The Wireline Heave Compensator (WHC) is an “active” heave compensation system designed to counter the vertical motion (heave) of a ship caused by waves, thus keeping subseafloor logging tools in a fixed position. Such devices are also commonly referred to as Wave Motion Compensators or WMCs.
Operationally, the WHC utilizes a vertical accelerometer positioned near the ship’s center of gravity to determine whether the ship is moving up or down with respect to the sea floor. The system itself consists of a block-and-tackle type device with one set of the pulleys (called the flying head) attached to the top of a pair of hydraulic pistons controlled by a programmable logic controller running a specialized program. When the system computes that the ship is moving down with a wave trough, it forces the flying head up so that the tools maintain their position; when it detects the ship moving back up at the next wave peak, it retracts the flying head to allow the tool string to maintain position with respect to the sea floor. This system is rated to maintain tool position within approximately +/- 1 ft. in up to 4.5m of heave (peak-to-peak). If the heave is greater than that limit, then it is normally considered unsafe to conduct logging operations.
The WHC is located on the wireline deck, on the starboard side of the vessel just above the drill floor, along with the winch module and the winch operator’s cabin. In addition to the WHC frame itself, there is also a Hydraulic Power Unit (HPU) that provides all of the mechanical energy (hydraulic power) to move the flying head and houses the programmable logic controller that controls the unit. There is a network link to a laptop located in the logging scientists’ office that allows fine tuning of the heave compensator remotely so that it can be adjusted and optimized for prevailing conditions at the beginning of each logging session.