Performance Assessment of Deep Penetration Offshore Piles Driven Into Calcareous Soils

ABSTRACT This paper presents the results of an installation monitoring and performance assessment program for offshore platform foundations in Campos Basin on the Brazilian Continental Shelf. Pile driving dynamic measurements were conducted during piling installation of the Enchova and Namorado platforms. Measurements were obtained on nine platform piles and during the restrike on six of those instrumented piles. The foundation piles were driven open-ended into complex site stratigraphies containing a shallow layer of calcareous sands/clays near the mudline followed by sandy silts and silty clays intermixed with layers of sand and shell fragments. This paper discusses the approach that was taken to evaluate hammer-pile-soil interaction for offshore driven piles. This included pre-field wave equation driveability analysis, field pile driving dynamic measurements, and post-field foundation assessment. This paper provides the offshore engineer with useful information such as measured driving system energy output and efficiency for both monitored Menck and Vulcan hammers, experienced gained during piling installation in calcareous soils, soil set-up effect, and an estimate of long-term pile capacity. INTRODUCTION Pile driving dynamic measurements were conducted during pile installation for the Enchova and Namorado platforms in the Campos Basin, offshore Brazil. Strain transducers and accelerometers were attached to the pile top during initial driving of nine platform piles and during the restrike on six of those instrumented piles. The foundation piles (60 to 72-in. OD with wall thicknesses varying from 1.5 to 1.75 in.) were driven open-ended into complex site stratigraphies containing a shallow layer of calcareous sands/clays near the mudline followed by sandy silts and silty clays intermixed with layers of sand and shell fragments. An approach was taken to assess the foundation performance of deep penetration offshore piles by incorporating:pre-field wave equation driveability analysis,field pile driving dynamic measurements, andpost-field CAPWAP (Case Pile Wave Analysis Program) analysis. The following sections of the paper describe in more detail each of the steps used in the foundation performance assessment. BACKGROUND INFORMATION The Enchova and Namorado sites are located in the Campos Basin, offshore the Brazilian Continental Shelf. The water depth is approximately 380 ft at the Enchova site and 476 ft at the Namorado site. An extensive site investigation program was conducted, including conventional offshore soil borings and in-situ cone penetrometer tests. A cemented calcareous sand layer is present at shallow depths underlain by a soft soil. Lessons had been learned from the construction of the Garoupa platform which is the first platform installed in the Campos Basin (10). The most severe problem encountered during the piling installation of the Garoupa platform was related to the calcareous soil. Significant pile self-weight penetration was observed due to the pile punching through the surficial calcareous sand layer and penetrating into the underlying silty clay layer. This resulted in some unexpected hazards and delays in the driving operation. Recognizing major uncertainties regarding foundation pile driveability and their in-service performance in calcareous soils, a pile installation monitoring and assessment program was planned during construction of the Enchova and Namorado platforms.