Platform Construction From an Adjacent Semisubmersible Vessel
Authors: Edwards W.R. 
Published: December 2007

Edwards, W.R., Occidental Petroleum (Caledonia), Ltd.

Semisubmersible support barges (SSSB) alongside platforms in the North Sea have made year-round construction 95% available. This concept was used successfully to develop the Piper and Claymore fields. A checklist is given for outfitting SSSB's for construction support.

Introduction

Traditionally, offshore platform construction has occurred from an adjacent vessel since the first platform was installed in the Gulf of Mexico in 1946. As platform size and complexity increased, so did the requirements for construction support. Fortunately, the normally placid waters of the Gulf of Mexico permitted use of placid waters of the Gulf of Mexico permitted use of conventional barge hulls for this service, with sufficient reliability to complete programs continuously.

The advent of exploratory drilling in the North Sea caused the evolution of more refined versions of the column-stabilized semisubmersible hull with which to conduct those drilling programs reliably. These units are designed to continue drilling in 35-ft seas and to keep station in waves of up to 100 ft. Unfortunately, vessel designs suitable for extensive rough-water platform construction have not kept pace with those for drilling, with the result that offshore construction programs were designed to be completed in the more predictable summer "weather window." This sort of planning requires duplicate provisions for personnel, systems, and equipment to assure timely completion, with resultant escalating costs and operational inefficiencies. Perhaps a classic example of the need to extend the weather window into year-round operation was the installation of Occidental Petroleum (Caledonia) Ltd.'s North Sea Piper platform jacket. Petroleum (Caledonia) Ltd.'s North Sea Piper platform jacket. This program was an early offshore effort by the Occidental consortium to develop their Piper and Claymore North Sea fields. This consortium - comprising Occidental Petroleum Corp. (36.5%), Getty Oil International (England) Ltd. (23.5%), Thompson Scottish Petroleum, Ltd. (20%), and Allied Chemical (North Sea) Ltd. (20%) - acquired the Piper Block (U.K. 15/17) and the Claymore Block (U.K. 14/19) in March 1972, and began an aggressive exploratory drilling program that resulted in discovery and definition of these two fields in 1973-74.

Piper and Claymore Field Developments Piper and Claymore Field Developments Piper Platform Construction Piper Platform Construction In June 1975, Occidental began installation of the Piper platform. In water 474 ft deep, at 58.5 deg. N latitude, this platform. In water 474 ft deep, at 58.5 deg. N latitude, this drilling production platform was designed to accommodate a total of 36 wells, with provisions for 30 production wells and 4 water and 2 gas injection wells. Platform production and support facilities were designed for production and support facilities were designed for 250,000 B/D oil production, 330,000 B/D water injection, and 70 MMscf/D gas processing.

The platform jacket, constructed in Scotland and weighing about 14,300 tons at launching, was towed to the field in June 1975, launched from the barge Intermac 600, upended, rapidly pinned with four 48-in. pin piles, and driven 110 ft into the seabed through four inner piles, and driven 110 ft into the seabed through four inner legs. Because of poor local soil conditions, the jacket design provided for six 60-in. primary piles per comer, driven to 115 ft, and an insert pile in each primary pile. The inserts were grouted into holes predrilled 380 ft below the mud line and tapered from 50 to 36 in.

JPT

P. 1679