Semi design - estimation of hull dimensions

Abstract

One of the processes early in the development of offshore oil and gas fields is the concept selection. Because semi-submersible and other alternative designs is very complex, a simplification in the early design phase could lead to time and cost savings. The objective of this thesis is to develop a simple spreadsheet, which estimates the hull dimensions and cost based on a few input parameters. The estimated hull must have sufficient stability, heave motions and air gap.

All the important hull parameters have been evaluated and data from relevant semis have been gathered. Based on these evaluations and data, the input parameters are selected and estimation equations for the parameters are developed. The input parameters for the spreadsheet are topside weight, weather condition and air gap margin. The estimations are mainly based on linear relationships and are shown to give reasonable values when compared to existing semi-submersibles. Especially the normal draft semis give very good estimations.

A parametric sensitivity study was conducted to look on the possibility to reduce heave motions. The analysis of different hull configurations showed that draft is the only parameter giving a noteworthy reduction. The necessity of reduced heave motions is the possibility to use steel catenary risers. The necessary reduction was chosen to be roughly 50 %. The study showed that a draft of 44 meters was necessary for this reduction, independent on semi weight. This deep draft alternative is included in the estimation spreadsheet.

To secure sufficient air gap, an air gap analysis was conducted for five semis generated with the estimation spreadsheet. The semis was analyzed for four different weather situations, which represent the most common locations for offshore oil and gas production. One of the locations is also analyzed in more detail. This detailed analysis show that the most critical weather situations are for periods below TP and the additional response can result in an insufficient air gap. In addition to estimate the required air gap, the spreadsheet accommodates the possibility to define the wanted air gap margin. The analysis also show that the estimated semis have good stability and motions.

With only three simple input parameters, the estimation spreadsheet combines the presentation of hull dimensions, weight, stability and cost in a good and user-friendly way.