The Trident Stabilized Vessel Concep for Offshore Drilling and Construction Operations
Authors: Stenger J.J. 
Published: October 2008

A new, twin-hulled vessel design has been evolved by the Author. This design, which emphasizes stability, mobility and station-keeping ability, can be used to provide a self-propelled, floating platform for such offshore operations as drilling, underwater construction, rig service, or other marine engineering applications. The shape of the hulls is such that waterline area is minimized, and the buoyant portions are below the level of maximum wave action. A pair of horizontal, airfoil-shaped struts between the hulls provides damping as well as improving the vessel's speed characteristics while under way.

A 133-foot vessel utilizing this design concept has been constructed and sea trial results have proved the salient features of the design.

Introduction

Stability in rough seas is an important design consideration for vessels used in offshore drilling, construction, and support operations. Particularly in areas such as the North Sea, where the prevailing conditions are frequently quite rough, significant problems develop in maintaining a floating drilling unit on station. Additionally, the rough seas often make the supplying of drilling units from smaller vessels very difficult. Both for deep offshore drilling vessels and for offshore supply vessels it is desirable to maximize stability, mobility, and station-keeping ability. Floating drilling vessels typically emphasize either stability at the expense of mobility, as in the "space frame" designs, or mobility at the expense of stability, as in the conventional "drill-ship" designs. Station-keeping is accomplished in shallower waters with anchors. However, for the deep offshore, a system for dynamic positioning becomes essential.

The expression, "dynamic positioning" refers to the capability of a floating unit to keep herself stationed over a fixed point on the seabottom within certain limits. If weather conditions do not become abnormal, it must be possible to stay over the chosen location with the available means of propulsion, and one must be able to vary the resultant thrust of the combined propulsion units on a continuous basis. This must be executed in a manner whereby the external forces of waves, wind and currents causing translations along the two horizontal axes and rotation around the vertical axis are opposed in such a way that the resultant motions will be kept within specified limits. With dynamic positioning, a drilling vessel is not limited by depth of water, in contrast to methods using spuds, legs, caissons or anchors to prevent drifting from a location.

The designer of an offshore drilling vessel must seek to minimize potential down-time due to bad weather conditions. To the designer, this means that translations along and rotations around the ship's three axes must remain within specified limits, and with all motions reversing continuously, accelerations must none the less remain small. Resonance must be prevented; consequently, the shape of the vessel should incorporate as much damping against translations and rotations as feasible.

In order to minimize the time and resultant cost involved in moving from one remote location to another, a drilling vessel's speed and mobility become important. Hence, drag should be minimized. Building costs are also very important.