NVIC 12-83: Research results on towing and fishing vessel stability

NVIC 12-83 15 Nov 1983 NAVIGATION AND VESSEL INSPECTION CIRCULAR No. 12-83 Electronic Version for Distribution Via the World Wide Web Subj: Intact Stability of Towing and Fishing Vessels; Research Results Ref: (a) Roach, C.D., "Tugboat Design," Trans. SNAME, l955. (b) Miller, E.R., Ankudinov, V., "Evaluation of Current Towing Vessel Stability Criterion and Proposed Fishing Vessel Stability Criteria - Task III," HYDRONAUTICS, Inc. Technical Report 7311-3 (NTIS No. AD A019-831). (c) McGowan, J. F., Meyer, R.B. ,"Has Stability Delayed the Delivery of Your Tug?," Marine Technology, SNAME, Jan. 1980.

Intact Stability of Towing and Fishing Vessels; Research Results — PURPOSE. The purpose of this Circular is to formally publish the results of Coast Guard

sponsored research on intact stability criteria of towing and fishing vessels. This Circular is not intended as regulation nor are the research findings offered as a substitute for regulation. This Circular will provide a medium for exchange of research information regarding intact stability criteria for the design and consultant community.

Intact Stability of Towing and Fishing Vessels; Research Results — APPLICATION. The stability criteria recommended by the researcher may be applied to tugboats,

offshore supply vessels, fishing vessels and other similar hull forms of approximately 90 to 180 feet in length which are engaged in towing operations. Application to vessels outside that range should not be used without further investigation.

Intact Stability of Towing and Fishing Vessels; Research Results — BACKGROUND.

a.

In the late 1960's, the Coast Guard published the "Weather Criterion" (46 CFR 170.170) for cargo and miscellaneous vessels.

This regulatory standard requires a minimum metacentric height (GM) as protection against the hazard of wind heeling forces.

The application of this criterion was originally intended for cargo ships of conventional size, arrangement and form. b.

A provision (46 CFR 170.173) is included in that regulation for establishing equivalent levels of safety for "special case" cargo and miscellaneous vessels to which the Weather Criterion does not apply.

Such vessels are those of unusual proportion and form, those which may carry cargo above decks, and those which may be involved in special operations such as towing.

NAVIGATION AND VESSEL INSPECTION CIRCULAR No. 12-83 c.

Since January 1974, the Coast Guard has been applying either of two towline pull criteria to vessels which are engaged in towing operations.

One of those is a GM or "static" criterion which was derived from a theoretical formula by RADM C.P.

Murphy which is discussed in references (a) and (c).

The other alternative criterion is a required moment area or "dynamic" criterion based on the same physical principle of capsize.

Both are offered as protection against the hazard of self-tripping. d.

There are no Federal regulatory stability standards for fishing vessels.

The Coast Guard has been active in the development of recommended standards at the International Maritime Organization (IMO).

Those international recommended standards have been published as IMO Resolution A.168 (NVIC 3-76) and the International Conference on Safety of Fishing Vessels, 1977. e.

In January 1976, a Coast Guard funded research project on the stability of towing and fishing vessels was completed.

The results of that project are published in a three volume report which is concluded in reference (b). f.

The research project surveyed the U.S. towing (including tugboats and offshore supply vessels) and fishing fleet and prepared detailed stability calculations for 51 such vessels as representative samples.

Four of those representative vessels were extensively model tested. g.

In the final report, reference (b), five stability criteria are recommended, all of which are published in Enclosure (1) of this Circular.

Intact Stability of Towing and Fishing Vessels; Research Results — DISCUSSION.

a. Regarding the application of any stability criterion to the complexity of a vessel/seaway system, it is generally accepted that total safety or reliability is not possible to attain. The prevailing philosophy is that a vessel should be provided with sufficient stability to survive extreme case service conditions if it is handled competently by the crew and if no equipment or material failure occurs. b. It must be understood that criteria which are based on a limited test program and analyses are necessarily tentative. c. The following guidelines and assumptions were used by the researcher in the development of the recommended intact stability criteria:

(1) The criteria are only to apply to towing, fishing and supply vessels which are of a size and form covered by the analysis or experiments that form the basis of the criteria.

(2) Stability criteria which apply to operations at sea should result in a stability level which reduces the probability of capsize for a properly handled vessel in a defined extreme sea condition.

(3) Proper handling of the vessel includes compliance with conditions assumed in the stability calculations (i.e., openings closed, tanks pressed up) and the avoidance of local areas where unusual conditions exist such as over shoals or in surf. NAVIGATION AND VESSEL INSPECTION CIRCULAR No. 12-83

(4) Only hazard situations which logically exist together, such as high winds with large waves, need to be considered. However, a margin should be added to the stability level predicted for capsize in extreme conditions to account for uncertainties in the analysis.

(5) Deterministic extreme sea conditions can be defined on the basis that, for wave lengths which are critical, the wave steepness is limited by wave breaking.

(6) The use of extreme conditions and margins in the criteria supplies a reasonable allowance for crew error or equipment failure in less than the most extreme conditions.

