Battle Turns in Shipworm War

by Douglas Page, ©  2002

Recent emphasis on reducing pollution in the nation’s harbors has not only helped the return of fish species, but paradoxically the cleaner waters has encouraged growth of marine creatures that feed on wooden pier pilings.

Extensive damage is done to submerged portions of wharf timbers and wooden boats by organisms known collectively as marine borers, particularly shipworms (Teredo navalis). Shipworms can destroy untreated timber in less than a year.

Wood used where it is subject to shipworm attach must be protected, either by control of moisture content, use of wood naturally resistant to pests, or chemical treatment. In addition, mechanical barriers (such as metal termite shields and caps on pilings, poles and posts) are sometimes used, but are usually ineffective.

Now, a more effective solution may be at hand. Marine engineers are at work developing a new composite technology that shows promise as a better way to protect wooden piers and marine structures from shipworms.

University of Maine engineering professor Roberto Lopez-Anido is perfecting a fiberglass and polymer composite shield designed to withstand hostile marine environments. In practice, the flexible shield is wrapped around pier pilings like a poultice, providing a durable barrier against shipworm infestation.

Although in the early stage of development, the research has been awarded a substantial grant from the Maine Sea Grant Program, and attracted the participation of Kenway Corporation, an Augusta, Maine, composites manufacturing company.

During lab tests, Lopez-Anido has exposed the material to seawater, weather extremes, and mechanical stresses in order to test its durability.

"We're calling the material a FRP (fiber reinforced polymer) shield," Lopez-Anido said. "It will provide strength to the wood pile and protect it from marine borers."

Lopez-Anido is a specialist in the application of FRP composites, so far mostly to highway bridges. His new pier shield, however, is fabricated and applied differently. The shipworm shield is designed to be placed around a piling from the mud line to the high tide line.

"We don't apply the shield above the high tide line because of problems with fungi that attack wood pilings in that area," he said. "We need to let the piling breathe, and shielding the piling above the high tide line could aggravate the fungi problem."

Lopez-Anido’s technology is currently working its way through patent channels. Nevertheless, he who hopes his waterfront repair technology will be ready for deployment and possible commercialization by the end of 2002.

While shipworms, also called termites of the sea, have been the scourge of wooden ships and piers for centuries, until recently they were unknown in colder waters, such as those of coastal Maine. The reasons for shipworm appearance along the Maine coast are unclear, although global warming may be implicated.

Kevin Eckelbarger, director of University of Maine's Darling Marine Center, believes that relatively warmer water temperatures could be playing a role. Shipworms in the U.S. traditionally flourish in the warm waters of the Pacific, Gulf, and South Atlantic coasts.

"Shipworms are abundant in warmer waters, but they can also tolerate low water temperatures and low salinity," he said. "Since warm water temperatures stimulate reproduction, warmer summers in recent years may have provided a more favorable habitat for the species in Maine."

Shipworms are really not worms at all, but marine mollusks similar to clams, with long, worm-like bodies with greatly reduced shells at the end. Floating larvae bore tiny holes on the surface of woody materials, and the adults use their shells to create larger tunnels inside the wood. Damage is often discovered too late to do anything except replace the infected structures.

A number of commercial products address the issue, each with their own advantages and disadvantages. "We’ve been using plastic pile wraps such as Pile Guard for quite awhile," said Ed Byrne, a senior civil engineer with the Port of San Francisco. While the use of pile wraps has been successful in extending the life of wood piles, the wraps have a limited life expectancy, and need to be replaced due to age and damage from floating debris.

Monterey, California's, Municipal Wharf #2 has used two different types of composite piles for repairs, including concrete filled fiberglass piles and recycled plastic piles with steel pipe cores. according to harbormaster Paul Dangreau.

"The recycled plastic piles were used on the fender pile line where the fishing boats offload, and the fiberglass/concrete core piles were used to replace failing wood piles under the yacht club and restaurant on Wharf 2, both with success," he said.

Coating structures with creosote and other protective chemicals can slow the attack, but creosote-resistant strains of boring animals soon appear. Plus, the chemicals eventually pollute the harbor themselves. Plastic coatings work but need laborious wrapping.

Anti-pollution efforts are themselves implicated in the shipworm problem. Shipworms have plagued humanity for hundreds of years, until relatively recently when it is believed the tar and oil that began appearing in harbor waters following the Industrial Revolution killed off many of the creatures. Ironically, the petroleum sludge even helped protect wooden marine structures by depositing a protective film. These petrol-substances were said to be so concentrated in some areas of some harbors that ships hulls could be treated merely by sailing into port.

However, a shipworm revival began to appear by the late twentieth century, when wooden piers in the cleaned-up harbors were beginning to show signs of renewed shipworm attack. The efficient intestinal bacteria of these organisms help them to reduce timber at rates that continue to amaze harbormasters. By the 1990s 12-inch diameter pilings were being reduced in diameter to only a few inches in less than two years. For reasons no one fully understands, 150 years ago it took as many as 17 years to destroy a piling so completely.