Title: UHMWPE — A High Impact Material Being Used to Replace Shipboard Steel Scuppers During Mid-Life Overhauls
Author(s): Brian Olin, Polymer Industries/UltraPoly and Gary Kozak, P.E., Life Cycle Engineering Inc.
Steel scuppers utilized on large amphibious ships and others to channel effluents down to the waterline are fabricated from steel pipe and have limited energy absorbing characteristics when impacted by tugs or when hitting piers. Joints between the steel sections frequently leak, causing corrosion and appearance problems. In addition to leakage between joints, there are no seals where the hull penetrations line up with the holes in the backside of the steel scuppers and there is additional leakage occurring here, especially when the penetrations are stacked above each other, which further erodes these openings causing even more corrosion and appearance problems. Whether installed in hull curvature areas or anywhere on the hull, steel scuppers are custom fabricated and become quite expensive to fabricate and to install. Attachment lugs must be located precisely along its full length from top to bottom. Because of heavy corrosion and the inability to withstand impacts with tugs and hitting piers, steel scuppers require constant maintenance and repairs.
The synthetic downcomers, a replacement to steel scuppers, have evolved and improved since the first generation design was first developed and installed on the USS Tarawa and USS Bataan. Some of the improvements included the elimination of unnecessary components including the deletion of welding flatbar to the hull and using elastomer strips from top -to-bottom of the installed system. In the second generation design, it also was no longer necessary to gang the lower high impact sections together before they could be installed which made for a very cumbersome installation in trying to fit the assembly onto the attachment studs. In this regard, the most important design improvement was to change the design of the lower high impact sections such that they would be CNC machined from solid UHMWPE stock into smaller sections rather than to form larger sections under heat and pressure. The benefit of this change was that it was no longer necessary to worry about the expansion problem of UHMWPE wanting to expand and return to its original flat shape when exposed to sunlight. More importantly, the new improvements made to the original synthetic downcomer designs make it more economical for ship installations by reducing material and installation costs. In addition, improvements were made to fabrication of the funnel by employing a molded process vs. having someone hand form from fiberglass. The appearance of the color of the funnels was improved as well.
The paper opens by discussing how synthetics have replaced steel scuppers on large amphibious ships and their performance to date. The basic types of synthetic downcomer installations for installation on most ships (A, B & C) will be discussed and the rationale behind this. It also discusses the characteristics of the synthetic downcomer design including the following:
• Installation Friendly - Higher impact lower section. It is now machined from solid material thus eliminating the expansion problem and making it more installation friendly.
• For Impact Resistance – Lower sections are designed for very high impact resistance.
• Corrosion Free -Watertight connections down to waterline.
• Cost Reduction - Elastomer strips deleted including the welded flatbar.
• Offers additional weight savings – This is high weight.
• Offers options for attachment - Welded studs or Click Bond especially to eliminate the need for gas freeing on tanks.
To date there have been a total of 8 shipboard installations as follows:
2003 – 9 synthetic downcomers were installed on the USS TARAWA (LHA-1) - First generation design.
2004 - 25 synthetic downcomers were installed on the USS BATAAN (LHD-5) - First generation design.
2007 - 1 synthetic downcomer on the USS Saipan (LHA-2) – First generation design
2007 – 1 synthetic steam downcomer was installed on the USS NASSAU (LHA-4) - Generation design N/A
2008 – 8 synthetic downcomers were installed on the NASSAU (LHA-4) - First generation design
2009 – 25 synthetic downcomers were installed on the USS KEARSARGE (LHD-3) - Second generation design
2013 – The installation of 25 on the USS ESSEX (LHD-2) - Second generation design.
2014 – The installation of 31 synthetic downcomers on the USS Wasp 9 (LHD-1) – Second generation design.
Based on the most recent installations, further improvements to the design will be offered in this paper.
To summarize, UHMWPE is now being used in other areas where steel is corroding and becoming heavily damaged to the point that it is not functioning as before. In addition to synthetic downcomers, some of these installations include LCU fenders, decking, batterboards and thin non-skid decking. On LCUs, the welded steel half rounds are being replaced in greater numbers now with equivalent sections of UHMWPE. Actual Impact testing and including FEA work is used to determine the final thicknesses and configurations of the synthetics needed to replace conventional metals and sustain the impact forces. New approved options will be included in this paper by the recently revised CIDs for synthetic decking and batterboards will also be discussed and highlighted. Other potential uses in the US Navy, including major commercial applications, will be presented including thin non-skid decking. A benefit in all of these applications to the US Navy has been synthetic downcomer and well deck system longevity at significantly reduced maintenance costs.