(7) Stability criteria which apply to normal operations, such as the towing vessel tripping criteria, should prevent capsize of a vessel which is not properly handled by its crew or which is subject to errors by others, up to arbitrarily defined limits.

(8) The resulting criteria should not be so stringent that large segments of the fleet could not satisfy them and still operate economically. d. There are two basic types of stability criteria, namely general criteria and specific criteria. The general type criterion, such as Rahola and the IMO criteria, published in NVIC 3-73, is usually applied to provide protection against many capsize hazards and is based on data from the casualty history of a group of vessels. A specific type criterion is one in which the measure and level of stability is defined to prevent a certain type of capsize hazard under given environmental conditions. Specific criteria should be applied as a set to take into account those hazards that a particular vessel may be expected to encounter. e. The recommended intact stability criteria in Enclosure (1) were developed on the concept of a set of specific criteria. The major reasons that the researcher did not choose the general criteria were:

(1) the questionable statistical base available for use in general experience criteria, and

(2) the lack of information on the effects of specific environmental conditions which is inherent with general criteria. f. The set of specific criteria recommended are as follows:

(1) Tow Tripping Criterion - This is directed at the hazard caused by action of the tow or assisted vessel relative to the towing vessel.

(2) Self Tripping Criterion - This is directed at hazards caused by improper application of power and rudder angle by the towing vessel.

(3) Low Speed Operation with Water on Deck Criterion - This is directed at the hazard of operations at low speed in head and following seas in combination with a buildup of water on deck causing large angles of heel. NAVIGATION AND VESSEL INSPECTION CIRCULAR No. 12-83

(4) Operation in Following Seas at Moderate and High Speed Criterion - This is directed at the hazards of operation in following and quartering seas at speed/length ratios of about 0.8 and higher.

(5) Wind Heel with Rolling Criterion - This is directed at the hazards of a vessel being subjected to a wind gust while rolling. g. The following notes apply to the above criteria:

(1) All five criteria are based on GZ curves calculated assuming a constant trim moment rather than constant trim with heel.

(2) In all cases, the vessel's full range of loading conditions (including free surface) should be investigated. Reference (c) contains recommendations for minimum loading conditions to be considered on tug and tug supply vessels. Unusual loading conditions such as topside icing should be included as appropriate.

(3) Special operating conditions other then towing, such as lifting heavy weights over the side, dragging fishing nets, off-center passenger load, etc. should be checked separately if appropriate.

(4) Both the tow tripping and self tripping criteria are intended to apply to conventional tug, tug supply and fishing vessels when involved in towing operations. The criteria apply to such vessel types with single or twin screw propulsion, with or without nozzles, but they do not apply to such vessels with paddle wheels or vertical axis propellers.

(5) The downflooding point for tow and self tripping should be considered the lowest edge of an opening giving access below deck (i.e., door, tank vent, engineroom vent, etc.) not fitted with an autoclosure. Downflooding for all other criteria should be any opening not fitted with a weathertight or watertight closure. Reference (c) provides further discussion of the location and type of downflooding points of vessels that tow. h. The recommended criteria as a set or individually may offer greater vessel design flexibility than existing general criteria. This is because each type of hazard faced and the specific design features which influence the degree of hazard are defined and tested. Therefore, for each specific criterion there are fewer unknowns and consequently a less conservative safety factor required. i. A fleet impact analysis in reference (b) determined that the recommended criteria required, on the average, 10 percent less GM than the existing USCG "static" towline pull criterion. No comparison was made to the USCG "dynamic" towline pull criterion.

Intact Stability of Towing and Fishing Vessels; Research Results — ACTION. Coast Guard personnel are encouraged to provide wide dissemination of these research

criteria, but are cautioned not to accept them in lieu of published regulatory standards. Designers and consultants are encouraged to consider these recommended criteria for specific design applications and to compare them to present standards for towing and fishing vessels. The Coast Guard would appreciate receiving comments on any operating experience, research, or economic impact studies related to the application or suitability of these criteria. NAVIGATION AND VESSEL INSPECTION CIRCULAR No. 12-83 End: (1) Stability Analysis Methods for Towing and Fishing Vessels. NON-STANDARD DISTRIBUTION: Ce: Baltimore (45); Alameda (40); Port Arthur, Honolulu, Seattle (35); Miami, Mobile, Long Beach (25); Norfolk, Jacksonville, Portland OR (20); Boston, Portland ME, Charleston, Anchorage (15); Cleveland (12), Cincinatti, Louisville, Memphis, Nashville, Paducah, Pittsburgh, St. Louis, Savannah, San Juan, Tampa, Galveston, Buffalo, Chicago, Detroit, Duluth, Milwaukee, San Diego, Juneau, Valdez 10); Providence, Huntington, St. Paul, Wilmington, Corpus Christi, Toledo (5). C:m New Orleans (140); New York (70); Philadelphia (35); Houston (25); St. Ignace (5); Sturgeon Bay (4). D:1 CG Liaison Officer MILSEALIFTCOMD M-65 STRAT MOB, CG Liaison Officer JUSMAGPHIL (1). ZTC-68