Defining the specification and moreover the limitations of Managed Pressure Drilling (MPD) equipment installations on Mobile Offshore Drilling Units (MODU’s) could be challenging from a functionality stand point. These challenges mainly arise from a wide range of functional or drilling parameters that these permanent or semi-permanent MPD installations are exposed to. With the ever increasing demand of using MPD, many of the existing and new built MODU’s are getting furnished with MPD equipment. Since the increase in demand of MPD equipment for the offshore market is attributed to the advantages that the MPD systems exhibit, typically for drilling wells that demonstrate narrow drilling margins, the MPD system significantly assist in managing bottomhole pressures, especially to mitigate ballooning, and to manage losses / kicks. Due to the potential exposure of the MPD installation to extreme environments, and to ensure proper utilization of these MPD systems within its design limits, it is necessary to define the Functional Design Specification (FDS) for the MPD equipment installed on MODU’s.
The FDS consists of four major calculation categories for the MPD equipment installations. These are, "surface line frictional pressure loss calculation", the "PRV sizing calculation", the "Riser degassing with MPD equipment as a function of MGS liquid and gas handling capacity" and lastly the "erosion calculation" of the piping that is installed as part of the MPD equipment on MODU’s. The development, methodology and approach of specific calculations that were performed to estimate the functional design specification, encompassing these four categories are illustrated in this paper.
The paper describes the method utilized to estimate friction loss through MPD surface lines based on specific calculations that estimate friction factors as a function of mud rheological properties, pipe geometry, length etc. The results obtained from the developed program to estimate frictional pressure loss in surface lines, were calibrated using fingerprinting data obtained from earlier installations on similar Drillships. Next, the FDS calculation uses the MGS (Mud gas separator) gas and liquid handling capacity to estimate the amount of gas that could be safely circulated out from the riser from above the closed BOP’s, without exceeding the MGS capacities and MPD equipment pressure limitations. The paper explains how the PRV sizing calculations were performed which help in specifying the limiting flowrates that the PRV can handle, as a function of mud weight and pressure differential based on API 521b. Lastly the paper illustrates the estimated erosion calculation based on the maximum flowrates that the MPD system could potentially be exposed to due to drilling and/or any limiting riser degassing events, using DNV recommended practices.