The paper will first give a brief summary on the technological developments of offshore loading systems and a prediction of future trends and challenges.

Based on previous studies of several different design concepts, the paper will deal with the methods applied in the analysis and factors contributing to the unavailability of offshore loading systems.

The following categories are discussed:

  • Environmental factors limiting the availability of the plant,

  • Structural failures, non-scheduled repairs and inspection.

  • Process system and operational failures and the associated repair, causing off hire.

The result of the availability analysis will be identification of components and conditions contributing to availability.

It will be shown how the knowledge in turn can be utilized in formulating redundancy and more specific design related criteria and guiding inspection and maintenance during operation. The use of the analysis in evaluating relative value of different design solutions will be discussed.

The last part of the paper will give specific guidance and recommendation on design, fabrication and operation of offshore loading system based on many years of involvement in all phases of offshore loading systems, i.e.

-research-conceptual evaluation-certification-in-service inspection-rule and guidance development.

Introduction

In October, 1981, the Norwegian government approved the initial development of today's most prosperous oil field on the Norwegian continental shelf the so-called "Golden Block".

With estimated reserves of 200 mill. tons of oil corresponding to a present value of US $48 billion, the reserves must be denoted as considerable by any standard.

The transport alternative for the shipment of oil from this source, located in the harsh environment and deep waters of North Sea, will be through two offshore loading systems (Fig. 1.).

Rather than representing the frontiers of utilization of today's offshore loading systems, these may be considered as typical applications.

The twenty years of development within offshore loading might therefore be regarded as remarkable.

Historical Development

Single-point mooring terminals have been developed over the years as a means of mooring large vessels for offloading or loading bulk cargoes without building complex port facilities. Lately their use has been extended to the export of crude oil from offshore oil fields, especially in hostile and deep environments such as the North Sea.

The principle common to all Single Moorings (SPM) in use today is that a shuttle tanker is moored to a permanent buoy by a braided synthetic rope or ropes as its bow. From a seabed pipeline, the product is discharged through a riser pipe or hose pipeline, the product is discharged through a riser pipe or hose to a swivel assembly at the buoy centre. A turntable or super-structure supporting pipework and mooring attachments, connects the swivel with flexible hoses which extend to the tanker's manifold system. In addition to product hoses, lines for ballast water, bunkers or pipeline flushing return may be fitted.

A wide range of buoys have been installed and the different types of terminals are mainly recognized by the anchoring system. The first generation and more simple floating terminals are fixed to the seabed by several chains and load onto the shuttle tanker through a floating hose.

These simple buoys will normally not achieve the desired reliability in a hostile environment and deeper waters. Structures more resistant to adverse weather conditions, such as articulated towers., are here required. An articulated tower is "anchored" to the seabed by a universal joint which allows angular movements in all directions.

Typical examples of today's generation is shown on Fig. 2, comprising.

Catenary Anchor Leg Mooring CALM, which is the traditional and most widespread offshore loading buoy. The first CALM design was installed in Dalaroe in Sweden in 1959. In the North Sea, the CALMs installed in the Ekofisk area in 1971 ensured the supply of oil from the first commercial oil field on the Norwegian shelf.

The pipeline end manifold is connected to the buoy by flexible hoses. The tanker is free to weathervane 360 deg around the buoy connected with a blow hawser. Crude oil is transferred from the buoy to the tanker via floating hoses.

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