Model Testing in Support of Molikpaq Deployment at Sakhalin Island
Authors: Davies M.H. , Readshaw J.S. , Sullivan M.A. , Wells H.C. 
Published: October 2008

Sakhalin Energy Investment Company has modified and deployed the Molikpaq as the first production unit in the Russian subarctic NE of Sakhalin Island. Four separate physical modeling programs were undertaken to define and assist in the modification of the structure. These programs included:Interaction of the Molikpaq with a submerged, rockarmoredberm;Wave loading and wave run-up on the Molikpaq/Spacer;Design modifications required for top-side facilities; andOptimization and construction of rock armor scour protection around the base of the Molikpaq/Spacer.

The various alternatives considered showed significantly different wave interaction patterns. For example, necessary alterations in base geometry were seen to have a large influence in the magnitude of the maximum forces, their distribution on the structure, the degree of overtopping and the details of required scour protection. The various studies undertaken have also examined the response of the Molikpaq to regular, irregular and multi-directional waves.

The extremely harsh environment at Sakhalin (shallow water and steep waves) has highlighted the utility of physical modeling in the engineering process. The modeling programs undertaken have provided essential engineering design insight and data which could not otherwise have been obtained.

Introduction

Sakhalin Energy Investment Company has modified and refurbished the Molikpaq, an Arctic offshore drilling caisson, originally designed for the Canadian Beaufort Sea, by adding a new steel pontoon base named the "Spacer".

The combined structure was deployed 16 km NE of the coast of Sakhalin Island in September of 1998 at the Piltun-Astokhskoye (PA) field. Details of the field, the overall design process and evolution, construction and deployment are provided in two companion papers1,2,3,4.

At the PA field, ice conditions are less severe than in the Canadian Beaufort and wave conditions dominated the design process (100 year return period Hs=9.8m, the water depth is about 31m).

Facilities

The studies of the interaction between the Molikpaq and the submerged rock berm and the wave loading, wave run-up studies and scour protection design were undertaken at the Multi-directional Wave Basin and the Irregular Wave Basin at the Canadian Hydraulics Centre (CHC) in Ottawa, Canada. The design modifications for top-side facilities were tested in the Offshore Engineering Basin at the Institute for Marine Dynamics (IMD) in St. John's, Newfoundland. Both CHC and IMD are part of Canada's National Research Council.

Berm Testing

Purpose

The initial deployment scenario for the Molikpaq at Sakhalinwas to place the Molikpaq on a rock-armored berm. This berm would elevate the Molikpaq so that sufficient freeboard would be maintained to minimize wave overtopping. Preliminary analysis indicated that the presence of a sloping berm would encourage wave breaking and thereby reduce wave impact on the platform. To fully evaluate and optimize this design, a detailed three-dimensional model study was required which could evaluate wave transformations around the berm and Molikpaq and determine rock armor requirements. The 3-D tests would involve massive amounts of material and relatively long construction times. To minimize 3-D test time, a small 2-D study was first undertaken to allow preliminary evaluation and optimization of the proposed design.