Andrew Area Development: Dropped Object Assessment for JPK’s 48” Caisson Clamp 1/3

TECHNICAL NOTE

To: Mohammad Nabavian

Cc: Shaun Harnden; David McCann; Colin Wilson; Nick Edwards

From: Swee Fook Yong

Date: 11th May 2010

Ref: 2425-AIG-TN-003, Rev A1

Re: Andrew Area Development - DETAILED Phase: Fatigue Assessment of Welded Top Caisson Guide Connection at EL(+) 13.0m

1. Introduction

It is proposed to install new Andrew Area Development (AAD) facilities, including a new 44” caisson, on the BP Andrew Platform for the AAD Project. A new 44” caisson is to be installed at centre east face (Frame 6) of the platform with the Dead Weight Support (DWS) at EL(-)23.600m. Other three caisson guides are to be located at EL(+)13.000m, EL(-)54.875 and EL(-)85.850. The guides at EL(-)54.875 and EL(-)85.850 are to be clamped on the existing jacket brace member while either clamped or welded options are proposed for the top guide at EL(+)13.000m. In order to support the Project for decision making on the top guide configuration, this Technical Note presents the fatigue assessment results of the welded top caisson guide configuration at EL(+)13.0m.

2. Stress Concentration Factors

The welded option for the top guide configuration at EL(+)13m is shown in Figure 2.1, where the box-shaped brace member of the top caisson guide support structure are to be welded onto the existing jacket brace member.

Figure 2.1: Welded Top Guide Configuration

Andrew Area Development: Dropped Object Assessment for JPK’s 48” Caisson Clamp 2/3

In order to accurately determine the local Stress Concentration Factors (SCFs) of the welded joint taking into account of the geometric contribution from the box-shaped brace member, a detailed finite element (FE) analysis of the top guide configuration was carried out. These SCFs have been utilised in the BP Andrew fatigue model, with 1530Te Andrew Area Development loads, for fatigue life assessment of this joint. Table 2.1 shows the local SCF at different hotspot positions with the effect of seal plate thickness.

Table 2.1: Local SCF at Different Hotspot Positions

The hotspot positions are illustrated in Figure 2.2.

Figure 2.2: Hotspot Positions

Min-plane

Mout-of-plane

Andrew Area Development: Dropped Object Assessment for JPK’s 48” Caisson Clamp 3/3

3. Fatigue Analysis Results

Since the no seal plate option provides the highest local SCFs, i.e. lowest fatigue lives, only local SCFs from 10mm and 25mm seal plate options are utilised in the fatigue model. Various S-N curves from DNV and HSE have been employed and the fatigue results are shown in Table 3.1. Sensitivity studies of the local SCFs in relative to axial, in-plane and out-of-plane bending moment on the fatigue life of the welded connection are also performed. Although the axial SCF on certain hotspot positions is high on either chord and brace side, this does not penalise the fatigue life of this joint due to minimal axial load from this top guide arrangement onto the existing jacket brace member. The main fatigue damage is from the out-of-plane bending moment of the box-shaped brace member. In accordance with the DNV (RP-C203) recommendations of S-N ‘D’ curve, the minimum fatigue life of the welded connection is 132 years at south brace connection. The governed hot spot position is located at the corner of the brace, i.e. Position 11 of Figure 2.2.

Fatigue Life

South Brace North Brace SN

Curve Unity SCF

10mm Seal Plate

25mm Seal Plate

Unity SCF

10mm Seal Plate

25mm Seal Plate

Remark

DNV C2 5800000 217 294 6490000 264 365

DNV D 3430000 132 178 3840000 160 220 DNV Recommended

DNV D - 149 - - 182 - With Axial SCF = 1

DNV D - 317 - - 409 - With IPM SCF = 1

DNV D - 21500 - - 21300 - With OPM SCF = 1

DNV E 1900000 77 103 2120000 93 127

DNV F 1050000 46 60 1170000 55 74

DNV F3 320000 17 22 358000 20 27

HSE D 1820000 74 99 2030000 89 127

HSE D - 136 184 - 165 228 No Thickness Effect

HSE F2 224000 13 17 251000 15 20

HSE F2 - 22 18 - 26 35 No Thickness Effect

Table 3.1: Fatigue Lives

4. Conclusions

The minimum fatigue life of the caisson top guide welded connection is 132 years at south brace, which is about 9 times higher than the proposed fatigue life of 15 years.