A Comprehensive Approach to Deepwater Marine Riser Management
Authors: Denison E.B. , Kolpak M.M. , Garrett D.L. 
Published: December 2007

A system for riser management has been developed and used that comprises (1) a variable buoyancy system, (2) hardware to hang off a disconnected marine riser near the keel of a drillships (3) a heave acceleration prediction capability, (4) instrumentation for monitoring heave acceleration and riser buoyancy, tension, pressure, vibrations, and inclination, and (5) a technique for pressure, vibrations, and inclination, and (5) a technique for making hang-off and disconnect decisions based on forecasts and real-time vessel motion measurements.

Introduction

When conducting a deepwater drilling operation from a dynamically positioned (DP) vessel, riser disconnect can become necessary at positioned (DP) vessel, riser disconnect can become necessary at any time. It can be caused either by loss of station or by heave in excess of tensioner/slip joint stroke capability. Drilling vessels supporting disconnected long marine risers in heavy seas are vulnerable toriser buckling,overloading the attachment point to the vessel,moonpool impact/riser bending, andriser handling problems.

All of these problems are minimal with a short riser because when drilling in relatively shallow water ( less than 1,000 ft [300 m]) with an anchored vessel, one can plan to disconnect and pull the riser for major storms. However, in 5,000- to 7,500-ft (1500- to 2300-m] water depths with a DP ship, this is not the case. Pulling the riser requires at least 36 hours, which approaches the range of reliable weather forecasts. One assessment of winter storms in the mid-Atlantic region (Wilmington Canyon) indicated that we would need to pull and secure the riser six or more times during the winter to prevent riser damage-if we had sufficient warning. The expected downtime for pulling and running the riser was unacceptable. Furthermore, the pulling and running the riser was unacceptable. Furthermore, the mid-Atlantic is an area of rapidly developing storms, which often can preclude pulling a long riser. Therefore, it became necessaryto develop a method of surviving storms without pulling the riser,to improve forecasting, andto develop a systematic, logical approach for operating the marine riser.

The specific constraints can be summarized asdrilling in the mid-Atlantic in 5,000 to 7,500 ft [1500 to 2300 m] of water in year-round weather,use of Discoverer Seven Seas drillshipsuse of semi-conventional riser system, andstartup of the drilling operation in less than 1 year.

Background

A neutrally buoyant riser is desirable for minimizing top tension requirements and riser stresses. However, if it is disconnected from the blowout preventer (BOP) stack or wellhead and suspended from the vessel, neutral buoyancy is detrimental. If the riser is attached to the vessel, it must move up and down with the vessel. Newton's Second Law of Motion is

where m is the total riser system mass, a is the vertical acceleration of the riser-the same as the vessel's heave acceleration, and F is the resultant vertical force on the riser. Obviously there must be a cyclic vertical force applied to the riser to make it follow the vessel. The upward force applied to the riser by the vessel is limited either by the strength of the attachment point or by the tensile strength of the riser. Buckling of long, slender tubes is well understood. Since compression in the riser can lead to failure, the vessel cannot be allowed to exert a large, downward force on it. The only practical way to achieve a net downward force is to give the riser system a net in water weight, which obviously cannot be concentrated near the top of the riser or the buckling problem will remain. Thus, it becomes apparent that heave acceleration of the vessel is the most important vessel motion. The force/mass ratio or weight/mass ratio of the riser determines whether it is capable of falling with the vessel and not being pushed or compressed. When the vessel accelerates upward, the tension at the top of the riser must include both the riser's in water weight and the force needed to accelerate it upward. Many techniques for increasing the weight and, therefore, the weight/mass ratio, have been proposed and/or developed:bare (unbuoyed) riser joints,variable buoyancy (air can) riser joints,heavy weights on the Lower Marine Riser Package (LMRP), andheavy drilling mud inside the riser.

JPT

P. 835