Rules for Classification of Ships, July 2006 Pt.7 Ch.1 Thruster Survey Issued 7 February 2007
This report contains DNV s overall interpretation of the consequences of the many new DNV rules impacting the DNV fleet today. Our goal is to help our customers stay up to date, while at the same time clearly state what DNV regards to be the motivation behind the changes, and point out the most important changes. Please note that the interpretations of the rules and regulations made by DNV and presented in this report are generic, meaning that special exceptions or novel designs are not included. To obtain information regarding such special cases, please contact DNV s experts listed in the contact section of this report. New Rules for Thruster Survey The DNV Rules of July 2006, in force from January 1 st this year, contain some changes with respect to the survey of thrusters. This document will attempt to highlight and comment the main changes. The previous Rules were defining the Thruster survey as full scope & reduced scope, which were as a main rule done at alternate surveys with a 2,5 year survey interval. Exceptions were given for low number of running hours. This principle remains but now without any exceptions. (See table on following pages) The Rules have three survey scopes, reduced and full scope with 2,5 year schedule and also a scope called overhaul. The latter is applicable whenever, no matter the reason, a thruster is dismantled. The overhaul survey has no due date. As in the previous rules, thrusters are subject to full survey at overhaul. Parts of the full scope survey have however been moved to the survey at overhaul. Among others NDT has been moved, since this is indeed only possible when the thruster is dismantled. The overhaul survey has no due date, since it is not our intension to regulate overhaul intervals in our Rules. Thus the normal full scope survey does not require extensive dismantling, unless deemed necessary by the Surveyor, e.g. due to suspected damages. On the following pages we have given a comparison between the previous and current rule versions.
206 Requirements for external survey are included in scope for bottom survey, see F207. Internal survey of outboard parts, see 207-208, is recommended to be carried out concurrent with bottom survey CURRENT Pt.7 Ch.1 Sec.1 A (July 06) 607 Propulsion Thruster survey is scheduled according to complete survey (2.5 years). It is generally recommended that the propulsion Thruster survey is carried out in conjunction with bottom survey. When the Propulsion thruster survey requires the ship to be out of the water the survey shall be carried out in conjunction with bottom survey in dry dock. As can be seen there is now a requirement for alignment between bottom survey and thruster survey, except if the Thruster survey may be carried out afloat (e.g. reduced scope by lifting the pinion and/or using fibre optics). The motives for this are: Avoid additional or unscheduled dry dockings To allocate sufficient time for a thorough survey To ensure that all major Class surveys are carried out during the time window of the renewal survey in order to start a new 5 year period with a clean sheet This harmonisation with bottom survey will not be enforced at the forthcoming Thruster survey, but all ship managers are strongly advised to implement this harmonisation as soon as possible to be aligned by the vessels subsequent Thruster survey. At the second Thruster survey after 2007-01-01 the vessel should have been able to plan ahead for this alignment. NB! By harmonisation is meant to actually carry out the scope of thruster survey concurrently with the scope of bottom survey. However, most thruster surveys already require the vessel to be in dock, even though the bottom survey scope is not required to be carried out concurrently. 207 Reduced scope 201 The reduced scope Reduced scope will still require opening up of inspection covers and in many designs these are under water and will require docking of the vessel. Some designs can be inspected when afloat by lifting the pinion and /or using fibre optics if the thruster in question is prepared for this. The inspection of the sealing arrangement is moved to Pt.7 Ch.1 Sec.5 A402: Thrusters for propulsion and thrusters for propulsion and steering shall have the sealing arrangement of propeller blades, propeller shaft and steering column examined. (Continued on next page)
208 Full scope is to be applied to the periodical survey following a reduced scope survey unless the running hours are less than 15 000 hours since last full scope survey, see 207. Requirements for survey with full scope are: the requirements to reduced scope as given in 207 visual inspection of relevant parts NDT of relevant parts running test 202 The full scope survey shall include: the reduced scope as given in 201 opening up of protection covers and more thorough inspection of power transmission gear (gear clearance to be measured), bearings (axial play to be measured), visible parts of shafts and general condition of housing internally or more thorough examination of drive motor rotor and stator condition (air gap to be measured) and associated equipment, shafts, bearings (rollers and bearing races to be inspected visually either by fibre optical means, or by other relevant means) and stator fixation arrangement examination of controllable pitch (CP)-mechanism oil transmission system and feed back system for wear down and damages, full stroke ahead and astern to be verified and correct blade position feed-back and indication verified examination of steering column and related sealing and bearings running test at MCR The option for doing reduced scope if running hours are less than 15,000 since last full scope has been removed. By removing the 15,000 running hours rule, there is no longer any doubt about when a full or reduced scope survey shall be carried out. We may however at the first upcoming survey grant exemptions for vessels who according to the old Rules would be due for reduced scope survey have the possibility to carry out reduced scope survey afloat are not at the same time due for bottom survey in dry dock This will be based on a case-by-case evaluation The requirement for NDT is moved to the overhaul survey. (Continued on next page)
209 Any opening up of thrusters shall be witnessed by a surveyor and should meet the requirements in 208. Previous rules only repeated the full scope survey items. This has now been made more relevant to expected situation and possibilities. 203 At overhaul of the thruster unit the following shall be included in addition to the survey requirements given in 202: all relevant parts of the components made accessible during overhaul shall be surveyed using adequate methods, such as visual inspection and MPI or DP, wear down measurements, etc. MPI shall be carried out of gear teeth and at least in way of stress raisers in the shafts proper assembly of the Thruster shall be verified proper gear mesh shall be documented in same extent as required for new Thruster We are of the opinion that the new rules are introducing a more relevant survey regime and will serve our customers better even though reduced scope as an alternative to full scope in case of low running hours has been removed. The need for unscheduled dry docking due to out of phase Thruster surveys should be stopped as a result of Pt.7 Ch.1 Sec.1 A607. For further information regarding survey of thrusters please contact MTPNO867@dnv.com