Repair Techniques of Riveted VesselsRobert G. Dvorak and James L. Brauer*
* Robert Dvorak, Hull Superintendent and James Brauer, General
INTRODUCTIONThe Manitowoc Marine Group repairs vessels that normally operate in the North American Great Lakes including 1000 foot long self-unloading bulk carriers that are of all welded construction. Due to the fresh water environment, we are able to work on operating vessels of riveted construction up to 100 years old.
We are currently involved in a seven year program to renew the underwater hull on a cement carrier built in 1906. We are also replacing approximately an 11 ft. wide by 300 ft. long section of port and starboard side shell plating on a ship originally built in Manitowoc in 1936, as a self-propelled oil tanker. This vessel was converted to a self-unloading cement barge in 1987 at Bay Shipbuilding and is one of the 18 vessels we have at our Bay Shipbuilding yard in Sturgeon Bay, Wisconsin for winter maintenance and repairs.
In addition, the variety of vessels in our yard provides first hand observation in the technological advances our industry has made from all riveted hulls, to welding in non-critical areas mixed with riveting on bilge and shear strakes, to all welded construction.
Our riveting crews have assisted with repairs in Rhode Island, Long Beach California, and Johore Baru, Malaysia. These repairs included fixing leaking rivets, caulking leaking seams, removing old rivets and re-driving new ones.
This presentation should provide you with information on what type of repairs can be done on rivets, caulking edges, and alternatives for riveted repairs.
INSPECTION AND SURVEYSA pre-docking survey or even sounding of tanks while the ship is floating can be used as an indicator for leaking rivets or seams. Are we making water? Where is it coming from? Crew reports and surveyors working for the ship owners typically handle this.
When the vessel is dry-docked for a hull survey, the inspection may reveal a variety of repair needs. The point and head of the rivet are examined for deterioration. The vessel may have deteriorated rivet points that are weeping, leaking or loose. Sprung caulking edges on the riveted lap of hull plates caused by minor groundings may be present. The internal survey will also give an indication of the condition of the rivet heads. This is valuable when rivet points on the exterior of the vessel are hard to see because of paint or rust. To test a loose rivet, a finger is placed on one side of the rivet head while tapping on the opposite side of the head with a small hammer. A loose rivet will move or vibrate.
The key areas on the rivet that deteriorate affecting the strength of the joint are the head and the countersunk point. If the head area is reduced in size greater than 20 to 25% of it's original size, replacement is required for active ships. Sometimes scale or wastage must be removed to see the actual size of the rivet head. The rivet point acts like a wedge to hold the outside and inside lap tightly together. When this point deteriorates so that part of the point is gone and areas of the plate countersink are visible, rivet failure is possible. Rivet points also lose the ability to be tightened when they deteriorate because of the loss of this wedge portion of countersunk rivet.
Although it can be advantageous to inspect for leaky rivets as soon as the hull is dry, the type of deterioration or loose rivets that require attention will still be noticeable for days after. Inspection will show wastage on the point of the rivet where it fills the countersink of the plate, or a continual dripping or seepage around the perimeter of the point is evidence of loose rivets. If corrosion has taken place so as to expose part of the countersunk plate or the rivet has developed a crack between point and countersink, repair or replacement should be undertaken. Rivet points can also be damaged as the hull scrapes along a dock wall or other hard objects. A more severe situation may require the complete renewal of a damaged plate and associated rivets.
A paper written in 1967 by Lieutenant Commander D. C. Cunningham entitled "Marine Riveting On the Great Lakes" is commonly referenced when repairing riveted vessels.
Cunningham states that "riveted butts in the way of the midbody deck and bottom plating deserve special attention due to the high longitudinal bending stress. Where there is evidence that a butt has started to "work" common practice calls for re-driving the entire butt or replacing it with a welded insert, cutting back to good metal at least a couple of feet on either side of the butt."
REPAIR OF OLD RIVETSFirst, we will address typical repairs made when the existing plate is satisfactory and only the rivets require attention. The rivet will either be repaired in place or removed and redriven. In the case of hull rivets, most repairs in place should be done from the outside of the ship in order to correct the source of the problem. Cunningham describes three methods of rivet repair: bobbing, frenching and ring welding.
