A Coupled Analysis Approach to the Assessment of Marine Drilling Systems

Abstract

Summary This paper describes the development of a new approach to marine-drilling-system analysis. From studies with traditional analysis techniques where components were treated independently, a decision was made to develop a new model. This new model was based on a requirement to analyze the system as a complete entity by "coupling" the system components. This new approach was used to analyze the behavior of a drilling riser and conductor. It has demonstrated the influence of conductor parameters on riser response and the factors that influence conductor design. Introduction In recent years, several studies (identified in subsequent references) have been published on the loading and response of individual marine drilling system components: subsea wellheads, conductors, and blowout preventers (BOP's). These studies have addressed such issues as wellhead failure and conductor stability but have tended to focus on one specific area of the marine drilling system instead of the system as a complete entity. The components were analyzed in a discrete way, and limitations to this technique were identified. The most important of these limitations were the relative difficulty in identifying the onset of coupled behavior between two components, the difficulty of isolating the most important parameters that govern the system behavior, and the need to model the end conditions for a system component accurately. This work confirmed the need to model the complete marine drilling system and showed that modeling could be done more efficiently with a new approach. At the same time, the need to bring the marine-drilling-system design within the scope of the drilling engineer's work was recognized. This led to the definition and implementation of a new procedure for the design and analysis of marine drilling systems. Requirement To Analyze Marine Drilling Systems The analysis of the marine-drilling-system riser, BOP, wellhead, and conductor (Fig. 1) is not recognized universally as part of the subsea well design. The general assumption has been that standard practices have been adequate and that conductor design is not part of the well design. This is reflected in drilling engineering texts where offshore conductor design is excluded from casing design.1 In some situations, the design of one or more marine-drilling-system components can be crucial to the integrity of the well. Deepwater Drilling. The growth of deepwater drilling (often in harsh environments with high currents) brought the need to analyze marine-drilling-system loading and response. In 1981, wellhead failure from fatigue caused by riser oscillation induced by high currents highlighted the inadequacy of the wellhead systems then available.2 Socket-type wellheads with full-moment transfer that eliminate this failure mode are now specified routinely and often are designated as "preloaded." Conductor Stability. The basis for design of reaction and preloaded subsea wellheads is to transfer the loads to the conductor, through the cement, and into the soil. Several instances of conductor movement have been recorded,3 although the causes have not always been identified. Analysis of subsea conductor loading and response therefore has become a new area of investigation for the drilling engineer. BOP's on Wellheads. The use of a two-stack wellhead system with a large-diameter low-pressure wellhead and a small-diameter high-pressure wellhead generally has been superseded by the use of a single large-diameter high-pressure wellhead and a BOP. The large-diameter high-pressure BOP stacks are much heavier than the small-diameter BOP's. Several subsea wells with two-stack wellheads have been completed or suspended, however, that need to be re-entered. In the North Sea, this almost inevitably requires running with large-diameter BOP onto a small-diameter wellhead. The feasibility of doing this and possible ways to mitigate additional loading from the larger equipment have to be considered in the well design. Long-Term Fatigue Life for Wellheads. Subsea wells that are part of the field development have to be designed with a fatigue endurance sufficient for the probable field development strategy. For example, it often is necessary to work over or redrill subsea wells. Thus, a well possibly could have a riser attached for more than 1 year. Fatigue therefore may be the predominant design criterion in an otherwise benign loading environment if only peak stress is considered. Consequences of Station-Keeping Loss. A 1989 U.K. Dept. of Energy safety notice4 advised operators to ensure that, in the event of a station-keeping failure in a rig attached to a subsea well, the marine drilling or workover system would not fail below the pressure-containing valves. This required a comprehensive system analysis from the top of the riser to the point of fixity of the conductor. Thus, in a number of cases, marine-drilling-system analysis is important and