Improving Dynamic Positioning Performance in the Deepwater, High-Current, Rough Water Environment
Authors: Shatto H.L. , Van Calcar H. 
Published: October 2008

ABSTRACT
Several modifications were made to the Discoverer Seven Seas dynamic positioning system in preparation for drilling in the Atlantic Baltimore Canyon, an area noted for severe winter storms, two knot currents and water depths to 7,500 feet. The modifications included the addition of long-baseline acoustic position measurement, active current and cross force compensations and a Kalman filter with optimum state vector estimation based upon simultaneous input of all available position data.
The system's improved accuracy, stability and reliability of position measurement, higher permitted gain, and active forces compensation have produced highly stable and accurate position control, particularly when making quick turns to put the vessel's bow into a storm. An added benefit is a significant reduction in fuel consumption. The baseline transponder deployment, grid calibration and recovery are all accomplished without adding rig time to the drilling operation.

INTRODUCTION
Late in 1981, Shell Offshore Inc., bidding in OCS Sale 59 with Amoco Production Company and Sun Exploration and Production Company, acquired several tracts in the Atlantic Baltimore Canyon area. These tracts, about 100 miles offshore, are subject to sudden and severe winter storms. With water depths ranging to 7,500 feet, the area occasionally experiences-currents up to two knots to depths of 1,500 feet during the passage of warm core rings.
Shell contracted with Sonar Offshore Drilling Inc., for use of the Discoverer Seven Seas. This drillship had been highly successful in establishing new water depth records over the several years prior to Shell's deep water program. These records can be attributed, at least in part, to the fine performance of the ship's dynamic positioning (DP) system. Its recent history (after initial debugging) of about two drive-offs or drift-offs per year from DP failure was in line with that of other prudently operated DP vessels. Nevertheless, in view of the record water depth and severe environmental conditions, expected, Shell undertook an evaluation of both the needs and the potential benefits of various candidate system improvements. Several improvements were deemed worthwhile in the areas of (1) position measurement reliability, and (2) accuracy and tightness of control.
Studies of station keeping requirement for the deepwater program showed that the allowable vessel excursion would be limited by the 50 ft. stroke available in the tensioners. In addition to vessel offset, the available stroke is consumed by variations in tide, riser tension, riser temperature, riser sag, vessel draft, vessel heave, riser space out, and clearance requirements after disconnect. Under the most adverse of these conditions of tide, heave, etc., the remaining stroke limits the allowable vessel offset to about 355 feet or 5.9 percent of 6000 ft. water depth (3.4° bottom riser angle). For these same conditions the alarm for final disconnect in event of a DP failure will be reached at less than 3 percent of water depth.