Analyses for Wave Induced Sea-Floor Movements
- Stephen G. Wright (U. of Texas)
- Document ID
- Offshore Technology Conference
- Offshore Technology Conference, 3-6 May, Houston, Texas
- Publication Date
- Document Type
- Conference Paper
- 1976. Offshore Technology Conference
- 5.1.3 Sedimentology
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Due to a paper numbering error, this number was previously assigned to apaper presented at the 1975 Offshore Technology Conference. Todistinguish between the two papers, the filename extension -A has been added tothe 1975 paper and the filename extension -B has been added to this paper.
Procedures are presented for calculating stresses and soil movements producedby seafloor pressures which are generated by ocean waves. The procedures arebased on the finite element method and provide a direct means to assess theeffects of gravity on wave induced soil movements. Separate procedures arepresented for calculating both cyclic and permanent soil movements.
A series of analyses are presented using the procedures described withrepresentative data for the site of a wave induced slide in South Pass Block 70of the Gulf of Mexico. These analyses show that the gravity stresses anddeterioration of soil properties under cyclic loads have an important effect onthe soil movements. The important parameters required to calculate realisticsea-floor movements with the finite element model are identified and additionalstudies are suggested.
Pressures induced on the sea-floor by large ocean waves can produce relativelyhigh stresses and movements in the underlying soils (9). The movements can beappreciably large in soft clays arid for water depths up to several hundredfeet. Movements can occur in even relatively flat slopes of one degree andless, and in at least one such case the destruction of an offshore oilproduction platform was apparently due to wave-induced soil movements and slopefailure (2, 15). As a result of failures of this type increased attention hasbeen directed in recent years to the prediction of wave-induced soil movementsas a routine part of the design of offshore structures in areas where suchmovements appear likely.
Recently the finite element method has provided one of the most usefulanalytical tools for the prediction of soil movements produced by waves. Wrightand Dunham first described a relatively simple model and finite elementprocedure for this purpose in 1972 (17). In this model the pressures producedon the sea-floor by waves were represented by a traveling pressure wave, andstresses and displacements in the underlying soil continuum were calculatedemploying nonlinear finite element analysis techniques. This procedure wassubsequently employed successfully by Shell Oil Company to evaluate soilmovements at an oil lease site in South Pass Block 70 of the Gulf of Mexico (1,3). A slide occurred at this site during Hurricane Camille in 1969 and theslide was apparently a result of the large waves accompanying the storm (2).The successful results of the study of the slide and other related studies haveprovided encouragement for use of finite element procedures during the pastseveral years as a tool for investigating sea-floor movements produced bywaves. The results of such studies have produced several important improvementsin the earlier finite element models. The purpose of this paper is to describesome of these improvements and to illustrate their significance in terms ofapplications to an area where wave induced sea-floor movements have beenrecorded.
One of the most important improvements to the finite element model has been toinclude the effects produced by gravity loads on the soil and a slopingsea-floor.
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