Effectiveness of Passive Heave Compensation in a Deepwater Environment

Abstract

Abstract The technical advantage of a method of providing heave compensation for deepwater installation operations is presented. This paper explores the use of passive heave compensation (PHC) as an alternative to active heave compensation (AHC) to improve seabed landing. The theoretical modeling of such systems is shown to increase vessel operability and therefore reduce installation time and costs. When a subsea package is to be installed on the seabed, the landing velocity or acceleration may be restricted due to various reasons, such as the possibility of disturbing the soil or damaging any sensitive components in the package. This may limit the permissible seastate for installation and restrict vessel operability. In order to improve operability, active heave compensation systems are often used to bring down the landing velocity and acceleration The effectiveness of passive heave compensation (PHC) for seabed landing is assessed analytically in a variety of global locations. It is concluded that passive heave compensation systems can be effective in many regions and applications where achieving soft landing of subsea packages is important during subsea installation to improve operability. This paper demonstrates that passive heave compensation systems are a viable alternative to active heave compensation systems. Introduction There is an increasing need to lower, install and retrieve larger packages to greater water depths from vessels offshore. As a result, there is increased scrutiny on the integrity of the cranes and winches during these operations, especially as these assets are used closer to their rated capacity. There also appears to be increasing demand on the integrity of installed packages and to position these within tighter tolerances on the seabed, calling for better prediction, control of package displacement parameters and line tension. The major displacement of the package during lowering, installation or retrieval operations is in the vertical (heave) direction of the vessel from which it is suspended. Compensating for the heave of the vessel from which the structure is installed can therefore reduce the displacement of the package. The motion parameters and line tension can also be influenced by introducing suitably tuned devices in the line which can additionally absorb any amplified dynamic loads. Package motions and line tensions assume importance at three key stages during a lowering operation; as the package passes through the splash zone, as it passes through resonance, and as it approaches touchdown on the seabed. In shallow water, shorter wire length causes the boom tip and the package to move in phase. However, in deeper water as the wire is much longer the package motions are no longer solely a function of boom tip motions, indeed the package can oscillate at the system (i.e. crane structure, wire and package) natural frequency.