The presented paper covers essential aspects and requirements for offshore loading systems. Response characteristics of typical concepts and associated design forces for the mooring and anchoring systems will be discussed. The need for model tests compared to anchoring and mooring systems in respect to strength and durability. The governing factors for the lifting efficiency including operational limitations with respect to weather conditions are considered. The paper concludes with some considerations from the certification point of view. Introduction Offshore loading systems for the initial temporary or permanent solution to the transportation of production to the shore is receiving increasing attention from operators in the North Sea. Offshore loading of oil as such is well known to the Industry, however, sufficient experience from hostile environments like the North Sea hardly exists. The few systems operating in the North Sea must be regarded as prototype and the next generation of offshore loading units now being developed will be able to incorporate a number of the valuable lessons learned. Reliability and efficiency are key words in selection of the most suitable system and this paper will focus on a number of engineering aspects of importance in this context. Figures and tables are purposely made in a schematic and general format to indicate the tendency rather than actual measured or calculated values. Articulated buoyant towers without storage capacity and large volume floating anchored storage vessels will be the main types of offshore loading systems discussed in this paper as there appears to be a tendency in the industry towards these types of concepts. A number of refined concepts utilizing one shuttle and one storage tanker whereby the latter is remotely connected to the buoy in different ways, articulated towers with two universal joints, one semisubmersible, etc., are proposed or even in use, but will not be subject to detailed comments here, as this is regarded as being outside the scope of this paper. Concepts The general concept which has proven successful for offshore loading is the single point mooring (SPM) idea. The tanker is moored with bowlines to a single point only, allowing the vessel to rotate freely (weathervane) around the buoy. The different developments may be divided into two main groups on the basis of the type of fixation used; CALM and SALM (see fig. 1). The catenary anchor leg mooring buoys (CALM) are the most commonly used offshore loading system. The simplest and most widely used being the single buoy mooring buoy (SBM).
This paper describes an experimental program undertaken to understand better the hydrodynamic loading of multitube production risers. Test results are described, and a generalization is given for engineering applications. Sample design calculations are performed. Introduction In recent years, much attention has been paid to analyzing the response of drilling risers. Most effort has been focused on the development of structural and analytical techniques to predict the stresses in riser systems. The analytical tools are, to a certain extent, calibrated by full-scale measurement. API published a comparison] of nine different, independent calculations of riser response that were in close agreement.Normally, drilling risers have a simple cross section for which the hydrodynamic forces are fairly well known. Extensive studies during many years have established insight into the flow patterns around a single smooth cylinder in constant flow and the corresponding forces. For multitube production risers the hydrodynamic load pattern is far more complex.Research is continuously improving the knowledge of flow-induced forces, particularly the influence of these parameters: high Reynolds number at prototype condition, effect of roughness, proximity effect of closely spaced cylinders, current/wave interaction, and effects of irregular seas.The state of the art regarding hydrodynamic loading on single cylinders in constant and oscillatory flow as a function of varying roughness has been published by Miller and Sarpkaya. Correspondingly, multiple cylinders have been considered by Zdravkovich, Bushnell, and Sarpkaya.To understand better the hydrodynamic loading of multitube production risers, Kongsberg Vapenfabrikk initiated an experimental program early in 1977. This program consisted initially of five riser configurations and one single cylinder to be tested in constant and oscillatory flow. A comparison with published data was made to check the experimental procedure and results against the state of the art. The experimental program later was extended by 18 additional geometries, sponsored by 10 oil companies as a joint industry program.The tests in constant flow were carried out at the Danish Hydraulic Inst. (DHI), while the oscillatory flow tests were performed at the Hydraulic Research Station (HRS). Det norske Veritas was consultant for the project and supervised the experimental work and the data interpretation procedures.This paper describes the experimental program, the test results, and a possible generalization for arriving at a simple engineering application of results.The discussion of sensitivity of important parameters in determining design loads is based on an actual design case. Design calculations performed by Kongsberg Vapenfabrikk for a production riser in deep water are presented. JPT P. 881^
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