Installation of Heavy Subsea Components Using Submerged Motion Compensator and Multi-Sheave Block
Authors: Mahone W.C. , van Luipen P. , Werenskiold C. 
Published: October 2008

The paper summarizes the results of a study which considered installation of heavy subsea production components on the sea floor. This conceptual study uses a new generation semi-submersible crane vessel in water depths of up to 800 meters. The installation method described utilizes a submersible sheave block with an underwater motion compensator. A computer model of the entire system, with and without an underwater motion compensator, was developed to predict the mot ion of each subsea component as it was landed in its location on the sea floor. The model includes all elements in the system and incorporates the heave and pitch mot ions of the crane vessel as well.

INTRODUCTION

The Subsea Atmospheric System (SAS) is a Mobil designed subsea hydrocarbon production unit undergoing final development by Mobil, Kvaerner and Statoil. This subsea production unit consists of several large components which are required to be installed on the sea floor. An installation system was developed to 1i ft a component in air, lower it through the air/water interface to the sea floor, and finally mate it with another production unit component. The installation system was designed to keep velocities within specified limits during the contact associated with mating. This lowering system consists of a submersible sheave block, an underwater motion compensator, and a wire rope grid to accurately position the component being installed.

Present day modified and new semi-submersible heavy lift crane vessels have the ability toreach water depths of up to 800 meters with their submersible sheave block. The multi-reeved wire rope of this block characteristically has a high spring stiffness, which readily transmits crane boom motions to the subsea production component during installation. The motions of the subsea component, induced by the crane motions, must be attenuated to reduce displacement and velocity amplitudes during touchdown and mating. This motion attenuation is provided by an underwater motion compensator, which IS placed between the component being lowered and the block. This compensated lowering system, as developed, is capable of handling weights of 330 tonnes submerged, with a maximum uncompensated load (compensating piston rod fully extended) of approximately 1000 tonnes.

SUBSEA COMPONENT DESCRIPTION

The SAS unit is a part of a deep water subsea production system which will drain hydrocarbon reserves using conventional subsea well drilling and hydrocarbon producing technology. As a production system, the SAS is discussed in Reference (1).

The SAS unit contains three heavy components, which require assembly on the sea floor. These are the monopile, the base template and the subsea work enclosure (SWE). Using an underwater hydraulic hammer, a single pile (monopile) is driven into the sea floor until three to four meters of it remains above the sea floor. This is achieved by lowering the monopile/hammer assembly to the sea floor with the block, but without the compensator. The monopile is 1 to 2 meters in diameter and 60 to 100 meters in length depending on soils.