Mike Efthymiou

COMPARISON OF LOADS PREDICTED USING "NEWWAVE" AND OTHER WAVE MODELS WITH MEASUREMENTS ON THE TERN STRUCTURE

Global wave loads measured on the Tern platform during a very severe storm have been compared with predictions made on the basis of three different models for wave kinematics. The first is the “Newwave” theory, a broadbanded, probabilistic-based model for the extreme waves of a random seastate. The second model involves complex time-domain simulation of random directional seas. Both these theories are used with the Morison equation and realistic force coefficients to predict global forces. The third wave model is the Stokes fifth-order theory with artificial values for the force coefficients, as used in conventional design practice.

Reliability and (RE)assessment of fixed steel structures

This paper reviews the structural integrity and reliability of fixed steel offshore structures with a focus on improved models and incorporation of these models in design standards. Technical achievements in four key areas are reviewed which, when combined, resulted in a step improvement in the calculation of structural reliability. The first area is the extreme environmental loading on an offshore platform; the second area is the joint occurrence of waves, winds and currents, i.e. accounting for the fact that these do not, in general, peak at the same time and do not act in the same direction. The third area is the estimation of the ultimate strength of a fixed steel platform, accounting for component strength, including the buckling and post-buckling behaviour and the uncertainty in system strength. The fourth and final area is the integration of the above models to estimate the probability of failure. The historical performance of platforms and the improvements in successive editions of API RP 2A are reviewed; reliability targets appropriate for different exposure levels and corresponding performance standards are developed, aimed at harmonizing design practices worldwide. A differentiation is recommended between permanently manned L-1 installations and manned-evacuated L-1 installations in the Gulf of Mexico; this is because the consequences of failure are considerably different.Copyright

Experimental Investigation of the Dynamic Behaviour of a Water Intake Riser

This paper considers the dynamic stability of free-hanging water intake risers. Suspended from a barge, these risers convey a great volume of cooling water, which is needed for offshore liquefaction process of natural gas. There is a contradiction between theoretical predictions and experiments for cantilever pipes pumping up water. Reported small-scale experiments did not show any instability, while theory predicts instability beyond a critical fluid velocity. To investigate whether the previous experimental setups did not allow to observe the instability or the pipe aspirating water is unconditionally stable, a new test setup was built which could attain a higher internal fluid velocity than the predicted critical velocities. A cantilever pipe of about 5 m length was partly submerged in water. The experiments clearly showed that the cantilever pipe aspirating water becomes unstable by self-excited oscillatory motion (flutter) beyond a critical velocity of water convection through the pipe. Below this velocity the pipe is stable, whereas above it, the pipe shows a complex motion that consists of two alternating types of motion. The first type is a nearly periodic orbital motion with the amplitude of a few pipe diameters and the second one is a quasi-chaotic motion with very small amplitude. Translating these results to offshore water intake risers, shows that for realistic internal flow velocities the riser might become unstable.© 2007 ASME

Instability of a Simplified Model of a Free Hanging Riser Conveying Fluid

Studying stability of a vertically suspended, fully submerged pipe conveying water (free-hanging water intake riser), researchers have found that if a critical flow velocity is reached, these pipes may flutter. However, there are different predictions for the value of this critical velocity. Researchers have mentioned values changing from infinitely small fluid velocities up to velocities which are unachievable in practice. The nonlinear hydrodynamic damping caused by the surrounding water seems to be crucial for correct description of the stability of the submerged riser aspirating fluid. In this paper, using the Morison’s equation, the nonlinear drag is taken into account as a function of the relative velocity between the current and the velocity of the riser itself. The nonlinear system is studied employing the Galerkin method. Ten-mode discretization turns out to be necessary to obtain an accurate result. It is shown that the current has a strong stabilizing effect. If the internal fluid flow exceeds a critical velocity the riser performs self-sustained oscillations with the amplitude smaller than one diameter.Copyright

Aspects of the stability of jack-up spud-can foundations

Abstract A three-level procedure for assessing jack-up foundation stability for more or less homogenous soils is described. The objective is to provide a rational framework for these assessments that ensures their safe operation in extended year-round operations and enables their use in deeper waters than at present. The three levels of the procedure have to be entered successively as long as foundation stability cannot be proven. The first level is a screening exercise and essentially replaces the well-known preload check. The second level compares factored foundation loads resulting from a structural analysis with foundation capacities obtained with ultimate bearing capacity formulae. The most refined third level assesses whether the displacements associated with these loads lead to an acceptable situation, i.e. capacity increase and/or load redistribution that does not result in collapse of the jack-up unit. Since, for maximum benefit, this third-stage analysis requires a non-linear foundation model to be linked with the structural package used: such a tool is provided in the paper. Examples are given to demonstrate the impact of the assessment procedure. This procedure forms part of the overall in-house approach to the assessment of jack-ups and has already been offered to the jack-up industry as part of the continuing efforts towards establishing common and accepted standards for jack-up assessments. Further developments have been identified and will be pursued.

Sakhalin Single Anchor Leg Mooring (SALM) Repair Project

In July 2008, Sakhalin Energy Investment Company (SEIC) had successfully retrofitted its SALM system offshore Sakhalin Island in the Russian Sea of Okhotsk following its Universal Joint (Uni-joint) connection failure in November 2007. This SALM system is part of the “Vityaz” Production Complex in which the produced oil is exported from the Piltun-Astokhskoye-A platform via a subsea pipeline and the SALM to the Okha floating storage and offloading (FSO) unit for subsequent loading onto tankers for delivery to customers. Yearly oil production using the SALM system starts normally in June and lasts till December, during the ice free period. Full year round production were planned using a pipeline replacement for the SALM system (on-stream early 2008). However, in the interim period, there was a significant production opportunity if the SALM could be reinstalled in time for the summer 2008 ice-free season. Another significant incentive for SEIC was an early gas flow through the offshore pipelines to the Onshore Processing Facility for commissioning purposes, which expedites first gas delivery for the whole project by an estimated 4 months. In order to make this production season possible, a unique and fast track project to reinstate the SALM was necessary. SEIC, Shell International E&P in Rijswijk and Single Buoy Moorings Inc. in Monaco, had jointly undertaken this project, incorporating proven engineering standards with an immense amount of creativity to meet these time constraints. The SALM project, that would normally take 2 years, has been hailed as a great success with design, fabrication and installation completed in just 7 months time. This paper describes the key challenges overcome to successfully deliver a fully working SALM system to an extreme time schedule. The emphasis is on a description of the technical challenges and the engineering repair solutions developed with reference to the constructibility, transportation, installation and operation. The challenges include: Re-engineering of the SALM Unijoint cast components to meet the limitations of the available foundries, implementation of a novel pull down installation method using a subsea linear winch, design and construction of independent contingency option, installation trials, worldwide logistics and project coordination. All these aspects form an integral part of the project execution strategy with prompt decision making to meet the required schedule.Copyright