Sunday, June 26, 2011

Rig Stability during tow or transit

Offshore rig stability is a complicated aspect of naval architecture which has existed in some form or another for past years. Historically, offshore rig stability adopted some of the ship stability calculations and for ships, it relied on rule-of-thumb calculations, often tied to a specific system of measurement. Some of these very old equations continue to be used in naval architecture books today, however the advent of the floaters including rig and ship, model basin allows much more complex analysis.

Stability standards for both ships and offshore rigs ( jackup, semi-submersibles ) are based on a two-tier approach:

• intact stability requirements, designed to ensure that the unit will withstand all expected environmental conditions when in its normal operating or survival condition, and while it remains
undamaged and watertight;

• damaged stability requirements, designed to ensure that the unit will not capsize in foreseeable environmental conditions, after undergoing a limited amount of damage or flooding, and will be capable of returning to the upright condition.

Two alternative approaches are normally adopted when defining damage: damage to any one compartment at any draught, or waterline damage, including breaching of internal watertight divisions between compartments. Both approaches have their strengths and weaknesses. An offshore unit designed to meet the any one compartment standard cannot necessarily be guaranteed to meet the waterline damaged standard, and vice versa.

NMD adopted a three-tier approach. The first two tiers were the established intact and damaged stability philosophies, and the third was a requirement that the unit should withstand loss of buoyancy from either the whole or a major part of one column, but without any requirement to return to the upright position. The objective in this case was to allow the crew time to evacuate the unit. This requirement was expressed in terms of providing a maximum angle of heel after a large loss of righting moment, and a minimum level of reserve buoyancy above the damaged waterline. The concept of providing some level of reserve buoyancy, beyond that necessary to meet basic code requirements, has since been widely accepted.

This information is intended to provide an adequate level of stability during routine operations of floating Installations. The aim is to take account of the most probable damage cases, in particular low energy collisions with supply vessels during loading, towing and anchor handling. Consideration should be given to carrying out an inclining test on the first unit of a design, when as near to completion as possible, to determine accurately the lightship weight and position of centre of gravity. The test will need to be conducted in accordance with an approved procedure.

For successive units of a design which are identical with regard to hull form and arrangement (with the exception of minor changes in machinery or outfit) detailed weight calculations showing only the differences of weight and centres of gravity may be acceptable. However, the calculated changes in weight and position of centre of gravity should be small, and the accuracy of the calculations confirmed by a deadweight survey.
Vessel Stability_1


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