VERIFICATION OF DESIGN AND CONSTRUCTION OF LARGE OFFSHORE PROJECTS
W. Rettedal, A. Gillesvik and T. Aarum, Statoil, Stavanger, Norway.
A. Vegge and A. Kvitrud, Norwegian Petroleum Directorate, Stavanger, Norway.
ABSTRACT
The Norwegian Petroleum Directorate, NPD requires the Operators to carry out extensive verification of all phases of a platform project. In particular the regulations state that the design of structures, or structural parts, of significance to the overall safety should be verified. For the verification there are also requirements with respect to personnel qualifications and independency.
This paper will discuss how the Operator may organise verification activities.
Contractual relationships will be discussed. Further, a discussion on how verification should be performed will be presented, i.e. by system review (procedures) or inspection, by separate analyses, design review or model testing. Criteria for when there is a need for extensive verification and what the benefits are will be adressed.
Furthermore, it should be evaluated whether the verification affects the time schedule and whether the schedule for the construction and design affects the quality of the verification activity.
The paper will present a couple of examples from verification work carried out for the design and construction of the Sleipner gravity base structure 2.
This project was unique in the sense that a very tight time schedule was adopted and the Opertor carried out an extensive verification throughout the design and construction phases. Model tests were carried out to verify design and construction in addition to more classical verification
MOORING SYSTEMS DESIGN BASED ON ANALYTICAL EXPRESSIONS OF CATASTROPHES OF SLOW MOTION DYNAMICS
Michael M. Bernitsas and Luis O.Garza-Rios, Department of Naval Architecture and Marine Engineering, The University of Michigan, Ann Arbor, Michigan
ABSTRACT
Analytical expressions of the necessary and sufficient conditions for stability of mooring systems representing bifurcation boundaries, and expressions defining the morphogeneses occurrings across boundaries are presented. These expressions provide means for evaluating the stability of a mooring system around an equilibrium position and constructing catastrophe sets in any parametric design space. These expressions allow the designer to select appropriate values for the mooring parameters without resorting to trial and error. A number of realistic applications are provided for barge and tanker mooring systems which exhibit qualitatively different nonlinear dynamics. The mathematical model consists of the nonlinear, third order maneuvering equations of the horizontal plane slow motion dynamics of a vessel moored to one or more terminals.
Mooring lines are modeled by synthetic nylon ropes, chains, or steel cables. External excitation consists of time independent current, wind, and mean wave drift forces. The analytical expressions presented in this paper apply to nylon ropes and current excitation. Expressions for other combinations of lines and excitation can be derived.
