This paper describes a new application for covering subsea flowlines with sand and gravel.
Two small diameter crude oil flowlines rapidly developed unexpected production problems which resulted from high viscosities and wax deposition caused by low transport temperature.
The Operator evaluated several possible methods for alleviating the problem. Retrofitting thermal insulation to the "As-laid" flowline was subsequently selected as a long term maintenance-free solution. In close cooperation with the Contractor, an innovative method of accurately applying sand and gravel to achieve good thermal insulation properties was developed and later executed with excellent results.
Using the experience gained here and from previous trials the Contractor further developed a variation of the method for areas where trenching is difficult or impractical due to adverse soil conditions. Subsequently this variation was used in practice by a second major Operator.
This paper includes an historical overview of the evolution in the use and techniques of the accurate placing of sand and gravel and also includes some theoretical aspects regarding thermal insulation properties when used in a sub-sea environment.
At the end of the 1987 and at the start of the 1988 offshore construction season, two pipeline insulation projects, using a sand and gravel combination, were executed in the northern North Sea. The Operators' motives for selecting such an insulation method were completely different.
The first project was performed after an unexpected production problem rapidly developed in newly constructed and operating crude oil pipelines which forced the Operator to take prompt remedial action. Under these circumstances, the insulation method has successfully demonstrated its capability to alleviate or solve low temperature related subsea production or topsides separation problems which are either developing or established in existing oil fields.
The second project was performed as the result of a successful trial completed early in 1987. The results of the trial demonstrated the viability of the technique to the Operator. Consequently the method was applied during the construction phase of a new satellite subsea well.
Under these circumstances the insulation method has demonstrated it can be successfully used for thermally insulating future pipelines when the necessity for such insulation is identified during the engineering phase of a project.
Both projects have shown a sand and gravel combination used for thermal insulation is a viable alternative, both technically and economically to more traditional pre-lay pipeline insulation methods such as neoprene cladding.
The method becomes especially attractive economically if bundled flowlines are installed, or stabilization and/or mechanical protection are also required.
The first project was executed in the Ness field, an oil producing reservoir in the northern North Sea in Block 9/13b, approximately 7.3 kilometers south-west of the Beryl Bravo Platform and operated by Mobil North Sea Ltd. Other participants are Texas Eastern North Sea Inc., Armerada Hess Ltd. and Enterprise (E &P) Ltd.
