Detailed Description :

As the Frigg field in 1999 was approaching the end of production life, Elf Petroleum Norge A.S. committed DORIS Engineering, former designer/builder of the Frigg CDP1 (1975) and TP1 (1976) platforms, for helping in evaluating, in terms of feasibility, risks, schedule and cost, several disposal options for these two platforms. ode contributed to the successful completion of these areas of the work through the secondment of key personnel into DORIS:

• Feasibility studies
• Structural Assessment
• Mechanical Assessment
• Naval Architectural Assessment
• Construction/Operational Assessment
• Risk assessment
• Contribution to environmental assessment
• Schedule assessment
• Cost assessment

CDP1 and TP1 are concrete gravity base structures (GBS). CDP1 comprises a raft foundation and an external lobate perforated shell filled with sand/gravel ballast and extending above sea level. A central shaft housing risers provides, together with columns fitted on top of the lobate shell, the support for the concrete deck beams. TP1 substructure is made of a caisson filled with seawater and topped with two columns supporting a steel MSF.

Five disposal options have been considered for each platform :

Option 1 : Leave in place. This option consists in leaving the concrete substructure in place after removing the topsides modules and MSF. Navigation marking must be implemented.

Option 2 : Leave in place, after removing the topsides modules/MSF and all steel items attached to the concrete structure. Removable steel items are understood as 'practically and reasonably accessible for removal'.

Option 3 : Partial removal. This option consists in leaving the lower part of the concrete structure in place, whilst dismantling, topling or removing the upper part down to -55m below sea level.

Option 4a : Complete removal. This option consists in removing both the topsides and the substructure, with a subsequent sinking offshore, in deep sea.

Option 4b : ditto 4A but after refloat, towing to a deep water sheltered site and full dismantling inshore/onshore.

Their feasibility has been evaluated, with due account to relevant regulations, along with risk assessment studies and schedule/cost estimates, so providing a reliable assistance to ELF for further selecting the preferred option.