requires suitable analytical tools. Standard Marine-Drilling-System Analysis Techniques Two additional broad approaches to marine-drilling-system analysis exist. All-Embracing Finite-Element Code. Use of a very powerful finite-element (FE) code to analyze the complete marine drilling system in detail is possible. Such a code has to handle the application of enviromental loads, hydrodynamic behavior, "gap element" wellhead behavior, and nonlinear behavior of cemented conductors. Separate Component Approach. The analysis of a marine drilling system can be simplified by separating this system into individual components and analyzing each one with a dedicated solution code. Thus, riser elements would be analyzed with a time- or frequency-domain riser-analysis program, wellhead behavior would be approximated to an equivalent beam-element model with nonlinear gaps, and conductor behavior would be modeled as a grouted pile with a standard soil/pile interaction program. Limitations of Standard Approaches Complexity of a Single FE-Code Solution. The single FE-code solution probably will be the most valid analysis. The complexity of the data preparation, however, makes this solution unsuitable for all but the most extreme and critical applications. It certainly is not suitable for routine application to well design. Deepwater Drilling. The growth of deepwater drilling (often in harsh environments with high currents) brought the need to analyze marine-drilling-system loading and response. In 1981, wellhead failure from fatigue caused by riser oscillation induced by high currents highlighted the inadequacy of the wellhead systems then available.2 Socket-type wellheads with full-moment transfer that eliminate this failure mode are now specified routinely and often are designated as "preloaded." Conductor Stability. The basis for design of reaction and preloaded subsea wellheads is to transfer the loads to the conductor, through the cement, and into the soil. Several instances of conductor movement have been recorded,3 although the causes have not always been identified. Analysis of subsea conductor loading and response therefore has become a new area of investigation for the drilling engineer. BOP's on Wellheads. The use of a two-stack wellhead system with a large-diameter low-pressure wellhead and a small-diameter high-pressure wellhead generally has been superseded by the use of a single large-diameter high-pressure wellhead and a BOP. The large-diameter high-pressure BOP stacks are much heavier than the small-diameter BOP's. Several subsea wells with two-stack wellheads have been completed or suspended, however, that need to be re-entered. In the North Sea, this almost inevitably requires running with large-diameter BOP onto a small-diameter wellhead. The feasibility of doing this and possible ways to mitigate additional loading from the larger equipment have to be considered in the well design. Long-Term Fatigue Life for Wellheads. Subsea wells that are part of the field development have to be designed with a fatigue endurance sufficient for the probable field development strategy. For example, it often is necessary to work over or redrill subsea wells. Thus, a well possibly could have a riser attached for more than 1 year. Fatigue therefore may be the predominant design criterion in an otherwise benign loading environment if only peak stress is considered. Consequences of Station-Keeping Loss. A 1989 U.K. Dept. of Energy safety notice4 advised operators to ensure that, in the event of a station-keeping failure in a rig attached to a subsea well, the marine drilling or workover system would not fail below the pressure-containing valves. This required a comprehensive system analysis from the top of the riser to the point of fixity of the conductor. Thus, in a number of cases, marine-drilling-system analysis is important and requires suitable analytical tools. All-Embracing Finite-Element Code. Use of a very powerful finite-element (FE) code to analyze the complete marine drilling system in detail is possible. Such a code has to handle the application of enviromental loads, hydrodynamic behavior, "gap element" wellhead behavior, and nonlinear behavior of cemented conductors. Separate Component Approach. The analysis of a marine drilling system can be simplified by separating this system into individual components and analyzing each one with a dedicated solution code. Thus, riser elements would be analyzed with a time- or frequency-domain riser-analysis program, wellhead behavior would be approximated to an equivalent beam-element model with nonlinear gaps, and conductor behavior would be modeled as a grouted pile with a standard soil/pile interaction program. Complexity of a Single FE-Code Solution. The single FE-code solution probably will be the most valid analysis. The complexity of the data preparation, however, makes this solution unsuitable for all but the most extreme and critical applications. It certainly is not suitable for routine application to well design.