The Manitowoc Marine Group would add frenching and bobbing to ring welding. This method uses the veeing of the edge of the rivet point perimeter tightened by the bobbing tool. When rivet conditions permit, this method can eliminate "hot work" on repairs to fuel bunkers and single skin tankers.
Care and foresight must be used in any repairs to rivets in place. A typical riveted seam has two rows of rivets spaced 4 to 5 inches apart and when you tighten one, the neighboring rivet can be loosened slightly. This procedure could go on forever as you tighten one and loosen the next one, etc. The key is not pounding too hard on the rivet and knowing when to stop chasing the weepers.
In his paper, LCDR Cunningham reminds us that "It is routine that a few of these will leak". Often rust can cause a weeping rivet to stop leaking in as little as 24 hours.
RIVET REPLACEMENTIn the case where loose rivets cannot be tighten by the previously mentioned methods, judgment comes into play on how extensive the problem is. Frenching and welding of the rivet points and welding of a caulking seam may be satisfactory in instances where the repair location on the vessel is not critical and number of rivets to be repaired is minimal. In other cases it may be necessary to remove and replace the rivet. Care must be used in burning out the old rivet to prevent damage to the existing plate and it's countersink. The rivet hole and countersink is then cleaned up by reaming to proper size and a light turn with the countersink. The new rivet is then driven, and neighboring rivets and caulking edges addressed with bobbing/frenching/re-caulking/etc.
CAULKING PLATE EDGESOn the riveted plate connection, water can penetrate the vessel two ways; through the loose rivet or from the caulking edge. The exposed edge of the outside plate is called the caulking edge. Caulking on steel vessels involves forcing an edge of the outside plate tight against the inside plate sealing the lap joint. This is accomplished with an air powered chipping hammer and a caulking tool that wedges the inboard part of the caulking edge toward the inside plate.
The first pass with the tool cuts a wedge shaped groove about 1/8" to 1/4" wide. The high side of the wedge shape is against the inside plate. On the second pass, the tool is turned 180¡ and used to force the wedge shape down flat and tight against the inside plate. The finished caulking edge has a slight step on it's inboard edge towards the inside plate.
Repairs to caulking edges of riveted plates are also done from the outside of the plate. If a gap greater than l/16" exists between the two plates, the edge can be heated and brought tight by means of a flattening hammer. This double headed hammer has a round end to be hit with a sledge hammer and a square end that rests against the plate. After the plate is heated enough to allow it's edge to be hammered back in place, one person holds the flattening hammer against the plate and another hits the round end with a sledge hammer. Finally the edge is recaulked and neighboring rivets are bobbed or frenched.
Caulking seams are welded in some instances depending on the seam location. The same situation holds true when repairing rivets in place. If you weld a caulked seam, you will have an effect on the neighboring rivets and they may required the bobbing/frenching treatment. Also, the seam will require caulking an additional 12" to 24" beyond the welded end. On smaller vessels, like tug boats, it can be more cost effective to weld the seam all around the ship and weld all the rivets. Continuous welding can be more efficient than doing a section and chasing the leak all around the ship. From a cosmetic point of view, welding seams and rivets may not be as desirable as the caulking, frenching and bobbing method. The later method does not alter the appearance of the vessel as much as welding does. However, for underwater areas on inactive vessels, frenching and welding will only be seen when the vessel is dry-docked. As always the decision rests with the ship owner on unclassed vessels. On vessels classed to haul cargoes, welding of caulking edges of riveted seams has not been allowed, except for small areas at transitions between riveting and welding. Typically this would be outside of the two-thirds mid-length of the vessel.
PLATE REPLACEMENTReplacing a shell plate on a riveted vessel occurs when the plate is damaged, or when it's condition has deteriorated to the point that renewal is required. The cost of replacing a plate that is riveted is at least 5 times more expensive and more time consuming than one that is completely welded. For this reason ship owners are always trying to minimize riveting. As an example, a basic task list is shown for replacing a welded plate versus a riveted plate with welded frames. An average plate is 3/4" by 96" by 288".
