and simulated in the riser analysis software RIFLEX for three riser column lengths: 311 m 800 m 1225 m The riser joints for each of the riser column length have the same diameters and submerged weight per unit length. Weather conditions, current profiles and sailing speeds were defined prior to running the simulations. The results obtained from RIFLEX is then further analyzed by calculating a usage factor, n, based on the combined loading equation from the DNV-OS-F201 standard. This equation is used to perform riser analysis using tension and bending moments, and the value of the calculated usage factor may not exceed a critical value.

The usage factor is calculated at two locations on each of the riser columns for all of the weather conditions. One of the locations is located at the top of the riser column where the riser column is attached to the drilling vessel. The second location is where the maximum dynamic bending moment takes place.

Based on the simulation results, it is found that the value of n increases with an Increase in sailing speed Increase in wave height Increase in riser column length

However, the value of n did not exceed the limiting value in any of the simulations. One can therefore conclude that it is safe to sail with a suspended marine drilling riser and subsea BOP for all of the weather conditions and sailing speeds used in the RIFLEX simulations.