Stability standards, tests, and information are covered in this section. In each instance, the presentation is divided into a brief background of the subject at hand and its applicability to inspected and/or uninspected oceanographic research vessels on either a required or recommended basis.
A basic understanding of the principals of Ship Stability is essential to the safe operation of any vessel, but particularly so for the operation of a research vessel which can be subject to greatly varying deck loads, towing loads and crane loads, all having an impact on stability. For these reasons it is important that the vessel operators understand the basic concepts of stability and that the operating institutions seek and use the services of a qualified Naval Architect whenever stability questions arise.
A vessel’s stability characteristics are based on its hull form, sail area (windage profile), weight (displacement), center of gravity, and free surface of liquids on board. Changes in any of these characteristics will impact the vessel’s stability.
Hull form is rarely changed during the life of a vessel. If a hull form change is needed for a particular operation, e.g., addition of a transducer pod, additional sea chests, etc., a naval architect can evaluate potential impacts to stability.
Sail area can change from voyage to voyage, most commonly by the addition of science or equipment vans. Some vessel’s stability booklets account for the addition of a van by including an alternate required GM curve based on the additional windage area. If a vessel’s existing stability documentation does not account for planned increases in sail area, a naval architect should be consulted to verify that stability criteria can be met.
Weight and center of gravity are typically determined by calculations that add loads to an approved lightship value (determined via stability test). Changes to the lightship weight must be documented in accordance with the guidance given in section 13.3.1 below for inspected research vessels.
Liquids in tanks can reduce a vessel’s stability. Stability letters and booklets typically document tank-loading requirements for a specific vessel including the number of tanks that are allowed to be slack at any one time.
Certain aspects of research vessel operations have adverse effects on stability and must be considered in the form of guidance in the Trim and Stability Booklet or by guidance provided by a competent naval architect:
Both required and recommended stability standards should, in general, be viewed as being minimal. In applying them to the design and operation of individual vessels, they should be upgraded as appropriate considering any unique aspects of the vessel’s mission requirements and/or design features pertinent to stability.
STABILITY STANDARDS: for the design, construction, and operation of oceanographic research vessels may be placed into one of two categories:
Required standards have been set forth by the USCG and by International Conventions (see next section). Generally these take the form of minimum righting energy requirements, minimum GM, and in the case of larger (inspected) vessels, damage stability requirements.
Additionally, vessels engaged in towing and lifting operations must meet additional criteria. Vessels that operate in areas where ice accretion is possible should meet stability criteria with icing loads.
Uninspected vessels with an assigned load line, although not subject to the requirements of Subchapter S, must still demonstrate adequate stability. The requirements of Subchapter S may be used for guidance. Alternatively, the guidelines for fishing vessels may be used, although these criteria may be more demanding than those outlined for oceanographic research vessels in Subchapter S.
STABILITY TESTS: include formal inclining experiments and, in some cases, rolling period tests. Inclining experiments are conducted to obtain “as inclined” data from which “light ship” displacement and centers of gravity can be derived to define the “light ship condition.” This experiment is normally conducted under the auspices of a qualified Naval Architect, and witnessed by the US Coast Guard or its designee, commonly ABS. Various loadings can then be added to this basic condition to obtain prescribed “service conditions” and associated stability information. This becomes the basis for a “Stability Letter” or “Stability Book”.
STABILITY INFORMATION: includes 1) specific information pertinent to the safe operation of a specific vessel and 2) general information, the understanding of which promotes the safe operation of vessels in a more general sense. Specific information is contained in “Stability Booklets” and “Stability Letters,” or their equivalents, which are carried on board. General information should also be carried on board and made readily available to all personnel on board having duties or functions, which may affect the vessel’s stability.
Principal references include Title 46 CFR 170 - Subchapter S, the International Maritime Organization (IMO) Code of Safety For Special Purpose Ships Resolution A.534 (13), and the Commercial Fishing Vessel regulations (46 CFR 28). The last reference concerns commercial fishing vessels but contains much information of value for uninspected oceanographic research vessels.
Requirements are based on sizes and service.
Inspected oceanographic vessels, including motor-driven vessels of 300 and over gross registered tons and steam ships over 65 ft long, must comply with stability criteria set forth in Title 46 CFR, Subchapter S, Parts 170 and 173.
