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In order to increase submarine operational availability in the face of significantly increasing costs and longer overhaul duration, the Navy introduced the SSN Engineered Operating Cycle maintenance concept. Application of this concept has resulted in extending the time between major shipyard overhauls from 43 months to the present 8~ months for the SSN 59~. 637, and 688 class attack submarines. This phased increase to an 84month operating cycle evolv~d from the early 1970s, based on engineering studies, technical reviews, monitoring of selected critical ship systems, and application of the resulting engineered maintenance requirements to ensure that these longer operating periods were feasible. Figure 1 shows the SEOC operating cycle.

In 198~. the Chief of Naval Operations requested that Commander Naval Sea Systems Command review the current 8~-month operating cycle to evaluate the feasibility of increasing the operating interval between overhauls. A comprehensive technical feasibility study was undertaken that included engineering analy.sis of 1500 components in 103 submarine systems, special at-sea testing, and material condition assessment on components being overhauled. In addition, SSN 688s systems’ performance was evaluated by the Submarine Monitoring. Maintenance and Support Office. As a result, NAVSEA determined that it was feasible to replace the regular 18-24 month non-refueling overhaul currently scheduled at the 8~ month point with a 10-11 month Depot Modernization Period. This new availability is the key to executing the new Extended Submarine Engineered Operating Cycle.

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This concept was approved by CNO in April 1987 for the SSN 688s — and SSNs 700 through 718. The cycle shown in Figure 1 is the maintenance and modernization plan for these submarines. The major difference between the regular overhaul cycle and the new extended operating cycle is that the non-refueling overhaul (current average duration or 23 months and coat of $121,000,000) is replaced by a Depot Modernization Period (duration ot 10-11 months and cost of $65,000,000). This modernization period is a labor intensive, welldefined, closely managed availability during which modernization and essential maintenance will be accomplished. Unlike overhauls, the submarine commences the availability with a minimum of deferred maintenance and no “open and inspect” scheduled on systems/components which are operating satisfactorily (i.e., “If it ain’t broke, don’t fix it”). The same scope of modernization as an overhaul is accomplished during the depot modernization period.


Having verified the technical feasibility of this concept, the next step was to establish a series of monthly planning conferences that started 20 months prior to the first depot modernization. Their objective was to identify all issues required to support the execution of the depot modernization. There were two major results: numerous problems were solved; and cooperation was fostered among all of the participants in the process.

Conferences started so far in advance of the first modernization period that the attention of senior shipyard management was focused on current availabilities. Fortunately, the Shipyard Commanders, in advance, appointed planning· personnel to the depot modernization effort, enabling a team spirit to grow and to prevail. The results were greatly enhanced by the teamwork and communications that evolved.

The conferences led which addressed specific shorten the modernization durations. Several examples to spin-off meetings technical issues to period’s maintenance include:

  • air induction diesel exhaust-valve flamespray modification to be moved outside of modernization period;
  • special hull treatment tile installation procedure improvements were implemented to reduce time and effort; and
  • it was determined that Weapons System testing could be accomplished in approximately 10 weeks as opposed to the 16-24 weeks experienced in overhaul.

The results were typical of the wide variety of other problem areas similarly treated. When it was determined that the modernization point would require a new set of test procedures, it was decided that the shipyard would test only those components on which work was accomplished. This meant that the shipyard’s Inactive Equipment Maintenance programs and the Ships-Force Preventive Maintenance program — which were inadequate to support the depot modernization requirements — were analyzed and a manual developed to serve as a foundation for making them responsive to this new concept for submarine overhauls.


The time line shown in Figure 2 represents the countdown to the depot modernization period. The combined Work Definition and Forces Afloat Meeting is held at A-13 months prior to the modernization start and is the first opportunity for the joint review of the work package by the customers, shipyard and Ship’s Force. Maintenance work is authorized by the Type Commander to be included in the work package. ShipAlt plans and material availability status is evaluated and the ShipAlt package is finalized and included in the work package.

A pre-test period is scheduled six to nine months prior to the depot modernization. It is conducted under the direction of the assigned shipyard and consists of at-sea (2-3 days) and inport (15-18 days) portions. The test period provides the opportunity to determine the material condition of the ship prior to the modernization period with the goal of having the ship enter the modernization period with a minimum of maintenance unknowns. A Deficiency Screening Conference is held at the conclusion of the test period. It screens each deficiency with emphasis on those which impact crew or ship’s safety or mission essentiality. Those deficiencies which are within the capabilities of forces afloat, are accomplished during the pre-depot modernization period upkeep. The deficiencies beyond forces afloat capability are assigned the depot modernization. Three test periods have been successfully conducted so far. Nearly 95% of the total deficiencies identified were within the capabilities of the . forces afloat to resolve.

The depot modernization upkeep, scheduled at 2-~ months prior to the modernization start provides the opportunity to ensure that the ship arrives at depot modernization with all systems operational. Major deficiencies not corrected are evaluated at the Pre-Arrival Conference for inclusion in the depot modernization as new work.

The ship’s force is extensively involved in the depot modernization. Crew training and certification requirements are the same as for an overhaul; though the availability is half the length. The compressed time requires intensive support of the shipyard by ship’s force. In addition, the ship’s force will retain control of ship’s systems, which remain operational to the maximum extent possible. .A.ll in all, ship’s force support during the modernization period will closely resemble an SRA for intensity, only longer in duration.

Although depot modernizations are well planned, they are not without risk. Factors which can increase the risk of delays include:

– untimely changes in the repair or modernization work package;

– unanticipated diversion or resources due to higher priority shipyard work;

– unanticipated failure to clear the existing backlog or current work;

– unusual fiscal constraints; and failure to control growth/new work.


The first depot modernization started in October 1988. The stakes are significant: 1.08 billion dollars in cost avoidance already realized; and 12 months or additional operating time for each ship.

Success is achievable as the concept is technically valid. The price to ensure success may be at the expense or lower priority shipyard work concurrent with the depot modernization. Failure to meet completion time is the major risk, but this risk has been reduced by extensive planning. It is believed that the introduction or this concept will have a far-reaching effect beyond the current depot modernization program. The concept has also provided many technical decisions and resolved issues that will immensely benefit the entire submarine force.

CDR M. E. House, USN
Mr. K. G. Troxell

Naval Submarine League

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