Deepwater Vertical Pipelay From a Dynamically Positioned MODU (1991)

ABSTRACT Exploration and production vessels have demonstrated the technical capability to operate in water depths greater than 3,000 feet. However, extended pipe lay technology to deep water is just emerging as existing pipe lay equipment is modified. Major modifications include providing vertical support of the pipeline and dynamic positioning capability. Dynamically Positioned (DP) Mobile Offshore Drilling Unite (MODU) already possess these features. Vertical welding procedure and pipe-span monitoring also must be developed. This paper examines the requirement to outfit a DP MODU for use in the vertical pipe lay mode, while maintaining the capability of working in the drilling mode. A model pipeline project based on field data for a Gulf of Mexico location is also presented showing that the modified DP MODU can lay pipe in deep water as economically as present pipe lay vessels working in shallower water. INTRODUCTION In today's search for new sources of oil and gas, interest is growing in deep water areas of the Gulf of Mexico and various oceans throughout the world. Exploration and production vessels have been technically capable of drilling wells in deep water for over a decade. Deep water pipeline technology has been evolving to allow an economical means for gathering the products from these new discoveries. In their paper which was based upon the results of Shell Oil Company's "Deepwater Pipeline Feasibility Study" (1974-1977) Langner and Ayers concluded that "...the J-Lay technique utilizing a converted dynamically positioned drilling chip or semisubmersible, or a purpose-built J-Lay vessel, has the greatest potential for extending offshore pipelines into extreme depths, say greater than 6000 feet" [1]. In 1990, Sedco Forex undertook a study jointly funded by Shell Offshore and Sedco Forex to develop the system of Vertical Pipelay from a Dynamically Positioned (DP) Mobile Offshore Drilling Unit (MODU). This paper presents the highlight of the study. The equipment specifications and accompanying procedures are a schematic development to demonstrate technical feasibility and economic attractiveness of the DP drillship conversion. Comparison of S-Lay and J-Lay Configurations The purpose of a stinger in the S-Lay configuration, Figure 1, is to control the deflection of the pipe in the overbend region above the pipeline inflection point in order to return the angle of the pipeline at the surface to the horizontal. Deeper water depths will result in a steeper lift-off angle of the suspended pipe span at the stinger tip. This will require the stinger to be longer and/or more curved to accommodate the greater arc of reverse curvature in the overbend region. Accordingly, greater stinger buoyancy and/or structural strength will be necessary to support the increased weight of the suspended pipe span [1]. Conversely, the J-Lay method, Figure 2, offers an alternative to the conventional lay barge in which the stinger requirements for deep-water are greatly reduced. The purpose of a stinger in J-Lay configurations is to change the angle at top of the pipeline to a vertical orientation.