New inspected oceanographic vessels are required to be inclined in accordance with inclining experiment details set forth in 46 CFR, Subchapter S, Part 170, Subpart F.
Any research vessel should be re-inclined any time a significant change in magnitude and/or location of a “light ship” weight occurs or there is a major change in hull shape. For inspected vessels the Coast Guard, in MTN 04-95, requires the following actions based on the amount of aggregate (magnitude of weight removed and weight added) weight change:
Inspected oceanographic vessels are required to carry the following stability information on board as set forth in Title 46 CFR, Subchapter S, Part 170, Subpart D:
The above reference does not specify that vessels engaged in towing are required to carry towing information pertinent to stability. Nevertheless, it is recommended that these vessels carry this information.
Uninspected oceanographic vessels engaged in international or foreign voyages and subject to load line assignment, as described in Chapter 14, are treated as inspected vessels with regard to stability tests and stability information.
An uninspected vessel subject to SOLAS requirements, i.e., over 500 GT Convention Tonnage yet under 300 GRT domestic tonnage will be subject to load line requirements and SOLAS stability requirements. The SOLAS requirements for vessels under 100m (328 ft) in length are not well defined, but compliance with the IMO Code of Safety For Special Purpose Ships would be a reasonable minimum requirement. For a load line, ABS would seek compliance with a “recognized” stability standard such as those outlined in Subchapter S for oceanographic vessels or, alternatively, the requirements contained in IMO’s Code of Safety For Special Purpose Craft.
An inspected vessel subject to SOLAS must meet the stability criteria of Subchapter S.
Other uninspected vessels that are not in “class” have no required stability standards unless they have an assigned load line, in which case ABS will require compliance with IMO A167/A206 with A562 or “recognized” criteria suitable to vessel type, such as Subchapter S criteria for oceanographic research vessels. Standards for these vessels under the RVSS are described below.
It shall be the Master‘s responsibility to maintain the vessel in a satisfactory stability condition at all times through control and management of liquid, solid and science loads. It is the Operating Institution‘s responsibility to insure that (1) current stability data are correct and available to the Master, (2) changes to the vessel are controlled and managed to insure compliance with all regulatory requirements and the recommendations of this section.
As a minimum, a deadweight survey shall be performed every five years. If the deadweight survey shows a shift in LCG over 1% and/or a change in weight over 10%, the vessel must undergo a complete stability test (inclining).
Un-classed and uninspected oceanographic vessels, while not required by regulation to undergo inclining experiments, will be inclined and have sufficient data and documentation to determine safe loading. These vessels will carry operators’ directives containing specific stability information equivalent to that required for inspected vessels.
Uninspected vessels may be divided into two groups: 1) vessels from 79 feet to 328 feet in length, and 2) vessels shorter than 79 feet.
Recommended intact stability standards for group (1) vessels are contained in IMO’s Code of Safety For Special Purpose Craft, which invokes IMO A167/A206 with A562
No firm criteria exist for recommended stability standards applicable to group (2) vessels. Again, criteria set forth in the fishing vessel regulations provide useful guidelines but one must be cautious in their direct use to establish stability standards for these small vessels and it may be necessary to increase IMO Resolution A.168 (ES.IV) criteria. While the basis for this increase has not been established, the practice of some European countries is to increase all criteria by twenty percent.
It is recommended that all uninspected oceanographic vessels carry general stability information on board. Consideration should be given to following the regulations for commercial fishing vessels in 46 CFR 28.
Instructions and data contained in Stability Booklets and Stability Letters, or their equivalents, should be set forth in a clear and concise manner to facilitate stability analysis either by hand or by use of a personal computer. In this regard, it is recommended that vessels be provided with user-friendly stability software for intact and damaged conditions (if feasible) and on board personal computers.
The RVOC Safety Training Manual contains chapters on Stability and on Load Lines and Watertight Integrity that could be used to help in understanding the principals covered by regulations and guidelines. The information provided includes diagrams and example calculations as well as useful safety practices that will minimize or eliminate adverse effects on stability. The North Pacific Fishing Vessel Operators Association (NPFVOA) also publishes a safety manual and a series of videotapes that are excellent training resources with regard to stability on smaller vessels.
For those interested in learning more about the basics of vessel stability, the following publications are recommended: