Evaluation of jack-up units in deeper water in the North Sea

Abstract

The aim of this thesis has been to investigate challenges and possible limitation associated with purpose-built jack-up units for weathering the ultra harsh environment of the North Sea. Two Site-Specific Assessments (SSAs) have been performed of a jack-up unit for all-year operations at the Johan Sverdrup field. The jack-up unit is a “similar” design as the MSC CJ- 70 Class drilling unit, which is able to operate at water depths up to 150 meters and drill wells down to 10 000 meters. She is the largest built jack-up class unit in the world. In addition to the Site-Specific Assessments (SSAs), a parameter study has been performed to investigate and quantify the sensitivities related to water depth, air gap, lightship weight, soil conditions and other effects related to uncertainties in stiffness, hydrodynamic loading, soil structure interaction, nonlinearities and statistical parameters. Analysis methodology and procedures for assessing jack-up units have been much debated over the last 30 years. This is mainly caused by the large extent of nonlinearities associated with estimating the jack-up responses, due to a wide number of significant uncertainties. Nonlinearities arise because these units are drag dominated marine structures with nonlinear hydrodynamic loading, nonlinear stiffness characteristics due to the soil structure interaction and to P-δ effects in addition to be dynamic sensitive structure. The responses are therefore non-Gaussian implying assessment of various statistical methods to determine the extreme maximum responses in a seastate. The nonlinear soil-structure interaction is one of the main uncertainties to overcome because of the complicated stiffness characteristics of the foundation, which is strongly nonlinear. The effects of foundation stiffnesses are significant when assessing a jack-up unit. In this study, a two-stage deterministic procedure has been used to perform the analyses. This analysis procedure is considered acceptable by the industry because it produces satisfactory results and treats associated nonlinearities in an adequate manner. This analysis method together with the more stringent requirements in the Norwegian Annex in the ISO 19905-1 standard have been the basis for the assessment. Results from the Site-Specific Assessments for the Johan Sverdrup field at 110 meter water depth show the purpose-built jack-up unit (MSC CJ-70 Class design) satisfies the regulatory requirements with respect to overturning stability, leg sliding, preload capacity, leg strength, leg holding system strength and foundation bearing capacity. The parametric studies show that some parameters have significant effect on the extreme responses due to increasing water depth, foundation fixity and leg length. Several other parameters investigated did have only minor effects on the extreme responses. Moving into much deeper water than 110 meter in the harsh environment of the North Sea is questionable due to the uncertainties inherent in the applied methods, the damping characteristics, nonlinear dynamic response estimations, the structural high yield strength capacities of the legs and the soil resistance capacities of the spudcans.