Standard Guide for Escort Vessel Evaluation and Selection
This guide presents some methodologies to predict the forces required to bring a disabled ship under control within the available limits of the waterway, taking into account local influences of wind and sea conditions. Presented are methodologies to determine the control forces that an escort vessel can reasonably be expected to impose on a disabled ship, taking into account the design of the ship, transit speed, winds, currents, and sea conditions. In some instances, this guide presents formulae that can be used directly; in other instances, in which the interaction of various factors is more complicated, it presents analytic processes that can be used in developing computer simulations.
Unlike the more traditional work of berthing assistance in sheltered harbors or pulling a “dead ship” on the end of a long towline, the escorting mission assumes that the disabled ship will be at transit speed at the time of failure, and that it could be in exposed waters subject to wind, current, and sea conditions.
The navigational constraints of the channel or waterway might restrict the available maneuvering area within which the disabled ship must be brought under control before it runs aground or collides with fixed objects in the waterway (see allision).
The escort mission requires escort vessel(s) that are capable of responding in timely fashion and that can safely apply substantial control forces to the disabled ship. This entails evaluation of the escort vessel's horsepower, steering and retarding forces at various speeds, maneuverability, stability, and outfitting (towing gear, fendering, and so forth). This guide can be used in developing escort plans for selecting suitable escort vessel(s) for specific ships in specific waterways.
The methodologies and processes outlined in this guide are for performance-based analyses of escort scenarios. This means that the acceptability of a vessel (or combination of vessels) for escorting is based upon the ability to control the disabled ship in accordance with specified performance criteria. This guide addresses four selected performance measures:
Towing—the ability to tow the disabled ship under specified parameters,
Stopping—the ability to stop the disabled ship within specified parameters,
Turning—the ability to turn the disabled ship within specified parameters, and
Holding steady—the ability to hold the disabled ship on a steady course under specified parameters.
The “specified parameters” are additional details that must be factored into the performance analysis. These parameters might be specified by a regulatory agency imposing the escort requirement, by a study group evaluating the feasibility of escorting in a particular waterway, or by the ship or escort vessel operators themselves to define the performance envelope of their vessels. Some examples of these parameters are:
A ship transit speed (at the moment of failure);
The failure scenario (rudder failure alone, or simultaneous rudder/propulsion failure, degree of failure, and so forth);
Navigational constraint within which the disabled ship must be brought under control (such as allowable advance and transfer, cross-track error, and so forth);
Wind, current, and sea conditions; and
Time delays, failure recognition, decision making, escort vessel notification, escort vessel positioning, achieving full power, and so forth.
The anticipated users of this guide are:
Ship owners/operators who are required to select escort vessel(s) that meet the performance measures addressed by this guide.
Escort vessel designers/operators who need to evaluate the performance capabilities of their vessels with respect to the measures addressed by this guide.
Regulatory agencies that have imposed the performance measures in this guide in a particular waterway to develop suitable escort vessel matrices for various sized ships in the waterway.
Enforcement agencies can use this guide to confirm/verify compliance with the performance measures (that is, that suitable escort vessel(s) are being selected).
Study groups can use this guide to explore the feasibility and effectiveness of escorting as a means of mitigating risk on a particular waterway.
This guide does not address the use of escort vessels with barge fleets or barge tows. However, some sections of this guide would be useful if an evaluation of escort vessels with barge shipments were undertaken. Paragraphs 5.4 and 5.5, and all of Section 6 would apply in this type of analysis.
The methodologies and processes presented in this guide will yield valid solutions to the performance measures. This means that the selected escort vessel(s) can reasonably be expected to control the disabled ship within the specified parameters. However, users are reminded that other circumstances surrounding the disabling incident may still preclude the escorts from safely responding (such as fire).
The methodologies in this guide are not necessarily the only ones that can be used to find solutions for the performance measures. There may be other analytic approaches that also will yield valid results. It is hoped that as these alternative methods are developed, they will be incorporated into this guide.
1.1 This guide covers the evaluation and selection of escort vessels that are to be used to escort ships transiting confined waters. The purpose of the escort vessel is to limit the uncontrolled movement of a ship disabled by loss of propulsion or steering to within the navigational constraints of the waterway. The various factors addressed in this guide also can be integrated into a plan for escorting a given ship in a given waterway. The selection of equipment also is addressed in this guide.
1.2 This guide can be used in performance-based analyses to evaluate:
1.2.1 the control requirement of a disabled ship,
1.2.2 the performance capabilities of escort vessels,
1.2.3 the navigational limits and fixed obstacles of a waterway,
1.2.4 the ambient conditions (wind and sea) that will impact the escort response, and
1.2.5 the maneuvering characteristics of combined disabled ship/escort vessel(s).
1.3 This guide outlines how these various factors can be integrated to form an escort plan for a specific ship or a specific waterway. It also outlines training programs and the selection of equipment for escort-related activities.
1.4 A flowchart of the overall process for developing and implementing an escort plan is shown in Fig. 1. The process begins with the collection of appropriate data, which are analyzed with respect to the performance criteria and in consultation with individuals having local specialized knowledge (such as pilots, waterway authorities, interest groups, or public/private organizations, and so forth). This yields escort vessel performance requirements for various transit speeds and conditions; these are embodied in the ship's escort plan. When the time comes to prepare for the actual transit, the plan is consulted in conjunction with forecast conditions and desired transit speed to select and dispatch the appropriate escort vessel (or combination of vessels). A pre-escort conference is conducted to ensure that all principal persons (ship master, pilot, and escort vessel masters) have a good understanding of how to make a safe transit and interact in the event of an emergency.
1.5 This guide addresses various aspects of escorting, including several performance criteria and methodologies for analyzing the criteria, as well as training, outfitting, and other escort-related considerations. This guide can be expanded as appropriate to add new criteria, incorporate “lessons learned” as more escorting experience is gained in the industry, or to include alternative methodologies for analyzing the criteria.
1.6 This guide addresses physical control of the disabled ship with the assistance of the escort vessel(s). Other possible functions, such as firefighting, piloting, or navigational redundancy, are outside the scope of this guide. Also, this guide was developed for application to oceangoing ships in coastal waterways; it is not suitable for application to barge strings in riverine environments.
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