Options to reduce sediment build-up in a surf zone trench protected by an open-ended cofferdam

Muller, Jacobus Johannes (2015-03)

Thesis ((MEng)--Stellenbosch University, 2015.

Thesis

ENGLISH ABSTRACT: When constructing a submarine pipeline, construction teams must work in the hostile environment in the ocean known as the surf zone. The surf zone is the area along a shoreline stretching between the first evident point of wave breaking and the beach line. In order to ensure that the pipeline is shielded from the imposing forces within the surf zone, engineers use a burial technique which leaves the pipeline length in the surf zone buried underneath the active seabed once construction is finished. Thus a temporary surf zone trench is dredged and protected by an open-ended cofferdam built using iron sheet piles. As a result of the incoming wave climate and the surf zone currents created by this wave climate, sedimentation in and around the trench becomes problematic. In this study alternative geometric layouts for the open-ended cofferdam protecting the surf zone trench are investigated, attempting to minimize the sediment build-up in and around the trench. This was done by using both a 3D qualitative physical model conducted at the CSIR in Stellenbosch, and numerical model using MIKE developed by DHI. However, this study only considers sediment build-up and not structural integrity and constructability of the cofferdam designs. Combining the observations of both the physical- and numerical models, a conclusion was drawn that a structure built perpendicular to the shoreline with a 45oextended arm built from the upstream edge of the cofferdam wall, is the most effective. No dimensions are given as the cofferdam design will change depending on the site specific characteristics. Also an increase in structure length will result in the mouth of the structure being located outside the active sediment zone, which leads to a longer period of time before the pipeline pathway is compromised by sediment.

AFRIKAANSE OPSOMMING: Tydens die konstruksie van 'n onderwaterse pyplyn, moet konstruksie spanne in 'n gevaarlike gedeelte van die see werk naamlik die brandersone. Die brandersone kan gedefinieer word as die area tussen die eerste punt waar branders breek en die strandlyn. Om die pyplyn te beskerm teen die kragte wat branders op dit uitoefen, gebruik ingenieurs 'n installasietegniek waar hul die brandersone seksie van die pyplyn onder die aktiewe seebodem begrawe. Om die tegniek te bewerkstellig, grawe kontrakteurs 'n sloot deur die brandersone en beskerm dit met 'n tydelike struktuur bekend as 'n kofferdam. As gevolg van die inkomende branders en die strome wat deur die branders aangedryf word, kan die opbou van sediment in, en rondom die sloot in die brandersone problematies word. Hierdie studie ondersoek alternatiewe uitlegte vir die tydelike kofferdam struktuur met die oog daarop om die opbou van sediment in, en rondom die struktuur te verminder. Die doel was nagestreef deur gebruik te maak van beide 'n 3-dimensionele fisiese model, gebou en gebruik by die WNNR in Stellenbosch, en 'n numeriese model wat op MIKE, ontwikkel deur DHI gedoen was. Let wel die studie het slegs die sediment beweging in die nabye area van die tydelike kofferdam struktuur in ag geneem en nie die praktiese implimentering en strukturele integriteit van die struktuur nie. Deur die observasies van beide die fisiese- en numeriese modelering in ag te neem, is die volgende gevolgtrekkings gemaak. 'n Struktuur wat loodreg met die strandlyn gebou is en met 'n 45o arm wat na die stroom-op kant toe uitstrek, was die mees effektiewe een. Geen dimensies is deurgegee nie aangesien die ontwerp sal verskil afhangende van die spesifieke area waar die projek aangepak word. Daar is ook gesien dat indien die struktuur langer gemaak word, sal die kontrakteur langer tyd h^e voordat daar sediment probleme in die brander sone sloot ondervind sal word.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/96922
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