Experience gained from the exploration, appraisal, and development of aNorth Sea block resulted in a drilling-engineering strategy that helps achievethe maximum benefit from each well. All wells are designed to accept a subseacompletion and to allow for redrilling. The number of well sites and flowlinepaths is minimized by the use of well clusters.


Since Well 9/13a-1 discovered the Beryl field in Block 9/13 in May 1972, 215miles [346 km] northeast of Aberdeen, 42 more subsea wells have been drilled inthe block, 12 of which have been completed. The early wells were for theappraisal of the original discoveries and for exploration in other areas of theblock. The success of the discovery resulted in the installation of twocombined drilling and production platforms. The Beryl A, a three-leggedconcrete platform with storage capacity in the base platform with storagecapacity in the base for more than 1 million bbl [158 990 m3] of oil, wasinstalled in 1976. In 1983, the Beryl B, a steel-jacket structure, wasinstalled with an oil-export line to the Beryl A platform. Oil export from theblock has always been by tanker loading from a single-point mooring. In 1986, the Ness field was discovered. by Well 9/13a-28 and came on stream with asubsea development in 1987 (Fig. 1).

From the very first well, the possibility of reusing the exploration andappraisal wells for subsea production by means of the platform facilities wasalways an option, and platform facilities was always an option, and wells thathad identified significant hydrocarbon potential were suspended pendingcompletion. Well 9/13a-20 was the first subsea development well drilled;several more subsea development wells were subsequently drilled. This patternof subsea developments that use the process and export facilities of a platformis common in North Sea oil production. production. Subsea development drillingcan be separated into three categories: completion of an exploration well, development drilling of wells as subsea producers, and redrilling ofexploration and development subsea wells. These well types all have differentobjectives. Thus, the best design for an exploration well may be unsuitable fora subsea completion and sidetracking may compromise the initial welldesign.

The nature of a subsea development implicitly involves the other disciplinesof pipeline, operations, and facilities engineering. pipeline, operations, andfacilities engineering. These also impose requirements that must be consideredin a well's drilling engineering. This paper examines how all the potentiallyconflicting requirements can be potentially conflicting requirements can besatisfied and how an integrated approach to exploration, appraisal, anddevelopment wells can lead to a cost-effective solution.

Well Locations

The first consideration for a subsea development well is the surfacelocation. In the early stages of field appraisal, before installation of anyfield facilities, this was not a problem. Wells were drilled vertically togeologic or reservoir targets. This policy, a haphazard approach for adevelopment strategy, led to considerable problems of access later in the fieldlife. The following criteria were subsequently adopted for locatingexploration, appraisal, or development wells within the block.

Simultaneous Access. The "footprint" of a moored rig is oftensignificant compared to well spacing. If the distance between wells is to beless than twice the length of the anchor radius, careful planning of the anchorpattern at each location is required to avoid pattern at each location isrequired to avoid the situation where a rig at one location precludes access toa rig at another location. precludes access to a rig at another location. Flowline Clearance. One requirement of a subsea development is a subseaflowline. In the case of an elaboration or appraisal well, the route of thisline may be unknown or undecided. Therefore, an exit route from a wellhead thatis clear of the anchor pattern should always be considered. Two other pointsmust be considered for flowline points must be considered for flowline routes. First, flowlines are rarely simple lines. A relatively small subsea developmentmay have production, water-injection, gaslift, and control umbilicals. Thecombined width of all these lines, allowing for spacing and installation, maybe appreciable. Second, avoiding a flowline is easy for a rig if the flowlineis close to the wellhead location (i.e., beneath the suspended segment of thecatenary) or beyond the anchor. If the mooring line is to cross the flowline, the flowline will need protection or the mooring line will requiremodification.


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