Abstract

Replacement of existing steel choke and kill lines on a drilling riser string with composite lines will result in weight savings that will allow third generation mobile offshore drilling units to operate in 6,000 feet of water depth without undergoing major upgrades. The design and analysis, fabrication and qualification testing of high pressure (15,000 psi) composite choke and kill lines is discussed. The objective is to develop a low-cost composite design of reliability equivalent to that of existing metal systems.

Introduction

Replacement of the steel choke and kill lines on a standard drilling riser string with composite tubulars will result in a 15%-20% weight saving. Table 1 is a weight comparison of a typical 4.5" steel choke and kill line with a comparable composite line. This straightforward substitution would allow a third generation mobile offshore drilling unit (MODU) to operate in water depths as great as 6,000 feet without major upgrades. One of the limiting factors in taking a MODU out into deepwater and ultra-deepwater is the deckload capacity of the vessel. Mooring lines, riser joints, hook, and variable loads all contribute to this weight. A variety of other limitations require third generation semisubmersibles to undergo major upgrades before they can operate in water depths as great as 6,000 feet.

For composite lines to be widely accepted, they must be designed to be 100% interchangeable with existing steel lines, with no modifications required to the riser pipe body, flange, brackets or buoyancy modules. When installed on a steel main riser pipe and flange joint, these composite lines create a hybrid drilling riser system. Such hybrid riser joints would then be completely interchangeable with the all-steel riser joints of a deepwater riser system.

The market focus for a hybrid riser system is very specific. The drilling and production units being targeted are those third generation rigs on the edge of deepwater that want to be able to drill in 6,000 feet of water without undergoing major upgrades. The use of hybrid riser joints will lighten deckload, hookload and craning requirements for these MODU"s. Equally important is the reduction in the volume of syntactic foam buoyancy needed to support the riser joints underwater. Foam modules with a smaller outside diameter can be used, allowing the hybrid riser to be run by many of the vessels operating on the fringe of deepwater which still have 49.5- inch rotary tables. Another advantage to the use of smaller diameter buoyancy is the narrower profile of the riser string in the water, which reduces the drag forces of the sea currents.

Composite Choke and Kill Line Requirements

The API specification for choke and kill systems (Ref. 1) states that: "Choke and Kill Lines are an integral part of the surface blowout prevention equipment required for well control. The kill line provides a means of pumping fluid into the well bore when normal circulation through the drill string cannot be employed.

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