scope:

General

The term Semi-Submersibles used in these Guidance Notes refers to:

• Column-Stabilized Drilling Units defined in the ABS Rules for Building and Clasing Mobile Offshore Drilling Units (MODU Rules)

• Column-Stabilized Units defined in the ABS Guide for Building and Clasing Mobile Offshore Units (MOU Guide)

• Column-Stabilized Installations in the ABS Rules for Building and Clasing Floating Production Installations (FPI Rules).

A semi-submersible is a column-stabilized floating offshore structure, which consists of a deck structure with large diameter support columns attached to submerged pontoons. In the design and operation of semisubmersibles, the air gap as defined in Subsection 1/3, also known as the deck clearance, is an important design parameter. A low air gap may lead to wave impact that the structures and equipment of a semisubmersible are not designed for and could potentially cause serious damage. Conversely, an excessive air gap could increase the cost, decrease the stability, reduce payload capacity and impair global performance.

These Guidance Notes provide procedures for the air gap analysis based on either the linear or the second order potential theories, address nonlinear effects on the air gap analysis, and identify industry common design practice. The theories and equations referenced in these Guidance Notes are based on the infinite water depth assumption. However, the procedures described in these Guidance Notes are in general also applicable to finite water depth condition.