Active Heave: The Benefits to Operations as Seen in the North Sea

Abstract

IADC Member Abstract Active Heave Compensation Systems have become significantly more prominent in Semi Submersible Drilling Rig operations over the last 18 months. The increasing number of Retro fit Active Heave Systems being introduced to the Semi submersible fleet has allowed actual comparisons and excellent field evaluation to take place with Semi Submersibles with Active Heave Systems against similar Semi Submersibles with Passive Compensation only. This recent Industry generated interest and increased Active heave utilisation has resulted in statistics being compiled by drilling contractors and operators, these show a reduction in Waiting on Weather time, benefits to a wide range of Drilling and Completion equipment and operations and ultimately savings in well costs. Introduction To describe the Active Heave (AH) System will require to briefly overview the Passive heave compensation system and the definition of the term Active when used as a prefix in this context. Passive Compensation The Passive Drillstring Compensator was designed to allow floating Drilling rigs to maintain a near constant weight on a load while in operation. (fig 1) The Passive Drillstring Compensator can be described as dampening or a low rate spring device used to reduce disturbance or load variation caused by wave motion. The spring or dampening device being a bank of Compressed air and the Mass being the Traveling Block Hook weight (normally a drillstring). (Fig 2 & 2a) The mass or load must be in contact with a surface of the well to react to the vessel motion, it is important to understand that a passive compensator will not react to vessel motion if the load is freely suspended from the floating drilling rig (paramount for the understanding of landing objects on the seabed) and also note that there is no direct ratio between load variation and dampening. Passive heave compensators generally have a load variation of between 8 to 12 % of total load, transcribe this to a Drill string weight or Mass of 450,000 pounds means that the hook load may vary as much as 50,000 lbs before the compensator will actually function correctly. Passive Compensators also are very sluggish when in use with light string weights or mass loads. Active terminology Active when used as a prefix in this context refers to the ability to react and maintain constant parameters, for example as a simile, The Williams Renault Formula 1 racing team had Active suspension fitted to their racing cars for a season, this in real time from input received from sensors adjusted the distance from the car to the race track to allow the car to give optimum traction performance. With this in mind we use the term Active as a prefix with the meaning of being able to react in real time to movement and keeping a high degree of stability. Active Heave Compensation therefore means the ability to keep the mass load (Drillstring) in a stable position in reference to the seabed or bottom of the well throughout all of the floating drilling rig's heave motion movement. The Active Heave Compensator system is made up of the following main components. (1) (figs 3, 3a & 4,4a)–Actuating accumulator cylinder–Heave sensors–Control unit (computer)–Operator Panel–Hydraulic Power unit There are 3 companies which provide Active Heave systems, Retsco International, Maritime Hydraulics and WestTech Gear Corp. All work on the same principle which is, heave sensors send a signal to central computer, the computer senses the wave height and frequency from the signal, it then instructs the hydraulic pumps to adjust hydraulic fluid in the active actuating cylinder, this then controls the position of the hook in real time to the vessels heave motion with a calibration of 80 to 93 percent of wave characteristics and active load variation of 1 to 2 percent. P. 207^