Structural Design of Floating Production and Drilling Platforms
Authors: Almeland I.B. , Lefranc Marc 
Published: October 2008

A number of floating production and drilling platforms have been built during recent years. These types of platforms appear to be favored for developing deep water fields. A floating platform is governed by the hydrodynamic considerations determining the size and geometry of the hull. In addition, the topside design is determined by production and operation requirements. It is often the structural engineer's task to integrate the sometimes divergent requirements. Significant structural issues are therefore important to consider when a floating platform's overall geometry is chosen.

INTROOUCTION

Floating platforms consist of two distinct parts; the hull and the topside, which often are analysed separately. Governed by hydrodynamics and layout requirements respectively, the hull and topside are structurally designed to work together. The hull will introduce bending moments and shear forces to the topside which must be designed to accommodate these forces. Fatigue may be a critical design issue for the topsides, in particular related to platforms with large displacements.

Ballast system requirements are important to accommodate, especially during the temporary phases. Ballasting is actively used in controlling hull deformation during deck mating, and for controlling the stability of the platform during all phases. The connections of the topside to the hull are of particular importance. Design requirements of the mating area include; strength, fatigue life, mounting access and minimizing hook-up work.

Floating production and drilling platforms are complex structures. The design is a compromise between hydrodynamic, operation and structural aspects. It is, however, important to consider the platform as an entity with interdependent components.

GLOBAL SIZING OF A FLOATING PRODUCTION PLATFORM

Compared to a fixed platform which carries all loads on legs standing on the sea floor, a floating platform carries the load by buoyancy of the hull. The hull can be formed in many shapes to satisfy the layout requirements of the deck and the safety aspect for a platform. The latest generation of floating production and drilling platforms has been designed with only four main columns. This design has met the needs for stability, for strength and functional requirements; and structurally speaking, has made a simpler design compared to the previous multicolumn concept, see fig.1.

The columns are tied together below the waterline by pontoons only or by a combination of pontoons and trusses. Main loads acting on the hull are wave forces, mooring forces, and the weight of the topside. The hull must be strong enough to withstand these loads. Special care must be taken for the dynamic loads which are critical for all connections in the hull.

When sizing a floating production platform, special attention must be given to the weight balance. This means that any tonne which is added to the platform must be compensated for by extra buoyancy in the hull. Accordingly, it is of major importance to keep the weight budget under control; otherwise it is easy to end up with a platform which does not serve the intended purpose. The volume distribution in the hull is another important factor.