Calibration of an offshore DPSH rig and interpretation of DPSH refusal

Stergianos, Theo John (2022-04)

Thesis (MEng)--Stellenbosch University, 2022.

Thesis

ENGLISH ABSTRACT: n this study two research projects were undertaken; namely the calibration of a novel offshore Dynamic Probe Super Heavy (DPSH) rig and the interpretation of DPSH refusal. A novel offshore DPSH rig was compared to a standard DPSH. The novel offshore rig had a chain driven hammer mechanism enclosed within a steel chamber, so it could operate in an underwater environment. Data obtained from field testing showed that the offshore DPSH rig produced similar penetration profiles to the standard DPSH. However, the DPSH n values from the offshore rig were consistently greater than the standard rig. This suggested that the offshore rig was less efficient in energy transfer. An energy dissipation ratio (𝐸𝐷𝑅) was calculated, which showed that the offshore device delivered roughly 60% of the energy delivered by the standard rig per 300 mm penetration. As the anvil and steel chamber are one, greater energy dissipation occurs. DPSH refusal conditions were investigated in relation to SPT (Standard Penetration Test) refusal conditions. Twenty-one sites across Southern Africa were assessed and classified as cohesive or non-cohesive based on site profiles. DPSH n were found to generally increase monotonically and then ‘spike’ (defined as an increase in blow count by a minimum factor of 1.25) at certain depths. ‘Spikes’ resulting in DPSH n greater than 50 were grouped according to a set of conditions relating to the average SPT refusal depth for each site. For the cohesive sites, 75% of ‘spikes’ were recorded at depths greater than or equal to one standard deviation above the mean SPT refusal depth. A further 13% of data points were recorded at depths greater than or equal to two standard deviations above the mean SPT refusal depths. Thus, for cohesive sites ‘spikes’ could be used as DPSH refusal criteria as they showed good agreement with the SPT refusal depths. For the non-cohesive sites, 15% of ‘spikes’ were recorded at depths greater than or equal to one standard deviation above the mean SPT refusal depth across the respective non-cohesive sites. A further 11% of ‘spikes’ were recorded at depths greater than or equal to two standard deviations above the mean SPT refusal depths. The majority (74%) were recorded at depths shallower than two standard deviations from the average SPT refusal depths. Most were likely due to rapid increase in rod friction as material collapsed onto the rods at random depths. Thus, for non- cohesive sites the DPSH appears to be a poor tool to ascertain the presence of competent layers.

AFRIKAANSE OPSOMMING: In die studie is twee navorsingsprojekte onderneem; naamlik die kalibrasie van ‘n nuwe onderwater Dynamic Probe Super Heavy (DPSH) tuig en die interpretasie van DPSH weiering. ‘n Nuwe onderwater DPSH tuig is vergelyk met ‘n standaard DPSH. Die nuwe onderwater tuig het ‘n kettinggedrewe hamermeganisme in ‘n waterdigte staalomhulsel gehuisves, sodat dit in ‘n onderwateromgewing kon werk. Data verkry uit veldtoetsing het getoon dat die onderwater DPSH tuig soortgelyke penetrasieprofiele as die standaard DPSH gelewer het. Maar, die DPSH n-waardes van die onderwater DPSH tuig was deurgaans groter as die standaardtoestel. Dit het voorgestel dat die onderwater DPSH tuig minder doeltreffend was in die oordrag van energie. ‘n Energiedissipasieverhouding (𝐸𝐷𝑅) is bereken, wat getoon het dat die onderwater DPSH toestel ongeveer 60% van die energie gelewer het per 300mm van penetrasie, in vergelyking met die standaard DPSH. Aangesien die aambeeld en staalomhulsel een is, vind groter energiedissipasie plaas. DPSH weiering toestande was ondersoek in verband met SPT (Standard Penetration Test) weiering toestande. Een-en- twintig terreine regoor Afrika was beoordeel en geklassifiseer as kohesiewe of kohesielose. Klassifikasie was gebaseer op die terreine se grondprofiele. Daar is gevind dat DPSH n-waardes oor die algemeen eentonig toeneem en dan ‘styg’ (gedefinieer as ‘n toename in DPSH n-waardes met ‘n minimum faktor van 1.25) op sekere dieptes. Stygings’ wat lei tot DPSH n-waardes groter as 50 is gegroepeer volgens ‘n stel voorwaardes wat verband hou met die gemiddelde SPT weiering diepte vir elke terrein. Vir die kohesiewe terreine is 75% van ‘stygings’ aangeteken op dieptes groter as of gelyk aan een standaardafwyking bo die gemiddelde SPT weiering diepte oor die verskeie kohesiewe terreine. ‘n Verdere 13% van datapunte is aangeteken op dieptes groter as of gelyk aan twee standaardafwykings bo die gemiddelde SPT weiering dieptes. Dus, vir die kohesiewe terreine kan ‘stygings’ as DPSH weieringkriteria gebruik word aangesien dit goeie ooreenstemming met die SPT weiering dieptes getoon het. Vir die kohesielose terreine is 15% van ‘stygings’ aangeteken op dieptes groter as of gelyk aan een standaardafwyking bo die gemiddelde SPT weiering diepte oor die verskeie kohesielose terreine. ‘n Verdere 11% van ‘stygings’ is aangeteken op dieptes groter as of gelyk aan twee standaardafwykings bo die gemiddelde SPT weiering dieptes. Die meerderheid (74%) is aangeteken op dieptes vlakker as twee standaardafwykings van die gemiddelde SPT weiering dieptes. Die meeste was waarskynlik as gevolg van ‘n vinnige toename in staafwrywing, aangesien materiaal op verskeie dieptes op die stawe neergestort het. Dus, vir kohesielose terreine blyk die DPSH ‘n swak hulpmiddel te wees om die teenwoordigheid van grondlae met relatiewe digtheid vas te stel.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/124709
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