ABSTRACT

With a new trend towards faster realization of capital investments for offshore hydrocarbon developments, the use of pre-drilled wells using a pre-installed template is becoming increasingly attractive. This method enables a more rapid production of oil from the field source and, in certain cases, lower overall development costs.

The development of docking technology over the last decade is examined with particular reference to' North Sea experience. The method of pin/sleeve indexing arrangement has become the more common method in recent years especially with the heavier Northern North Sea jackets.

This approach usually involves mating of two or more docking sleeves on locating piles adjacent to the template, prior to grounding of the jacket. The accurate prediction of the forces between the piles and sleeves becomes of paramount importance not only in detail structural design, but in the overall assessment of the limiting sea states for installation.

This paper describes the analytical methods that have been developed for the prediction of the docking forces on jackets during installation. Both frequency domain and time domain approaches are adopted to cope with nonlinearities such as pile-sleeve separation. The analytical approach also allows for operations where the jacket is indexed to the template using a Semi-Submersible Crane Vessel SSCV.

The method involves the solution of motions for coupled two body systems (e.g. a jacket and an SSCV) during the installation and shows results of pile loads, limiting sea states and some correlation with model test data for two case studies.

The paper concludes by examining the sensitivities of docking loads to the approach and the analytical methods adopted and proposes some future developments in docking technology.

INTRODUCTION

Recent economic constraints imposed on the offshore industry have demanded rapid recovery of capital investment. Conventional field development has been based on series scheduling, i.e. the jacket and topsides is installed and commissioned prior to drilling. Such an approach results in longer payback times and higher costs. However, if the drilling can be performed in parallel to the design and construction of the facilities, then production schedules and hence costs may be substantially reduced.

Parallel development of a field can be achieved by installing a drilling template two to three years in advance of the facility installation. Thus, as detailed design and construction of the facility is undertaken, several or all wells can be pre-drilled using a drilling rig and capped off. The facility is then installed over the drilling template, pre- drilled wells are tied back and re-entered using the platform's own equipment allowing several wells to produce oil on commissioning of the platform facilities.

This approach however imposes new constraints on the installation of the jacket. Position control becomes of prime importance to ensure that the pre-drilled wells align with the conductor framing in the facility within acceptable tolerances. Table 1 compares typical setting tolerances of a jacket over a pre-drilled template as against that of a conventional jacket.

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