OPERATION SEQUENCE FOR PLATE REPLACEMENT
In most of the shell plate replacement on riveted vessels over 600 feet in length, plate connections riveted and welded are replaced in kind. That is bilge and shear strakes are riveted; strakes in between are welded. Most frame connections are welded.
RIVETING OPERATIONRiveting operations (Items 11 & 13) usually require 5 people to install the rivets; Driver, Heater, Bucker or Holder On, and 2 Passers.
The heater handles logistics of the rivets, the different lengths and types required. If the rivet is too short, it will not fill the countersink or the point will be too flat. If the rivet is too long, the rivet will be so large that the driver has to remove excess stock from the rivet point during forming. This adds to his work and slows the process. When driving 300 to 600 rivets a day on piece work, seconds saved for each rivet add up. A kerosene fired forge is used to heat the rivets to a light yellow color. Rivets should be driven in the upper range of 1000-1950oF (Cunningham, 1967).
The heater paces the crew by tossing hot rivets to be caught in a funnel by the outside passer at a pace that is comfortable to the driver. The outside passer takes the hot rivet and passes it though a small hole in the shell plate to the inside passer.
The inside passer puts the hot rivet in the rivet hole. The bucker then engages the head of the rivet with a pneumatic hammer similar to the one the driver uses but with a die shaped to conform to the head of the rivet. The bucker is responsible for keeping the rivet tight to the shell plate and making sure the rivet head is not deformed during driving. He should also sound the rivets to check for any loose ones before the crew moves to a new location.
When the bucker has the hot rivet in the rivet hole, the driver forms the rivet to fill the countersink and finishes the point of the rivet. To do this, he uses a 100 PSI pneumatic hammer with slightly concave die.
Ideally the driver leaves a small sliver of stock to be removed as he does his final forming of the rivet point. This insures that the rivet hole and countersink are filled and leaves a slight convex point on the rivet. When all rivets are driven, the edges of the new plate are caulked. The pace is set so that when the driver finishes a rivet a fresh one is in the next rivet hole about 3 seconds later.
TESTING REPAIRSTesting of riveted work can be done with water or air test. Water is the preferred method. First the inside of the plate is visually inspected for appearance of rivets, and weld size and quality. Then a water hose is used to spray the inside of the plate while the outside is inspected for leaks. Air testing would involve the tank being pressurized with air to 2 PSI. A water leg or similar safety device must be used to prevent over pressurizing of the tank. Testing soap is applied on rivets and caulking edges. Bubbles will develop where there are leaks. Repairs can be made and retested until the vessel is watertight.
If time allows, a pre-test with water will help to seal any small weepers before the final test for water tightness. Any weeping rivets or leaks in the caulking seams can be repaired by additional caulking or bobbing of the rivet point. Inspectors should be aware that small weepers will seal up.
Internal rivets on watertight connections between tanks can be sealed by the same methods used outside the shell plate. Bounding angles that connect the frames and keelsons to watertight bulkheads can be tightened by caulking.
On riveted ships showing signs of deterioration, good paint systems and other methods to prevent electrolysis, such as cathodic protection, and checking for grounds in electrical systems have proven to arrest the situation.
CONCLUSIONSRivets can be welded in certain locations on classed vessels under the following conditions:
What you choose to do with your ship is up to you. We are capable of providing the services you require to accomplish your rivet repairs. To paraphrase a famous San Franciscan, Paladin. "Have riveting gun. Will travel"
AcknowledgmentsThe authors wish to acknowledge the assistance provided by Nancy Anschutz and Eugene Ehlers, Bay Shipbuilding Co., and Christine Randall, Head Curator, Door County Maritime Museum, Sturgeon Bay, Wisconsin.
ReferencesThearle, Samuel J. P., The Modern Practice of Shipbuilding in Iron and Steel, William Collins, Sons, & Company, Ltd., 1886
Haliday, George V. Swanson, W. E., Ship Repair and Alteration, Cornell Maritime Press, 1942
"Specifications for Riveting", Part 1 - Steel Construction for Vessels of the U. S. Navy, U. S. Government Printing Office, October 1940
"Marine Riveting on the Great Lakes", D. C. Cunningham, LCDR, USCG,
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