Development and Testing of an Active Motion Compensation System for Production System Support

Abstract

ABSTRACT An active motion compensation system has been successfully tested which will allow hardware such as control pods and choke modules to be lowered and mounted to seabed installed hardware from a DP monohull multi-service vessel. The compensation system attenuates vessel vertical heave effects on the liftline and sub sea equipment in up to sea state five. This system enables an MSV to provide cost effective support to deepwater production facilities by allowing shock and vibration sensitive sub sea equipment to be run, set and recovered from the MSV. Testing instruments monitored the vessel, liftline, a subsea test package and the compensator control. Offshore testing is essential to determine the system's capabilities and suitability for intended purpose. INTRODUCTION Production systems are being installed sub sea at greater depths and in more severe climates. At depths greater then 1,000 FSW, dynamically positioned (DP) monohull multipurpose service vessels (MSV) can provide cost-effective support, work overs and maintenance to control and maintain sub sea systems. A monohull MSV equipped with a motion compensation system can provide a cost-effective alternative to drill ships and semi submersibles. The motion compensator pre-engineeringspecified the operating parameters, including a testing program to coincide and guide system development, and confirm that operating parameters were met. The design and operating parameters are as follows:Working depth of 4,000 FSWHandle loads from 2,000 to 40,000 poundsNot sensitive to load variationsVessel heave periods of 6 to 13 secondsDouble heave amplitude of 15 feetMaintain sub sea package +/- six inches from a fixed referenceMaximum velocity of sub sea package is 1 foot/second (ft/sec)Interface with existing winch systemFit in deck space allotted Due to the wide load range specified, the requirement to set down and recover equipment, and the level of compensation required, an active motion compensation system was selected rather than a passive unit. The handling system layout, including the winch, sheaves, and motion compensator is shown on Figure 1. Figure 2 shows the details of the motion compensator hydraulic skid. The active motion compensator's hydrauliccylinder drives a traveling sheave assembly that interfaces with the liftline. In the compensating mode as the vessel heaves down the cylinder rod retracts, shortening the liftline. As the vessel heaves up the cylinder rod extends paying out cable. An accelerometer package mounted by the over boarding sheave measures vessel vertical motion. The compensator control for processing accelerometer outputs 'is located on the bridge. outputs are double integrated to determine vessel heave displacement. This signal regulates cylinder hydraulics so the traveling sheave mirrors vessel heave. The motion compensator control process is illustrated in Figure 3.