In recent years, offshore exploration efforts were extended into deeper and deeper water located further and further from shore in less and less hospitable environments. Consequently, not all of the fields discovered were feasible - economically and/or technologically - to develop and to produce using established methods. To improve this situation, a produce using established methods. To improve this situation, a transportation system was developed which allowed crude-oil tankers to be loaded via a single-point mooring buoy, SPM, located near a producing platform -thus, the need for expensive, deep-water pipelines was reduced. platform -thus, the need for expensive, deep-water pipelines was reduced. Despite this innovation plus additional improvements, many discovered fields still remain in the marginal category for conventional evaluation. Therefore, it is necessary to optimize the total production/storage/ transport system before consistently rational decisions can be made regarding field development.

This paper presents a method for predicting the behavior of an SPM system - considering the effects of tanker size, number of tankers, deviations from schedule, weather, loading rate, storage capacity and production rate. The primary descriptive factor determined by this approach is the operating efficiency of the storage/transport system which constitutes a constraint on the production system. Since the production system can be optimized on an economic basis while recognizing well productivity and reservoir performance, it is possible to specify the productivity and reservoir performance, it is possible to specify the optimal configuration for the entire system - and to assure the compatibility of the upstream and downstream elements.

The analysis of the SPM system, a procedure for optimizing the production system and a discussion of the upstream-downstream coupling - and its effect on the optimization of the entire system - are presented in this paper. A simple, but complete, example is also included. paper. A simple, but complete, example is also included

Introduction

Crude oil produced from offshore wells has historically been transported to shore by submarine pipelines-except when barges have been used in fairly shallow waters where fair weather prevailed. However, recent discoveries in deep water located far from shore - particularly, in the northern North Sea, in the western Mediterranean and on the Grand Banks east of Canada-have forced reconsideration of this historical approach. Since it was frequently discovered that a submarine pipeline simply was not feasible on an economic basis (inadequate reserves, excessive distance to shore, legislated burial requirements) and or on a technical basis (water depth beyond limits of equipment, unacceptable bottom conditions possibility of iceberg scour), a number of offshore loading systems possibility of iceberg scour), a number of offshore loading systems were devised to permit tanker transport to replace pipelines. These systems were given the generic name of single-point mooring system, SPM, although they were more precisely designated in the industry ELSBM, SALM, SPAR. The SPM concept was not new; it was previously used in conjunction with large, onshore export terminals for loading Cankers in protected. shallow water. But, extensive modifications were required for its use in exposed, deep water.

The SPM system was combined, in theory and in practice, with several types of production platforms-piled tower (steel), gravity stabilized (concrete and/or steel) or floating (semisubmersible or tension leg). Different storage facilities were also proposed-integral (gravity stabilized) or external (SPAR, permanently moored or "tethered" tanker). Consequently, an array of production/storage transport systems - all of which were adequate for the expected environmental and reservoir conditions - was often specified for each potential development site. However, the decisions, or non-decisions, that were then made regarding the system configuration were rarely based on the objective consideration of all of the optimal alter-natives-both good and bad decisions, fortuitously, were ultimately enhanced by increased crude-oil prices.

THE PROBLEM

The development of offshore productive capacity, particularly in a frontier area, requires a timely decision based on limited and uncertain information. To make that decision-that is, to proceed with a specific system configuration -in a consistent and intelligent manner, it is necessary, but not sufficient, to consider all of the feasible alternatives. Sufficiently requires that each of the alternatives be optimized on the same economic bases. Therefore, the general problem is to determine the optimal configuration for a particular type of system. In this paper, however, the problem is limited to the optimization of those feasible alternatives that specify offshore tanker loading through an SPM system.

THE MODEL

To solve the problem posed, an economical model that faithfully interrelates the system components is required. It must be easy to understand and to use-and, it should allow for the use of judgement when data is not available.

The first requirement is a physical model which will represent the behavior of the total system. It must be simple enough to be imbedded in the economic model, but it must also be flexible enough to be realistic. Therefore, let us assume that Figure 1 adequately represents the system to be considered and proceed to examine its elements.

If we assume a simple storage model similar to that shown in Figure 2, the problem is deterministic, but it is also trivial.

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