API RP 95F-2006 pdf download.Interim Guidance for Gulf of Mexico MODU Mooring Practice—2006 Hurricane Season.
4 Mooring Design Criteria 4.1 CURRENT DESIGN CRITERIA API RP 2SK 3rd Edition (2005) provides the basis for mooring design in the Gulf of Mexico for both MODUs and permanent installations. Appendix I contains a listing of key MODU mooring design parameters extracted from API RP 2SK. 4.2 RECOMMENDED MODIFICATIONS FOR GULF OF MEXICO MODU MOORINGS 4.2.1 Design Environment Return Period For operations during the hurricane season, the design environment return period should be established from the following princi- ples: • The return period should not be less than 10 years for the design of any mooring system based on site-specific metocean study • A minimum 1-minute wind speed of 64 knots shall be used even if site-specific studies indicate a lower value • In the absense of site-specific data, the environmental parameters presented in Section 11 shall be used • Data used in prepairing site-specific metocean parameters shall be inclusive of weather information for the period 1950 through 2005 • A site specific assessment as described in Section 5 shall be conducted for the specific drilling operation and loaction. Miti- gation strategies should be considered during the design of the mooring system 4.2.2 Anchor Capacity Anchor holding capacity (for all types of anchors) shall be considered in the design of the mooring system. Anchor selection should be based upon capacity, availablity, and protential to minmize damage to subsea infrastructure should an anchor failure occur in condition such as: • A marine installation such as a pipeline, which lies in the dragging path between the anchor and the MODU • A mooring line that crosses another mooring line • Density or importance of seafloor or water column infrastructure merits a higher safety margin Unless site-specific soil data is available, upper and lower bound soil conditions shall be considered.
5 Site-Specific Assessment 5.1 ASSESSMENT The probablity and consequences of a MODU losing station when operating at any location within the US Gulf of Mexico should be assessed. The intent of the assessment process is to identify the characteristics of the area near the drilling operation, options related to mooring component selection and mooring system design, and mitigation opportunities prior to the finalization of the design, installation, and operation of the mooring system. The resulting mooring system should lead to an acceptable risk scenario through minimization of potential consequences due to the loss of function in a mooring component or mooring system or the reduction in the probablity that a mooring component or the mooring system will lose function for a given MODU and operation. In performing the assessment, one souce of infrastructure information is the Gulf of Mexico infrastructure map maintained by the Minderals Management Service. 5.2 ASSESSMENT REQUIREMENT Risk is defined as: Risk = [Probability of an adverse effect occurring] x [The consequences associated with that event] The risk can be reduced either by reducing the probability of experiencing an incident, or by reducing the consequences of that incident should it occur. A fundamental part of reducing the risk associated with MODU operations is to ensure that all parties have a clear understanding of their “Risk Exposure.”
5.2.1 Checklist Approach The checklist approach is a simple evaluation methodology that allows the stakeholders to assess, on a relative basis, a level of risk the well operations represent. The intent of this approach is to be more conservative by comparison to a rigorous analysis. However, the checklist can be completed with the routinely available information and data that would be expected to be available to the Operator and Drilling Contractor. An example checklist approach is included in Appendix II. The data portion within the checklist describes the attributes of the location and the drilling vessel mooring system. These include, for example, the local surface and subsea infrastructure, the gen- eral location, the type of mooring system to be used, and the months that the drilling operation is planned. A checklist approach should be based on three dimensions: 1. Consequence factors based on location (infrastructure that could be damaged in the event of a mooring failure) 2. Design of mooring components and system 3. Likelihood of exceeding design conditons 5.2.2 Full Risk-Based Approach The full risk-based approach is commonly used by a number of Operators. The process contains a series of steps to formally assess the risk at any given location. Due consideration should be given to the time required to complete this process. The steps can be summarized as: • Definition of Location and Well Parameters • Identification of Local and Distant Infrastructure • Undertaking a Hazard Identification (HAZID) Study • Likelihood of Mooring Failure • Quantification of the Consequences of Failure (e.g., through Event Tree Analysis) • Risk Mitigation • Documentation