Ocean Wave Forces on a Small-Diameter Adjustable-Clearance Test Pipe
Authors: Grace R.A. 
Published: October 2008

The effect of pipe gap and roughness on wave forces was studied offshore ofHonolu1u using a special pipe rig. Test swell involved a local hurricane as well as distant storms. This paper emphasizes peak horizontal and vertical forces. A new technique, using previous flow history, is proposed for accurately predicting such forces from the measured kinematics.

SUBMARINE PIPES EXPOSED IN THE SURF ZONE

The Danger from Heavy Wave Surge submarine pipeline engineer may, by ignorance or design, leave a pipe unburied through all or part of the storm wave surf zone. In the latter regard, budgetary and/or environmental constraints may preclude burial through hard substrates. Both of these factors, for example, applied to the St. George sewage outfall in the Pribilof Islands of Alaska. This line was anchored to studs cemented into the seabed, just as exposed intake lines have been at the Monterey Bay Aquarium in California and at Keyhole Point, Hawaii.

It is instructive to mention what can happen to such an exposed pipe untethered under heavy wave action. Consider a situation that occurred off the Hawaiian Island of Oahu (Honolulu's location) during the passage of Hurricane Iwa in November 1982. Near-breaking waves, with periods probably in the 11- to 13-second range, crossed virtually perpendicular to a pair of pipelines linking an offshore tanker mooring to the Chevron Hawaiian Refinery at Barbers Point. These lines are at about a 40 degree angle to the shoreline.

The steel pipes, with thin concrete weight coats, were originally buried from shore out to a depth of 16 feet, then allowed to lie along the seabed to the tanker mooring in 70 feet of water. After the 20-inch bunkering line was thrown up and over the 30-inch crude-oil-unloading pipe in an early-1960's storm, concrete was poured in 1963 around/between the pipes - and model straps added [1]. This linkage extended from the offshore end of pipe burial to a depth of45 ft, leaving a final 2900 ft distance with the pipes not connected.

During Hurricane Iwa, the 20-inch line moved very little. The larger line, down-wave of it, moved laterally sufficiently far that its end was pulled shoreward 131 feet. From a zero perpendicular offset at the termination of its tie-in with the smaller pipe, the larger pipe's spacing gradually increased to 410 feet, 1800 ft. from the end in a depth of roughly 50 ft., and then decreased to 24 feet at its end. Pipe damage was fortunately minor, attesting to the forgiving nature of steel pipes.

In another case, the weighted but non-anchored high-density polyethylene Green Point outfall at Cape Town, South Mica, was heavily damaged by a storm in May 1984, then totally destroyed by storm waves in mid-1989, leaving sewage pouring into the sea near the adjacent sea wall [2].

Pipe Clearance

In the design of the Green Point, Monterey Bay Aquarium, and Keyhole pipes, attempts were made to set their undersides above the seabed - in order to decrease wave-induced forces. Gaps under submarine pipes also occur uriintentiona1ly because of spanning across outcrops or differential seabed erosion [3-5].