SS: MODU Anchor-Regional Analysis of Seafloor and Near Surface Sediments for MODU Anchors
Authors: Berger William J. , Harper Melissa , Oyebode Olaniyi 
Published: July 2009

A significant challenge of predicting Mobile Offshore Drilling Unit (MODU) anchor performance is the lack of detailed sediment information. There are several proprietary and non-proprietary sources of seafloor and shallow sediment information. The various sources of sediment information can be merged into a comprehensive regional database. A regional model can be developed for use in the planning of MODU anchors prior to drilling. This paper will document the methodology of the various types and format of sediment information, and the use of that information for planning MODU anchors prior to drilling. In addition, this paper will demonstrate similarities and discrepancies between the regional model and site-specific data. The sources of sediment information originate from academic universities, government databases, joint industry projects, and individual companies. Regional studies include sediment classification with some chemical and physical characteristics. Site-specific samples are often collected as box cores, drop cores, piston cores, and rotary soil borings. The properties of the samples can be geographically represented in a Geographic Information System (GIS). The application of available sediment data can facilitate improved predictions of MODU anchor performance. Regional information can be classified within a site-specific area. Generalities can be inferred from the regional data, and the results can be supplemented by available seismic data. In areas without site-specific geotechnical data, the regional information correlated with seismic data can provide more detail when planning MODU anchor layouts prior to drilling. The use of readily available regional information can be a very cost-effective approach when designing a MODU anchor layout. The regional model can also be used to refine a site-specific geotechnical field program. Additional site-specific information can be analyzed and compared to the regional geologic model and refined accordingly.

Introduction

The design of MODU anchors should be based on reliable, site-specific information concerning seafloor features and sediment properties. Sediment properties of interest in MODU anchor planning include, but are not limited to, shear strength, bulk density, water content, and grain size. These sediment properties may be divided into categories that describe the bulk sediment characteristics or the individual grain properties. Different methods of sediment and data collection are used depending upon the properties to be measured. For example, in-situ measurements are ideal for certain bulk sediment properties that require an undisturbed sample, whereas grab sampling of seafloor sediments may be necessary for accurate grain size measurements. The acquisition of sediment property information is expensive and time-consuming; however, a substantial body of sediment information already exists for the northern Gulf of Mexico (GoM). Much of this public information has been amassed into searchable GIS databases (ie. usSEABED, Buczkowski et al., 2006).

Utilizing available regional data is an efficient, cost-effective approach to designing a MODU anchor layout. Generalized regional models based on existing data can be used to refine a site-specific geotechnical field program. Regional models are particularly useful in frontier areas of the GoM where existing well data is limited or non-existent.